20 June 2024, Volume 37 Issue 3
    

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  • Wear and Rolling Contact Fatigue Behavior of Locally Laser Cladded Rail Material Using Self-fusible Alloy
    DING Haohao, XIE Tianxing, WANG Wenjian, ZHU Yi, YANG Yi, GUO Jun, LIN Qiang
    China Surface Engineering. 2024, 37(3): 1-13. https://doi.org/10.11933/j.issn.1007-9289.20230830002
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    With an increase in the axle load of trains, damage to the rails becomes more severe, decreasing their service life. Thus, local repair could be a solution for rail surface damage. Laser cladding is a relatively new additive manufacturing technology that can be used for local damage repair. However, wear and rolling contact fatigue (RCF) damage behaviors, particularly the damage at the boundary between the clad and unclad zones, have not been thoroughly explored. Thus, a pothole is cut off from the U75V rail sample (a roller sample) to simulate the local damage on the rail. Ni-, Fe-, and Co-based self-fluxing alloy powders (F103, Fe-Cr, Fe-58, Stellite 21, Stellite 22, and Stellite 23) are laser cladded at the pothole using a CO2 laser with a rectangular spot size of 7 mm × 1 mm, a laser power of 1.9 kW, a scan speed of 200 mm / min, and a powder feed rate of 15 g / min. The microstructure and hardness of the locally repaired rail materials are analyzed. The wear and RCF behaviors of the laser-repaired rail samples are studied using the twin-disc rolling test with a maximum contact pressure of 1.1 GPa, a slip ratio of 0.75%, and a rotational speed of 500 r / min. The number of cycles for each rolling test is 105 . The friction coefficient, wear rate, depth, plastic deformation, and damage morphology are analyzed. The results showed that the cladded sample could be divided into three regions in the depth direction on the cross section or in the rolling direction on the surface: the clad zone, heat-affected zone, and substrate. Fine eutectic and dendritic structures are formed in the laser cladding. The hardness is higher than that of the substrate. The microstructure of the Ni-based clad is coarse, and its hardness is low. The microstructural sizes of the Fe-and Co-based clads are small. The hardness of the Fe-based clads is high and that of the Co-based clads has an intermediate value. During the rolling test, the friction coefficient exhibits an increasing trend during the running-in period and then remains stable. The stable friction coefficient is approximately 0.4 and shows no evident difference for samples with different clads. After the rolling test, the wear rates of the samples with Ni-and Fe-based clads are high, and those of the samples with Co-based clads are low. The wear depth in the clad zone is smaller than that in the unclad zone. The surface hardness of the samples is increased after testing, and plastic deformation of the microstructure is observed in the cross section. Compared with the uncoated zone, the thickness of the plastic deformation layer in the laser cladded zone (that is, clads) is smaller. With an increase in the original hardness of the cladding, the hardness after testing is increased; however, the hardening ratio and plastic deformation layer thickness are decreased. The damage mode of the laser-repaired rail is predominated by fatigue wear. In the unclad zone, the RCF crack length is large whereas the crack angle is small. In the clad zone, the crack length is decreased whereas the crack angle is increased. The RCF damage at the boundary between the clad and unclad zones is the most severe. The crack angle and depth at the boundary are greater than those in the clad and unclad zones. Comparing the six studied clads, notably, the Stellite 21 (Co-based) cladded sample presents a lower friction coefficient, smaller wear depth difference between the cladded and uncladded zones, and better RCF resistance, making it more suitable for laser repair of local rail damage. The research results can provide theoretical and technical guidance for the application and optimization of laser cladding technology for local rail repair.
  • Inhibition Mechanism of Laser-cladding Porosity Defects Based on Steady-state Magnetic-field Damping Effect
    HU Yong, MENG Qingxin, WANG Liang, ZOU Pengjin, LIU Yunfeng, ZHANG Qunli, YAO Jianhua
    China Surface Engineering. 2024, 37(3): 14-24. https://doi.org/10.11933/j.issn.1007-9289.20231111002
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    Laser cladding, which is characterized by a small heat-affected zone, a low dilution rate, a wide range of material applications, and the ability to achieve excellent metallurgical bonding with a substrate, has gradually been applied in fields such as aerospace and energy. As one of the primary defects in laser additive manufacturing, pores significantly affect the mechanical performance under dynamic loads. A method assisted by a steady magnetic field to suppress pores without changing the laser-processing conditions is employed in this study. A multiphysics model for pore transport under a magnetic field is constructed to elucidate the inhibitory mechanism of a steady-state magnetic field on pore defects. The flow-field-distribution patterns in the molten pool, the pore-transport trajectories, the pore distribution, and the internal element distribution under various magnetic-field intensities are systematically investigated. A steady magnetic field is positioned on both sides of a ductile iron (QT-400) substrate with a carbon content of approximately 3.5 wt.%. Austenitic stainless steel (AISI 316L) powder with a particle size of 50–110 μm is used. The pores are observed using an optical microscope, and the elemental distribution is analyzed using an energy dispersive spectrometer. A two-dimensional transient finite-element model is established using the Comsol6.0? multiphysics coupling analysis software. Without applying a steady-magnetic field, the maximum surface flow velocity of the molten pool is approximately 0.137 m / s. As the magnetic flux density increases to 1.2 T, the induced Lorentz force generated by the external steady magnetic field within the molten pool enhances the viscous effect of the fluid, thus reducing the flow velocity within the molten pool to 0.054 m / s. Subsequently, the transport of pores in the molten pool model is considered and the trajectories of their movement are calculated. Without an external field, pores with diameters of 40 and 80 μm at the front of the molten pool exhibit reciprocating helical motion within the molten pool. As the pore diameter increases to 120 and 160 μm, the buoyancy exerting on them increases, and under the combined action of fluid drag force, they propagate toward the surface of the clad layer. Under steady magnetic-field conditions, as the magnetic-field intensity increases, the trajectories of the rear pores transition to a vertical-upward movement and eventually remain within the molten pool. The front pores undergo periodic motion, thus rendering it challenging for them to be expelled from the molten pool. At a magnetic field intensity of 0 to 0.6 T, the inhibitory effect of the magnetic field on the pores is not prominent. However, as the magnetic-field intensity increases to 1.2 T, the porosity decreases from 13.357% to 7.768%, thus indicating that the steady magnetic field suppresses the pores. Further analysis of the distribution patterns of the elements in the molten pool shows that under a steady magnetic field, the color gradient of Fe is more prominent compared with the case when no magnetic field is applied. Applying a magnetic field increases the disparity in elemental content between the cladding layer and substrate. This augmentation reduces the dilution effect of the substrate elements on the clad-layer composition and is primarily attributed to the magnetic damping effect, which decreases the fluid velocity, inhibits the entry of external oxygen into the molten pool, and hinders the formation of pores through the combination of carbon. Consequently, the number of pores is reduced. The results of this study can provide a theoretical foundation for controlling pore defects in processes such as magnetic-field-assisted laser cladding, welding, and repair.
  • Microstructure and Tribocorrosion Properties of Cr-W-Mo-V Coating Fabricated via Laser Hot-wire Cladding
    WANG Yanfang, ZHOU Xuejing, SONG Zihan, SHI Zhiqiang
    China Surface Engineering. 2024, 37(3): 25-36. https://doi.org/10.11933/j.issn.1007-9289.20230831001
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    Owing to the development of marine oil and gas resources, drilling and exploitation equipment are affected by coupled damage from corrosion and wear. The development of wear-and corrosion-resistant coating materials as well as preparation technologies guarantees the safe and reliable operation of marine equipment. Laser cladding is a promising surface modification technology that is widely used in machine-component repair and functional coatings. However, the limitations of the deposition and material-utilization rates of the conventional laser cladding technology restrict its widespread use. Laser hot-wire cladding (LHWC) is a promising, efficient, and energy-saving coating-deposition process. In this study, Cr-W-Mo-V steel coating is prepared on the surface of a 20 steel substrate using LHWC technology. The microstructure and phase composition of the coating are analyzed using a ZEISS optical microscope, an XRD-7000 X-ray diffractometer, and a JSM-7200F scanning electron microscope. The microhardness of the coating is measured using an HV-100A microhardness tester. The dry friction factor, polarization curve, and electrochemical impedance of the coating are measured using a reciprocating electrochemical corrosion friction and wear tester (MFT-EC4000, HUAHUI). The loss of wear marks is measured using three-dimensional morphometry, and the microscopic morphology and elemental distribution of the wear marks are observed using scanning electron microscopy and energy dispersive spectroscopy. The open potential, dynamic polarization curve, and constant-potential polarization curve of the coating at various friction load in 3.5 wt.% NaCl solution are analyzed and the tribocorrosion mechanism is discussed. The prepared Cr-W-Mo-V steel coating is uniform and compact-without cracks, pores, or other defects-and is primarily composed of carbide, martensite, and residual austenite phases. The microstructures are primarily columnar and cellular crystals. The average hardness of the cladding layer is 780 HV0.1, which is approximately 6.5 times the hardness of the substrate. The friction factor of the coating is between 0.15 and 0.18, and the friction factor and wear amount of the coating are smaller than those of the substrate. The wear volume of the coating increases from 2.36× 10?3 to 1.33×10?2 mm3 as the friction load increases; meanwhile, the wear mechanism changes from abrasive wear to abrasive wear, and from adhesive wear and oxidation wear. The free-corrosion potential of the cladding coating is ?0.386 V and the free-corrosion current density is 3.45×10?6 A / cm2 , both of which are higher than those of the substrate. Additionally, the impedance arc radius of the coating is much larger than that of the substrate, thus indicating that the coating possesses excellent corrosion resistance compared with the substrate. In the 3.5 wt.% NaCl solution, as the friction load increases, the open-circuit potential of the coating decreases from ?0.22 to ?0.35 V, the friction factor decreases from 0.2 to approximately 0.13, the corrosion potential of the coating decreases from ?0.4 to ?0.45 V, and the corrosion current density increases slightly, thus indicating that the friction significantly promotes corrosion. As the applied potential increases, the corrosion current density of the cladding layer increases and the friction factor decreases, thus indicating that corrosion clearly promotes wear. A synergistic effect is indicated between corrosion and wear. In conclusion, Cr-W-Mo-V steel coating with a compact structure, a uniform microstructure, and excellent tribocorrosion performance is successfully prepared via hot-wire laser cladding. It can be used for the surface modification of components used in oil and gas drilling as well as in exploitation equipment used in harsh tribocorrosion environments.
  • Optimization of Process Parameters in a Laser Fe / Ti3SiC2 Composite Coating and Its Microstructure and Properties
    JIANG Ping, ZHU Xiebin, ZHANG Wei, LIU Hongwei, ZHANG Zhong, ZHANG Zhao
    China Surface Engineering. 2024, 37(3): 37-45. https://doi.org/10.11933/j.issn.1007-9289.20231009004
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    Traditional hard coatings are typically prepared on the surfaces of crankshafts, shingles, and other friction parts to improve wear resistance. However, these coatings have been shown to have no wear-reducing effect on their counterparts, and the overall wear-reducing effect is poor. In this study, Fe / Ti3SiC2 wear-resistant and friction-reducing composite coatings were prepared on the surfaces of specimens of 45 steel using high-speed laser cladding technology under different process parameters. The objective was to achieve a friction vice that improves the wear resistance of the workpiece and reduces the wear of dyadic parts. The hardness of the composite coatings was examined under different process parameters using a Vickers microhardness test. Friction wear tests of the composite coatings under different process parameters were conducted at room temperature using a friction wear machine, and the wear mark morphology was characterized by scanning electron microscopy. Elemental analysis of some specimen areas was performed using self-contained energy dispersive spectroscopy. The wear amount of each coating on the pin of the grinding specimen was recorded as a criterion, and the mean value and extreme deviation of each process parameter were calculated to optimize the process parameters of the high-speed laser melting of the Fe / Ti3SiC2 wear-resistant and friction-reducing composite coatings. X-ray diffraction and optical microscopy were utilized to examine the physical phases and cross-sectional morphology of the composite coatings under different process parameters, and the effects of these process parameters on the organization and properties of the coatings were investigated.The optimal combination of process parameters for the composite coatings was estimated to be a laser power of 2.5 kW, powder feeding amount of 15 g / min, scanning rate of 14 mm / s, and coating microhardness of 591.7 HV0.2. The macroscopic morphology of the cross-section of the single-pass cladding layer of the coating in the laser power was constant. When the scanning rate was too fast or the amount of powder delivery was too large, the dilution rate of the coating decreased whereby the coating showed an morphology, which in turn prevented the coating and substrate from forming a good metallurgical bond. The combination of coating and substrate was mainly composed of columnar, dendritic, and planar crystals, but the size of the organizational structure of the coating changed under different process parameters. With an increase in laser power, the input heat increased and the degree of subcooling decreased such that the grains coarsened. With a suitable increase in the scanning rate and amount of powder delivery, the fusion layer of powder particles was subjected to a lower heat and the rate of subcooling increased, which led to a refinement of the grains. A 30-min friction wear test at room temperature and under a 30-N load showed that the composite coatings under different process parameters exhibited different abrasion patterns. By contrast, the composite coatings under the optimal process parameters showed the best friction performance, where the amounts of wear of the coating and paired parts were 0.4 and 0.7 mg, respectively. Compared with the amount of wear of the matrix of non-fusion-coated composite coatings under the same friction wear test parameters, the wear amount of the composite coating was reduced by 94%, whereas that of the couple was reduced by 65%. By contrast, the Fe-based coating without Ti3SiC2 under the same parameters did not reduce the wear amount on the couple despite an increase in abrasion resistance; the wear amount on the couple was increased due to its own hardness. These results showed that the addition of composite coatings under appropriate process parameters, greatly improving the wear resistance of the workpiece surface while reducing wear on the dual parts. Thus, the performance of the entire friction system was systematically improved under the high-performance wear-resistant friction-reducing composite coatings. This study solves the technical problem wherein traditional hard coatings, despite enhancing the wear resistance of the workpiece, increase the wear of the spouse parts.
  • Microstructure and Properties of Ultra-high-speed Laser Cladding Ni-cBN / (NiCoCr)94Al3Ti3 Coating
    WANG Xinsheng, LUO Jifeng, LI Yang, MOU Honglin, TONG Yonggang, XING Zhiguo, CAI Zhihai, WEI Shizhong, YU Yueyang
    China Surface Engineering. 2024, 37(3): 46-56. https://doi.org/10.11933/j.issn.1007-9289.20230712001
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    Titanium alloys have high strength, low density, and excellent corrosion resistance and are important alloy materials in the aerospace field; however, their weak surface wear resistance restricts their application and further improvement in the aviation industry to a certain extent. To improve the wear resistance of titanium alloy surfaces, the application scenarios and service life of titanium alloy parts should be expanded. The wear resistance of the high-entropy alloy coating was improved by particle-strengthening technology. After adding 0, 5, 10, 15, and 20 wt.% Ni-coated with cBN particles to (NiCoCr)94Al3Ti3 alloy powder, ultra-high-speed laser cladding technology was used to prepare Ni-coated cBN particle-reinforced (NiCoCr)94Al3Ti3 wear-resistant coating on the TC11 matrix. The influence of the Ni-coated cBN particle content on the wear resistance of the coating was investigated using XRD, SEM, EDS, hardness, friction, and wear analyses. Phase analysis of the five coatings showed that the content of the Ni-coated cBN particles was greater than 15 wt.%, and the FCC phase summit of the high-entropy alloy was weakened, indicating that Ni-coated cBN particles can reduce the ductile phase in the coating. Scanning electron microscopy and energy spectrum analysis showed that the content of Ni-coated cBN increased, the Cr in the coating was polarized, the Cr-rich phase structure was a thin strip and constantly aggregated and grew, and the Cr-rich phase belonged to the hard phase, which is consistent with the phase analysis results. The increase in Ni-coated cBN particles leads to an increase in the hard phase inside the coating and a decrease in the toughness phase. The hardness values of five coatings were measured. The hardness curve of the Ni–CBN coating showed that the hardness of the coating top was higher and the hardness of the cross-sectional binding area was lower owing to the uneven distribution of the particles. The accumulation and growth of the Cr-rich hard phase in the coating indicated that the hardness of the coating increased. However, the truss effect of cBN accumulation weakened the compactness of the coating and promoted an increase in its porosity, which affected the test hardness of the coating. Therefore, the average microhardness test results of these five coatings showed an initial upward trend, followed by a downward trend with an increase in the Ni-coated cBN particles. When hard cBN particles were added to the coating, hard particle points appeared during wear, resulting in an increase in the roughness of the friction surface. Therefore, the friction factors of the five coatings showed an upward trend with an increase in the number of Ni-coated cBN particles, and the high hardness characteristics of cBN sustained most of the grinding force. The larger the number of cBN particles, the smaller the friction contact area between the grinding pair and the coating. Therefore, the wear amounts of the five coatings gradually decreased with increasing Ni-coated cBN particle content, and the wear resistance of the coatings gradually increased. Among the five wear-resistant coatings, the cBN coating with 15 wt.% Ni has the best overall performance. The adhesion between the coating and substrate is strong, as is the adhesion between the cBN particles and coating. The coating exhibited the highest average microhardness, reaching 1 024 HV0.5 with a friction factor of 0.534 and a wear volume of 0.017 mm3 . The wear resistance of the coating is 2.8 times that of the cBN particle coating without the addition of Ni. The addition of Ni-coated cBN particles can improve the wear resistance of coatings and provide a reference for the research and application of cBN in wear-resistant coatings.
  • Effect of Picosecond / nanosecond Pulsed-laser Polishing on Surface Integrity of Selective Laser Melting TC4
    LI Liangliang, MU Jianwei, LIU Yanmei, XU Jiwen, LI Jinlong, LI Pengfei, HUANG Shu
    China Surface Engineering. 2024, 37(3): 57-66. https://doi.org/10.11933/j.issn.1007-9289.20231212001
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    The TC4 titanium alloy is widely applied in various fields owing to its low density, low thermal conductivity, corrosion resistance, and stable high-temperature mechanical properties. In this study, picosecond and nanosecond pulsed lasers are used in laser polishing to investigate the effects of different laser-polishing methods on the surface properties of additively manufactured titanium alloy. Pulsed-laser polishing, as a novel surface-processing technology, primarily focuses on precision polishing to achieve superior surface roughness and performance while avoiding laser-induced substrate overheating. Researchers have conducted polishing treatments using nanosecond and picosecond pulsed lasers on different materials and confirmed that pulsed-laser polishing significantly reduces surface roughness, refines the grain structure of melted pools, enhances the microhardness of polished surfaces,and alters the mechanical properties of samples. However, comparative studies pertaining to the polishing of the same thin-wall structure material are limited, thus rendering it challenging to analyze the thermal effects of different lasers and their impact on performance. Based on the average surface roughness and depth of the heat-affected zone as targets, after optimizing hundreds of parameter sets, the optimized processing parameters for picosecond pulsed lasers are determined to be as follows: spot diameter, 15 μm; scanning speed, 2 000 mm / s; power, 16 W; frequency, 500 kHz; pulse width, 10 ps; and serpentine scanning path. Meanwhile, the optimized processing parameters for nanosecond pulsed lasers are as follows: spot diameter, 25 μm; scanning speed, 2 000 mm / s; power, 70 W; frequency, 200 kHz; scanning spacing, 0.08 mm; dot engraving time, 0.1 ms; and serpentine scanning path. A comparative study is performed using picosecond and nanosecond pulsed lasers to investigate their effects on the surface morphology, surface roughness, depth of the heat-affected zone, elemental distribution, oxidation degree, and tensile properties of laser-melted TC4 titanium alloy thin-wall structures. The results indicate that both types of lasers can significantly improve the surface quality of additively manufactured TC4 titanium alloy, i.e., the original roughness of 3.52 μm is reduced to 0.71 and 0.66 μm via picosecond and nanosecond lasers, respectively. The nanosecond pulsed laser exhibits higher thermal accumulation, thus resulting in a higher surface oxidation degree and a deeper heat-affected zone of approximately 34.20 μm, which is approximately 40.9% higher than that afforded by the picosecond pulsed laser. During laser polishing, the thermal accumulation of picosecond pulsed lasers is minimal and residual unmelted powder particles do not melt during polishing, whereas the thermal accumulation of nanosecond pulsed lasers is significant, thus causing residual unmelted particles to melt during laser polishing. The surface layer after nanosecond pulsed-laser polishing exhibits only a few minor cracks, which is attributable to thermal stress caused by high thermal accumulation. Although the thermal accumulation of the picosecond pulsed laser is smaller and the depth of the heat-affected zone is shallower, within the heat-affected zone, the oxidation degree due to picosecond-laser processing is slightly higher than that due to nanosecond-laser processing. Results of mechanism analysis show that the spot diameter of the picosecond pulsed laser is extremely small and that under the action of a Gaussian spot, the energy density at the center of the laser heat source is extremely high. Consequently, the molten pool splashes during polishing, thereby removing protruding material and resulting in a small amount of remelting, which reduces the surface roughness. The nanosecond pulsed laser relies primarily on the thermal accumulation of the overall spot to melt the original surface. Under the effect of Marangoni flow in the melt pool, the molten material flows toward the depression area, thus improving the surface roughness and resulting in a larger depth of the heat-affected zone. Using picosecond and nanosecond pulsed lasers for the laser melting and polishing of TC4 titanium alloy thin-wall structures, this study elucidates two different mechanisms of pulsed-laser polishing, thus providing new insights and methods for preparing high-surface-quality metal components via additive manufacturing.
  • Nanosecond-laser Texturing of Aerospace Aluminum-alloy Surface and Its Bonding Properties
    FENG Jiecai, LI Cheng, WANG Xing, JIANG Meng, DU Changlin, XU Congcong, ZHANG Yilian, TIAN Yingzhong
    China Surface Engineering. 2024, 37(3): 67-77. https://doi.org/10.11933/j.issn.1007-9289.20230925002
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    Owing to the development of aerospace technology, conventional aluminum-alloy surface-treatment technology can no longer satisfy the demands of industrial production for environmental protection and strength consistency. In this regard, nanosecond-laser texturing is an ideal surface-treatment technology that offers the advantages of high processing consistency, zero environmental pollution, and no damage to the processing substrate. However, studies regarding the nanosecond-laser texturing of LY12CZ aluminum alloy to enhance bonding performance are few, and the law of process parameters on micropore formation remains unclear. Therefore, the effects of process parameters on the micro-morphology of aluminum alloy are investigated and the relationship between the bonding-performance-enhancement mechanism and micro-morphology is determined. Specifically, a YLPN-2-20x500-300-5M nanosecond pulse laser is used to laser process a 25 mm×15 mm aluminum-alloy sample. The single-factor-analysis method is adopted to investigate the effects of laser energy (E), laser pulse width (τ), and laser frequency (f) on the morphology of micropores. Laser texturing is performed under a hole spacing of 100 μm, a marking speed of 40 mm / s, and a jump speed of 5 mm / s. E, τ, and f are varied separately to observe their effects on the surface morphology of micropores under a metallographic microscope. A three-dimensional profilometer is used to inspect the processed samples, and the diameter and depth of the micropores are recorded. The analysis software SensoVIEW is used to obtain the average results and construct a graph showing the effects of E, τ, and f on the hole diameter and depth. The formation rules and mechanisms of the micropores are analyzed based on observation results. The tensile shear strength of the sample is measured in accordance with GB / T7124-2008. The tensile shear strength of adhesives (rigid material to rigid material) is determined, and the average results are obtained. The optimal processing parameters are selected after comparing the shear strength, and the relationship between the micro-morphology and shear strength is analyzed. The result shows that E is the main factor affecting the surface microstructure of aluminum alloys. As E increases, the diameter and hole depth increase gradually. When τ is 20–120 ns or the pulse frequency is increased to 10–90 kHz, the hole diameter and depth increase. When τ exceeds 120 ns and increases, the hole depth decreases, whereas the hole diameter increases and then decreases gradually. When f increases above 70 kHz, the hole depth and diameter increase. Meanwhile, when lower E and τ values or higher f values are selected, the aluminum alloy in the laser-processing area flows from the edge to the center of the molten pool. This causes a buildup of molten metal, thus forming in a bulge in the center of the pool. After the aluminum alloy solidifies, a Mexican-hat shape with a convex center and concave edges is formed on the aluminum-alloy specimen. Its hole depth and diameter are small, thus rendering the adhesive’s bonding effect unsatisfactory. Consequently, the bonding performance of the aluminum alloy is subpar. When the appropriate E, τ, and f are selected, the aluminum alloy in the laser-processing area forms a crater-like microstructure owing to Marangoni convection. The micropores exhibit large diameters, large depths, small micropore spacings, and a volcano-like microstructure around the micropores, thus facilitating the penetration of the adhesive and resulting in a “pinning” effect. Consequently, the shear strength of the aluminum-alloy bonding increases to 16.77 MPa, which satisfies industrial production requirements (10 MPa). The recommended process parameters are as follows: laser pulse energy, 1.8 mJ; pulse frequency, 70 kHz; pulse width, 240 ns; ablation times, 5; and micropore spacing, 100 μm. The results of this study serve as useful reference for the high-quality nanosecond laser texturing of LY12CZ aluminum-alloy surfaces.
  • Edge-effect Suppression Method for Microcorrugated Sheets by Laser Thermal Stress Forming
    YAO Zhehe, FAN Weixin, CHEN Zhimin, ZHANG Qunli, YAO Jianhua
    China Surface Engineering. 2024, 37(3): 78-88. https://doi.org/10.11933/j.issn.1007-9289.20230731001
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    Laser thermal stress forming, which is a contactless and highly flexible manufacturing technology, can minimize problems such as springback, cracking, and wrinkling caused by mold or tool manufacturing, thus providing a precision manufacturing solution for microcorrugated sheets. However, microcorrugated sheets are prone to edge effects due to the uneven distribution of energy input and constraint differences during the scanning process. Because of the multipass and multicycle scanning involved in the laser thermal stress forming process of microcorrugated sheets, the surface distortion and residual stress caused by the previous scanning affect the thermal deformation in the subsequent scanning. This effect is amplified by the superposition of edge effects during multiple scanning, which exacerbates the instability of microcorrugated sheet forming. Therefore, the suppression of edge effects in the laser thermal stress forming of microcorrugated sheets should be a major focus. In addition, the mechanism of the surface distortion caused by multipass coupling is complex. In this study, the effects of laser process parameters on the edge effect are analyzed by numerical simulations and experimental studies. A varying velocity round-trip coupling scanning strategy is proposed to suppress the edge effect and is also applied to microcorrugated sheet forming, as this provides a reference for the analysis and suppression of edge effects in multiple scanning. The heat accumulation and displacement distribution at the free end after scanning are analyzed through numerical simulations under various scanning strategies. An experimental platform of laser thermal stress forming is constructed. The specimen used in the experiments is 304 stainless steel sheets with dimensions of 30 × 30 × 0.4 mm. The study employs a 500-W oscillator continuous fiber laser with a laser power of 150–450 W, velocity range of 10–70 mm / s, and scanning number range of 5–20. Response surface analysis (RSA) is used to analyze the effects of different laser process parameters on the edge effect. Suppression of the edge effect using the varying velocity round-trip coupling scanning strategy is then conducted, and the displacement of the free end of the forming sheet is determined. To observe the local morphology of the formed end of the sheet, a local 3D solid contour is acquired using confocal microscopy. According to the RSA, laser process parameters are critical factors that affect the deformation of bending parts. The smallest change in relative angle with constant-velocity scanning are obtained when the scanning number and velocity are 15 and 50 mm / s, respectively. Furthermore, the results show that a varying velocity strategy (an early-stage scanning velocity of 30 mm / s combined with a later-stage scanning velocity with 50 mm / s) exhibits a smaller change in relative angle as compared with constant-velocity scanning (a scanning velocity of 30 mm / s), with a reduction of approximately 63.0%. Based on the suppression of the varying velocity strategy, a varying velocity round-trip coupling scanning strategy is proposed, and the mechanism of edge-effect suppression under different strategies is investigated by numerical simulations and experimental studies. The study finds that the varying velocity round-trip coupling scanning strategy effectively reduces terminal heat accumulation and improves the temperature field distribution in the scanning process. Both the round-trip and varying velocity scanning strategies can improve the edge effect within a certain range of processes. In addition, the coupling of these two strategies can achieve further optimization because the temperature fields at both ends of the sheets can be controlled. Furthermore, the relative angle change is reduced by 77.8% using the varying velocity round-trip coupling scanning strategy. Finally, laser thermal stress forming of trapezoidal corrugated sheets with depth–width ratios of greater than 0.75 is achieved. Under this strategy, a flat profile appears on one side of the trapezoidal corrugated sheet wall, and a smaller height difference can be observed in the flow channel forming. Both numerical and experimental results reveal that round-trip scanning restrains the edge effect in the forming by balancing the temperature field at the beginning of the scanning line, whereas the varying velocity suppresses the end displacement height by reducing the terminal heat accumulation at the scanning line. Laser process parameters are also significant factors that affect the deformation of sheets. In summary, the varying velocity round-trip coupling scanning strategy can effectively suppress the edge effect of forming parts and realize the precision forming of microcorrugated sheets with large depth–width ratios, thus providing a reference for high-quality microcorrugated sheet forming.
  • Research Status of Doping Modification of Bismuth Sodium Titanate Based Lead-free Piezoelectric Ceramics
    ZHU Hefa, XING Zhiguo, GUO Weiling, DONG Lihong, WANG Haidou, DONG Han, HUANG Yanfei
    China Surface Engineering. 2024, 37(3): 89-102. https://doi.org/10.11933/j.issn.1007-9289.20230518003
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    Lead-free piezoelectric ceramics based on bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) are increasingly popular in aerospace, naval sonar, high-speed trains, and electronic devices due to their environmental friendliness and outstanding ferroelectric capabilities. To address the challenge of high coercive fields and enhance electrical performance, doping modification of BNT-based ceramics introduces a morphotropic phase boundary (MPB) featuring both rhombohedral and tetragonal phases, effectively reducing the coercive field and significantly boosting electrical properties. This modification is essential for advancing the performance of sodium bismuth titanate-based lead-free piezoelectric ceramics. This paper provides an extensive review of the latest advancements in the field of BNT-based lead-free piezoelectric ceramics, with a focus on multi-component modifications, A / B-site ion doping, and rare earth ion doping. Findings indicate that incorporating appropriate components into BNT-based ceramics facilitates the formation of an MPB, which not only reduces the coercive field but also significantly improves the piezoelectric and ferroelectric properties of these materials. However, despite these advancements, piezoelectric ceramic development represents just a fraction of the piezoelectric materials landscape, with vast potential for further exploration. Doping with A-and B-site ions in BNT ceramics aims to maintain cellular structure stability, aligning with the consistency of ionic radius and electric valence. A-site doping mitigates the volatilization of Bi and Na elements and eases sintering challenges, significantly enhancing piezoelectric and ferroelectric properties while reducing the coercive field. B-site ion doping, through Ti4+ substitution, introduces defects and A-site vacancies, improving the piezoelectric constant d33. While these modifications have significantly advanced the structure and performance of BNT-based ceramics, issues with temperature stability remain, limiting their immediate practical application. Rare earth ion doping introduces light-emitting capabilities to BNT-based lead-free piezoelectric ceramics alongside piezoelectric improvements, significantly affecting their photoelectric properties. These diverse modification strategies collectively elevate the performance of BNT-based lead-free piezoelectric ceramics, paving the way for further research and potential practical applications. To advance the electrical performance of BNT-based lead-free piezoelectric ceramics, future research should focus on the material's intrinsic properties, specifically uncovering the physical nature of the quasi-isotropic phase boundaries and their role in enhancing electrical performance. This involves examining the changes in piezoelectric, ferroelectric, and dielectric properties across the MPB phase boundary range, as well as their dynamic evolution under external electric fields. Additionally, the research should explore the physical mechanisms by which component adjustments influence the stability of the material's phase structure, the regulation of the ferroelectric domain structure, and piezoelectric properties. Through these studies, the goal is to develop high-performance lead-free piezoelectric ceramics and facilitate their industrialization.
  • State of the Art on Self-lubrication of Boron Carbide Ceramics
    ZHANG Wei, ZHANG Jie
    China Surface Engineering. 2024, 37(3): 103-114. https://doi.org/10.11933/j.issn.1007-9289.20230922001
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    Self-lubrication of boron carbide (B4C) ceramics significantly influences their tribological properties. However, systematic studies on the mechanism and techniques of self-lubrication of boron carbide ceramics have not been conducted extensively. Boron carbide has high hardness (Vickers hardness is 36 GPa); thus, boron carbide ceramics are potential candidate materials for use as wear-resistant components. However, the friction coefficient of boron carbide ceramics is high, which increases the energy consumption of the frictional system; thus, the widespread application of boron carbide ceramics is limited. Self-lubrication can prevent contamination caused by external lubricants, revealing that the self-lubrication mechanism of boron carbide ceramics can provide a feasible reference for solving the high friction coefficient of boron carbide ceramics. Currently, three methods are mainly used for the self-lubrication of boron carbide ceramics: preoxidation, the addition of solid lubricants, and the construction of a relief structure on the surface. Preoxidation refers to the high-temperature oxidation treatment of boron carbide ceramics in advance in an air environment, which forms an oxide layer comprising B2O3, H3BO3, or both on their surfaces. After the oxidation treatment, H3BO3 with a layered crystal structure or B2O3 generated on the surface of boron carbide ceramics causes a lubrication effect. The residual graphite formed during oxidation may also perform a lubrication function. Although boron carbide ceramics can achieve self-lubrication after a preoxidation treatment, the effect of the preoxidation treatment on the wear rate of boron carbide ceramics is unclear. The reason for applying a solid lubricant is to add a material with a layered crystal structure to the boron carbide ceramic matrix. During the sliding process, the solid lubricant peels off from the boron carbide ceramic matrix and smears on the worn surface of the boron carbide ceramics, forming an external lubricating layer. The wear rate of boron carbide ceramics with solid lubricants is related to the nature of the counterbody. The construction of a surface relief structure involves introducing a second phase with relatively low hardness into the boron carbide ceramic matrix and using the difference in hardness between the boron carbide grains and silicon carbide grains to generate concave and convex surface morphologies in situ during the sliding process. On the one hand, the relief structure formed on the surfaces of boron carbide ceramics can trap wear debris generated during the sliding process, reducing the friction coefficient and abrasive wear. On the other hand, the relief structure can reduce the actual contact area between the boron carbide ceramics and the counterbody, which not only decreases the number of contact points between the boron carbide ceramics and the counterbody, thereby reducing the force required to break these connections, but also reduces the adhesive wear of the boron carbide ceramics. Therefore, the friction coefficient and wear rate of boron carbide ceramics can be reduced by constructing relief structures on their surfaces. Although these self-lubrication methods have technical limitations, they can still achieve self-lubrication of boron carbide ceramics under certain operating conditions, resulting in a decreased friction coefficient of the tribopairs and decreased energy consumption of the friction system. This review summarizes the state of the art on the self-lubrication of boron carbide ceramics in recent years based on the author’s research results and proposes future research directions on the self-lubrication of boron carbide ceramics. This review provides useful guidance for the design, research, and application of self-lubrication of boron carbide ceramics.
  • Preparation and Self-healing Anticorrosion Properties of Photothermal Responsive MXene-based Polyurethane Coatings
    WANG Bo, WU Lianfeng, FENG Huimeng, ZHAO Zhipeng, LI Wen, CHEN Shougang
    China Surface Engineering. 2024, 37(3): 115-124. https://doi.org/10.11933/j.issn.1007-9289.20230629001
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    Coatings are prone to scratching and cracking defects during service, which result in passive failure and reduced protective performance. The development of self-healing anticorrosive coatings is necessary for the corrosion protection of marine engineering materials. In this study, MXene two-dimensional nanomaterials were successfully synthesized using an etching and stripping method, and MXene polyurethane self-healing coatings with rich hydrogen bonds were synthesized using a one-pot method. The microstructure of the MXene was investigated using scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and X-ray photoelectron spectroscopy. The results showed that the aluminum layer of Ti3AlC2 was successfully etched by acid etching and ultrasonic stripping to obtain a two-dimensional Ti3C2 MXene material. Self-healing polyurethane coatings containing numerous hydrogen bonds were synthesized using Polytetramethylene ether glycol as the soft segment, isophorone diisocyanate and toluene diisocyanate as the hard segments, and Dimethylglyoxime as the chain extender. MXene was doped into the coating as a photothermal agent to improve the repair efficiency and protection performance. The synthesis of PU and PU / MXene was confirmed using Fourier transform infrared spectroscopy. The photothermal properties of the coatings were characterized using a near-infrared laser emitter and thermal imager. The temperature of the PU / MXene coating surface increased by more than 20 ℃ within 1 min, reaching 43.1 ℃, which was significantly higher than the 29 ℃ of the PU coatings, demonstrating that the composite coating doped with MXene exhibited a good photothermal conversion performance under near infrared light. The self-healing process of the coating was monitored using confocal laser scanning microscopy. The healing rate of the PU / MXene coating was higher than that of the PU coating. The excellent photothermal conversion of MXene facilitated the motion of the coating segments and the hydrogen bond recombination process, thereby improving the self-healing performance. The self-healing and anticorrosion properties of the coatings were investigated via electrochemical testing. The initial low-frequency (0.01 Hz) impedance mode value of the PU / MXene coating samples before scratching was close to 109 ?·cm2 , which is significantly higher than that of the PU coating (107 ?·cm2 ), and the corrosion resistance was better. After the first scratching, the low-frequency impedance mode value of the coated sample sharply decreased to below 105 ?·cm2 , and the corrosion resistance decreased. After the repair, the low-frequency impedance mode value was restored to an impedance value close to the initial level. The scratch and photothermal self-healing cycle of the PU / MXene coating were tested for five times, and the low-frequency impedance modulus could be restored to approximately 108 ?·cm2 . However, the impedance mode values of the PU-coated samples after the repair were significantly lower than those of the PU / MXene-coated samples. The MXene / PU coating exhibited excellent cycle repair performance, which significantly improved the service life. The electrochemical impedance test results show that the low-frequency impedance modulus of the coating sample can be maintained above 108 ?·cm2 . When immersed in a 3.5 wt.% sodium chloride solution for 30 d, the low-frequency impedance mode value of the PU / MXene coating can remain above 108 ?·cm2 , whereas the PU coating is only higher than 106 ?·cm2 , indicating that the anticorrosion property of the PU / MXene coating can be maintained for an extended period. Two main reasons are responsible for why the PU / MXene coatings exhibit improved long-term anticorrosion performance. On the one hand, this is attributed to the excellent barrier properties of the two-dimensional materials; on the other hand, the self-repair of the coating constantly repairs microcracks and reduces the infiltration of corrosive media. The experimental results showed that a polyurethane self-healing coating with a good photothermal response and long-term anticorrosion was prepared using MXene as a photothermal conversion agent. This study provides a theoretical basis for the practical application of photothermal responsive polyurethane self-healing coatings on marine engineering equipment.
  • Effect of Laser Remelting on Microstructure and Corrosion Resistance of Plasma Sprayed AlCoCrFeNi High-entropy Alloy Coating
    DONG Tianshun, LIU Jianhui, MA Qingliang, LIU Qi, FU Binguo, LI Guolu, LU Pengwei
    China Surface Engineering. 2024, 37(3): 125-133. https://doi.org/10.11933/j.issn.1007-9289.20230725001
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    The high cost of producing multi-principal component high-entropy alloys (HEAs) has limited their widespread application, despite their superior properties. Applying HEA coatings to conventional metals can harness these exceptional properties while conserving precious metal resources. Nonetheless, HEA coatings frequently exhibit defects like pores and cracks, which significantly impair their functionality. Research has demonstrated that laser remelting can effectively mitigate most of these defects, refining the coatings' microstructure and enhancing their overall performance. Although existing studies on laser-remelted HEA coatings have primarily concentrated on their microstructure, mechanical attributes, and wear resistance, the impact of laser remelting on their corrosion resistance remains less explored. This investigation assessed the corrosion resistance of an AlCoCrFeNi HEA coating applied to AISI 1045 steel via plasma spraying, followed by laser remelting. The coatings' microstructures, both pre-and post-remelting, were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), with a particular focus on the elemental distribution at the coating-substrate interface. Phase analysis was conducted using X-ray diffraction (XRD), while transmission electron microscopy (TEM) provided insights into the microstructural details of both coatings. Electrochemical and immersion corrosion tests evaluated the coatings' resistance to corrosion. The findings revealed that laser remelting substantially reduced the defects present in the plasma-sprayed coating, decreasing porosity from 4.8% to a negligible 0.3%. This process also converted the mechanical bonding between the coating and substrate into a stronger metallurgical bond. Despite the remelting process, the elemental composition of the coating remained close to an equimolar ratio, consistent with HEA definitions. The laser-remelted coating exhibited a predominance of the BCC solid solution phase, alongside minor FCC phase precipitates, with a higher BCC content than the original sprayed coating. This resulted in a uniform and dense microstructure, characterized by dendritic and interdendritic patterns. Electrochemical tests, including polarization curve analysis and electrochemical impedance spectroscopy, indicated that laser remelting significantly enhances the corrosion resistance of the AlCoCrFeNi HEA coating in a 3.5% NaCl solution. Laser remelting significantly enhanced the corrosion resistance of the HEA coating, evidenced by an increase in self-corrosion potential from –0.421 6 V to –0.282 1 V and a reduction in corrosion current density from 4.809×10?7 A / cm2 to 1.475×10?7 A / cm2 . Long-term immersion tests further confirmed the superior corrosion resistance of the laser-remelted coating compared to the plasma-sprayed coating. The improved performance is attributed to the elimination of large pores and visible cracks that characterized the surface of the sprayed coating. These defects allowed electrolyte penetration to the coating-substrate interface, facilitating electrochemical reactions. Additionally, electrolyte infiltration led to significant Cl?aggregation within the pores, hindering the formation of a protective passive film on the sprayed coating's surface. Laser remelting addressed these issues by effectively sealing the pores and cracks, enabling the formation of a uniform and dense passivation film that significantly impedes electrolyte penetration. The process of combining plasma spraying with laser remelting produces HEA coatings with fewer defects and enhanced corrosion resistance, offering valuable insights for broadening the application of HEAs in various industries.
  • Effect of Ultraviolet Radiation on the Wear Performance of Self-lubricating Joint Bearing Liner
    YE Qiang, TAN Deqiang, ZONG Fatao, HU Yue, HE Qiang, YANG Wenfeng, ZHOU Changchun
    China Surface Engineering. 2024, 37(3): 134-144. https://doi.org/10.11933/j.issn.1007-9289.20230513001
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    PTFE / Kevlar fabric liner type self-lubricating materials, extensively utilized in aircraft, face the challenge of enduring strong ultraviolet (UV) radiation when operating at high altitudes throughout the year. Simultaneously, these self-lubricating joint bearing liner materials, used externally on aircraft, encounter high-intensity friction conditions along with substantial UV exposure. Despite this, there is a paucity of studies focusing on the wear behavior of PTFE / Kevlar fabric liners following UV irradiation. This gap highlights the importance of investigating the impact of intense UV exposure on the wear characteristics of self-lubricating plain bearing liners. In this paper, the UV aging tester model HZ-2008A was employed to subject PTFE / Kevlar fabric liner materials to UV radiation at an intensity of 1 W / m2 for varying durations. Post-irradiation, the liner samples underwent reciprocating friction testing using the MXW-01 tester. To examine the macroscopic damage morphology of the abrasion marks on the liner material, a stereo microscope (SM) and a three-dimensional optical profiler (3D-OM) were utilized. Furthermore, the micro-morphology of surface damage and chemical state within the abrasion damage area were analyzed using a field emission scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Reflectance micro-infrared spectroscopy (ATR) was applied to analyze the microstructure of PTFE before and after UV irradiation. The study reveals that the wear behavior of PTFE / Kevlar fabric liners against GCr15 bearing steel balls through 5,000 cycles of reciprocating friction initially decreases and then increases with prolonged UV irradiation. Notably, the liners subjected to short-term irradiation (T=100 h and 200 h) exhibited the least wear, with the volume of friction loss and the maximum depth of abrasion marks being only two-thirds of those observed in liners that were not irradiated or were irradiated for longer durations (T=300 h and 500 h). This indicates that UV irradiation's primary impact on the wear performance of self-lubricating spherical bearing liners involves a reduction in PTFE fibers' plastic deformation and their capacity to form transfer films. In comparison to non-irradiated liners, those exposed to short-term irradiation experienced less fiber fracture and formed a "transfer film" that was not sufficiently hard, thus protecting the underlying PTFE fibers from frictional effects. Conversely, extended irradiation resulted in the wear of the surface layer of brittle PTFE fibers, restricting the underlying fibers from undergoing dendritic plastic deformation and limiting the formation of transfer films to small areas. The study further elucidates that the fundamental reason UV irradiation affects the wear performance of the liners is due to changes in the PTFE fibers post-irradiation, such as a decrease in the absorption rate of the -CF2- group, reduction in molecular weight, decrease in crystallinity, and a transition from ductility to surface layer embrittlement. Consequently, the wear mechanism of the liner material shifts from fatigue and adhesive wear to abrasive wear as irradiation time increases. By irradiating PTFE / Kevlar fabric liner samples without considering the intensity of UV irradiation, the research highlights the significant influence of UV exposure on the wear resistance of PTFE / Kevlar fabric liners, showing a trend of initial decrease followed by an increase in wear damage with extended UV irradiation time. This investigation into the effects of UV irradiation on the tribological properties of self-lubricating PTFE / Kevlar fabric liners for aviation applications is innovative. It extends the understanding of textile material applications in UV-exposed environments and offers insights for material development and process optimization of self-lubricating joint bearing liners. The findings provide a foundation for monitoring the long-term performance of PTFE / Kevlar fabric liner parts in aircraft operating in high-altitude conditions, contributing to the stable operation and maintenance of such aircraft.
  • Preparation of Nano TiO2 Superhydrophobic Coating Based on Palmitic Acid Modification
    LI Zhiyong, SHENG Wei, ZHENG Haikun, HAO Xiaoru, ZHOU Jiahui
    China Surface Engineering. 2024, 37(3): 145-156. https://doi.org/10.11933/j.issn.1007-9289.20230524001
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    Several methods are available for preparing superhydrophobic surfaces. However, their practical applications are limited owing to imperfections in their modification and hydrophobicity mechanisms. Therefore, investigating the mechanisms underlying the formation of superhydrophobic surfaces is crucial. Additionally, the preparation method and material selection for superhydrophobic surfaces have a significant impact on cost and environmental considerations. In this study, we chose palmitic acid (PA)-modified titanium dioxide (TiO2), which has a low surface energy and is environmentally friendly and cost-effective. Our objective was to prepare a highly efficient superhydrophobic surface using this material and analyze its modification and hydrophobicity mechanisms. Experimentally, we prepared suspensions with varying modification ratios and applied them to polished aluminum surfaces using a two-step spraying method. The chemical reactions between TiO2 nanoparticles and PA were analyzed using a Fourier infrared spectrometer. Scanning electron microscopy was used to characterize the morphologies of the superhydrophobic surfaces. A contact angle meter was used to evaluate the wetting performance of the samples by measuring their surface contact and rolling angles. To further investigate the modification mechanism and effect of different modification ratios on the surface wetting behavior, we utilized a molecular dynamics simulation method. Different models consisting of varying numbers of molecule-modified nano-TiO2 (101), were constructed. Additionally, various surface-wetting models have been developed to simulate different wettabilities. The COMPASS II force field was then employed for molecular dynamics simulations to analyze and study the modification mechanism and microscopic wetting behavior through system configuration. The root-mean-square displacement and radial distribution functions were calculated to obtain meaningful results. Overall, this research aims to elucidate of the mechanism underlying superhydrophobic surface modification and provide insights into the influence of different modification ratios on the wetting behavior at the molecular level. Validation using macroscopic experiments and microscopic molecular dynamics simulations confirmed that different modification ratios result in distinct micro-nanostructures with significant impact on surface wettability. Through characterization tests, the optimal modification ratio was identified to be 0.2 g of the PA weight and 1 g of TiO2 weight. At this ratio, a superhydrophobic surface was successfully prepared, exhibiting a contact angle of 164.4° and a rolling angle of 2°. Meticulous analysis revealed that bonding between PA and nano-TiO2 occurred via hydrogen bonding, followed by subsequent dehydration condensation reactions that culminated in the formation of ester bonds. When the head of the PA molecules is grafted onto the TiO2 surface, the tail is expelled and oscillates in a swaying motion. This unique adsorption mechanism creates a hydrophobic film composed predominantly of alkyl chains, which effectively transforms the inherent hydrophilic properties of the surface. Notably, the optimal modification ratios not only yield surfaces with low surface energy but also facilitate the development of a hierarchical micro-nanostructured surface, thereby augmenting the superhydrophobic characteristics. By combining macroscopic experiments and microscopic molecular dynamics simulations, this study analyzed the mechanism of PA-modified nano-TiO2, along with the impact of different modification ratios on surface wettability and hydrophobicity. These findings further refine our understanding of superhydrophobic surfaces, which hold immense significance for their preparation and research. Nevertheless, one limitation became apparent in this study, primarily concerning the opaque white appearance of the coating derived from PA-modified TiO2. Unfortunately, this monochromatic attribute significantly restricts extensive application. Consequently, the development of a coating with the ability to assume a diverse range of colors or achieve transparency would undoubtedly yield substantial benefits, facilitating a more versatile utilization of superhydrophobic surfaces. This aspect should be further explored in future studies to enhance the applicability of these surfaces in various practical settings.
  • Tribological Properties of FeCoNiCrMnAl0.5(Si0.5) High Entropy Alloy Melted via Vacuum Arc Melting
    LIU Jian, PENG Zhen, WANG Haidou, LI Jing, TONG Yonggang, XIAO Yi
    China Surface Engineering. 2024, 37(3): 157-164. https://doi.org/10.11933/j.issn.1007-9289.20230530004
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    Vacuum arc melting (VAM) technology stands as a pivotal technique in the synthesis of High-entropy alloys (HEAs), offering distinctive benefits and wide-ranging potential applications. Primarily, VAM is executed under high-vacuum conditions, which mitigates oxidation and gas contamination at elevated temperatures, facilitating the creation of high-purity HEA materials. These high-purity alloys are crucial in sectors like aviation, aerospace, and electronics, where material integrity significantly impacts performance and reliability. The vacuum setting plays a vital role in eliminating gases and impurities from metals, thus elevating the quality and properties of the resulting materials. Moreover, VAM technology is characterized by its high degree of controllability. The manipulation of process variables such as electrode spacing, arc current, and atmospheric conditions allows for precise tailoring of the alloy's composition and microstructure. This precision is instrumental in exploring the relationship between the composition of HEAs and their properties, enabling the development of custom alloys designed to fulfill specific requirements across various industries. HEAs are recognized for their exceptional attributes, including superior strength, enhanced ductility, and outstanding corrosion resistance. Their complex crystal structures endow them with a performance edge over conventional alloys, making VAM a critical method in advancing the development and application of HEAs. Therefore, HEAs produced via VAM technology present vast application potential across multiple domains. In materials science, HEAs are poised to revolutionize the fabrication of advanced structural components, including engine parts and aerospace structures, by leveraging their superior physical and chemical characteristics. These properties make HEAs ideal for enduring the rigors of high temperature, high pressure, and the multifaceted challenges of aviation and aerospace operational environments. Specifically, the incorporation of HEAs into aircraft engine components can significantly enhance high-temperature strength and wear resistance, leading to notable improvements in engine efficiency and longevity. Beyond aviation and aerospace, HEAs find applicability in sectors such as high-speed rail and nuclear energy equipment, offering innovative pathways for the advancement of engineering structural materials. In the energy sector, the exceptional thermal conductivity and resistance to high temperatures of HEAs make them suitable for use in combustion devices, heat exchangers, and as thermoelectric materials. The paradigm shift in design concepts allows HEAs to outperform traditional alloys by exhibiting a combination of high strength, hardness, superior corrosion resistance, and high-temperature endurance, thus opening up expansive industrial applications. The composition of HEAs, including the selection of elements and their proportions, plays a critical role in determining their microstructure and resultant properties. This direct relationship between elemental composition and alloy characteristics underscores the importance of precision in the design and production of HEAs, highlighting the transformative potential of VAM technology in the field. This study explores the impact of non-metallic elements on the wear resistance of FeCoNiCrMnAl0.5 HEAs. Utilizing VAM technology, FeCoNiCrMnAl0.5 (Si0.5) HEA ingots were prepared to examine the influence of Si on the alloy's microstructure, hardness, and dry friction characteristics. The investigation revealed that the FeCoNiCrMnAl0.5 alloy exhibits a singular FCC phase structure characterized by equiaxed dendritic morphology with a grain size ranging from 20–30 μm. Conversely, the addition of Si results in the FeCoNiCrMnAl0.5Si0.5 ingot featuring both FCC and BCC phases, alongside a reduced grain size of 10–20 μm. Notably, the hardness of the FeCoNiCrMnAl0.5Si0.5 ingot significantly increases to 750.7HV0.1 from 185.8HV0.1 observed in the FeCoNiCrMnAl0.5 ingot, enhancing its wear resistance by over tenfold. These findings demonstrate that Si effectively facilitates a phase transformation from FCC to BCC, refines grains, and markedly boosts the hardness and wear resistance of HEAs. The role of Si in enhancing HEAs' properties suggests a broader applicability in strengthening the wear resistance of HEA coatings. Moreover, VAM technology showcases scalability and environmental sustainability, supporting large-scale production to fulfill industrial demands. This method exhibits a lower energy requirement compared to traditional metallurgical processes, minimizing waste generation and environmental pollution, thereby contributing to reduced carbon emissions and endorsing sustainability. In summary, VAM technology emerges as a pivotal approach in the fabrication of HEAs, holding vast potential across materials science, aviation, aerospace, energy, and other engineering disciplines. It promises to accelerate the development and application of novel materials, propelling industrial advancement and economic growth.
  • Microstructure and Tribological Performance of Cu Matrix Composites Reinforced with Cu-modified WC Particles
    XU Yuxuan, WANG Xing, GUO Yuefen, ZHOU Haibin, ZHOU Peiyu, KANG Li, DENG Minwen, YAO Pingping
    China Surface Engineering. 2024, 37(3): 165-174. https://doi.org/10.11933/j.issn.1007-9289.20230508001
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    Tungsten carbide (WC) particle-reinforced composites are widely used owing to their excellent mechanical properties. The limited solubility of WC in Cu and the weak mechanical interface between the WC particles and the Cu matrix are the main factors contributing to the suboptimal tribological performance of these composites. To address this issue, the surface modification of WC particles is crucial for optimizing the interface and enhancing the bonding strength between the particles and the matrix. In this study, porous spherical WC particles were modified with Cu via spraying and sintering. Cu matrix composites reinforced with five different amounts of Cu-modified WC particles (0 and 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, and 25 wt.%) were prepared using a powder metallurgy technique. The mechanical and tribological properties of the Cu matrix composites were tested using a hardness tester and a UMT3 friction tester. Nanoindentation and scanning electron microscope (SEM) line scanning were used to characterize the interface of the Cu-mdified WC particles and Cu matrix. Macro-and microstructural characterization and tribological performance analysis were performed on Cu matrix composites reinforced with Cu-modified WC particles. The results of this study indicate that the Cu-modified WC particles were effectively embedded in the Cu matrix, resulting in the formation of a submicron diffusion layer at the interface between the modified particles and the matrix. This modification process led to a 33% increase in elastic resilience and a 20% increase in hardness at the interface. Microcracks within the particles as well as between the particles and Cu matrix contribute to an increase in porosity. Among the tested composites, the Cu-matrix composite reinforced with 15 wt.% Cu-modified WC particles exhibited superior tribological performance. The friction coefficients of this composite showed minimal fluctuations, stabilizing at approximately 0.75, with wear volumes decreasing to 0.075 mm3 . The worn surfaces of 15 wt.% Cu-modified WC reinforced composite displayed fairly smooth contours, the shallowest grinding crack, and large-scale continuous films. Furthermore, as the Cu-modified WC content increased, the main wear mechanism shifted from adhesive to abrasive wear at 15 wt.%, and eventually to fatigue wear at 25 wt.%. The surface friction transfer layers of the composites were characterized by the presence of Cu, Fe, O, and W. The Fe from the sliding friction pairs and absorbed oxygen contributed to the formation of a tribological film. Additionally, the presence of the modified particles causes deflection in the direction of the cracks and reduces their expansion at the corresponding locations. This research demonstrates a significant improvement in the interfacial bonding strength between the Cu-modified WC particles and Cu matrix. A composite material with 15 wt.% Cu-modified WC effectively inhibits adhesion wear and fatigue wear, resulting in excellent tribological properties. The utilization of Cu-modified WC particles to reinforce Cu-matrix composites offers a promising approach for optimizing the interface between WC and the Cu matrix, thereby enhancing the tribological properties of Cu-matrix composites. This paper has significant implications for the practical application of Cu-modified WC particles in Cu-matrix composites and provides a theoretical foundation for further research in this area.
  • Ultra-low Wear Mechanism of Molybdenum Disulfide / carbon Composite Films under Vacuum Wide Load
    CHENG Zhiqiang, LI Chunyan, GAO Kaixiong
    China Surface Engineering. 2024, 37(3): 175-184. https://doi.org/10.11933/j.issn.1007-9289.20230328001
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    The evolution of space equipment has been progressing to support heavier loads and longer durations of operation, necessitating the advancement of higher-performance lubricating materials. Currently, molybdenum disulfide (MoS2) films are predominantly used under low vacuum loads (＜ 0.5 GPa), underscoring the urgent need for developing MoS2 composite films that can perform under a wider range of vacuum loads, including medium and high loads. In this study, MoS2 / diamond-like carbon (DLC) composite films were carefully fabricated using non-equilibrium magnetron sputtering technology. A variety of analytical methods,such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy (TEM), and vacuum friction testing, were utilized to thoroughly assess the structure, morphology, tribological properties, and wear mechanisms of the films. The SEM images clearly show that the DLC film's surface is densely structured, consisting of closely packed small particles of similar sizes, without noticeable defects such as cracks or holes. Conversely, the surface of the MoS2 film features a worm-like structure, leading to a surface that is not smooth. When comparing the surface morphology of the MoS2 / DLC composite film to that of the DLC film, it is observed that both surfaces are composed of uniform small particles; however, the composite film’s particles create a rough, island-like structure. This is attributed to the amorphous growth of the DLC film, which disrupts the one-dimensional growth pattern of MoS2. Compared to the MoS2 film, the cross-sectional organization of the composite film shows an improvement, with a less pronounced columnar structure leading to a denser film structure. The presence of amorphous carbon prevents the formation of the columnar structure in MoS2, effectively mitigating the issues of pores, cracks, and other defects in the MoS2 film. Notably, the XRD diffraction peaks of the composite film were primarily observed at the (002) crystal face, with amorphous carbon aiding the film's growth and promoting a preferred orientation on this crystal face, which enhances its lubrication effectiveness. The hardness of the composite film increased significantly to 8.13 GPa, marking an eightfold improvement over the pure MoS2 film. Additionally, the surface roughness of the composite film was significantly reduced to 1.66 nm, in contrast to the higher surface roughness of 5.89 nm exhibited by the pure MoS2 film. The hardness of the MoS2 / DLC film also showed a significant increase when compared to the MoS2 film. The inherent low hardness of the MoS2 film, which leads to high wear rates, is effectively countered by the addition of carbon, increasing the composite film's hardness and thereby reducing its wear rate. The elastic recovery rate of the composite film was also found to be improved over the MoS2 film. The integration of DLC with MoS2 modifies the MoS2 film's structure, incorporating the DLC film's high hardness advantage into the composite, enhancing its performance under medium to high loads. XPS analysis confirmed that the composite film is predominantly composed of 2H-MoS2, favoring effective lubrication. To evaluate the tribological properties of the composite film, comprehensive testing was conducted under a wide range of vacuum loads, from 0.73 GPa to 1.27 GPa, showing the film's ability to consistently maintain a low friction coefficient (0.02–0.06) and a low wear rate 10?10 mm3 ·N?1 ·m?1 ). Comparative analysis has shown that, relative to MoS2 alone, the composite film significantly lowers both the friction coefficient and wear rate by three orders of magnitude. Further detailed examination revealed that the composite film is capable of undergoing a graphitization transformation, which leads to the creation of ordered graphite structures. These structures effectively lubricate MoS2 with a (002) orientation, a process induced by both the catalytic effect of MoS2 and contact stress. The development of layer-intercalated low shear stress graphite structures and layered MoS2, facilitated by the catalytic influence of MoS2 and contact stress, was identified as crucial for attaining both a low friction coefficient and wear rate. The incorporation of amorphous carbon into the composite film plays a significant role in enabling it to sustain a low friction coefficient and an ultra-low wear rate, even within a vacuum environment. Moreover, this study not only contributes practical insights but also offers valuable theoretical guidance for the future application, design, and development of MoS2 / carbon composite films.
  • Induced Co-deposition of Co-Fe Alloy on Silicon Steel Surface
    XING Hongxuan, HU Xianwei, LI Jidong, WANG Yaowu, ZHAO Chenxu
    China Surface Engineering. 2024, 37(3): 185-194. https://doi.org/10.11933/j.issn.1007-9289.20230611001
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    Most scholars have investigated silicon steel coated with an insulating varnish and a magnesium silicate substrate covering the surface of silicon steel. However, before silicon steel is coated with an insulating varnish, it is exposed to an uncontrolled environment, which significantly increases the risk of corrosion. Therefore, investigating processes that enhance the corrosion resistance of silicon steel without affecting its soft magnetic properties is crucial. In this study, the process of preparing Co-Fe soft magnetic alloy plating by induced co-deposition on the surface of silicon steel is proposed to address the problems of corrosion resistance and soft magnetic properties of silicon steel. In addition, sodium saccharin(C7H8NNaO4S) is a commonly used additive in the plating process of sodium saccharin in the sulfur element; although the content of a very small amount of the plating layer and its soft magnetic properties have a negative impact, non-toxic and non-polluting sodium tartrate, instead of sodium saccharin, was used in this study. After preparing the Co-Fe soft magnetic alloy coating, the microstructure of the coating and change of elemental content were studied in detail using scanning electron microscopy and energy dispersive spectroscopy; the phase structure composition of the coating and peak diffraction intensity were analyzed using X-ray diffraction; and the soft magnetic properties were investigated using the hysteresis line measured by vibrating-sample magnetometry. The process parameters such as cobalt sulfate concentration,deposition current density, pH, and sodium tartrate concentration were optimized step by step by the above test methods, and the optimal process parameters were selected and the optimal coating structure was constructed by analyzing the microstructure of the plating layer and the variation of the elemental content of the plating layer. The results show that the optimum process parameters are 20 g·L?1 cobalt sulfate, 45 g·L?1 sodium tartrate, pH=8, and 0.3 A·cm?2 current density, and the plated layer has a dense morphology and uniform grain distribution. In addition, the phase structure of the plated layer was Co7Fe3 with no significant difference in the preferred orientation for each process parameter. Cobalt content that is excessively high or low in the coating has a negative effect on the soft magnetic properties; when the cobalt content reaches 65.11%, the saturation magnetization strength reaches 209 emu·g?1 ((A·m2 ) / kg), the coercivity is below 4 Oe(1 Oe=79.6 A / m), and the plating has optimal soft magnetic properties, which meets the national requirements of the soft magnetic properties of silicon steel. In the corrosion test, the corrosion solution used in previous studies was a NaCl solution with a mass fraction of 3.5%, but the working environment of the silicon steel of the transformer core was exposed to air or was in a harsh acidic environment. Therefore, the corrosion solution we selected was a vented acidic solution (HCl) with pH 3, 4, and 5, and vented deionized water, to simulate the actual production and storage environment and to ensure that the oxygen was saturated. The test studies on corrosion performance revealed that the corrosion potential of cobalt-iron alloy plating was positively shifted by 0.012 V and the self-corrosion current density was reduced to 2.86 μA·cm?2 in pH = 5 acidic solutions, which provided more optimized corrosion resistance. Compared with the traditional silicon steel surface coating, insulating varnish, and magnesium silicate substrate research, the electrodeposition of Co-Fe soft magnetic alloy is a new type of protection method, and the future surface treatment of silicon steel has reference significance.
  • Influence of Rolling Amount on the Surface Integrity of 300M Steel Bolt Backing Groove
    YUE Meiru, GUO Xiaoguang, KANG Renke, ZHU Xianglong, LIU Hongwei, WANG Ziyue
    China Surface Engineering. 2024, 37(3): 195-203. https://doi.org/10.11933/j.issn.1007-9289.20230531001
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    Because of their high strength, precision, and hardness, 300M steel bolts are widely used in the assembly of various aircraft structural parts. Owing to long-term service in extreme environments, such as severe vibrations and large temperature differences, fatigue damage is prone to occur at the bolt-backing groove, resulting in major safety accidents. Rolling processing, which has dual functions of surface finishing and surface strengthening, is the primary processing technology for bolt surface strengthening. The selection of rolling parameters has significant influence on surface integrity and fatigue life. The rolling amount is an important parameter in the rolling process. To effectively improve the surface quality and performance of the bolt, a rolling strengthening test of the bolt-back groove under different rolling amounts was conducted on a self-developed special rolling tooling for bolts. The effect of the rolling amount on the surface integrity of the 300 M steel bolts was studied. Based on the previous test, the rolling amounts were selected as 0.05 mm, 0.08 mm, and 0.11 mm, respectively. The surface roughness, surface morphology, microhardness, residual stress, and surface microstructure of a 300 M steel bolt after back-groove rolling were tested using a Zygo surface roughness tester, super depth-of-field microscope, microhardness tester, X-ray stress diffractometer, and scanning electron microscope. The results showed that with an increase in the rolling amount, the surface roughness initially decreases and then increases. When the rolling amount is 0.08 mm, the surface roughness Sa reaches the minimum value of 0.058 μm, which is 75 % lower than that before rolling. The surface morphology of the bolt reaches the optimal state when the rolling amount is 0.08 mm, the turning marks basically disappear, and the surface is the most polished. As the rolling amount increases, the surface quality is destroyed, and a large number of pits and ploughs appear. The rolling process causes plastic deformation of the bolt surface structure, and the martensite grains are elongated and refined. Based on the results of the local orientation difference, the KAM value of the bolt after rolling increases from 0.619° to 0.875°, proving that the rolling process introduces high-density dislocations on the surface of the bolt. As the rolling amount increases, the content of the small-angle grain boundary increases, and the degree of plastic deformation of the bolt increases gradually. When the rolling amount is 0.11 mm, the depth of plastic deformation layer reaches 166 μm. In the surface layer of the bolt, the grain boundaries become blurred owing to severe plastic deformation. Cold work hardening and plastic deformation significantly increase the microhardness of the bolt surface, with a maximum value of 660 HV, which is 16 % higher than that of the matrix. Within 0-400 μm, the microhardness gradually decreases as the distance from the rolling surface increases. The rolling process introduces a large residual compressive stress on the bolt surface. When the rolling amount is 0.11 mm, the dislocation density between the microstructures reaches the maximum value, and the residual compressive stress reaches 848.4 MPa, which is 662 % higher than that without rolling. In this study, the surface integrity of a 300 M high-strength steel bolt cutter groove after rolling is systematically studied, and the influence of the rolling amount on the surface quality is analyzed, which can provide a reference for process personnel to optimize bolt rolling process parameters. Because the fatigue life of bolts is closely related to the surface roughness and residual stress, the research results can also be applied to the analysis of bolt fracture causes and prediction of fatigue life, providing technical support for the anti-fatigue manufacturing of high-strength bolts.
  • Equivalent Pulse Heat Transfer Model and Process Analysis for Laser Marking Based on Spot Overlapping
    DENG Kai, CAI Song, TAO Nengru, XIONG Xianwen, CHEN Da, XU Wensheng, YU Fan
    China Surface Engineering. 2024, 37(3): 204-219. https://doi.org/10.11933/j.issn.1007-9289.20230528001
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    Currently, laser surface marking technology is extensively utilized. Establishing a heat transfer numerical model for line marking to predict material ablation depth holds significant importance for advancing laser marking technology. The energy coupling in the marking process is intricately complex, primarily due to the multitude of pulses needed for pattern marking and the variance in spatial positions and timing of each pulse's irradiation. This complexity makes direct calculation of the heat conduction process during marking challenging. This paper presents an analysis of the combined effects of spot overlap and energy accumulation. It identifies that during the laser lens's movement along the processing path, numerous instances of similar spot overlap patterns occur. From this observation, two distinct methods for calculating spot overlap are derived. In real-world applications, the influence of the first spot overlap calculation method on the entire line marking process is minimal and can be disregarded, allowing the second method to be utilized for further calculations. Given that the time interval between the actions of any two adjacent pulses in space remains constant, the material's energy absorption attenuation can be precisely determined through the cumulative effect of pulse energy. Consequently, by averaging the total energy, the equivalent single pulse energy and equivalent pulse heat transfer numerical model for each spot area are derived. Laser surface marking experiments were conducted on acrylonitrile / butadiene / styrene copolymer plates (ABS plates), followed by ultrasonic cleaning to prepare the material surface for analysis. The processed surfaces were then evaluated using a surface roughness measuring instrument and a three-dimensional ultra-depth-of-field microscope. The effectiveness of the proposed equivalent pulse heat transfer numerical model for line marking was assessed by comparing the measured depths to the predicted values, taking into account the behavior of the ABS plates at various temperatures. Surface roughness was utilized as a metric to evaluate the quality of the marking. An in-depth analysis was performed to understand the discrepancies between the experimental and theoretical results, bridging theory with practical outcomes. The findings are as follows: the material's ablation depth increases with the overlap rate, ranging from 91% to 99%, due to the effects of laser scanning velocity v or laser pulse frequency f, with the highest material removal rate observed at a 99% overlap rate. Conversely, the ablation depth decreases as the overlap rate increases when considering the effect of spot diameter D, with the highest removal rate occurring at a 91% overlap rate. In scenarios influenced by three key parameters, an equivalent laser power density of I1=3.88W / μm2 generates significant recoil pressure, enhancing material removal and optimizing the energy efficiency of the laser marking process. In surface marking, the optimal coverage of the target area is crucial for achieving high-quality results. If the distance between lines exceeds the width of the line segment groove, then the overlapping of line segments fails to entirely cover the target area, leading to incomplete marking. Conversely, when the distance between lines is less than the width of the groove in the line segment, excessive material overlap can cause expansion and deformation, compromising the surface quality. However, setting the distance between lines to closely match the width of the groove ensures complete coverage of the target area without compromising surface quality, resulting in an effective marking outcome. This study further explores the dynamics of energy coupling in the spatial overlapping area of light spots and the accumulation of energy from adjacent pulses over time. By employing the concept of light spot diameter overlap, the overlap rate for each light spot area was estimated. Utilizing the principle of equivalent effect, an equivalent pulse heat transfer numerical model was developed, offering a more straightforward and efficient method for calculating the ablation depth of laser markings.
  • Structural Evolution and Friction Loss Effect Analysis of TiB2 / WS2 Composite Films under Annealing Treatment
    LIU Jinlong, LI Hongxuan, JI Li, LIU Xiaohong, ZHANG Dingjun
    China Surface Engineering. 2024, 37(3): 220-231. https://doi.org/10.11933/j.issn.1007-9289.20230302003
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    The exploration of structural evolution and failure patterns in WS2-based composite films, especially regarding their friction and wear reduction capabilities under harsh aerospace conditions, still requires further investigation. To widen their application across various temperature ranges, TiB2 / WS2 composite films, aimed at adapting to a broad temperature domain, were deposited onto silicon wafers and 718 high-temperature alloy substrates using a magnetron sputtering method. These films were then annealed at 200 ℃, 450 ℃, and 600 ℃ in atmospheric conditions. Detailed analyses of the films, both before and after the annealing processes, were performed using a range of analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, nanoindentation, scratch testing, and ball-on-disk high-temperature friction testing. The study focused on the effects of annealing temperatures, ranging from 0 to 600 ℃, on the films' elemental composition, microstructure, hardness, and frictional wear properties. It was determined that the films, deposited by non-equilibrium magnetron sputtering, were characterized by a cabbage-like morphology on the surface, a columnar growth structure in the cross-section, and an amorphous state overall. An increase in annealing temperature was associated with an accelerated decomposition rate of sulfur elements, a significant reduction in the S / W ratio, a noticeable rise in film oxidation, and a wear rate that decreased initially but then increased with the annealing temperature. At an annealing temperature of 200 ℃, the films exhibited the lowest number of surface defects and the most compact structure, which slightly enhanced mechanical properties and maintained stable tribological performance, demonstrating effective friction and wear reduction (with a coefficient of friction ＜0.075 and wear rate of 9.21 × 10–6 mm3 / m·N). Excellent tribological properties were exhibited by the film during dynamic and continuous temperature increases (RT of approximately 600 ℃), and thereby, excellent thermal stability and continuous lubricity were realized. Mechanisms of destabilization in composite films before and after undergoing annealing treatments at various temperatures were observed: A low and stable coefficient of friction was maintained by the unannealed film during the friction process due to the formation of a shear-slip friendly WS2 (002) crystal orientation structure. Increased densities and significantly improved hardness, along with optimal tribological properties, were observed after the annealing treatment at 200 ℃. A significant fluctuation in the coefficient of friction was displayed by the composite film after the annealing treatment at 450 ℃, as WS2 starts to oxidize at 450 ℃, leading to the formation of a small amount of oxide phases WO3 and TiO2 on the composite film surface. These oxide phases, being higher in the coefficient of friction than the lubricant phase WS2, partake in the friction process, causing the coefficient of friction to fluctuate and gradually increase, and the wear rate to rise. Failure of the composite films annealed at 600 ℃ occurs within a very short period, attributed on one hand to the destruction of the microstructure and mechanical properties by high-temperature annealing, and on the other hand to the instantaneous failure of the S element in the composite films due to a high temperature decomposition rate of 92.9% and the difficulty in forming the lubricant phase during the friction process in the absence of the S element. The evolution of frictional wear loss at different temperatures was clarified by investigating the changes in the microstructure of the composite films before and after the annealing treatment.
  • Microstructural, Frictional, and Wear Properties of CrN / TiAlN Multilayer Coatings
    TIAN Feng, DUAN Haitao, PAN Lin, XU Xinyu, QIN Jianglin, WEI Qinqin, LUO Guoqiang
    China Surface Engineering. 2024, 37(3): 232-241. https://doi.org/10.11933/j.issn.1007-9289.20230919003
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    Special equipment vehicles often operate under high-speed, high-impact, or other adverse environmental conditions. The surface of a rotary shaft seal on the chassis of this type of vehicle is prone to wear and tear, which can lead to sealing failure. An important method for extending the service life and prolonging the maintenance interval is the preparation of hard coatings on the working surfaces of seals. The aim of this study is to improve the wear resistance and extend the service life of rotating axes. The rotary shaft on the vehicle chassis is generally made of 45# steel. Therefore, we prepared hard coatings with different structures on a 45# steel substrate that was ground, polished, and cleaned using ultrasound and alcohol in advance. A single-layer CrN coating and multilayer CrN / TiAlN coatings with thicknesses of support layers were prepared on the 45# steel via physical vapor deposition. The microstructure, mechanical properties, and friction and wear properties of the coatings were analyzed using X-ray diffractometry, scanning electron microscopy, surface profilometry, nanoindentation, and reciprocating tribometry. The single-layer CrN coating was 0.87 μm thick, with hardness of 19.49 GPa and an elastic modulus of 160.53 GPa at a minimum. The multilayer CrN / TiAlN coatings had significantly higher hardness and elastic modulus values, which increased with the thickening of the support layer. The hardness and elastic modulus of the 4CrN / TiAlN coatings reached 39.86 and 386.72 GPa, respectively. The main reason for the increased hardness of the CrN / TiAlN multilayer coatings was Al doping. The addition of Al can refine the grain and cause crystal lattice distortion of nitride, which increases the resistance to dislocation movement and generates fine crystal strengthening and solid solution strengthening effects. In the friction and wear tests in an air atmosphere, the single-layer CrN coating was soon worn. With an increase in the thickness of the support layer, the average friction coefficient of the multilayer CrN / TiAlN coatings initially increased and then decreased, and the cross-sectional area of the wear scar and the maximum wear depth decreased. The 4CrN / TiAlN coatings exhibited optimal frictional abrasion performance, with an average friction coefficient of 0.792 5 and a cross-sectional area of wear scar of 315.09 μm2 . The wear mechanisms of the 4CrN / TiAlN coatings were abrasive and slight adhesive wear. Increasing the thickness of the support layer could reduce the plastic deformation of the substrate, bridge the difference in the physical properties between the coatings and the substrate, decrease coating cracks, and improve the frictional and wear performance of the coatings. Multilayer coatings can strengthen the binding force between the film and substrate and reduce the residual stress in the coating. In multilayer structures, the interface between the two layers prevents dislocation glide, stress concentration, and cracking and detachment of the coating. It is easier to control the thickness of CrN / TiAlN multilayer coatings prepared via physical vapor deposition than that of conventional electroplated chromium coatings. Moreover, the former yields a significantly better surface hardness and modulus and is a more efficient and eco-friendly preparation process than the latter. With excellent wear resistance, multilayer coatings enable a rotary shaft seal to provide a long-term sealing effect under complex environmental conditions and high-speed rotation, thus extending the service life of the seal. In this study, we obtained CrN / TiAlN multilayer coatings with outstanding performance by controlling the coating thickness. These findings are expected to significantly improve the wear resistance of the working surface of the rotary shaft seals of vehicles.
ISSN 1007-9289　CN 11-3905/TG
Supervised by: China Association for Science and Technology
Sponsored by: Chinese Mechanical Engineering Society
Edited and Published by: Editorial Office of China Surface Engineering
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