20 December 2023, Volume 36 Issue 6
    

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  • Research Status of Plasma Spraying to Prepare Coating Absorption and Plasma Carburizing Technology
    GAO Yukui, ZHENG Xiangyuan, GONG Sijie
    China Surface Engineering. 2023, 36(6): 1-15. https://doi.org/10.11933/j.issn.1007-9289.20230104001
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    With the development of radar detection technology, the demand for the stealth performance of equipment has increased. The stealth technology can significantly enhance the survivability of military equipment and personnel, improve combat efficiency, and establish greater control on the battlefield. The traditional methods for preparing radar-absorbing coatings are complex and inefficient. Plasma spraying has gained widespread application as a thermal spraying technique in the preparation of absorbing coatings owing to its simple process, broad applicability, and high controllability. The surface condition of the materials significantly impacts their performance. Plasma carburizing, as a surface treatment process, plays a vital role in improving the surface strength and wear resistance of materials. This study introduces the basic principles of plasma spraying as well as the influence of basic coating process parameters such as powder feed rate, output power, spraying distance, and spraying speed on the quality of the coatings. Research has shown that when the powder feed rate remains constant, excessively high or low spraying power can lead to a decrease in the coating quality. An excessively short spraying distance can result in poor adhesion between the coating and the substrate, whereas an excessively long distance can reduce the spraying efficiency and coating density. Therefore, the proper control of process parameters in plasma spraying directly and significantly impacts the coating quality. Subsequently, recent research results in the preparation of radar-absorbing coatings using plasma spraying are summarized. Adjusting the plasma spraying process parameters significantly affects the absorption performance. For instance, a higher spraying power can hinder the melting and deposition of the powder, leading to increased porosity and reduced dielectric constants, that in turn affect the absorption performance. In the case of composite coatings, the content of the constituent materials can be adjusted to improve the microwave absorption performance and enhance the conductivity and impedance matching, resulting in excellent absorption and mechanical properties. Additionally, temperature plays a substantial role in microwave absorption, with certain coatings demonstrating outstanding absorption performance at elevated temperatures, presenting promising prospects for high-temperature microwave absorption applications. Furthermore, this study provides an introduction to the development of traditional carburizing heat treatment methods and new carburizing heat treatment technologies. Traditional carburizing methods involve the treatment of materials followed by quenching that creates a surface with higher hardness while maintaining a softer core, thereby enhancing the surface hardness and wear resistance. The new vacuum carburizing technology uses hydrocarbons as a carbon source to treat material surfaces at pressures below atmospheric levels, minimizing deformation and avoiding issues such as surface oxidation and decarburization. However, it still faces challenges such as long processing times and high treatment temperatures. Although high-temperature carburizing improves production efficiency, it can lead to significant grain coarsening during the carburization process, affecting the fatigue performance of the material. Plasma carburizing technology uses the principle of gas discharge to cause carbon ions to collide with the surface of the material, forming a high-quality carburized layer. During ion carburizing, neutral molecules and charged particles move directionally on the surface of the material under an electric field, resulting in a higher concentration of active carbon atoms on the surface of the material than during vacuum carburizing. This substantially enhances the carburization efficiency. As surface modification technologies continue to evolve, low-temperature plasma carburizing techniques have also achieved certain research progress. The study provides an overview of the development and current status of plasma carburizing, highlighting the substantial influence of processing time and heating temperature on the performance of the carburized layer. Plasma spraying and carburizing technologies have matured over time. However, in the future, apart from adjusting the basic spraying process parameters, exploring new control methods for the electromagnetic properties, mechanical performance, and microstructure of materials regarding plasma spraying and carburizing will better meet the increasingly stringent requirements of radar-absorbing coatings. Further understanding of the mechanisms of residual stress, grain boundaries, and dislocations in materials through plasma carburizing processes is also necessary. This study also provides prospects for the future research and development of these two surface modification technologies, offering valuable insights for industries involving critical component surface modifications, such as aerospace and military equipment.
  • Research Status on Numerical Simulation of Spraying with Gas Discharge Heat Source
    LI Tianhao, HUANG Yanfei, LIU Ming, BAI Yu, WANG Haidou, MA Guozheng, GUO Weiling
    China Surface Engineering. 2023, 36(6): 16-35. https://doi.org/10.11933/j.issn.1007-9289.20230129001
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    Gas discharge generates the arc or plasma that acts as a heat source. Arc spraying and plasma spraying using the generated energy as a heat source are collectively called gas discharge heat source spraying. When spraying occurs, the arc or plasma discharge combined with the carrier gas excites the jet with high-temperature and high-speed plasma, which melts the powder or wire material; finally, the molten droplet is deposited on a substrate to form a coating. Notably, various interactions occur between the jet and molten droplets during the in-flight process, such as melting and fragmentation of the droplets and the drag effect between the droplet and jet. However, real-time monitoring of the jet and molten droplets is difficult owing to the high temperature and harsh environment during the spraying process. As an emerging computational method, numerical simulation can be used to study the transient the field and the physical behavior of molten droplets during the spraying process; thus, numerical simulation is an important tool and area of focus. In this study, the differences and potential drawbacks of models in the simulation of two spraying processes, arc spraying and plasma spraying, are studied, and the characteristic distribution of different jets, state of the droplets, and mechanisms of acceleration and heating of in-flight molten droplets are investigated. In the arc spraying process, wire materials with different thermal conductivities cause the static temperature at the cathode to be much higher than that at the anode, and the velocity distribution of the jet flow is more diffused in the wire plane. The use of a massive wire can increase the velocity and temperature of the molten droplets; however, this might exacerbate asymmetric melting of the wire. Therefore, core wires are expected to be more promising for future applications. Most models consider the interaction of the individual droplets and jet flow instead of droplet groups, and the improved two-fluid model does not consider the thermophoretic force; therefore, further adjustments and advanced models are required. Plasma spraying has been more extensively studied than arc spraying. This study focuses on the following key scientific issues in the spraying process before coating deposition: On one hand, the energy source used for plasma spraying, excitation, and plasma flow directly affect the state of the materials. Thus, determining an appropriate turbulence model based on the flow state is the first important issue. The plasma two-temperature model using the non-local thermal equilibrium very closely reflects the actual working conditions and has become the most suitable base model because the electromagnetic processes at the cathode and anode crucially affect plasma formation and the plasma characteristics, thereby affecting the electromagnetic properties of the cathode / anode and sheath regions, such as the arc reattachment behavior and anode wear. The physical parameters of in-flight molten droplets have been evaluated in several studies using theoretical analyses, experimental demonstrations, and numerical simulations. The effects of various process parameters were analyzed, providing an effective guide for achieving the desired experimental results and for determining the optimal parameters. Finally, the mass and heat transfer processes in molten droplets in the jet flow were systematically analyzed. For example, the droplet goes through the following stages: acceleration and heating, constant temperature and velocity, deceleration and cooling in the jet flow, and fragmentation via vibration breaking as the main mechanism. However, numerical simulations still have limitations such as insufficient simulation accuracy and a lack of in-depth research. Future research on multi-droplets modeling of arc spraying, simulation of the turbulence and electromagnetic properties of plasma spraying, and numerical simulation of plasma transfer arc spraying, a new technique that is becoming increasingly mature, can potentially provide directives for advancing the simulation of gas discharge heat source spraying. This study summarizes the evolution of each physical process model in the numerical simulation of gas discharge heat source spraying since the emergence of thermal spraying simulation, and provides theoretical guidance for the experiments by combining the results of each stage of evolution.
  • Research Progress in Synergetic Corrosion and Protection of Pipelines in Oil and Gas Production Environment
    LIU Zhengyu, CAO Xueqian, WANG Shaolong, ZHANG Guang’ an, XUE Qunji
    China Surface Engineering. 2023, 36(6): 36-48. https://doi.org/10.11933/j.issn.1007-9289.20230215001
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    Pipeline corrosion in CO2 / H2S environments at high temperatures and pressures is a critical problem in oil and gas fields that must be solved urgently CO2, H2S, and Cl- are common corrosion media in oil and gas pipelines. CO2 changes the chemical properties of water in a solution and decreases the pH value of the solution, which promotes cathodic hydrogen evolution reaction. Corrosive ions dissociated from H2S exhibit strong penetrability, which accelerates pitting and sulfide stress cracking. The barrier function and catalytic effects of Cl? reduce the adhesion of corrosion products, which accelerates metal corrosion. The synergistic corrosion between CO2, H2S, Cl? , and external factors, such as temperature, pressure, pH value, water content, and flow rate, leads to the severe corrosion of pipelines. These external factors impact the corrosion process by influencing the morphology of the corrosion products. A harsh corrosion environment loosens the corrosion products and weakens the barrier effect of the corrosion products on the corrosive medium, thereby accelerating the corrosion of the substrate. Thus, it is necessary to investigate the synergistic corrosion mechanisms of corrosive media and external factors and to propose measures to mitigate pipeline corrosion. In view of the corrosion problem of metal pipelines encountered during oil and gas exploitation, the roles of CO2, H2S, and Cl- in the synergistic corrosion process are summarized in this paper. In addition, the effects of external factors, such as temperature, pressure, pH value, water content, and flow rate, on the morphology and composition of corrosion products under actual working conditions are discussed. The characteristics of existing pipeline corrosion protection technology and processes are discussed. Doping with alloy elements can modify the composition of the corrosion products and improve the corrosion morphology and compactness of the corrosion product layer. Doping with elements, such as Ni, Cr, Mo, and Ti, can effectively slow the pitting and stress corrosion cracking of metals. Plasma diffusion and coating technology can be applied to prepare a dense protective layer to absorb a part of the corrosion medium and decrease the corrosion rate. The corrosion resistance of a metal is effectively improved by infiltrating interstitial atoms into the metal surface using chemical heat treatment techniques, such as nitriding, carburizing, and carbonitriding. The addition of a corrosion inhibitor can slow the cathodic or anodic reaction of the pipeline, or form an adsorption layer to decrease the corrosion rate. However, traditional measures for protecting oil and gas pipelines, such as the use of corrosion-resistant alloys and corrosion inhibitors, are limited owing to their high operating costs and environmental pollution. The preparation of advanced coatings on the inner surfaces of pipelines is expected to achieve a compromise between corrosion protection and cost, and is one of the most effective methods for protecting metal pipelines. Ni-P coatings effectively decreases the corrosion rate by absorbing the corrosive medium to generate corrosion products. Diamond-like carbon films prepared via hollow cathode plasma-enhanced chemical vapor deposition on the inner surfaces of pipelines are also considered one of the most effective methods for future oil and gas field pipeline protection because of their excellent chemical inertness and minimal chemical reactions with acids, alkalis, and salts. Finally, future directions for the advancement of pipeline protection technology in oil and gas fields are proposed. It is necessary to further investigate the synergistic corrosion effect between corrosive media and external factors, and simulate the corrosion environment under actual conditions to effectively protect pipelines in oil and gas production environments. This can be achieved by conducting systematic experimental tests on existing protection technologies, such as plasma diffusion and coating technology, and using corrosion inhibitors.
  • Deposition and Properties of DLC Film on the Inner Wall of a 10-m-Long Stainless-steel Tube
    JIN Pengli, TIAN Xiubo, GONG Chunzhi
    China Surface Engineering. 2023, 36(6): 49-56. https://doi.org/10.11933/j.issn.1007-9289.20230227003
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    During industrial production, the inner wall of a tube is often corroded and worn by the conveying material; therefore, it must be protected. Currently, research on the coating of the inner wall of a tube is limited, and research regarding the performance of the inner wall of a large-aspect-ratio tube is lacking. Thus, this study attempts to solve the problem of a diamond-like carbon(DLC) film on the inner wall of a long tube. Herein, the internal surface of a 304 stainless-steel tube, which is 10 m long and 100 mm in diameter, is coated with DLC using plasma-enhanced chemical vapor deposition(PECVD). The hollow cathode discharge of gas into the tube, brightness of the film, contact angle, surface hardness, friction factor, and Raman spectroscopy results are characterized using different tests. During the preparation of the inner coating, the plasma spectrum in the 300-800 band at position 3 of the pipe is analyzed using a spectrometer and optical focusing platform. After the film is prepared, its surface brightness is measured using a color difference meter to determine the L*, a*, and b* values of the film, where L* represents the brightness, a* represents red and green, and b* represents yellow and blue. Scanning electron microscopy(SEM) is used to observe the section morphology of the film. The surface roughness of the film is measured using atomic force microscopy(AFM). A friction and wear-testing machine is used to test the wear resistance of the film. A GCr 15 ball with a diameter of 3 mm is selected as the grinding pair, and the experimental parameters of load, rotational speed, and test time are set as 3 N, 200 r / min, and 120 min, respectively. A nanoindentation hardness instrument is used to measure the nanohardness of the film. A measuring meter for the static contact angle of water is used for measurement; the measurement requires 2 μl of each drop. Each sample is tested three times, and the average value is calculated as the final test result. The film is measured by a laser Raman spectrometer using a 532 nm laser source from 800 to 2 000 cm–1, and a Gaussian curve is employed to fit the Raman spectrum. The results demonstrate that in the coating process of the inner wall, the first step is the entry of Ar gas into the pipe for glow discharge cleaning. Ionization and excitation occur during the process of Ar gas discharge, thereby producing Ar+ and Ar0 ; additionally, C2, H, and CH decompose from acetylene. The SEM results of the cross-sections of the films at different positions reveal that the cross-section of the film is uniform and dense, with no cracks, pinholes, or other defects. Further, no cracks are observed in the binding transition between the film and matrix, thickness of the film at different positions, and thickness of the film near the ends of the pipeline, thus indicating that the plasma density in this region and the deposition rate of the film are high. The largest L* and ΔE* values of the film are 37.4 and 1.9, respectively. The AFM results demonstrate larger bulges on the film surface at positions 3 and 7, which result in high surface-roughness, and uniform distribution of the bulges at positions 3 and 9. The water contact angles of the films are smaller at both ends. The results of the wear tests indicate that the friction factor of the film decreases in the first 20 min from 0.225 to approximately 0.175. For a prolonged time, the friction factor remains stable at 0.17. The hardness of the film near the ends of the pipe is slightly higher. Further, no damage or film-peeling is noted in the pin-on-disk test, and the film is characterized as having a good wear resistance. The Raman spectra of the films at different positions reveal that in contrast to positions 1 and 9, the G peaks at positions 3 and 7 exhibit a shift toward the peak level, thus indicating a high stress in the film at both ends. Considering the distribution trend of ID / IG in the film, a low ratio of ID / IG is determined at both ends of the tube. The glow discharge and DLC deposition in the long tube with a large length ratio provides theoretical support and technical guidance for the uniform coating of the walls of long pipes.
  • Effects of Pulsed Bias Duty Cycle on the Microstructure and Properties of TiCrN Films
    WEI Yongqiang, GU Yanyang, FAN Mengyuan, YANG Jiale, ZHANG Huasen, ZHANG Xiaoxiao, ZHONG Sujuan, LIAO Zhiqian
    China Surface Engineering. 2023, 36(6): 57-67. https://doi.org/10.11933/j.issn.1007-9289.20221230001
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    With the requirements of high-speed steel material cutting and machining are increasingly in high advanced manufacturing fields, there is an urgent need using nitride films to improve the hardness of the substrate materials and wear resistance and other comprehensive properties. Meanwhile the service life of high-speed steel materials is extended. The effects of the pulsed bias duty cycle on the microstructure and properties of TiCrN films were investigated to optimize the deposition process parameters and improve the properties of TiCrN films. TiCrN films subjected to different pulsed bias duty cycles were deposited onto M2 high-speed steel (HSS) substrates and Si wafers using the arc ion-plating method. The surface morphology, elemental composition, phase structure, and nanohardness of the TiCrN films were examined using scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, and nanohardness indentation. The corrosion behaviors and tribological properties of the coated and uncoated M2 HSS samples were examined using an electrochemical workstation and a pin-on-disk tribometer at room temperature. Potentiodynamic polarization curves were used to calculate the self-corrosion potential and self-corrosion current density of the tested samples in a 3.5 wt.% NaCl solution. With an increase in the pulsed bias duty cycle from 10% to 60%, the amount of macroparticles on the TiCrN film surfaces decreased, and the surface quality improved. At pulsed bias duty cycle of 10%, the maximum amount of macroparticles was 175, whereas at pulsed bias duty cycle of 60%, the minimum amount of macroparticles was 85. The thicknesses of the TiCrN films ranged from 517.4 to 623.8 nm. The TiCrN film thickness showed a trend of increasing at pulsed bias duty cycles of 10%-30% and decreasing at pulsed bias duty cycles of 30%-60%. At pulsed bias duty cycle of 30%, the thickness of the TiCrN film reached the maximum value of 623.8 nm. At pulsed bias duty cycle of 60%, the minimum thickness was 517.4 nm. At pulsed bias duty cycle of 10%, the Cr content reached 33.9 at.%, the grain size of the TiCrN film reached the minimum of 12.692 nm, and the nanohardness and elastic modulus reached maximum values of 29.22 and 407.42 GPa, respectively. At pulsed bias duty cycle of 30%, the Cr content reached the minimum of 33.07 at.%, the grain size of the TiCrN film reached the maximum of 15.484 nm, the stable friction factor was 0.9, and the nanohardness reached the minimum of 25.83 GPa. All TiCrN films deposited under different pulsed bias duty cycles showed preferred orientations in the (220) crystal plane, and the diffraction peak intensity gradually increased as the pulsed bias duty cycle increased from 10% to 40%. However, the intensity of the (220) crystal orientation diffraction peak decreased when the pulsed bias duty cycle exceeded 40%. The nanohardness of the TiCrN films under different pulsed bias duty cycles exceeded 25 GPa, which is more than 2.8 times of that of M2 HSS. Potentiodynamic polarization curves showed that TiCrN films subjected to different pulsed bias duty cycles exhibited improved corrosion resistance. Compared with the M2 HSS substrate, the corrosion resistance of the TiCrN films showed that the corrosion potential increased by approximately 0.556-0.642 V, and the corrosion current density decreased by more than one order of magnitude. Friction factor curves plotted using the pin-on-disk wear test results, as well as optical microscopy observations of wear trace width and morphologies, indicated that the TiCrN films exhibited significant wear resistance compared to the uncoated M2 HSS substrate. The wear scars on the TiCrN films were more uniform, and the number of furrows decreased significantly. At pulsed bias duty cycle of 20%, the factor of friction and the abrasion width of the TiCrN films reached minimum values of 0.68 and 0.63 mm, respectively. The potentiodynamic polarization curve for the 20% cycle showed that the self-corrosion potential (Ecorr) of the TiCrN film reached the maximum of 0.330 V (vs. SCE), and the self-corrosion current density (icorr) reached the minimum value of 0.255 μA / cm2 . At pulsed bias duty cycle of 20%, the corrosion resistance was the highest, and the corrosion rate was the lowest. Compared with the M2 HSS substrate properties, the hardness, corrosion resistance, friction, and wear properties of the TiCrN films with different pulse bias duty cycles improved significantly. The Cr content and ion bombardment were the main factors that influenced the microstructure and properties of the TiCrN films. These results provide experimental basis for optimizing the hard films deposition process. TiCrN films have a better application future for the properties improvement of cutting tool materials.
  • Tribological Behavior of TiN Coatings Deposited by Plasma-enhanced Magnetron Sputtering Against Aluminum
    XIE Qi, LIU Ziyi, FU Zhiqiang, KANG Jiajie, ZHU Lina, SHE Dingshun
    China Surface Engineering. 2023, 36(6): 68-78. https://doi.org/10.11933/j.issn.1007-9289.20221231001
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    The friction and wear performances of aluminum extrusion die surfaces are important factors affecting the quality of aluminum or aluminum alloy products and the life of extrusion dies. The deposition of wear-resistant films on the surface of extraction dies is one of the most effective strategies, and the friction and wear performances of different coatings against aluminum have been previously investigated. However, studies regarding the influence of the coating microstructure on the tribological behavior of the same coating material when applied against aluminum remain limited. Plasma-enhanced magnetron sputtering introduces an extra electron-emitting source into conventional magnetron sputtering equipment to obtain a densified and controllable plasma around the substrates, producing wear-resistant coatings with similar compositions but significantly different microstructures. The TiN coating, which is a widely used wear-resistant coating, is selected as an example and deposited by plasma-enhanced magnetron sputtering at various substrate bias currents to obtain coatings with different microstructures. The effect of the substrate bias current on the microstructure, mechanical properties, and tribological behavior of the TiN coatings against aluminum is systematically investigated to further optimize the deposition process of the wear-resistant TiN coating applied on aluminum extrusion dies. The TiN coating is prepared using plasma-enhanced magnetron sputtering under varied substrate bias currents of 0.1 A, 1.5 A, 3.0 A, and 4.5 A. The chemical compositions of the TiN coatings are analyzed using X-ray photoelectron spectroscopy (XPS). The surface and cross-sectional morphologies of the coatings are observed using scanning electron microscopy (SEM). The 3D surface microstructure and surface roughness of the coatings are studied using atomic force microscopy (AFM). The phase structures of the coatings are determined using X-ray diffraction (XRD). The comprehensive mechanical properties and tribological behavior of the TiN-coated samples against aluminum are investigated using a nano-indenter and a rotary ball-on-disk friction and wear tester, respectively. The surface morphologies and chemical compositions of the wear tracks are analyzed using laser confocal microscopy, SEM, and EDS. The results show that the variation in the substrate bias current has little impact on the chemical composition of the TiN coatings deposited by plasma-enhanced magnetron sputtering, and all the coatings have a nearly stoichiometric composition. The cross-sectional microstructure of the TiN coating is gradually refined with an increasing substrate bias current, and the surface microstructure of the coating is consisting of island-like microprojections. When the substrate bias current increases from 0.1 A to 4.5 A, the size and amount of the microprojection are gradually decreased along with the surface roughness (from 77.67 nm to 15.67 nm). The preferred growth along the TiN(111) direction dominates in all the coatings, and it is further enhanced when the substrate bias current reaches 3.0 A. The grain size of the TiN coating is pronounced decreased from 44 nm to 11 nm as the substrate bias current increases from 0.1 A to 1.5 A, and the comprehensive mechanical properties of the TiN coating are significantly improved. When the substrate bias current is further increased, the effect of the substrate bias current on the grain size and the mechanical properties of the coatings becomes unobvious. Adhesive and abrasive wear are dominated in the wear process against aluminum of TiN-coated samples, and the friction-reduction and wear-resistance performance of the TiN-coated samples is negatively correlated with the aluminum adhesion area. In conclusion, the substrate bias current plays an important role in controlling the surface cross-sectional microstructure, grain size, mechanical properties and tribological behavior of the TiN coatings deposited by plasma-enhanced magnetron sputtering. When deposited at a substrate bias current of 1.5 A, the TiN coating with excellent mechanical properties and a rough surface microstructure has the lowest friction factor and wear rate of 0.41×10?15 and 3.03×10?15 m3 / (N·m), respectively. This study is theoretically significant and practically valuable for the research and development of high-performance and long-life protective coatings on the surfaces of aluminum-forming dies.
  • Tribological Behaviors of Polycrystalline Diamond Sintered by Titanium-coated Diamond Particles under Humid Condition
    SHA Xiaohua, LI Jinyan, ZHOU Bo, YUE Wen
    China Surface Engineering. 2023, 36(6): 79-89. https://doi.org/10.11933/j.issn.1007-9289.20221216001
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    The polycrystalline diamond (PCD), sintered at a high temperature (1 460-1 500 ℃) and pressure (5-5.5 GPa), consists diamond particles and a cobalt binder. PCDs are widely used in geological and petroleum drilling systems, such as thrust bearings and drill bits, owing to their high hardness, toughness, thermal conductivity, and wear resistance. In geological drilling, PCD tools are used in a complex boundary lubrication environment in which the relative humidity (RH) significantly affects the tribological behavior and drilling efficiency. Coating diamond particles with strong carbide-forming elements has been proposed to strengthen the interfacial bonding between the diamond and cobalt binder in PCDs, which influences the tribological behavior. However, the tribological behavior of a PCD sintered by coated diamond particles at different RH levels, and the corresponding mechanisms, have yet to be studied in detail. A titanium (Ti) coating with a thickness of ~500 nm was deposited onto diamond particles via magnetron sputtering. The Ti-coated diamond particles were then sintered at high temperature and pressure to synthesize the Ti-PCD. The tribological properties of the Ti-PCD were studied at 5%-50% RH. The Ti-coated diamond particles and Ti-PCD were analyzed by scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, impact tests, and tribological tests. The wear morphology of the samples was analyzed by an optical microscope, a Nano Map-D three-dimensional white-light interferometer, and Raman spectroscopy. The results showed the formation of a titanium carbide transition layer between the diamond grain and cobalt binder in the Ti-PCD. The as-obtained titanium carbide phase promoted the tribological behavior of the Ti-PCD under humid conditions, including the reduction of the friction factor and enhancement of the wear resistance. The friction factor of the Ti-PCD tested at different RH levels had a run-in period and steady state. The friction factor significantly fluctuated during the run-in period and stabilized after ~8 min. The steady friction factor exhibited a rising trend with the RH level, increasing from 0.034 at 5% RH to 0.073 at 50% RH. The steady friction factor of the Ti-PCD at 5%-50% RH was affected by the carbonaceous transfer film on the worn silicon nitride surface. A low steady friction factor was generally accompanied by a high transfer film-covering fraction. A continuous transfer film induced by carbon hybridization was formed at 5% RH, leading to the lowest steady friction factor. A carbonaceous transfer film was formed by the layer-shearing action of massive tiny diamond grains exfoliated from the Ti-PCD surface, which significantly reduced the friction factor. The Ti-PCD surface was lyophobic, and the H2O dissociative passivation was weak. This facilitated the formation of the carbonaceous transfer film, which reduced the friction factor by 30% compared with that of pristine PCD (P-PCD) in humid environments. The wear rate of the Ti-PCD decreased from 2.4 × 10?11 mm3 /(N·mm) at 5% RH to 4 × 10?12 mm3 /(N·mm) at 50% RH. Diamond exfoliation during the sliding operation dominated the wear loss of the PCD, leading to massive spalling pits on the wear track. The Ti-PCD wear was much milder than that of the P-PCD, exhibiting the same turning trend as the RH level. The wear rate of the Ti-PCD at 5% RH was significantly lower than that of P-PCD (9.1 × 10?11 mm3 / (N·mm)). The high-temperature and high-pressure sintering of Ti-coated diamond particles significantly enhanced the wear resistance of the Ti-PCD. The titanium carbide phase inhibited diamond exfoliation by strengthening the interface bonding, which involved transforming the mechanical interaction between the diamond grains and cobalt binder into chemical bonding, thereby enhancing the wear resistance of the Ti-PCD. These results indicate that the tribological behavior of the Ti-PCD in humid environments can be significantly improved by introducing Ti-coated diamond particles, which mainly transfer the interfacial state of diamond particles and Ti-PCD. The Ti-coated diamond particles were used to synthesize the anti-friction Ti-PCD. The results showed that the interface state between the diamond grains and cobalt binder was strengthened by interfacial bonding, strength, and toughness reinforcement. This will be significant in the fabrication and application of efficient and durable drilling equipment.
  • Effect of Detergent, Dispersant, and Zinc Dialkyldithiophosphate on the Tribological Properties of Amorphous-C Films
    ZHANG Shaojun, YUE Wen, WANG Yanyan, HUANG Haipeng, ZHANG Guang’an
    China Surface Engineering. 2023, 36(6): 90-99. https://doi.org/10.11933/j.issn.1007-9289.20221229002
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    The solid-liquid composite lubrication system is a key technology for achieving high-fuel-economy and high-durability engine systems. Diamond-like carbon (DLC) films have high hardness, a low friction coefficient, and good biocompatibility, thereby being widely applied in engine system, involving valve lifters, piston rings, etc. It is well known that the performance of oil lubricants is highly correlated with the chemical composition of the grinding surface materials. Previous studies have shown that Fe2+ from ferrous materials directly takes part in the tribochemical reactions of oil lubricants. DLC films exhibit chemical inertness, in contrast to traditional ferrous materials. However, the current formulas for engine oil lubricants are universally developed for ferrous materials,and a special lubricant formula for DLC films needs urgent improvement. The extreme-pressure antiwear agent zinc dialkyldithiophosphate (ZDDP), the detergent calcium persulfonate (OBCaSu), and the dispersant polyisobutylene succinimide (PIBSI) are the most widely used additives in formulated lubricants. However, there is still limited research on the tribological properties of DLC films lubricated with the above additives. In this work, amorphous carbon (a-C) films were prepared via nonequilibrium magnetron sputtering, and the tribological properties of the a-C films under boundary lubrication conditions between ZDDP and OBCaSu (ZDDP+OBCaSu) and between ZDDP and PIBSI (ZDDP+PIBSI) were evaluated using a CSM tribometer. The tribochemical reactions were analyzed using Raman spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy (EDS), combined with the full formula (FF) oil and GCr15 steel, to explore the tribological mechanism of a-C films. The morphologies of worn surfaces were determined with a three-dimensional surface profilometer, and it can be seen that the main wear mechanism of a-C films against steel balls is abrasive wear. Under various lubrication conditions, the results indicate that the worn surfaces of the a-C films undergo graphitization compared to the unworn surfaces, which is beneficial for achieving a low friction coefficient. Phosphate tribofilms are formed on the worn surfaces of GCr15 steel and a-C films under ZDDP, ZDDP+OBCaSu, and ZDDP+PIBSI lubrication conditions. Under a ZDDP+OBCaSu lubrication condition, the composite tribofilms of Ca3(PO4)2 and Zn3(PO4)2 on the worn surfaces of GCr15 steel and a-C films can improve their wear resistance and lubrication performance, which was confirmed by the low friction coefficients and wear rates. Moreover, similar friction coefficients and wear rates on the surfaces of GCr15 steel and a-C films are obtained. Therefore, the tribochemical reaction of the ZDDP+OBCaSu lubrication has less dependence on the surface materials. Under ZDDP+PIBSI lubrication, the tribological properties of GCr15 steel and a-C films decrease, resulting in high friction coefficients and high wear rates. According to the EDS energy spectrum, it can be derived that the strong dispersion of PIBSI is not conducive to the formation of phosphate tribofilms. Under FF lubrication conditions, the tribological properties of GCr15 steel and a-C films decreased compared to those under ZDDP, ZDDP+OBCaSu, and ZDDP+PIBSI lubrication conditions, which may be influenced by the lubricant concentrations and other lubricants. In this work, the tribological behavior and tribochemical reaction mechanism of traditional oil lubricants on the surfaces of GCr15 steel and a-C films are studied. Overall, the results show that the changes in the friction coefficients and wear rates of GCr15 steel and a-C films with various lubricants are similar. However, the a-C films are not sensitive to the lubricants. That is, compared to GCr15 steel, the tribological performance of the a-C films under different lubricants fluctuates by only a minor degree. Therefore, there is a need to develop more suitable oil lubricant formulas for a-C films. This work serves as a guidance for future design and development of lubricants and a-C films for energy savings and fuel efficiency.
  • Abrasive Wear Mechanism of the Diamond-like Carbon Composite Microstructure Surface
    LI Xingliang, YUE Wen, KANG Jiajie, MENG Dezhong, WANG Chengbiao
    China Surface Engineering. 2023, 36(6): 100-107. https://doi.org/10.11933/j.issn.1007-9289.20221209002
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    For more than one hundred years, researchers have been investigating the tribological behavior of certain materials. Their studies concentrated mostly on adding lubricants or modifying the material surface. Unfortunately, these methods do not always guarantee effectiveness in special conditions, such as under abrasive wear. The application of surface texturing on rubbing pairs has been demonstrated to improve the abrasive wear property of certain materials. Titanium and its related alloys are one such type of materials that exhibit excellent properties and are widely used in important fields such as weapon equipment, civilian products, and the chemical industry. However, applications of titanium and its related alloys are limited by its easy adhesion and poor wear resistance under abrasive wear conditions. Therefore, it is of great significance to study the friction reduction and wear resistance performance of surface-textured titanium under abrasive conditions. Many scholars have conducted extensive research on the friction and wear properties of textured surfaces. However, the quantitative relationship between the texture edge and wear resistance property has not been further investigated. The purpose of this study is to investigate the effect of the texture edge on abrasive wear performance. The substrate selected is a commercial pure titanium grade 2 (TA2), which has poor tribological behavior under abrasive wear. To prepare the diamond-like carbon (DLC) composite textured surface, an indirect laser processing method (i.e., laser processing before DLC film deposition) is chosen to prevent the DLC film from being damaged by laser exposure. Dimple textures are prepared on the surface of TA2 using laser processing technology. The dimple texture parameters are designed by using orthogonal analysis to equalize sampling so that the results do not depend on subjective factors. Afterward, DLC film is deposited on the dimple textured surface using magnetron sputtering. The DLC composite textured surfaces are obtained through the above processing methods. First, the friction and wear performances of DLC composite textured surfaces are analyzed based on simulation and experimental methods. The stress distribution of the dimple textured surface under dry friction is studied using finite element analysis. In addition, the friction and wear properties of DLC composite textured surfaces under lunar soil conditions are studied using an MS-T3000 friction and wear tester, and the effect of dimple edges on tribological performance is studied. Second, various testing methods are used to analyze the micro properties of the worn surface. Surface analysis technologies, such as scanning electron microscopy, energy dispersive spectrometry, and Raman analysis, are used to study the microstructure, chemical characteristics, and molecular structures of the DLC surfaces before and after the tribotest. Finally, important conclusions are drawn by analyzing the mechanism of friction and wear. The analysis results show that the maximum stress appears at the edge of the texture. Moreover, a mathematical model is established to obtain the formula for the edge density of the texture on a unit area. Under the same friction conditions, the average friction factor of the DLC composite micro-textured surface is lower than that of the untreated surface. The wear rates of samples with different texture parameters are listed in order. Meanwhile, the micro morphology and tribochemical properties of the worn surface are studied. The results reveal that the DLC composite textured surface can effectively play an important role in reducing friction and wear. The dimple density exhibits a noticeable influence on the friction coefficient of the surface while the dimple texture edge density greatly influences on the wear rate. The research results can provide a theoretical and design basis for the application of titanium under abrasive wear conditions. Furthermore, it also has importance for the development of certain materials.
  • Effects of Physical Vapor Deposition on the Microstructure of Atmospheric Laminar Plasma-sprayed Mo Coatings
    ZHANG Huiyu, LIU Senhui, DENG Shijie, LI Changjiu, LI Chengxin
    China Surface Engineering. 2023, 36(6): 108-116. https://doi.org/10.11933/j.issn.1007-9289.20230310001
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    Relevant studies based on atmospheric plasma spraying have proven that laminar plasma jets have the characteristics of a high length, low velocity, and high energy density, which can effectively heat particles by prolonging their dwell time in the plasma jet. Previous studies have shown that increasing the particle temperature can effectively improve the interlayer bonding rate. During laminar plasma spraying, the particle and substrate temperatures can be smoothly improved by prolonging the particle dwell time and heating the substrate in situ. However, the coating still exhibits a low bonding rate and contains numerous pores. Therefore, studying the deposition mechanism of laminar plasma-sprayed coatings is critical. In this study, the deposition mechanism of a laminar plasma-sprayed Mo coating with a high melting point is analyzed, and the general deposition behaviors of metal and ceramic coatings are deduced by analogy. Further, the structures of the Mo coatings under three spraying parameters are characterized and analyzed using scanning electron microscopy. The main variables of the three spraying parameters are the spray, distance, and powder feeding rates. The tests reveal that coatings with different structures are obtained under the three spray parameters. When the spray distance is short and powder feeding rate is low, numerous protrusions arise on the surface of the coating and several pores are observed inside the coating. When the spray distance is short and powder feeding rate is high, the surface of the coating is flat and the internal bonding is good. When the spray distance is short and powder feeding rate is low, the surface of the coating is flat; however, numerous unbound interfaces appeares inside the coating. During the spraying process, the substrate temperature can reach up to 650 ℃, thus indicating that the spray distance controls the degree of in-situ heating of the substrate by the plasma jet (substrate temperature and vapor phase content in plasma jet). The presence of numerous fluffy structures on the coating surface and inside the coating indicates that the powder feeding rate controls the average heat input of the particles (temperature and evaporation of the particles). Therefore, during the spraying process, the evaporation of molybdenum oxide from the surface of the particles leads to a large composition of the gas phase in the plasma jet. Under the in situ heating effect of the plasma jet and high-temperature particles on the substrate, the molybdenum oxide vapor can adhere to and deposit on the coating surface during or after the plasma jet sweeps the substrate, thus affecting the subsequent deposition of the molybdenum particles and changing the microstructure of the coatings. The structure of the coating is related primarily to the evaporation of the molybdenum particles; a higher substrate temperature and stronger evaporation is more likely to result in the coating exhibiting a porous island structure, whereas a weaker particle evaporation is more likely to result in the coating exhibiting a layered structure. Therefore, to obtain a dense metal coating with a low oxidation by laminar plasma spraying, low powder feeding rate and short spray distance must be ensured. Our results suggest that laminar plasma spraying can yield coatings with a low oxidation content and high density by changing the spraying parameters. Thus, the study demonstrates that laminar plasma spraying has the potential to achieve atmospheric physical vapor deposition in an atmospheric environment.
  • Interface Bonding Behavior and Fractal Characteristics of Supersonic Plasma Spraying Coatings under Different Process Parameters
    WANG Rui, LI Jiaqi, WANG Haidou, MA Guozheng, LIU Jian, SUN Xiaofeng, GUO Yanbao, ZHAO Haichao
    China Surface Engineering. 2023, 36(6): 117-125. https://doi.org/10.11933/j.issn.1007-9289.20230331001
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    Supersonic plasma spraying is a process that uses an extremely high-energy-density supersonic plasma jet to heat and accelerate the spraying of materials to obtain high spray quality. The formation and presence of heterogeneous interfaces significantly affect the operational performance of refurbished parts. In this study, a Ni60A coating renowned for its robustness and wear resistance is employed as the substrate. The morphology of the highly disorganized and irregular coating / substrate bonding interface is quantitatively characterized by fractal theory. Additionally, the relationship between the interface state and the bonding strength of the supersonic plasma coating is explored. A comparative test is performed to generate coatings with distinct deposition morphologies. This is achieved by controlling the spraying distance and current to vary the melting and flight characteristics when the particles make contact with the matrix. The influences of parameters such as spraying distance, spraying current, and other process variables on the morphology and bonding strength of the coating / substrate bonding interface are studied. Fractal dimensions are calculated using the FracLac plug-in and box-counting methods. Furthermore, the corresponding relationships between the morphology of the binding interface, binding strength, and fractal dimensions are investigated. The results show differences between the pretreated substrate and the bonding interface of the coating / substrate after the spraying process, affecting the bonding strength of the coating. Correspondence related to the deposition process of the coating is found between the surface morphology and fractal dimension. The supersonic plasma-sprayed nickel-based alloy coating and substrate are predominantly mechanically bonded. The bonding occurs though different forms including mosaic, anchoring, spreading, occluding, and compound types. Among these, the mosaic and anchoring types feature barbs, enhancing the contact area between the matrix and the coating. This increased contact area serves to enhance stress distribution, making it more uniform and effectively dispersing concentrated stress. Within a certain range, as the fractal dimension increases, the morphology of the mosaic and anchoring types increases, and the contact area between the substrate and coating also expands. This expansion leads to improved distribution of concentrated stress and a more uniform stress distribution, thereby improving the bonding strength. The spraying distance has a more significant effect on the fractal dimension than the spraying current. At spraying distances of both 80 and 100 mm, an increase in the spraying current from 400 to 500 A initially leads to a reduction in the fractal dimension to a minimum of 1.115, followed by a subsequent increase. At a spraying distance of 120 mm, the flight time of the particles in the plasma flame flow increases, and the fractal dimension of the coating interface exhibits an initial increase followed by a subsequent decrease as the current increases. There is a positive correlation between the fractal dimension of the interface and the bonding strength of the coating when the tensile method is used to measure the coating bonding strength. As the fractal dimension increases within a certain range, the porosity at the coating / substrate interface decreases and the bonding strength increases. During the tensile process, cracks propagate readily at the interface where bonding strength is comparatively weaker. In addition, the presence of voids inside the coating leads to stress concentration, initiating cracks which eventually propagate, destabilize, and expands at the interface, culminating in fracture formation. Within a certain range, there is a positive correlation between the fractal dimension and bonding strength, indicating a degree of dependence between the bonding strength and the fractal dimension of the bonding-interface topography. However, the existence of a functional relationship between the fractal dimension and bonding strength, as well as the extent of the positive correlation, require further exploration. Therefore, studying the fractal characteristics of the coating / substrate bonding behavior is of great significance in the evaluation of coating quality.
  • Effect of Laser Alloying TiC Doping on the Hot Corrosion Behavior of Plasma Sprayed 8YSZ Thermal Barrier Coatings
    ZHANG Panpan, SUN Yuhai, SUN Lei, LI Bo, ZHANG Qunli, YAO Jianhua
    China Surface Engineering. 2023, 36(6): 126-134. https://doi.org/10.11933/j.issn.1007-9289.20221209001
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    Double-layered thermal barrier coatings (TBCs), which are composed of a top ceramic coating and a bonding coating, are widely used in the industry to reduce the surface working temperature of hot components. The primary materials used for the ceramic coating have been 6-8 wt.% Y2O3 partially stabilized ZrO2 (6-8YSZ), which has excellent performance of thermal insulation, high temperature resistance, low thermal conductivity, and effective thermal protection effect on metal substrates. However, traditional plasma-sprayed TBCs contain numerous pores and microcracks, and they are susceptible to corrosive salt penetration at high temperatures, leading to premature peeling failure. Doping with self-healing materials and laser post-treatment methods can effectively improve the hot corrosion resistance of TBCs. Therefore, this study aims to examine the effect of laser alloying on the hot corrosion behavior of plasma-sprayed TBCs. First, a NiCrAlY bonding coating is prepared on the surface of an Inconel 718 nickel-based superalloy via atmospheric plasma spray (APS) technology. An 8YSZ ceramic coating is then applied on the NiCrAlY bonding coating. Finally, self-healing TiC is melted on the plasma-sprayed 8YSZ coating by using a 1 kW fiber-coupled laser. The hot corrosion behaviors of the plasma-sprayed and laser-alloyed TBCs are investigated by immersion in 25% NaCl + 75% Na2SO4 mixed salt at 900 ℃ for 4 h. The weight losses of the plasma-sprayed and laser-alloyed TBCs following hot corrosion are examined. The microstructures of the plasma-sprayed and laser-alloyed TBCs before and after hot corrosion are studied using scanning electron microscopy. X-ray diffraction is used to characterize the phase composition of each coating, and energy-dispersive spectroscopy is used to analyze the elemental compositions. A high-precision electronic balance is used to measure the weights of the plasma-sprayed and laser-alloyed TBCs before and after hot corrosion, and the weight loss due to hot corrosion is determined. The results shows that the surface of the laser-alloyed TBCs is smoother. A few segmented microcracks are distributed on the laser-alloyed TBCs, which exhibits dense microstructure. The main corrosion products of the plasma-sprayed TBCs are needle-shaped Y2(SO4)3 particles and m-ZrO2. Meanwhile, only a small amount of corrosive salt penetrates the interior of the laser-alloyed TBCs, and its corrosion products are Y2(SO4)3 and a small amount of TiO2. After hot corrosion, the volume fraction of m-ZrO2 in the plasma-sprayed TBCs is 18.2%, whereas that in the laser-alloyed TBCs is only 8.1%. It is advisable to avoid the formation of m-ZrO2 during the preparation of 8YSZTBCs. If a detrimental phase transformation of t-ZrO2 to m-ZrO2 occurs, the original pores and cracks will serve as the starting points for stress relief, further exacerbating crack propagation and providing a pathway for the infiltration of corrosive salts, ultimately leading to coating delamination. On the one hand, the microstructure of the laser-alloyed TBCs is denser; this can prevent the penetration of corrosive salts into the interior of the coating. On the other hand, the surface roughness of laser-alloyed TBCs is lower, leading to lowered contact area with corrosive salts. Additionally, the self-healing material TiC undergoes an oxidation reaction during the hot corrosion test, resulting in partial self-healing of the pores and microcracks through volume expansion, further reducing the occurrence of hot corrosion reactions and the formation of harmful m-ZrO2 phases. Compared with plasma-sprayed TBCs, the laser-alloyed coatings shows an improvement in hot corrosion resistance by 55.5%.
  • Influence Mechanism of the Quasicrystalline Phase Content on the Corrosion Resistance of AlCuFe-based Coatings
    LI Wei, ZHANG Yuxin, YU Ang, JIN Guo, WANG Hao
    China Surface Engineering. 2023, 36(6): 135-144. https://doi.org/10.11933/j.issn.1007-9289.20230315002
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    Considering the development of China’s strategic goal of “ocean power”, aluminum alloy components, such as the deck and keel used on the ship surface, face the risk of corrosion failure during long-term service. Currently, the method of preparing a film or coating on the surface of aluminum alloys is typically used to isolate the corrosive medium and effectively protect the matrix material. Among these, thermal spraying technology is widely used for the preparation of anti-corrosion coatings on the surface of aluminum alloys owing to its simple operation and because it is not limited by the size of the parts. Aluminum reference crystal materials have a high strength and hardness owing to their unique structure and excellent properties such as a low friction and corrosion resistance. Thus, they have high potential as protective coating materials on the surface of a new generation of aluminum alloys; however, the poor bonding strength and corrosion resistance in the preparation of quasicrystalline (QC) coatings limit their application. Therefore, in this study, AlCuFe and AlCuFeSc QC coatings are prepared on aluminum alloy substrates using explosive spraying technology based on the long-term service conditions of aluminum alloy parts in a marine environment to improve the corrosion resistance of aluminum alloys and increase their service life. First, the microstructures and phase structures of the powder and coating are characterized using scanning electron microscopy and X-ray diffraction. Subsequently, the binding force and corrosion resistance of the coating are analyzed using a tensile testing machine and electrochemical workstation, and the influence of the QC phase content and corrosion resistance of the coating are studied. The tests reveal that the powder and coating are composed of a Ⅰ-Al65Cu20Fe15 QC phase and small amount of the β-AlFe phase. The AlCuFe and AlCuFeSc coatings prepared by explosive spraying have a high density and demonstrate good bonding with the substrate; the bonding strengths of the coatings are determined to be 51.9 MPa and 51.2 MPa, respectively. After annealing at 700 ℃, the content of the coating QC phase increases from 49%, 38% to 93.2%, 92.5%, respectively. In addition, the corrosion resistance of the QC coating before annealing is similar to that of the substrate,and the self-corrosion current density of the annealed QC coating is only one-fifth of that of the aluminum alloy substrate. A further comparison of the surface morphologies of the coatings with different QC contents reveales that as the QC content increases, the flatness of the coating surface increases and the pore distribution decreases, which can effectively prevent the entry of corrosive media. This suggests that an increase in the QC content enhances the corrosion resistance of the coating. The salt-water static hanging test simultaneously demonstrates that no apparent corrosion area formes on the annealed coating surface after 336 h, and the aluminum oxide passivating layer that forms on the surface provides a good protection for the substrate as the content of the QC phase increases. The stability of the QC phase I improves by the introduction of the Sc element into the Al-Cu-Fe icosahedron. This study provides a new idea for the corrosion protection of aluminum alloy surfaces used in ships and a basis for the preparation and application of aluminum reference crystal coatings.
  • Effect of Contact Electrification During Water Droplet Impact on the Wettability of PTFE Surfaces Modified by Radio Frequency Plasma
    LI Yupeng, MENG Xiangren, LIU Yuxia, HUO Lei, LEI Mingkai, STARINSKIY Sergey V, TEREKHOV Vladimir
    China Surface Engineering. 2023, 36(6): 145-154. https://doi.org/10.11933/j.issn.1007-9289.20230106002
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    A water droplet impacting a solid surface is a natural universal phenomenon. The impact behavior of water droplets and their effect on the superhydrophobicity of anti-wetting surfaces are important for the practical application of anti-wetting materials. In this study, radio frequency plasma modification methods including oxygen plasma treatment (OPT) and octafluorocyclobutane (C4F8) plasma polymerization deposition (PPD) are used to fabricate superhydrophobic surfaces on polytetrafluoroethylene (PTFE) substrates. Micro / nanocone arrays with different heights and spacing distances and fluorocarbon films with a low surface energy are fabricated on superhydrophobic PTFE surfaces by OPT for 3, 10, and 20 min, and PPD for 1 min. The height and distance of the cone structure on the PTFE surfaces increase as the OPT duration increases. The transition from a nanocone to a microcone array is achieved on the PTFE surfaces when the OPT duration is increases to 10 min. Complete droplet rebound behavior is achieved on superhydrophobic PTFE surfaces with a micro / nanocone array. The water contact angle increases, and the rolling angle decreases on the superhydrophobic PTFE surfaces as the OPT duration increases. Superhydrophobic surfaces with microcones having a wide spacing distance and low array density preserve a low adhesive force on water droplets under static conditions. The contact time increases and the restitution coefficient decreases when the water droplet impactes the superhydrophobic surfaces with an increased OPT duration. Superhydrophobic PTFE surfaces with nanocones having a low spacing distance and high array density exhibit a low adhesive force to the water droplet during droplet impact. Changes in the wettability and impacting behaviors of the water droplets are investigated after water droplet impact by changing the number of impacting droplets. The contact electrification on the PTFE surfaces with micro / nanocone arrays at different heights and spacing distances is analyzed by measuring the accumulated potentials of the PTFE surfaces and accumulated charge quantity of the impacting droplets. The water contact angle decreases and the rolling angle on the PTFE surfaces increases as the number of impacting droplets increases up to nine. A water contact angle of lower than 150°and rolling angle of lower than 10°are observed on the PTFE surfaces with the micro / nanocone array after the water droplet impactes with droplet number nine. The contact time of the impacting water droplet increases and its restitution coefficient decreases as the number of impacting droplets increases. The accumulated potentials of the PTFE surfaces and charge quantity of the impacting droplets increase. The attractive force of the accumulated negative charges on the water droplets resultes in the damage of superhydrophobicity on the PTFE surfaces and a change in the water droplet impacting behaviors. The PTFE surfaces fabricated by OPT for 3 min and PPD for 1 min preserve the nanocones at a low spacing distance and high array density. The PTFE surfaces with nanocones having a high array density preserve the highly accumulated negative charges after the water droplet impact under the increased number of impacting droplets. The high surface potential causes a remarkable increase in the wettability of the PTFE surfaces as well as a change in the contact time and restitution coefficient of the impacting water droplets. The accumulated negative charge and surface potential of the PTFE surfaces with the microcones having a wide spacing distance and low array density by OPT for 20 min and PPD for 1 min are low after droplet impact under the increased number of impacting droplets. The influence of the droplets on the wettability of the PTFE surfaces and the changes in the contact time and restitution coefficient of the impacting water droplets are weakened. In conclusion, this study demonstrates the effect of contact electrification on the superhydrophobic stability of surfaces with different textures under water droplet impact.
  • Effects of Laser Shock Peening on Low Temperature Carburizing of AISI9310 Gear Steel
    SONG Jingdong, HE Weifeng, LUO Sihai, CAO Zhenyang, LIANG Xiaoqing
    China Surface Engineering. 2023, 36(6): 155-162. https://doi.org/10.11933/j.issn.1007-9289.20221231004
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    AISI 9310 steel is a kind of high-strength carburized steel with good toughness. Owing to its material properties, this steel is usually used to fabricate gear parts. Gear tooth surface is prone to wear and contact fatigue damage during service processing. Therefore, to effectively resolve the resistance properties and for the synergistic strengthening of the wear and contact fatigue properties, the AISI 9310 steel sample was processed by laser shock peening (LSP) and then treated by low temperature gaseous carburization (LC). The carburized layer and cross-sectional crystallographic characteristics were imaged using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD); subsequently, the cross-sectional hardness was measured. The following results were obtained. After LC treatment, a white carburized layer, which was approximately 14-μm thick and uneven, was induced on the 9310 steel matrix surface. The maximum hardness achieved for the carburized layer of 9310 steel was about 305.67 HV with the depth of work hardening being 300 μm. The maximum hardness of the LCed sample was enhanced by 27.56% compared to the as-received sample. However, with pre-LSP treatment, the thickness of the carburized layer of 9310 steel was improved to approximately 23 μm and the maximum hardness to approximately 328.87 HV with the depth of work hardening being 700 μm. The maximum hardness of the LSP-LCed sample was enhanced by 5.46 % compared to the LCed sample. However, in comparison, the pre-LSP treatment improves the thickness of the carburized layer by 64.3%, the maximum hardness by 23.17 HV, and the depth of work hardening by 133%. The underlying reasons for these enhancements are as follows. Generally, LSP treatment induces plastic deformation and improves the proportion of low angle grain boundary (LGB); this enhances the diffusion behavior of the carbon atoms, and consequently improves the hardness of the LCed layer and the depth of work hardening. After pre-LSP treatment, the carbon diffusion behavior and hardness of LC were enhanced. Specifically, combining pre-LSP and LC processing results in cross-sectional work hardening because LC alone can hardly influence the Kernel average misorientation (KAM) and proportion of LGB of as-received 9310 steel. In other words, pre-LSP improves the KAM of the LCed sample by 15.38% (from 0.52° to 0.60°), and the depth from 0-100 μm. Moreover, pre-LSP enhances the KAM of the LCed sample by 15.79 % (to 0.66°) and the depth from 100-200 μm. Finally, the low angle grain boundary was measured. Notably, LC does not affect the proportion of the low angle grain boundary and the cross-sectional distribution for 9310 steel. On the contrary, pre-LSP processing evidently enhances the proportion of the low angle grain boundary. At the depth of 0-100 μm, pre-LSP enhances the total proportion of the LGB of the LCed sample by 13.04% (from 36.8% to 41.6%). Moreover, the total proportions of the LGB of the LCed sample were enhanced from 36.8% to 55.8% and from 38% to 46.2% for the depth ranging from 100-200 μm and 200-300 μm, respectively. Based on the above results, the main conclusions to enhance the carbon atoms diffusion behavior are as follows. Pre-LSP enhances the carbon diffusion behavior of LC by inducing plastic deformation via increased KAM and increasing the proportion of low-angle grain boundary. Consequently, the easier carbon diffusion behavior of LC could induce the thicker carburized layer, the harder work hardening level, and even improves the thickness of work hardening layer. The problem of carbon diffusion enhanced by microstructure defects induced by LSP pretreatment is preliminarily solved. This resolution would provide technical support for LSP compound strengthening to extend the service life of key components of aviation gear.
  • Effect of Thickness on Structure and Property of Black Microarc Oxidation Coatings Prepared on Different Aluminum Alloys
    LIU lei, LI Laishi, WU Yusheng, WANG Yuzheng, ZHANG Wei, BAO Zebin, QIAO Ziping
    China Surface Engineering. 2023, 36(6): 163-177. https://doi.org/10.11933/j.issn.1007-9289.20221206001
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    Aluminum alloys are widely used because of their low density and remarkable strength; however, they are prone to corrosion and wear. Dyeing microarc oxidation technology has gained widespread attention as an emerging surface treatment method. In this study, black microarc oxide coatings are prepared on the surfaces of three aluminum alloy substrates (7075, 7A04, and 2A12) using the constant-current method. The microstructure, hardness, wear resistance, and corrosion resistance of black microarc oxide coatings on the surfaces of the aluminum alloy substrates are investigated using scanning electron microscopy, X-ray diffraction, microhardness testing, friction wear testing, and electrochemical experiments. The black graphite dye improved the microscopic morphology of the film layer and reduced the micropore sizes. Microcracks are significantly reduced, resulting in increased layer thickness without a significant decrease in density. In addition, the change in the aluminum alloy matrix did not alter the structure of the black microarc oxide coatings. The black microarc oxide coatings are mainly composed of α-Al2O3 and γ-Al2O3, with the thickness increasing as γ-Al2O3 gradually transformed into α-Al2O3. The thickness of the microarc oxide coatings are significantly influenced the hardness of the coating layer. For the 7075 aluminum alloy, the average hardness of the black ceramic oxide coating with a thickness of 40 μm is approximately 645 HV, increasing to 731 and 780 HV when the thicknesses of the black microarc oxide coating reached 50 and 60 μm, respectively. The hardness values increased to 790 and 916 HV when the black coating thicknesses reached 50 and 60 μm, respectively. For the 2A12 aluminum alloy, the hardness of the 60-μm-thick black ceramic-oxide coating is significantly higher than those of the 50- and 40-μm-thick coatings, with maximum and minimum hardness values of 904 and 572 HV, respectively, for the 40-μm-thick black ceramic-oxide coating. As the coating thickness increased, the dense density of the coating increased significantly. The average porosity rate decreased to 0.8%, the proportion of α-Al2O3 in the coating increased, and the wear resistance increased significantly. Moreover, the frictional factor of the composite coating decreased from 0.6 to 0.3 with the self-lubrication effect of the black ceramic particles, and the frictional curve tended to change more smoothly. The thickness of the black microarc oxidation coating increased from 40 to 60 μm, and the wear resistance of the black oxide coating on all three aluminum alloy substrates increased significantly. The electrochemical results also show that the corrosion resistance of the three aluminum alloys with black microarc oxide coating thicknesses of 50 and 60 μm is superior. The polarization curves indicates that the corrosion current density of the 60-μm-thick black microarc composite coating is two orders of magnitude lower than that of the 40-μm-thick black microarc oxide coating, and the self-corrosion potential can be increased by 200 mV. The black microarc oxide coatings of the 7A04 and 2A12 aluminum alloys are exhibited improved corrosion resistance, close to the excellent corrosion resistance of the white microarc oxide coating of the 7075 aluminum alloy. The corrosion current density of the 60-μm-thick black microarc oxide coating is two orders of magnitude lower than that of the 40-μm-thick coating, and the self-corrosion potential can be increased by 200 mV. The passivation performance is improved, and the corrosion resistance of the coating is increased significantly.
  • Plasma Treatment on the Activation Effect of the Inner Wall of CFRP Cylindrical Parts
    WAN Junhao, DAN Min, HUANG Jiajun, TANG Guoqing, HUANG Yi, JIN Fanya
    China Surface Engineering. 2023, 36(6): 178-185. https://doi.org/10.11933/j.issn.1007-9289.20230317001
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    To improve the corrosion resistance of the inner walls of carbon fiber-reinforced plastic (CFRP) cylindrical components to special gases, metal coatings can be applied to their inner surfaces. However, carbon-fiber composite surfaces exhibit high chemical inertness, low catalytic activity, and low bonding strength with metal atoms. To address this, the surface of the CFRP matrix undergoes plasma activation prior to coating with the aim of improving the bonding strength between the matrix and the metal coating. By increasing the chemical activity of the CFRP inner-wall surface, an ideal interfacial morphology is obtained, forming the basis for creating high-performance metallized coatings on the surface. Prior studies has successfully employed Hall ion source for activating planar CFRP materials. However, this technological approach is not suitable for the plasma activation of the inner walls of cylindrical parts due to the size mismatch between the components and the Hall ion source. Plasma activation of the inner-wall surface of the cartridge is performed before the coating of the inner wall takes place, by replacing the power supply of the coating equipment with a radio frequency (RF) power supply and modifying the cartridge installation. An RF glow discharge is used to activate the CFRP inner-wall surface. A column electrode is inserted into the cylinder, with one end connected to the RF power supply and the other to the vacuum chamber. The vacuum chamber achieved a level of less than 3 mPa through the pumping system. The working gas is introduced until the desired air pressure is reached, and the RF power supply is activated to generate plasma by glow discharge, effectively activating the inner-wall surface of the cylindrical components. Post-activation, the RF power supply is replaced with a magnetron power supply. Argon gas is introduced to initiate magnetron sputtering, resulting in the application of a metal coating to the inner wall of the cylindrical component. The effects of the plasma treatment parameters on the activation process, including air pressure, discharge power, treatment time, and ion type, are investigated using contact angle tests and infrared spectral analysis. The results show that the plasma activation effect is evident on the inner-wall surface of the CFRP cylinder after the RF glow discharge plasma treatment. Consequently, the contact angle of both liquids on the CFRP substrate decreases significantly, while the surface energy and percentage of polar components increase significantly. Under identical process parameters, the activation effect initially increases and then decreases with rising air pressure, whereas it increases consistently with an increase in discharge power and treatment duration, with the most favorable results observed for oxygen plasma activation. The most significant effect is achieved when the parameters included a discharge pressure of 0.5 Pa, an RF discharge power of 500 W, a treatment duration of 60 min, and the use of oxygen plasma. Under these conditions, the contact angles for water and diiodomethane decrease substantially from 71.29°, 49.36°to 4.93° , 5.39°, respectively. The surface energy increases from 38.85 mJ·m?2 to 74.73 mJ·m?2 . The inactive bonds of the plasma-treated carbon fiber composites, including C-H and C≡C, are broken, the number of aldehyde and carboxyl groups with C=O increases, and the wettability is greatly improved. A comparison of the surface microscopic morphology before and after activation is performed using scanning electron microscopy (SEM). The impurity particles that have adhered to the carbon fiber surface and between the carbon fibers are completely removed, establishing favorable interfacial conditions, and thereby enhancing the bonding strength of the metal coating. The film-based bonding force between the activated CFRP substrate and the metal film increases from less than 0.1 MPa to 0.49 MPa.
  • Effects of Process Parameters on Formability and Microhardness in Plasma Arc Additive Manufacturing of Ti-6Al-4V Alloy
    WU Xiangju, GUO Dengji, LIN Jianjun, WANG Xujin
    China Surface Engineering. 2023, 36(6): 186-194. https://doi.org/10.11933/j.issn.1007-9289.20221231003
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    Titanium alloys offer the advantages of low density, high specific strength, and good corrosion resistance, making them widely used in aerospace, ocean engineering, military medicine, and other fields. However, the high cost, low thermal conductivity, and low elastic modulus of titanium alloys result in poor quality, low efficiency, and high cost of traditional processing, which seriously restricts their application and development. Plasma-arc additive manufacturing technology provides a cost-effective solution with high deposition and material utilization rates, which is crucial for the production of large and complex parts. However, owing to the high energy density of the plasma arc and the low thermal conductivity of titanium alloys, the formability of titanium alloy-deposited parts is compromised, resulting in the growth of coarse columnar grains. In addition, the characteristics of many parameters and the difficulty in controlling plasma-arc additive manufacturing limit the rapid formulation of additive manufacturing process parameters that meet mechanical standards. The influence of plasma arc additive manufacturing of Ti-6Al-4V alloy process parameters on formability, microstructure, and microhardness was investigated by orthogonal experiments, metallographic analysis, and characterization of the relationship between the microstructure and mechanical properties. The experiment was conducted in an inert argon gas environment using a plasma arc additive manufacturing system, which consists of a Kuka robot, main power supply, plasma power supply, and wire feeding system. The main process parameters included deposition speed, wire feeding speed, pulse base current, pulse peak current, pulse frequency, and duty cycle. The three main evaluation parameters of formability were evaluated using the melting width, reinforcement, and aspect ratio of the deposited layer as indicators. In addition, the average grain size and microhardness were used as indices to evaluate the effect of microstructure on mechanical properties. The results indicate that the degree of influence of the plasma arc process parameters on the formability is as follows: base current (Ib) ＞ peak current (Ip) ＞ duty cycle (Idcy) ＞ wire feed speed (TWFS) ＞ deposition speed (Ts) ＞ pulse frequency (FP). Ib has the greatest influence on the deposited width, deposited height, and formability of a single layer, with a more pronounced effect when Ib is 50%-70% Ip. The deposition speed and duty cycle exhibited the following relationships: the faster the deposition speed, the smaller the width and height of the deposition layer. The effect of the duty cycle on the width and formability of the single-pass deposited layers was positively correlated. The effect of process parameters on the average grain size was Ts＞FP＞TWFS＞Ib＞Ip＞Idcy, with larger deposition speeds resulting in smaller grain sizes. Pulse frequency was the second most influential parameter on average grain size, demonstrating that pulse disturbance aids in grain refinement. Moreover, the degree of influence of the process parameters on microhardness was Ts ＞ Idcy ＞ TWFS ＞ Ib ＞ FP ＞ Ip. The deposition speed had the greatest influence on average grain size and microhardness, with Ip having limited influence on these two aspects. Although the influence of the deposition speed on microhardness was the greatest, the degree of influence was only 4%, indicating that the influence of the selected plasma-arc process parameters on microhardness was not significant. These findings provide a theoretical basis for plasma arc additive manufacturing and additive remanufacturing processes and offer technical support for the rapid repair of damaged parts in applications such as field mining machinery, marine ships, engineering equipment platforms, and petroleum and chemical equipment.
  • Heat and Mass Transfer Behavior of Pulsed Arc Plasma of Duplex Stainless Steel
    ZHANG Zhiqiang, GOU Qingze, LIU Bo, ZHANG Tiangang, LU Xuecheng, XU Lianyong
    China Surface Engineering. 2023, 36(6): 195-204. https://doi.org/10.11933/j.issn.1007-9289.20221224001
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    Duplex stainless steels contain nearly equal proportions of ferrite (δ) and austenite (γ). Its unique organizational characteristics enable it to combine the good plastic toughness and uniform corrosion resistance of austenitic stainless steel with the high strength and stress corrosion resistance of ferritic stainless steel. Consequently, it is widely used in marine engineering, petrochemical, and other important energy fields. Oil and gas pipelines are the most reliable and cost-effective means for safely transporting energy over long distances. To extend the service life of duplex stainless-steel oil and gas pipelines, arc additive remanufacturing has been developed based on overlay welding technology with special advantages such as high efficiency and low cost, and thus has broader application prospects. Hence, it is necessary to have the capability to effectively control the heat and mass transfer characteristics of the arc plasma and to elucidate the interaction mechanisms between the arc plasma and droplets. These factors have significant effects on the stability, forming quality, and microstructure of the arc additive repair process of duplex stainless steel. However, the high enthalpy, strong arc light characteristics, and non-equilibrium physicochemical reactions of arc plasma impede the quantitative analysis of the heat and mass transfer mechanisms of arc plasma via in situ testing methods. A multiphysics coupling simulation model of arc-droplet integration was established in this study. The model is based on the theories of electromagnetism, fluid dynamics, thermodynamics, and in situ experiments using a high-speed camera and electrical signal acquisition, while considering the behavior of metal vapor in plasma. The Eulerian multiphase flow model was used to improve the computational convergence of the two-phase mixing region of the arc plasma and metallic droplets. The physical fields of the gas and metal were solved separately using two sets of governing equations, and a species transport model was used to calculate the distribution of the metal vapor. This study thus sought to investigate the heat and mass transfer behavior of a pulsed arc plasma with duplex stainless steel and reveal the interaction mechanism between the arc plasma and droplets. A cycle in the pulsed arc additive manufacturing process of duplex stainless steel was selected to study the heat and mass transfer behavior. The temperature field, velocity field, metal vapor behavior, and experimental results at six characteristic moments of the arc plasma additive manufacturing process were analyzed separately. The results indicate that the peak arc plasma temperature was distributed on both sides of the droplet axis and was positively correlated with the current. The temperature distribution on the substrate surface was not uniform owing to the asymmetric effect of the arc plasma during necking to the transition stage of the droplet. In addition, the results of the flow-field distribution of the arc plasma were similar to those of the temperature field. However, the velocity peaks at different instances were not only related to the corresponding values of current but also to the transition states of the droplets. With the droplet transition, both the high-temperature and high-speed regions of the arc plasma were compressed toward the substrate. Before the droplet necking, the iron vapor was gradually compressed toward the axis as the current increased, and the mass fraction below the droplet could reach 100%. Following the droplet necking, the high concentration of iron vapor above and below the droplet increased the electrical conductivity of the plasma, which in turn accelerated the droplet transition. On the substrate surface axis, the mass fractions of iron vapor at different instances of times were between 20% and 60%. The simulation results of the pulsed arc plasma with duplex stainless steel droplets were generally consistent with the experimental results in terms of heat and mass transfer behavior, although complex electromagnetic thermal effects occurred between the arc plasma and the molten metal.
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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