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作者简介:

王浩宇,男,1996年出生,硕士研究生。主要研究方向为摩擦学和材料服役行为。E-mail:haoyu_t@163.com

通讯作者:

蔡振兵,男,1981年出生,博士,研究员,博士研究生导师。主要研究方向为摩擦学、表面工程和材料服役行为。E-mail:czb-jiaoda@126.com

中图分类号:TG156;TB114

DOI:10.11933/j.issn.1007−9289.20221017001

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目录contents

    摘要

    CoCrWSi 涂层由于其优异的耐高温氧化性能,有望成为汽轮机零部件的防护涂层之一。但是关于该涂层在汽轮机真实服役环境中的高温冲击磨粒磨损行为却鲜有报道。以汽轮机阀门部件材料 SA-182F92 为基体,制备 CoCrWSi 防护涂层。利用自研的高温沙粒冲击试验机,研究 CoCrWSi 涂层在沙蚀环境中的高温冲击磨损行为,通过冲击动力学响应和磨痕形貌来评价该涂层的耐冲击磨粒磨损性能。结果表明:CoCrWSi 涂层具有耐高温冲击磨粒磨损性能,具体表现为相同冲击次数下,CoCrWSi 涂层样品的磨损面积、磨损体积和最大磨痕深度比基体样品小数倍,CoCrWSi 涂层样品的能量吸收量和吸收率均小于基体样品。在高温沙粒环境下,冲击的过程中会有大量的沙粒嵌入磨痕表面,沙粒具有的不规则棱角会切削磨痕表面,进而磨痕表面可以观察到大量犁沟。在高温沙粒环境下,基体与涂层的磨损机理为塑性变形和磨粒磨损。不同的是,软化的基体在冲击区域边缘有明显的隆起,发生更严重的塑性变形;而涂层在冲击过程中虽没有完全剥落,但涂层内部萌生了微裂纹,磨痕表面也有部分涂层剥落。研究结果是在模拟汽轮机真实服役状况下得出的,试验参数如加热温度、沙粒(杂质)、摩擦副材料等均依据汽轮机涂层应用的实际工况进行选择,对 CoCrWSi 涂层在汽轮机零部件上的应用有一定指导意义。

    Abstract

    With the development of steam turbines, the key parts of the steam turbine blades will suffer from mechanical damage, including friction wear and solid erosion, owing to the influence of high-temperature fluid and impurities. Therefore, materials used for the high-temperature components of steam turbines are required to have excellent thermal strength, oxidation resistance, and other comprehensive properties. Usually, high-temperature protective coatings are sprayed on these key components via various processes to improve the physical and chemical properties of the substrate at high temperatures. CoCrWSi coating is expected to become a protective coating for steam turbine parts owing to its excellent high-temperature oxidation resistance. However, there are few reports on the high temperature impact abrasive wear behavior of this coating. The CoCrWSi protective coating was prepared based on the steam turbine valve component material SA-182F92. The high temperature impact wear behavior of CoCrWSi coating in a sand erosion environment was studied by using self-developed high-temperature sand impact tester. The impact wear resistance of the coating was evaluated by dynamic response and impact wear scar morphology. The dynamic response data of the impact process, including the impact velocity curve and impact force curve, were obtained through the corresponding sensors. On the macro level, an optical microscope was used to observe the wear condition of the sample surface. On the micro level, SEM was used to observe the micro morphology of the surface and section of the wear scar, EDS was used to analyze the element distribution of the surface and section of the wear scar, and white light interferometer was used to measure the profile of the wear scar to obtain the maximum wear depth, area, and volume. The results show that the impact return velocity and impact force of the coating and substrate decrease with the increase in impact cycles, which is caused by the increase in contact area and decrease in stress during the impact. At the same impact cycles, the impact return velocity and impact force of the coating are greater than those of the substrate. The CoCrWSi coating has high-temperature impact abrasive wear resistance, which is specifically manifested in that under the same impact cycles. The wear area, wear volume, and wear scar depth of the coating samples are several times smaller than those of substrate samples, and the energy absorption and absorption rate of the coating samples are less than those of the substrate samples. In a high-temperature sand environment, a large number of sand particles will be embedded into the wear scar surface during the impact, and the irregular edges and corners of sand particles will aggravate this cutting effect. Then, a large number of grooves can be observed on the wear scar surface. In the high temperature sand environment, the wear mechanism of the substrate and coating is plastic deformation and abrasive wear. The difference is that the softened substrate has an obvious bulge at the edge of the impact area, resulting in serious plastic deformation. Although the coating did not completely peel off during the impact, micro cracks sprouted in the coating, and some coatings peeled off on the surface of the wear scar. The result was obtained under the condition of simulating the actual working condition of the steam turbine. The test parameters, such as heating temperature, sand material (impurities), and friction pair, were selected according to the actual working condition of the application background of the steam turbine coating, which has certain guiding significance for the application of CoCrWSi coating on steam turbine parts.

  • 0 前言

  • 随着高参数汽轮机的发展,由于受到高温、高温流体、应力以及杂质等的影响,汽轮机叶片、阀门等关键部位会发生腐蚀或机械损伤(包括磨损,固粒侵蚀等)[1-3],故应用于汽轮机高温部件的金属材料要求具有优异的热强性能、抗高温腐蚀、抗氧化等综合性能[4]。通常在这些关键部件上采用各种工艺(包括火焰喷涂、激光熔覆、化学气相沉积等) 喷涂高温高性能涂层,以改善基体在高温下的理化性性能,同时起到防腐、耐磨的作用[5]

  • 关于新型钴基耐高温高性能涂层的制备及性能已经有了一些研究,SIDHU 等[6-7]在不同基材上采用 HVOF 技术制备了 Stellite-6 高温合金涂层,揭示了涂层提高热腐蚀性能的原因并探讨了其高温氧化行为,发现喷涂工艺、基体材料、熔融盐配比等其中任何条件改变对结果都会有明显的影响。路世盛等[8]采用激光熔覆技术,通过添加 Ni 基过渡层在球墨铸铁表面制备 Co 基涂层,采用高温摩擦磨损试验机测试不同温度下 Co 基涂层与球墨铸铁的摩擦磨损性能,结果表明制备的 Ni-Co 复合涂层能有效提高球墨铸铁表面的高温摩擦学性能和耐腐蚀性能。YANG 等[9]制备不同 W 含量的钴基合金涂层,然后测试涂层在高温下的硬度和抗磨性能,结果表明,钴基合金镀层的摩擦因数随 W 含量的增加而减小,磨损量随 W 含量增加而增加。在高温油润滑条件下,当 W 含量为 12%时,钴基合金涂层的摩擦因数最低。王晓明等[10]基于相平衡法制备 Hf 掺杂 NiCoCrAlY 高温涂层,利用 X 射线衍射仪(XRD) 对该涂层进行物相检测,利用扫描电子显微镜 (SEM)和电子背散射衍射(EBSD)研究合金的微观结构和晶粒取向,通过显微硬度、纳米压痕和压缩试验来表征其力学性能,结果表明,掺杂改性后的 NiCoCrAlYNbMoHfTa 涂层显微硬度显著提高,表现出更加优异的磨损防护性能。CoCrW 涂层具有优异的耐高温磨损、高温腐蚀和高温氧化性能,应用于各种承受高温、磨损、腐蚀、氧化等恶劣工况条件的零部件表面强化,如高温下工作的涡轮机密封件和密封阀、内燃机排气阀及阀座上[11]。而在 CoCrW 合金中加入 Si 阻止了含 Co 化物的形成,并稳定了以 Cr2O3 为主的均匀氧化膜,形成的 Cr2O3和 SiO2 降低了高温下的氧化速率[12]。曹晓英等[13] 采用超音速火焰喷涂工艺制备 CoCrWSi 系高温抗氧化涂层,并对喷涂态涂层金相组织、显微硬度、结合强度、开裂韧性等性能进行分析。涂层经静态氧化和热震试验后,CoCrWSi 系涂层内部无裂纹,热稳定性好,氧化过程为未熔颗粒边界包覆氧化,涂层表现出优异的高温稳定性。本文以汽轮机阀门部件材料 SA-182F92 为基体,制备一定厚度的 CoCrWSi 涂层,利用自研的高温沙粒冲击试验机,研究 CoCrWSi 涂层在沙蚀环境的高温冲击磨损行为,通过冲击动力学响应和冲击磨痕形貌来评价该涂层的冲击磨粒磨损性能。

  • 1 材料与方法

  • 1.1 试验材料及制备

  • 试样由东方电气集团东方汽轮机有限公司提供。采用超音速火焰喷涂工艺方法在 SA-182F92 马氏体型耐热钢(以下简称 F92 钢,试样规格为 20 mm×10 mm×10 mm)基体表面制备 CoCrWSi 系高温抗氧化涂层,喷涂粉末为粒度为 15~45 μm 的 Co28Cr4WSi 粉末。基体的主要化学成分(wt.%)为 0.1%C、0.54%Mn、9.34%Cr、0.47%Mo、1.78%W、 0.31%Si。喷涂前对基体进行除油处理,然后进行喷砂活化处理,采用进口超音速火焰喷涂系统制备涂层。为降低涂层表面粗糙度,制备后的涂层经过一定磨削处理。磨削后测得涂层的面粗糙度(Sa)为 1.38 μm。试验前基体和涂层的表截面形貌如图1 所示,由图1a 可以看出基体和涂层的表面粗糙度相差不大,从图1b 中可以看出涂层膜基界面间结合性良好,无明显缺陷。试验前基体与涂层在常温和 630℃下的硬度测试结果如表1 所示,由此可知高温下基体和涂层都存在材料软件现象(硬度下降)。

  • 图1 F92 基体和 CoCrWSi 涂层的表截面形貌

  • Fig.1 Surface and cross-section morphology of F92 substrate and CoCrWSi coating

  • 表1 基体与涂层的在常温和 630℃下的硬度

  • Table1 Hardness of substrate and coating at RT and 630℃

  • 1.2 试验装置及操作流程

  • 如图2 所示,在动能控制冲击磨损试验机的基础上添加沙粒装置和高温马弗炉装置,用来实现高温沙粒环境中的冲击运动。试验机具体运行流程为:在电源的驱动下,音圈电机做线性往复运动。撞击之前,音圈电机通过弹簧拉杆使动能块向右移动,当速度达到最大速度(冲击速度)时,动能块与弹簧拉杆分离,并在撞击试样后以一定的速度(回弹速度)回弹;撞击之后,音圈电机通过弹簧拉杆使动能块向左移动,直到它返回到初始位置,从而完成整个循环的冲击动作。在开始试验之前,先接通高温马弗炉电源,加到预定温度后,打开并调节流量调节阀,使得漏斗中的沙粒以恒定的流速流出,由于沙粒进入后带走一部分热量,这时需要边加沙粒边等待温度达到预定值,待达到预定温度之后在启动音圈电机开始试验。在试验过程中,导轨上的动能块能在弹簧的推拉下自由移动,位移和力传感器分别检测动能块撞击前后冲头的速度和试件所受的冲击力。冲击过程中的能量损失 ΔEi 可以通过计算冲击前后动能块动能的差值来获得。

  • 图2 高温沙粒冲击磨损试验机示意图

  • Fig.2 Schematic of impact wear test machine

  • 1.3 试验方案

  • 本次试验的试样为 F92 钢和以 F92 钢为基体的 CoCrWSi 涂层,试验变量为冲击次数,设定为 5× 103、104、2×104、5×104。冲击头采用直径为 4.76 mm 的氮化硅陶瓷球(显微硬度为 2 273 HV),沙粒采用的天然石英砂(ωSiO2>98%),然后经人工破碎和筛选得到粒径为 380 μm(约 40 目)的颗粒(考虑汽轮机固粒侵蚀中杂质可能为锅炉中带入的 SiO2 杂质),试验温度为 630℃(汽轮机部件服役的环境温度),冲击动能块质量为 700 g,初始冲击速度为 150 mm / s。试验结束后需要观察冲击磨损产生的凹坑大小和形状,判断有无涂层剥落。

  • 1.4 分析与测试

  • 试验开始前,采用维氏硬度计(型号为 AFFRIDAKO 300)测量基体与涂层在常温和 630℃下的硬度。试验结束后,采用光学显微镜(型号为 VHX-1000C)观察样品表面磨损情况,采用 JSM6610LV 型扫描电子显微镜观察磨痕表截面的微观形貌,采用 EDS 分析仪(型号为 OXFROD X-Max 80)观察磨痕表截面的元素分布,利用白光干涉仪(型号为 Contour GT)测量磨痕的轮廓,得到磨损最大深度、面积和体积。

  • 2 结果与讨论

  • 2.1 冲击动力学响应

  • 一般而言,运动总是伴随着能量转换。除了动能和变形,在冲击过程中还存在其他形式的能量转换,如摩擦热、裂纹扩展。普遍认为,在低速撞击条件下,热能的一部分可以忽略不计。本研究假设冲击能量仅转化为变形能、动能和材料去除过程中消耗的能量。因此,在碰撞过程中,冲击能量仅转化为变形能、动能和材料去除过程中消耗的能量,能量耗散量包含试样塑性变形和材料去除过程中消耗的能量,所以能量耗散量等于撞击前后动能的差值[14-15]。当温度、动能块质量和冲击速度等其他变量一定时,基体与涂层在不同冲击次数下的冲击动力学响应曲线如图3 所示。采集的冲击速度和冲击力均为不同冲击次数下最后一次冲击时记录的数值。如图3a 和 3b 所示,动能块撞击试样前的初始速度均为 150 mm / s,随着冲击次数的增大,涂层与基体的冲击返回速度都有减小的趋势,这使得速度变化值 δ |V|变大,表明冲击次数增大后,冲击前后能量的耗散量增大,有更多的能量被冲击试样吸收(图4)[14]。当冲击次数 N<104 时,基体的回弹速度明显大于涂层,当 N>104 时,两者的回弹速度趋于相等。这表明试验初期,在撞击基体时能量的耗散量比较大,即每次撞击时基体能吸收更多的能量(见图5)。在持续的冲击下,可能由于磨损区域产生了加工硬化,表面强度增加,故冲击次数大于一定值时,基体与涂层每次吸收的能量相近,故两者的回弹速度趋于相等。图3c 和 3d 分别为不同冲击次数下基体与涂层的冲击力-时间响应曲线,从中可以获取最大冲击力和接触时间。由于基体与涂层的材料力学性能不同,所以两者产生了不同的冲击力波形和接触时间[15]。在此试验条件下,基体与涂层的接触时间分别为 4 ms 和 3.5 ms。如图3c 和 3d 所示,随着冲击次数的增加,基体与涂层的冲击力反而有减小的趋势,这是因为冲击初期,冲击头的接触面积较小,则应力集中,故采集到的冲击力较大,随着冲击次数增加,一方面试样由于塑性变形使得接触面积变大,应力减小导致冲击力减小;另一方面,随着冲击次数的增加,在高温环境下材料软化后,沙粒嵌在了磨损区域表面,沙粒本身具有较多的棱角,能切削和挤压磨损区域表面,由于切削力的存在导致应力不再集中,使得采集到的冲击力减小[16]

  • 图3 F92 基体与 CoCrWSi 涂层在不同冲击次数下的动力学响应曲线

  • Fig.3 Dynamic response curve of F92 substrate and CoCrWSi coating under different cycles

  • 图4 显示了冲击过程中能量的变化情况,可以看出冲击能量随着冲击次数的增大而增大,涂层的能量损失 ΔEi 从冲击次数为 5×103 时的 3.54 mJ 增长到 5.46 mJ,增长了 54.24%。这部分能量主要用于试样的塑性变形和材料的去除。图5 是不同冲击次数下基体与涂层的冲击能量吸收情况。随着冲击次数的增加,无论是基体还是涂层,能量吸收量和吸收率均有增长的趋势。

  • 图4 F92 基体与 CoCrWSi 涂层在不同冲击次数下的冲击能量变化曲线

  • Fig.4 Impact energy curve of F92 substrate and CoCrWSi coating under different cycles

  • 图5 F92 基体与 CoCrWSi 涂层在不同冲击次数下的冲击能量吸收情况

  • Fig.5 Impact energy absorption of F92 substrate and CoCrWSi coating under different cycles

  • 2.2 磨痕形貌分析

  • 图6 显示了基体和涂层在不同冲击次数下的磨痕形貌。磨损区域可以分为冲击磨损区域和冲蚀磨损区域,冲蚀区域位于冲击磨损区域周围。冲击磨损区域是撞头能直接接触的区域,试样的塑性变形主要集中在这个区域,冲蚀区域是具有一定动能的沙粒冲击试样形成的。如图6 所示,基体和涂层的磨损区域随着冲击次数的增加而增大。在相同冲击次数下,基体的磨损情况明显比涂层的严重。这表明 CoCrWSi 涂层具有抗高温冲击磨粒磨损的性能。

  • 图6 F92 基体和 CoCrWSi 涂层在不同冲击次数下的磨痕形貌

  • Fig.6 Wear scars of F92 substrate and CoCrWSi coating under different cycles

  • 图7 为不同冲击次数下基体和涂层的三维磨痕形貌图,可以看出冲击后基体的表面粗糙度明显比涂层的大,而且基体在冲击区域边缘有明显的隆起,在冲击区域外有明显的点蚀现象。而涂层虽然也有明显的塑性变形,但是无隆起现象。这是因为基体硬度比涂层低,在高温下基体更容易软化,所以在冲击过程中产生更严重的塑性变形,继而有隆起现象[17]。这也体现了 CoCrWSi 涂层具有抗高温冲击磨粒磨损的性能。

  • 图7 F92 基体和 CoCrWSi 涂层在不同冲击次数下的磨痕三维形貌

  • Fig.7 Three-dimensional morphology of wear scars of F92 substrate and CoCrWSi coating under different cycles

  • 图8 为 F92 基体与 CoCrWSi 涂层在不同冲击次数下的磨痕截面轮廓。从中可以看出,基体和涂层有明显的塑性变形现象,并且两者的磨痕深度随着冲击次数的增大而增大。相同冲击次数下,涂层由于冲击所形成的最大磨痕深度明显小于基体,但当冲击次数足够大时,涂层与基体的最大磨痕深度差距并不大,这可能因为在冲击过程,磨损区域出现加工硬化现象,使磨痕表面硬度提高,从而减缓了磨损[1518]

  • 图8 F92 基体与 CoCrWSi 涂层在不同冲击次数下的磨痕截面轮廓

  • Fig.8 Cross-sectional profile of wear scars of F92 substrate and CoCrWSi coating under different cycles

  • 图9 分别是基体和涂层磨损面积和体积的统计图,从中看出随着冲击次数的增大,基体和涂层的磨损面积和磨损体积都增加,冲击次数从 5×103 增长到 5×104 时,基体的冲击磨损面积由 1.81×106 μm 2 增长到 3.88×106 μm 2,增长了约 2.14 倍,冲击磨损体积更是由 10.68×106 μm 3 增长到 60.79×106 μm 3,增长了约 5.69 倍。这是因为氮化硅陶瓷球的直径是恒定的,当沙粒嵌在磨损表面时,沙粒能够接触到距离磨痕中心点更远的位置,所以当冲击次数增加时,磨损面积和磨损深度也就会随之增加[16]。相同的冲击次数下,基体的磨损面积、磨损体积和最大磨痕深度明显比涂层大。当冲击次数 N=5×104 时,基体的冲击磨损面积为 3.88×106 μm 2,是涂层 2.41×106 μm 2 的 1.61 倍,冲击磨损体积为 60.79×106 μm 3 ,是涂层的19.56×106 μm 3 的3.11 倍。这也体现CoCrWSi 涂层具有抗高温冲击磨粒磨损的性能。

  • 图9 F92 基体与 CoCrWSi 涂层在不同冲击次数下的磨损面积和体积

  • Fig.9 Wear area and volume of F92 substrate and CoCrWSi coating under different cycles

  • 图10 为冲击次数 N=5×104 时,F92 基体与 CoCrWSi 涂层表面的 SEM 形貌和 EDS 图,表2 为磨痕表面不同位置处的化学成分。从图10a 和图10b 中可以明显看到由冲击磨损造成的冲击区域和沙粒影响的沙蚀区域。在基体与涂层的磨损区域(表2 中的 ACDF 点)均检测出大量的 Si 元素,表明在冲击的过程中,有大量的沙粒嵌入磨痕表面,而且在磨痕表面观察到大量犁沟,说明冲击过程中有切削作用,而沙粒具有的不规则棱角也会加剧这种切削作用[19],所以表明在高温沙粒环境下,基体与涂层的磨损机理为塑性变形和磨粒磨损[20]。图10b 中,除了检测出大量的 Si 元素,还检测出 Co 和 W 元素,但并且没有检测出 Fe 和 Mn 元素,表明 CoCrWSi 涂层在冲击的过程中并没有完全剥落。这也体现了 CoCrWSi 涂层具有抗高温冲击磨粒磨损的性能。

  • 图10 F92 基体与 CoCrWSi 涂层磨痕表面的 SEM 形貌和 EDS 结果(N=5×104

  • Fig.10 SEM morphology and EDS result of surface of F92 substrate and CoCrWSi coating (N=5×104)

  • 表2 磨痕表面不同位置的化学成分(质量分数)

  • Table2 Chemical composition at different positions on the surface of the wear scar (wt. %) .

  • 图11 为冲击次数 N=5×104 时,CoCrWSi 涂层截面的 SEM 形貌和 EDS 结果,表3 为磨痕截面不同位置的化学元素成分。从图11a 的电镜图中可以明显看出基体与涂层均发生明显的塑性变形,且涂层内部有微裂纹产生,磨痕表面也有部分涂层剥落,但膜基界面间结合紧密,无明显间隙。从图11b 的 EDS 结果中可以看出涂层并没有完全剥落。这说明在冲击过程中,当塑性变形到一定程度时,涂层中会产生额外的内应力,诱发微裂纹的萌生,并且位于磨痕表面的涂层在沙粒的切削作用下剥落[21]

  • 图11 CoCrWSi 涂层磨痕截面的 SEM 形貌和 EDS 结果(N=5×104

  • Fig.11 SEM morphology and EDS result of cross section of CoCrWSi coating (N=5×104)

  • 表3 磨痕截面不同位置的化学成分(质量分数)

  • Table3 Chemical composition at different positions of cross section of wear scar (wt. %) .

  • 图12 为 F92 基体和 CoCrWSi 涂层的磨损机理图。基体与涂层的磨损机理均为塑性变形和磨粒磨损。在冲击的过程中,有大量的沙粒嵌入磨痕表面,沙粒具有的不规则棱角会对磨痕表面产生切削作用。不同的是,高温下软化的基体在冲击区域边缘有明显的隆起现象,发生更严重的塑性变形;而涂层在冲击过程中虽然没有完全剥落,但涂层内部有微裂纹萌生,磨痕表面也有部分涂层剥落。

  • 图12 F92 基体与 CoCrWSi 涂层的磨损机理图

  • Fig.12 Wear mechanism diagram of F92 substrate and CoCrWSi coating

  • 3 结论

  • 系统研究 CoCrWSi 涂层在沙蚀环境中的高温冲击磨损行为,通过冲击动力学响应和磨痕形貌分析来评价该涂层的耐冲击磨粒磨损性能,得到如下结论:

  • (1)CoCrWSi 涂层具有耐高温冲击磨粒磨损性能,具体表现为相同冲击次数下,CoCrWSi 涂层样品磨损面积、磨损体积和磨痕深度比基体样品小数倍,CoCrWSi 涂层样品的能量吸收量和吸收率均小于基体样品。

  • (2)在高温沙粒环境下,冲击过程中会有大量沙粒嵌入磨痕表面,沙粒具有的不规则棱角会切削磨痕表面。基体与涂层的磨损机理为塑性变形和磨粒磨损。不同的是,软化的基体在冲击区域边缘有明显的隆起现象,发生更严重的塑性变形;而涂层在冲击过程中虽然没有完全剥落,但涂层内部有微裂纹萌生,磨痕表面也有部分涂层剥落。

  • (3)研究结果是在模拟汽轮机真实服役状况下得出的,相关试验参数如加热温度、沙粒(杂质)、摩擦副材料等均依据汽轮机涂层应用的实际工况进行选择,对 CoCrWSi 涂层在汽轮机零部件上的应用有一定指导意义。后续将继续探究该涂层在其他高温异物流体环境中的摩擦磨损行为。

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