超音速火焰喷涂Ni60-40WC涂层高频感应重熔处理及其水润滑摩擦磨损性能

High Frequency Induction Remelting of Ni60-40WC Coating Prepared by High Velocity Oxy-Fuel Spraying and Its Water-Lubricated Wear Resistance

为进一步提高重熔涂层的性能,采用超音速火焰喷涂与高频感应重熔复合工艺在17-4PH马氏体沉淀硬化不锈钢上制备了孔隙率低且含WC硬质相的Ni60-40WC涂层,利用金相显微镜、扫描电子显微镜、X射线衍射仪、电子探针和维氏显微硬度计表征了涂层显微组织及特性,采用摩擦磨损试验测试了涂层的水润滑磨损性能。结果表明:感应重熔促使涂层组织均匀化,涂层与基体界面元素互扩散形成了连续的界面冶金结合;随着感应电流增大,重熔程度增强,界面扩散区增厚,从860 A的厚约5μm增加至920 A的10μm左右;界面区不锈钢基体侧产生了30～50μm厚的热影响区,由马氏体转变为铁素体;感应重熔对涂层显微硬度改善不明显,可主要归结为Cr_7C_3和Cr_(23)C_6等硬质相弥散析出作用,但重熔涂层比磨损率显著降低,仅约为喷涂态涂层的1/3;喷涂态涂层磨痕呈现出典型WC颗粒脱落及破碎导致的犁沟形貌特征的磨粒磨损,感应重熔涂层磨痕光滑平整,为轻度磨粒磨损,摩擦系数降低了1个数量级;涂层扁平粒子之间、硬质相与Ni60合金组织之间的元素扩散,有效增强了涂层内聚力,抑制了摩擦接触中WC颗粒的脱落,改变了摩擦磨损机制。

Ni60-40 WC coatings were prepared by high velocity oxy-fuel spraying( HVOF) spraying process on 17-4PH martensitic precipitation hardening stainless steel,combined with high frequency induction remelting post-treatment. The microstructures and microhardness of the coatings were characterized by optical metallographic microscope( OM),scanning electron microscope( SEM),X-ray diffractometer( XRD),electron probe microanalyzer( EPMA) and Vickers indention tester. The wear test was performed on a block-on-ring tribometer under deionized water lubrication. Results showed that the induction remelting led to the microstructure homogenizing of the coating and coating-substrate interface metallurgical bonding due to element diffusion in the interface as well as within coatings. The thickness of the diffusion zone at the coating-substrate interface was about 5 μm at the remelting current of 860 A,and increased to about 10 μm at a higher current of 920 A due to the enhanced element diffusion at expected higher temperature and longer melting state. The induction remelting process generated a30-50 μm depth heat-affected zone on the substrate surface,in which microstructure of 17-PH steel was transformed into ferrite from original martensite. Besides,the induction remelting had a minor effect on microhardness of the coatings,which was attributed to the precipitation of some dispersed Cr7C3 and Cr23C6 hard phases. And meantime the specific wear rate of induction remelted coatings greatly reduced,about 1/3of that of the as-sprayed coatings. The as-sprayed coatings presented a typical abrasive wear behavior with WC hard phase pull-out and fragmentation,leaving continuous ploughing scratches on the wear track,whereas the remelted coatings had a much smoother wear track of mild abrasive wear and one order lower sliding friction coefficient under water lubrication. The element diffusion at the interfaces between the deformed splats and between WC hard phases and Ni60 matrix,effectively improved the coating cohesion,leading to the transition of wear mechanism by suppressing the WC particle pull-out in the frictional contact.