超音速火焰喷涂WC-17Co涂层氧化行为研究

Oxidation Behavior of WC-17Co Coatings Prepared by HVOF

WC-17Co涂层由于其优异的耐磨性能,被广泛用于工件的摩擦防护,但其在较高温度下服役存在过早氧化失效的问题。采用超音速火焰喷涂在45#钢表面制备WC-17Co致密涂层,将其置于400、500、600和700℃的温度下进行恒温热暴露,研究超温服役氧化行为对其结构及性能的影响。采用X射线衍射、扫描电镜和显微硬度计等手段表征喷涂态及不同氧化温度下涂层的物相和微观结构的演变,对其物相和性能变化进行讨论。研究结果表明:喷涂过程中WC的分解及过冷使喷涂态WC-17Co涂层形成少量的Co基非晶;400℃热暴露后,涂层物相和结构无明显变化,当热暴露温度提高到500℃以上时,涂层表面CoWO_(4)、WO_(3)和Co_(3)O_(4)等氧化相开始生成,在700℃氧化处理2 h后,氧化物生长层增厚到10μm;氧化促使涂层内部的Co元素向表面扩散,导致涂层内部WC硬质相的浓度提高,故内部涂层的显微硬度也大幅提高,在距离表面50μm深度涂层显微硬度增加到1400 HV_(0.3)以上;表层显微硬度升高则主要是由于氧化相的生成。高温氧化后,由于Co粘结相的减少使涂层断裂韧性降低,涂层在高速摩擦环境下,疏松的氧化物层易粉化失效,故WC-17Co涂层的服役温度应保持在500℃以下。

WC-17Co is widely used in friction protection of workpiece due to its excellent wear resistance. In this paper,dense WC-17Co coating was prepared on 45# steel surface by High Velocity Oxy-fuel spraying.It was subjected to constant temperature heat treatment at 400 ℃,500 ℃,600 ℃ and 700 ℃ to study the influence of oxidation behavior on its structures and properties. X-ray diffraction, scanning electron microscopy and microhardness tester were used to characterize the phase change and microstructure evolution of the coating at different oxidation temperatures. The results showed that the decomposition and undercooling of WC during the spraying process led to the formation of a small amount of Co-based amorphous in the sprayed WC-17Co coating. When the heat treatment temperature was above 500 ℃,the main oxidation phases such as CoWO_(4),WO_(3) and Co_(3)O_(4) were formed on the coating surface. The oxide growth layer grew to 10 μm after being oxidized at 700 ℃ for 2 h. Co diffused toward the coating surface from the inner coating due to the oxidation reaction,leading to the increase of the concentration of WC within the coating. The microhardness values of the inner coating were significantly increased to more than1400 HV_(0.3) at a depth of 50 μm from the coating surface. The increase of surface microhardness was mainly due to the formation of multiple oxidation phases. After high temperature oxidation,the reduction of Co bonding phase reduced the fracture toughness of the coating,and the loose oxide layer on the surface would lead to atomize under high-speed friction conditions. Therefore,it is concluded that the service temperature of WC-17Co coating should be kept below 500 ℃.