超音速火焰喷涂制备纳米和微米结构WC-η涂层的耐熔锌腐蚀性能

以钨氧化物、钴氧化物和炭黑为原料, 通过原位还原碳化反应制备纳米WC-η(η为Co₃W₃C、Co₆W₆C等缺碳相)复合粉, 粉末平均粒径为155 nm。该复合粉经团聚造粒制备得到具有高致密性和良好流动性的热喷涂粉末。以此纳米结构和商业化的微米结构低碳WC-12Co粉末作为喂料, 通过超音速火焰喷涂制备硬质合金涂层。结果表明, 纳米结构涂层中生成了一定量等轴状的W₂C相, 裂纹主要沿晶界或相界面扩展, 而微米结构涂层中除W₂C外还含有较多的W相, 主要包裹在WC颗粒表面, 穿晶断裂比例较高, 裂纹扩展路径较平滑。由于纳米结构涂层组织致密、晶粒细小、界面积大, 因此比微米结构涂层具有更高的硬度和断裂韧性。两种涂层在熔融锌液中浸泡200 h后, 微米结构涂层中产生了较多的横向和纵向裂纹, 导致材料的大面积剥落和基材腐蚀; 纳米结构涂层中没有发生锌的浸蚀, 在局部产生了少量纵向裂纹, 裂纹间隙被钨钴氧化物所填充, 反而抑制了熔锌对涂层的腐蚀, 因此纳米结构涂层表现出更高的耐熔锌腐蚀性能。

Corrosion Resistance of HVOF-sprayed Nano- and Micon-structured WC-η Coatings against Molten Zinc

WC-η (i.e. Co₆W₆C, Co₃W₃C, etc) composite powder was synthesized by in situ reduction and carbonization reactions using tungsten oxide, cobalt oxide and carbon black as raw materials. Mean particle size of the as-synthesized composite powder is about 155 nm. Thermal spraying powder with a high density and an excellent flowability was prepared using WC-η composite powder by agglomeration and heat-treatment. Cemented carbide coatings were then fabricated by high velocity oxy-fuel (HVOF) spraying technique using nanostructured and commercially available low-carbon WC-12Co thermal spraying powders as feedstock. The results show that equiaxed W₂C particles form at certain amount in the nanostructured coating while cracks propagate mainly along grain boundaries and phase interfaces. However, the micron-structured coating still contains W besides W₂C, which mainly distribute in outer surface of WC grains. The micron-structured coating has stronger tendency to get transgranular fracture. As a result, the cracks go forward along a relatively straight path. Due to higher density, finer grain size and much larger interface area, the nanostructured coating has simultaneously higher hardness and fracture toughness as compared to the micron-structured coating. After immersion into molten zinc for 200 h, the micron-structured coating suffers from more serious cracking along the directions perpendicular and parallel to the interface between coating and substrate, which causes the large-scale exfoliation of coating material and the corrosion of substrate. In contrast, zinc diffusion is not observed in the nanostructured coating and only a few cracks perpendicular to the coating/substrate interface are locally produced. Moreover, the inner surfaces of the crack are covered with tungsten and cobalt oxides, which inhibit corrosion of the nanostructured coating in molten zinc. Therefore, the nanostructured coating has a promising corrosion resistance against molten zinc.