Relationships between feedstock structure, particle parameter, coating deposition, microstructure and properties for thermally sprayed conventional and nanostructured WC–Co

Abstract

Thermal spray cermet based on tungsten carbide has been widely used due to its excellent wear resistance. The features of both carbide and binder phases are essential factors which determine the performance of cermet coating. The thermal cycling of WC–Co spray particles up to a temperature over the melting point of binder phase during thermal spraying involves the decarburization of carbide. The decarburization of carbide becomes severe with the decrease of carbide particle size, which makes it difficult yet to deposit a dense nanostructured WC–Co with a limited decarburization by thermal spraying. The decarburization not only reduces the wear-resistant phase but also leads to the formation of brittle Co–W–C ternary binder phase. Moreover, the limited decarburization involves the deposition of spray particle at a solid–liquid two-phase state with carbides at a solid state and metal binder in a molten state during spraying. High velocity impact of two-phase droplets as in high velocity oxy-fuel spraying (HVOF) results in the formation of a dense cermet coating and on the other hand leads to the possibility of rebounding of wear-resistant solid carbide particles. In this review article, the microstructural features of thermal spray WC–Co are examined based on the effect of the decarburization of tungsten carbide. The decarburization mechanisms of tungsten carbide are discussed for the control of decarburization of carbide. The effects of carbide particle size on the deposition process, adhesion of HVOF coating and wear performance of WC–Co coating as well are examined based on a solid–liquid two-phase deposition process. It is demonstrated that WC–Co cermet with different sizes of WC particles should be deposited by different processes. Moreover, the deposition of nanostructured WC–Co by thermal spraying and recent advances on the cold spraying of nanostructured WC–Co are introduced. The cold spraying with the proper design of spray powders will become promising process to deposit nanostructured WC–Co with pure cobalt binder with the hardness comparable to a sintered bulk and even high toughness of 18.9MPam1/2. The pure metastable metal binder phase evolved in the deposit makes it possible to deposit hard cermet through healing the non-bonded interfaces in the coating by post-spray annealing.