Deposition of a dense coating with solid particles by cold spraying requires sufficient deformation of impacting particles and previously deposited underlying particles. The cermet particles and subsequent coating with a high hardness are difficult to deform upon impact. To increase the ability of deformation, the cermet spray particles with a porous structure design is proposed to fulfill the requirements of deformation on impact. To understand the deposition mechanism, the deposition behavior of single WC-Co spray particles impacting on the substrates with different hardnesses during cold spraying were examined using WC-12Co powders with different porosity. The substrates include stainless steel, nickel-based self-fluxing alloy coatings were employed to examine the effect of substrate deformation on the cermet particle deposition. It was found that using two porous powder of the porosity of 30% and 44% the WC-Co cermet particles were deposited on the substrate of different hardness from 200Hv to 800Hv. The deposition of the particles is mainly attributed to the deformation of powders themselves. The properly designed porous cermet powder with certain hardness is necessary condition to deposit hard WC-Co cermet coating.

The carbide decomposition and the dissolution of carbide into the binder accompanying with thermal spraying of cermets can be eliminated during cold spray deposition of WC-Co. However, the limited deformation of hard impacting cermet particles and impacted coating makes it difficult for conventional thermal spray powders to continuously build up on impact in cold spraying. The porous structure of WC-Co powders provides the powder particles with certain deformability on high velocity impact, which benefits the continuous building-up of coating. In this study, three nano-structured WC-12Co powders with different porous structures and apparent hardness were employed to deposit WC-Co coatings on stainless steel substrate by cold spraying. The deposition characteristics of three powders of different porosity levels of 44%, 30% and 5% were investigated. It was found that WC-Co coating is easily built-up using the porous powders with WC particles bonded loosely and a low hardness. The microhardness of WC-12Co coatings varied from Hv400 to Hv1790 with powders and spray conditions, which depends on the densification effects by impacting particles. With porous WC-Co powders, the fracture of particles on impact may occur and low deposition efficiency during cold spraying. The successful coating building up at high deposition efficiency depends on the design of powder porous structure.