Building up WC-Co coatings by cold spray: a finite element simulation

Abstract

The impact of porous WC-12wt%Co and WC-17wt%Co particles onto homologous, smooth surfaces is simulated through a coupled Eulerian-Lagrangian, three-dimensional finite element model, in order to study the build-up mechanisms of cold sprayed WC-Co coatings. Deformation of hardmetal particles upon impact involves compaction and spreading of a rim, whose bottom face experiences heating, close to or even above the melting point of the matrix, and develops large equivalent plastic strain. The rim therefore adheres strongly to the underlying hardmetal. SEM observations of cold sprayed WC-Co particles deposited onto a smooth hardmetal surface show metallurgic bonding of the heated matrix and mechanical fixation as WC grains from the particle indent and penetrate into the underlying material. Little deformation and heating are instead seen in the middle of the contact surface, where adhesion is limited. Deformation of the particle requires the criterion for ductile failure of the hardmetal to be exceeded. This means the deformed rim is also extensively failed and ejects fragments as it spreads. Increasing the momentum of the particle enhances the spreading of the rim and promotes its bonding to the underlying surface, but also causes ejection of larger fragments, as well as the elastic storage of a greater amount of rebound energy, which can result in eventual rebounding of the entire particle or its poorly bonded central portion. It is therefore inferred that the deposition efficiency of cold sprayed hardmetals cannot increase linearly with impact velocity, and it is further impaired with stiff, dense particles and/or with very hard materials (e.g. WC-12Co as compared to WC-17Co).