The Elastic Modulus of Cold Spray Coatings: Influence of Inter-splat Boundary Cracking

It is well established that cold spray coatings exhibit substantially lower elastic modulus as compared to bulk material of the same composition. It has also been observed that the heat treatment of the cold spray coatings results in a significant increase in the elastic modulus of the coating. To check whether the presence of inter-splat cracks is responsible for the above behavior, a wide variety of metallic materials (Cu, Ag, Zn, Nb, Ta, Ti, and 316L stainless steels) in the powder form have been deposited on a mild steel substrate using the cold spray technique. These coatings in both as-coated and heat-treated conditions have been characterized for their porosity, extent of inter-splat boundary cracking, hardness, and elastic modulus. Results indicate that the elastic modulus of the coatings are substantially lower than the bulk value and also that the heat treatment of the coatings consistently increase their elastic modulus values. It has been shown that the reduction in elastic modulus of cold spray coatings can be related to the extent of inter-splat boundary cracking. Further, it has been shown that the standard models relating elastic modulus to the crack density are capable of explaining the observed modulus in the case of cold spray coatings in the as-coated and heat-treated conditions.