Adhesion of cold sprayed soft coatings: Effect of substrate roughness and hardness

When depositing a soft material (e.g. pure Cu) onto a harder substrate (e.g. stainless steel) by cold spraying, the attainable coating adhesion depends on both the hardness and the roughness of the substrate. To decouple their respective influences, the present work focuses on a model system of cold sprayed copper onto AISI 304 steel surfaces with differently adjusted hardness and roughness values. AISI 304 stainless steel plates were either polished or grit-blasted to different roughness, and then employed as substrates either as-is, i.e. in the “hardened” condition, or in the “softened” vacuum-annealed state. SEM analyses and nano-indentation tests showed that grit-blasting causes severe plastic deformation of the surface, where any visible austenite grain structure is lost up to a depth of some tens of microns. The corresponding microstructures are work-hardened, reaching a hardness of about 350–400 HV0.001. Vacuum annealing induced recrystallization and restored the original, lower hardness (≈200 HV0.001) of AISI 304, without changing the surface roughness, as obtained by the grit-blasting. Through cavitation tests on single-particle deposition experiments, it was found that cold sprayed Cu particles' adhesion was lower on roughened and hardened substrates than on polished surfaces. In contrast, the situation was reversed for roughened and annealed (“softened”) substrates, showing an increase in adhesion of cold-sprayed Cu particles with increasing roughness up to a maximum at Rz of about 34 μm. The latter trend was confirmed by bond strength testing of complete Cu coatings onto “softened” substrates, while the maximum adhesion on hardened substrates was attained with Rz = 18 μm. The underlying mechanisms are discussed with respect to particle deformation and the substrate surface topographies needed to trap areas of mutual shear instabilities of the particles and the substrate. Identifying the conditions that ensure metallurgical bonding then should help to provide a more general understanding of deposit adhesion in cold spraying.