Metallization of ceramic coatings: Effect of cold gas spray parameters and roughness of the ceramic substrate on metallic layer properties

The deposition of a metallic layer on top of a plasma-sprayed (PS) ceramic coating, for further deposition of a PVD thin layer to enhance the tribological properties of the layer composite, is a challenging task. Since high adhesive strengths and limited phase transitions are required, cold spray (CS) is the most suitable process for metallization. However, there is a lack of knowledge about the influence of CS process parameters and the roughness of the ceramic substrate on the CS metallic layer properties. In this work, titanium (Ti) coatings (as metallic layers) were deposited by a high-pressure cold spray (HP-CS) process at different parameters on top of PS stabilized-zirconia coatings (7YSZ). The effects of the CS parameters on the hardness, adhesive/cohesion strengths, and thicknesses of ceramic and metallic layers were investigated. Additionally, single Ti splats were collected on polished ceramic substrates and their bonding, as well as their deformation and flattening behavior, were analyzed. Finally, the influence of ceramic substrate roughness on the thickness and roughness of the subsequent metallic layer was investigated. It was found that the deformation of Ti splats is responsible for the bonding with the ceramic substrate, with no deformation at the substrate side, and that the bonding area increases with the increased deformation degree of the particles. Increased propellant gas temperature and pressure of the CS process lead to increasing the bonding strength between ceramic and metallic layers, improving the hardness, and decreasing the porosity level of the metallic layer. Furthermore, it was found that the higher roughness of ceramic substrates leads to the production of thicker metallic layers, but has no effect on the roughness of the respective metallic layers.