Effect of process parameters on the properties of Stellite-6 coatings deposited by Cold Gas Dynamic Spray

Abstract

This study analyzes the effects of process parameters on mechanical and tribological properties of Stellite-6 coatings obtained by Cold Gas Dynamic Spray (CGDS). The gas pressure and temperature as well as the speed of the deposition torch were considered as significant process parameters. The aim is to overcome some technical issues arising in the cold spray deposition of hard anti-wear metallic coatings, such as stellite, due to their high strength and melting point. A High-Pressure CGDS equipment was used and systematic studies were carried out for a deeper understanding of the effects of all investigated process parameters. A particular focus has been put on the substrate temperature, that can be regarded as an indirect process parameter. This latter, in fact, was monitored in-situ during deposition, by infrared thermography (IR). The microstructure was analyzed by scanning electron microscopic observations. Mechanical properties were analyzed by instrumented micro- and nano-indentation measurements. Hardness (H) and Young's modulus (E) were considered as affective parameters to estimate the inter-particle cohesion strength and the work hardening of the coating. Results revealed that the substrate temperature, that is affected by the process parameters, plays a fundamental role in the coating formation process. In particular, the lower the scanning speed the higher the quality of the coatings, in terms of compactness, hardness and elastic modulus. Finally, tribological properties of the coatings were analyzed by pin-on-disk tests. Wear coefficient is strongly related to the strain-to-failure ratio (H/E) that is higher when high values of scanning speed were chosen. In fact, it was observed that decreasing scanning speed elastic modulus increases more quickly if compared with trend of the hardness response. In conclusion, it was demonstrated that both mechanical and tribological properties of CGDS Stellite-6 coatings are mainly affected by the impact temperature of the particle-substrate system and effective deposition windows were identified.