Coating deposition in cold spray process
Koh, Pak Keng
Date of Issue2016-04-15
School of Mechanical and Aerospace Engineering
Cold spray technology is an emerging spray coating process that uses high velocity (kinetic energy) impact instead of high temperature melting (as in thermal spray) for its coating mechanism. With its unique working principle, the coating produced has numerous advantages over conventional thermal spray processes such as high density, low oxide content and minimal or no change in microstructure of the coating. Its distinctiveness presents immense opportunity in providing a new platform to address wear, erosion, corrosion and environmental attack issues. Preliminary study demonstrates the technical viability of the cold spray process as an alternate repair method to address corrosion issues for gas turbine engine fan cases. However, being a relatively new coating process, the effect of pre and post coating processes on the properties of the coatings needs to be better understood. The effect of grit blasting of the substrate on the mechanical properties and microstructures of the cold spray coatings was first investigated. The results showed that grit blasting may not enhance the microhardness or tensile adhesion strength of the cold spray coating and may be eliminated as a pre-coat process. The effect of post coating heat treatment was also evaluated. The results indicated that both stress relief and annealing processes reduce as well as homogenize the microhardness of the coatings while the tensile adhesion strength was unaffected by stress relief but improved significantly when subjected to annealing. In addition, components that are cold sprayed usually come in complex shapes and sizes. It is difficult to ensure that the spray angle is consistently normal to the surface of the component. As such, the effect of the spray angle on the microhardness, tensile adhesion strength, and microstructure was examined. Results indicated that the effect of off-normal spraying has minimal impact on tensile adhesion strength and a positive effect on the microhardness of the coating. Furthermore, the relative coating efficiency of the cold spray process at various spray angles was also evaluated. Significant detrimental impact on the relative coating deposition efficiency was observed for spray angles smaller than 60°. The deposition phenomenon of the coating, during build-up on a stationary substrate, at different dwell times was also experimentally investigated. The mound-like built up was cross sectioned and viewed with microscopy and showed delamination from the substrate. The delamination phenomena were, however, absent in continuously sprayed coatings. The study postulated that the built-up of compressive residual stresses on the coating have resulted in the contraction of the coating and subsequent delamination from the substrate. The scope of the study has the intent of establishing optimum and preferential conditions for cold spray processes in general. This will pave the way for the industrial application of such a technology in a cost effective and efficient manner.