MMCs Coatings with High SiC Volume Fraction Retention by Pulsed-Gas Dynamic Spraying

 Metal matrix composites (MMCs) are used in fields as diverse as aerospace and electronics with aluminum alloys widely used as the matrix material due to their high strength/density ratio. In particular, the addition of silicon carbide particles (SiC_p) to aluminum alloy matrix result in increased strength and wear resistance. Al–SiC_p composite coatings have largely been produced by plasma spraying. In blended aluminum alloy-SiC_p feedstock powders the latter are preferentially lost compared to the former. This is attributed to the difference in melting temperatures between the SiC_p and the aluminum alloy and poor wettability of SiC_p by aluminum. It was reported that increasing the SiC_p content in the feedstock powder beyond some value does not increase its volume fraction in the coating, thus limiting the possible SiC_p volume fraction in the coatings.
In this work, Al-12%Si–SiC_p composite coatings have been produced by the Pulsed-Gas Dynamic Spraying (P-GDS) process using feedstock powders with various SiC_p %vol. In the P-GDS process, shock waves generated at a constant frequency are introduced in a spray gun containing feedstock particles injected at the same frequency. The feedstock particles are accelerated and heated by the resulting unsteady high velocity medium temperature flow. Upon impact with the substrate, the matrix particles plastically deform on the substrate and around the SiC_p to form the composite coating. The effect of the processing conditions on the MMC coating microstructure and the SiC_p volume fraction retained is investigated using different analysis techniques (OM, SEM, microhardness and bond strength measurements). It was found that the P-GDS process, due to its fluid flow physics, allows retaining in the coatings a large percentage of the SiC_p present in the feedstock powder compared to other spray techniques. A detailed dynamic analysis of the coating formation is presented in order to explain this behavior.