Vacuum plasma sprayed alumina—titania coatings

Abstract

Vacuum plasma spraying is gaining acceptance as a source of high quality coatings Tor many materials on a wide range of substrates. The process produces coatings that exhibit excellent wear, corrosion resistance and high-temperature behavior. In this study, alumina-titania powder was deposited on graphite using controlled vacuum spraying to minimize the porosity of the sprayed ceramic. Experiments were conducted using a statistical, fractional factorial design parametric study. Operating parameters were varied around the typical spray parameters in a Taguchi design of experiments to display the range of plasma processing conditions and their effect on the measured coating characteristics. The Taguchi design evaluated the effect of three plasma processing variables on the measured response: current, primary gas flow and secondary gas flow. The coating qualities evaluated were thickness, porosity, surface roughness and microhardness. This paper discusses the influence of processing parameters on as-sprayed coating qualities. The best coatings were produced with a high power setting and a high ratio of secondary gas flow to the total gas flow. The paper also discusses the effect of thermal cycling on ceramic samples in an inert atmosphere ( i.e. helium). Results indicated no degradation of the coating. A vacuum plasma sprayed ceramic coating was applied to graphite to demonstrate the feasibility of using the coating to protect graphite from oxidation.