The behaviour of amorphous silica coatings at high temperatures in aggressive environments

Amorphous silica coatings produced by plasma assisted and laser chemical vapour deposition (PACVD and LCVD) on Incoloy 800H and 21/4 Cr 1 Mo ferritic steel were exposed in air and in simulated coal gasification atmospheres (CGA) for periods of up to two years at temperatures between 450°C and 900°C . In some cases interlayers of TiN were used to promote adhesion and to reduce interdiffusion between the coating and substrate. PACVD silica coatings deposited onto Incoloy 800H provided outstanding corrosion protection at 450°C , but, at higher temperatures interaction with the substrate was observed which could be reduced by the presence of the TiN interlayer. However, these coatings would only adhere to the surface of 21/4 Cr steel in the presence of the TiN interlayer. Protection of both steels at 450°C in the CGA using the LCVD silica coatings was dependent upon coating density, the presence of a TiN interlayer, and silica thickness in the range 0.35 - 3.5 µm. Adhesion of the silica layer to the Incoloy 800H substrates was good and, in this case, did not depend on the presence of a TiN interlayer. Differences between the individual coatings are interpreted in terms of residual stresses and interfacial chemistry