Corrosion-resistant silica coatings obtained by plasma-assisted chemical vapour deposition

Amorphous silica coatings produced by plasma-assisted chemical vapour deposition (PACVD) provide remarkable long-term corrosion protection in aggressive oxidizing and sulphidizing environments for a range of alloys up to 1000 °C. In certain aspects, however, the coatings have not performed as well as expected. With low-chromium-containing ferritic steels the coatings became less effective above 450 °C owing to spallation, while deterioration of coating performance on some chromia-forming alloys above 700 °C originated from solid-solid reactions between the coating and alloy constituents, such as chromium, manganese, aluminium and titanium.Rectification of these difficulties could be achieved by changing the interlayer between the substrate and the coating, which was an oxide layer formed by substrate pre-oxidation. Titanium nitride (TiN) was selected as an alternative and was produced by a PACVD coating procedure. Initial evaluation, during 1000 h tests in air, has revealed that the TiN interlayer improved silica (SiO2) coating adhesion such that protection of 214wt.%Cr−1wt.%Mo ferritic steel was extended up to 600 °C, while interfacial reactions between SiO2 and the nickel-based superalloy IN-939 were prevented at 800 °C.