Si-C films on alloys : characterization of ageing behavior in air

Abstract
Coatings of silicon rich silicon carbide are obtained in a plasma assisted device from tetramethylsilane and argon mixture, around 760K under reduced pressure on metallic alloys : FeCrAl and NiCrAl. The atomic composition of the deposits is determined by microprobe analysis and adherence is evaluated by scratch test measurements. These results are discussed in relation with previous determinations obtained when identical films are deposited on low carbon steel substrates. All samples exhibit a similar behavior which denotes a good adherence. The formation of a diffusion zone is responsible for this trend. The qualitative behavior of the coated substrate in air at high temperature (1273 K) is investigated in comparison with the uncoated substrate through cross section examinations by different analysis (EDS mapping, WDS profiles). The heat treatment (10h, 100h) induces a strong diffusion which is discussed. It is evidenced that the silicon carbide based films drastically change the oxidation mechanism (mainly in the case of NiCrAl) and tend to improve the resistance of the substrate against an oxidizing atmosphere. Futhermore, some solutions to improve the lifetime of such metal/ceramic junctions are suggested. As for example, the interest of an alumina interlayer between SiC coating and FeCrAl substrate is expenmentally shown.