INTERACTION BETWEEN SILICON-NITRIDE AND SEVERAL IRON, NICKEL AND MOLYBDENUM-BASED ALLOYS

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile. © 1979 Chapman and Hall Ltd.