Crystallization and oxidation behaviour of Mo-Si-N coatings

Mo-Si-N coatings with different nitrogen concentrations were produced by nitrogen alloying simultaneously during sputter deposition from a planar magnetron MoSi₂ target with nitrogen plasma onto steel substrates.
The ratio of molybdenum and silicon concentrations was 0.5. The nitrogen concentrations of the samples were 20, 35, and 50 at.% and the concentration profile was uniform througout the film thickness. The initial microstructure at all nitrogen concentrations involved was amorphous and the crystallization temperature was strongly dependent on nitrogen concentration. The sample with 50 at.% N was still amorphous after annealing at 1000°C whereas the sample with 35 at.% N was crystallized at 740-760°C and the sample with 20 at.% N at 660-680°C. Even this was higher than the crystallization temperature of a pure MoSi₂ coating, about 500°C. The oxidation behavior of the Mo-Si-N coatings with 50 at.% N was determined in wet oxidation conditions from 400 to 1000°C by measuring oxygen concentration on the surface using a nuclear reaction ¹⁶O(d,p)¹⁷O or non-Rutherford scattering of He⁺. The excellent oxidation resistance of this coating on low carbon steel was confirmed up to 800°C, above which diffusion of nitrogen occurred, resulting in crystallization and degrading of the beneficial properties. The oxidation behavior of the Mo-Si-N coating can be explained in terms of amorphous microstructure and silicon-nitrogen interaction.