Physical properties and high-temperature oxidation resistance of sputtered Si3N4∕MoNx nanocomposite coatings

Abstract

This article reports on structure, phase composition and high-T oxidation resistance of sputtered Mo–Si–N films. These films were dc reactively sputtered using an unbalanced magnetron equipped with a MoSi2 alloyed target in a mixture Ar and N2. A continuous increase of partial pressure of nitrogen pN2 from 0to0.6Pa makes it possible to produce two groups of composites: (1) MoSix+a-Si3N4 and (2) a-Si3N4+MoNx. The composites of the first group are crystalline and contain a low amount of the a-Si3N4 phase. On the contrary, the composites of the second group are amorphous and the a-Si3N4 phase dominates in these films. Sputtered films were characterized using XRD, EPMA, microhardness measurements, thermogravimetric measurements and SEM. It was found that the thermal annealing of Mo–Si–N films in flowing air at temperatures Ta⩾900°C results in a loss of the film mass (Δm<0). This loss of weight is due to the decomposition of MoNx>1→Mo+N(g) and the formation of volatile MoOx oxides, which diffuse out of film. This process results in (i) the formation of thin porous oxide surface layer and (ii) the loss of film mass. A very low (Δm≈0.01mg∕cm3) decrease of the film mass is obtained in the case when the Mo–Si–N film contains a large (>60vol%) amount of Si3N4 phase and stoichiometric (x=1) or substoichiometric (x<1) MoNx nitride. In these films the loss of weight does not increase with increasing Ta up to 1300°C. This fact demonstrates the high-T oxidation resistance of the a-Si3N4∕MoNx<1 composite. The temperature Ta=1300°C is not a physical limit of the high-T oxidation resistance of the a-Si3N4∕MoNx⩽1 composite but only the limit of Si substrate used in our annealing experiments. The microhardness H of the a-Si3N4∕MoNx<1 composite is also quite high and achieves approximately up to 25GPa.