Physical properties and high-temperature oxidation resistance of sputtered Si_3N_4/MoN_x nanocomposite coatings

摘要

This article reports on structure, phase composition and high-r oxidation resistance of sputtered Mo-Si-N films. These films were dc reactively sputtered using an unbalanced magnetron equipped with a MoSi_2 alloyed target in a mixture Ar and N_2. A continuous increase of partial pressure of nitrogen p_(N_2) from 0 to 0.6 Pa makes it possible to produce two groups of composites: (1) MoSi_x +a-Si_3N_4 and (2) a-Si_3N_4+MoN_x. The composites of the first group are crystalline and contain a low amount of the a-Si_3N_4 phase. On the contrary, the composites of the second group are amorphous and the a-Si_3N_4 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 T_a ≥900℃ results in a loss of the film mass (Am<0). This loss of weight is due to the decomposition of MoN_(x >1) → Mo+N_((g)) and the formation of volatile MoO_x. 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 ≈ 0.01 mg/cm~3) decrease of the film mass is obtained in the case when the Mo-Si-N film contains a large (>60 vol %) amount of Si_3N_4 phase and stoichiometric (x=1) or substoichiometric (x< 1) MoN_x. nitride. In these films the loss of weight does not increase with increasing T_a up to 1300℃. This fact demonstrates the high-r oxidation resistance of the a-Si_3N_4/MoN_(x< 1) composite. The temperature T_a=1300℃ is not a physical limit of the high-r oxidation resistance of the a-Si_3N_4/MoN_(x≤ 1) composite but only the limit of Si substrate used in our annealing experiments. The microhardness H of the a-Si_3N_4/MoN_(x< 1) composite is also quite high and achieves approximately up to 25 GPa.