Properties of reactively sputtered W–Si–N films

Recently, it has been found that ternary nitrides of the type Me–Si–N (Me=Ti, Zr, Ta, W, etc.) containing a low (≤10 at.%) amount of Si can form nanocomposites with enhanced hardness. However, there is insufficient knowledge on properties of these nitride films with a high content of Si. This article reports on the d.c. reactive magnetron sputter deposition of W–Si–N films with a high (≥20 at.%) content of Si and their physical and mechanical properties. Films were characterized by XRD, EDX, microhardness, elastic recovery, resistance to plastic deformation, macrostress σ and thermogravimetry. On the basis of a systematic investigation of the properties of reactively sputtered W–Si–N films it was found that (1) according to the elemental composition, which can be controlled by a variation of the partial pressure of nitrogen (pN2), W–Si–N films are nanocomposites composed of a mixture of different phases, either WSi2+W+Si3N4 or Si3N4+WNx, when produced at low and high values of pN2, respectively; (2) the 4 to 5 μm thick W–Si–N films produced at high values of pN2 exhibit (i) an amorphous structure, (ii) the highest (≈35 GPa) hardness and (iii) a relatively low (≈−1.6 GPa) compressive macrostress σ, (3) the maximum hardness Hmax does not depend on the energy density Ebi delivered to the growing film by bombarding ions but the value of Ebi must be higher than a minimum energy Ebimin≈0.1 MJ/cm3, (4) H and σ of the hardest W–Si–N film do not depend on (i) the film thickness h up to 15 μm and (ii) the deposition temperature Ts up to 500 °C and (5) the oxidation resistance is achieved up to approximately 800 °C. Special attention is devoted to correlations between the film hardness and the content of Si3N4 in the film. The W–Si–N films with a higher content of Si and a higher N/Si ratio exhibit a higher H. Correlations between the hardness H, effective Young's modulus E*=E/(1−ν2), elastic recovery We and the ratio H3/E*2 are also given (here ν is the Poisson's ratio). The main conclusion resulting from this work is a finding that the high (≥60 vol.%) content of Si3N4 in the W–Si–N film is not a sufficient condition to achieve the oxidation resistance exceeding 1000 °C. The thermal stability of individual phases from which the Si based (Me–Si–N) nitride film is composed must be ensured too.