Thermal behaviour of hard nanocomposite coatings within the W–Si–N system in oxidant and protective atmospheres

This paper reviews the current knowledge on the thermal annealing of W–Si–N sputtered films in both protective and oxidant atmospheres. Firstly, sputter deposited W films are presented as a particular case of single nc-metallic films and their thermal behaviour is analyzed. Afterwards, the thermal stability and the oxidation resistance of W–Si–N system are considered. Special attention is paid to amorphous films. In a protective atmosphere, it is shown that after crystallization the hardness can be higher than those of as-deposited crystalline films with similar nanocomposite structure. The hardest films present a nc-W/a-Si3N4 nanocomposite structure type. The key factor for thermal oxidation resistance is the Si content. The higher the Si content the lower the oxidation rate is. The excellent thermal behaviour in oxidant atmospheres is attributed to either the formation of a protective surface layer of SiO2 or the hindering of oxygen diffusion along the grain boundaries due to the Si–N phase.