Influence of RF substrate bias on properties of reactive sputtered TiAIN films

A thin coating of TiAlN was deposited on a high speed steel substrate by reactive sputtering. The effect of varying the rf substrate bias on microstructure, chemical composition, and mechanical properties of these films was investigated. The deposition rate and critical N₂ flowrate decreased with increasing bias voltage. A stoichiometric composition of TiAlN was obtained and the chemical composition was found to be independent of bias voltage if the N₂ flowrate exceeded a critical value. Large grained microstructure was developed with increased bias voltage, probably because of the larger size and smaller number of nuclei formed. A denser, harder film was obtained with increasing bias voltage, but adhesive strength decreased as substrate bias increased owing to the residual stresses developed. Optimum wear resistance occurred at a bias voltage of 80 V; this was probably a result of the combined effects of hardness and adhesive strength.