Phase formation and phase separation in multiphase thin film hard coatings

Recent studies on thin films and coatings containing multiple phases have demonstrated their potential for improving hardness and tribological performance. Coatings for these applications are typically used in the as-deposited condition, so it therefore becomes important to understand the phase formation and separation processes that occur during deposition. In this work, we demonstrate that thermodynamic considerations, particularly free energy-composition diagrams, can help predict the phase separation process for a given film composition. The results of numerous experimental studies of phase formation in co-deposited thin films are reviewed with the goal of rationalizing the observed phases in terms of thermodynamic principles. Additional kinetic considerations are also discussed and a model is devised where phase separation takes place in a thin surface layer followed by crystallization of one or both phases. The fact that phase separation must precede crystallization has important implications for the microstructure that develops and the crystallinity of the phases formed.