Decomposition and phase transformation in TiCrAlN thin coatings

Metastable solid solutions of cubic (c)-(TixCryAlz)N coatings were grown by a reactive arc evaporation technique to investigate the phase transformations and mechanisms that yield enhanced high-temperature mechanical properties. Metal composition ranges of y < 17 at. % and 45 < z < 62 at. % were studied and compared with the parent TiAlN material system. The coatings exhibited age hardening up to 1000 °C, higher than the temperature observed for TiAlN. In addition, the coatings showed a less pronounced decrease in hardness when hexagonal (h)-AlN was formed compared to TiAlN. The improved thermal stability is attributed to lowered coherency stress and lowered enthalpy of mixing due to the addition of Cr, which results in improved functionality in the temperature range of 850–1000 °C. Upon annealing up to 1400 °C, the coatings decompose into c-TiN, bcc-Cr, and h-AlN. The decomposition takes place via several intermediate phases: c-CrAlN, c-TiCrN, and hexagonal (β)-Cr2N. The evolution in microstructure observed across different stages of spinodal decomposition and phase transformation can be correlated to the thermal response and mechanical hardness of the coatings.