Influence of spinodal decomposition and fcc→w phase transformation on global and local mechanical properties of nanolamellar CVD fcc-Ti1-xAlxN coatings

Abstract

Recently, it was shown that annealing of nanolamellar CVD fcc-Ti1-xAlxN at temperatures of 1000-1200 °C results in the formation of complex phase fields consisting of still intact nanolamellar face centered cubic (fcc) zones, side by side with non-lamellar fully decomposed and transformed fcc and wurtzite (w) zones. It can be assumed that the observed phase fields and their microstructure strongly correlate with their mechanical properties. Consequently, this work focuses on the investigation of the effects of spinodal decomposition and fcc→w phase transformation of a nanolamellar CVD fcc-Ti0.2Al0.8N coating on the corresponding global and local mechanical properties. The sequence of spinodal decomposition and fcc→w phase transformation of a compact coating sample was investigated by in situ high temperature synchrotron X-ray diffraction up to a maximum temperature of ~1250 °C. Conventional nanoindentation experiments on the surfaces of samples annealed between 900 to 1300 °C in vacuum were performed to illustrate the age hardening and overaging behavior. Finally, the influence of the observed phase fields on the local mechanical properties was investigated by correlative SEM/EBSD and nanomechanical mapping experiments on a cross-section of a coating annealed at 1050 °C. Maps of the lateral microstructure, phase composition, Young´s modulus and hardness of the coating were successfully obtained with a resolution of ≤100 nm. The lateral phase fields could be clearly identified and correlated with the observed mechanical properties. The results indicate that age hardening of nanolamellar CVD fcc-Ti0.2Al0.8N coatings occurs homogeneously, while overaging is associated to the fcc→w transformation and thus, locally confined.