Influence of phase transformation on the damage tolerance of Ti-Al-N coatings

In contrast to earlier work, where we focused on the evolution of fracture toughness upon annealing of free-standing Ti-Al-N films using cantilever bending experiments, here we study the practically more relevant case of thin Ti-Al-N films firmly attached to a stiff substrate. We found - experimentally and by continuum mechanics modelling using the Transformation Field Analysis - that the formation of even tiny fractions of hcp (ZnS-wurtzite type) AlN during decomposition of supersaturated fcc (NaCl-type) Ti-Al-N gives rise to high in-plane compressive stresses in both the matrix and the precipitates. These eigenstresses impede the formation and propagation of cracks as evidenced by cube corner nanoindentation experiments - the measured fracture toughness (KIC) increases from the as-deposited value of ∼4.4 MPa√m to ∼8.0 MPa√m when annealed at 900 °C. The (apparent) toughening effects combine with less pronounced inherent toughening and concomitant age hardening effects to enhanced damage tolerance of annealed Ti-Al-N.