A mechanistic approach of oxidation resistance, structural and mechanical behaviour of TiAlN coatings

Although TixAl1-xN has been thoroughly studied, it is still unclear how TixAl1-xN coatings annealed for extended duration behave from a structural point of view, and if these structural changes are linked with oxidation mechanisms. This work aims to fill this gap by studying the chemical, structural and mechanical behaviour of TixAl1-xN coatings (0.38 × 0.58) deposited by dual bipolar magnetron sputtering, annealed in air and vacuum for an extended duration (up to 16 h) at temperatures ranging from 600 °C to 800 °C. X-ray photoelectron spectroscopy confirms previously reported results, with the formation of an Al2O3/TiO2 bilayer during oxidation, that is shifted to higher temperatures when increasing Al-content. X-ray diffraction demonstrates that Al-rich coatings undergo spinodal decomposition at lower annealing temperature/duration, but with the stabilisation of fcc-TiN and fcc-AlN. For Ti-rich films, the spinodal decomposition occurs at higher annealing temperature/duration, but the rapid transformation of fcc-AlN into w-AlN occurs, which is detrimental for the mechanical properties and the thermal stability. This study demonstrates that the oxidation mechanisms of TixAl1-xN are process- and time-independent, and occur independently from the structural changes. Two behavioural maps summarising the oxidation mechanisms combined with the structural behaviour of TixAl1-xN at 600 °C and 800 °C are proposed.