An in-operando evaluation identified the oxidative failure mechanism of TiN hard coatings during extreme thermal cycling

[Display omitted] •The failure mechanism of passivated TiN films determined by in-operando monitoring.•In-operando monitoring by Raman and optical microscopy, in air and Ar, up to 650 °C.•Initially, anatase forms from pre-existing anatase-rich nucleation centers.•These develop into domed bulges of condensed N2 gas in the sub-surface.•During cooling, cracks propagate from bubbles, leading to film delamination. We determine the definitive failure mechanism of passivated TiN films by in-operando monitoring extreme temperature (650 °C) induced microstructure, chemical, and mechanical changes. Films were in-operando characterized using Raman and optical microscopy, in air and argon, and post-mortem using nanoindentation, XPS, XRD, and microscopy. Initially, TiO2 anatase forms from pre-existing anatase-rich nucleation centers. These develop into domed bulges of condensed N2 gas in the sub-surface. This initial oxidation is limited to the surface. However, during cooling, cracks propagate from the bubbles, leading to wide-area delamination of the films. These cracks provide a pathway for further destructive bulk oxidation to rutile. This insight into failure mechanism provides deposition protocol for coatings operated under demanding conditions.