In situ and ex situ studies of microstructure evolution during high-temperature oxidation of ZrN hard coating

Microstructure evolution during oxidation of ZrN coating at 650 °C was investigated by in situ and ex situ methods. The results reveal the simultaneous occurrence of epitaxially growing tetragonal ZrO2 near the ZrO2-ZrN interface and transformation from preformed tetragonal to monoclinic phase close to the oxide surface. The significant volume expansion associated with tetragonal → monoclinic transformation plays a major role in producing interconnected microcracks and pores across the oxide scale, which provide fast diffusion paths of oxygen and hence lower the oxidation resistance of ZrN coating.