Comparison of microstructure and phase transformation for nanolayered CrN/AlN and TiN/AlN coatings at elevated temperatures in air environment

CrN/AlN and TiN/AlN multilayer coatings with modulation period of 4 nm and thickness ratio equal to 1.0 were manufactured by RF magnetron sputtering. Both films were annealed at temperatures of 800 °C in air for 1 h and then for an additional 9 h. Both coatings in as-deposited and after heat treatment were evaluated with a transmission electron microscope (TEM) equipped with EDS. After heat treatment at 800 °C for 1 h, a thick oxide layer around 260 nm was formed on the surface of the TiN/AlN coating. The oxide layer which formed on the coating was composed of three different regimes, including Al-enriched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO2 layer 30–80 nm thick above the nitride coating and in between, mixed nano-crystalline Al2O3 and TiO2 films. In comparison, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating. This amorphous or nanocrystalline oxide layer identified by EDS was a metal-deficient oxide, in which Al2O3 and Cr2O3 were mixed together forming a solid solution. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures.