Oxidation resistance of protective coatings studied by spectroscopic ellipsometry

Spectroscopic ellipsometry is applied to determine the oxidation resistance of TiN and CrN coatings. This technique proves to provide accurate measurements of the oxide layer thickness in a convenient, fast, and nondestructive manner. The magnetron-sputtered thin films were heated in air in the temperature range between 673 and 925 K. The verification with scanning electron microscopy and x-ray reflectometry shows an excellent agreement between these complementary techniques. The rate constant k of the oxidation is directly related to the film morphology in TiN thin films. While understoichiometric films with a dense fine-grained structure exhibit a moderate k of 4 nm2/s at 773 K, overstoichiometric films with a pronounced columnar structure oxidized over seven times faster. The nonstoichiometry in TiNy and CrNy leads to a reduced activation energy for oxidation compared to stoichiometric compounds.