Influence of varying nitrogen partial pressures on microstructure, mechanical and optical properties of sputtered TiAlON coatings

Abstract

In recent years, TiAlON coatings have gained increasing interest owing to their excellent mechanical properties, chemical stability and tunable optical properties. Within this work, a series of TiAlON coatings was grown by magnetron sputter deposition using constant oxygen and increasing nitrogen partial pressures. Oxygen concentrations between 29 and 52 at.% were obtained, while only nitrogen contents of up to 13 at.% could be detected using a combined approach of energy and wavelength dispersive X-ray spectroscopy. With increasing nitrogen partial pressure, the (Al + Ti)/(O + N) ratio decreased from 1.74 to 0.59. All coatings exhibited a dominating fcc-(Ti1-xAlx)ν(O1-yNy)ξ structure with additional fractions of amorphous oxides, as determined by X-ray diffraction and complementary TEM investigations on selected samples. These observations were corroborated by ab initio calculations, which also precluded the presence of a rutile-based structure and demonstrated that a defected fcc-phase containing vacancies on the metal sublattice exists up to high oxygen contents. Using nanoindentation, the highest hardness could be observed for the coating with an (Al + Ti)/(O + N) ratio of 0.78. With increasing oxygen content, a decreasing refractive index and extinction coefficient were found by spectroscopic ellipsometry. However, the behavior of the optical properties remained metallic-like.