Influence of oxygen impurities on growth morphology, structure and mechanical properties of Ti–Al–N thin films

Unintended impurities, such as oxygen, can significantly influence the growth morphology, structure, and mechanical properties of many materials. Therefore, we study the origin and impact of oxygen impurities (between ~ 0.3 and 1.3 at.%) on sputtered Ti_1-xAl_xN thin films by using targets with three different oxygen impurity levels and modifying the deposition conditions such as the base pressure. The oxygen impurity content of our coatings was always (by around ≥ 0.1 at.%) above that of the target used. Detailed atom probe tomography as well as secondary ion mass spectroscopy indicate a homogeneous distribution of the oxygen impurities in the intermediated regions of our face centered cubic structured Ti_0.50Al_0.50N coatings. Except for higher oxygen levels at the interface to the substrate, which is more pronounced when the target is not sputter-cleaned prior to the deposition, and at the coating surface, there are no detectable oxygen variations at the grain or column boundaries. Based on our results we can conclude that the growth orientation of our coatings changes from random (for the cleanest coatings with ~ 0.3 at.% O) to pronounced 111 (for the coatings with the highest oxygen content of ~ 1.3 at.% O) with increasing oxygen impurity content. Thereby, also the coherently diffracting crystallite sizes increase from around 35 to 100 nm, the growth morphology changes from dense columnar grains to more open, porous columnar grains, and thus the hardness decreases from around 35 to 20 GPa. Our study highlights the importance of well-defined deposition conditions and high quality targets for developing high performance thin films. - Highlights: • Influence of oxygen impurities on Ti–Al–N thin films was investigated. • Oxygen impurity contents in the coatings stay ≥ 0.1 at.% above the target contents. • SIMS, ERDA, and APT confirmed homogeneous distribution expect surface near regions. • Orientation changes from random to 111 with increasing O content (0.3 to 1.3 at.%). • Accompanied increase in grain sizes leads to decreased hardness values — 35 to 20 GPa.