Thermal stability and mechanical properties of Ti-Al-B-N thin films

Abstract

The increasing demand in machining and drilling operations calls for new developments of protective coatings with outstanding properties. Meeting these challenging requirements, nitride based thin films have been well established and investigated. In recent years, this development has led - from binary transition metal nitrides (e.g. TiN, CrN) - to multinary alloyed thin films. The oxidation resistance and high temperature hardness of TiN was significantly improved through the addition of aluminum, to form supersaturated c-Ti1−xAlxN thin films. This system allows for age-hardening processes (via spinodally formed cubic Al- and Ti-rich domains) and increased oxidation resistance due to the formation of protective Al2O3-containing oxide scales. In this study, we investigate the influence of boron addition to further improve the already excellent thermal and mechanical properties of Ti1−xAlxN, using reactive magnetron sputtering of B-alloyed Ti1−xAlx targets. In addition to detailed experimental investigations, we study the preferred lattice sites of boron and the influence on phase stability by density functional theory (DFT) calculations. The calculations suggest that energetically, boron prefers the metal sublattice (hence, the formation of (Ti1−xAlx)1−yByN is preferred over Ti1−xAlxByN1−y). The cubic-to-wurtzite phase transition is unchanged for Ti1−xAlxByN1−y solid solutions, but shifts to lower Al contents for (Ti1−xAlx)1−yByN solid solutions, with increasing B. Furthermore, the experimental results show, that due to the addition of boron, the hardness of a w-Ti1−xAlxN thin film significantly increases to ~31GPa after annealing at 1000°C.