Effects of low boron concentrations on the thermal stability of hard coatings

Hard coatings of the systems CrN and TiN were deposited by reactive cathodic arc evaporation of the pure metals as well as of boron microalloyed metallic cathodes. The effective boron concentrations in the coatings amounted to 4 and 1 at.% for Cr-B-N and Ti-B-N, respectively. Industrially deposited (Ti,Al)(N,C) cathodic arc multilayer coatings served as reference samples. As-deposited samples and samples tempered under atmospheric conditions at 700°C for 1 h were fractured and investigated by scanning electron microscopy. In the as-deposited state the low boron concentrations caused grain refinement, as observed in coating fracture cross-sections. The coating resistance against oxidation and oxygen diffusion as a result of low boron concentrations could particularly be improved for the Ti-based system.