Evaluating the microstructure and performance of nanocomposite PVD TiAlBN coatings

Physical vapour deposition (PVD) TiAlBN coatings can exhibit excellent wear resistance, with optimised coating compositions demonstrating a 150% increase in lifetime compared to standard TiAlN coatings in wet-cutting drill tests. For various TiAlBN compositions deposited by electron-beam evaporation, the stoichiometry and relative phase composition were determined using X-ray photoelectron spectroscopy (XPS) and the microstructure was examined by transmission electron microscopy (TEM). Al was found to substitute for Ti into the cubic TiN structure. In accordance with their position in the Ti(Al)BN phase diagram, all coatings exhibited a three-phase composition of (Ti,Al)N+BN+TiB2. The TiB2 content was very small and the microstructure was effectively that of a (Ti,Al)N and BN dual-phase coating. Optimum drilling performance was obtained for a coating with a phase fraction of approximately 90% (Ti,Al)N and 10% BN. The microstructure can be described as nanocrystalline (Ti,Al)N grains separated by an intergranular amorphous BN phase, in which the average (Ti,Al)N grain size and grain separation was determined to be 26 and 3 nm, respectively. The presence of a compliant intergranular phase permits some degree of grain displacement under load, reducing the elastic modulus, leading to greater toughness and wear resistance.