Thermal stability and structural evolution of quaternary Ti-Ta-B-N coatings

Nanostructured hard coatings are becoming fundamental in many application areas due to their highly enhanced mechanical properties compared to conventional materials. Among others, novel quaternary systems are attracting increasing attention since they are expected to further improve endurance of such coatings. In this work, combined experimental and ab initio analysis of thermal stability and structural evolution of quaternary Ti-Ta-B‐N coatings with Ta content 0-40 at.% prepared by reactive magnetron co‐sputtering is presented. All prepared Ti1-x-yTaxByN coatings in the as-deposited state exhibit amorphous structure, which is supported by ab initio calculations. Structural development of Ti-Ta-B-N coatings induced by vacuum annealing leads to the formation of a nanocomposite system consisting of fcc-TiN, possibly fcc-TaN phase and disordered a-BTiy(Taz)Nx phase. At further increase of annealing temperature above 1100 °C, other nitride, boride and diboride crystalline phases begin to form. This process is accompanied by release of nitrogen from all samples, phase transformations and grain coarsening. Calculations of formation and cohesive energies for various phases of possible Ti-Ta-B-N decomposition products as well as molecular dynamics simulations of the Ti1-x-yTaxByN system at high temperatures were performed in order to give deeper insight into the structural evolution and thermal stability of the investigated alloy. Increased value of hardness at higher Ta content and low elastic modulus as well as high elastic recovery of the Ti-Ta-B-N coatings are also reported in this work and show a promising way to improve toughness of nitride coatings.