A correlative experimental and theoretical investigation of phase formation and thermal stability of a quaternary Cr_0.69Ta_0.20B_0.11N coating

In recent years, CrTa(B)N coatings deposited by physical vapor deposition have gained increasing interest due to their high hardness and good wear resistance. However, a detailed investigation of the microstructure of quaternary CrTaBN coatings both at ambient conditions and at elevated temperature is lacking in literature. Therefore, the present study provides an overview of the temperature-dependent phase formation and microstructure of a Cr0.69Ta0.20B0.11N coating grown by cathodic arc deposition. The face-centered cubic Cr1-x-yTaxByN solid solution was found to be the only crystalline phase present in the as-deposited coating, with the B atoms exclusively forming B-N bonds. Both, atom probe tomography and transmission electron microscopy suggest that the B concentration is increased at the grain boundaries compared to within the grain. Annealing CrTaBN at 1000°C provokes no change in the crystalline phase composition, but results in a reduction of the B content due to the formation of volatile B-containing compounds. In addition, B-metal bonds form upon annealing as a consequence of the CrN→Cr2N+N decomposition. The here reported formation of B-N bonds in the as-deposited state and the annealing induced formation of B-metal bonds are consistent with density functional theory calculations of relaxed corresponding configurations as the energy differences between them are minute. The obtained findings provide a detailed insight into the phase formation of the given complex material system.