Structural, tribo-mechanical, and thermal properties of NbAlN coatings with various Al contents deposited by DC reactive magnetron sputtering

DC reactive magnetron sputtered NbzAlyNx coatings with various Al contents (y/(y + z)) ranging from 0 to 0.75 were deposited using different ratios of the currents applied to the Al and Nb targets in order to systematically control their chemical composition and crystalline structure, and to enhance their mechanical, tribological and thermal stability performance. The film microstructure and properties were assessed by various complementary techniques such as electron probe microanalysis, X-ray diffraction, nanoindentation, ball-on-disc test, and scanning electron microscopy. It was found that the solubility limit of Al in the NbN lattice is in the range 0.44 < y/(y + z) < 0.60. The NbzAlyNx coatings present low compressive residual stress (≤ 3 GPa), and their hardness increases up to 31 GPa with increasing y/(y + z) up to 0.40. The observed hardening is attributed to the solid solution mechanism. For y/(y + z) > 0.44, the hardness of the NbzAlyNx coatings is reduced due to the formation of the soft hexagonal AlN phase. In addition, the coatings exhibit interesting tribological properties: the coefficient of friction is in the range of 0.74 to 0.85, and wear rate is in the range of 4.8-9.1 × 10− 6 mm3/Nm. Thermal stability shows a dependence on the Al content; specifically, the Nb0.33Al0.17N0.50 coating was found to be thermally stable up to 700 °C.