Effect of Ni content on the microstructure and mechanical behaviour of CrAlNiN coatings deposited by closed field unbalanced magnetron sputtering

Ni incorporation is known to impose an intriguing influence on the composition, microstructure and mechanical properties of nitride-based coatings prepared by physical vapour deposition. This work examined the effect of varying Ni concentrations, controlled by NiCr alloy target current (I-NICr), on the microstructures and properties of CrAlNiN coatings deposited on to M2 tool steel substrates. These coatings were deposited by closed field unbalanced magnetron sputtering (CFUMS). The composition and structure of the as-deposited coatings were investigated by XPS, XRD, FIB, and TEM. Residual stresses were determined using the XRD-sin(2)psi method. Nanoindentation tests were performed to assess mechanical properties of the CrAlNiN outer layer. At relatively low I-NICr values, below 3 A, the grains within CrAINiN layer underwent refinement, yet maintained a columnar structure. At higher I-NICr, values, from 3 to 5 A, the columnar grains transitioned into more equiaxed grains. Hardness values of 25-28 GPa at low I-NICr values were associated with the high residual compressive stress, solid solution hardening and grain refinement.