Scratch response and tribological behaviour of CrAlNiN coatings deposited by closed field unbalanced magnetron sputtering system

CrAlNiN coatings were deposited on M2 tool steels using closed-field unbalanced magnetron sputtering. The coating microstructure transformed from coarse columnar grains to fine equiaxed grains with increasing Ni content, which was accompanied by a general decrease in both hardness (H) and elastic modulus (E_r ) values. The scratch tests showed both scratch toughness and adhesion strength were improved for coatings with higher ratios of H/E_r and H³/E_r² , whereby higher critical loads (L_C1 and L_C2 ) are required to initiate cohesive and adhesive failure. For the coatings with coarser columnar grains, the improved scratch toughness may be ascribed to tortuous crack deflection patterns. Improved wear resistance was found in the columnar grained coatings, which also exhibited the higher H/E_r and H³/E_r² values. The inferior wear resistance in the equiaxed grained coatings was suggested to arise from these coatings being more susceptible to grain pull-out processes during abrasive sliding. The wear debris composed of tribo-oxides serving as additional capacity for load bearing via re-distribution of the applied stress. This was believed to boost the wear resistance of the coatings.