Influence of Al on the microstructure and mechanical properties of Cr–Zr–(Al–)N coatings with low and high Zr content

Low Zr (S1) and high Zr (S2) quaternary Cr–Zr–(Al–)N coatings with increasing Al content were deposited by d.c. reactive magnetron sputtering. The structure, fracture cross-section morphology and mechanical properties of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, scratch testing and Vickers micro-indentation testing. All the coatings present an fcc NaCl-type B1 structure; in the low Zr content coatings, the diffraction peaks shift towards higher angles as the Al content increases. The grain size is approximately constant in a range from 6 to 8 nm, except for high Zr content films where a significant decrease in crystalline order is observed (grain size ~ 2.5 nm). In both series, the microstructure changed from equiaxed to columnar with increasing Al content. The highest hardness and strongest adhesion values were achieved in coatings with lower Zr and Al content. Conversely, the coatings with high Zr and the highest Al content exhibited an abrupt decrease in hardness, adhesion strength and toughness.

► The coating structure changes from equiaxed to columnar with increasing Al content.
► The mechanical properties of low Zr coatings exhibit little change with Al content.
► High Al leads to a decrease in crystalline degree and hardness in high Zr coatings.
► KIC from Vickers indentation agrees with H3/E*2 ratios and scratching critical load.