Influence of substrate bias on structure and mechanical properties of arc evaporated (Al,Cr)2O3 and (Al,Cr,Fe)2O3 coatings

Alloying already small amounts of Fe to powder-metallurgically produced Al-Cr cathodes leads to an increased hexagonal phase fraction in cathodic arc evaporated (Al1−x−yCrxFey)2O3 films. This observation can mainly be attributed to the nucleation of hexagonal crystallites onto smaller Cr- and Fe-rich droplets or favourable spots at the surface of larger macroparticles. As a consequence, the coatings feature a non-homogeneous microstructure consisting of finer-grained cubic and pronounced-columnar hexagonal crystallites. Taking this morphology into consideration, we improve the growth kinetics by varying the bi-polar pulsed substrate bias and elucidate its impact on the film microstructure and mechanical properties, thereby comparing in detail the Fe-free and 5 at.% Fe (Al0.7Cr0.3)2O3-based coatings. Upon increasing the substrate bias from −60 V to −160 V only marginal changes in the phase composition are observed, whereas a distinct transition in the stress state from tensile to marginally compressive occurs, directly affecting the mechanical properties. This results in a hardness improvement by 100% (from 12.5 to 25 GPa) for the Fe-free and 40% (from 16.5 to 23 GPa) for the Fe-alloyed coating. The indentation moduli increase by ~ 24% from 210 ± 21 to 260 ± 17 GPa.