Role of droplets and iron on the phase formation of arc evaporated Al–Cr-oxide coatings

•Characterisation of macroparticles in (Al,Cr)2O3 and (Al,Cr,Fe)2O3 coatings•Correlation between macroparticles and subsequent film growth•Flat-shaped Al-rich droplets result in metastable cubic growth.•Small spherical Fe- and Cr-rich droplets can be associated with hexagonal growth.

Investigations on Al-rich (Al,Cr)2O3 cathodic arc evaporated thin films showed that the hexagonal phase fractions can be increased significantly by alloying with small amounts of Fe. Based on detailed transmission electron microscopy studies we show that the hexagonal phase fraction in (Al0.76Cr0.24)2O3 and (Al0.70Cr0.25Fe0.05)2O3 coatings correlates with the presence of small spherical Cr- and Fe-enriched macroparticles. The smaller these particles are, the more likely the crystallising regions on these particles exhibit a hexagonal α-structure. This effect is clearly more pronounced in Fe-alloyed (Al,Cr)2O3 coatings. Contrary, the crystallites on macroparticles with a flattened shape—being typically Al-rich—or on rather large macroparticles, primarily exhibit a cubic structure.