On the phase evolution of arc evaporated Al-Cr-based intermetallics and oxides

(Al,Cr)2O3 solid solutions in corundum-structure are of great potential for protective applications facing severe mechanical and thermal conditions. In order to elucidate the influence of cathode composition and reactive gas flow on the phase formation of (Al1‐xCrx)2O3, Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, and Al0.25Cr0.75 cathodes were utilised to synthesise coatings with and without oxygen reactive gas. Transmission electron microscopy studies reveal successive microstructural modifications from intermetallic coatings towards oxides with M2O3 stoichiometry. The transition from substoichiometric (suggesting intermetallic γ2-Al8Cr5, AlCr2, and/or Cr structures with distorted lattices due to oxygen incorporation) to stoichiometric oxides is connected with the formation of metastable cubic (Al,Cr)2O3 phases for Al-rich compositions. With decreasing Al-content of the cathodes, the critical oxygen flow for this transformation increases (i.e., 380 sccm for Al0.75Cr0.25, 400 sccm for Al0.70Cr0.30, and 420 sccm for Al0.50Cr0.50). Only when using the high-Cr containing Al0.25Cr0.75 cathode, the coatings directly crystallised with corundum-type (Al,Cr)2O3-solid solutions for O2 flow rates above ~ 570 sccm.