Phase composition and transformations in magnetron-sputtered (Al,V)2O3 coatings

Abstract Coatings of (Al 1-x V x ) 2 O 3 , with x ranging from 0 to 1, were deposited by pulsed DC reactive sputter deposition on Si(100) at a temperature of 550 °C. XRD showed three different crystal structures depending on V-metal fraction in the coating: α-V 2 O 3 rhombohedral structure for 100 at.% V, a defect spinel structure for the intermediate region, 63–42 at.% V. At lower V-content, 18 and 7 at.%, a gamma-alumina-like solid solution was observed, shifted to larger d-spacing compared to pure γ-Al 2 O 3 . The microstructure changes from large columnar faceted grains for α-V 2 O 3 to smaller equiaxed grains when lowering the vanadium content towards pure γ-Al 2 O 3 . Annealing in air resulted in formation of V 2 O 5 crystals on the surface of the coating after annealing to 500 °C for 42 at.% V and 700 °C for 18 at.% V metal fraction respectively. The highest thermal stability was shown for pure γ-Al 2 O 3 -coating, which transformed to α-Al 2 O 3 after annealing to 1100 °C. Highest hardness was observed for the Al-rich oxides, ~24 GPa. The latter decreased with increasing V-content, larger than 7 at.% V metal fraction. The measured hardness after annealing in air decreased in conjunction with the onset of further oxidation of the coatings.