Thermal stability of hard transparent AlxCoCrCuFeNi oxide thin films

Multi-element AlxCoCrCuFeNi (the molar ratio of x = 0.5, 1, and 2) oxide thin films were deposited by reactive sputtering of homogeneous alloy targets. The as-deposited films comprise single cubic-(c-) spinel crystalline phase. The films post-annealed at 500 °C for 5 h in air reveal no phase transformation and no obvious grain growth. Those post-annealed at higher temperature, 700 and 900 °C, show possible formation of tetragonal-(t-) and orthorhombic-(o-) spinel phases resulting in a c/t/o mixed phase as well as precipitates of copper-rich oxide nanorods on the film surface. The amount of c-spinel in the mixed phase increases with increasing x value. The density of the precipitates is dependent on both x value and annealing temperature. Hardness increases for films annealed at 500 °C with the highest value of hardness of 25.7 ± 2.3 GPa. Higher annealing temperatures result in formation of cracks and holes on the films that make the nanoindentation measurements unreliable.