Effects of negative substrate bias voltage on the structure and properties of aluminum oxide films prepared by DC reactive magnetron sputtering

Aluminum oxide thin films (AlxOy) were deposited onto Ni-based superalloy substrates by DC reactive magnetron sputtering using Al target in an Ar and O2 gas environment. Subsequently post-deposition annealing was carried out in air ambient at 500 °C for 1 h with heating rate of 25 °C/min and then some annealed samples were heated at 800 °C for 1 h to simulate the application environment. Effects of the negative substrate bias voltage (Vb) on the structure and properties of the as-deposited, the annealed and the heated films, such as the deposition rate, elemental composition, surface morphology, microstructure, optical transmittance and microhardness, have been studied. It is found that with the increase of negative Vb (0 V, −20 V, −30 V and −40 V), the deposition rate decreases sharply and so do the O/Al ratios of the as-deposited films. While for the annealed films, the O/Al ratios show an increasing trend. All the as-deposited and annealed films are amorphous. However, the heated films are all crystallized and the crystallization gets weaker with the negatively increasing Vb. Grain refinement for the as-deposited films and loose crack surface for the annealed ones are found in the process of negatively increasing Vb to −40 V. And all the annealed films exhibit higher transmittance but lower microhardness than the as-deposited ones.