Relation of hardness and oxygen flow of Al2O3 coatings deposited by reactive bipolar pulsed magnetron sputtering

Aluminum oxide thin films are widely used because of their excellent properties, especially in terms of chemical, thermal, abrasive and corrosive resistance. But many properties of alumina films are significantly deposition parameters dependent. Since different applications and environments demand different kind of properties in thin films, it is important to determine the influence of the deposition parameters on the alumina film properties. In this work, different alumina structures were deposited by means of reactive, bipolar, pulsed, magnetron sputtering. In order to find the appropriate parameter combination to synthesize crystalline alumina (for this investigation γ-Al2O3), substrate temperature, power density at the target and oxygen flow were varied. The γ-Al2O3 films were synthesized at 650 °C, 0.2 Pa, 800 W, 1:4 duty cycle, 19.2 kHz, and 11–12% oxygen flow. The structure and morphology of the deposited Al2O3 films were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Since the coating hardness is a decisive factor for many applications, the aim of this paper was to investigate the influence of the oxygen flow on the alumina hardness. It was observed that the hardness and the structure of the PVD-deposited alumina coatings are significantly oxygen flow dependent. The hardness of the alumina films was determined by nanoindentation. It varied between 1 and 25.8 GPa. The hardness increased by increasing oxygen flow until the target reached the poisoned state, where a hardness reduction was clearly observed.