Thermal stability of sputtered Al2O3 coatings

Al2O3 has a high potential as a hard compound for wear and corrosion protection because of its chemical inertness, high corrosion resistance and hardness. This work focuses on the influence of ion bombardment on the thermal stability of sputtered Al2O3 films. An industrial scale sputter system equipped with bipolar pulsed magnetrons was used to grow coatings at 640 °C in an argon-/oxygen atmosphere under different ion bombardment conditions. To evaluate the thermal stability, heat treatments were done in vacuum combined with differential scanning calorimetry. The crystal structure was examined by X-ray diffraction and nanoindentation was used to determine coating hardness.The structure of the coatings grown on silicon substrates is either predominantly X-ray amorphous for low ion bombardment conditions or γ-Al2O3 structured for enhanced ion bombardment. For iron substrates, the formation of γ-Al2O3 is fostered. Two different transformation sequences were found, both ending in the formation of the thermodynamically stable α-Al2O3. While the γ to α-transformation on coatings deposited on iron foil occurs via the transition phase δ-Al2O3, coatings deposited on silicon transform directly into α-Al2O3. The amorphous coatings transform at lower temperatures than the coatings with γ-Al2O3 structure in the as deposited state. Hardness values of 10 GPa for the amorphous coating, 14 GPa for γ-Al2O3 and 22 GPa for α-Al2O3 were measured.