Thermally activated transformations in metastable alumina coatings prepared by magnetron sputtering

An as-deposited metastable structure of alumina coatings can be of essential importance for the transformation processes occurring in the coatings during their annealing. The present study focuses on thermal stability and phase transformations in magnetron sputtered alumina coatings with different as-deposited structures characterized by different ratios of an amorphous a-Al2O3 to crystalline γ-Al2O3 phase. Purely amorphous, purely γ-phase and γ-phase/amorphous coatings were investigated by a combination of differential scanning calorimetry (40 °C/min in air) and ex-situ X-ray diffraction analyses. Indentation testing and optical characterization were also carried out. In the coatings with a purely or partially amorphous as-deposited structure two exothermic transformations occurred. An a-to-γ transformation proceeded at ~ 900 °C while a γ-to-α transformation proceeded at ~ 1200 °C. In the crystalline γ-phase as-deposited coating the γ-to-α transformation was the only exothermic transformation observed at the same temperature of ~ 1200 °C. Independently of the as-deposited structure, the γ-to-α transformation was accompanied by the formation of the intermediate metastable θ phase without any detectable heat released. Measured changes in transformation enthalpies for the a-to-γ and γ-to-α transformations were close to each other while the activation energy was higher for the γ-to-α transformation. Hardness, effective Young's modulus and refractive index showed a tendency to increase with a decreasing amount of an amorphous phase in the coating.