Effect of the substrate temperature on the structure and properties of Al 2O 3 layers reactively deposited by pulsed magnetron sputtering

Abstract

Alumina coatings were reactively deposited on steel substrates by pulsed magnetron sputtering at substrate temperatures ( T s) of 330–760 °C. Investigations into the structure and morphology of the layers were made via XRD and SEM techniques, respectively. As to the layer properties, the hardness was determined by nanoindentation, and the residual stresses were derived from the bending of the coated substrates. At substrate temperatures of less than 330 °C the Al 2O 3 layers are amorphous to X-rays, whereas γ-Al 2O 3 is detected at a substrate temperature T s ≈ 480 °C. A further increase in substrate temperature to 560 °C results in the formation of a pronounced texture of γ-Al 2O 3. A phase mixture of textured γ- and α-Al 2O 3 is deposited at T s ≈ 690 °C. At T s ≈ 760 °C the layer consists completely of α-Al 2O 3 with crystallite sizes of about 1 μm. The occurrence of the crystalline γ phase at 480 °C is linked with a pronounced increase in hardness from 10 to 19 GPa. The layer hardness of pure α-Al 2O 3 amounts to 22 GPa and corresponds to the hardness of the bulk material.