Effect of the substrate temperature on the structure and properties of Al2O3 layers reactively deposited by pulsed magnetron sputtering

Alumina coatings were reactively deposited on steel substrates by pulsed magnetron sputtering at substrate temperatures (T-s) of 330-760 degrees 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 degrees C the Al2O3 layers are amorphous to X-rays, whereas gamma-Al2O3 is detected at a substrate temperature T-s approximate to 480 degrees C. A further increase in substrate temperature to 560 degrees C results in the formation of a pronounced texture of gamma-Al2O3. A phase mixture of textured gamma- and alpha-Al2O3 is deposited at T-s approximate to 690 degrees C. At T-s approximate to 760 degrees C the layer consists completely of alpha-Al2O3 with crystallite sizes of about 1 mu m. The occurrence of the crystalline gamma phase at 480 degrees C is linked with a pronounced increase in hardness from 10 to 19 GPa. The Layer hardness of pure alpha-Al2O3 amounts to 22 GPa and corresponds to the hardness of the bulk material.