Effect of plasma activation on the phase transformations of aluminum oxide

The deposition of protective, anticorrosive and wear resistant layers onto large-area metal substrates by physical vapor deposition techniques is gaining increasingly in industrial significance. This contribution deals with the deposition of aluminum oxide onto stainless steel. The aluminum oxide was deposited by reactive high-rate electron beam evaporation. Plasma activation of the vapor took place via a hollow-cathode plasma. The substrates were coated at a temperature of 500 and 700 degrees C and subsequently postannealed in vacuo at 600, 800, 1000 and 1200 degrees C. At a substrate temperature of 500 degrees C with and without plasma activation Al2O3 is deposited with different densities and amorphous to X-rays. At 700 degrees C without plasma activation, however, gamma-Al2O3 is found in an amorphous matrix whereas plasma activation results in alpha-Al2O3 in an amorphous matrix. The effects of plasma activation and post-annealing temperature on the sequence of Al2O3 phase transformations were investigated. The hardness of the layers increases noticeably with plasma activation and post-annealing temperature. In addition, layers deposited under plasma activation exhibit better thermal stability with respect to volume contraction. Phase identification was done using X-ray diffraction, and transmission electron microscopy with selected area diffraction. The morphology of the layers was investigated by scanning electron microscopy and the hardness was measured by nanoindentation.