Structure and properties of Al2O3 layers deposited by plasma activated electron beam evaporation

The effects of process parameters substrate temperature, degree of plasma activation and pulsed bias voltage on the structure and properties of Al2O3 layers deposited by electron beam (EB) evaporation at high deposition rates between 1.5 and 3 µm/min are presented. Layers deposited by reactive EB evaporation without plasma activation are amorphous to X-rays and have a porous columnar microstructure up to a substrate temperature of 700°C. The hardness of these layers ranges only to between 2.5 and 8 GPa. A drastic improvement of the layer quality can be achieved by an intense plasma activation during deposition process using a low voltage electron beam of a hollow cathode arc discharge (Hollow cathode Activated Deposition; HAD process). Even at low substrate temperatures, dense, amorphous layers with a hardness of 12 GPa could be deposited. At an enhanced substrate temperature of 700°C the layers consist of the nanocrystalline gamma phase with a grain size between 12 and 15 nm. The additional application of a pulsed bias voltage of 75 V results in the formation of a pronounced <110> fiber texture of columnar gamma nanocrystallites with a lateral grain size between 30 and 40 nm. The development of the texture in the growth direction was investigated by cross-section TEM investigations. A further increase of bias voltage led to a line broadening of the X-ray diffraction reflexes, which can be interpreted by higher defect densities and lower grain size. The hardness of the dense, nanocrystalline gamma Al2O3 layers ranges between 20 and 22 GPa. Thus the application of the process for the deposition of wear resistant layers for cutting tools can be taken into account.