Structure of superhard nanocrystalline (Ti,Al)N layers deposited by reactive pulsed magnetron sputtering

Stoichiometric Ti(sub 1-x)Al(sub x)N layers with 0.55 < x < 0.57 have been deposited by adjusting working point and pulse times for reactive pulsed magnetron sputtering of aluminum and titanium target. By variation of the substrate bias voltage between floating potential and 100 V, the hardness of the layers is drastically raised up to 38 GPa. The aim of the present work is the identification of structural reasons for the achievement of these very high hardness values. XRD investigations with grazing angle of incidence have revealed that the layers consist of two phases with predominantly cubic rock salt and minor amounts of hexagonal wurtzite structure, respectively. The structure of the layer with the maximum hardness has been additionally investigated by cross-section TEM. It can be shown that the microstructure consists of repeatedly interrupted columns with a lateral size of about 25 nm. Within these columns, globulitic nanocrystallites with a grain size between 6 and 12 nm are present. In addition the TEM investigations have revealed that the layer is composed of alternating aluminum rich and titanium rich layers with a period of about 2.9 nm. It is concluded that the maximum hardness values are mainly caused by the presence of this defined superlattice structure, which hinders the formation and movement of dislocations. By GD-OES depth profiling of the chemical composition it can be shown that the layers possess a good oxidation, by the formation of a thin aluminum oxide passivation layer.