Tempering behaviour of TiB2 coatings

Abstract

The compound TiB₂ has been proposed as a wear resistant coating for tools because of its high hardness, but for practical applications it is also necessary to know its thermal and oxidation behaviour. For this reason in the present investigation TiB₂ coatings were magnetron sputtered on to two different steel substrates and annealed in vacuum or air up to 800°C. After deposition the TiB₂ coatings were nanocrystalline with a crystallite size ranging from 15 to 30 nm. The coatings deposited at low bias voltage showed a strong [001] fibre texture. With increasing bias the grain size decreased and the strong texture was lost. These structural modifications influenced the layer hardness, which decreased from about 30 to about 25 GPa at - 200 V bias, even though these lower hardness layers had higher residual compressive stresses. The oxidation of TiB₂ started at 600°C. At this temperature the formation of TiO_x was detectable. Moreover, the coating strength was overcome and a characteristic crack pattern was generated. This temperature therefore limits the practical application of TiB₂ coatings.