Characterisation and performance of partially filtered arc TiAlN coatings

It is now being recognised that the level of residual stress in physical vapour deposited (PVD) coatings is yet another important physical property of thin film PVD coating parameters that can critically affect their performance. In this study, titanium aluminium nitride (TiAlN) coatings were deposited using a dual source filtered arc evaporation system with a view to determine the effect of bias voltage increased from −50 to −250 V, the residual stress in the coating as well as the adhesion and cutting tool performance. The X-ray diffraction methods of Bragg–Brentano and glancing angle parallel beam (sin2ψ) were used to study the texture and residual stress in the as-deposited coatings. The results showed that as the bias voltage increased from –50 to –250 V and the residual stress increased from 7.67 to 11.81 GPa (compressive). However, little change in residual stress was observed with an increase in arc current from 75 to 175 A. All coatings exhibited a preferred orientation in the {111} direction, however, a reduction in the {111} intensity was observed for the coating deposited at –50 V. Coating hardness was observed to increase from 26.3 to 31.7 GPa when the bias voltage was increased from –50 to –150 V. However, no further increase was observed at –250 V. No effect on hardness was noted for any change in arc current. Scratch adhesion results showed little effect on the bias voltage with critical load values of 46, 48 and 43 N for the −50, −150 and −250 V biased coatings, respectively. However, increasing the arc currents above 75 A resulted in a reduction of critical load from 48 N for 75 A to 36 and 37 N for 125 and 175 A arc currents, respectively. Damiler Benz Rockwell ‘C’ adhesion tests carried out also revealed a similar trend. The Al content of the coating was found to decrease with increasing bias voltage but increase with an increase in arc current. Accelerated drill life tests suggested that an increase in residual stress associated with the higher bias voltages does affect tool life. For arc current variation, the balance between intrinsic and extrinsic stresses that contribute to the total residual stress may affect cutting tool life.