Structure, mechanical and tribological properties of sputtered Ti1–xAlxN coatings with 0.5≤x≤0.75

Ti1–xAlxN hard coatings are already successfully applied for cutting tool applications with extreme conditions. Special emphasis in this field is laid on high hardness, oxidation resistance and superior tribological properties. The aim of this work is to present a comprehensive study on the influence of the Al content on coating microstructure and related mechanical and tribological properties. A DC magnetron sputtering system was used to deposit coatings in an Ar+N2 discharge at constant N2 partial pressure and bias voltage onto high-speed steel substrates. Ti1–xTAlxT targets with atomic ratios xT=0.5, 0.6, 0.67 and 0.75 were used. By X-ray diffraction and transmission microscopy the fcc single-phase coatings at low Al contents and dual-phase or hcp coatings at higher Al contents are investigated in detail. Hardness measurements showed high values of 33 GPa for x=0.54 in the coating, decreasing with increasing Al to values of 19 GPa at x=0.76. Friction coefficients against stainless steel balls were high at room temperature with values around 1.5, but decreased significantly at higher temperatures to 0.88 at 700 °C. The wear performance was better for dual-phase and hcp coatings with high Al contents compared to fcc coatings. This investigation shows clearly the relations between target and coating composition, where the resulting structure specifies their mechanical and tribological properties.