Effect of Al/Ti ratio on the mechanical properties and tribological behaviours of TiAlN coatings deposited by multi-arc ion plating method

This article presents a systematic study of the effect of Al/Ti ratio of the TiAlN coatings deposited on high-speed steel W18Cr4V substrates on the mechanical properties and tribological behaviours. Three different Al/Ti-ratio TiAlN coatings, with the targets of Ti(60)Al(40), Ti(50)Al(50), and Ti(25)Al(75), were prepared using the multi-arc ion plating deposition method. X-ray diffraction analyses revealed that all three coatings exhibited a face-centred cubic crystal structure. AES analyses showed that the chemical compositions of the coatings were different from those of the targets, and the Al/Ti ratios were changed during the deposition. Mechanical and wear tests showed that the Al/Ti ratio had a profound effect on the strength and tribological properties of the coatings. Using nano-indentation techniques, the measured nano-hardnesses and Young's moduli increased with increasing the Al/Ti ratio. Scratch tests showed that lower Al/Ti ratio improved the adhesion strengths of the coatings. Wear tests proved that the coefficients of friction were reduced by increasing the Al/Ti ratio. Moreover, two different balls, steel, and ceramics, were separately tested on the TiAlN-coated steel discs using a conventional dry ball-on-disc sliding wear apparatus. Both friction coefficient curves and micro-graphs revealed distinctly different wear mechanisms. It was found that the adhesive wear behaviour was dominant between the TiAlN coating layers and steel, while the abrasive and fatigue wear behaviour prevailed in the case of the ceramic ball. The evolutions of the coating micro-structures during the wear tests were examined by scanning electron microscope. All experimental results suggested that highest Al content, Ti(14.8)Al(85.2)N coating, had the best combined mechanical and tribological properties.