Research on the oxidation resistance and ultra-high frequency thermal fatigue shock failure mechanisms of the bilayer and multilayer nano-coatings on cemented carbide tools

The multilayer nano-coating shows excellent thermal stability and oxidation resistance which could be attributed to the multielement composition and multilayer structure. It is widely used in the machining of hard-to-cut materials, such as titanium alloy. The formation of serrated chip during cutting process of Ti-6Al-4V would result in the ultra-high frequency thermal fatigue shock on the coating surface, and this could induce the thermal fatigue failure of coating. In this study, the ultra-high frequency (20 kHz) thermal fatigue shock testing was developed to investigate the thermal fatigue failure mechanisms of the bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating. It was achieved by using the pulsed laser system to irradiate the coating surface under different shock cycles, which could produce alternating thermal stress on coating surface. This creates a close simulation of the actual working conditions in high-speed machining of Ti-6Al-4V with serrated chip produced. Moreover, the high temperature oxidation experiments were performed to study the oxidation resistance of coating. The failure mechanisms of coating/substrate system under the constant thermal loads and dynamic thermal loads were compared and analyzed. The cutting performance of related coated tools was evaluated after turning Ti-6Al-4V. The correlation between the thermal fatigue shock resistance of coatings at ultra-high frequency and the wear behavior of coated tools was built. It can be concluded that the multilayer TiSiN/TiAlN nano-coating showed better properties of the resistance to thermal fatigue shock in machining Ti-6Al-4V.