Reactivity of PVD cathodic arc coated hardmetal tools with Inconel 718: Correlation between diffusion couples and tool wear in drilling tests

The reactivity of Al_0.66Ti_0.33N, Al_0.31Ti_0.62Si_0.07N, Al_0.60Cr_0.40N, and Al_0.38Cr_0.54Si_0.07N cathodic arc-coated hardmetal tools with Inconel 718 has been evaluated by using diffusion couple experiments at 900 °C and 1100 °C. As expected, a strong reaction occurs at these temperatures between Inconel 718 and uncoated hardmetal samples leading to the formation of η phases; mainly M₁₂C and M₆C carbides. PVD coatings act as diffusion barriers limiting the presence of such reactions as long as they maintain their structure in contact with Inconel 718 at high temperatures. Diffusion couple experiments confirm that Cr-containing coatings are significantly more unstable than Al_0.66Ti_0.33N or Al_0.31Ti_0.62Si_0.07N materials, thus leading to notable differences in cutting performance. The relation between diffusion couple results and drill wear mechanisms in drilling Inconel 718 with different coating materials are also analysed. The best results are obtained either with Al_0.66Ti_0.33N or Al_0.31Ti_0.62Si_0.07N coated drills, where the coating remains longer time at the cutting edge leading to more efficient protection, especially at the drill corner.