Annealing of Diamond-Coated Tools: A Flexible Approach to Enhancing interfacial Fatigue strength and Milling Performance

Diamond coatings deposited on cemented carbide tools are extensively employed in the machining of non-ferrous materials, including aluminum alloys and composites. A key issue for ensuring a sufficient diamond-coated tool life, especially in milling, is the fatigue strength of diamond coating-substrate interface. Nano-crystalline diamond (NCD) coatings deposited on cemented carbide tools are characterized by high residual stresses. The high level of residual stresses in the diamond film structure are attributed to epitaxial and thermal expansion coefficients mismatch of the diamond coating and its cemented-carbide substrate. Thus, the potential for reducing residual stresses in diamond films deposited on cemented carbide inserts, with the aim of improving their effective interfacial fatigue strength and wear resistance, is of high importance.

NCD coatings were deposited on cemented carbide inserts, and a subset of the coated tools was subsequently annealed under vacuum to decrease residual stresses within the film. Inclined impact tests at ambient temperature were performed on both as-deposited and annealed diamond-coated tools to evaluate their effective interfacial fatigue strength. Depending on the applied impact load, damage initiated at the film–substrate interface after a certain number of impacts, leading to coating detachment and lifting. Residual stresses within the diamond films were quantified through finite element method (FEM) analysis of the impact imprints. In addition, milling experiments were conducted using aluminum foam as the workpiece material to assess the cutting performance of the coated tools. A clear correlation was identified between the interfacial fatigue strength of the diamond coatings and their residual stress state as influenced by annealing. The obtained results show that an impressive enhancement of the effective interfacial fatigue strength and milling performance of diamond-coated tools can be achieved by decreasing the structural residual stresses via appropriate adjustment of the annealing duration and temperature.