Oxidation behavior of the TiAlN hard coating in the process of recycling coated hardmetal scrap

Abstract

Recycling coated hardmetal scraps is becoming increasingly important for tungsten resource recovery. However, the coatings in these materials are one of the biggest problems, especially Al-containing coatings. In this study, discarded TiAlN-coated WC–Co hardmetal tool tips were isothermally oxidized at 900 °C in air, during which the final oxide, phase transition and microstructure evolution were investigated. Milled powders below 0.15 mm were completely oxidized in 180 min, and pieces of coatings were found in the final oxides. White WO₃ was mainly distributed on defect-rich areas of oxide scale surfaces. Furthermore, the final oxide scale was triple-layered, mainly consisting of the WO₃-concentrated outmost layer, the Al₂O₃-concentrated middle layer, and the TiO₂-concentrated inner layer. It is different from the bi-layered Al₂O₃/TiO₂ oxide scale that appeared for a new TiAlN-coated hardmetal during an oxidation resistance test. This was attributed to the defects in hardmetal scraps, which provided a fast pathway for element diffusion and volatilization of WO₃. Consequently, it was impossible to remove Al₂O₃ completely.