Selective Stripping Processes for Al-Cr-N Hard Coatings on WC-Co Cemented Carbides

Abstract

Hard coatings like (Al,Cr)N are commonly used to protect cemented carbide cuttings tools from wear and corrosion. While such coatings increase the tool-lifetime, they also hinder the recycling effort of damaged tools. We report a new recycling strategy for coated cemented carbide tools by applying an interlayer between an arc evaporated Al0.7Cr0.3N coating and the WC-Co substrate. By selectively dissolving this interlayer in a concentrated basic solution, the coating spalls off the substrate, which is left intact. Four bases have been tested as saturated solutions: LiOH, NaOH, KOH, and CsOH, of which NaOH showed the highest reactivity. It was therefore used for subsequent investigations. Three different methods have been tested: (1) a metallic Al-doped interlayer region between substrate and coating can be removed in 3 h at 140 °C. Depositing an (2) Al0.7Cr0.3 or (3) Al0.9Cr0.1N interlayer leads to a significantly faster removal compared to the first method. Hereby a substrate could be fully de-coated in 4 h at only 110 °C. The reaction time was also decreased by pre-treating the samples in concentrated HCl, HNO3, or H2SO4 solutions at 90 °C for 10 min before subjecting the samples to the NaOH bath. Such treated substrates were fully de-coated in 3 h at only 90 °C. While HNO3 oxidized the substrates significantly, HCl and H2SO4 caused only negligible substrate damage.