Designing 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 Al0.7 Cr0.3 N 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) Al0.7 Cr0.3 or (3) Al0.9 Cr0.1N 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