Murakami and H₂SO₄/H₂O₂ Pretreatment of WC-Co Hard Metal Substrates to Increase the Adhesion of CVD Diamond Coatings

Abstract
Distinct improvement in adhesion of diamond coatings on hard metal substrates was achieved by following substrate surface pretreatment [1] : First WC was removed from the substrate surface by etching with Murakami solution (K₃[Fe(CN)₆] in KOH), after which the Co binder network was etched with H₂SO₄/H₂O₂ solution. Then diamond coatings were deposited on the substrates by hot-filament CVD. Apparently during the H₂SO₄/H₂O₂ treatment of the Co binder a thin CoO / CoSO₄ film forms, which prior to the diamond deposition would be reduced to CoS by the hydrogen and carbon species involved in the diamond synthesis. Since CoS is a stable compound at the diamond daposition temperatures, its Co vapor pressure and the Co surface mobility should be substantially lower than that for metallic Co. Thus the diamond/hard metal interface is no longer influenced detrimentally by the metallic Co binder. This explanation has however not yet been confirmed by analytical methods such as SIMS and XPS. An alternative explanation would be the increased roughness of the hard metal surface resulting from the removal of the Co binder and the formation of deep grooves on the interface which could lead to the improved adhesion. This however does not explain why Co migration and other detrimental effects are not observed.