Novel conversion annealing pretreatment for improved deposition of diamond coatings onto WC-Co cemented carbide

To maintain the substrate strength and toughness of WC-Co cemented carbide as well as improve its adhesion with a diamond coating deposited by hot filament chemical vapor deposition (HFCVD), a novel conversion annealing pretreatment was studied. The pretreated substrates, as-nucleated, and as-grown diamond coating specimens were characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and Rockwell hardness tests. It was found that conversion treatment could effectively convert the Co in the surface layer into intermetallic (Co3W, Co7W) and/or Co-rich compound (M6C, M12C) phases. The conversion layer underwent a re-carburization process during the nucleation stage of HFCVD. The intermetallic phases returned to the gamma-Co (W) phase which will be encapsulated by the freshly formed WC layer. This is beneficial for seeding growth and new diamond nuclei formation. However, those Co-rich carbides consumed much carbon atoms to decompose and react to form a lot of free gamma-Co phase during the nucleation stage, which promoted severe graphitization at the growth interface during HFCVD, and caused the film to peel off. A high-quality microscale diamond coating with an adhesion of the HF 2-3 level could be deposited onto the conversion layer with only the WC and Co3W phases. (C) 2021 Elsevier B.V. All rights reserved.