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

• Published in: Journal of alloys and compounds 2022-02, Vol.893, p.162325, Article 162325
• Main Authors: Peng, Jihua, Zeng, Jiwei, Xiao, Yang, Li, Weiqiu
• Format: Article
• Language: English
• Subjects: Adhesion; Annealing; Carburization (corrosion); Carburizing; Cemented carbides; Chemical vapor deposition; Conversion annealing; Decomposition reactions; Diamond coating; Diamond films; Graphitization; HFCVD; Intermetallic phases; Nucleation; Photoelectrons; Pretreatment; Raman spectroscopy; Rockwell hardness tests; Spectrum analysis; Substrates; Surface layers; Tungsten carbide; WC-Co cemented carbide; X ray photoelectron spectroscopy
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2021.162325

•Changing the surface microstructure of hard metal YG6 by conversion annealing.•Adjusting the constituent phases of conversion layer to be Co3W and WC.•Encapsulating γ-Co(W) transformed from Co3W during HFCVD initiation stage.•Depositing diamond coating with adhesion HF 2–3 onto YG6 without chemical etching. [Display omitted] 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 γ-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 γ-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.