Effect of Low-Pressure Plasma Nitriding with Hollow Cathode Discharge on the Surface Microstructure of WC-Co Cermet

WC-Co cermet was plasma-nitrided with the assistance of a hollow cathode ion source at 400 degrees C under a vacuum of 3-8 Pa. Hot film chemical vapor deposition (HFCVD) of a diamond coating was carried out on the nitrided specimen, without chemical etching. Scanning electronic microscopy, electron probing microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the surface microstructure of the nitride specimens and the coatings. A thin surface conversion layer with a specific structure was formed, in which the primary Co binder was transformed into Co-rich particles. The Co-rich particles consisted of a gamma-Co core and a Co4N outer layer. This specific surface conversion layer significantly suppresses the out-diffusion and catalytic graphitization of Co during HFCVD. The existent phase, morphology, and density distribution of Co compounds can be tuned by varying the nitriding parameters, such as gas media, ionization ratio, bombardment energy flux, and nitriding duration.