Surface treatment on tungsten carbide substrate prior to hot filament chemical vapour deposition diamond coating

Abstract

Deposition of a large and uniform distribution of diamond grains with good adherent and high quality of diamond coating using chemical vapour deposition (CVD) technique is a challenge. Large diamond grains reduce the adhesion strength between the diamond coating and substrate material. The aim of this research is to develop a large and uniform distribution of grain size with high quality and good adhesion strength of diamond coating coated on WC–6wt% Co for grinding application. The research started with the determination of suitable etchant and etching time that can provide high surface roughness and lower Co content of WC substrate. The best concentration of SiC (174 µm) of 1, 5 and 10 g/l mixed with diamond (0.5 µm) powders with a concentration of 0.8 g/l for seeding was also determined. The diamond coating was performed for 30 hours using hot filament CVD unit. The field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectra results indicate that all the coated samples have well faceted grains of (111) and (220) morphologies with high quality (>99% purity) of diamond coatings. The atomic force microscopy (AFM) shows the grain size formed was in the range of 1-6 µm with a density of above 108 grains/cm2. Sample treated with HNO3 + H2O2 for 60 seconds however had the highest adhesion strength measured by sand blasting. It is due to the lowest surface Co content and highest diamond grain density. The WC substrates etched with this etchant then seeded with 1 g/l of SiC mixed with 0.8 g/l of diamond powders was found to have diamond coating with sharp peaks with uniform height and gaps between diamond grains when observed using FESEM and AFM. This condition fulfilled the requirement for grinding application and at the same time having the highest quality (99.496% purity) and adhesion strength. To determine the grinding performance, WC grinding wheels were fabricated and treated with HNO3 + H2O2 for 60 seconds, seeded with 1 g/l of SiC mixed with 0.8 g/l of diamond powders and diamond coated for 30 hours. The grinding process was performed on WC-2wt% Co work piece using ultra-precision grinding machine at different feed rates of 0.015, 0.030, 0.045, 0.060 and 0.075 mm/min respectively. The diamond grain sharpness of the wheel was found unaffected after analysed using FESEM while the surface finish (Ra) of the work piece was finer from 0.020 µm to 0.007 µm. When compared with the commercial diamond bonded wheel grinding at 0.20 mm/min, the work piece surface finish is almost the same. However, the diamond bonded wheel was severely damaged with clogged grinding chips, broken-off and dislodged diamond particles. Meanwhile, at the diamond coated wheel, only clogging was observed. As a conclusion, mix seeding of diamond (5 µm, 0.8 g/l) and SiC (175 µm, 5 g/l) has produced good diamond coating properties for grinding application which is at par with the diamond bonded wheel.