Substrate has an essential influence on the growth process of diamond grains and the quality of the diamond coating. In this paper, the diamond coatings on Co-cemented tungsten carbide (WC-Co) and silicon nitride (Si3N4) substrates were deposited by hot filament chemical vapor deposition (HFCVD) technique to study the effect of growth process on adhesive strength of coating. The WC-Co and Si3N4 substrates were treated by etching and polishing process, respectively. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the diamond coatings. Furthermore, the adhesive strength and tribological properties of the as-deposited diamond coatings were evaluated by indentation tests and dry sliding tests. The results showed that the diamond coating deposited on WC-Co had a considerable stronger adhesive strength and a higher friction coefficient than that on Si3N4, which may partly result from the flat surface morphologies of Si3N4 substrate that was due to no etching pretreatment. Thereafter, a novel etching method for Si3N4 surfaces by hydrofluoric acid (HF) was proposed to roughen the surface, and a new diamond coating with enhanced adhesive strength was fabricated on Si3N4. Hence, for the hard substrate, etching the surface, rather than polishing the surface, played an importance role on improving the adhesion strength of diamond coating.

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The strong adhesive strength is essential for the widespread applications of diamond coating in the mechanical field. In this paper, a novel method combining conventional chemical vapor deposition (CVD) diamond technique with solid particles was proposed to modify the interface material properties of substrate to improve the adhesive strength of the diamond coating. Prior to deposition, wet dispersion was adopted to uniformly distribute solid particles on the substrate, with which the diamond coatings with the WC, TiC and BN particles embedded at the interface were fabricated on Co-cemented tungsten carbide (WC-Co) substrate. In addition, the pure diamond coating was also fabricated for comparison. The as-deposited diamond coatings were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. The results indicated that the diamond coating with TiC exhibited the highest residual stress, which may be due to the larger lattice mismatch between TiC and diamond as compared to WC and BN. The indentation test suggested that WC and BN particles were favorable for the improvement of the adhesive strength and the crack resistance of diamond coating, while this phenomenon was not observed in the diamond coating with TiC particle. This might be caused by the large lattice parameter, high lattice mismatch, and high thermal expansion coefficient mismatch of TiC as compared with diamond. Furthermore, the lattice parameter of additional particles might be an important factor to determine whether or not the adhesive strength of diamond coating can be enhanced, because it determined the compressive stress or tensile stress generated inside diamond coating. Hence, interface modification of substrate by dispersing the solid particles with low lattice parameter or similar material properties of diamond may be an effective and convivence approach to improve the adhesive strength of the diamond coating.

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