NUCLEATION AND GROWTH OF DIAMOND FILMS ON NI-CEMENTED TUNGSTEN CARBIDE .2. EFFECTS OF DEPOSITION CONDITIONS

Diamond films were deposited by hot-filament chemical vapor deposition (HFCVD) on substrates made of WC sintered with 6 wt% of Ni. The as-ground substrates were scratched with diamond powder (S samples) or scratched and wet-etched (SE samples), Diamond synthesis was carried out at substrate temperatures ranging between 600 degrees and 1050 degrees C, and using 1.0% or 2.0% CH4 in H-2. The diamond nucleation density, as measured by scanning electron microscopy (SEM) and automatic image analysis (AIA), did not significantly change in the 600 degrees-900 degrees C temperature range, while at substrate temperatures higher than 900 degrees C a steep decrease of the density of nuclei was observed and attributed to the thermal annealing of nucleation sites. The activation energy of the growth process was measured and found to be 21 +/- 2 kcal/mol, Neither nucleation density nor growth rate were affected by an increase of CH4 concentration in the feed gas, while a lack of crystallinity was observed at the higher methane concentration, Raman analysis showed that phase purity of the films was affected mainly by the substrate temperature: the lower the temperature, the better the film quality. The presence of Ni on the substrate surface did not induce the preferential formation of non-diamond carbon phases, as confirmed by comparing the Raman spectra obtained from both S and SE substrates, As a comparison, continuous films were deposited on scratched WC-5 wt% Co substrates under the same experimental conditions. The results indicated that the use of Ni as a binder is preferable to Co.