High-temperature oxidation behavior of nanocrystalline diamond films

In this study, high-temperature stability of nanocrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD) was investigated through differential thermal analysis/thermal gravimetric analysis (DTA/TGA), thermal analyses, visible and UV Raman analysis, and XPS analysis. Nanocrystalline diamond films with crystalline size of about 25 nm were obtained, which possess high density of grain boundaries with a high sp2-bonding of non-diamond carbon. In the initial stage of oxidation, grain boundaries with non-diamond carbon and graphite phase were preferentially etched away by oxygen, which carries on with a faster rate. Then, a slower oxidation rate was continued through reacting the carbon atoms in rigid diamond structure with oxygen. The activation energies for former and the latter, calculated from the thermal analysis, are 195 and 217 kJ/mol, respectively. Both the results of in-situ Raman spectra and XPS spectra indicate that the carbon with sp2-bonding significantly decreased with increasing temperature and soaking time.