CARBURIZATION OF TUNGSTEN AND TANTALUM FILAMENTS DURING LOW-PRESSURE DIAMOND DEPOSITION

The carburization behavior of tungsten and tantalum filaments in a CH4-H2 gas mixture during hot-filament low-pressure diamond deposition was investigated at different filament temperatures, methane concentrations and gas pressures. Tungsten filaments were fully converted to tungsten carbide-sub-carbide (WC-W2C) at 2000-degrees-C in 24 h. Variation of the methane concentration between 0.5% and 1.0%, and gas pressure between 10 and 40 Torr had little or no effect on the carburization rate. The tantalum filament took longer to convert to tantalum carbide-sub-carbide (TaC-Ta2C). The carburization rate of the tantalum filament increased with increasing methane concentration, gas pressure and filament temperature. At higher methane concentrations, tantalum filaments were coated with carbon at 2000 and 2200-degrees-C, but at 2400-degrees-C no carbon coating was observed. For tungsten filaments under these conditions (2000-degrees-C, 1% CH4 and pressures of 10 and 40 Torr), no carbon coating was observed at all. The possible influence of the carburization process on diamond deposition is discussed.