IMPROVEMENT OF TA FILAMENT FOR DIAMOND CVD

In this study, Ta wires 0.5 mm in diameter were carburized for 10-200 min at 2500 degrees C in H-2-0.5-3% CH4, 30 Torr, during which time th e electric resistance and elongation of the filament were measured. Af ter carburization, bending tests and S.E.M. observations were performe d. Carburization proceeds in the sequence of Ta, Ta2C and TaC, forming concentric circular carbide layers in cross-section. Filament bending strength initially decreases to minima of 2-20 kgf/mm(2) at about 20- 60 min. However, it recovers up to 30-60 kgf/mm(2) with further carbur ization. Filament embrittlement can thus be overcome through full carb urization. Carburization time at the minimum strength, t(b), roughly c orresponds to the time at which the Ta2C layer disappears from the fil ament cross-section. Complete carburization results in filament expans ion of about 5-8%. Electric resistance initially increases to a maximu m at about 10-20 min, 2-3 times of which is roughly equal to t(b). (C) 1998 Elsevier Science S.A.