Characterization of carburized tantalum layers prepared in inductive RF plasma

Tantalum surfaces were carburized in order to improve their mechanical prop erties and corrosion resistance. The carburized layers were produced in an inductively coupled radio-frequency (r.f.) plasma using argon/methane or ar gon/methane/hydrogen mixtures, and substrate temperatures of between 773 an d 1123 K,while the main process variables were: total gas pressure, p (6-10 0 torr); power, P (0.3-2.0 kW); CH4 concentration (0.1-0.8 vol.%); hydrogen concentration, C-H2 (1-50 vol.%); and process duration, t (0.5-20 h). The carbide phases and film composition were determined by X-ray diffraction, A uger electron spectroscopy, and temperature-programmed desorption using a m ass spectrometer. The mechanical properties were measured by microindentati on and microscratch techniques, and the corrosion resistance was examined b y impedance analysis. For the same treatment time, it was observed that the thickness of the carburized layer and the phase content (TaC or Ta,C) were different for three distinct ranges of fabrication conditions: (a) p < 30 torr and/or P < 1000 W, and a thin layer less than or equal to 1 mum thick was formed with a steep diffusion profile; (b) 40 < p < 60 torr and 1400 < P < 1600 W, and a thick carbide layer (several microns) was formed, mainly consisting of a TaC phase with uniform chemical composition and the highest hardness (H-e similar to 25 Cpa); and (c) p similar to 100 torr and/or P s imilar to 2000 W when carbon films were formed. The effect of these fabrica tion conditions on corrosion resistance, surface chemical composition, TaC: Ta2C phase ratio, and the carburizing mechanism are discussed. (C) 2001 Els evier Science B.V. All rights reserved.