Characterization of carburized tantalum layers prepared in inductive RF plasma

Tantalum surfaces were carburized in order to improve their mechanical properties and corrosion resistance. The carburized layers were produced in an inductively coupled radio-frequency (r.f.) plasma using argon/methane or argon/methane/hydrogen mixtures, and substrate temperatures of between 773 and 1123 K, while the main process variables were: total gas pressure, p (6-100 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, Auger electron spectroscopy, and temperature-programmed desorption using a mass spectrometer. The mechanical properties were measured by microindentation and microscratch techniques, and the corrosion resistance was examined by impedance analysis. For the same treatment time, it was observed that the thickness of the carburized layer and the phase content (TaC or Ta₂C) were different for three distinct ranges of fabrication conditions: (a) p < 30 torr and/or P < 1000 W, and a thin layer ≤1 μm 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_v ∼ 25 GPa); and (c) p ∼ 100 torr and/or P ∼ 2000 W when carbon films were formed. The effect of these fabrication conditions on corrosion resistance, surface chemical composition, TaC:Ta₂C phase ratio, and the carburizing mechanism are discussed.