Carburizing of tantalum by radio-frequency plasma assisted chemical vapor deposition

Tantalum carbide (TaC) has great potential as an alternative to tantalum and tantalum oxide for applications requiring thermal stability and corrosion resistance. In this study TaC layers were produced by inductive rf plasma-assisted chemical vapor deposition that combines diffusion with chemical vapor deposition. The maximum temperature of the tantalum substrates measured during a 6 h processing time was 900 degrees C using Ar-CH4 or Ar-CH4-H-2 gas mixtures. The microstructure of the layers was characterized by X-ray diffraction and Auger electron spectroscopy, and the mechanical properties were studied by micro- and nanoindentation and by microscratch techniques. A close correlation among the carburizing parameters, the microstructure, the mechanical behavior of the layers, and the corrosion resistance was found. The best performing films, several mu m thick, consisting of TaC phase with the highest hardness (similar to 25 GPa), were obtained under the following conditions: input power of 1400 W, pressure of 40-60 mbar, and substrate located at the center of the rf coil. The effect of gas composition, gas pressure, and substrate temperature on the layer composition, the TaC/Ta2C phase ratio, and the mechanical characteristics and chemical stability is presented and discussed. A mechanism of carburizing of tantalum in an inductive rf plasma is proposed. (C) 2000 American Vacuum Society. [S0734-2101(00)10504-4].