On the mechanism of intensified plasma-assisted processing

Intensified plasma-assisted processing (IPAP), is a surface modification technique developed recently in our laboratory. Plasma intensification is accomplished by triode discharge and can be utilized for low-pressure, low-temperature diffusion treatments and synthesis of a range of advanced compounds. The energetic flux of ions and neutrals generated in IPAP can create highly favorable surface conditions producing significant improvements in properties ranging from thermal and chemical stability to wear resistance. The objective of the present study was to investigate further the role of energetic particle bombardment on the modification process. IPAP nitriding experiments were carried out on Ti-6Al-4V substrates at specific nitrogen flux levels and flux energies. Parallel experiments were performed by low-energy, broad-beam nitrogen implantation at comparable ion energies and dose levels. Nitrogen concentrations and penetration depths were determined by depth profiling using AES and microhardness measurements. The experimental results were used to derive effective nitrogen diffusivities to assess the effect of the energetic particle bombardment on the diffusion process. For both IPAP and ion implantation, nitrogen diffusion into titanium nitride and solid solution layers is greatly accelerated compared to conventional ion nitriding. IPAP is almost as effective as low-energy ion implantation in enhancing nitrogen penetration when the two processes use ions of similar energies and dose levels. The energetic neutrals present in IPAP appear to further enhance the growth of surface nitride layers. Analysis also indicates that an optimal ion energy may exist.