Effect of Nitrogen Feedgas Addition on the Mechanical Properties of Nano-Structured Carbon Coatings

Chemical vapor deposition was employed to grow nano-structured carbon films on a titanium alloy (Ti-6Al-4V) using different N₂/CH₄ feedgas ratios in a balance of H₂. Of particular interest is a determination of how the film hardness changes with N₂/CH₄ feedgas ratio and how this measurement correlates with the film structure as determined using micro-Raman spectroscopy, X-ray diffraction, and atomic force microscopy. We find that the broad Raman peak at 1550 cm^-1 (attributed predominantly to tetrahedral amorphous carbon) becomes more intense and sharp with increasing N₂/CH₄ ratio and that the relative concentrations of CH₄ and N₂ are critically linked to the hardness of the film. The combination of high film hardness, low surface roughness, and good toughness is highly desirable (especially for deposition on metals) and these properties can be optimized by manipulation of the N₂ and CH₄ feedgas concentrations.