Influence of internal diffusion barriers on carbon diffusion in pure titanium and Ti-6Al-4V during diamond deposition

Diamond coatings were deposited on pure titanium and Ti-6Al-4V, at a temperature in the range of 600-750 degrees C, in a microwave plasma from CH4/H-2 and CO/H-2 mixtures. The influence on carbon diffusion of different intermediate layers, especially tungsten, niobium, titanium nitride and pure titanium previously deposited on titanium alloys by physical vapor deposition (PVD) is reported. These intermediate layers are always composed of at least two sub-layers: (1) an internal diffusion barrier and (2) an external titanium layer that allows some carbon diffusion to be maintained. After diamond deposition, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) observations coupled with energy-dispersive X-ray (EDX) analysis of the final multilayer systems allow us to determine the diffracting phases, their lattice parameters and the efficiency of the different barriers. The carbon diffusion coefficients in the titanium carbide phase and in the alpha-titanium solid solution are deduced from an experimental study carried out on pure titanium with or without an underlying diffusion barrier. The results are compared to the carbon diffusion in Ti-6Al-4V alloy. This work permitted us to calculate the carbon concentration profiles in both pure titanium and Ti-6Al-4V substrates. (C) 1999 Elsevier Science S.A. All rights reserved.