Residual stress in polycrystalline diamond / Ti-6Al-4V systems

Polycrystalline diamond coatings were deposited on Ti---6Al---4V alloy by HF-CVD, at fixed temperature (650 C) for different deposition times. During the process, thick titanium carbide layers were formed at the metal/diamond interface. X-ray diffraction (XRD) methods were used to assess coating quality, phase composition, texture, and residual macrostress of the diamond/TiC/Ti system. For a better evaluation of the residual stress present in each phase, three independent measurements were performed with synchrotron radiation (SR-XRD). The measured residual strain could be interpreted in terms of a simple axially uniform residual stress model: σ11 = σ22, σ33 = 0, σij = 0 (i ≠ j). Irrespective of film thickness, the residual stress was very intense, compressive both in the diamond layer (approx. −6.5 GPa) and in TiC (approx. −1.4 GPa), and tensile in Ti---6Al---4V (approx. 70 MPa). The high residual strain in the diamond layer affected the results of texture measurements using the traditional pole figure method; more reliable results were obtained by measuring the integrated intensity, rather than peak maximum intensity, as a function of tilting angles.