Residual stress in diamond coatings by Synchrotron Radiation XRD

Synchrotron Radiation X-Ray Diffraction (SR XRD) was used to study the residual stress field in diamond coatings produced by hot filament CVD on titanium substrates; taking advantage of the unique features of SR and instrumental geometry of Daresbury's station 2.3 diffractometer, it was possible to measure the residual strain in diamond, titanium and interfacial TiC, on the same sample without moving it. High flux, tunable wavelength (0.5-3 Angstrom) and narrow instrumental profile were essential to perform the experiment. Results showed the presence of an intense compression in diamond (-2.8 GPa), and a tension in TiC and a-Ti (+0.39 GPa and +0.11 GPa, respectively), due to thermal stresses produced by the high temperature CVD process. Diamond was preferentially oriented along [hhO], with large (40-80 nm) and almost defect free crystallites, as demonstrated by the very low microstrain. On the contrary, highly disordered TiC and TiH2 crystals formed at the interface; the high shear stress component developed at the interface is the main factor controlling coating adhesion. Beyond this particular case of study, the present work demonstrated the great potentiality of SR XRD for the characterisation of surfaces and thin films for industrial applications.