| The hardness, elastic modulus, subcritical crack growth and fracture toughness of chemical vapor deposited (CVD) polycrystalline diamond films have been investigated on thick ({dollar}sim{dollar}100 to 300 {dollar}mu{dollar}m) free-standing films with regard to the composition, microstructure, failure mechanisms and measurement techniques. The rationale for this study was the uncertainty in measuring these properties in previous research and the variability in the composition and microstructure of the material, which may affect these properties. Two predominant micro-hardness measurement techniques, namely Vickers and Knoop indentation, were employed. Existing Young's modulus measurement techniques such as dynamic resonance and nano-indentation were reviewed for modulus measurement on these films. The validity of indentation fracture toughness measurement for CVD diamond films using micro-hardness indentation has been established based on comparison with the conventional method of tensile testing of pre-notched compact-tension samples. The fracture toughness, {dollar}Ksb{lcub}rm c{rcub}{dollar}, of diamond was measured using indentation methods and for the first time by the tensile testing of pre-notched fracture-mechanics type compact-tension samples. Measured {dollar}Ksb{lcub}rm c{rcub}{dollar} values were found to be between 5 and 7 MPa-m{dollar}sp{lcub}1/2{rcub}{dollar} by either method. Studies on subcritical crack growth (i.e., at stress intensities less than {dollar}Ksb{lcub}rm c{rcub}{dollar}) indicated that CVD diamond is essentially immune to stress-corrosion cracking under sustained loads in room air, water and acid environments. Extensive studies of the microstructure and mechanisms of failure were conducted. A commonly known toughening mechanism for ceramics by weakening the grain boundary in order to promote intergranular failure and grain bridging, has been implemented to improve the toughness of CVD diamond films. Several films with nominally the same thickness but small differences in their non-diamond content were studied and a significant variation in the toughness measurements was observed. Raman spectroscopy provided the most sensitive measure of the relative non-diamond carbon content in diamond films. Correlation of fracture toughness with changes in non-diamond carbon content and microstructure was established by studying the crack path, fracture surfaces and the location of non-diamond phase using atomic force microscopy, high magnification scanning electron microscopy and transmission electron microscopy. |
