YOUNGS MODULUS AND POISSONS RATIO OF CVD DIAMOND

The elastic modulus E of diamond is often set equal to 1/s11 = 1050 GPa, which assumes that it does not vary much with orientation, and many authors use nu = 0.2 as an appropriate average value of Poisson's ratio, which is incorrect. In fact, since the elastic constants of diamond are known with great accuracy, it is a straightforward matter to derive exact numbers for E and nu that take into consideration the stress direction and the intrinsic anisotropy as well as the crystalline configuration. For diamonds synthesized by chemical vapor deposition (CVD) we find that in a first approximation the Hershey-Kroner-Eshelby averaging procedure yields acceptable numbers, EBAR = 1143 GPa and nuBAR = 0.0691, which are quite compatible with available experimental evidence. Our measurements of the biaxial modulus E' = E/(1 - nu) make use of the bulge test method to characterize the elastic behavior of both microwave-power-and hot-filament-assisted CVD diamond films. High quality deposits yield E' almost-equal-to 1180 and 1220 GPa for randomly orientated and (110)-textured deposits respectively; these results confirm that state-of-the-art deposits exhibit elastic properties that are in accord with the measured stiffnesses of natural single-crystal diamond. The residual hydrogen content strongly impacts the elastic behavior and appears to be responsible for the degradation of the modulus observed in this and previous work.