Young's modulus and Poisson's ratio of CVD diamond

Abstract

The elastic modulus E of diamond is often set equal to 1 s 11 = 1050 GPa , which assumes that it does not vary much with orientation, and many authors use v = 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 v 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-Kröner-Eshelby averaging procedure yields acceptable numbers, E ̄ = 1143 GPa and v ̄ = 0.0691 , which are quite compatible with available experimental evidence. Our measurements of the biaxial modulus E′ = E (1 − v) make use of the bulge test method to characterize the elastic behavior of both microwavepower- and hot-filament-assisted CVD diamond films. High quality deposits yield ′ E ≈ 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.