Strength of optical quality polycrystalline CVD diamond

Abstract

The three-point loading method is used to the measure the fracture strength σ f of polycrystalline CVD diamond plates with thickness in the range of h = 0.06–1.0 mm. Optical quality samples grown in a microwave plasma using CH4-H2 gas mixtures show an inherently nonuniform structure, the crystallite size varying (increasing) by 1–2 orders of magnitude in traversing from the substrate side to growth side. The value of σ f approaches ≈ 2200 MPa for the thinnest film when the fine-grained (substrate) side is under tensile stress, reducing with plate thickness down to ≈ 600 MPa at h ≈1000 μm. The strength is approximately a factor of two lower for the substrate side under tensile stress. In general, the material tested follows Hall-Petch relationship—a stress increase with grain size reduction. The fracture statistics are analyzed using a Weibull distribution, and a Weibull modulus m of 6.4 and 4.5 is found for the growth and substrate side under tension, respectively. Young’s modulus E = 1072 ± 153 GPa for polycrystalline diamond is evaluated from the same tests.