Direct measurement of Young’s modulus, fracture strength and fracture toughness of nanocrystalline diamond foil by means of tensile testing

A tensile testing method to directly determine the elastic as well as the fracture properties of self-supporting nanocrystalline diamond foil (NCDF) is introduced. NCDF is the strongest ceramic foil material known, offering a bending strength of more than 4 GPa . The previously presented bending test reflects the influence of the surface morphology on the mechanical properties well, but is limited to a small near-surface volume. Hence only a small number of different flaws in the material can be detected. To investigate the bulk properties of this novel material a tensile testing method, where the stress is evenly distributed over the whole cross-section of the sample, was developed and validated. The interpretation of the tensile strength by the statistical Weibull approach resulted in values of ∼535 MPa and 470 MPa for NCDF grown on silicon and copper templates, respectively. The combination of tensile testing with digital image correlation allows for the direct determination of Young’s modulus. As compared to methods using resonance frequencies this technique does not have limitations regarding the thickness of the specimen. Young’s modulus was found to yield 624 ± 88 GPa for NCDF synthesized on polished silicon and 536 ± 65 MPa for NCDF from an etched silicon template. Furthermore, by stressing the double-edge-notched specimen in this set-up, the fracture toughness was determined to be image.