Determination of linear and nonlinear mechanical properties of diamond by laser-based surface acoustic waves

The laser-based excitation of surface acoustic wave (SAW) narrowband trains and broadband pulses is described. SAW detection with a piezoelectric foil transducer was used to determine the Young's modulus of rough nano- and microcrystalline diamond films with linear SAWs. For high quality films ~ 1000 GPa is approached, near the stiffness of ideal polycrystalline diamond of 1143 GPa, indicating that this property is predominantly controlled by the diamond crystallites and not the grain boundaries. With strongly nonlinear SAW pulses developing shocks, fracture experiments were performed for well-defined crystallographic geometries of single-crystal silicon. The critical stresses of several gigapascals and strains of about 0.01 realized in these measurements are sufficient to determine the strength of nano- and microcrystalline diamond films, as well as natural single-crystal diamond. Their strength is one to two orders of magnitude smaller than the ideal tensile (shear) strength of 95 GPa (93 GPa), and thus reaches only several gigapascals, accessible by the pulsed SAW method. The surprisingly low strength of diamond materials indicates that the quality of the grain boundaries and defects plays an essential role in the fracture process.