Tensile and fatigue strength of free-standing CVD diamond

Chemical vapour deposited (CVD) diamond is finding increased application and it is important to study its strength and fatigue properties. The present paper describes research on a batch of di-electric (optical) grade CVD material. The procedure in the experiments was to use some samples from a batch to obtain an average failure load. Other samples were then stress cycled to a chosen fraction of this average load. If they survived 107 cycles, they were considered to be immune to fatigue at that load, and their strengths were measured using a three-point bend test. Some samples survived at least 95% of their critical fracture stress for 107 cycles without fatiguing. In other experiments, a 4-point (double torsion) rig was used to stress samples with macro-flaws in different environments. The observation that fatigue does not reduce the strength of CVD diamond in normal environments adds to its attractiveness for various applications.