Structure and properties of high-temperature annealed CVD diamond

Effects of high temperature, up to 1700 deg C, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06-1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 deg C. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5-20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460-1640 deg C), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion.