SYNTHETIC DIAMOND CRYSTAL STRENGTH ENHANCEMENT THROUGH ANNEALING AT 50 KBAR AND 1500 DEGREES-C

High-pressure, high temperature (HPHT) annealing of synthetic type I d iamond crystals at 1200-1700 degrees C and 50-60 kbar was found to ind uce aggregate-nitrogen dissociation and metal coalescence as well as h eal diamond lattice dislocations. For crystals with low levels of meta l inclusions, HPHT annealing was observed to increase the average comp ressive fracture strength of the crystals by apparently strengthening the strongest crystals of the population. Crystals with high metal-con tent, or otherwise of low quality, are weakened by annealing. Strength ening is believed to occur by locally stabilizing the diamond lattice by healing lattice dislocations as well as dispersing nitrogen within the lattice. A general model is presented that ties together these res ults with those of other researchers.