MECHANICAL-PROPERTIES OF NATURAL DIAMONDS AT 1200-DEGREES-C

The hardness, fracture toughness and dislocation mobility of two natural diamond single crystals were studied at 1200 degrees C. Tests were carried out on the (011) plane in type IIa zones, in zones which contained only B nitrogen defects (type IaB) and in zones containing A, B and B' nitrogen defects (type Ia). Two types of indentation were observed. In a specimen with alternating type Ia and type IIa zones, indentations of ductile type were formed. The specimen did not exhibit lateral cracks, and long slip lines were revealed around the indentation. The second specimen with zones of types Ia, IaB and IIa exhibited brittle indentations. The indentation and the region around it showed the formation of lateral and ring cracks, and slip lines were much shorter than in the first specimen. This discrepancy between the samples cannot be explained by the influence of A, B and B' nitrogen defects, as zones of types Ia and IIa were present in both specimens. It is probably due to different dislocation densities in the specimens. It was found that internal tensile stresses cause a dramatic degradation in fracture toughness, even at 1200 degrees C.