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

The hardness, fracture toughness and dislocation mobility of two natur al diamond single crystals were studied at 1200 degrees C. Tests were carried out on the (011) plane in type IIa zones, in zones which conta ined only B nitrogen defects (type IaB) and in zones containing A, B a nd B' nitrogen defects (type Ia). Two types of indentation were observ ed. In a specimen with alternating type Ia and type IIa zones, indenta tions of ductile type were formed. The specimen did not exhibit latera l cracks, and long slip lines were revealed around the indentation. Th e second specimen with zones of types Ia, IaB and IIa exhibited brittl e indentations. The indentation and the region around it showed the fo rmation of lateral and ring cracks, and slip lines were much shorter t han in the first specimen. This discrepancy between the samples cannot be explained by the influence of A, B and B' nitrogen defects, as zon es 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 fractur e toughness, even at 1200 degrees C.