HIGH-TEMPERATURE YOUNGS MODULUS OF POLYCRYSTALLINE DIAMOND

The Young's modulus of polycrystalline diamond grown by microwave assi sted chemical vapour deposition was determined by a dynamic three-poin t bending measurement between room temperature and 750 degrees C. The room temperature Young's modulus was approximately one-half of the the oretical value of 1143 GPa. The lower Young's modulus was traced back to voids and microcracks and therefore a smaller effective sample cros s-section, Also, the density of the the polycrystalline samples of 3.3 87 g cm(-3) was smaller compared with single crystal diamond. Up to te mperatures of 600 degrees C the Young's modulus rt?mains approximately constant, At higher temperatures the onset of diamond etching in air leads to a strong reduction of the Young's modulus. At 750 degrees C t he Young's modulus drops linearly with -10.4 GPa/min to one-thired of its initial value before sample fracture occurs. The onset of diamond etching al these temperatures was proved by thermogravimetric measurem ents. (C) 1997 Elsevier Science S.A.