Temperature dependence of Young's modulus and degradation of chemical vapor deposited diamond

Temperature dependent measurements of Young's modulus were performed for th e first time on black and transparent bulk material of chemical vapor depos ited (CVD) diamond by a dynamic three point bending method in a temperature range from -150 to 850 degrees C. The CVD specimens correspond to a room-t emperature Young's modulus of single crystal diamond (1143 GPa). A lower Yo ung's modulus of polycrystalline diamond is caused by crystal imperfections and impurities. At temperatures between -150 and 600 degrees C (black type ) or -150 and 700 degrees C (transparent type) the Young's modulus is only slightly temperature dependent and decreases monotonically with an average temperature coefficient of -1.027x10(-4) K-1, which is much higher than the oretically expected. At higher temperatures the bending stiffness and appar ent Young's modulus of the diamond beams are drastically reduced to one thi rd of the initial value before fracture occurs due to oxygen etching effect s in air. The onset temperature of this degradation phenomenon and the rate of decline are dependent on grain size, texture and the crystal lattice im perfections of the CVD diamond material. (C) 1999 American Institute of Phy sics. [S0021-8979(99)04723-4].