RELATIVE OXIDATION BEHAVIOR OF CHEMICAL VAPOR-DEPOSITED AND TYPE-IIA NATURAL DIAMONDS

Thermal oxidation characteristics of chemical vapor deposited (CVD) diamond films prepared by the hot filament method along with (111) and (100) oriented type IIa natural diamond wafers were investigated in flowing oxygen at atmospheric pressure and in the temperature range 973-1173 K by thermogravimetry. Partially oxidized samples were also analyzed by x-ray diffraction, Raman spectroscopy, and electron microscopy. On oxidation, diamond films attached to the silicon wafer turned black, while free standing diamond films did not undergo any color change. Both x-ray diffraction and Raman spectroscopy failed to identify the transformation of diamond to nondiamond carbon forms. Electron microscopy and thermogravimetry indicated that CVD films were least resistant to oxidation followed by (111) surface and (100) surface of natural diamond. The oxidation rates of the CVD films which were dominated by (111) faces are close to those of (111) oriented natural diamond wafers. The apparent activation energies for the oxidation of the films, (111) and (100) oriented wafers are 229, 260, and 199 kJ/mole, respectively, suggesting that the films and the wafers oxidize by direct reaction between diamond and oxygen to CO and CO2.