Oxidation behavior of chemically vapor-deposited Sic in11. Experimental Procedure CO-CO, atmospheres (0.1 MPa j was investigated using a thermogravimetric technique at temperatures from 1823 to 1923 K. Active or passive oxidation was observed CVD-SIC (p type) plates were prepared on graphite substrates using SiCI,, C,H,, and H, as source gases. Specimens for oxidation tests were o.5 mm in thickness and mm in diamedepending On temperature and coZ/co partial pressure ratio ('C02/'CO)*The for the tranter. All specimens were polished with diamond paste to 1 ,,m and then ultrasonically cleaned in acetone and ethyl alcohol.
pCOi/pCOMass changes in the specimens were continuously measured by thermogravimetry at temperatures from 1823 to 1923 K. The specimens were suspended from an electrobalance (Model R-100, Cahn Instruments, Cerritos, CA) in an alumina reaction sition was 10' times as large as a theoretical value calculated from the Wagner mode'. In the passive Oxidation above 1873 K, SiO, bubbles were grown-The expansion and ruPture of bubbles caused cyclic rapid mass gain and mass loss.
Active oxidation behavior of CVD-SIC in CO-CO, atmospheres was investigated using a thermogravimetric technique in the temperature range between 1823 and 1923 K. The gas pressure ratio, PcO,/P,,, was controlled between and lo-' at 0.1 MPa. Active oxidation rates (mass loss rates) showed maxima at a certain value of Pc,,,/Pc,, (PcoJPco)*. In a PcoJPco region lower than the (Pco!Pco)*, a carbon layer was formed on the S i c surface. In a Pco,/PcO region higher than the (PcoJPcc,)*, silica particles or a porous silica layer was observed on the SIC surface.
The oxidation behavior of chemically vapor-deposited silicon nitride in N,-0, and Ar-0, atmospheres was studied using a thermogravimetric technique at temperatures 1823 to 1923 K. Active oxidation was observed at low oxygen partial pressures. The active oxidation rates increased with increasing oxygen partial pressure (Po) up to a certain Pol, and then passive oxidation occurred. The transition oxygen partial pressures from active to passive oxidation were determined. The rate-controlling step for the active oxidation could be oxygen diffusion through a gaseous boundary layer near the Si,N, surface. Decomposition of Si,N, does not seem to be associated with the mass loss behavior. The Wagner model was employed to explain the oxidation behavior.
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