KINETICS AND MECHANISMS OF HIGH-TEMPERATURE CREEP IN CHEMICALLY VAPOR DEPOSITED AND REACTION-BONDED SILICON-CARBIDE

The principal objective of this dissertation research has been the study of the constant compressive stress creep of NC-430 reaction-bonded SiC and a chemically vapor deposited (CVD) SiC in order to determine their creep characteristics and controlling creep mechanisms when subjected to the stresses and temperatures employed in this research. The experiments were performed in a constant stress high-temperature creep apparatus at temperatures ranging from 1673K to 2023K and stresses from 110 MN/m('2) (16,000 psi) to 220 MN/m('2) (32,00 psi). An extensive TEM study was conducted on both materials in the as-received and as-crept conditions as one means of determining the controlling creep mechanism. The results of the TEM study were correlated with the predicted creep mechanism(s) derived from a comparison of the experimental results with theoretical models.; The stress exponent, n, in the equation (epsilon)(,ss) (PROPORTIONAL) (sigma)('n), was calculated to be 5.7 for the NC-430 SiC. For the CVD SiC it was determined to be 2.3 below 1923K and 3.7 at 1923K. The activation energies for creep were determined to be 7.37 (+OR-) 0.20 eV/atom for the NC-430 SiC and 1.81 (+OR-) 0.05 eV/atom for the CVD SiC. Various creep mechanisms have been evaluated with respect to these results plus those of the TEM study. For the NC-430 SiC, the controlling creep mechanism has been determined to be dislocation glide/climb that is controlled by climb. At temperatures between 1673K and 1873K the controlling creep mechanism for CVD SiC is thought to be dislocation glide controlled by overcoming the Peierls stress. At 1923K dislocation glide/climb controlled by climb appears to become the dominant mechanism in CVD SiC.