Effect of water vapor on the oxidation and thermo-mechanical behavior of SiC and Si(3)N(4) ceramics

The oxidation and mechanical behavior in the presence of air and water vapor are studied for two SiC (Hexoloy SA, SA 93) and two {dollar}rm Sisb3Nsb4{dollar} ceramics (NT 164, SN 88) at high temperatures. The test environments used for this study consist of air with {dollar}0, {lcub}sim{rcub}1.2,{dollar} and 6.4 v/o {dollar}rm Hsb2O{dollar} at temperatures from {dollar}1000spcircrm C{dollar} to {dollar}1350spcircrm C.{dollar} The oxidation exposure times are up to 500 hours. The strength measurements for flexural dynamic fatigue tests are made at temperatures from {dollar}1038spcircrm C{dollar} to {dollar}1350spcircrm C{dollar} and at actuator speeds of {dollar}8.4times10sp{lcub}-2{rcub}{dollar} and {dollar}8.4times10sp{lcub}-5{rcub}{dollar} mm/s ({dollar}sim{dollar}200 and {dollar}sim{dollar}0.2 MPa/s). Step stress rupture tests are performed at {dollar}1350spcircrm C{dollar} and {dollar}1288spcircrm C{dollar} for a SiC ceramic and at {dollar}1288spcircrm C{dollar} and {dollar}1150spcircrm C{dollar} for a {dollar}rm Sisb3Nsb4{dollar} ceramic.; Oxidation and crystallization of oxidation phases are accelerated by the presence of water vapor. However, the effects of water vapor on oxidation are reduced with oxidation time for Hexoloy SA SiC, while the effects of water vapor are increased with oxidation time for NT 164 {dollar}rm Sisb3Nsb4.{dollar} The oxidation of NT 164 {dollar}rm Sisb3Nsb4{dollar} are the most affected by the presence of water vapor and SN 88 {dollar}rm Sisb3Nsb4{dollar} is the least affected.; AES depth profiles across a ball crater ground into the oxide scale show that the shape of the oxygen concentration profile changed as a function of time. Even though the amount of weight gains for SN 88 {dollar}rm Sisb3Nsb4{dollar} in air and in wet air are very close after 500 hours oxidation at {dollar}1350spcircrm C,{dollar} a much thicker oxide scale forms in the presence of wet air. It is determined that oxidation is controlled by oxygen diffusion through the grain boundary in wet air; but in air the rate controlling step is a dissolution reaction of {dollar}rm Sisb3Nsb4{dollar} grains. Crystallization and purification of grain boundary hinders grain boundary diffusion, while water vapor accelerates the dissolution of {dollar}rm Sisb3Nsb4{dollar} grains. Catastrophic oxidation is observed in SN 88 {dollar}rm Sisb3Nsb4{dollar} ceramics at {dollar}1000spcircrm C.{dollar} This is attributed to volume expansion due to oxidation of the grain boundary oxynitride phase.; Water vapor has a beneficial effect on the flexural strength of the three candidate materials due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrate that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases (allowing more time for the healing reaction). SA 93 SiC ceramics show increased slow crack growth susceptibility at the lower temperatures in air.; NT 164 {dollar}rm Sisb3Nsb4{dollar} and Hexoloy SA SiC was used for step stress rupture testing. Times-to-failure are always longer in wet air for both ceramics. The beneficial effects of water vapor are greater for NT 164 {dollar}rm Sisb3Nsb4{dollar} than for Hexoloy SA SiC. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores are the primary mechanism for slow crack growth for NT 164 {dollar}rm Sisb3Nsb4.{dollar} Stress corrosion cracking from pre-existing pores or oxidation pits are the primary mechanism for slow crack growth of Hexoloy SA SiC. (Abstract shortened by UMI.)...