| Single-crystal MoSi{dollar}sb2{dollar} and four polycrystalline MoSi{dollar}sb2{dollar} materials prepared by three different preparation techniques (hot pressing, arc melting, and vacuum plasma spraying) were isothermally oxidized for 100 hours at 1500{dollar}spcirc{dollar}C in 1 atm. oxygen. After an initial period which varied among the samples, these materials exhibited parabolic oxidation behavior and their parabolic rate constants ranged from {dollar}0.4 times 10sp{lcub}-9{rcub}{dollar} to {dollar}2.0 times 10sp{lcub}-9{rcub}{dollar} g{dollar}sp2{dollar}cm{dollar}sp{lcub}-4{rcub}{dollar}hr{dollar}sp{lcub}-1{rcub}{dollar}, indicating that preparation method does not have a significant effect on the oxidation behavior. Short-time oxidation runs showed that the higher initial oxidation rate in two of the materials did not correspond to variations in the crystallization rate of the scale from amorphous silica to cristobalite, but instead correlated to higher impurity levels of Al and Fe which diffused into the scale within 0.25 hr. A {dollar}rm Mosb5Sisb3{dollar} interlayer between the oxide and the MoSi{dollar}sb2{dollar} was observed in the single crystal but not in the polycrystalline materials, indicating the importance of grain boundary diffusion in delaying the formation of the Mo{dollar}sb5{dollar}Si{dollar}sb3{dollar} interlayer.; Four MoSi{dollar}sb2{dollar}-based composites containing 40 vol.% SiC, 5 vol.% SiC. 10 vol.%SiC-10vol.% partially stabilized zirconia, and 20 vol.% ZrO{dollar}sb2{dollar}, respectively, were also oxidized at 1500{dollar}spcirc{dollar}C in 1 atmosphere oxygen. With all four composites, the Al and Fe impurities rapidly diffused into the oxide scale, which after 100 hours had regions of high impurity content near the gas-oxide interface and regions of high purity silica near the oxide-composite interface. The 40% SiC and 5% SiC containing composites had isothermal oxidation growth rates of {dollar}4.4 times 10sp{lcub}-9{rcub}{dollar} and {dollar}2.9 times 10sp{lcub}-9{rcub}{dollar} g{dollar}sp2{dollar}cm{dollar}sp{lcub}-4{rcub}{dollar}hr{dollar}sp{lcub}-1{rcub}{dollar}, respectively. The 10vol.%SiC-10vol.% partially stabilized zirconia composite exhibited parabolic oxidation behavior over 100 hours but showed complicated scale reactions with the dissolution of yttria from the zirconia to form a yttrium silicate within 0.25 hr. and zircon formation within 2 hours. The 20% ZrO{dollar}sb2{dollar} material showed logarithmic oxidation behavior, a loss of integrity of the initial composite structure, and the formation of internal porosity. |
