| The focus of this Ph.D. dissertation is on understanding the chemical processes responsible for oxidation of ZrB2-SiC ceramics at 1500°C. The initial research centered on the effect of processing conditions (hot pressing time and temperature) on the microstructure and mechanical properties of ZrB2 containing 30 volume percent SiC. Analysis showed that the strength of ZrB2-SiC was limited by the maximum size of the SiC inclusions in the ZrB2 matrix. Subsequent research has focused on the oxidation behavior of ZrB2-based ceramics. During oxidation up to 1500°C, formation of a borosilicate layer provided passive oxidation protection by forming an oxygen diffusion barrier. SEM investigation revealed the formation of a four layered structure, which included a SiC-depleted layer. Due to the high vapor pressure of SiO (g) under reducing conditions, that exists at the interface between the SiO2 (1) surface layer and unoxidized ZrB2-SiC, SiC was believed to be preferentially vaporizing, leading to the formation of the SIC-depleted region. Vaporization of SiO (g) was verified experimentally by exposing ZrB2-SiC to a CO/CO 2 mixture that provided an oxygen partial pressure of ∼ 10 -10 Pa. The evolution of the surface structure on ZrB2-SIC, as it was heated from room temperature to 1500°C in air, was examined by exposing the specimens to flowing air at 800, 1000, 1200, 1400, and 1500°C followed by SEM/EDS and XRD analysis. Graphite was added to ZrB2-SiC in an attempt to reduce the formation of the SiC-depleted layer. An increase in the activity of carbon in the ceramic, and the corresponding change in the partial pressure of CO (g) below the silica layer, was thought to inhibit the formation of a SiC-free layer during oxidation. Oxidation of ZrB 2-SiC-C at 1500°C showed that graphite was stable beneath the SiO 2-rich outer layer. Thermodynamic analysis supported by experimental results was performed to justify the stability of graphite in the SiC-free region. |
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