Improving the oxidation resistance of diboride-based ceramics
"Oxidation behavior has restricted the development of ZrB₂-based ceramics for aerospace and hypersonic flight vehicles applications. The research presented in this dissertation focuses on the effect of transition metal (TM) additives on oxidation behavior of ZrB₂ ceramics. In the first stage of the research, the effect of Nb additions on the morphology of the oxide particles and stability of the protective B₂O₃ glassy layer, which formed on the top surface during oxidation, was investigated. Addition of Nb increased the thickness of the glassy layer and, as a result, improved the oxidation resistance of ZrB₂ after oxidation at 1500⁰C. Next, the oxidation behavior of nominally pure ZrB₂ and (Zr,W)B₂ after oxidation at temperatures ranging from 800 to 1600⁰C was studied. Two oxidation stages before and after significant evaporation of B₂O₃ at about 1100⁰C were recognized for nominally pure ZrB₂. Higher stability for the WO₃-B₂O₃ glassy layer compared to pure B₂O₃ resulted in a shift in the onset of the second oxidation regime toward higher temperatures for (Zr,W)₂ specimens and resulted in higher oxidation resistance for (Zr,W)B₂ compared to nominally pure ZrB₂. In the third stage of the research, the effects of TM-oxides such as WO₃, Nb₂O₅, or ZrO₂ on weight loss and structure of B₂O₃ glasses was studied. Thermogravimetric analysis performed on (TM-oxide)-B₂O₃ glasses indicated that TM-oxide additions reduced the evaporation of B₂O₃. Since no change in the structure of the glasses was detected, it was concluded that the increased stability of (TM-oxide)-B₂O₃ glasses compared to pure B₂O₃ was due to the lower activity of B₂O₃ in (TM-oxide)-B₂O₃glasses. Finally, comparison of the effects of W, Mo, or Nb on oxidation behavior of ZrB₂ at 1600⁰ showed that Mo and Nb were the most effective additives for improving the oxidation resistance of ZrB₂"--Abstract, page iv.
Ultra-high Temperature Ceramics, Zirconium diboride, Materials Science and Engineering
Missouri University of Science and Technology
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