Specimens of ZrB2 containing various concentrations of B4C, SiC, TaB2, and TaSi2 were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation
resistances were studied by scanning thermogravimetry over the range 1150 - 1550 degree C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content.
Tantalum additions to ZrB2-B4C-SiC in the form of TaB2 and/or TaSi2 increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi2 proved to be a more
effective additive than TaB2. Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This
interior layer was less porous in tantalum-containing compositions.
The oxidation resistances of ZrB2 containing SiC, TaB2, and TaSi2 additions of various concentrations was studied using
isothermal thermogravimetry at 1200, 1400, and 1500 degree C, and specimens were further characterized using x-ray diffraction and
electron microscopy.
Increasing SiC concentration resulted in thinner glassy surface layers as well as thinner ZrO2 underlayers deficient in silica. This silica deficiency was argued to occur by a wicking process of interior-formed borosilicate liquid to the initially-formed borosilicate liquid at the surface. Small (3.32 mol%) concentrations of TaB2 additions were more effective at
increasing oxidation resistance than equal additions of TaSi2. The benefit of these additives was related to the formation of
zirconium-tantalum boride solid solution during sintering, which during oxidation, fragmented into fine particles of ZrO2 and TaC.
These particles resisted wicking of their liquid/glassy borosilicate encapsulation, which increased overall oxidation resistance. With
increasing TaB2 or TaSi2 concentration, oxidation resistance degraded, most egregiously with TaB2 additions. In these cases, zirconia dendrites appeared to grow through the glassy layers,
providing conduits for oxygen migration.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/28208 |
| Date | 09 January 2009 |
| Creators | Peng, Fei |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Dissertation |
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