REFRACTORY DIBORIDE COMPOSITES

The diborides of hafnium and zirconium are highly resistant to ablation at extreme temperatures under oxidizing conditions but are brittle and have poor thermal shock resistance. A potential solution to this problem is reinforcment with diboride fibers to create tough, thermal shock resistant diboride fiber-diboride matrix composites. The proposed work will involve the fabrication of ZrB2-Zrb2 composites by the forced-flow chemical vapor infiltration (CVI) of low density carbon fiber mat preforms. The 40 to 50 um diameter fibers will be formed in situ by depositing ZrBf2 onto the 9 um diatmeter corbon fibers to give final fiber volumes of 40 to 50%. This will be followed by depositing thin layers of BN on the fibers to promote the debonding needed for toughness and them forming the ZrB2 matrix by CVI. The experimental work will be guided by thermodynamic and transport analytical modeling methods developed in a previous program. In addition to the ZrB2 CVI development, experiments will be performed to determine if SiC particulate can be added to the composites by slurry infiltration to improve oxidation resistance. Specimens will be characterized by light and acanning electron microscopy, and toughness and strength will be evaluated by pressurized ring tensile tests.