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As-received, yttria-doped (4.2 wt % Y203) single crystals of zirconia were heated to ?21OO0C in air. Cube-shaped samples with faces perpendicular to (100) axes on the basis of the pseudocubic symmetry were cut from the cryst:als. X-ray and electron diffraction indicated that the crystals are polydomain with [OOl] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (pcerpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400°C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along (100) on the hasis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000°C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was -12 and -8 MPa . m"*, respectively. Preliminary experiments on po1:ycrystalline tetragonal zirconia samples containing 5.4 wt% Y 2 0 3 and sintered at 22100°C also showed no evidence of the nionoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa . m"'. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness. [

Fiber push-out tests have been performed on a ceramic matrix composite consisting of Carborundum sintered SIC fibers, with a BN coating, embedded in a reaction-bonded SIC matrix. Analysis of the push-out data, utilizing the most complete theory presently available, shows that one of the fiber/coating/matrix interfaces has a low fracture energy (one-tenth that of the fiber) and a moderate sliding resistance T = 8 MPa. The debonded sliding interface shows some continuous but minor abrasion, which appears to increase the sliding resistance, but overall the system exhibits very clean smooth sliding. The tensile response of a fullscale composite is then modeled, using data obtained here and known fiber strengths, to demonstrate the good composite behavior predicted for this material.

Mechanical properties of a developmental high strength and high toughness SiC, Generation I SX, have been evaluated under a Department of Energy (DOE)/Oak Ridge National Laboratory (ORNL) subcontract. The mechanical properties determined included flexural strength, tensile strength, and fracture toughness at room and elevated temperatures. Stress rupture, dynamic fatigue and creep at elevated temperatures also have been evaluated. The strength limiting factors have been identified at room and elevated temperatures. The strength controlling mechanisms are discussed. The microstructure-mechanical property relationship has been established.

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