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Deformation (constitutive relations) of free-standing, thick thermal barrier coatings of sprayed ZrO2-8Wt% Y2O3 was determined at ambient temperature in both pure tension and pure compression using cylindrical bar test specimens. The material exhibited both significant nonlinearity and hysteresis in its load-strain curves, The load-strain relations in four-point uniaxial flexure were determined from tension and compression sides and were compared with individual pure tension and compression constitutive data. Effect of sintering on deformation behavior was significant, resulting in a dramatic change in constitutive relation. Cyclic fatigue testing of the coating material in tension-tension at room temperature showed an insignificant susceptibility to fatigue, similar to the slow crack growth behavior of the material in flexure in 800 C air.

Strength of three continuous fiber-reinforced ceramic composites, including SiC/CAS-11, SiC/MAS-5 and SiC/SiC, was determined as a function of test rate in air at 1100 - 1200 C. All three composite materials exhibited a strong dependency of strength on test rate, similar to the behavior observed in many advanced monolithic ceramics at elevated temperatures. The application of the preloading technique as well as the prediction of life from one loading configuration (constant stress-rate) to another (constant stress loading) suggested that the overall macroscopic failure mechanism of the composites would be the one governed by a power-law tyw of damage evolution/accumulation, analogous to slow crack growth commonly observed in advanced monolithic ceramics. It was further found that constant stress-rate testing could be used as an alternative to life prediction test methodology even for the composite materials at least for the short range of lifetime.

Single crystal oxides such as yttria-stabilized zirconia (Y2O3-ZrO2), yttrium-aluminum-garnet (Y3Al5O12, or YAG), magnesium oxide (MgO) and sapphire (Al2O3) have been considered as refractive secondary concentrator materials for high temperature solar propulsion applications. However, thermal mechanical reliability of the oxide components in severe thermal environments during space mission sun/shade transitions is of great concern. In this paper, critical mechanical properties of these oxide crystals are determined by the indentation technique. Thermal shock resistance of the oxides is evaluated using a high power CO, laser under high temperature-high thermal gradients. Thermal stress fracture behavior and failure mechanisms of these oxide materials are investigated under various temperature and heating conditions.

The error associated with strength measurements made with fixed-roller and V-grooved roller figures was studied with a variety of roller and figure material combinations. Both the fixed-roller and the V-grooved fixtures yielded an error in stress of about 4 to 7%. They also exhibited an appreciable amount of load strain hysteresis. Dynamic fatigue testing of 96 wt.% alumina verified the error: the fatigue strength was overestimated by 7 to 9% for the fixed roller configuration. The source of error is exclusively attributed to friction between the rollers and the specimen surface, as verified by previous investigators.

An attempt to determine high temperature "inert" strengths of several silicon nitride ceramics was made by using dynamic fatigue testing at the very high stressing rate of 2 x 106 MPa/min. The fatigue strengths of all the silicon nitride materials exhibited a well-defined convergence to the corresponding room-temperature inert strengths as stressing rate was increased. The result showed that the high temperature Inert" strength of a ceramic material can be obtained by using sufficiently high stressing rates (greater or = 2 x 10(exp 6) MPa/min) at which fatigue strength converges to a specific value. The specific value can be close to the room-temperature strength of the material.

The slow crack growth of a hot-pressed silicon nitride was determined at 1300 C in air using dynamic fatigue testing under both uniaxial and biaxial stress states. Good agreement in fatigue parameter exists between the data obtained from uniaxial and biaxial loading conditions. A reasonable prediction of dynamic fatigue from one stress state to another was made using the recently developed CARES/LIFE computer code.

Previously derived solutions of fatigue strength as a function of preloading were verified by applying preloads to elevated temperature dynamic fatigue tests of 96 wt% alumina at 1000 C and NC 132 silicon nitride at 1100 C. The technique was found very useful in identification and control of the governing failure mechanism when multiple failure mechanisms, such as slow crack growth, creep and oxidation occurred simultaneously at elevated temperatures.

The slow crack growth parameters of a hot-pressed silicon nitride were determined at 1200 and 1300 C in air by statically, dynamically and cyclicly loading bend specimens. The fatigue parameters were estimated using the recently developed CARES/Life computer code. Good agreement exists between the flexural results. However, fatigue susceptibility under static uniaxial tensile loading, reported elsewhere, was greater than in flexure. Cyclic flexural loading resulted in the lowest apparent flexural fatigue susceptibility.

The crack healing behavior of a commercial, MgO-containing, hot pressed Si3N4 was studied as a function of temperature in oxidizing and inert annealing environments. Crack healing occurred at a temperature 800 C or higher due to oxidation regardless of crack size, which ranged from 100 microns (indentation crack) to 1.7 mm (SEPB precrack). The resulting strength and apparent fracture toughness increased at crack healing temperature by 100 percent and 300 percent, respectively. The oxide layer present in the crack plane was found to be highly fatigue resistant, indicating that the oxide is not solely silicate glass, but a mixture of glass, enstatite, and/or cristobalite that was insensitive to fatigue in a room temperature water environment.

The R-curve and fatigue behavior of a silicon nitride with large, highly elongated grains were compared to those of a silicon nitride with smaller, more equiaxed grains. The former was found to have higher toughness and a slightly rising R-curve which enhanced fatigue resistance, while the latter had constant toughness with crack length. The R-curve was attributed to crack deflection and grain pull-out. The microstructure did not influence the dynamic fatigue properties of the two ceramics, as both had good resistance to dynamic fatigue. The cyclic fatigue resistance of the large-grained Si3N4, however, was surprisingly low.

The dynamic fatigue behavior of 30 vol pct SiC whisker-reinforced silicon nitride was determined in flexure as a function of temperature from 1100 to 1300 C in an air environment. The fatigue susceptibility parameter n decreased from 88.1 to 20.1 with increasing temperature from 1100 to 1300 C. A transition in the dynamic fatigue curve occurred at a lowest stressing rate of 2 MPa/min at a temperature of 1300 C, resulting in a considerably lower value of n = 5.8. This transition was primarily due to creep that was enhanced by a combination of high temperature and very slow deformation rate. The fatigue resistance of natural flaws at 1100 C was found to be greater than that of artificial flaws produced by indentation.