High-Temperature Strength Behavior of Hot-Pressed Si3N4: Evidence for Subcritical Crack Growth

The high-temperature strength of commercial hot-pressed Si₃N₄ was obtained for (1) two materials with different impurity contents, (2) the weak and strong material directions, (3) air and Ar ambients, and (4) different stressing rates. Strength degradation occurred at a lower temperature for the less pure material; both material directions exhibit the same rate of strength degradation. The testing ambient did not affect strength. The strength at temperatures ∼1200°C depended strongly on stressing rate. The presence of rough, crack-shaped topographical features on the fracture surface and the observation of large cracks that formed during stressing are reported as evidence for subcritical crack growth at high temperatures. It is hypothesized that accelerated creep caused by grain-boundary sliding at preexisting crack fronts is the mechanism responsible for the observed subcritical crack growth.     

Corrosion Behavior of Single-Crystal Alumina in Argon, Air, and Water Vapor Atmospheres at 1700-2000°C

The results for the corrosion of alumina single crystals at 1700-2000°C in argon, argon/water vapor, air, and air/water vapor for 10 h are reported. There were no obvious weight and volume changes after corrosion. White spots were observed on the surfaces of the specimens after corrosion tests. The initial temperature for the appearance of these white spots was 1800°C for argon and air, 1900°C for argon/water vapor, and 2000°C for air/water vapor. These white spots were likely formed by internal impurities, which diffused outward to the surface and coalesced at high temperatures. There was no evidence of corrosion damage inside the specimens. The flexural strength of the specimens was clearly enhanced after the corrosion tests and showed no evident relation to the corrosion conditions. This increase in strength after corrosion was likely due to the healing of surface machining flaws. The surface flaw healing temperature for alumina crystals was higher than 1400°C.     

Atmospheric Effects on Compressive Creep of SiC-Whisker-Reinforced Alumina

A SiC-whisker-reinforced alumina composite was crept in compression at 1200° to 1400°C in an air ambient and in nitrogen. The data were described by a power-law-type constitutive relation. The measured value of the stress exponent was n = 1 at 1200°C and n = 3 at 1300° and 1400°C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n = 1 were associated with diffusional creep and unaccommodated grain-boundary sliding, while values of n = 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of SiC and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion.     

Comparison of the Creep and Creep Rupture Performance of Two HIPed Silicon Nitride Ceramics

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long-term mechanical reliability of a commercially available and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260° and 1370°C utilizing button–head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavitation at multigrain junctions. Two-grain cavitation was also observed in the commercial material. Failure in both materials resulted from the evolution of an extensive damage zone. The failure times were uniquely related to the creep rates, suggesting that the zone growth was constrained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significantly improved compared to those of the commercial material was attributed to the absence of cavitation along two-grain junctions in the developmental material.     

Effects of In Vitro Aging of Magnesia-Stabilized Zirconia

The intent of this project was to evaluate any changes in the flexure strength of magnesia-partially-stabilized zirconia when aged in distilled water for time periods of 6, 12, and 18 mos. The aging conditions were autoclaved specimens in air (AA) or distilled water (AW) and nonautoclaved specimens in distilled water (NW) at stressing rates of 10.0, 1.0, or 0.1 mm/min. The statistical analysis showed no significant difference between the aging times. The aging-time data were then pooled and a statistical analysis run between the different aging conditions (AA, AW, and NW) and stressing rates. The specimens tested in air were significantly stronger than those tested in water autoclaved or nonautoclaved. For the pooled data, all stressing rates were statistically significant from each other.     

Stress Rate Dependence of Fracture Strength in Pre-Cracked Silicon Nitride Ceramics Doped with Ytterbium Oxide

High-temperature dynamic fatigue behavior has been investigated in 6 wt% ytterbium oxide and 2 wt% alumina-doped silicon nitride ceramics by nitrogen gas pressure sintering. The specimens were pre-cracked by Vickers indentation to prevent creep damage and to ensure dynamic fatigue dominating. The tests were performed in four-point flexure in air at temperatures of 1000°, 1200°, 1300°, and 1400°C and by varying the loading rate from 1, 0.5, 0.1–0.01 mm/min at each temperature. The analyses were conducted by plotting fatigue stress against loading rate at each testing temperature in double logarithm coordinates. The material was found to be the least susceptible (the highest slow crack exponent number N ) to slow crack growth at 1200°C, as reflected by the comparison of the plot slopes for the four testing temperatures. The explanation and analyses take into consideration the grain-boundary phase crystallization, crack healing, and oxidation during testing evidenced by X-ray diffraction and transmission electron microscopy. The fracture surfaces were characterized by three well-defined zones, namely zone I, II, and III, referring to the pre-cracked area, slow crack growth area, and fast fracture area, respectively.     

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.     

Stress-Oxidation Behavior of a Carbon/Silicon Carbide Composite in a High-Temperature Combustion Environment

A carbon/silicon carbide composite with a silicon carbide coating was prepared by chemical vapor infiltration. Stressed oxidation testing was performed on the composites in a self-built high-temperature combustion environment. The gas in this environment contained oxygen, steam, carbon dioxide, and some nitrogen. Test conditions were controlled at temperatures of 1300°, 1500°, and 1800°C, and the stress was sustained at 40, 80, 120, 160, and 200 MPa. The effect of combustion environment and applied load on stress-oxidation behavior was discussed by analyzing the residual strength and weight loss. The morphology of the fracture surface of the tested specimens was observed by scanning electron microscopy. The high-temperature combustion environment and the high sustained stress above 80 MPa enhanced the material failure and led to strength reduction by determining crack openings and thus oxidation of fibers. However, sustained stress below 80 MPa resulted in no strength degradation after exposure for 10 min at 1500°C.     

Effect of Environment on the Stress-Rupture Behavior of a Carbon-Fiber-Reinforced Silicon Carbide Ceramic Matrix Composite

Stress-rupture tests were conducted in air, under vacuum, and in steam-containing environments to identify the failure modes and degradation mechanisms of a carbon-fiber-reinforced silicon carbide (C/SiC) composite at two temperatures, 600° and 1200°C. Stress-rupture lives in air and steam-containing environments (50–80% steam with argon) are similar for a stress of 69 MPa at 1200°C. Lives of specimens tested in a 20% steam/argon environment were about twice as long. For tests conducted at 600°C, composite life in 20% steam/argon was 30 times longer than life in air. Thermogravimetric analysis of the carbon fibers was conducted under conditions similar to the stress-rupture tests. The oxidation rate of the fibers in the various environments correlated with the composite stress-rupture lives. Examination of the failed specimens indicated that oxidation of the carbon fibers was the primary damage mode for specimens tested in air and steam environments at both temperatures.     

Elevated-Temperature-Delayed Failure of Alumina Reinforced with 20 vol% Silicon Carbide Whiskers

Alumina composites reinforced with 20 vol% SiC whiskers were exposed to applied stresses in four-point flexure at temperatures of 1000°, 1100°, and 1200°C in air for periods of up to 14 weeks. At 1000° and 1100°C, an "apparent" fatigue limit was established at stresses of ∼ 75% of the fast fracture strength. However, after long-term (>6 weeks) tests at 1100°C, some evidence of crack generation as a result of creep cavitation was detected. At 1200°C applied stresses as low as 38% of the 1200°C fracture strength were sufficient to promote creep deformation and accompanying cavitation and crack generation and growth resulting in failures in times of <250 h.     

Strength Distribution in Commercial Silicon Carbide Materials

Four types of commercial silicon carbide samples from different sources were characterized in terms of baseline strength and strength distribution (reliability). Four-point flexural strength of each material was determined on 30 test bars, 5.1 by 0.64 by 0.32 cm, for a reliable estimate of the Weibull modulus values. The results show that the average strength of these sintered silicon carbide samples ranged from 380 to 482 MPa (55 to 69 ksi) at room temperature and 307 to 470 MPa (45 to 68 ksi) at 1370°C (2500°F). Considerable variations in strength were found among specimens of each material. Baseline Weibull modulus values ranged from 8 to 11 at room temperature and 7 to 11 at 1370°C (2500°F). The strength scatter clearly reflected flaw variability, which must be minimized to improve reliability in sintered silicon carbide materials.     

Evaluation of Mechanical Stability of a Commercial Sn88 Silicon Nitride at Intermediate Temperatures

Dynamic fatigue and stress rupture tests in four-point bending were conducted on a commercially available SN88 silicon nitride ceramic at temperatures in the range 700°–1000°C in air. The objective of the present study was to elucidate the failure of SN88 silicon nitride ceramic nozzles arising from a critical crack initiated at the intermediate temperature airfoil region during an engine field test. Results of dynamic fatigue tests indicated that SN88 silicon nitride tested at a stressing rate of 30 MPa/s exhibited little change in characteristic strength at the various test temperatures. However, SN88 silicon nitride exhibited a significant degradation in mechanical strength when tested at 0.003 MPa/s at temperatures indicative of a great susceptibility to slow crack growth, especially at 850°C. SEM and XRD analyses indicated that the mechanical instability of SN88 silicon nitride at intermediate temperatures resulted from the transformation of secondary phase(s) from oxidation. These phase transformations were accompanied by a large volume change, which led to the generation of large local residual tensile stresses. As a result, extensive damage zones were formed, which led to a substantial degradation of mechanical strength and reliability. Microstructural examination of failed SN88 airfoils indicated that a similar damage zone was formed in the regions exposed to intermediate temperatures during engine testing. Consequently, the ultimate failure of these vanes was attributed to the loss in mechanical strength from the damage zone formation.     

Steady-State Creep Behavior of Si–SiC C-Rings

Because of the ease of experimental setup as well as economics in sample preparation, C-ring specimens are sometimes chosen for the evaluation of mechanical behavior. In this paper, the long-term creep of siliconized silicon carbide (Si–SiC) C-rings is investigated. Creep tests on a number of Si–SiC C-rings were carried out under constant compressive loads at 1300°C in air. Load-point displacements were continually monitored as a function of time, thereby establishing the steady-state regime as a function of load and ring geometry. Optical micrography on the postcrept specimens was performed to obtain damage zone sizes. A simple curved beam theory was employed to analyze the stress state developed throughout the body during steady-state creep. Loadpoint displacement rates were numerically calculated using both geometric and energy methods. Observed damage zone sizes and shapes within the specimen agreed with those predicted theoretically. Results obtained on the stress solutions are useful as local loading parameters in the study of high-temperture fracture behavior of a cracked C-ring.     

High-Temperature Strength Behavior of Synroc-C

Comparative measurements have been made of the high-temperature flexure strength characteristics of synroc-C in air and argon environments. The stress–strain curves for synroc show a large deviation from linearity with increasing temperature in both environments, indicating a brittle–ductile transition. Strength is relatively constant at ≤800°C, followed by a discernible increase, with a peak at ∼920°C in air and 940°C in argon, and then a dramatic drop-off. The strengthening response is explored with reference to microstructural changes, in particular oxidation effects, and the implications of the observations are discussed.     

Improvement of Tensile Creep Displacement Measurements

A water-cooled copper shield is shown to minimize local temperature variations (hence, regulate the refractive index of the air) in the optical path traversed by a laser beam used to measure creep displacements in a tensile specimen. Both the scatter and the reliability of the elevated-temperature tensile creep displacement measurements have been significantly improved from ±5 to ±0.5 μm, which is comparable to that obtained at room temperature. Results achieved with this modification were demonstrated with a hot isostatically pressed (HIPed) silicon nitride ceramic containing 4 wt% Y₂O₃ as a sintering additive tested at 1370°C in air. Results also indicate that the creep rates can be independent of the specimen geometry (and size) and the gripping system.     

Mechanical Properties of Polycrystalline Lithium Orthosilicate

Room-temperature mechanical properties and high-temperature creep deformation of lithium orthosilicate (Li₄SiO₄) were studied. Elastic constants, flexural strength, and fracture toughness were determined for specimens with densities between 68% and 98% of theoretical. Critical quenching temperature and thermal-shock resistance parameters for 90% dense specimens were also measured. High-temperature creep deformation was investigated by a constant-strain-rate test in an argon atmosphere at temperatures between 750° and 1025°C and strain rates ranging from about 10⁻⁶ to 10⁻³ s⁻¹. At 950°C and above, the stress exponent, n , was determined to be 3.6, with a creep activation energy of 715 kJ/mol. Selected results obtained for Li₄SiO₄ are compared with results obtained for other Li-containing ceramics that are under consideration as candidates for fusion reactor breeder blankets.     

Creep Behavior of a SiC-Whisker-Reinforced Alumina

Creep tests in four-point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20-vol%-SiC-whisker-reinforced alumina and unreinforced single-phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain-boundary sliding by the SiC whiskers.     

Compressive Creep of SiC-Whisker-Reinforced Al2O3

Compressive creep of SiC-whisker-reinforced Al₂O₃ composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples.     

Microstructure, Strength, and Oxidation of a 10 wt% Zyttrite-Si3N4 Ceramic

Hot-pressed Si₃N₄ doped with 10 wt% zvttrite as a sinterine aid was studied. An equiaxed, fine-grainid microstructure was predominant, with no apparent porosity. Bend strengths were determined at room temperature and high temperatures (up to 1370°C/2500°F). Oxidation was measured by weight gain at 1370°C in air. The resulting material exhibited very good room-temperature strength (755 MPa/110 ksi). The work showed that room-temperature strength can be improved significantly by using controlled Si₃N₄ powder with 10 wt% zyttrite. High-temperature strength (514 MPd75 ksi) at 1370°C was nearly double that of hot-pressed Si₃N₄ (NC-132). The oxidation resistance at 1370°C was also higher than that of NC-132.     

Effects of Crystal Orientation and Temperature on the Strength of Sapphire

The flexure and compressive strengths of sapphire are dependent on crystal orientation and temperature. Most notably, the c -axis compressive strength decreases below the tensile strength at temperatures >400°C and falls to 2% of the room-temperature compressive strength at 800°C. Loss of compressive strength complicates the interpretation of flexure tests. Four-point flexure specimens with no component of c -axis compression increase in strength at temperatures >500°C; however, specimens that have c -axis compression decrease in strength. It has been observed that c -axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhombohedral planes causes fracture that leads to mechanical failure.