Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.     

Tensile Creep and Creep-Strain Recovery Behavior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites

The tensile creep and creep strain recovery behavior of 0° and 0°/90° Nicalon-fiber/calcium aluminosilicate matrix composites was investigated at 1200°C in high-purity argon. For the 0° composite, the 100-h creep rate ranged from approximately 4.6 × 10⁻⁹ s⁻¹ at 60 MPa to 2.2 × 10⁻⁸ s⁻¹ at 200 MPa. At 60 MPa, the creep rate of the 0°/90° composite was approximately the same as that found for the 0° composite, even though the 0°/90° composite had only one-half the number of fibers in the loading direction. Upon unloading, the composites exhibited viscous strain recovery. For a loading history involving 100 h of creep at 60 MPa, followed by 100 h of recovery at 2 MPa, approximately 27% of the prior creep strain was recovered for the 0° composite and 49% for the 0°/90° composite. At low stresses (60 and 120 MPa), cavities formed in the matrix, but there was no significant fiber or matrix damage. For moderate stresses (200 MPa), periodic fiber rupture occurred. At high stresses (250 MPa), matrix fracture and rupture of the highly stressed bridging fibers limited the creep life to under 70 min.     

Polishing Behavior and Surface Quality of Alumina and Alumina/Silicon Carbide Nanocomposites

The response of Al₂O₃ and Al₂O₃/SiC nanocomposites to lapping and polishing after initial grinding was investigated in terms of changes in surface quality with time for various grit sizes. The surface quality was quantified by surface roughness ( R _a ) and by the relative areas of smooth polished surfaces as opposed to rough as-ground areas. Polishing behavior of the materials was discussed in terms of SiC content and grain size. It was concluded that nanocomposites are more resistant to surface damage than Al₂O₃, and this behavior does not depend on the amount of SiC in the range 1–5 vol%. SiC addition ≥1 vol% is enough to produce a noticeable improvement in surface quality during lapping and polishing.     

R-Curve Behavior in a Silicon Carbide Whisker/Alumina Matrix Composite

R -curve measurements were performed on a SiC whisker/Al₂O₃ matrix composite. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that bridging by intact whiskers in the crack wake was the mechanism responsible for the rising R -curve behavior.     

Creep Deformation of an Alumina Matrix Composite Reinforced with Silicon Carbide Whiskers

Whisker-reinforced ceramic composites with enhanced fracture toughness properties are being developed. The creep behavior of such a composite was studied. The introduction of silicon carbide whiskers significantly improves the creep resistance of polycrystaline alumina.     

Diffusional Crack Growth and Creep Rupture of Silicon Carbide Doped with Alumina

This study deals with tensile creep and crack growth behavior of silicon carbide doped with alumina at 1400°C. Excellent creep resistance was observed for stresses from 150 MPa to 200 MPa. From the creep exponent of 1.4 and the activation energy of 320 kj/mol, the principal creep mechanism was Coble creep. The creep failure was caused by slow crack growth from a preexisting flaw. The crack was found to grow subcritically along grain boundaries almost in isolation. The relation between the time–to–failure and the applied stress was well treated by a diffusive crack growth model, and the threshold stress of this material at 1400°C was estimated at 165 MPa.     

Particle/Matrix Interface and Its Role in Creep Inhibition in Alumina/Silicon Carbide Nanocomposites

This study focuses on interfacial bonding between intergranular silicon carbide particles and an alumina matrix, to determine the creep inhibition mechanism of alumina/ silicon carbide nanocomposites. It is revealed that the silicon carbide/alumina interface possesses much stronger bonding than the alumina/alumina interface through three approaches: investigation of fracture toughness and fracture mode and consideration of internal thermal stresses acting at grain boundaries, estimation of equilibrium thickness of intergranular glassy films by force balance, and direct observation of grain boundaries by TEM. The rigid bonding of alumina/silicon carbide interfaces causes inhibition of vacancy nucleation and annihilation at the interfaces, causing remarkably improved creep resistance of the nanocomposite.     

Silicon Nitride/Silicon Carbide Nanocomposite Materials: II, Hot Strength, Creep, and Oxidation Resistance

The mechanical properties of Si₃N₄/SiC nanocomposite materials that contained nanosized intercrystalline SiC dispersions that originated from different starting powders and were made via different fabrication routes were studied in the temperature range of 1400°-1550°C. The strength retained at 1400°C was between 70% and ∼100% of the room-temperature strength. Both creep and oxidation resistance were very high and were comparable to or better than those of the best Si₃N₄-based materials published previously. The effect of SiC particles on the creep properties can be understood in terms of a recent model of dilatational creep; however, the model invokes a series of microstructural, micromechanical, and chemical modifications.     

Mechanical Behavior of Alumina Reinforced with Carbon-Coated Silicon Carbide Whiskers

SiC whiskers with 0, 20, and 50 Å carbon coatings were incorporated into an alumina matrix to modify residual thermal stress and interfacial bonding. Composites were characterized using triaxial X-ray diffraction for residual stress determination and electron microscopy to explore interfacial chemistry. Fracture toughness and R -curve behavior were examined for short and long crack lengths. Uncoated SiC whiskers optimized strength, fracture toughness, and R -curve behavior of these composites. A graphite interphase at the whisker/matrix interface decreased contributions to crack bridging without promoting additional toughening by whisker pullout.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

Tensile Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Sliding Wear of Alumina/Silicon Carbide Nanocomposites

The wear resistance of four Al₂O₃/SiC nanocomposites that contained SiC particles of varying average size (40, 200, and 800 nm) was studied under dry sliding conditions and compared with the results obtained in unreinforced alumina. The wear rate of the alumina and the nanocomposites of equivalent grain size increased as the contact load increased; however, the nanocomposite wear resistance at high contact loads was better than that of the alumina by a factor of 3–5. The wear resistance of the nanocomposites of submicrometer grain size was fairly independent of the contact load, and their wear resistance at high contact loads was up to two orders of magnitude better than that of the alumina. The mechanisms responsible for these behaviors were discussed in terms of the microscopic wear mechanisms that were observed on the worn surfaces.     

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.     

Constitutive Equations for the Creep of a Silicon Carbide Whisker-Reinforced Polycrystalline Alumina Composite Material

The θ -projection parametric method is used to analyze the creep strain-versus-time data, obtained in four-point flexure, for a 25 wt% silicon whisker-reinforced polycrystalline alumina composite material. The results, used in conjunction with a suggested value of the activation energy for the creep-rate-controlling process for this material of about 620 kJ/mol, led to a postulate that grain-boundary sliding is the rate-determining mechanism. In addition, the use of the results in obtaining the values of relevant design creep parameters (namely, life or strain) is illustrated.     

Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.     

Fracture Resistance Behavior of Silicon Carbide Whisker-Reinforced Alumina Composites with Different Porosities

Fracture resistance behavior was characterized for SiC-whisker-reinforced alumina composites with porosities ranging from 0.6% to 11.5% The composites were hot-pressed from an Al₂O₃ powder with 25 wt% SiC whiskers. Strengths of individual specimens were measured in four-point flexure either for natural flaws or for Vickers-indentation flaws as a function of radial crack size. Indentation crack sizes were controlled with indentation loads which varied between 2 and 200 N. A novel method of analysis of these measurements indicates that the fracture resistance of these composites increases as a function of crack extension, a rising R curve. This behavior is interpreted in terms of tractions from both crack-bridging whiskers and interlocking grains, which develop in the wake of the crack tip as it extends. A decrease in porosity raises the level of fracture resistance, but has a negligible effect on the relative steepness of the R curve. The sizes of natural flaws which causes failure in flexure testing were also estimated from analysis of the data.