Push-Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push-pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle-number-based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time-based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress-imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.     

Oxidation of Silicon Carbide Fibers During Static Fatigue in Air at Intermediate Temperatures

The static fatigue of SiC-based fiber bundles and single fibers has been examined in previous papers, with emphasis placed on the analysis of the stress–rupture time data, and on the modelling of delayed failure from slow crack growth. The present paper investigates the oxidation of the fibers during static fatigue, at temperatures in the intermediate temperature range (500°–800°C). Two oxidation-induced phenomena have been evidenced: the formation of a thin silica film at the surface of fibers and the delayed failure of fiber bundles and single filaments. The stress–rupture time data are interpreted with respect to the chemical and structural characteristics of fibers, and to the oxide film growth rate. The structural analysis of the fibers was carried out using scanning electron microscopy and Auger electron spectroscopy. Delayed failure was found to result from slow crack propagation from surface defects, as a result of the consumption of the free carbon at grain boundaries and the local stresses induced by the SiC→SiO₂ transformation at the crack tip. The respective contributions of these phenomena to static fatigue are discussed.     

Steam Oxidation Resistance of Silicon Carbide Castables at Elevated Temperatures

Silicon carbide castables of different SiC contents (86％ and 71％,by mass) were prepared using white fused corundum,silicon carbide particles and fines,activated...     

Reactions of Silicon Carbide and Silicon(IV) Oxide at Elevated Temperatures

The reaction between SiC and SiO₂ has been studied in the temperature range 1400–1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a high-temperature mass spectrometer was used for this study. Two systems were studied—1:1 SiC (2 wt% excess carbon) and SiO₂; and 1:1:1 SiC, carbon, and SiO₂. In both cases the excess carbon forms additional SiC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SiC and SiO₂ to form SiO( g ) and CO( g ) in approximately a 3:1 ratio. These results are interpreted in terms of the SiC-O stability diagram.     

Etching of Silicon Carbide Materials at Elevated Temperatures in a Nitrogen-Based Gas

Two sintered SiC-based materials were heat-treated for 150 h at 1300°C in a nitrogen-based gas (1.2% H₂, 0.6% CO) at a total pressure of 130 Pa. Sintered SiC samples were also preoxidized and then exposed to this gas under the same conditions to evaluate the protective nature of an SiO₂ scale. In this atmosphere, SiO gas and cyanogens are predicted to form, rather than SiO₂. Experimental studies confirmed that etching of sintered SiC occurs. Preoxidation does not provide protection from etching, because of the rapid removal of SiO₂ by H₂ as H₂O and SiO.     

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 H z , it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.     

Active Corrosion of Sintered α-Silicon Carbide in Oxygen–Chlorine Gases at Elevated Temperatures

The active corrosion of sintered α-silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl₂ by volume with additions of O₂ or H₂ has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine-containing gas mixtures, but the corrosion was suppressed by an active-to-passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H₂ even when the chlorine potential was high. The concentration of oxygen necessary to produce the active-to-passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide.     

Carbon Inclusions in Sintered Silicon Carbide

The carbon additions in the pressureless sintering of SiC are commonly used for the removal of SiO₂ layers on the starting powders. In practice, it is common to add more C than is necessary for stoichiometric removal to ensure a complete deoxidation. As a result, inclusions of excess free C are a general feature of the microstructure of sintered SiC. This phenomenon was studied by high-resolution Auger electron spectroscopy on ultra-high-vacuum-exposed fracture surfaces as well as by high-resolution transmission electron microscopy of B- and C-doped materials.     

Deformation of Alumina/Titanium Carbide Composite at Elevated Temperatures

The deformation behavior of an Al₂O₃/30 wt% TIC composite in uniaxial tension was evaluated under vacuum over the temperature range of 1300° to 1550°C. The Al₂0₃/TiC composite exhibited the maximum elongation of 66% at an initial strain rate of 1.19 X l0^-4 s^-1 at 1550°C. The stress exponent calculated from peak stresses of true stress-true strain curves at 1500^OC was 3.8, which was in good agreement with that obtained by changing the crosshead speed during the tension test. The apparent activation energy at 20 MPa was 853 kJ/mol. In addition the deformation of the Al₂O³/TiC composite in uniaxial tension at elevated temperature was accompanied by cavitation.     

Physical Characteristics of Titanium Carbide Type Cermets at Elevated Temperatures

The principal purpose of this investigation was to determine the creep behavior and strength at 1200° to 1850°F. of cermets which may be used in the high-temperature areas of aircraft. Initially a series of bending tests was made on variations of Kennametal 150-type Kentanium containing from 5 to 30% nickel binder. The 5, 15, and 20% compositions were the strongest and the most creep resistant. Because of the need for both thermal and mechanical shock resistance, tensile creep tests were then made on K150-type Kentanium compositions containing 20, 25, and 30% nickel binder. A limited number of tests also were made on Kennametal composition K162B and on Firth Sterling composition FS-27. The cermet K162B was the most creep resistant and the strongest. Linear thermal expansion determinations were made on Kennametal compositions K138A, K138, K151, and K151A. Their coefficients of expansion between room temperature and 1205°C. ranged from 8.1 × 10⁻⁶ to 8.6 × 10⁻⁶ per °C. Metallo-graphic examinations were made of all the tensile-tested specimens; in general, the test strains were too small to cause any noticeable deformation or changes in microstructure.     

High-Frequency Ultrasonic Characterization of Sintered Silicon Carbide

High-frequency 60- to 160-MHz ultrasonic nondestructive evaluation was used to characterize variations in density and microstructural constituents of sintered SiC bars. Ultrasonic characterization methods included longitudinal velocity, reflection coefficient, and precise attenuation measurements. The SiC bars were tailored to provide bulk densities ranging from 90% to 98% of theoretical, average grain sizes ranging from 3.0 to 12.0 μm, and average pore sizes ranging from 1.5 to 4.0 μm. Velocity correlated with specimen bulk density irrespective of specimen average grain size, average pore size, and average pore orientation. The attenuation coefficient was found to be sensitive to both density and average pore size variations, but was not affected by large differences in average grain size.     

An Aqueous Gelcasting Process for Sintered Silicon Carbide Ceramics

An aqueous gelcasting process for the preparation of dense as well as porous-sintered SiC ceramics has been described in this paper. A commercial silicon carbide powder coated with phenolic resin was used in this investigation. For the purpose of comparison, a pure SiC powder was also studied. ς potential and viscosity studies revealed that the pure SiC powder requires an electro-steric stabilization, whereas the phenolic resin-coated powder requires an electrostatic stabilization in order to produce their corresponding aqueous slurries with high solids content. Thermogravimetry and differential thermal analysis techniques have been used to study the decomposition behavior of phenolic resin. Aqueous slurries containing 25–50 vol% SiC powder were gelcast and sintered at 2150°C for 1 h. The sinterability of gelcast SiC samples was found to be highly influenced by the SiO₂ formed on the surface of SiC during aqueous processing, as confirmed by the Fourier transform infrared spectroscopy study. The results obtained from various characterization techniques suggest that in order to make dense SiC parts with >3.13 g/mL bulk density (a theoretical density of 97.5%) by an aqueous gelcasting process, the starting phenolic resin (∼5%)-coated SiC powder should possess a median particle size of <11.0 μm, surface area of >3.2 m²/g, a compact (green) density of >1.67 g/mL, and a B content of >0.5%. Further, by using polyethylene granules and organic foaming agents, sintered SiC foam with a porosity of >80%, a compressive strength of >16 MPa and a coefficient of thermal expansion of 4.574 × 10⁻⁶/°C between 30° and 700°C can be prepared by an aqueous gelcasting process, followed by sintering at 2150°C for 1 h.     

Static and Cyclic Fatigue of Alumina at High Temperatures

Fatigue behavior of alumina at 1200°C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading. A variety of loading wave forms were applied during the cyclic tests. Cyclic lifetime is found to be cycle shape dependent and controlled by the duration of the hold time at the maximum tensile stress in a cycle. Cyclic loading with a higher strain rate and a short duration of maximum stress during each cycle provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. The time to failure for cyclic loading with a longer hold time at maximum stress is very close to the static loading lifetime. Viscous boundary phase may be the primary contributor to the improved cyclic fatigue resistance for cyclic loading with a short duration of maximum stress.     

Structural Reliability of Yttria-Doped Hot-Pressed Silicon Nitride at Elevated Temperatures

The strength of yttria-doped hot-pressed silicon nitride was investigated as a function of temperature, time, and applied load. Data collected at 1200°C are presented in the form of a strength-degradation diagram for an applied stress of 350 MPa. At this temperature, the behavior of yttria-doped hot-pressed silicon nitride is found to be superior to that of magnesia-doped hot-pressed silicon nitride, in which creep results in the formation of microcracks that lead to strength degradation. By contrast, the yttria-doped material does not suffer from microcrack formation or strength degradation at 1200°C. Strength degradation does occur at higher temperatures and, as a consequence, an upper limit of 1200°C is recommended for yttria-doped hot-pressed silicon nitride in structural applications.     

Auger Analysis of Hot-Pressed and Sintered Silicon Carbide

Intergranular and transgranular chemistries of hot-pressed and sintered silicon carbides were investigated by Auger electron spectroscopy. Results indicated major differences in grain-boundary compositions between the two. Hot-pressed silicon carbide displayed a complex intergranular chemistry. Sintered silicon carbide displayed grain facets that were free of impurities and additives. The observed intergranular chemistries for both silicon carbides are discussed in terms of their relation to the processing methods.     

Viscous Glass Crack Bridging Forces in a Sintered Glassy Alumina at Elevated Temperatures

Time-dependent crack bridging forces were measured at temperatures from 1150° to 1275°C in a vitreous bonded alumina using the rear-notched, short double cantilever beam technique. Glassy ligaments were observed to bridge the crack even after the crack entered the rear notch. The crack bridging force exhibited both a temperature and displacement rate dependency. A model was developed that describes this temperature and time dependence of the bridging forces that result from the viscous glass bridges.     

Dynamic and Static Fatigue Behavior of Sintered Silicon Nitrides: II, Microstructure and Failure Analysis

The microstructures and failure mechanisms of a sintered silicon nitride, tested under dynamic and static fatigue conditions, were studied. For longer test times at 1000°C, all specimens showed a slow crack growth mechanism of failure, with failure generally initiating at a defect. Even more importantly, these specimens contained cracks in addition to the failure fracture which extended into the material surrounding the initiation site. The cracking always appeared to occur within an amorphous grain-boundary phase.     

Stress Rupture of an Enhanced Nicalon/Silicon Carbide Composite at Intermediate Temperatures

The stress rupture characteristics of an enhanced Nicalon/SiC composite at 900°C have been examined. This temperature has been identified as being in the regime wherein oxidation embrittlement is operative. The enhancement of the composite involves the use of a coating around the fiber tows, comprising a C-rich matrix and B-containing particulates. The efficacy of this oxidation protection scheme has been evaluated by comparing the stress rupture characteristics with those of both Nicalon/SiC composites without the enhancement and the fibers alone. Such comparisons indicate that a substantial portion of the strength loss is attributable to a degradation of the fibers, and that the performance of the enhanced material is marginally better than that of the reference (nonenhanced) composite. Moreover, at stress levels greater than the matrix cracking limit, oxidation embrittlement occurs rapidly and the rupture times (several hours) are short in relation to the targeted service lives of most ceramic composite components. The mechanisms associated with the embrittlement have been identified using scanning electron microscopy and Auger spectroscopy.