Alumina Volatility in Water Vapor at Elevated Temperatures

The volatility of alumina in high-temperature water vapor was determined by a weight loss technique. Sapphire coupons were exposed at temperatures between 1250° and 1500°C, water partial pressures between 0.15 and 0.68 atm in oxygen, a total pressure of 1 atm, and flowing gas velocities of 4.4 cm/s. The water vapor pressure dependence of sapphire volatility was consistent with Al(OH)₃( g ) formation. The enthalpy of reaction to form Al(OH)₃( g ) from sapphire and water vapor was determined to be 210 ± 20 kJ/mol, comparing favorably to other studies. Microstructural examination of tested sapphire coupons revealed surface rearrangement consistent with a volatilization process.     

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.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.     

Cyclic Fatigue Behavior of an Alumina Ceramic with Crack-Resistance Characteristics

The behavior under cyclic tension—tension loading of an alumina ceramic with pronounced crack-bridging (R-curve) characteristics is studied. Tests on disk specimens with indentation cracks reveal no failures below the static fatigue limit. Theoretical predictions of the stress-lifetime response, based on the premise that environmentally assisted slow crack growth is the sole factor determining lifetime, are consistent (within experimental scatter) with the data. The results indicate that there is no significant cyclic degradation from potential damage to the bridges, at least in the short-crack region pertinent to strength properties.     

Reaction of Uranium Hexafluoride Gas with Alumina and Zirconia at Elevated Temperatures

Alumina and zirconia are considered for potential use in uranium hexafluoride (UF₆) based nuclear gaseous-core reactors. The corrosion reactions of these oxide ceramics in high-temperature UF₆ environment were investigated. Chemical thermodynamic analysis of the reactions was performed by using a computer data base and code package, FACT, to identify stoichiometric products. An alumina reaction tube in a flowing loop test unit was used to expose alumina and calcia-stabilized zirconia samples to UF, at temperatures from 873 to 1473 K and pressures of about 20.0 to 20.7 k Pa. Chemical reaction rates were measured by a discontinuous gravimetric method. X-ray diffraction and EDS analyses and SEM microstructural examination identified corrosion products and the morphology of the exposed surfaces. Results indicate that reactions were not inhibited or slowed down for either oxide. At 1273 K and below, alumina samples formed stable AIF₃ scales and showed acceptable compatibility with UF₆. The rate of UF₆ reaction with alumina and weight loss dramatically increased at 1473 K. Zirconia samples failed to resist degradation and reacted rapidly with UF₆ at 1073 K. The maximum service temperatures for alumina and zirconia in UF₆ environment do not seem to exceed 1273 and 973 K, respectively.     

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.     

Tribological Behavior of Ti2SC at Ambient and Elevated Temperatures

The tribological properties of Ti₂SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al₂O₃) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10⁻⁴ mm³·(N·m)⁻¹ the specific wear rate (SWRs), was high. However, against Al₂O₃, at ∼5 × 10⁻⁵ mm³·(N·m)⁻¹ and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti₂SC/Inc718 and Al₂O₃ tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10⁻⁵ mm³·(N·m)⁻¹. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti₂SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al₂O₃ in the wide temperature range from ambient to 550°C.     

Liquid-Film-Assisted Formation of Alumina/Niobium Interfaces

Alumina has been joined at 1400°C using niobium-based interlayers. Two different joining approaches were compared: solid-state diffusion bonding using a niobium foil as an interlayer, and liquid-film-assisted bonding using a multilayer copper/niobium/copper interlayer. In both cases, a 127-μm-thick niobium foil was used; ∼1.4-μm- or ∼3-μm-thick copper films flanked the niobium. Room-temperature four-point bend tests showed that the introduction of a copper film had a significant beneficial effect on the average strength and the strength distribution. Experiments using sapphire substrates indicated that during bonding the initially continuous copper film evolved into isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. Film breakup appeared to initiate either at niobium grain boundary ridges or at asperities or irregularities on the niobium surface that caused localized contact with the sapphire.     

Fracture of Alumina Tubes under Combined Tension/Torsion

A fracture criterion for Al₂O₃ tubes is investigated for various loading paths under combined tension and torsion. Experimental data are compared with the Weibull statistical fracture theory. As the stress state approaches a shear dominant state, a tensile principal stress at fracture from the Weibull theory underestimates strengthening effects. By including the effect of the minimum principal stress in the tensile principal stress criterion, an empirical fracture criterion is proposed. Agreement between the proposed criterion and experimental data for Al₂O₃ tubes under combined tension/torsion and balanced biaxial tension is very good.     

Fracture Behavior of Alumina/Monazite Multilayer Laminates

Monazite (LaPO₄) has been proposed as an interphase to promote debonding between the reinforcement and the matrix during the fracture of oxide-based composites. The correlation between fracture behavior and micromechanical properties in model alumina/monazite (Al₂O₃/LaPO₄) multilayer laminates has been investigated in this study. The delamination fracture energy (Γ_i) was dependent on crack length, which is consistent with previous results; the initial value of Γ_i was ∼10 J/m². The interfacial frictional sliding resistance increased as the normal stress on the interface increased. Using a Coulombic friction model, the coefficient of static friction between the Al₂O₃ and LaPO₄ layers was determined to be 0.63. The influence of Γ_i and flaw size in the Al₂O₃ layers on fracture path has been predicted, using an existing model, and confirmed experimentally. The results indicate that, in addition to satisfying energy-based fracture criteria, several other factors affect whether LaPO₄ is a suitable interphase for oxide composites.     

疲劳与蠕变复合作用下聚苯乙烯的断裂行为

研究了聚苯乙烯在疲劳／蠕变复合作用下的断裂行为，研究结果表明，其疲劳／蠕变曲线与纯蠕变曲线十分相似，因蠕变加载作用时间缩短和疲劳载荷变化更频繁，导致在较小的应变下结束普弹应变阶段并进入延迟弹性变形的平台变阶段，随最大载荷加载作用时间延长，断裂寿命减小。     

Microstructure of Alumina Composites Containing Niobium and Niobium Aluminides

The microstructures of niobium-based alumina composites prepared by pressureless sintering of compacts of attrition milled Al₂O₃, Nb, and Al powder mixtures were studied. The addition of a small amount of Al is assumed to assist in rapid sintering. X-ray diffraction analyses show that Al₂O₃, Nb, NbO, and the intermetallics AlNb₂ and AlNb₃ are present in the composites. Electron microscopy studies confirm the existence of these phases and reveal dense, fine-grained (≤500 nm) composites. Al₂O₃ and Nb grains form the matrix. NbO occurs as grains and additionally as small particles within Al₂O₃ grains and at Al₂O₃/Al₂O₃ grain boundaries. The intermetallic AlNb₂ and AlNb₃ phases do not exceed 300 nm in size if they occur at grain boundaries, and possess even smaller dimensions when occluded within Al₂O₃ grains or located at Al₂O₃ triple junctions. While the niobium intermetallics are expected to form during the heating cycle before reaching the sintering temperature, the NbO is assumed to form during the cooling cycle due to precipitation of oxygen dissolved in the niobium.     

Corrosion Behavior of Alumina Ceramics in Caustic Alkaline Solutions at High Temperatures

The corrosion rate and changes in the microstructure and fracture strength of alumina ceramics (93.0% Al₂O₃ and 99.5% Al₂O₃) were studied in 0.1 m to 25 m NaOH solutions at 150°C to 200°C, where m = mol/(kg of H₂O). The attack of the caustic alkaline solution started at the grain boundaries. Consequently, the corrosion resistance increased with decreasing SiO₂ content in Al₂O₃ ceramics, and the corrosion resistance of 99.5% pure Al₂O₃ was similar to that of Si₃N₄ ceramics. Since large pits are formed by corrosion, the surface area increased first and the apparent corrosion rate increased with time in the initial stage of the corrosion. The corrosion rate of Al₂O₃ increased linearly with increasing NaOH concentration, and the activation energy was 102 kJ/mol. The fracture strength of corroded Al₂O₃ decreased monotonically as the degree of dissolution of alumina increased.     

Cyclic Fatigue-Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti₃SiC₂ ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti₃SiC₂ materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K _th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K _th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti₃SiC₂ microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.     

Creep and Creep Fracture in Hot-Pressed Alumina

The creep and creep fracture behavior of two hot-pressed aluminas are presented, for both flexural and tensile testing. Steady-state power-law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single-phase materials such as Al₂O₃. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super-plastic behavior.     

Indentation Fatigue of High-Purity Alumina in Fluid Environments

The influence of environment on the cyclic fatigue behavior of a high-purity alumina bioceramic was investigated using the repeated indentation technique. Tests were conducted in the presence of water, a variety of alcohols, toluene, and simulated physiological fluid environments. The results show that these environments do not have any detectable effect on the damage produced by single indentations, but those containing water cause a significant degradation in cyclic fatigue resistance which cannot be quantified in terms of known subcritical crack growth behavior in static fatigue. It is concluded that the effects of fluid environments on the growth of cyclically driven cracks must be an integral part of the mechanism responsible for cyclic fatigue in ceramics.     

Fracture Resistance of Highly Textured Alumina

Textured alumina has been fabricated previously by either hot-deformation processing, to produce moderate texture, or templating with aligned platelets, to produce stronger texture. Fracture-toughness measurements on ceramics fabricated by hot deformation have indicated only a modest improvement in toughness compared with that of untextured ceramics, while measurements on more strongly textured ceramics have been very limited. In this work, a simplified process for fabricating highly textured alumina was developed, using a solvent-based slurry, tape casting, and liquid-phase sintering. Grain size was tailored to maximize the likelihood of grain bridging and crack deflection. Image analysis was used to characterize morphologic texture, and X-ray pole-figure analysis was used to measure crystallographic texture. Fracture tests revealed significant changes to the crack path as a result of the texture. However, the apparent fracture resistances measured using single-edge notched-beam samples were similar for textured and untextured ceramics. The lack of apparent toughening resulting from texturing is discussed in light of previous results.     

Influence of Grain Size on the Indentation-Fatigue Behavior of Alumina

The surface fracture behavior of a high-purity, high-density alumina, as a function of grain size (3, 5, and 9 μm), was investigated using an indentation-fatigue technique. Increasing the grain size reduced the threshold for crack nucleation, reduced the resistance to surface spalling, and increased the volume of materials lost per spalling event. These results are explained in terms of residual stresses and fatigue damage.     

Comparison of Methods to Determine the Fracture Toughness of Three Glass-Ceramics at Elevated Temperatures

Two standardized methods for measuring k _Ic in ceramics are compared for use at high temperatures (precracked beam and surface crack in flexure). Results from a third technique involving the measurement of cracks around Vickers indentations are also presented. Three dental glass-ceramics, differing primarily in microstructure, were used as model materials in this study. They emphasized the importance of microstructure in determining high-temperature k _Ic values and the challenges in measuring them. The measured fracture toughnesses decreased with temperature and increased with imbedded grain size for all three methods.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.