Elastic Moduli of Glasses at Elevated Temperatures by a Dynamic Method

The elastic constants of thirteen glasses were measured by a dynamic method at elevated temperatures. Both Young's modulus and the shear modulus were determined and from these Pois-son's ratio was calculated as a function of temperature. Fused silica, Vycor-brand glass, and Pyrex-brand glass had positive temperature coefficients of elastic moduli, whereas all the other glasses showed negative coefficients. Poisson's ratio was found to rise with temperature in all thirteen glasses. This is interpreted as an indication of an approach to the liquid state.     

Sliding Wear of Oxide Ceramics at Elevated Temperatures

Sliding wear tests of sintered alumina and mullite consistently showed that the wear loss significantly decreased at 800°C and above by an order of magnitude. Microscopy of the room-temperature wear surfaces revealed a feature suggesting material removal by brittle fracture. Microscopy of the wear surface at 1000°C revealed that the immediate vicinity of the wear surface consisted of a very fine grain size polycrystalline structure. The zone below this consisted of heavily deformed grains containing dense dislocation networks forming a cellular structure. The results suggest that, at high temperatures, dynamic recrystallization at the wear surface forms the fine grain size structure which suppresses further material removal.     

Double-Shear Geometry for the Deformation and Flow of Ceramics at Elevated Temperatures

Simple-shear deformation is essential to the study of the time-dependent rheological behavior of materials in atomistic and/or molecular terms, for the change in shape under simple shear is not accompanied by any change in volume. A novel test specimen with double-shear geometry is proposed for the study of viscoelastic and elastoplastic deformation and flow of ceramic materials at elevated temperatures. The external load is applied to the specimen in compressive mode, which overcomes several difficulties in high-temperature testing. In order to scrutinize the reliability, reproducibility, and the self-consistency of experimental results obtained in the double-shear geometry, the linear viscoelastic deformation and flow of soda-lime glass are examined at temperatures greater than the glass transition point. It is concluded on the basis of experimental observation that the double-shear geometry proposed in the present study produces ideal simple-shear deformation at elevated temperatures in a relatively easy way and will provide an important tool for characterizing the high-temperature deformation and flow of ceramic materials.     

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.     

Precise Temperature Measurement in Debye-Scherrer Specimens at Elevated Temperatures

A general method was developed for precisely measuring the temperature of a Debye-Scherrer specimen at elevated temperatures. The method consists of measuring the lattice parameters versus temperature of a primary standard which forms one leg of a calibrated thermocouple, with both specimen and thermocouple junction in the X-ray beam, and then mixing the internal standard in powdered form with the powdered Debye-Scherrer specimen so that the lattice parameters of the internal standard directly indicate temperature. Gold was used as the primary standard, platinum as the other thermocouple leg, and magnesium oxide as the Debye-Scherrer specimen. The platinum/gold thermocouple was calibrated from 0° to 1000°C by the National Bureau of Standards. The lattice parameters (in angstroms; corrected for refraction in Ni Kα radiation) versus temperature are expressed by the equations: Gold (25° to 1000°C)

Magnesium oxide (25° to 932°C)

The temperature precision attained with the gold standard was estimated to be 1.5°C.     

Thermal Properties of Transparent Yb‐Doped YAG Ceramics at Elevated Temperatures

This work presents the thermal properties of ytterbium‐doped yttrium aluminium garnet (Yb:YAG) transparent ceramics at elevated temperatures in dependence on the dopant concentration and on temperature. Transparent polycrystalline Yb:YAG ceramics were prepared by solid‐state reaction of oxide powders sintered under high vacuum. The dopant amount varied from 0 to 20 at.% of Yb. Thermal diffusivity of the sintered samples was measured by the laser and xenon flash methods at temperatures ranging from room temperature to 900°C. Both the thermal diffusivity and thermal conductivity values decreased with increasing dopant content, and until 500°C a decrease was observed also with increasing temperature. When available, the measured values were compared to data published in literature, and were found to be in good agreement. Based on the measured values, empirical relations in the form of shifted power laws are proposed for the temperature dependence of thermal diffusivity.     

Measurement of Fiber/Matrix Interfacial Shear Stress at Elevated Temperatures

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic-matrix composites and one metal-matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differences.     

In Situ Measurement of the Elastic Moduli of Glass-Ceramic Thick Films

A line-focus acoustic microscope and a depth-sensing nanoindenter have been used to directly measure the elastic moduli of glass-ceramic thick films on copper and copper/invar/copper substrates. Both techniques proved successful in determining Young's modulus of the films. Measured moduli of the films were close to those of bulk glass-ceramic values for both techniques. The nanoindenter produced slightly higher values than the acoustic microscope.     

Note on Failure of Ceramic Materials at Elevated Temperatures Under Impact Loading

The failure of Al₂O₃, MgO, Pyrex-brand glass, soda-lime-silica soft glass, and a triaxial semi-vitreous whiteware body under impact loading has been observed in some cases from room temperature to 1600°C. The pendulum energy required to fracture these materials remains constant or decreases with temperature over the temperature region of possible use.     

Application of Hertzian Tests to Measure Stress–Strain Characteristics of Ceramics at Elevated Temperatures

A new method for evaluating the elastic–plastic properties of ceramics from room temperature up to the onset of creep based on Hertzian indentation testing is proposed. Indentation stress–strain curves are compiled for representative alumina and zirconia ceramics at prescribed temperatures. Deconvolution of the indentation stress–strain curves for each material provides a measure of Young's modulus, yield stress, and work-hardening coefficient as a function of temperature, enabling construction of true stress–strain curves. The stress–strain curves flatten out with increasing temperature, in accordance with an expected increased plastic response at elevated temperatures.     

Reactions of Zirconia and Titanium at Elevated Temperatures

The reactions between zirconia and titanium were studied by (1) weight change and bulk density determinations, (2) chemical analysis, (3) metallographic and petrographic analyses, (4) microhardness determinations, (5) lattice parameter measurements, (6) electron probe analysis, and (7) thermal conductivity determinations. These studies were made on specimens which had been fired in vacuum in the temperature range 1200° to 2000°C. Experimental data indicated that titanium contents up to 4 atomic % were retained at room temperature as a substitutional solid solution in zirconia. In titanium-rich binary alloys, zirconium and oxygen were retained in solid solution in titanium for zirconia contents up to approximately 10 mole %. In this case, the zirconium entered the titanium lattice substitutionally and the oxygen went into interstitial positions.     

Deformation of Polycrystalline MgO at Elevated Temperatures

Stress-strain curves for five types of polycrystalline MgO are presented as a function of temperature. All types were nominally dense and pure but differed in grain size, composition, and porosity. Above 1200°C deformation occurred by grain boundary shearing accompanied in some cases by slip; below 800°C, specimens fractured primarily by grain boundary parting with little permanent strain. Between 800° and 1200°C, one type deformed plastically by slip; the other four types were brittle. The observed behavior is analyzed in terms of the presence of mobile dislocations, the resistance to dislocation motion, and the strength of grain boundaries.     

Strengths of Ceramic Fibers at Elevated Temperatures

Tensile strengths were measured and Young's moduli were estimated for two SiC-based and three oxide ceramic fibers for temperatures from 25° to 1400°C. The SiC-based fibers were stronger but less stiff than the oxide fibers at room temperature and retained more of both strength and stiffness to high temperatures. High-temperature strengths of the SiC-based fibers were limited by internal void formation and oxidation; those of the oxide fibers were limited by softening of an intergranular glassy phase.     

先驱体转化-热压单向Cf/SiC复合材料的高温弯曲力学行为

研究了采用先驱体转化－热压烧结制备的单向C1／SiC复合材料室温,1573,1723,1923K温度下力学行为,并从显微结构的特征分析了单向Cf/SiC复合材料高温力学行为的变化原因,结果表明：C1/SiC复合的室温,1573,1723,1923K温度下弯曲强度分别为550,392,394,574MPa,弯曲模量分别为157,148,132,83GPa,Cf/SiC复合材料破坏时,其破坏方式将从室温和573K的分层断裂向1723K,1923K的脆性断裂转化,Cf/SiC显微结构的分析表明,在纤维周围和大晶粒间存在着大量的有一定结晶程度的玻璃相,它在高温时的软化对Cf/SiC复合材料的高温强度和弯曲模量变化规律起到重要的支配作用。     

Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.     

Directional Dilatation of Crystal Lattices at Elevated Temperatures

The X-ray measurement of lattice parameters of crystals at elevated temperatures provides a means of determining the true thermal expansion of a crystal. In this investigation, a furnace in which temperatures above 1200°C. could be accurately maintained and a specially designed circular back-reflection X-ray camera were employed in measuring the thermal dilatation of crystals representative of the cubic, hexagonal, rhombohedral, and tetragonal systems. Thermal-expansion values up to 1200°C. are reported for magnesia, alumina, zinc oxide, rutile, beryllia, calcium oxide, and strontium oxide and for magnesia-alumina, magnesia-chrome, zinc-alumina, and zinc-chrome spinels.     

Oxidation Behavior of Titanium Boride at Elevated Temperatures

The oxidation behavior of dense TiB₂ specimens was investigated. Hot-pressed TiB₂ with 2.5 wt% Si₃N₄ as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB₂ exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO₂( s ) and B₂O₃( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO₂ and an outer layer mainly composed of B₂O₃. When the oxidation temperatures were higher than 1000°C, gaseous B₂O₃ was formed along with crystalline TiO₂ by the oxidation process. In this case, the surface was covered with large TiO₂ grains imbedded in a highly textured small TiO₂ matrix.