Segregation Effects at Grain Boundaries in Fluorite-Structured Ceramics

The atomic-scale structure, composition, and chemistry of grain boundaries in two fluorite-structured ceramic materials were characterized by a combination of Z -contrast imaging and electron energy-loss spectroscopy (EELS). In the case of a symmetric 24° [001] tilt bicrystal of yttria-stabilized-zirconia (YSZ), a shift in the zirconium M -edge onset and a change in the yttrium and zirconium M -edge ratios at the boundary indicate an increase in the number of electrons in the boundary plane. A detailed study of the structure and composition indicates that this is caused by an increase in the number of oxygen vacancies in the grain boundary core that is partially compensated by yttrium segregation. Studies of grain boundaries in an industrial Gd-doped ceria ceramic reveals similar changes in vacancy/dopant profiles indicating that these effects may be generic to grain boundaries in fluorite-structured materials.     

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.     

STEM Analysis of Grain Boundaries in Cemented Carbides

There is controversy over whether the cobalt binder of liquid-phase sintered WC forms a continuous skin over the WC particles or whether the carbide grains form a continuous skeleton. It is shown that the atomic Co/W ratio in the 20-Å grain boundaries is >3 times larger than that in the grains, supporting the former model.     

Effect of Grain Boundaries on Plastic Deformation of Sodium Chloride

Sodium chloride single crystals were joined at a melt zone with a hot-wire technique to produce bicrystals of controlled crystallographic orientation. The plastic deformation properties of the bicrystals were measured in bending while the birefringence was observed with a petrographic microscope. The birefringence at the boundary was interpreted to indicate that significant deformation across the grain boundaries occurred. The stress required for deformation across the boundary was found to increase with increased angle between crystallographic axes. Twist boundaries were found to be more resistant than tilt boundaries. Behavior was consistent with the dislocation model for grain boundaries.     

Superplasticity-like Deformation of Nanocrystalline Monoclinic Zirconia at Elevated Temperatures

The deformation behavior of nanocrystalline monoclinic ZrO₂ polycrystals (nanocrystalline MZP) was studied at 1273–1373 K in compression tests. The deformation of nanocrystalline MZP was characterized by stress exponent ( n = 2.5), grain-size exponent ( p = 2.5), and apparent activation energy ( Q = 350 kJ/mol). The values of n and p were similar to the superplasticity of high-purity Zn-22% aluminum alloy. The strain rate of nanocrystalline MZP was faster than that of Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) at temperatures lower than the monoclinic-tetragonal transition temperature. The strain rate of MZP gradually approached to that of Y-TZP as the temperature increased to the transition temperature. The comparison of present data with published data suggested that trace amount of impurities affected the deformation behavior of MZP.     

Effect of Grain Size on the Sliding Wear and Friction of Alumina at Elevated Temperatures

The sliding friction and wear of three different grain-size aluminas were studied from room temperature through 1000°C. The coefficient of friction revealed two distinct regions of decrease with increased temperature, with a transition at ∼700°C. Below 700°C, the coefficient of friction decreased rapidly with increased temperature (∼10^-3/°C). However, above 700°C, the decrease was more gradual (∼10^-5/°C). This was believed to be related to a brittle-to-ductile transition at the wear surface. The coefficient of friction was only weakly dependent on grain size, because the largest grain sizes exhibited slightly higher friction coefficients. However, the specific wear loss of the aluminas increased with increased grain size at room temperature and at 600°C, both below the 700°C transition. The primary mechanism of wear was ascertained to be brittle microfracture along grain boundaries. At 1000°C, above the 700°C transition, the specific wear loss was significantly decreased and appeared to be independent of the alumina grain size. At 1000°C, the wear surfaces developed a thin layer of fine grains formed by dynamic recrystallization. The grain size within the thin layer was in agreement with the previously reported grain-size/Zener-Hollomon parameter relationship.     

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...     

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.     

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.     

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.     

Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K ₁₀ for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, K ₁₀× 2.25 MPa * m^1/2 at 1200° and ∼1.75 at 1400°C. On oxidation for 10 h at 1200°C, K ₁₀ drops to ∼1.75 MPam^1/2 at 1200° and ∼1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.     

Space Charge Segregation at Grain Boundaries in Titanium Dioxide: II, Model Experiments

A quantitative study of space charge solute segregation at grain boundaries in TiO₂ is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part I. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are "special" in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor- and donor-doped compositions, the Frenkel energy (assumed to be lower than the Schottky energy in TiO₂) can be separated into its individual terms. An average value for the titanium vacancy formation energy of g_vTi = 2.4 eV and an upper limit to the titanium interstitial formation energy of g_Tii = 2.6 eV are obtained.     

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.     

Effect of Grain Boundaries on Thermal Conductivity of Silicon Carbide Ceramic at 5 to 1300 K

The thermal conductivity of a SiC ceramic was measured as 270 W·m⁻¹·K⁻¹ at room temperature. At low temperatures ( T < 25 K), the decrease in the conductivity was proportional to T ³ on a logarithmic scale, which indicated that the conductivity was controlled by boundaries. The calculated phonon mean free path in the ceramic increased with decreased temperature, but was limited to ∼4 μm, a length almost equal to the grain size, at temperatures below 30 K. We concluded that the thermal conductivity of the ceramic below 30 K was influenced significantly by grain boundaries and grain junctions.     

Detection of Boron Segregation to Grain Boundaries in Silicon Carbide by Spatially Resolved Electron Energy-Loss Spectroscopy

Boron segregation to grain boundaries in SiC was directly observed for the first time by using spatially resolved electron energy-loss spectroscopy methods. The hot-pressed, fully dense material was doped with 0.3 wt% of boron and was free of other additives, except for 2 wt% of free carbon. The detection of boron was achieved in the difference spectra at all the grain boundaries that were examined. Its interfacial excess was in the range of 15–29 atoms/nm², or approximately one monolayer. Concurrently, silicon depletion occurred at these boundaries, although to a lesser extent (−13.5 atoms/nm² on average), which indicated that boron mainly replaces silicon and bonds with carbon at the grain boundary. These findings validate the dual role of boron at the grain boundary for promoting densification via improved grain-boundary diffusivity while maintaining a covalent grain boundary without an oxide phase.     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

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.     

Role of Solute Segregation at Grain Boundaries During Final–Stage Sintering of Alumina

The addition of minor amounts of MgO or NiO to Al₂O₃ inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of solute at the grain boundaries, causing a decrease in the grain–boundary mobility. The segregation of MgO or NiO at the grain boundaries can be inferred from the results of the microhardness studies presented and is substantiated by autoradiographic experiments and also by lattice parameter determinations as a function of grain size.     

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.     

Microstructure-Property Relations of Hot-Pressed Silicon Carbide-Aluminum Nitride Compositions at Room and Elevated Temperatures

A series of SiC-AlN compositions of 0, 10, 25, 50, 75, 90, and 100 mol% AlN were hot pressed at 2100°C for a 1 h soak at a pressure of 35 MPa under vacuum. 2H-wurtzite SiC-AlN solid-solution structures were formed for compositions with 25-100 mol% AlN. The associated lattice parameters for these solid solutions followed Vegard's law. The microstructures varied with composition; the number of needlelike grains decreased for compositions up to 25 mol% AlN and the amount of equiaxed grains increased for compositions with 25–100 mol% AlN. Densities for all the specimens were >99% of the theoretical density. Coefficients of thermal expansion varied from 4.80 × 10^-6/°C to 6.25 × 10^-6/°C in the 20°-1400°C range. Young's moduli varied from 451 GPa to 320 GPa at room temperature (RT) and retained 98%, 96%, and 94% of their RT values at 500°, 1000°, and 1250°C, respectively. These three properties correlated linearly with composition. RT microhardness varied from 21.6 GPa to 11.2 GPa and correlated linearly with composition within the solid-solution range. Flexural strengths increased from 487 MPa to 604 MPa from 0 mol% AlN to 25 mol% AlN and then decreased to 284 MPa for 100 mol% AlN. At 1250°C, flexural strengths decreased from 90% to 65% of the RT values. Fracture toughness increased from 3.6 MPa·m^1/2 to 4.2 MPa·m^1/2 from 0 mol% AlN to 10 mol% AlN and then decreased to 2.5 MPa·m^1/2 for 100 mol% AlN.