Influence of an Amorphous Second Phase on the Properties of Yttria-Stabilized Tetragonal Zirconia Polycrystals (Y-TZP)

Y-TZP ceramics with various amounts of glassy intergranular phase were fabricated in order to investigate the effect of this amorphous phase on the properties of TZP. The presence of a liquid silicate phase not only enhanced sintering but also controlled the evolution of distinctly different grain morphologies, dependent on the amount of liquid phase present. The influence of the glass phase on the transformability of grains is discussed with respect to the room-temperature fracture toughness and surface degradation at 250°C.     

Creep Behavior of a Model Refractory System MgO-CaMgSiO4

The effects of the presence of a silicate boundary phase on the high-temperature creep behavior of a model refractory system MgO-CaMgSiO₄ (monticellite, CMS) were studied at 1200° to 1450°C. A change in the dominant mechanism of deformation was determined with increasing temperature and decreasing applied stress. It was concluded that, at 1200°C, deformation is controlled by a dislocation mechanism in the MgO framework, whereas at higher temperatures creep is the result of simultaneous mechanisms but dominated by viscous deformation of the silicate boundary region.     

Surface Crystal Formation During Acid Corrosion of Phosphate-Doped Lead Silicate Glass

The effect of small P₂O₅ additions on the acid corrosion behavior of lead silicate glass was studied at 22°C. Crystals of penta-lead phosphosilicate, a phase heretofore synthesized only by high-temperature melt techniques, formed on the surface of the phosphate-doped glass during the corrosion process. No crystals were observed to form on the pure lead silicate glass. Crystallization appears to result from the controlled release of phosphorus and lead at the glass surface and/or from the reactive catalytic properties of a silica gel surface film. Apparent lead diffusion rates into the acid solution for the phosphate-doped lead silicate glass are much less (by a factor of 11,3) than for the pure lead silicate glass. Application of these findings is suggested in two areas: formulation of low cation-release glass and glaze compositions and the synthesis of high-temperature phases in aqueous solution at ambient temperatures.     

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite( p ) content the high-temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite( p . Furthermore, 10 vol% SiC( w ) reinforced 15 vol% mullite/TZP composites improved the high-temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high-temperature strengthening is attributed to load transfer from TZP matrix to SiC( w ) and mullite particulates. Significant whisker pull-out and interface debonding were also observed on the fractured surfaces when SiC( w ) was coated with Al₂O₃ film.     

Microstructure Analysis and Stress-Strain Behavior of a Model Refractory System MgO-CaMgSiO4

The effects of a silicate boundary phase on the high-temperature stress-strain behavior of MgO materials have been studied using a model system MgO-CaMgSiO₄ (monticellite, CMS). Specimens were fabricated by hot-pressing, or by hot-pressing and annealing, and contained up to 15% silicate phase. The boundary phase consisted primarily of CMS. The stress-strain behavior of these specimens in the range 1200° to 1400°C was strongly dependent on the degree of continuity of the grain boundary phase and on the temperature.     

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP)

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y₂O₃ and ZrO₂, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.     

Tricalcium Silicate and Its Stability Within the System CaO-SiO2

A portion of the system CaO-SiO₂ has been reinvestigated by high-temperature microscopy. Direct observations at the liquidus establish a primary-phase region for tricalcium silicate, which is shown in a revised phase diagram to melt incongruently. The tricalcium silicate primary-phase field extends from the eutectic with dicalcium silicate at 2050 °C., 69.5% CaO, to the incongruent boundary with lime at 2070°C., 71.5% CaO, with a possible error in composition of ± 0.25%. The estimated accuracy of these temperatures is ±3° relative to one another, and ± 10° relative to the melting points of alumina and dicalcium silicate, for which the values 2040°C. and 2130°C. taken from the literature have been assumed.     

Effect of Heat Treatment on Rapidly Quenched AgI-Based Silver Orthoborate Glasses Containing Large Amounts of AgI

The effect of heat treatment on the twin-roller rapidly quenched 75AgI·18.7Ag₂O6.3B₂O₃ glass was investigated by differential scanning calorimetry (DSC), high-temperature X-ray diffraction (XRD), and field-emission-type scanning electron microscopy. The glass had an inhomogeneous microstructure with dispersed particles 40-60 nm in diameter at room temperature. On the other hand, island regions of several hundred nanometers with fine dispersed particles about 20-30 nm in diameter were observed in the glass after heating to 120°C. DSC and high-temperature XRD measurements revealed that crystallization occurred at around 120°C, which is lower than the α-β phase transformation temperature (147°C), to form α-AgI in the glass. The crystallization of α-AgI from the glass below the α-ß phase transformation temperature strongly supports the possibility of the existence of α-AgI nuclei in AgI-based silver orthoborate glasses. Validating the existence of AgI microcrystals supports the microdomain model for superionic AgI-based glasses.     

Redistribution of a Grain-Boundary Glass Phase during Creep of Silicon Nitride Ceramics

The compressive creep behavior of a high-purity silicon nitride ceramic with and without the addition of Ba was studied at 1400°C. Two distinct creep stages were observed during high-temperature deformation of both materials. Transmission electron microscopy (TEM) has been used to characterize the intergranular glass film thickness. Statistical analysis of a number of grain-boundary films indicates that the film thickness is confined to a narrow range in the as-sintered materials. However, the mean thickness is greater in the Ba-doped ceramic than in the undoped material. The standard deviation of the film thickness of a given material is considerably larger after creep than before. We conclude that the grain-boundary glass phase is redistributed during creep, suggesting that viscous flow of the glass phase is responsible for the first stage of the creep process.     

Microstructural evolution in nanocrystalline Y–TZP containing a silicate glass

Nanocrystalline yttria stabilized tetragonal zirconia polycrystal (NC–Y–TZP) powders were coated with a sodium strontium silicate glass using the sol-gel technique. The compact powders were sintered at 1400°C for 1 h resulting in dense nanocrystalline pellets with 5, 10 and 15 vol.% glass. Short pre-sintering heat treatment at 800°C caused the full stabilization of the tetragonal phase while long treatments led to the devitrification of the glassy phase. The mean grain size decreased from 196 nm in the glass-free specimen to 140, 110 and 100 nm in the specimens containing 5, 10 and 15 vol.% glass, respectively. The grain size distribution width also decreased with increase in the glass content. TEM revealed the morphological changes of the highly facetted polyhedral grains in the glass-free specimens into the round-shape grains with increase in the glass content. The effect of the glass on the phase formation, densification and microstructure have been discussed.     

High-Temperature Tensile Deformation of Glass-Doped 3Y-TZP

Amorphous grain boundary phases in 3-mol%-yttria-stabilized zirconia ceramics (3Y-TZP) were studied to determine the influence of intergranular amorphous silicate phases on tensile superplasticity at temperatures of 1300–1500°C. Controlled additions (1 wt%) of compositionally distinct barium silicate and borosilicate phases were used. The initial grain sizes of the pure, barium silicate added, and borosilicate-added samples were 0.45, 0.55, and 0.55 μm, respectively. Systems with added barium silicate and borosilicate glass both exhibited a 60% reduction in flow stress as compared with pure 3Y-TZP, with the lower-viscosity barium silicate system exhibiting a slightly greater reduction in flow stress. The higher-viscosity borosilicate glass/3Y-TZP materials exhibited the greatest elongation to failure, while the barium silicate/3Y-TZP materials had the least elongation. Yttrium was found to segregate to grain boundaries in the pure and borosilicate-containing samples, and both yttrium and barium were found to segregate to grain boundaries in the barium silicate containing samples. No silicon was observed along two-grain boundaries in any of the samples, even those containing pockets of glass. The difference in deformation behavior may be due to a combination of viscosity of the glass addition, grain boundary segregation, and grain boundary bond character.     

Fabrication of Textured Alumina by High-Temperature Deformation

Texture formation via high-temperature deformation in sintered alumina was investigated. Fine-grained, normal-purity-alumina sintered bodies deformed under stresses up to 80 MPa in a temperature range of 1200°–1300°C. Fine, disklike grains formed in the equiaxial fine-grained matrix during high-temperature deformation and aligned unidirectionally via material flow during deformation. Highly textured sintered alumina bodies were obtained via high-temperature deformation and further annealing.     

Tetragonal Zirconia Polycrystals Reinforced with SiC Whiskers

The microstructure and the mechanical properties of hot-pressed tetragonal ZrO₂ polycrystals (TZP) reinforced with up to 30 vol% SiC whiskers were studied. The SiC whisker-TZP composites were stable under the hot-pressing conditions at 1450°C. Annealing in an oxidizing atmosphere at ∼1000°C resulted in glass formation and microcracking caused by whisker oxidation and transformation of the ZrO₂ grains near the whiskers to monoclinic symmetry. The fracture toughness was markedly improved by the dispersed whiskers (∼12 Mpa·m^1/2 at 30 vol% SiC) compared to the values measured for the matrix (∼6 Mpa·m^1/2). The flexural strength of the hot-pressed TZP-30 vol% SiC whisker composite at 1000°C (∼400 MPa) was twice that of the TZP matrix.     

High-Temperature Deformation of Hypostoichiometric 239PuO2

The high-temperature deformation of hypostoichiometric ²³⁹PuO₂ was investigated using constant-displacement-rate compression tests. Considerable ductility (10 to 25% true strain) was observed at 800° to 1500°C. Yield stresses decreased with increasing temperature and decreasing strain rate. Strain-rate sensitivity values increased from 0.011 at 800° to 0.205 at 1500°C, whereas deformation activation energies were 121 to 157 kcal/mol. Deformed microstructures showed no evidence of grain-boundary cracking, suggesting that dislocation motion is the dominant high-temperature deformation mechanism.     

Crystallization in a Barium-Containing Magnesium Aluminosilicate Glass-Ceramic

Crystallization in a Ba-containing magnesium aluminosilicate (Ba-MAS) glass-ceramic was studied using transmission electron microscopy. Ba-MAS slabs were hot-pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ-cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba-rich silicate glass. The high-quartz solid solution generally acted as a precursor for the nucleation of the stable β -phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygonization of β -cordierite.     

Tricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder Diffraction

The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T₂ type structure with disordered SiO₄ tetrahedra was observed, and an average structure for the subcell ( P     , a = 11.7416(2) Å, b = 14.2785(2) Å, c = 13.7732(2) Å, α= 105.129(1)°, β= 94.415(1)°, and γ= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed.     

Effect of Microstructure on High-Temperature Compressive Creep of Self-Reinforced Hot-Pressed Silicon Nitride

An experimental self-reinforced hot-pressed silicon nitride was used to examine the effects of microstructure on high-temperature deformation mechanisms during compression testing. At 1575–1625°C, the as-received material exhibited a stress exponent of 1 and appeared to deform by steady-state grain-boundary sliding accommodated by solution-reprecipitation of silicon nitride through the grain-boundary phase. The activation energy was 610 ± 110 kJ/mol. At 1450–1525°C for the as-received material, and at 1525–1600°C for the larger-grained heat-treated samples, the stress exponent was >1. Damage, primarily in the form of pockets of intergranular material at two-grain junctions, was observed in these samples.     

Dynamic and Static Fatigue of Silicate Glasses

Fatigue of silicate glasses was studied using both static and dynamic tests. Static fatigue data for acid-etched soda-lime silicate glass determined at 74°F in 50 and 100% rh can be represented by a single universal fatigue curve (UFC). The UFC for acid-etched glass does not lie on the UFC of Mould and Southwick, which was determined for abraded soda-lime silicate glass; the acid-etched glass was less susceptible to static fatigue. The susceptibility of acid-etched soda-lime silicate glass to static fatigue differed little from that of pristine E-glass and fused SiO₂ fibers. Dynamic fatigue data for soda-lime, E, borosilicate, and fused quartz glasses agreed well qualitatively and quantitatively with fundamental crack velocity data for these glasses; the dynamic fatigue theory of Charles was used in the comparison. The theoretical implications of these results are discussed.     

Internal Nucleation of Highly Undercooled Magnesium Metasilicate Melts

Crystallization and vitrification in undercooled, fine magnesium silicate droplets, with compositions ranging from 34.5 ≤ wt% MgO ≤ 39.9, were examined following containerless drop tube processing. From an initial phase assemblage of a mixture of the metasilicate (MgSiO₃) polymorphs orthoenstatite and clinoenstatite, three morphological powder types were observed following processing: unmelted shards, glass spheres, and melted/recrystallized spheres. The primary phase in the powders processed at a maximum temperature of ∼1650°C is the high-temperature metasilicate polymorph protoenstatite, with metastable forsterite (Mg₂SiO₄) also appearing. The melted/recrystallized spheres have the uniform, submicrometer texture of a glass ceramic, decisively different from the surface crystallization textures normally seen for melts/glasses of these compositions. Transmission electron microscopy results indicate that the glass-ceramic texture occurs because the process technique allows a liquid-phase immiscibility to precede crystallization. The phases and textures developed during containerless solidification processing of these metsilicate compositions are analyzed thermodynamically; the minimum amount of undercooling required for amorphous phase separation is evaluated using the metastable extensions of the forsterite + liquid and the silica-rich, twoliquid miscibility phase boundaries. The application of metastable phase diagram analysis is demonstrated as an effective guide for identifying potential compositions for development of novel glass-ceramics.     

Leaching Behavior of Sodium from Fine Particles of Soda–Lime–Silicate Glass in Acid Solution

The leaching behavior of Na from soda–lime–silicate glass was investigated by preparing glass powders with average particle diameters of 53 and 19 μm, and leaching in HNO₃ at 90°−140°C. A new theoretical equation for Na leaching from a spherical particle is proposed based on the assumption that a rate-determining process is the three-dimensional self-diffusion of Na in glass. The diffusion constant ( D ) of Na in glass was obtained by fitting the experimental data to a theoretical equation. The values of D and activation energy obtained are comparable to those obtained in other studies on larger particles.