Hindrance of Grain Growth in Al2O3 by ZrO2 Inclusions.

Alumina and Al₂O₃/ZrO₂ (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements. Within the temperature range studied, the ZrO₂ inclusions exhibited sufficient self-diffusion to move with the Al₂O₃ 4-grain junctions during grain growth. Growth of the ZrO₂, inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4-grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al₂O₃ grains. This condition was observed for compositions containing ≤2.5 vol% ZrO₂, where the inclusions did not fill all 4-grain junctions. Exaggerated grains consumed both neighboring grains and ZrO₂, inclusions. Grain-growth control (no abnormal grain growth) was achieved when a majority (or all) 4-grain junctions contained a ZrO₂ inclusion, viz., for compositions containing ≥5 vol% ZrO₂. For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO₂ inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single-phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.     

Grain Growth in Very Porous Al2O3 Compacts

Extensive grain growth was observed by scanning electron microscopy in very porous Al₂O₃ compacts, even at densities <40% of theoretical. After ∼7% shrinkage at 1700°C, the grain size increased from ∼0.3 to 0.51 μm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.     

Exaggerated Grain Growth in ZrO2-Toughened Al2O3

Annealing of ZrO₂-toughened Al₂O₃ (ZTA) at elevated temperatures causes growth of both the intergranular ZrO₂ particles and the Al₂O₃"matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase.     

Faceted Grain Boundaries in Al2O3

Different types of faceted grain boundaries in commercial poly-crystalline Al₂O₃ were examined. Three distinct cases, namely the basal twin, the rhombohedral twin, and other special high-angle grain boundaries, are discussed, comparing theoretical models and experimental observations. It is shown that the faceting of the boundaries can be associated with their migration. A mechanism for the separation of a σ= 13 grain houndary from a pinning void, small grain, or second-phase particle is illustrated. It is also shown experimentally that faceting of high-angle boundaries can occur on a scale of Σ 5 nm, and that, theoretically, faceting most likely also occurs on an atomistic scale. Throughout the theoretical analysis the basal plane is predicted to be a favored facet plane, which is confirmed by experimental observation.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

Coupled Grain Growth Effects in Al2O3/10 vol% ZrO2

Coupled crystallization has been observed in the Al₃O₃/10%-ZrO₂ system by heating an amorphous precursor Al /Zr copolymerized alkoxide network structure. A finely divided two-phase material results which stabilizes tetragonal ZrO₂ to 1700°C and exhibits an unprecedented microstructure. During crystallization, the grain growth of ZrO₂ is coupled to the γ→α phase transformation of Al₂O₃.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

Vapor Growth of Al2O3 Bicrystals

Bicrystals containing symmetrical tilt boundaries with either the
or the
direction as the rotation axis were vapor-grown at 1740°C using the reaction 2AlCl₃( g )+3CO₂( g ) + 3H₂( g )→.Al₂O₃( s ) + 6HCl( g ) + 3CO( g ). Specimens as large as 3 by 5 by 20 mm were produced in 24 h. Chemical purity of the specimens is high, with <0.1 ppm total cation impurities, as determined by activation analysis.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

Journal. Pseudoelastic Behavior in Ce-TZP Al2O3 Ceramics

The pseudoelastic behaviour of 12 mol% Ce–TZP + 10 wt% Al₂O₃ ceramics in the loading–unloading cycles was found for the first time at room temperature, and corresponding reversible tm martensitic transformation mechanism was clearly identified the possibility of reversible ferroelastic domain switching at room temperature was also discussed.     

Effects of Chemical Inhomogeneities on Grain Growth and Microstructure in Al2O3

Effects of chemical inhomogeneities and single-crystal seeds on normal and discontinuous grain growth were investigated in both undoped and MgO-doped Al₂O₃. The chemical impurities in the samples were exsolved at a lower temperature than the sintering temperature and measured by SEM/EDS to determine the correlation between the distribution of impurities and the microstructure in Al₂O₃. A feature of this study was the use of clean-room processing and firing procedures to maintain sample composition at the levels initially present in the starting powders. As the local concentrations of chemical impurities (i.e., Si, Ca) increased, the grain boundary–grain boundary dihedral angle distribution widened, with many angles at 180°, the grain-size distribution widened, and pore–boundary separation was enhanced. Discontinuous grain growth was observed in regions of undoped Al₂O₃ containing the largest Ca and Si concentrations. It is suggested that doping with MgO solute reduces the effects of impurities on grain growth by increasing the bulk solubility and decreasing interfacial segregation of impurities, especially Si, and by narrowing the distribution of grain boundary–grain boundary dihedral angles.     

Kinetics of Growth of Al2O3 Whiskers

The kinetics of the growth of Al₂O₃ whiskers by the reaction of Al with a trace amount of H₂O in an H₂ atmosphere were studied. The activation energy in the region of constant growth rate was ∼77 kcal mol⁻¹. This activation energy value, in the light of thermodynamic calculations and other theoretical considerations, suggests that the rate-determining step in the growth of these whiskers is the dissociation of H₂O molecules on the surface of Al₂O₃.     

Microstructure Development in Al2O3-Platelet-Reinforced Ce-ZrO2/Al2O3 Composites

Composites containing Ce-ZrO₂, Al₂O₃, and aligned Al₂O₃ platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Dynamic Fracture Characterization of Al2O3 and SiCw/Al2O3

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al₂O₃ ceramic and 29 vol% SiC_w/Al₂O₃ composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.     

Surface Modification of Al2O3/TiC Cutting Ceramics

Cutting ceramics with a graded TiC-rich surface have been obtained by an in situ reaction. Al₂O₃/TiC ceramics, prepared by reaction sintering, have been subjected to various thermal posttreatments at atmospheric pressure. The temperature, dwelling time, and reaction atmosphere have been varied. It has been observed that a TiC-rich surface develops in CO-containing atmospheres. The microstructure of the bulk remains unaffected. Thermal treatment under a CO atmosphere at 1600 ° C for 1 h resulted in a dense TiC layer on the ceramic. A schematic model is proposed for this layer formation process.