Strength Characteristics of Transformation-Toughened Zirconia

Mechanisms of failure in a transformation-toughened MgO-partially-stabiltzed ZrO₂ are identified using in situ observations. The observations are related to measured stress-strain curves, strengths under various loading conditions, strength degradation from surface damage, and comparative strengths and toughnesses of toughened and overaged materials. Both reversible and irreversible tetragonal-to-monoclinic transformations, as well as microcracking, are associated with nonlinear stress-strain curves. The influence of this nonlinear response on stresses produced by flexural loading is evaluated. Failure is preceded by stable growth of microcracks, resulting in R -curve behavior and damage-tolerant strength characteristics. The implications of R -curve response for strengthening and toughening characteristics are discussed.     

Strength Limitations of Transformation-Toughened Zirconia Alloys

Two mechanisms are proposed for the limitation of strength in transformation-toughened zirconia ceramics: that the strength is limited by the critical stress to induce the tetragonal-to-monoclinic zirconia phase transformation and that the inherent R -curve behavior of these materials limits the strength. Experimental evidence to support both mechanisms is presented which agrees tolerably well with theoretical predictions. However, for materials with larger initial flaws and higher toughness the experimental results favor the R -curve argument.     

氧化铝陶瓷与金属间钎焊强度的提高

研究了含Al2O395%的BK95-1型真空致密的氧化铝陶瓷。其玻璃相组成中包括SiO2、Al2O3、MgO和B2O3。查明,向陶瓷配料中加入SrO和CaO等氧化物可使陶瓷的烧结温度下降50~70℃,能使其中出现金属化层之前的零件淬火温度下降100~130℃,金属化烧结温度下降30~40℃。此时金属陶瓷的钎焊强度提高20%￣30%,这是因陶瓷的玻璃相与Mo-Mn-Si系金属化面层之间发生强烈反应所致。     

氧化铝粒体在κ-至α-相变过程的发育现象

本研究观察以三水铝矿石[(Gibbsite,Al(OH)3)]为原料获得之氧化铝微粒在κ-至α-相变过程中之粒体成长行为.相变过程存在临界晶径与基础晶径现象.前者之大小对κ-与α-相均约为35nm,后者则为60 nm.由于所得α-Al2O3粒体存有原始粉末粒体的外型,因此如要由三水氧化铝获得奈米级(小于100 nm)α-Al2O3粉末,则于热处理前应将三水氧化铝先行细化.     

Phase Transformation and Thermal Conductivity of Hot-Pressed Silicon Carbide Containing Alumina and Carbon

Phase transformation and thermal conductivity of hot-pressed β-SiC with Al₂O₃ and carbon additions were studied. Densification rate was a complex function of both Al₂O₃ and carbon. Simultaneous additions of Al₂O₃ and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high-temperature hot-pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other.     

Effect of Alumina Addition on the Tetragonal-to-Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y₂O₃ (PSZ) containing 0 to 12 wt% Al₂O₃ was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al₂O₃. The grain size was independent of the addition of Al₂O₃. Both Y₂O₃ and Al₂O₃ segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y₂O₃–Al₂O₃ could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al₂O₃.     

Crack Branching in Transformation-Toughened Zirconia

Crack propagation and branching were investigated in calcia partially stabilized zirconia aged at 1300°C for various times. The crack-branching radii were measured and used to calculate the apparent stress intensity factors at crack branching ( K₈ ) which increase with increasing aging time. The K_B/K_IC ratios increased with aging time with a maximum of 5.3. This maximum ratio is much greater than the ratios previously observed for ceramics.     

Strength of Partly Sintered Alumina Compacts

Pressed compacts of sized alumina powders were studied to determine the dependence of strength of compacts as sintering begins and proceeds through the initial stage. By carefully controlling the time and temperature of heat treatment in a helium atmosphere, it was determined that the strength-controlling feature of these porous compacts is the area of the interparticle boundaries. Under isothermal conditions the strength was approximately proportional to time of sintering to the 2/7 power as described by theory. From these relations, it is possible to develop the semi-empirical relation between strength and porosity that predicted zero strength at a porosity equivalent to the unfired condition of the powder compact. Although not a confirmation of the grain boundary sintering model, the strength and porosity data are more consistent with the grain boundary diffusion model for initial sintering of alumina than with bulk diffusion models.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

Fracture Origin and Strength in Advanced Pressureless-Sintered Alumina

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.     

Laminar Ceramics Utilizing the Zirconia Tetragonal-to-Monoclinic Phase Transformation to Obtain a Threshold Strength

Ceramic laminates have been fabricated with thin layers, containing a mixture of unstabilized zirconia (MZ-ZrO₂) and alumina (Al₂O₃), sandwiched between thicker layers of alumina that contain a small fraction of Y₂O₃-stabilized tetragonal ZrO₂ to inhibit grain growth. The MZ-ZrO₂ undergoes a tetragonal-to-monoclinic phase transformation during cooling to produce biaxial compressive stresses in the thin layers. Cracks that extend within the thicker alumina layers can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. Laminates composed of Al₂O₃ layers 315 ± 15 μm thick and Al₂O₃/MZ-ZrO₂ layers 29 ± 3 μm thick exhibit a threshold strength of 507 ± 36 MPa, regardless of the MZ-ZrO₂ content, for volume fractions ≥0.35. These results, piezospectroscopic stress measurements, and microstructural observations suggest that microcracking produced during the transformation reduces the magnitude of the compressive stresses achieved, which in turn limits the magnitude of the threshold strength.     

Arrest of Fatigue Cracks in Transformation-Toughened Ceramics

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate fatigue crack growth in a transformation-toughened ceramic. The effective stress-intensity factor is calculated during crack growth, because it is believed to determine the crack-growth rate similar to the Paris-type growth law. For certain combinations of transformation strength and load, the effective stress-intensity factor decreases to zero, indicating crack arrest. A detailed parametric study of this phenomenon reveals that the applied load and minimum transformation strength parameter necessary to cause crack arrest are linearly related, independent of initial crack length. This suggests that a threshold stress similar to the endurance limit in the conventional stress/life (S/N) approach should be used instead of the threshold stress-intensity factor in the design of transformation-toughened ceramics against fatigue.     

Role of Grain Size in Strength Variability of Alumina

Relationships between grain size and strength variability in alumina were investigated. Results are presented for an Ultra-high-purity alumina with a narrow grain size distribution and an equiaxed grain morphology at three grain sizes ranging from 5 to 27 μ.m. Vickers indentations are used to introduce controlled flaws into the specimens. It is found that increasing the grain size leads to enhanced flaw tolerance owing to a rising R -curve. However, contrary to recent theoretical predictions, no reduction in strength variability is found with increasing grain size. It is proposed that increasing variability in local fracture toughness with grain size offsets any improvement in reliability. This suggests that an upper bound to the reliability of grain bridging materials may exist.     

Kinetics of Phase Change in Explosively Shock-Treated Alumina

The effect of shock treatment on the kinetics of transformation of transition-phase alumina has been examined. Alumina powder, consisting of spherical particles produced by melting and quenching, was shock-treated at 13- to 26-GPa maximum pressure and subsequently heat-treated at 1000° and 1050°C for various times. At 1050°C, the transformation of unshocked powder was typical of other aluminas with an incubation period of 60 min before significant conversion occurred. In contrast, in the shock-treated material, no incubation period was observed and the α-phase content increased exponentially with time. The growth kinetics of the shocked powder corresponded to a linear growth process with an activation energy of 482 kJ/mol. TEM examination revealed α-phase morphologies consistent with linear growth.     

Increasing Wet Green Strength of Alumina Body During Microfabrication by Colloidal Isopressing

Colloidal Isopressing involves formulating a slurry with a weakly attractive particle network that can be pre-consolidated to a high relative density by pressure filtration and still retain fluid-like characteristics. The pre-consolidated slurry is injected into an elastomeric mold and isopressed. Isopressing rapidly converts the slurry into an elastic body that can be removed from the mold without shape distortion. Not only is this process rapid, but since the water saturated compact produced by this method does not shrink during drying, it can also be converted into a green body without a long drying period. It is demonstrated that micron-size surface features, such as 5 μm wide channels with a depth/width ratio of 2, can be rapidly produced on the surface of alumina powder compacts. The fracturing of thin vertical portions of a micro-patterned surface during pressure release and demolding has been an obstacle to obtaining micron-sized features with high aspect ratios. A method is shown here that enables the fabrication of such features by strengthening the saturated isopressed body. It is shown that concentration controlled gelation of a poly(vinyl alcohol)–Tyzor^® Triethanolamine Tritanate (TE) additive effectively increases the strength of the elastic, isopressed body, saturated with water, while maintaining the low viscosity of the pre-consolidated slurry, which is required for transferring the pre-consolidated slurry into a rubber mold prior to isopressing.     

Preliminary Results on Ozonation Enhancement by a Perfluorinated Bonded Alumina Phase

A preliminary study was undertaken in order to determine the efficiency of ozonation in the presence of perfluorooctylalumina (PFOA) for the removal of organic substances from aqueous solutions by ozone. PFOA was synthesized by reacting hydrated alumina with pentadecafluorooctanoic acid. A monomolecular layer of non-polar perfluorinated alkyl chains, orientated away from the surface of alumina, capable of dissolving ozone and organic molecules, was obtained. The efficiency of ozonation alone and ozonation in the presence of PFOA or Al₂O₃ was compared. PFOA was found to significantly enhance the degradation level of organic substances in comparison with ozonation alone. Al₂O₃ did not show any catalytic activity.     

Sigmoidal Indentation–Strength Characteristics of Polycrystalline Alumina

The fracture properties of a series of three polycrystailine aluminas are examined using indentationstrength and double cantilever beam techniques. The indentation-strength response is shown to be sigmoidal with concave-down behavior at small indentation loads and concave-up behavior at large indentation loads. A model is developed for the general response, combining increasing toughening by ligamentary bridging at small crack lengths and increasing residual-stress relief by lateral cracking at large indentation loads. The model is fit to the strength data and used to deconvolute the underlying toughness variation and predict the intrinsic strength of the materials. Direct measurements of toughness using the "long-crack" double cantilever beam geometry are shown to overestimate the toughness variations effective during "short-crack" strength tests.     

高强度超轻填料──氧化铝空心微珠

介绍氧化铝空心微珠的用途、性能及生产过程。     

Electrical Breakdown Strength of Alumina at High Temperatures

The dc electrical strength of sapphire and poly crystalline alumina was studied up to 1400°C. The electrical strength was essentially identical for both materials. It was > 10⁶ V/cm at room temperature and decreased gradually with temperature up to 900°C (2.6×10⁵ V/ cm), then dropped rapidly to 2×10⁴ V/cm at 1400°C for a sample thickness of ∼ 100 μm. The electrical strength decreased with the sample thickness. It was inversely proportional to the thickness for samples thicker than ∼ 600 μm at 1200°C. The breakdown behavior was explained on the basis of a thermal breakdown model.