Strength improvement in beta″ alumina by incorporation of zirconia

Beta alumina ceramic electrolytes for use in Na/S batteries are inherently weaker than most engineering ceramics due to the presence of weakly-bonded conduction planes in the crystal structure and to difficulties in controlling grain growth during firing. Substantial improvement in microstructural control is obtained by incorporation of monoclinic zirconia (m-ZrO₂) or partially stabilized zirconia (PSZ) resulting in increases in strength and fracture toughness to around 350 MPa and 4 MPam^1/2, respectively. PSZ may adversely influence the electrical resistivity of the ceramic owing to the presence of impurities. With most zirconia powders a high level of retention of tetragonal zirconia (t-ZrO₂) is obtained at levels of addition up to 15% by weight ZrO₂. At these levels ZrO₂/-Al₂O₃ ceramics show low resistivity and stable resistance in Na/S cells.     

New Y-TZP powders for medical grade zirconia

There is interest in using zirconia for biomedical applications as ballheads for total hip prostheses. Two potential types are under discussion:partially stabilized zirconia (PSZ) and tetragonal zirconia polycrystals (TZP)materials. Because of its enhanced material properties, TZP stabilized withyttria is favourable. To eliminate high amounts of natural radioactiveimpurities, the precursors are purified. The kind of precursor and purificationmethod determine the powder impurity level. The disadvantage of Y-TZP is thatthe hydrothermal decomposition reaction method is that it depends very stronglyon the grain size and the distribution of the stabilizing yttria within thezirconia grains. Thermodynamical and kinetic investigations on high puritycoprecipitated and yttria-coated zirconia powders show different behaviours.Y-TZP materials based on yttria-coated zirconia powders show excellentmechanical strength of more than 1000 MPa, a Weibull modulus of up to 20 a!nd a fracture toughness of 9 MPam. The material properties of Y-TZP ceramicsbased on coprecipitated powders and prepared under the same conditions are lessattractive. It is expected that materials based on yttria-coated zirconia willshow enhanced properties compared to materials derived from coprecipitatedpowders. Therefore Y-TZP materials derived from yttria-coated powders are veryattractive as medical grade zirconia.     

Multiphase composites of tetragonal zirconia agglomerate dispersed into alumina and alumina-zirconia matrices

Multiphase composites of yttria- and ceria-doped tetragonal zirconia agglomerates (10–50 m) dispersed into an alumina or alumina-zirconia matrix were sintered at 1500–1600 °C in air, followed by post-Hot Isostatic Pressing (HIP) at 1450°C and 150 MPa in an Ar gas atmosphere. The relative density of the recovered composites was above 98% of the theoretical density. By chemically etching on the surface of zirconia agglomerates, the sinterability of composites was apparently improved; and no microcracks nor pores were observed at the interface of agglomerate and matrix. According to scanning electron microscopy (SEM) observation, tetragonal and tetragonal-monoclinic zirconia agglomerates were highly dispersed into the alumina or alumina-zirconia matrix. The multiphase composites containing 10 vol% spherical agglomerates demonstrate the relatively low value of bending strength, < 400 MPa, and a high value of fracture toughness, > 11 MPa m^1/2. The crack propagation introduced by Vickers indentation was efficiently suppressed and deflected by the agglomerates.     

Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, K, for fracture initiation in cyclic tension is as low as 3.4 M Pam^1/2, and catastrophic failure occurs at K=6.6 M Pam^1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of static failure modes. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y₂O₃. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.     

The spalling of long bars as a reliable method of measuring the dynamic tensile strength of ceramics

An analysis of the spall bar test as a reliable method of determining the dynamic tensile strength of brittle materials is presented. The method is based on the propagation and reflection of elastic waves in bars. Failure occurs when compressive waves are reflected into tensile ones on reaching a free end. The study analyses the hypotheses needed to obtain the true tensile strength with this experimental technique, referring to the requirements of the material and of the experimental procedure. The analysis is complemented with numerical simulations of the testing procedure. The correct way to determine the true dynamic tensile strength of ceramic materials is outlined. Finally, the results of tests of some ceramic materials, three different aluminium oxides, an alumina reinforced with zirconia, silicon carbide and boron carbide are presented.     

A supersaturation model for the degradation of sodium β/β″-aluminas

A mechanism is proposed for the sodium degradation of / alumina electrolytes. It is based on the Na⁺ supersaturation of feeder grains peripheral to flaws in the liquid-sodium/ electrolyte interface. This supersaturation is brought about by the focusing of the current by the flaw and leads to local oxidation of oxygen ions in the aluminate structure. This process leads to the formation of colour-centres and sodium atoms in the grains. The latter coalesce to form sodium colloids, microcracking the microstructures. These microcracks join the originating flaw and promote its extension. A model is presented which demonstrates the feasibility of the proposed degradation mechanism and it is discussed in light of other associated physical phenomena.     

Oxide layer development under thermal cycling and its role on damage evolution and spallation in TBC system

The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y₂O₃) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 °C in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating micro-decohesion and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.     

A Universal Laser-Pulse Apparatus for Thermophysical Measurements in Refractory Materials at Very High Temperatures

This paper presents a new experimental technique enabling thermophysical measurements to be carried out at very high temperatures in a very simple and small pressurized vessel in which the sample is heated by a continuous wave laser, and subsequently subjected to a short temperature pulse. The adopted method is essentially an extension of the laser-flash technique, widely used for thermal diffusivity measurements, whereby, in addition, the heat capacity and, hence, the thermal conductivity, , are simultaneously evaluated from the pulse analysis. Results are presented for the thermal diffusivity and heat capacity of graphite, zirconia, and uranium dioxide up to temperatures above 3000 K.     

Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks

Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks are considered. The work extends the earlier analysis of 2-D crack-microcrack interactions to the 3-D configurations.After analysing simple elementary interaction events (involving only one microcrack) we solve the interaction problem for a number of sample arrays (containing up to 50 microcracks)-realizations of certain microcrack statistics.Statistical aspects of the problem are examined. The interaction effects are found to fluctuate, even qualitatively (from shielding to amplification) along the crack front: the intervals of reduced stress intensity factors (SIFs) alternate with local peaks of SIFs that enhance local front advances. Thus, no statistically stable effect of stress shielding is found (at least, for the microcrack statistics considered): the toughening by microcracking, if it exists, may be due to a statistics of the microcrack centers which is biased towards shielding configurations or to expenditure of energy on nucleation of new microcracks, rather than elastic interactions with them. Similarly to the 2-D case, stochastic asymmetries in the microcrack field produce noticeable secondary modes on the main crack (i.e., modes II and III under mode I loading); this may be partially responsible for crack kinking and an irregular crack path.The short range interactions (several microcracks closest to the main crack tip) play a dominant role. Their impact on the main crack is quite sensitive to the individual microcrack locations and cannot be adequately reproduced by modelling the short range microcracking zone by an effective elastic material of reduced stiffness.The interaction effects in 3-D are found to be weaker than in 2-D.     

Some properties of mullite powders prepared by chemical vapour deposition

The sinterability of mullite (3Al₂O₃·2SiO₂) powder prepared by chemical vapour deposition was examined to improve the conditions for fabricating dense mullite ceramics. The starting powder contained not only mullite, but also a small amount of -Al₂O₃ (Al-Si spinel) and amorphous material. Although the compressed powder was fired at a temperature between 1550 and 1700 °C for 1, 3 and 5 h, the relative densities of the sintered compacts were limited to 90%: (i) due to the creation of pores/microcracks during the solid state reaction (1100–1350 °C), and (ii) due to restriction on the rearrangement of grains because the amount of liquid phase (1550–1700 °C) was insufficient. Calcination of the starting powder was effective for preparation of easily sinterable powder with homogeneous composition. When the compact formed by compressing the calcined powder at 1400 °C for 1 h was fired at 1650 °C for 3 h, the relative density was raised up to 97.2%; moreover, mullite was the only phase detected from the sintered compact. The sintered compact was composed of polyhedral grains with sizes of 1–2 m and elongated grains with long axes of 6 m.     

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 m and a zirconia grain size of 0.2 m. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the alumina phase stage shifted to lower temperature. During pressureless sintering the to alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature.     

Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO<Subscript>2</Subscript>phase diagram

Composites of hydroxylapatite (HA) with pure zirconia, and 3 and 8% Y₂O₃ in zirconia, were pressure-less sintered at temperatures from 900 to 1300C, and hot-pressed at 1200C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000,C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200,C, CaZrO₃ was formed. These reactions and transformations were interpreted in terms of the ZrO₂-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ sintered at 1300,C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering.     

Tensile ductility behaviour of fine-grained alumina at elevated temperature

High-temperature tensile ductility behaviour of polycrystalline fine-grained alumina is shown to be classified into four regimes, depending on flow stress: (1) fast-crack growth regime, (2) single-crack growth regime, (3) microcracks growth regime, and (4) superplastic-crack growth regime, in the order of decreasing flow stress. The unique tensile ductility behaviour observed for each fracture regime is related to the type of damage accumulation. A fracture mechanics model is applied to interpret the tensile ductility of alumina in the superplastic-crack growth regime. The model correctly predicts the observed linear decrease in the true fracture strain with an increase in the logarithm of flow stress. In addition, the model is in quantitative agreement with the increase in the true fracture strain with decreasing grain size when compared at a given stress. The enhancement of tensile ductility in alumina by dilute MgO additions is attributed to an increase in the surface energy and/or decrease in the grain-boundary energy which resists the fracture process. On the other hand, the enhancement of tensile ductility in alumina by addition of a second phase of zirconia is attributed to an increase in the amount of alumina–zirconia grain boundaries which have a low grain-boundary energy. © 1998 Chapman & Hall     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Observation of deformation and damage at the tip of cracks in adhesive bonds loaded in shear and assessment of a criterion for fracture

The evolution of damage at the tip of cracks in adhesive bonds deforming in shear was monitored in real time using a high-magnification video camera. Brittle and a ductile epoxy resins were evaluated, with the bond thickness t being an experimental variable. An extensive zone of plastic deformation developed ahead of the crack tip prior to fracture. In the case of the brittle adhesive, for relatively thick bonds tensile microcracks formed within that zone. Increased loading caused the microcracks to grow from the interlayer to the interface, which led to a complete bond separation after interface cracks emanating from adjacent microcracks linked. In contrast, for the ductile adhesive the crack always grew from the tip. Strain gradients tended to develop there when the bond thickness was large.The adhesive shear strain was determined from fine lines scratched on the specimen edge. For both adhesives, the average crack tip shear strain at crack propagation rapidly decreased with increasing t. This effect was attributed to the changing sensitivity of the bond to the presence of flaws; thicker bonds can accommodate larger microcracks or microvoids which cause greater stress concentration. For a given bond thickness, the critical crack tip shear strain agreed well with the ultimate shear strain of the unflawed adhesive previously determined using the napkin ring shear test [12]. This suggests that the ultimate shear strain is a key material property controlling crack growth. The critical distortional strain energy/unit area of the unflawed adhesive W _s was determined from the area under the stress-strain curve in the napkin ring test. Good agreement between W _s and the adhesive mode II fracture energy was found for all joints tested except for relatively thick bonds. For the particular case of an elastic-perfectly plastic adhesive, the agreement above implies % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaGqaciaa-Deada% WgaaWcbaacbaGaa4xsaiaa+LeacaGFdbaabeaakiabg2da9iaa-Dfa% daWgaaWcbaGaa83CaaqabaGccqGHHjIUcaWF0bGaeqiXdq3aaSbaaS% qaaiaa-LhaaeqaaOGaeq4SdC2aaSbaaSqaaiaa-zgaaeqaaaaa!463A!\[G_{IIC} = W_s \equiv t\tau _y \gamma _f \].     

Microstructural and chemical stability of Y-ZrO2 reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of -alumina reinforced with 10 vol% of tetragonal partially stabilized 3 mol% Y₂O₃-ZrO₂ (3Y-ZrO₂) and with 10 vol% of cubic 8 mol% Y₂O₃-ZrO₂ (8Y-ZrO₂) in molten sulfur or molten Na₂S₄ has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 °C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into -alumina and immersed in molten Na₂S₄. Destabilization without incorporation into -alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of -alumina enhanced zirconia destabilization in that -alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y₂O₃ from partially stabilized 3Y-ZrO₂. From XRD analyses, changing from partially stabilized 3Y-ZrO₂ to cubic 8Y-ZrO₂ in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO₂ suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y₂O₃ and the corrodants. In order to avoid depletion destabilization of zirconia in -alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO₂ is used.     

Pressureless sintered ATZ and ZTA ceramic composites

Alumina-20 wt% zirconia (ATZ) and zirconia-20 wt% alumina (ZTA) composites were prepared by conventional sintering of commercial powders, with average particle sizes in the range 0.35–0.70 m. Sintering at 1650 °C for 4 h resulted in final densities up to 96%. Bending strength and hardness increased with the final density. The tetragonal volume fraction was strongly dependent on both the final density and tetragonal grain size. The relatively high fracture toughness of 9 MPa m^1/2 was associated with the highly dense microstructure consisting of tetragonal grains of the critical size.     

Effect of barrier layer thickness and composition on fracture toughness of layered zirconia/alumina composites

Composites of yttria or ceria-partially-stabilized zirconia with layers of either alumina or a mixture of 50% by volume of alumina and zirconia were fabricated by sequential centrifuging of powder suspensions. This method allowed formation of layers with thickness of 10 to 70 m. In both cases (Y-ZrO₂ and Ce-ZrO₂ matrices), a significant increase in fracture toughness, work of fracture and bending strength was observed only for composites with barrier layers made of a pure alumina. A crack deflection in alumina layer was found to be the main mechanism responsible for an increase in mechanical properties. For confirmation this thesis, no increase in the transformation zone width was observed. As it was shown, crack deflection angle was dependent on alumina layer thickness. Higher deflection angles for a thicker alumina layers were found. Explanation of this phenomenon was given by determination of residual stress distribution in barrier layers made by piezospectroscopy. A correlation between the crack deflection angle and the difference of stress between the layer boundary and the centre of the layer was noticed. The residual stresses observed are a result of thermal expansion mismatch between alumina and zirconia and thermal anisotropy of alumina. Shrinkage mismatch, especially in the case of Ce-ZrO₂ and Al₂O₃, as a third source of stress is suggested.     

Electrochemical ZrO2 and Al2O3 coatings on SiC substrates

SiC was electrochemically coated with ZrO₂ and with Al₂O₃ from 0.1 m aqueous solutions of metal-nitrate-hydrates with ethanol added. Amorphous zirconia and alumina coatings were formed with current densities from 10 to 70 mA cm^–2, and deposition durations of 1–60 min. The as-deposited coatings contained microcracks caused by drying shrinkage. Sintering of zirconia at 900 °C for 1 h and of alumina at 1200 °C for 2 h in air was accompanied by crystallization to a mixture of tetragonal and monoclinic phases in the former and to -alumina in the latter. The absence of intermediate phases between the coatings and the substrates and the good adherence of the sintered coatings indicate the high-temperature stability of these coatings.     

Deformation and failure of adhesive bonds under shear loading

Experiments have shown that certain mechanical properties can be greatly enhanced when a material is stressed while under tight spatial constraint. In this work, the post-yield behaviour of brittle and ductile epoxy resins used as thin adhesive bonds was determined using the napkin ring shear test. Real-time observations of the deformation in the bond as well as SEM post-failure analysis were employed to gain information on the failure process. The complete stress-strain histories of the adhesives were established for bond thicknesses ranging from the micrometre level up to values large enough to expose the bulk properties. The most dramatic variations occurred for the ultimate shear strain, _f; for the brittle adhesive, _f increased by over 30-fold relative to the bulk material when the bond thickness, t, was decreased to a few micrometres. Experimental evidence and analytical considerations suggest that the decline of _f with t was due to premature bond failure caused by tensile microcracks or voids that were formed in the interlayer during loading, with the specific _f versus t relationship being a mere reflection of the variations in the degree of stress concentration at the tip of the flaws. The astonishingly large value of _f (i.e. 2.8–3.4) found for the brittle epoxy in the micrometre thickness range, is believed to represent the intrinsic shear strain of this material.