Transparent Armor Ceramics as Spacecraft Windows

The slow crack growth parameters of several transparent armor ceramics were measured as part of a program to lighten next generation spacecraft windows. Transparent magnesium aluminate (spinel, MgAl₂O₄) and AlON exhibit superior slow crack resistance relative to fused silica, which is the historical material of choice. For spinel, slow crack growth, strength, and fracture toughness are significantly influenced by the grain size, and alumina‐rich phases and porosity at the grain boundaries lead to intergranular fracture in coarse grain spinel. Functions describing the required mass for a desired window life imply that transparent ceramics can lighten window panes from a slow crack growth perspective.     

Evaluation of mechanical reliability of zirconia-toughened alumina composites for dental implants

Zirconia-toughened alumina composites containing 0–30 vol% of 3Y-TZP were fabricated by sintering at 1600 °C for 2 h in air. The effect of the 3Y-TZP content on the mechanical properties and microstructure of the alumina ceramics was investigated. The fracture toughness and biaxial flexural strength increased as the 3Y-TZP content increased. The Young's modulus decreased with 3Y-TZP content according to the rule of mixture, while the hardness showed the contrary tendency. The Weibull modulus of the Al₂O₃ with 20 vol% 3Y-TZP composite is higher than that of alumina. The residual hoop compressive stress developed in ZTA ceramic composites probably accounts for the enhancement of strength and fracture toughness, as well as for the higher tendency of crack deflection. No monoclinic phase and strength degradation were found after low temperature degradation (LTD) testing. The excellent LTD resistance can be explained by the increased constraining force on zirconia embedded in alumina matrix.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Preparation and properties of in-situ mullite whiskers reinforced aluminum chromium phosphate wave-transparent ceramics

In-situ mullite whisker reinforced aluminum chromium phosphate wave-transparent ceramics were designed and prepared. The phase transformation, microstructure, mechanical and electrical properties of the ceramics were investigated, and the mechanisms of in-situ growth and toughening were discussed. Results indicated that the in-situ growth of mullite whisker significantly improved the mechanical properties of the matrix, especially the high temperature flexural strength. The room temperature flexural strength, 1000 °C flexural strength and fracture toughness of the ceramics were 135.60 MPa, 121.71 MPa and 4.52 MPa m^1/2. After sintering at 1500 °C, the optimum properties of ε^'_r, tanδ and microwave transmittance at region 8–12 GHz were <3.6, <0.03 and>80%, respectively. The sinterability of ACP matrix was improved by the in-situ process of high mullization above 1450 °C. Using ACP binder as the raw material can avoid the phase transformation from B-AlPO₄ to T-AlPO₄. The synthesized mullite whiskers played a role in toughening by whiskers fracture, crack deflection and whisker pulling out.     

Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB₂-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB₂ grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB₂ grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB₂-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB₂-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB₂-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB₂. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.     

Microstructure and high temperature 4-point bending creep of sol–gel derived mullite ceramics

Four-point bending creep behavior of mullite ceramics with monomodal and bimodal distribution of grain sizes was studied in the temperature range of 1320–1400 °C under the stresses between 40 and 160 MPa. Mullite ceramic with bimodal grain size distribution was prepared using aluminum nitrate nonahydrate as alumina precursor. When γ-Al₂O₃ or boehmite were used as alumina precursors, mullite grains are equiaxial with mean particle size of 0.6 μm for the former and 1.3 μm for the latter alumina precursor. The highest creep rate exhibited the sample with monomodal morphology and grains in size of 0.6 μm, which is about one order of magnitude greater than that for the monomodal morphology but with grains in size of 1.3 μm. The highest activation energy for creep (Q = 742 ± 33 kJ/mol) exhibits mullite with equiaxial grains of 1.3 μm, whereas for sample with smaller equiaxial grains the activation energy is much smaller and similar to mullite ceramics with bimodal grain morphology. Intergranular fracture is predominant near the tension surface, while transgranular more planar fracture is predominant near the compression surface zone.     

High-toughness silicate ceramic

A silicate ceramic that is similar to porcelain and exhibits a maximum toughness of 4.6 MPa m^1/2 was obtained by tape casting from kaolin and 3 vol% of alumina fibers. Improved toughness and strength are achieved with the organized micro-composite microstructure that results from preferential orientation during the shaping of kaolinite particles and fibers in-plane of layers. During sintering, typical nucleation and growth processes of mullite produce specific microstructural characteristics, such as bulk zones, oriented fibers and large interfacial zones between the fibers and the bulk. Toughening is attributed to the decreased crack energy in the bulk ceramic, in which a dense and organized network of short mullite occurs, and in interfacial zones containing a superimposed network of large mullite. The silicate ceramic that is reinforced by only 3 vol% of the alumina fibers is strong (95 MPa) and tough (4.6 MPa m^1/2); although these properties are often mutually exclusive.     

Effect of mullite additions on the fracture mode of alumina

This work analyses the effect of mullite additions on the fracture mode of alumina. Mullite is proposed as an alternative to SiC for the second phase particles because the thermal expansion mismatch between alumina and mullite is of the same sign and order as that between alumina and SiC. Three alumina–5 vol.% mullite composites formed by alumina matrices with similar average grain sizes in the micrometric range (≈1 μm) and second phase sub-micrometric (50–350 nm) and nanometric mullite (<50 nm) particles located at grain boundaries and triple points were prepared. The fracture mode of the alumina matrix changed from predominantly intergranular to predominantly transgranular. This change became more significant as the size of the sub-micrometric fraction of mullite particles decreased.     

A comparison of microstructure and mechanical properties of 12Ce-TZP reinforced with alumina and in situ formed strontium- or lanthanum hexaaluminate precipitates

Composites of 12 mol% Ce-TZP and 30 vol% of alumina 12Ce-30A, strontium hexaaluminate 12Ce-30SA6 and lanthanum hexaaluminate 12 Ce-30LA6 were produced by a mixing and milling method, shaped by slip casting and subsequently sintered in air at 1500–1600 °C. The microstructure, phase composition and mechanical properties of the materials were determined. 12Ce-30A showed the highest hardness, toughness and strength closely followed by the 12Ce-30LA6 composite, the 12Ce-30SA6 showed high toughness at moderate strength and low hardness. Phase analysis hinted at strong differences in the efficiency of transformation toughening. For 12Ce-30A contributions of dilatation and shear were almost fully exploited, 12Ce-30SA6 showed an intermediate efficiency while in 12Ce-30LA6 subcritical transformation was observed. The interaction of cracks with the reinforcement showed specific characteristics for each hexaaluminate. Beside high resistance to subcritical crack growth observed in all materials, 12Ce-30A and 12Ce-30LA6 showed uncommonly high values of crack tip toughness.     

Evaluation of effects of crack deflection and grain bridging on toughening of nanocrystalline SiO2 stishovite

Nanocrystalline SiO₂ stishovite has a fracture toughness higher than 10 MPa m^1/2 due to the toughening mechanism by fracture-induced amorphization. In order to identify other toughening mechanisms which may operate simultaneously, we evaluated effect of crack deflection on fracture toughness. The median deflection angle of nanocrystalline stishovite was lower than polycrystalline ceramics such as silicon nitride and Y-TZP. While the crack deflection can contribute to fracture toughness to some extent, it cannot be the major origin of high fracture toughness of nanocrystalline stishovite. We discussed also the role of grain bridging from the relation between the fracture toughness and grain size.     

Influence of porosity on the mechanical behaviour of single phase β-TCP ceramics

Processing and mechanical behaviour of fine grained (diameter ≈0.5–3 µm) and pure β-TCP materials with different levels of porosity (up to 19%) is described. Pores with diameters, d₅₀ ≈13–14 µm were formed fromcorn starch during sintering. Comprehensive mechanical characterisation –Young´s modulus, strength and toughness– has been done paying special attention to toughness determined in stable fracture tests. The dependence of Young´s modulus and strength with porosity was well fitted to the minimum solid area models while toughness values did not. The competitive processes occurring during fracture impede the degradation of toughness associated to the decrease in Young´s modulus as porosity increases. Materials present similar values of the critical energy release rate G_IC, which describes crack initiation. A maximum of the specific fracture energy, G_F, which averages crack propagation, has been obtained for the material with the highest porosity.     

Preparation and thermal shock resistance of corundum-mullite composite ceramics from andalusite

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.     

Ytterbia–neodymia–costabilized TZP—Breaking the limits of strength–toughness correlations for zirconia?

TZP ceramics were manufactured by hot pressing of pyrogenic zirconia nanopowder which was costabilized by 1 mol% ytterbia and 2 mol% neodymia (1Yb–2Nd–TZP) via the nitrate route. The evolution of microstructure, phase composition and mechanical properties with variation of sintering temperature from 1200 °C to 1400 °C was investigated. 1Yb–2Nd–TZP consists of a bimodal microstructure of small very transformable tetragonal grains and large cubic grains. At intermediate sintering temperature the materials combine a bending strength of 1250 MPa with a fracture resistance >13 MPa √m. The high threshold stress intensity of 7 MPa √m indicates high resistance to subcritical crack growth. An increase in fracture resistance before the crack tip induced by compressive residual stress shifts the strength–toughness correlations to higher values than previously considered possible.     

Processing and mechanical response of highly textured Al2O3

Dense, high quality textured alumina was fabricated by templated grain with only 0.14 wt% (SiO₂ + CaO) and 1–15 wt% tabular alumina templates From stereological measurements, texture fraction was 95% and unaffected by template loading or dopant concentration. Due to the excellent template alignment during casting, the full-width at half the maximum (FWHM) of the rocking curve was exceptionally low at 4.6°. Samples with the largest templated grains (1% templated alumina) have much lower strength (300–320 MPa) than those with smaller templated grains (400–500 MPa). Predominantly transgranular crack paths were observed in fracture surfaces both parallel and perpendicular to the oriented grains. The fracture toughness was anisotropic with slightly higher toughness perpendicular to the basal surface than parallel the direction grain basal surface. Surprisingly, the lowest density sample (93%) tested, which started with 15 wt% templates had the highest strength of 511 MPa ± 0.45 and highest fracture toughness of 4.58 ± 0.44 MPa m^1/2 when measured perpendicular to the basal surface of the grains.     

Flexural strength evaluations and fractography analyses of slip cast mesoporous submicron alumina

Mesoporous submicron α?Al₂O₃ green compacts were fabricated using slip casting of ultrafine alumina powders. The pre-sintered samples were sintered in air atmosphere at 1300 °C, 1350 °C, 1400 °C, and 1500 °C to obtain a variety of grain morphologies namely submicron equiaxed and micro rod structures. The resulting grain diameters lie between ～ 270 nm and ～ 1590 nm and total porosity fraction between 0.05% and 13%. The room temperature flexural strength (σ) evaluations and fractography analyses of sintered alumina samples were performed. It was observed that the total porosity fraction dictates the flexural strength as compared to grain diameter. Further, it was found that the flexural strength exhibited a decreasing trend for an increase in the total porosity fraction, and proved to be a better effective parameter than open porosity fraction. The fractography analyses suggest that samples sintered at 1300 °C and 1350 °C predominantly underwent intergranular fracture, while those sintered at 1400 °C and 1500 °C underwent a mixture of intergranular and transgranular fracture.     

Investigating the effect of andalusite on mechanical strength and thermal shock resistance of cordierite-spodumene composite ceramics

For increasing working stability of cordierite-spodumene composite ceramics for solar heat transmission pipeline, andalusite was utilized as modified additive to improve mechanical strength and thermal shock resistance of the composite ceramics. The effects of andalusite on densification, mechanical strength, thermal stability, phase composition and microstructure were studied. The experiment results showed that andalusite significantly influenced bending strength and thermal shock resistance of the composite ceramics. Especially, specimen B1 with 5 wt% andalusite sintered at 1400 °C achieved the best performances. The linear shrinkage, water absorption, apparent porosity, bulk density and bending strength were 5.62%, 0.02%, 0.06%, 2.19 g cm^?3 and 104.94 MPa, respectively. After 30 thermal shock cycles (wind cooling from 1100 °C to room temperature), the residual strength of the specimen increased to 110.65 MPa, accompanying with ?5.44% strength loss rate. The XRD and SEM analysis illustrated that mullite grains with short rod-like shape could prevent crack growth of inter-granular fracture to enhance bending strength of the specimens. Furthermore, the generation of β-spodumene grains with low thermal expansion coefficient after thermal shock improved thermal shock resistance of the composite ceramics. It is considered that the cordierite-spodumene composite ceramics with high densification, good mechanical strength and excellent thermal stability can be a potential material for high temperature thermal transmission pipeline in solar thermal power generation.     

Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method

Alumina has high heat resistance and corrosion resistance compared to other ceramics such as silica or mullite. However, for its application to refractory bricks, its high thermal conductivity must be reduced. To reduce this thermal conductivity by increasing the porosity, a GS (gelation of slurry) method that can produce high porosity solid foam was applied here to produce the alumina refractory brick. This method was successfully applied to produce alumina foam with high porosity and thermal conductivity of the foam is evaluated. At room temperature, the thermal conductivity was about 0.12 W/mK when the foam density was 0.1 g/cm³. At elevated temperature above 783 K, thermal conductivity of the foam was strongly affected by heat radiation and increased with increasing temperature, in contrast to the thermal conductivity of alumina itself, which decreased with increasing temperature. The alumina foams developed here achieved sufficient thermal insulating properties for use in refractory bricks.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Particle-stabilized ultra-low density zirconia toughened alumina foams

Zirconia toughened alumina (ZTA) is one of the leading engineering ceramics; it is used in a wide range of components and products in applications for which high strength, high toughness, and high temperature stability are needed. The particle-stabilized direct foaming method has lately become a subject of particular interest. Nevertheless only a few studies on combining ZTA ceramics and particle-stabilized direct foaming have been reported. Therefore, in this study, ultra-low density ZTA foams having single strut wall thickness, cell size ranging from 80 μm to 200 μm, and above 90% porosity were successfully fabricated via the particle-stabilized direct foaming method. Valeric acid was used as particle surface modifier to render the particles partially hydrophobic, which stabilized the air/water interface of the ZTA foams. The sintered foams maintained compressive strength up to 8 MPa with porosity of 90%.