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.     

Conductivity and strength behaviour of aluminawhisker-zirconia composites

Yttria stabilized zirconia (8 mol%) composites were fabricated by tape casting with either alumina powder or alumina whiskers, and pressureless sintered. Sintering behaviour, ionic conductivity and mechanical strength were analysed. For all compositions analysed, increasing alumina content reduced the sintered density. For whisker-reinforced zirconia, the rigid whiskers prevented matrix densification along their axis. The ionic conductivity was measured by the complex impedance method from 500–1000 °C and the activation energy for ionic conduction calculated over that range. The ionic conductivity of the alumina-zirconia composites decreased with increasing alumina content as expected by the rule of mixtures. However, the ionic conductivity of the whisker-zirconia composites decreased more than expected possibly due to contamination from the whiskers. The strength of the whisker-zirconia composites was also found to be affected by the porosity. At 5 vol%, the average strength was measured at 39.9 kgf mm^–2, which decreased to 24 kgf mm^–2 at 20 vol%.     

Synthesis of polycrystalline alumina-zirconia fibre using chelated aluminium-zirconium precursor

Polycrystalline alumina-zirconia fibres were successfully synthesized by pyrolysis of preceramic fibres formed from mixed aluminium-zirconium chelate compounds. Ethyl 3-oxobutanoatodiisopropoxyaluminium (EOPA) was reacted with zirconium tetrabutoxide (TBZ) in the presence of glacial acetic acid yielding a polymeric product. The infrared absorptions from 500–625 cm^–1 due to Al-O and Zr-O bonds changed from sharp to coalesced bands by treatment with acetic acid. The signal at 40 p.p.m. in the ²⁷Al spectra of EOPA-TBZ increased in intensity on treating with acetic acid. The viscosity of the polymeric product increased as the amount of acetic acid increased. The viscosity of the precursor decreased on increasing the measurement temperature from 60 °C to 75 °C. The precursor polymer pyrolysed at 800 °C in air showed a broad X-ray diffraction of -alumina, and crystallized in a mixture of -alumina and tetragonal zirconia at 1000 °C. The median diameter of tetragonal zirconia in the -alumina matrix was 33 nm, when EOPA-TBZ (Al/Zr=9/1) was heat treated at 1300 °C for 1 h. The precursor fibres were pyrolysed at 1300 °C to yield fine-grained fibres of -alumina including tetragonal zirconia, which was confirmed by Raman microprobe spectroscopy.     

Sol-Gel Synthesis and Thermal Evaluation of Cordierite–Zirconia Composites (Ce–ZrO2, Y–Ce–ZrO2)

Cordierite and cordierite–zirconia composites (ceria- and yttria–ceria-stabilized zirconia) were prepared by sol-gel processing for different compositions. The precursor powders of these composites were studied using analytical techniques such as thermogravimetric analysis, differential thermal analysis, X-ray diffraction, and infrared spectroscopy for different temperatures to investigate the crystallization behavior of the material. It was observed that the cordierite–zirconia composite powders are crystallized as -cordierite and tetragonal zirconia when sintered at a temperature above 1200°C in the presence of zircon in the cordierite matrix. Powder morphologies of cordierite–zirconia composites have also been investigated.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Preparation of zirconia-toughened bioactive glass-ceramics

Bioactive glass-ceramics toughened by tetragonal zirconia polycrystal (TZP) were prepared by hot-pressing mixed powders of the MgO-CaO-P₂O₅-SiO₂ glass and TZP containing 20 to 80% alumina. The bending strength and the fracture toughness of the composite materials were improved compared with those of the material without TZP. These composites showed high bending strengths (400 to 500MPa) and high fracture toughness ( 2.8MPa m^1/2). The existence of a crack deflection mechanism was observed by scanning electron microscopy. After soaking in simulated physiological solution at 100 °C, no phase transformation from tetragonal to monoclinic of TZP in the composites and no degradation in bending strength occurred.     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Coating of hydroxyapatite on various substrates via hydrothermal reactions of Ca(edta)2- and phosphate

Hydroxyapatite was coated on various substrates such as 12 mol % ceria-doped tetragonal zirconia (12Ce-TZP), 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), alumina, monetite coated titanium (Ti/CaHPO₄) and calcium titanate coated titanium (Ti/CaTiO₃) via hydrothermal reactions of Ca(edta)^2- and 0.05 M NaH₂PO₄ at initial pH 6 and 160–200 °C for 0.5–6 h. Rod-like particles of hydroxyapatite precipitated to form film on the substrates above 160 °C. The morphology of the film changed significantly depending on the characteristics of substrate, i.e. hydroxyapatite entirely coated the surfaces of 12Ce-TZP, Ti/CaHPO₄ and Ti/CaTiO₃ plates, but sparsely deposited on 3Y-TZP and Al₂O₃ plates. Film thickness increased with time (ca. 20 and 90 m on 12Ce-TZP plates for 0.5 and 6 h, respectively, at pH 6 and 200 °C). The adhesive strength of the film for the substrate was in the order, 12Ce-TZP/hydroxyapatite(28 MPa) > Ti/CaTiO₃/hydroxyapatite (22 MPa) > Ti/CaHPO₄/hydroxyapatite (9 MPa). © 2001 Kluwer Academic Publishers     

Strength characterization of yttria-partially stabilized zirconia/alumina composite

The flexural strength of yttria-partially stabilized zirconia/alumina composite in the sintered and hot isostatically pressed condition (Super PSZ) was evaluated as a function of temperature (20–1300°C in air environment), applied stress and time. Failure was essentially governed by the presence of processing defects such as zirconia or alumina agglomerates. The sudden decrease in fracture strength at relatively low temperatures (400–600°C) is believed to be due to the stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 300–1000°C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000°C at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Filter cake forming and hot isostatic pressing for TZP-dispersed hydroxyapatite composite

The combination of a filter cake forming process and hot isostatic pressing was applied to prepare hydroxyapatite composites containing dispersed tetragonal zirconia polycrystal (TZP) with high strength and toughness. Fine TZP powder was dispersed into as-synthesized hydroxyapatite slurry, formed with the filter cake process and hot isostatically pressed at 800–1150 °C at 100 MPa for 2 h. The temperature needed for densification increased with increasing TZP content; 1100 °C was needed to fully densify the composite with 26.8 wt% TZP. No phase change was found in TZP nor in the hydroxyapatite phase up to the maximum temperature examined in hot isostatic pressing. Significant phase change was found in specimens annealed in air at 1200 °C. The strength and toughness achieved were respectively 190 MPa and 2.3 MPa m^1/2. These values were approximately 20% and 100% higher than the corresponding values for hydroxyapatite ceramics without TZP particle dispersion.     

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.     

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Sinterability of magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 m; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.     

Improved thermal shock resistant refractories from plasma-dissociated zircon

Dissociated zircon (DZ), produced in a plasma furnace recombined and sintered to about 92% of the theoretical density in the range 1300 to 1500° C The work-of-fracture of DZ increased from 20J m^–2 to 73 J m^–2 with additions of 10 wt% monoclinic zirconia particles which had a mean diameter of about 13m. Thermal shock data showed that crack propagation in DZ/ZrO₂ composites was stable.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Rare-earth doped zirconia fibres by sol-gel processing

Polycrystalline zirconia fibres, doped with 2–8 mol% of oxides of trivalent lanthanum, praseodymium, neodymium, samarium, gadolinium, and dysprosium (in decreasing cation size), were prepared by spinning of acetate-derived sols and baking the gel fibres thus obtained at 900–1300 °C for 1 h. The larger sized dopants lanthanum, praseodymium and neodymium (Group A) gave rise to tetragonal zirconia, with or without cubic zirconia, at 900 °C which converted partly or fully to monoclinic zirconia, in certain cases accompanied by a cubic zirconate phase at higher temperatures. The smaller sized dopants samarium, gadolinium and dysprosium (Group B) generated only tetragonal or cubic, or both polymorphs of zirconia, depending on the cation type, concentration and temperature. This stabilization of higher symmetry polymorphs with Group B dopants was associated with relatively large crystallite size (especially when calcined at 1300 °C). The maximum tensile strength values of usable fibres calcined at 1300 °C were found to decrease with increasing size in dopant dysprosium > gadolinium > samarium > neodymium, praseodymium, lanthanum=0). Although all the dopant cations were larger in size than Zr⁴⁺ (in the same oxygen coordination), the relative closeness in size of Group B cations with Zr⁴⁺ was considered to be the reason behind the obtained differences in properties.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Fabrication and plastic deformation of Y-TZP/alumina nanocomposite ceramics containing oxynitride glass

Dense yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) +28 vol% alumina nanocomposite ceramics with and without 17 vol% oxynitride glass were fabricated at 1380°C using microwave sintering. The specimens were uniaxially compressed in the temperature range 1250 to 1400°C. Strain rates as high as 10^–4 (s^–1) were measured at 1350°C and 90 MPa in the glass-free specimens with the stress exponent of 1.5. Similar strain rates were measured at lower compressive stresses in the counterpart glass-containing specimens. The stress exponent in the glass-containing specimens changed from 1.0 at 1250°C to 2.0 at higher temperatures. Dynamic grain growth of the alumina grains was inhibited in the presence of the oxynitride glass. Plastic deformation at lower temperatures in glass-containing alloy occurred by cooperative grain boundary sliding, aided by viscous flow of the grain boundary glassy phase. The changes in the deformation behavior at higher temperatures were related to crystallization of the glass and simultaneous plastic deformation by grain boundary sliding.     

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.     

Enhanced performance of fluorine substituted hydroxyapatite composites for hard tissue engineering

Dense fluorine (F) substituted hydroxyapatite composites with yttria-doped zirconia (Y-TZP) and/or alumina (Al₂O₃) were successfully fabricated without applying pressure at 1400 °C for 3 h. The suppression of decomposition via the formation of a fluor-hydroxyapatite (FHA) solid solution allowed the sintered body to reach full density. Such fully densified FHA-composites exhibited improved mechanical properties, such as strength, toughness, and hardness, having values of more than 2–4 times higher than those of pure HA or HA-composites. The proliferation behavior of osteoblast-like cells on the FHA-composites showed no cytotoxicity and comparable cell viability to that observed in pure HA for up to 10 days.