Microstructure and mechanical properties of cordierite ceramics toughened by monoclinic ZrO2

The effects of m-ZrO₂ addition on the mechanical behaviour of the cordierite ceramics were studied. Below 10 vol % of m-ZrO₂ content, zircon (ZrSiO₄) was formed as a second phase at the expense of all m-ZrO₂ due to the reaction between ZrO₂ and silica in the cordierite. The sintered density was improved with ZrO₂ addition through glass phase formation along grain boundaries, and maximum sinterability was obtained at 4.6 vol % m-ZrO₂. When sintered at 1300 °C for 4 h, the flexural strength at 4.6 vol % ZrO₂ was 190 MPa, compared with 55 MPa for the pure cordierite. Fracture toughness was gradually enhanced from 1.75 MPa m^1/2 to 2.4 MPa m^1/2 with m-ZrO₂ addition up to 10 vol %, which could be explained partly by thermal expansion mismatch between cordierite and the second-phase ZrSiO₄ and partly by crack deflection at the cordierite-ZrSiO₄ interfaces.     

Thermal shock fracture behaviour of ZrO2 based ceramics

Thermal shock fracture behaviour of alumina, mullite, silicon carbide, silicon nitride and various kinds of zirconia based ceramics, such as magnesia partially stabilized zirconia (Mg-PSZ), yttria and ceria doped tetragonal zirconia polycrystals (Y-TZP and Ce-TZP), Y-TZP/Al₂O₃ composites and yttria doped cubic stabilized zirconia (Y-CSZ), was evaluated by the quenching method using water, methyl alcohol and glycerin as quenching media. Thermal shock fracture of all materials seemed to proceed by the thermal stress due to convective heat transfer accompanied by boiling of the solvents under the present experimental conditions. Thermal shock resistance of zirconia based ceramics increased with increasing the fracture strength, but that of Y-TZP and Y-TZP/Al₂O₃ composites was anormalously lower than the predicted value.     

Effect of cerium addition on phase transformation and microstructure of cordierite ceramics prepared by sol-gel method

Low-temperature sintering of cordierite ceramic depends on the phase transformation into cordierite and the properties depend on its microstructure. In the present work, the effect of cerium on the phase transformation and microstructure of cordierite ceramics prepared by sol-gel method is studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in order to lower the sintering temperature and improve the properties of cordierite ceramic with the addition of cerium. It is observed that the cerium addition obviously lowers the crystallization temperature of -cordierite while slightly raises that of -cordierite. The lowest temperature for cordierite transformation, which approaches the crystallization temperature of -cordierite, is achieved in the sample containing 4 wt% of cerium, implying a possibility to lower the sintering temperature of cordierite ceramics. The Ce-contained ceramics show a biphasic microstructure that is dependent on sintering temperature. Sintered below 1300°C, a cordierite-CeO₂ microstructure is present; while sintered at the temperature above 1300°C, appears a cordierite-glass microstructure, of which the amount of glass phase is limited to a small extent. Since the addition of 4 wt% cerium to this MgO-Al₂O₃-SiO₂ system substantially enhances the densification of cordierite ceramics and lowers the sintering temperature to the level of around 1000°C, it makes the ceramics suitable for such applications, where the low-temperature sintering is required, as the substrates for electronic circuit and the catalytic supports (with oxygen storage capacity) for cleaning of automotive exhaust emissions.     

Mechanical properties and fracture behaviour of ZrO2-Y2O3 ceramics

Stabilized ZrO₂-Y₂O₃ ceramics have been prepared with varying grain sizes and microstructures with the help of different preparation techniques. Bi₂O₃ has been added as a sinter aid to some of the samples. This results in a certain amount of a zirconia-rich second phase. For Bi₂O₃-free samples the fracture toughness (K _lc), and therefore the fracture energy, increases with decreasing grain size. A linear relation with the inverse square root of the average grain size is found. The highest value ofK _lc amounts to 4.1 MPa m^1/2. Fracture toughness values of 1.9±0.2 MPa m^1/2 are measured for Bi₂O₃ containing materials. The fracture surfaces are intergrannular for Bi₂O₃-containing and transgranular for Bi₂O₃-free samples, respectively.     

Mechanical behaviour of MgO-partially stabilized ZrO2 ceramics at elevated temperatures

Transformation toughened partially stabilized zirconia ceramics containing magnesia exhibit quite high fracture toughness (K _lc 8 MPa m^1/2) at temperatures of up to 500° C. The observed temperature dependences of the toughness and the fracture strength are consistent with that of the transformation behaviour. The high toughness of these materials results in a significant reduction in the sensitivity of the flexure strength to crack size increases. Exposure of these materials at 1000° C for prolonged periods results in flexure strength changes associated with the generation of the monoclinic phase by tetragonal precipitate destabilization and eutectoid decomposition. However, when exposed at 500° C, neither the phase contents nor the flexure strength are altered for exposures of up to 1000 h.     

Pressureless sintering and characterization of cordierite/ZrO2 composites

Cordierite/ZrO₂ composites with 5 to 25 wt% ZrO₂ were fabricated by conventional powder mixing and pressureless sintering method. Their densification behavior, microstructure, mechanical and thermal properties were studied. By dispersing 25 wt% (9.57 vol%) ZrO₂, densified cordierite/ZrO₂ composite with a relative density of 98.5% was obtained at an optimum sintering condition of 1440 °C and 2 h. ZrO₂ particles were homogenously dispersed within matrix grains and at the grain boundaries. The intragranular particles were finer than 100 nm and the intergranular particles were coarser. Both fracture strength and toughness could be enhanced more than two times higher, compare to those of monolithic cordierite, by dispersing 25 wt% ZrO₂ into the cordierite matrix. The toughening mechanism in the present composites was mainly attributed to martensitic transformation due to ZrO₂ dispersion. Electronic Publication     

Effect of BaO and SiO2 addition on PTCR BaTiO3 ceramics

The influence of Ba-excess and liquid phase sintering with SiO₂ on the electrical conduction and microstructure in PTCR BaTiO₃ has been investigated. Dense (95–96%), small grain (5–10 m) PTCR materials were obtained in Ba-excess (Ba/Ti=1.006) BaTiO₃. The materials exhibit low room temperature resistivity _RT (10⁰–10² ·cm) and high PTCR respond (more than 5 orders). Solid state sintering was found to inhabit the semiconducting and PTCR behavior in Ba-excess materials. Liquid phase sintering, using SiO₂ in the Ba-excess BaTiO₃, resulted in low _RT and significant PTCR response. Through domain observation, interior Polaron deficient zones were found in samples which exhibit limited liquid phase sintering, leading to non-uniform directional domains and low charge carrier mobility. Proper control of the SiO₂ concentration was found critical for obtaining uniform directional domain microstructures and low _RT.     

Influence of microcracking and homogeneity on the mechanical behaviour of (Al2O3 + ZrO2) ceramics

Quantitative methods for estimations of microcrack density and dispersion homogeneity give evidence of microcrack generation on macrofracture as the dominating mechanism responsible for toughening. The efficiency of energy dissipation at one microcrack density is higher in zirconia-toughened alumina than in pure Al₂O₃ ceramics. Deterioration of dispersion homogeneity results in the promotion of subcritical crack growth and low strength due to large flaw sizes at instability.     

Effect of ZrO2 addition on the mechanical properties of porous TiO2 bone scaffolds

This study aimed at the investigation of the effect of zirconium dioxide (ZrO₂) addition on the mechanical properties of titanium dioxide (TiO₂) bone scaffolds. The highly biocompatible TiO₂ has been identified as a promising material for bone scaffolds, whereas the more bioinert ZrO₂ is known for its excellent mechanical properties. Ultra-porous TiO₂ scaffolds (> 89% porosity) were produced using polymer sponge replication with 0–40 wt.% of the TiO₂ raw material substituted with ZrO₂. Microstructure, chemical composition, and pore architectural features of the prepared ceramic foams were characterised and related to their mechanical strength. Addition of 1 wt.% of ZrO₂ led to 16% increase in the mean compressive strength without significant changes in the pore architectural parameters of TiO₂ scaffolds. Further ZrO₂ additions resulted in reduction of compressive strength in comparison to containing no ZrO₂. The appearance of zirconium titanate (ZrTiO₄) phase was found to hinder the densification of the ceramic material during sintering resulting in poor intergranular connections and thus significantly reducing the compressive strength of the highly porous ceramic foam scaffolds.     

Design and fabrication of porous ZrO2/(ZrO2 + Ni) sandwich ceramics with low thermal conductivity and high strength

3Y–ZrO₂/(3Y–ZrO₂ + Ni) sandwich ceramics were fabricated through cold isostatic pressing and pressureless sintering. Porous 3Y–ZrO₂ ceramics with large connecting open pores and permeability were used as interlayers for insulation, whereas outer metal–ceramic layers were used as bearing loads. Microstructures and properties of the porous ZrO₂ and ZrO₂/(ZrO₂ + Ni) sandwich ceramics were investigated in detail. The ZrO₂/(ZrO₂ + Ni) sandwich ceramics exhibited better mechanical properties than the monolithic porous ZrO₂ ceramics at the same low thermal conductivity (approximately 0.85 W/m K). The mechanical properties of the sandwich ceramics were influenced by metal toughening and sintering-induced residual thermal stress.     

Effects of ZrO2 addition on phase stability and photocatalytic activity of ZrO2/TiO2 nanoparticles

ZrO₂/TiO₂ nanoparticles with various Zr/Ti ratios (0–0.9) were prepared by a polymer complex solution method (PCSM). The prepared samples were characterized using transmission electron microscopy (TEM), the Brunauer, Emmett & Teller (BET) method, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The ZrO₂/TiO₂ photocatalyst showed a high specific area and small crystal size. The XRD pattern for the Zr/Ti = 0.1 sample indicated that the addition of ZrO₂ stabilized the anatase phase of TiO₂ up to 800 °C. The photocatalytic activity of Zr/Ti = 0.1 sample was higher than that of the TiO₂ sample and commercially available Degussa P25. The high photocatalytic activity can be attributed to stronger adsorption in the visible light region, higher specific area, smaller crystal size and increased surface OH groups.     

The effect of ZrO2 and TiO2 on solubility and strength of apatite–mullite glass–ceramics for dental applications

The effect of ZrO₂ and TiO₂ on the chemical and mechanical properties of apatite–mullite glass–ceramics was investigated after sample preparation according to the ISO (2768:2008) recommendations for dental ceramics. All materials were characterized using differential thermal analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. X-ray fluorescence spectroscopy was used to determine the concentrations of elements present in all materials produced. The chemical solubility test and the biaxial flexural strength (BFS) test were then carried out on all the samples. The best solubility value of 242 ± 61 μg/cm² was obtained when HG1T was heat-treated for 1 h at the glass transition temperature plus 20 °C (T_g + 20 °C) followed by 5 h at 1200 °C. The highest BFS value of 174 ± 38 MPa was achieved when HG1Z and HG1Z+T were heat-treated for 1 h at the T_g + 20 °C followed by 7 h at 1200 °C. The present study has demonstrated that the addition of TiO₂ to the reference composition showed promise in both the glass and heat-treated samples. However, ZrO₂ is an effective agent for developing the solubility or the mechanical properties of an apatite–mullite glass–ceramic separately but does not improve the solubility and the BFS simultaneously.     

Mechanical properties of alkoxy-derived cordierite ceramics

Mechanical properties of cordierite ceramics prepared by the controlled hydrolysis and following polycondensation of aluminium, magnesium and silicon alkoxides were investigated in great detail. Flexural strengths of- and-cordierite ceramics are about 60 and 100 MPa, respectively. The flexural strengths of these ceramics are mainly influenced by cracks arising from thermal mismatch between - and -cordierite precipitated during sintering. High fracture toughness of-cordierite ceramics prepared by this method is ascribed to the fine microstructure of the ceramics. The high-temperature flexural strength of-cordierite ceramics is little reduced below 1000° C because of high purity of the ceramics.[/p]     

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.     

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

Abstract

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.     

Thermal shock resistance of ZrO2 based ceramics

Thermal stress fracture behaviour of zirconia based ceramics are described. Although partially stabilized zirconia (PSZ) and tetragonial zirconia polycrystals (TZP) ceramics show superior mechanical properties such as high fracture strength/fracture toughness, the thermal shock resistance of zirconia ceramics is anomalously low. The thermal stress fracture mechanism and improvement of the thermal shock resistance are discussed.