Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Effective diffusion constant and adsorption constant of synthesized alumina,zirconia, and alumina–zirconia composite material

In this study, Al₂O₃, ZrO₂, and Al₂O₃–ZrO₂ composite materials were prepared with the sol–gel technique. X-ray diffraction analysis, differential scanning calorimetry–thermogravimetry, scanning electron microscopy–energy-dispersive X-ray spectrometry, nitrogen adsorption isotherm measurements, and helium pycnometry were used to characterize the resultant materials. Effective diffusion coefficients of helium and hydrogen and the adsorption equilibrium constant of hydrogen in the resultant materials were determined using single-pellet moment technique. The effective diffusivities of helium and hydrogen in both ZrO₂ and Al₂O₃–ZrO₂ composite pellets were found to be smaller than the value found for Al₂O_3, due to the lower tortuosity factor values of the Al₂O₃ pellet. It was found that hydrogen was weakly adsorbed on all resultant materials.     

Physical characteristics and sintering behavior of ultrafine zirconia–ceria powders

Ultrafine zirconia–12 mol% ceria powders have been prepared by the coprecipitation technique. The azeotropic distillation with n-butanol has been carried out to ensure complete elimination of the residual water in the precipitate. This procedure has proved to be quite effective in preventing the formation of agglomerates, which are responsible for inhomogeneities in the sintered microstructure, and for non-densification at low temperatures. The crystallization of the solid solution occurs at 430 °C as determined by thermal analyses. The specific surface of the calcined powder is 127.9 m² g⁻¹ and the pore size distribution exhibits only a maximum at approximately 9 nm. Total shrinkage of the compacted powder reached 30% at 1200 °C. Sintered specimens show six bands characteristics of the tetragonal phase in the Raman spectrum. Specimens with apparent densities >95% of the theoretical density and average grain size of 230–400 nm were obtained after sintering at 1200 °C.     

Sintering,microstrusture and hardness of different alumina–zirconia composites

Two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were used to produce alumina (Al₂O₃)-zirconia (ZrO₂) slip cast composites. The influence of the substitution of Al₂O₃ either by different Al-doped Y-PSZ contents or 50 vol% Y-PSZ on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al₂O₃ and the different composites at temperatures in the range of 1100–1600 °C was studied and related to the sample hardness. An increase in the sintering rate was observed when Al-doped Y-PSZ increased from 22 to 50 vol% or when 50 vol% Y-PSZ was substituted by 50 vol% Al-doped Y-PSZ. 50 vol% ZrO₂ was the most effective concentration to reduce the rate of Al₂O₃ grain growth in the final sintering stage; the Al₂O₃ grain growth began at lower temperatures and became greater with decreasing the Al-doped Y-PSZ content. On the contrary, the ZrO₂ grain growth slightly increased with increasing the Al-doped Y-PSZ concentration. However, for 50 vol% Al-doped Y-PSZ a smaller ZrO₂ grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al₂O₃ grain size of the sintered samples became greater than about 1 µm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400 °C and 1500 °C for Al₂O₃ and the composite with 10.5 vol% Al-doped Y-PSZ, respectively.     

Detection of tetragonal zirconia in alumina–zirconia powders by thermoluminescence

The thermoluminescence (TL) after excitation by UV or X-rays radiation of alumina-zirconia powders is investigated. The composites present five of the characteristic peaks of zirconia at −170, −145, −90, 0 and 95 °C. After a thermal treatment of mixed oxides, a new peak is observed at −35 °C in TL. This peak reveals the presence of stabilized tetragonal zirconia in the material. Moreover by comparing this analysis with those realised by X-ray diffraction (XRD), it can by shown that the TL has one better limit of detection than the XRD.     

Effects of washing and calcination–milling on ionic release and surface properties of yttria stabilized zirconia

Crystallographic features, physical properties and ionic release from yttria stabilized zirconia (YSZ) in suspension were studied by means of XRD, TEM, light-scattering particle size, BET, ICP and zeta potential analysis. It was found that Zr, Y, Na, and to a lesser extent Ca, Hf and Pd leach from 8 mol% YSZ powder. The impurities present increase the zeta potential of suspensions made from as-received YSZ. A trace amount of tetragonal phase observed in 8 mol% YSZ persists following washing and calcination–milling. Dislocations and crystallographic defects together with fractured crystals which form during milling of the calcined powder should lead to the formation of more broken bonds; as a result the surface of the particles can support higher surface charge density. Washing and calcination–milling lead to a shift of the isoelectric point of 8 mol% YSZ from pH 8.4 to pH 6.3 and 6.8, respectively. Due to higher chemical stability and previously shown positive impacts on microstructure and performance of fuel cells, use of calcined YSZ can be more advantageous than as received powder.     

Processing and characterisation of fine crystalline ceria gadolinia–yttria stabilized zirconia powders

For low temperature SOFCs the yttria stabilized zirconia (YSZ)-coated ceria is a promising candidate for replacing YSZ-electrolyte. An important requirement for the co-firing feasibility of such a configuration is the densification of ceria at low temperatures (<1400°C). Fine crystalline gadolinia doped ceria (CGO)-powder readily sinterable at 1250°C was synthesized by co-precipitation with oxalic acid of 0·05 M and crystallization in methanol at 200°C for 6 h. The fabrication and characterisation of solid solution phases with a graded composition (CGO)_x(YSZ)_1−x, to be used as an interlayer between YSZ and CGO, in order to avoid delamination, were also studied and discussed. CGO_xYSZ_1−x powders, prepared by the glycine combustion method, required higher sintering temperatures (1500°C) to densify, while they showed significantly lower ionic conductivity than YSZ and CGO, attributed to the large lattice deformation and scattering of oxygen ions.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

Elastic properties and damping behavior of alumina–zirconia composites at room temperature

Alumina–zirconia composite ceramics (AZ composites) have been prepared in the whole range of compositions from pure alumina to zirconia (in steps of 10 vol.%) by slip casting, followed by sintering at 1350 °C and microstructural characterization via the Archimedes method (relative densities 0.93–0.99). Young's modulus has been measured at room temperature via the impulse excitation technique (IET) and, after appropriate porosity correction (linear, power-law, exponential), found to be in good agreement with the Hashin–Shtrikman bounds. The damping factor (internal friction), which has been measured for dense AZ composites (also via IET at room temperature), is found to increase with increasing zirconia content. Damping factors measured for porous AZ composites with porosities 25–71%, prepared with corn starch as a pore former, have been found to depend only slightly on porosity, unless the porosities are extremely high (>70%). At these porosities, however, where the Young's moduli approach zero, the damping factors exhibit a steep increase.     

Fractal analysis of cracks in alumina–zirconia composites

The crack paths, induced by Vickers indentation in alumina–zirconia composites, were analyzed using fractal geometry. The fractal dimension n_S was calculated for each crack. This parameter refers to a corresponding three-dimensional fracture surface and indicates how its geometry varies by changing the magnification. An interesting correlation between K_IC and n_S was found: it suggests that the samples with high percentages of alumina and also the pure zirconia are characterized by an intergranular fracture mode, while the composites with high zirconia content present a transgranular fracture mode. This result is confirmed by analyzing the energies of fracture calculated using both the classical and fractal approaches. The results obtained in this research not only made it possible to understand the fracture behavior of the analyzed composites, but also confirmed the good potential of fractal analysis to explain complex mechanisms such as those involved in the fracture of brittle materials.     

Mullite–zirconia–zircon composites: Properties and thermal shock resistance

In the refractory field mullite and zirconia are the basis of materials used in the glass industry or when high chemical stability and corrosion resistance are necessary. In this work various mullite–zirconia/zircon compositions were investigated to improve the thermal shock (TS) resistance of dense composites produced by slip casting and sintering at 1600 °C. Zircon (SiZrO₄) acts as bonding phase and its thermal decomposition adds zirconia and silica to the material. Resultant composites were characterized by density and dilatometric measurements, XRD and SEM techniques. TS behavior was tested by quenching in water with quenching temperature differentials ΔT from 400 to 1200 °C. The degree of damage after the TS was experimentally evaluated through the variation of the elastic modulus E which is measured by the excitation technique. The severity of the TS test and the effect of the number of thermal cycles on E for each ΔT employed were determined.The tested materials retained their original mechanical properties for temperatures below a critical temperature ΔT_c near 600 °C. Materials quenched from ΔT of 1000 °C showed as much as 30% reduction in E indicating the important microstructure damage. The TS resistance improved with increasing zircon addition to 35 wt% in agreement with the behavior predicted from R parameter for crack initiation.     

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO₃/ZrO₂, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO₄/ZrO₂) is compared with a Pt/WO₃/ZrO₂ catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO₃/ZrO₂ catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.     

In-situ sol–gel synthesis of zirconia networks in flexible and conductive composite films

The conductive and stretchable films with improved hardness are suitable for the fabrication of flexible electronic devices. This study describes the sol–gel technique to prepare a novel conductive and flexible film consisting of epoxidized natural rubber (ENR), doped polyaniline (PD) and zirconia. The zirconia networks were directly synthesized in-situ in ENR/PD solution and flexible conductive composite film with improved hardness was obtained. The morphology study revealed the size of PD decreased significantly from 77.89 ± 43.95 nm to 4.32 ± 1.13 nm and highest electrical conductivity of 1.9 × 10⁻³ S/cm was achieved with 10 wt.% percolation threshold of zirconia precursor. The binding energy of Zr3d_5/2 and Zr3d_3/2 decreased, suggesting that zirconia was converted to the lower oxidation state. Furthermore, the shape of PD changed from spherical to rod-like structure with root mean square value of 2 nm, while the hardness and reduced modulus improved to 1.72 MPa and 36.7 MPa, respectively.     

Ceria–zirconia mixed oxides: Synthetic methods and applications

The primary objective of this review was to illustrate the significance of ceria–zirconia (CZ) mixed oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. In particular, we were interested in bringing together the recent literature pertaining to these mixed oxides with catalysis perspective. The most prominent application of CZ mixed oxides is in three-way catalysis (TWC) as oxygen storage and release material for several years by replacing cerium dioxide as it shows better efficiency and a high thermal stability. Doping with zirconium oxide, as it is alone a non-reducible oxide, makes the CZ mixed oxide a highly reactive, thermally stable, and more reducible with elevated oxygen storage capacity (OSC) that are important for TWC applications. Apart from the TWC use, the CZ mixed oxides have a huge number of applications, as a direct component or a support, ranging from water–gas shift reaction, reforming of hydrocarbons, dehydration of alcohols, CO₂ utilization, catalytic combustion of pollutants, fine chemicals production, photocatalysis, and so on. All these applications are mainly dependent on three parameters of the mixed oxides, namely, OSC or redox nature, acid–base properties, and crystalline phases. Besides, most of the applications are influenced by the physical properties such as specific surface area, pore volume, pore diameter, crystallite size, and so on. In this review, many details pertaining to the synthesis of these mixed oxides by various conventional and non-conventional methods, their characterization by several techniques, and their application for various reactions of energy and environmental significance, as reported in the literature, are assessed.     

The effect of the surface nanostructure and composition on the antiwear properties of zirconia–titania coatings

This study describes the preparation, surface imaging and tribological properties of titania coatings modified by zirconia nanoparticles agglomerated in the form of island-like structures on the titania surface. Titania coatings and titania coatings with embedded zirconia nanoparticles were prepared by the sol–gel spin coating process on silicon wafers. After deposition the coatings were heat-treated at 500 °C or 1000 °C. The natural tendency of nanoparticles to form agglomerates was used to build separated island-like structures unevenly distributed over the titania surface having the size of 1.0–1.2 μm. Surface characterization of coatings before and after frictional tests was performed by atomic force microscopy (AFM) and optical microscopy. Zirconia nanoparticles were imaged with the use of transmission electron microscopy (TEM). The tribological properties were evaluated with the use of microtribometer operating in ambient air at technical dry friction conditions under normal load of 80 mN. It was found that nanocomposite coatings exhibit lower coefficient of friction (CoF) and considerably lower wear compared to titania coating without nanoparticles. The lowering of CoF is about 40% for coatings heated at 500 °C and 33% for the coatings heated at 1000 °C. For nanocomposites the wear stability was enhanced by a factor of 100 as compared to pure titania coatings. We claim that enhanced tribological properties are closely related to the reduction of the real contact area, lowering of the adhesive forces in frictional contacts and increasing of the composite hardness. The changes in materials composition in frictional contact has secondary effect.     

Processing of different alumina–zirconia composites by slip casting

Two Al₂O₃–ZrO₂ mixture preparation routes: classical powder mixing and addition of a Zr (IV) precursor solution to a well dispersed Al₂O₃ suspension, were used to produce alumina (Al₂O₃)–zirconia (ZrO₂) slip cast composites. For the conventional powder mixing route, two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were employed. The influence of the zirconia content and the solid loading on the rheological properties of concentrated aqueous Al₂O₃–ZrO₂ slips were investigated. The density of green samples was studied and related to the degree of slip dispersion. In addition, the influence of the processing conditions on the density and microstructure development of sintered samples were investigated. By using the Zr (IV) precursor route, nano-sized ZrO₂ (ZN) particles homogeneously distributed on the Al₂O₃ particle surfaces were obtained; however, it let to aggregates of some Al₂O₃ particles with very fine ZrO₂ uniformly distributed. The viscosity and yield stress values of Al₂O₃–ZN suspensions were markedly higher than those of Al₂O₃–Al-doped Y-PSZ and Al₂O₃–Y-PSZ ones, for all the compositions and solid loading studied and resulted in a less dense packing of cast samples. However, for the composite with 10.5 vol% ZN a high sintered density and a smaller ZrO₂ grain size distribution compared with the conventional powder mixing route could be obtained.     

Zirconia toughening of mullite–zirconia–zircon composites obtained by direct sintering

Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological role, because they enlarge the design capability of the manufacturer in properties and behaviors.Some mullite–zirconia–zircon composites were recently processed and characterized which presented satisfactory properties for structural applications under severe chemical and thermomechanical conditions. The objective of the present work is to study the influence of the starting composition in the mechanical and fracture properties of mullite–zirconia–zircon composites, with different microstructures, obtained by direct sintering of binary mixtures of electrofused mullite–zirconia (MZ) and micronized zircon. The materials were consolidated by slip casting of concentrated aqueous suspensions in plaster molds from a wide range of powder compositions (between 15–85 wt% and 85–15 wt% of the two raw materials used).Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD-Rietveld analysis.At low proportion, the zircon was thermally dissociated. The ZrO₂ was a product of this reaction and also influenced the mechanical and fracture properties of these materials through several combined mechanisms, principally as a result of the development of microcracks due to the volume change of the zirconia grains caused by the martensitic transformation during the cooling of these composites from sintering temperature.Composites prepared with higher MZ in the starting powders showed a higher fracture toughness and initiation energy. Microstructure consisting of mullite as a continuous predominant phase in which zircon and zirconia grains were distributed showed better mechanical and fracture properties.     

Influence of zircon particle size on conventional and microwave assisted reaction sintering of in-situ mullite–zirconia composites

Mullite–zirconia composites were fabricated by reaction sintering of ZrSiO₄ and α-Al₂O₃ using conventional heating and microwave processing. The powder mixtures were prepared from sub-micron zircon powders with three different particle sizes and CIPed as coin shaped samples. The samples sintered both in a muffle furnace and microwave furnace. The open porosities, bulk and true densities were measured. Phase transformations were characterized by X-ray diffraction and microstructures were evaluated by scanning electron microscopy. The effects of zircon particle size on the in-situ transformation system and mullitization was evaluated for both methods. As a result, decreasing zircon particle size decreases the in-situ transformation temperature for 25 °C (1575 °C) in conventional heating. Microwave assisted sintering (MAS) lowers the transformation temperature at least 50 °C by lowering the activation energy more efficiently and gives better densification than conventional sintering. Furthermore, milling also produces structures having finer mullite grains.     

Fatigue and fracture characteristics of a fine-grained (Mg,Y)–PSZ zirconia ceramic

The fatigue and fracture characteristics of a partially-stabilized fine-grained zirconia with spinel additions, (Mg,Y)–PSZ, were studied. Fracture toughness, crack growth resistance curves and fatigue crack growth (FCG) behavior, under both sustained and cyclic loading, were evaluated. Mechanical fatigue effects were clearly evidenced by (1) remarkable crack growth rate differences under cyclic and static loading and (2) significant loading ratio effects. Comparing the cyclic and the static FCG behavior allows to deduce a higher cyclic fatigue sensitivity of the fine-grained (Mg,Y)–PSZ with respect to a commercial peak-aged Mg–PSZ used as a reference material. By in situ observation of crack extension under cyclic loading, the fatigue mechanisms could be resolved. Mechanical degradation of bridging ligaments, as already known for coarse-grained Mg–PSZ, is one source of cyclic fatigue. An additional source attributed to the particle dispersed microstructure of the (Mg,Y)–PSZ is the interaction between crack faces and hard spinel particles. The sensitivity of (Mg,Y)–PSZ and Mg–PSZ to cyclic fatigue is discussed in terms of the respective microstructures, prevalence and operativity of distinct mechanical fatigue mechanisms.     

Effect of addition of surface modified nanosilica into silica–zirconia hybrid sol–gel matrix

An organic–inorganic hybrid sol (MZ) comprising a methacrylate functionalized silane matrix (M) and zirconium-n-propoxide (Z) was prepared using sol–gel technique. Two methodologies were adopted to modify the hybrid sol for generating nanocomposite coatings viz., (a) addition of acrylic surface modified silica nanoparticles (N) of diameter ～20 nm to the sol to enhance their compatibility with the hybrid sol–gel matrix and (b) in-situ formation of a three dimensional silica network by addition of tetraethoxy silane (T) to the sol MZ. In the first methodology, the sols were prepared with six different weight ratios of the nanoparticles to the sol, i.e. 0, 0.01, 0.05, 0.1, 0.25 and 1 which were labelled as MZ+Nx where x=0, 1, 2, 3, 4 and 5 respectively. The prepared sols were dip coated on 100 mm×100 mm polycarbonate substrates followed by thermal curing at 130 °C. The coatings were characterized for their mechanical properties like pencil scratch hardness, scratch resistance using scratch tester, nanoindentation hardness, and abrasion resistance as well as visible light transmittance. FT-IR studies were also carried out on heat-treated gels derived from the sols. A maximum pencil scratch hardness of 3H was obtained for the MZ+T coatings and these coatings withstood a critical load of 4.3±0.7 N before failure during scratch test. The maximum nanoindentation hardness of 3.8±0.01 GPa was obtained for the MZ+N5 coatings. The abrasion resistance of MZ+T coatings was higher when compared to MZ+N0 and MZ+N5 coatings. The scratch and nanoindentation hardness were seen to be better for an in-situ formed –Si–O–Si– network in the hybrid sol when compared to those obtained from coatings generated by external addition of acrylic surface modified silica nanoparticles. The difference in properties was attributed to the level of interaction between the nanoparticles and hybrid sol–gel matrix.