Composite materials with ZrO2–TiO2 matrix and ZrO2 containing ceramic fibres

Abstract

The aim of the work reported in the present paper was to obtain composites consisting of a ZrO₂–TiO₂ matrix reinforced with ceramic fibres containing 12 wt-% ZrO₂, which are resistant to temperatures higher than1500°C. The resulting ceramic matrix consisted of 95 wt-% ZrO₂, partially stabilised with CaO, and 5 wt-% rutile TiO₂. A ceramic fibre content of 0·82 vol.-% was used and for the matrix, several grades of ZrO₂ partially stabilised with CaO were explored, prepared by dry and wet grinding for various grinding periods. Composites were prepared by uniaxial die pressing at 350 MPa and sintered at two temperatures: 1360°C for 1 h and 1500°C for 3 h. The resulting composites showed the following range of properties: total drying and firing shrinkage 0·4–3·3%; apparent density 3·51–3·96 g cm^-3; porosity 25–34%; water absorption 6–10%; bend strength 12–43 MPa. The optimum ZrO₂ grades were determined based on physical and mechanical properties, and on structural determinations carried out by thermodifferential and thermogravimetric analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). SEM evaluation illustrated the increase in average size of crystallites typical of ZrTiO₄ solid solution as a function of temperature, from 2 μm at 1360 up to 14 μm at 1600°C, and of their tendency to sinter.     

Sintered ZrO2–TiO2 ceramic composite and its mechanical appraisal

This work mainly considered the effect of different TiO₂ additions and of sintering temperatures on the structural change, densification and mechanical properties of ZrO₂–TiO₂ ceramic composites obtained by cold compaction and subsequent sintering. The results demonstrated that the structural transformation happens from pristine monoclinic zirconia into tetragonal zirconia, amount of cubic phase in as-obtained ZrO₂–TiO₂ specimens could be distinguished as well. The increasing concentration of TiO₂ addition facilitated lower the sintering temperature and densification of ZrO₂ matrix. The grain growth and bulk density of ZrO₂–TiO₂ ceramic composites varied with the sintering temperatures and dopant concentrations. Full evaluation of the role of TiO₂ addition and sintering temperature on the mechanical properties of ZrO₂–TiO₂ samples was carried out in terms of Vickers hardness, flexural strength and fracture toughness. In particular, the ZrO₂ matrix with a value of 5 wt % TiO₂ generated the desired flexural strength and fracture toughness at the sintering temperature of 1400 °C.     

Mechanical and functional properties of Al2O3–ZrO2–MWCNTs nanocomposites

Multi-walled carbon nanotubes (MWCNTs) are often reported as additives improving mechanical and functional properties of ceramic composites. However, despite tremendous efforts in the field in the past 20 years, the results are still inconclusive. This paper studies room temperature properties of the composites with polycrystalline alumina matrix reinforced with 0.5–2 vol.% MWCNTs (composites AC) and zirconia toughened alumina with 5 vol.% of yttria partially stabilised zirconia (3Y-PSZ) containing 0.5–2 vol.% of MWCNTs (composites AZC). Dense composites were prepared through wet mixing of the respective powders with functionalised MWCNTs, followed by freeze granulation, and hot-pressing of granulated powders. Room temperature bending strength, Young's modulus, indentation fracture toughness, thermal and electrical conductivity of the composites were studied, and related to their composition and microstructure. Slight increase of Young's modulus, indentation fracture toughness, bending strength, and thermal conductivity was observed at the MWCNTs contents ≤1 vol.%. At higher MWCNTs contents the properties were impaired by agglomeration of the MWCNTs. The DC electrical conductivity increased with increasing volume fraction of the MWCNTs.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Densification and properties of ZrO2 nanoparticles added magnesia–doloma refractories

In this work the effect of nano- and microZrO₂ addition on the densification and hydration resistance of MgO–CaO refractories was investigated. 0, 2, 4, 6 and 8 wt% ZrO₂ was added to MgO–CaO refractories that contain 35 wt% CaO. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. Results show that with addition of ZrO₂ the bulk density and hydration resistance of the samples increased while apparent porosity decreased. Also the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also it revealed that the nanoZrO₂ addition was more effective than microZrO₂ due to its higher activity.     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

Phase diagram study and thermodynamic modeling of the Na2O–Al2O3–ZrO2 system

The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was experimentally studied at 1500°C–1650°C by a classical equilibration/quenching method and differential thermal analysis followed by X-ray diffraction phase analysis and electron probe micro-analysis. A sealed Pt crucible was utilized to prevent the volatile loss of Na₂O during high-temperature phase equilibrium experiments and the hydration upon quenching. The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was revealed for the first time. Based on the present experimental data and available binary modeling results in literature, the thermodynamic modeling of the ternary system was performed using the Calculation of Phase Diagram method and the phase diagram of the entire the Na₂O–Al₂O₃–ZrO₂ system was constructed and the optimized thermodynamic properties for all solids and liquid phase within the ternary system were obtained.     

Carbothermal production of ZrB2–ZrO2 ceramic powders from ZrO2–B2O3/B system by high-energy ball milling and annealing assisted process

Zirconium diboride (ZrB₂)-zirconium dioxide (ZrO₂) ceramic powders were prepared by comparing two different boron sources as boron oxide (B₂O₃) and elemental boron (B). The production method was high-energy ball milling and subsequent annealing of powder blends containing stoichiometric amounts of ZrO₂, B₂O₃/B powders in the presence of graphite as a reductant. The effects of milling duration (0, 2 and 6 h), annealing duration (6 and 12 h) and annealing temperature (1200–1400 °C) on the formation and microstructure of ceramic powders were investigated. Phase, thermal and microstructural characterizations of the milled and annealed powders were performed by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). The formation of ZrB₂ starts after milling for 2 h and annealing at 1300 °C if B₂O₃ is used as boron source and after milling for 2 h and annealing at 1200 °C if B is used as boron source.     

Praseodymium-doped cubic Ca–ZrO2 ceramic stain

The cubic Ca–ZrO₂ structure has been used as host for preparing a yellow ceramic stain using praseodymium as dopant. Samples with Ca_xPr_0.1Zr_0.9−xO₂ (X=0.14, 0.17, 0.2) compositions have been prepared by the ceramic method (CE) and by several gel processing techniques: the colloidal method (CG), the gelatine method (GE), the citrate method (CI) and a polymeric route (PG). Fired samples have been evaluated as ceramic pigments following enameling with the powders in ceramic glazes. The results show that a yellow ceramic pigment is obtained in all samples without significative differences among the different methods and compositions. This yellow ceramic pigment has been identified as a Pr–(Ca–ZrO₂) solid solution.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Interfacial microstructure and mechanical properties of ultrasonic-assisted brazing joints between Ti–6Al–4V and ZrO2

Ultrasonic-assisted brazing (28.8 kHz, 180 W) was introduced to achieve high-quality joints between Ti–6Al–4V alloy and ZrO₂ ceramic with Al-5wt.%Si brazing filler in air. The interfacial microstructure of intermetallic compounds (IMCs) and the phase constitution of joints ultrasonic-assisted brazed at 700 °C for different ultrasonic time were investigated in detail using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer and X-ray diffractometer. When the ultrasonic time was 20s, the average shear strength of the joint reached a maximum value of 90.68MPa and two important types of IMCs Ti(Al,Si)₃ and Ti₇Al₅Si₁₂ formed at the interface between the Ti–6Al–4V and the filler. Ultrasonic played a vital role in the formation of Si segregation regions at the interface near the Ti–6Al–4V and in the centre of the joint. A change in the Ti molar fraction, resulting from the cavitation effect of ultrasonic on the surface of Ti–6Al–4V, increased the chemical potential gradient of Si across the joint. Meanwhile, driven by the ultrasonic standing wave field in the liquid Al-5wt.%Si filler, Si atoms moved to the ultrasonic antinode-plane spontaneously. Ultrasonic-assisted brazing proved to be an effective method of joining Ti–6Al–4V and ZrO₂ with Al-5wt.%Si filler.     

Preparation and characterization of Al2O3–25 mol% ZrO2 composites

Al₂O₃–ZrO₂ (AZ_x), with 25 mol% ZrO₂ content, was prepared using the co-precipitation method. Synthesized powders were characterized by thermal reaction using a differential thermal analysis technique (TG–DTA) and were investigated by phase formation using X-ray diffraction. It indicated that the reaction occurred at 850 °C; cubic (c)-ZrO₂ phase and Al₂O₃ were obtained. By increasing temperature to 1100 °C, tetragonal (t)-ZrO₂ phase was detected. The Al₂O₃–25 mol% ZrO₂ was sintered for 2 h in the temperature range of between 1300 and 1600 °C. The majority phases of ceramics were m-ZrO₂ and α-Al₂O₃, although a t-ZrO₂ phase also appeared as a minor phase and decreased with higher temperature. Moreover, morphology and particle size evolution have been determined via the SEM technique. SEM showed that the particles of powder are agglomerated and basically irregular in shape. An SEM micrograph of ceramics exhibits uniform microstructure without abnormal grain growth.     

Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites

ZrB₂ powder was coated with 5% ZrOC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO₂ retention, improving ZrB₂ densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB₂ composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO₂ content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB₂-based composite systems.     

Electrostatic and electrosteric stabilization of aqueous slips of 3Y–ZrO2 powder

Yttria-doped zirconia powder (3Y-ZrO₂) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH₃) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.     

Comparative Study on Partial Oxidation of Methane over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 Catalysts

The partial oxidation of methane has been studied by sequential pulse experiments with CH₄ O₂ CH₄ and simultaneous pulse reaction of CH₄/O₂ (2/1) over Ni/CeO₂, Ni/ZrO₂ and Ni/Ce–ZrO₂ catalysts. Over Ni/CeO₂, CH₄ dissociates on Ni and the resultant carbon species quickly migrate to the interface of Ni–CeO₂, and then react with lattice oxygen of CeO₂ to form CO. A synergistic effect between Ni and CeO₂ support contributes to CH₄ conversion. Over Ni/ZrO₂, CH₄ and O₂ are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed oxygen to produce CO, which is composed of the main path for the partial oxidation of methane. The addition of ceria to zirconia enhances CH₄ dissociation and improves the carbon storage capacity. Moreover, it increases the storage capacity and mobility of oxygen in the catalyst, thus promoting carbon elimination.     

Free Radical Formation in ZrO2–SiO2Sol–Gel Derived Catalysts

Zirconia–silica mixed oxides have been prepared by the sol–gel method using zirconium butoxide and tetraethoxysilane as starting materials. The samples were sulfated using sulfuric acid as alkoxide hydrolysis catalyst or by impregnation with sulfuric acid of a sample previously prepared using HCl as alkoxide hydrolysis catalyst. In thermally treated samples, an intense EPR signal was observed for the sample prepared using HCl, while the EPR signal was lower in the sulfated samples. FTIR–pyridine adsorption spectra showed that Brønsted and Lewis acidity is developed in the sulfated samples. The formation of two types of Brønsted sites depending upon the sulfating method used is proposed. The catalytic activity of the samples was evaluated in isopropanol andn-butanol dehydration. The mechanism by which sulfation is achieved and the Brønsted-type site formed are discussed.     

TG and DTA investigation of ZrO2–SO 4 2– catalysts exposed to hexane, methylcyclopentane and cyclohexane

ZrO₂– catalysts with different sulfur contents were analysed with thermal methods coupled with mass spectrometry after exposure to mixtures of hexane, methylcyclopentane, and cyclohexane in argon. The reaction of the hydrocarbons led to carbonaceous deposits, but an important part of hydrocarbon remained chemisorbed as well. Heating these samples in He atmosphere provoked the decomposition of these deposits with evolution of CO₂ and CO, and also of SO₂ and SO. At the same time, COS was evidenced in the reaction products. The release of these molecules occurred below the activation temperature of the catalysts. The behavior of the catalysts depended both on reactant molecule and sulfur content. The analyses clearly evidenced the oxidation ability of ZrO₂–SO catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.