E.G. Butler

Nanostructured ceramic powders by hydrothermal synthesis and their applications

Abstract Nanoparticles of 3:2 mullite (3Al2O3·2SiO2)+5 wt.% zirconia, boehmite (γ-AlOOH) and pure ZrO2 suitable for applications in the field of monolithic/fibre reinforced ceramic composites, nanoceramics and ceramic fibre coating, respectively, have been produced by hydrothermal synthesis (HS) at moderately low temperatures. The effect of the process parameters or starting precursor materials properties on the particle size, shape and structure have been examined by particle size measurements, TEM and scanning electron microscopy. The mullite powders synthesised tend to be highly irregular in morphology with a wide particle size distribution from 55 nm to 7.1 μm, whereas the boehmite (γ-AlOOH) particle morphology and size are determined by the pH value of the initial hydrothermal solution. As the pH changes from low (acid) to high (alkaline), the boehmite morphology changes from a “needle-like” one to a “platelet” one. At a pH value of 10, the boehmite particles exhibit platelet morphologies, with a maximum face dimension of about 40 nm and a thickness of approximately 5 nm. The synthesised boehmite particles were successfully used to produce monolithic alumina components at lower sintering temperatures with sub-micrometer grain sizes. Near-spherical shape monoclinic zirconia particles with dimensions in the range of 20–60 nm were also produced by HS. The successful application of the synthesised powders in the field of mesoporous fibre-reinforced ceramic composites, high-density monolithic ceramics and ceramic fibre coatings (formation of weak inter-phase between fibre and matrix) are also presented.

Oxide CMCs: interphase synthesis and novel fibre development

Strategies which have been used in the synthesis of high temperature interphases with debond capability in oxide/oxide systems have been: (i) the formation of layered oxides (β-aluminas or magnetoplumbites) with preferred crystal orientation due to in-situ interface reactions between phases deposited from vapour or liquid precursors and (ii) the deposition on fibre surfaces of complex oxides (vanadates and phosphates, principally of the rare earths) from colloidal precursors or by magnetron sputtering. This paper is primarily concerned with the latter. The constitution and thermal stability of these interphases has been studied with reference to potential fibres and matrices (alumina, YAG and mullite). Included within the program are newly developed single phase mullite fibres and examples of these are also presented.

Deposition of zirconia sols on woven fibre preforms using a dip-coating technique

Abstract The technique of depositing zirconia coatings onto woven fibre mats has been investigated in detail. The application of a coating to the fibre is essentially one of the easiest methods of providing a fibre–matrix interface with desired properties. Such coatings can act as reaction barriers or as fibre–matrix debond interfaces. This particular coating method, which is the dip-coating of fibre tows in zirconia sols, does not require sophisticated apparatus and, therefore, has great potential as a standard coating technique for use with woven fibre systems. In this work, an “in-house” produced zirconia sol, synthesised using hydrothermal processing, has been found to be the most successful coating material. Both alumina and alumina-silica woven fibre mats were used as the substrate materials. It has been shown that the zirconia sol employed can be deposited successfully in a single coating step, as a thin (1–2 μm) coating with minimal bridging of the fibre mats. The critical coating parameters when using this dip-coating technique have been discussed in detail.

Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing

Abstract Mullite fibre (Nextel 720™)-reinforced mullite ceramic matrix composites (CMCs) with zirconia weak interface were fabricated from heterocoagulated nano-size boehmite/amorphous silica powder particles dispersed in water, using electrophoretic deposition (EPD) and pressure filtration (PF). The nano-size mullite precursor was first prepared and characterised in terms of short-range particle–particle interactions and particle size distribution. Woven Nextel 720 mullite fibres were first desized and then coated with hydrothermally derived zirconia using dip-coating. EPD was performed under constant voltage conditions with varying deposition times, to infiltrate the dispersed powder suspensions into mullite fibre preforms, enabling the parameters necessary for good deposition of stoichiometric mullite to be established. EPD formed bodies were further consolitated using PF. The EPD/PF prepared green body specimens were dried under controlled atmosphere conditions before being sintered at 1200°C for 2 h in air. Mullite fibre mats were fully infiltrated using EPD parameters of 12 V DC applied voltage with 4 min deposition time, then eight EPD infiltrated fibre mats were further consolidated together using PF. The resulting CMC produced contained 35 vol% fibre loading and showed 81% theoretical density aftersintering at 1200°C for 2 h.

Co-extrusion of Al2O3/ZrO2 bi-phase high temperature ceramics with fine scale aligned microstructures

Abstract Structural ceramic composites comprising continuous fibrillar microstructure are produced using sol-based technology which involves the extrusion, at room temperature, of a two-phase material (Al 2 O 3 /ZrO 2 ) resulting in an aligned bi-phase structure which is then multiple co-extruded to reduce the lateral dimensions of the phases. Two sol-derived pastes of differing chemistry (γ-AlOOH as alumina source and zirconia) are co-extruded in parallel, and layed-up in closed-packed linear array to form a heterogeneous macro-plug for subsequent extrusion with or without zirconia coating. The second and third extrusion steps produce a filament with markedly reduced lateral paste dimensions provided that the flow properties of the chemically different pastes are similar. The resulting extrudates in the form of continuous green monofilaments, are subsequently laid up in a mould where the structure is pressed and consolidated in desired shape, then pressureless sintered in air to form the multi-phase component. The developed process allows the microstructure to be controlled at a nanometer scale within each extruded filament and after the 3rd stage co-extrusion, each filament size within the final extrudate is reduced to ≈ 65 μm.

Zirconia-toughened alumina ceramics of helical spring shape with improved properties from extruded sol-derived pastes

High strength (1270 MPa) and tough (>9 MPa m1/2) zirconia-toughened alumina (ZTA) ceramics with a helical spring shape were produced from sol-derived pastes comprising a mixture of boehmite (γ-AlOOH) and nano-size zirconia powders using extrusion. It is shown that the mechanical response of the extruded ZTA ceramics can be controlled/altered by manipulating the microstructure.

Processing and structures of bi-phase oxide ceramic filaments

Two phase ceramic filaments have been made by the co-extrusion of different green mono-filaments, each extruded mono-phase being produced from two differing material sources. The first was based on sol technology and the second based on oxide ceramic powder (+ thermoplastic binder) processing. Initially, mono-phase filaments were produced and re-extruded to produce bi-phase filaments which were then sintered to produce ceramics with aligned fine microstructures. The scale and fineness of the final sintered structure and the orientation of the grains in each phase (alumina or zirconia) have been determined by SEM and TEM and have been found to depend on the type of original precursor used, the powder grain size added and the temperature of heat treatment chosen. The study seems to demonstrate the viability of this novel multiple sol-gel based extrusion process to obtain optimised engineering structures for use at high temperatures.

Microfabrication of Al2O3/ZrO2 bi-phase ceramics with continuous fibrillar microstructure by co-extrusion

In the last two decades, new materials have been required for lightweight rigid structures with potential for high temperature operations [1, 2]. The evolution of intermetallic alloys and monolithic ceramics has been encouraged by the needs of the aerospace industries and the realization that the more conventional alloy systems have reached a development limit in relation to high temperature stability and deformation resistance. The technical limitations of these monolithic ceramics were due either to their low fracture toughness or a high temperature ceiling dictated by oxidation and creep-cavitation initiated by sintering liquid residues. Continuous fiber-reinforced ceramic matrix composites (CMCs) are the best solution for high-risk engineering components in which high specific stiffness and high temperature tolerance are required (rocket nozzles, engine flaps, hot structures in aircraft and engines ext.) [3–5]. However, the current limitations for CMCs are the lack of a commercial fiber of reasonable cost and a thermal stability above 1200 ◦C and the lack of a flexible fabrication route in which the matrix achieves neartheoretical density within a woven fiber preform. Therefore, this work presents an innovative processing route while is designed to overcome these limitations while retaining a high proportion of the beneficial properties of both monolithic ceramics and CMCs. Co-extrusion can be defined as the passing of two or more pastes through the same die to manufacture a green body of constant cross sectional area. It has been widely used recently to produce multilayer ceramics [6], multilayer tubes [7], alumina and PbO-containing ferroic ceramics [8], zirconia and stainless steel metalceramic pipes [9] and fine-scale alumina, mullite and ZTA components [10–14]. One of the main advantages of using a co-extrusion technique is the significant reductions in the number of processing steps. It has also been proven that deliberately introduced weak interfaces in a laminar structure suppress the catastrophic failure and increase the fracture toughness and work of fracture by the operation of debonding and crack deflecting mechanisms [15]. The main objective of the present work is to demonstrate the feasibility of forming multiphase aligned fibrillar microstructures from nano-size sol particle precursors using an innovative multiple co-extrusion process. To achieve this, two sol-derived high solidsloading pastes of alumina and zirconia were coextruded in parallel (with the presence of a weak zirconia interface), and layed-up in a closed-packed linear array to form a heterogeneous macro-plug for subsequent extrusion assuming that the flow properties of the chemically different pastes are similar. Boehmite (γ -AlOOH) sol (Remal A20, Remet corp., USA) was used as the alumina source. The sol had average particle size and solids-loading of 40 nm and 20 wt%, respectively. The received sol was stable at a pH value of 4. The received boehmite sol was seeded with 2 wt% of the total mass using ultrafine (α-Al2O3 (30 nm, BDH Chemical, UK, high purity polishing powder) powders. The flow chart for the seeding process, followed by paste preparation is given in Fig. 1. To seed the boehmite sol, the seeding powder was first dispersed in distilled water and then added into boehmite sol. 1 wt% glycerol and celacol were also added in order to minimize the surface roughness of the extrudate and increase the green strength, respectively. 1 wt% celacol was first dissolved in water at 85 ◦C and then added to the seeded sol in order keep it plastically deformable during the multiple extrusion stage. The seeded sol was first stirred magnetically for 10 h and then ultrasonic agitation was employed at 15 kHz for 3 h for further dispersion of any particle agglomerates which might be present [16]. The final sol composition i.e., boehmite + 2 wt% seeding powder + 1 wt% glycerol + 1 wt% celacol was ball-mixed for 2 days using high purity zirconia balls. The mixed seeded sol was then vacuum filtered in order to obtain a gel structure. The resulting soft white gel was further compacted using pressure filtration apparatus to squeeze out the excess water, and obtain an extrudable paste. For the preparation of zirconia paste, zirconia sol was prepared using ultrafine and high purity zirconia powders (average grain size 30 nm, VP zirconia, Degussa Ltd., Germany) with the addition of 3 mol% yttria. Kinetically stable and well dispersed zirconia sol having 20 wt% solids-loading was prepared by the addition of a small amount of zirconia to the water, while the suspension was magnetically stirred. The best pH value in order to obtain the maximum stability was found to be 8.5 and this pH value was maintained using ammonia. 1 wt% glycerol and celacol were added to the prepared zirconia sol. 1 wt% cyclohexanone (C6H10O)

Microstructurally controlled mullite ceramics produced from monophasic and diphasic sol-derived pastes using extrusion

Mullite ceramics with controlled microstructure in terms of grain size/shape, pore and glassy phase content were produced from sol-derived pastes using extrusion. Particular attention has been given to the development of a continuous process which is suitable for the preparation of high-solids-loading mullite pastes from two different starting mullite precursors, namely, diphasic and molecular mixed mullite sols. A combined processing technique comprising vacuum filtering and pressure filtration was introduced in order to obtain extrudable mullite pastes from low solids-loading colloidal sols. It is shown that glassy phase free stoichiometric 3:2 mullite (3Al2O3·2SiO2) with fine (0.94 μm) equiaxed grain microstructure is achievable from monophasic precursors after pressureless sintering at 1400°C for 3 h using the developed technique which can control both the sol-derived paste microstructure and process parameters. It is also found that the room and high temperature (1300°C) flexural strength and toughness of extruded mullites are mainly controlled by the grain size, the presence and location of glassy phase, nano-inclusions and pores at the grain boundaries. Pressureless sintered mullite derived from the monophasic sol-derived pastes provides flexural strength values of 345 and 277 M Pa for room temperature and 1300°C, respectively.