Novel Ceramic Processing Routes

Conventional Ceramic Processing

Conventional ceramic processing of polycrystalline ceramic components requires a number of stages, including manufacture of the powder, its calcination, milling, grading and mixing, incorporating additives, shape forming, drying and densification, and finally heat treatment and machining. Since each of these steps affects the final ceramic properties they must all be understood, and a more holistic approach is required when processing ceramics compared to metals and polymers, for example. While current knowledge of conventional ceramic processing is by no means complete, it is at a reasonable level and much research and development work is aimed at ‘novel’ processing. In this article, recent developments in the fabrication of powders, polycrystalline ceramics, single-crystal fibres and ceramic matrix composites (CMC) are outlined, along with novel uses of fine powders in refractories, the creation of a new formulation which could significantly alter the whitewares industry, and the application of rapid prototyping to ceramics.

Novel Powder Processing

Most ceramics are produced by the controlled sintering of shaped powder preforms. Powders are becoming finer and purer, enabling more control over the resulting ceramic properties. Finer (colloidal) powders have larger surface areas and sintering, driven by surface area reduction, occurs at lower temperature or requires shorter times.

The use of finer powders also means that thinner polycrystalline layers can be made, useful in capacitor and thin film technology in microelectronic applications. Techniques being used in a novel fashion for the production of ceramic powders include sol-gel processing, combustion synthesis, fusion and Dimox (directed metal oxidation).

Refractory Aluminosilicates

Sol-gel processing was developed in the 1950s for the production of radioactive powders of UO2 and ThO2 for nuclear fuels, without generation of large quantities of dust. It achieves this by producing gels from liquids and then calcining to form the product, which can be crystalline or amorphous, dense or porous, bulk solids, fibres, thin films or powders. Sols are simply dispersions of solid particles with at least one dimension between 1nm and 1µm in a continuous liquid. Gels consist of a continuous solid skeleton enclosing a continuous liquid so that producing a gel from a sol requires linking the solid phase in the system.

The term sol-gel includes products made from both inorganic colloidal particles suspended in aqueous solutions (particulate systems) and via alkoxides (metal-organic liquids) which can be partially hydrolysed and then polymerised into a gel (polymeric systems). Research at the University of Sheffield by has focused on the production of refractory aluminosilicate glass ceramics such as cordierite (Mg2Al4Si5O18), celsian (Ba2Al2Si2O8), and barium osumilite (BaMg2Al6Si9O30). Glass powders of these compositions have been fabricated by both melt and sol-gel routes and the evolution of the microstructures examined using electron microscopy.

The crystallisation initiates both at the original powder particle ‘surface’ and at the fibre/matrix interface. An alternative route to fabrication of such highly refractory powders is under development in a joint project between the University of Sheffield and British Glass using plasma melting. Temperatures in excess of 2000°C are generated in the plasma zone, which can be concentrated to avoid melt-container reaction. British Glass is operating an experimental plasma melter of 50kg per hour capacity in Sheffield.

Applications

These materials are being considered for uses in gas turbines for power generation and aerospace industries, in particular when reinforced with silicon carbide fibres. Hot pressed or hot isostatically pressed CMCs are strong, lightweight, heat resistant and damage tolerant, and are expected to form the thermal protection system on the next generation of space shuttles (the Skylon).

Tioxide Specialities has developed a novel coating method for ZrO2 powders which produces a dense ceramic, resistant to hydrothermal degradation. Tetragonal zirconia polycrystals (TZPs) exhibit disastrous loss of strength and toughness when subjected to a moist environment in the temperature range 100°C to 600°C. However, by processing the ceramic to give nanoscale zoning of the yttria stabiliser, the ceramic retains high toughness and excellent resistance to hydrothermal degradation. The resulting microstructure of the material reveals a monoclinic, yttria depleted, grain core while the tetragonal grain shell is yttria rich.

Single Crystal Fibres

Single crystal fibres offer distinct advantages for high-temperature applications, since degradation by grain growth or creep of grain boundary phases is impossible. In the EFG (edge defined film-fed growth) process, a fibre is grown from an induction heated melt through a small diameter molybdenum crucible and wound continuously onto spools.

Single crystal alumina fibres are produced by this process (and marketed as Saphikon) as well as a variety of shaped sapphire components such as substrates for silicon-on-sapphire wafers for integrated circuits, arc tubes for lighting, hollow fibres as optical waveguides for medical uses of lasers, and (one of the most familiar commercial applications) as abrasion resistant windows for supermarket laser scanners.

Combustion Synthesis

Combustion synthesis, also termed self sustaining high temperature synthesis (SHS), uses high temperature reactions to form powders or dense components, with large potential energy savings in a similar manner to the thermit process once used for welding iron and in incendiary bombs. The heat evolved during self-sustaining (once initiated) exothermic reactions can be used to form single phase or composite ceramics. The reaction is typically initiated at the top of a pellet and proceeds as a reaction wavefront down the sample to form the products. For example, a pellet of fine silicon metal and carbon black, inductively heated in graphite to 1200°C, will ignite exothermically to give temperatures of 2250°C at the pellet centre, leading to formation of SiC.

Other ceramics prepared by this route include borides, carbonitrides, nitrides and YBa2Cu307-x superconductors. A similar exothermic reaction mechanism is believed to occur in the formation of MoSi2 powder by mechanicochemical synthesis of silicon and molybdenum powders. In this technique, the powders are milled in a high energy ball mill for several hours until the particles are fine enough that the surface and strain energy available will sustain the reaction. Composites of Saphikon fibres in a MoSi2 matrix are currently being examined for various applications in the hot sections of jet combustion chambers, heat exchangers and high temperature filters.

Fine Powder Additives To Refractories

The addition of fine powders to refractories to improve their properties is an increasing trend. Metal powders (aluminium, silicon, and magnesium, and mixtures such as Al-Mg) have been added to MgO-graphite bricks for lining basic oxygen steelmaking furnaces since the 1970s, to improve oxidation resistance by gettering oxygen, and aid hot strength by forming ceramic bond phases such as MgAl2O4 and Mg2SiO4 at high temperature.

More recently, boron-containing ceramic powders including ZrB2, B2O3, and B4C have been used. B4C functions by gettering oxygen leading to formation of boric acid, H3BO3, which then reacts with MgO to form a viscous glass that reduces the refractories’ permeability, effectively acting as an internal glaze. CA (calcium aluminate) bonded monolithic (unshaped) refractories are used for an increasing number of heat retention linings in reheat furnaces, ladle and tundish backing, blast furnace runners and repair of blast furnace throat, cone and stack sections, as well as the water-cooled delta sections of electric arc furnaces. Fine, colloidal SiO2 (a by-product of silicon or ferrosilicon production) and reactive calcined Al203 added to CA castable refractories react to form elongated mullite grains in the bond phase which strengthens the structure and improves hot strength.

A New Look At Whitewares

A major problem with conventional whitewares is that the clay particles become aligned macroscopically in plastically-formed and slip-cast wares, and microscopically in those that are pressed from powder. The anisotropic firing shrinkage caused by clay alignment on the macroscale results in shape distortion, whereas on the microscale enlarged pores are formed, which reduce strength.

New body formulations have been developed by researchers at the University of Sheffield (UK) that have low clay contents, about one third of those used conventionally to reduce anisotropic shrinkage. The new formulations can be formed by pressing and casting. Coupled with newly developed binders that can be rapidly removed by a combination of water leaching and thermal debinding, injection moulding of certain shapes has become a practical possibility.

Glazing of whitewares is receiving much attention because of recent legislation in the USA concerning the use of PbO. However, PbO-containing glazes, and those recently developed to replace them containing Bi2O3, are easily scratched and abraded in service. The traditional glaze on hard porcelain is scratch resistant. Hard porcelain is glazed with a raw glaze, fired on at temperatures around 1400°C. Recently, it has been shown to be possible to glaze at temperatures some 200°C lower than is employed conventionally by using a sintering only approach using a fritted (powder) glaze with a tailored particle size.

One of the new low-clay whitewares has been formulated to have a thermal expansion coefficient to match that of the hard porcelain glaze, and it can be glazed at temperatures up to around 1300°C. This whiteware is based on anorthite (CaAl2Si2O8) and has a similar appearence to bone china. With a scratch resistant glaze it will be more serviceable than bone china, and as it has a high strength it should be more resistant to chipping than hard porcelain.

Primary author: William Lee

Source: Materials World, Vol. 4 No. 2 pp. 64-67, February 1996.

For more information on Materials World please visit The Institute of Materials.

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