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Most ceramic fabrication processes begin with finely ground powder. Oxides such as alumina (Al2O3), magnesia (MgO) and zirconia (ZrO2) occur naturally,

but have to be purified by chemical processing before use as engineering ceramics. Silicon carbide (SiC) is manufactured by reacting SiO2 sand with

coke (C) at high temperature and silicon nitride is also synthesized industrially, usually by reacting silicon powder with nitrogen at 1250 to 1400°C. Before consolidation, the powders are milled and graded into size (diameter of the order of 1 µm). They are then blended so that the subsequent shaping operation leads to material of optimum properties. The next stage is one of shape- forming, for which there are a number of possible processes.

Pressing requires the powder to be premixed with suitable organic binders

and lubricants and preconsolidated so that it is free flowing. It is then compacted in a die to form small shapes such as crucibles and insulating ceramics for electrical devices.

Slip casting is effected by suspending the ceramic particles in a liquid

(usually water) and pouring the mixture into a porous mould (usually plaster) which removes the liquid and leaves a particulate compact in the mould. An organic binder is usually present in order that the casting has sufficient strength to permit its removal from the mould before the firing operation.

Plastic forming is possible if sufficient (25 to 50 vol%) organic additive

is present to achieve adequate plasticity. Injection moulding and extrusion may then be employed.

Strong, useful ceramic products are produced after the final densification

by sintering at high temperature. Sintering brings about the removal of pores

between the starting particles (accompanied by shrinkage of the component), combined with strong bonding between the adjacent particles. The primary mechanisms for transport are atomic diffusion and viscous flow. In some cases, hot die pressing is employed, whereby pressure and temperature are applied simultaneously to accelerate the kinetics of densification. Only a limited number of shapes can be produced by this technique, however.

The thermodynamic driving force for sintering is the reduction in surface energy (γ) by the elimination of voids. A spherical void of radius 2r will experience a closure pressure P given by:

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Thus, the smaller the void, the greater the closure pressure. Efficient sintering is, therefore, promoted by the use of precursor powders of fine particle size. The diffusional process requires the presence of lattice vacancies and ceramics of covalent bonding have a very high formation energy for vacancies and, therefore, exhibit low solid state diffusion rates, giving poor densification properties. Atom transport is predominantly by grain boundary diffusion so, again, a fine grain-size is essential for efficient densification. In most cases, a ‘densification aid’ or grain growth inhibitor is added to the ceramic to achieve maximum density and minimum grain size. In the case of carbide and nitride ceramics these are metal oxides; LiF is added to alumina and magnesia for this purpose. The additives segregate to the newly-formed grain boundaries during sintering and increase the diffusion coefficient by forming low melting-point or low viscosity glass phase. The best sintering aids also suppress grain growth in the component, which would otherwise lead to a reduction in the number of diffusion paths, thus slowing the densification rate. Voids would become ‘stranded’ in large grains, with no fast pathway for mass transfer, and remain as a likely source cracking when the component is under stress in service.

The microstructure of a pure, polycrystalline engineering ceramic can be seen by polishing, etching and magnifying. The important features are the grain size and the degree of porosity, and a dense ceramic is thus similar in microstructure to a polycrystalline metal (Fig. 4.3). Table 4.3 summarizes the properties of some of these materials. The data give a general indication

4.3 Pure polycrystalline alumina (Courtesy of Dr R. L. Riley and Dr M. Miranda-Martinez.)

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of typical strengths, but test data from current suppliers should be used for analytical design or life prediction calculations.

4.3.2

Alumina

Many hundreds of tons of alumina (Al2O3) powder is produced annually

from the mineral bauxite and used in the manufacture of porcelain, crucibles, wear-resistant parts such as cutting tools and grinding wheels, medical components and a variety of other components. It forms ionic crystals of hexagonal structure, Fig. 4.4, with close-packed layers of oxygen ions, with the Al3+ ions occupying interstices such that each is surrounded by six O2– Table 4.3 Properties of some engineering ceramics

Property Units Alumina Zirconia Silicon Silicon nitride carbide (RBSN) (RSSC) Modulus of rupture MPa 300–400 200–500 200–350 450 Compressive strength MPa 3000 2000 2000 2000 Young’s modulus GPa 380 138 150–180 400 Thermal expansion 10–6_K–1 _{8.5 8 2.6 4.5} coefficient Thermal conductivity W m–1 _K–1 _{25.6 1.5 12.5 100} Toughness (Gc) J m–2 25 80 10 25 + + Empty hole Al3– O2

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ions. One third of the Al3+ sites remain empty, so that the overall ionic charges are in balance.

Reducing the grain size increases both the fracture strength and the toughness of alumina (Fig. 4.5). The manufacturing conditions for fine grain size (low temperatures, short time) are in conflict with those to minimize porosity (high temperatures, long time) so that a compromise has to be made by using sub-micron powder particles as a starting material and by the addition of 0.05–0.2% magnesia (MgO) which prevents grain growth by ‘pinning’ the alumina grain boundaries. Hot pressing at 1350–1800 K may enhance sintering, so that a shorter time is required at the sintering temperature of 1850–2000 K.

4.3.3

Zirconia

Zirconia (ZrO₂), an engineering ceramic of growing importance, is again an ionically bonded material and it exhibits three distinct crystal phases. Above 2300°C it is cubic, between 1150 and 2300°C it is tetragonal and below 1150°C it has a monoclinic structure. The cubic form consists of zirconium ions on a face-centred cubic lattice (Fig. 4.6) with oxygen ions occupying certain holes in the structure. ZrO₂ undergoes a 3.5% volume expansion during cooling below 1000°C due to its change in crystal structure to monoclinic and this causes catastrophic failure of any part made of pure polycrystalline zirconia. Addition of CaO, MgO or Y₂O₃ to the zirconia

4.5 Showing increase in strength with decreasing grain size in alumina.

Bending strength (MN m – 2) 600 400 200 0 1 2 5 10 20 50 100 200 Grain size (_µm)

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results in a cubic crystal structure that is stable over the complete temperature range and does not undergo a phase transformation. This is referred to as

stabilized zirconia.

Stabilized zirconia has a low fracture toughness and a poor resistance to impact. By not adding enough CaO, MgO or Y2O3 to stabilize the ZrO2

completely and by careful control of processing, mixtures of the stabilized cubic phase and the metastable tetragonal phase that have very high fracture toughness are achieved. This type of material is referred to as partially

stabilized zirconia (PSZ).

Transformation toughening

A suitable microstructure consists of a matrix of cubic zirconia containing a dispersion of particles of metastable tetragonal zirconia. The transformation of the small zirconia particles from tetragonal to monoclinic zirconia is inhibited by the elastic constraint of the surrounding matrix. Ahead of a propagating crack in such a material, there is a dilatational stress field; this interacts with the constraining stress field around a metastable particle and initiates transformation. Transformation will occur to some distance within the stress field and, thus, behind the crack tip, there will be a wake or process zone of transformed particles (Fig. 4.7). The volume expansion of these particles acts as a crack closure strain and thus reduces the stress intensity at the crack tip. This means that a further stress has to be imposed to continue crack propagation and the failure stress (and hence the toughness) increases. Partially stabilized zirconia can have fracture strengths of about 600 MPa with fracture toughnesses of around 8–9 MPa m1/2.

O2–

Zr4–

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Zirconia has other properties which make it a very interesting engineering ceramic. As seen in Table 4.3, it has a very low coefficient of thermal conductivity (1.5 W m–1 K–1 compared with 25.6 W m–1 K–1 in the case of alumina) together with a very high thermal expansion coefficient (8 × 10–6 K–1), which is two or three times that of most ceramics and almost the same as cast iron or steel. This makes zirconia a candidate for insulating engine components, since any coatings will not have the severe problems of thermal expansion mismatch found with other non-metallic surface layers.