Ceramics materials for the present

What springs to mind when you think about ceramics? Pottery and whitewares, or a highly versatile engineering material? Unlike the ceramics of the pottery and whitewares industries, advanced engineering ceramics require highly refined raw materials, rigorous compositional control and strictly regulated forming and processing routes. Most engineering ceramics are based on relatively simple combinations of carbon, oxygen, nitrogen, boron, aluminium and silicon.

The common features exhibited by ceramic materials (a function of very strong ionic and covalent bonding) include: high strength; particularly in compression; thermal and electrical insulation; and low toughness. The wider range of properties, however, is not widely appreciated. For example, whilst ceramics are perceived as electrical and thermal insulators, ceramic oxides (initially based on Y-Ba-Cu-O) are the basis for high temperature superconductivity. Diamond, beryllia and silicon carbide have better thermal conductivities than aluminium and copper. Control of microstructure can overcome inherent stiffness to allow the production of ceramic springs, and ceramic composites have been produced with a fracture toughness about half that of steel.

The majority of ceramic components are produced by sintering (firing) pre-formed powders. The powder preforms are usually referred to as green state and numerous powder-forming processes have been developed. Usually the green state, once formed, is dried and sintered to produce the final component.

However, the powder consists of solid hard, brittle particulates, so it is difficult to consolidate in a die by pressure alone. A binder is usually added to enhance the flow properties of the powder. The binders used vary according to the process to be used and the desired properties of the final product.

Once the ceramic powders have been formed to produce the green shape component, they are approximately 50-70% dense. They are also relatively weak. To impart strength, the green state components are fired.

Initial heating (up to 250 C) volatilises any organic processing additives and decomposable constituents. As the temperature increases to approximately 60% of the melting point (or maximum firing temperature), consolidation, or sintering of the ceramic powders begins and is usually accompanied with shrinkage (solid state sintering). Sintering can be assisted (decreasing temperature or time requirements) by the deliberate addition of additives which will react to produce lower melting point secondary phases (liquid phase sintering).

Compared to metals and plastics, ceramics are hard, non-combustible and inert. Thus they can be used in high temperature, corrosive and tribological environments. In addition, many ceramics exhibit superior electromagnetic, optical and mechanical properties under these environments.

Ceramics can be organised into categories in terms of composition (oxide, carbide and so on) or by function. The general properties of the main ceramic types are given in Table 1, with Table 2 indicating typical applications.

Alumina has a high melting point (2050 C), high hardness, is electrically insulating and can be produced in a wide variety of shapes and purities (typically from 95 to 99%, where the remainder is composed of a mixture of glasses). The most widespread applications of alumina utilise its electrical insulating properties, ie as electronic packages, thermal heads for fax machines and in automotive spark plugs. However its inertness to the body also result in it being used in artificial hips, whilst its hardness also leads to its application as ceramic tools.

Zirconia has a melting point of 2700 C; however it undergoes a three-stage phase change between 1000 C and 2370 C accompanied by significant changes in volume, which confer relatively high toughness to the material (about half that of steel). For this reason zirconia is used in oxygen sensors, fuel cells, scissors, knives, and tool parts.

Silicon nitride and SiAlON (alumina substituted into silicon nitride) offer high hardness, low density, good strength and a low coefficient of thermal expansion. This relatively new addition to the ceramics family is finding use in gas turbine stator blades, burners and nozzles; diesel engine pistons, cylinders and exhaust valve heads; welding nozzles; bearings and seals.

Aluminium nitride is highly stable in non-oxidising atmospheres to temperatures in excess of 2000 C, and its lack of wetting by molten metals means that it is predominantly used in the refractories industry. However, its high thermal conductivity also leads to its use in semi-conductors and electronic substrates.

Boron nitride comes in two forms: cubic and hexagonal. The hexagonal form resembles graphite and is a soft material good for heat and corrosion resistant products and electrical insulation. The cubic form gives properties similar to diamond and its high hardness leads to its use in cutting tool applications.

Silicon carbide is known for its strength, abrasion resistance, high thermal conductivity and resistance to corrosion leading to its use in refractory bricks and tiles; pump parts; and heat exchangers for the power generation industry. However it is also being investigated for the electronics industry since, with small additions of BeO, it transforms from a semi-conductor to an insulator. With its low density and high hardness, silicon carbide is also found in the military sector as armour.

The implementation of ceramics into critical components is dependent on both appropriate material selection and by fulfilling design criteria. Design of a particular component is based on two fundamental requirements: the purpose of the component and the operating environment. Considerations that must be made include the loading on the component, chemical, electrical and thermal requirements.

Two key principals should be considered during the design process: the specification of working conditions and requirements (including loads and temperature); and the specific properties of this class of materials. In particular, the look at properties should consider: high brittleness (lack of ductility) compared with other structural materials; high strength in compression (lower in tension, bending and torsion); susceptibility to impact and point loads; low thermal conductivity (and hence low resistance to thermal shock); sensitivity to stress concentrations, abrupt changes in shape, cross-section, notches, corners and sharp edges (all of which should be avoided); and flaws, which result in low strength, cracks and other faults.