Dough moulding for resilient ceramics

Materials World magazine
,
1 Feb 2009
Heat shields for cars made from new ceramic

UK researchers have created low-cost ceramic compounds with improved thermal and impact properties by applying a fibre reinforcing technique used for polymer moulding composites. These ceramics could have applications in automotive heat shields or composite aircraft wings.

Production is inspired by the dough moulding compounds often used for polymer matrices, in which a liquid resin is reinforced with glass or carbon fibres to form a ‘dough’ like material. This mix is then compression moulded into the required shape. ‘[Our research] brings the fast production technology from polymer composites, but uses a ceramic matrix to create prepeg moulding compounds,’ explains Dr Guogang Ren of the University of Hertfordshire, who worked with researchers at Queen Mary, University of London, on this project.

The ceramic dough moulding compounds (CDMCs) incorporate calcium carbonate, calcium phosphate, and an aluminium trihydroxide hardener. Liquid sodium silicate acts as the matrix resin. Increased strength and a thicker texture is achieved by adding alkali resistant glass particles composed of 16% zirconia.

In trials, a three-millimetre plate moulded from the CDMC can absorb 55J of energy without shattering. This improves on normal advanced ceramic composites, says Ren.

‘The strength and modulus are not as high as advanced composites,’ he notes. ‘However, the ductility and impact properties are greatly improved.’ The temperature performance of this material is reported to be equivalent to that of polymer DMCs.

Saving costs

One of CDMC’s biggest benefits over traditional advanced ceramic composites is the lower cost of the materials it uses, as well as the ease of production, says Ren. Advanced composites usually include aluminium oxide, silicon carbide, silicon nitride, as well as carbon, alumina or Niclon fibres. ‘The complicated processes used for [these] composites, such as physical or chemical vapour deposition, push the costs even higher, plus the high temperature moulding or sintering,’ he notes.

‘The lowest possible cost for CDMC, to create a fire barrier up to 1,000ºC, is only £2.5-3.5/kg. If it is on a larger scale, such as over 20t per batch, it should be lower than £3/kg’.

The biggest drawback of this system is the use of the cheaper liquid metal resin, sodium silicate, which is soluble and increases the water content of the compound to around 25-30%. This can affect the stability. However, says Ren, ‘if the hardening process takes place under certain pressures (0-10MPa) and temperatures (120-200ºC), the water will be released within an hour’.

So far, the group has investigated the use of this material in fire doors and heat barriers, and is interested in receiving financial support to help scale the process up.