Microstructural Characterisation Of Vitrified Ceramics

EPSRC research grant GR/L71780

Summary of the final report

Principal Investigators: W E Lee and Y Iqbal

Traditional ceramics such as bone china and porcelain densify by a process of vitrification in which large volumes of liquid (up to 60vol%) are produced by melting and dissolution of the clay, flux and filler. The mechanisms by which this liquid forms by melting and reaction of the raw materials has been elucidated in an EPSRC-funded ROPA award. The evolution of the microstructures in model porcelains made from kaolin clay, feldspar flux and quartz or alumina fillers and bone chinas made from kaolin clay, bone ash, feldspar and quartz have been examined by quenching samples from increasing temperatures up to and above vitrification and characterising the microstructures using scanning and transmission electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The systems examined experimentally were modelled closely on commercial compositions whose microstructures were also fully characterised using these techniques. The results reveal the critical importance of the level of mixing of the raw materials and that local regions of the microstructure which contain different levels of the raw materials behave differently leading to formation of quite different phases and morphologies. The composition of the liquid formed (and hence its viscosity) has a critical effect on phase evolution. This understanding should enable closer control of microstructures during fabrication of vitreous ceramics and improved product homogeneity.

When kaolin clay is mixed with bone ash and feldspar unfired but shaped bone china bodies comprised three regions: predominantly bone ash agglomerates, clay agglomerates with a little bone ash (<70μm), and a matrix of smaller (from submicron to 10μm) mixed clay, feldspar and bone ash particles. On firing these 3 regions with different bone/clay ratios produced liquid of different composition and viscosity. Spheroidal ß-tricalcium phosphate (TCP) crystals coming from bone ash mixed with clay and/or feldspar are in a less viscous liquid than those in unfluxed bone ash and so are able to grow to much larger sizes. The major phase in the clay relicts was acicular anorthite (CaAl₂Si₂O₈). The glass seen throughout the microstructure has variable composition at temperatures < 1400°C indicating that dissolution reactions are ongoing and the bodies are far from equilibrium. Commercial bone china bodies reveal identical features.

In porcelain low temperature firing (<600°C) reveals an inhomogeneous microstructure consisting of pure clay relict agglomerates, clay relict agglomerates containing feldspar and a matrix mixture of fine quartz, clay and feldspar particles. In the evolving microstructures these 3 regions contained different types of mullite necessitating a refinement of the terminology defining types of mullite formed in vitreous ware. The pure clay relicts contained fine primary 2Al₂O₃.1SiO₂ (Type I) mullite, feldspar-penetrated clay relicts contained secondary 3Al₂O₃.2SiO₂ mullite with a granular morphology (Type II) and the matrix of fine clay, feldspar and quartz contained elongated secondary 3Al₂O₃.2SiO₂ mullite (Type III as well as granular Type II). As well as shape the mullite crystal size changes increasing from regions containing Types I to III mullite due to the increasingly fluid liquid enhancing crystal growth. Commercial porcelains contained identical features.

Contact:

Prof W E Lee (now Imperial College, London),
Department of Engineering Materials,
University of Sheffield,
Mappin Street, Sheffield S1 3JD
Telephone: 0114 222 5502
Fax: 0114 222 5943