What is a Glass Ceramic?

Glass ceramic materials were first developed at the Corning Glass Works and share properties of both the parent glass material and polycrystalline materials. In this article we look at how they differ from glass materials, how they are produced, some typical compositions and their applications.

Glass Materials

When we think of glass materials, we tend to think of windows in buildings, houses and transportation. While this is true, many different kinds of glasses exist, with varying compositions.

Glass materials are generally transparent and very brittle (when not heat treated). The transparency is a result of the lack of grain boundaries and pores in the structure of the glass. This lack of grain boundaries also leads to the brittleness, as cracks can propagate unhindered.

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Another characteristic of glasses is their lack of order on their structure i.e. the atoms and molecules are randomly arranged. This can be evidenced in an x-ray diffraction (XRD) analysis, where a glass will display no clearly defined peaks. A crystalline material on the other hand displays order in its structure and will generally produce well-defined peaks when analysed by XRD.

The lack of crystallinity in their structure leads to them being referred to as amorphous or vitreous.

Glass Ceramic Materials

Glass ceramic materials have the same chemical compositions as glasses but differ from them in that they are typically 95-98% crystalline by volume, with only a small percentage vitreous. The crystals themselves are generally very small, less than 1µm and most often very uniform in size. Furthermore, due to their crystallinity and network of grain boundaries, they are no longer transparent.

Properties of Glass Ceramic Materials

Glass ceramic materials are typically characterised by:

• High strength
• High impact resistance
• Low co-efficient of thermal expansion, sometimes even negative co-efficient of thermal expansion
• Good resistance to thermal shock
• A range of optical properties, from translucent to opaque and sometime opalescence.

Production of Glass Ceramic Materials

Glass ceramic components are formed using the same processes that are applicable to glass components. To convert them from a vitreous glass material into a crystalline glass ceramic material they must be heat treated or devitrified.

Devitrification can occur spontaneously during cooling or in service, but is most commonly incorporated to produce glass ceramics. It involves heating the formed glass product to a temperature high enough to stimulate crystals to nucleate throughout the glass. The temperature is then increased, which induces growth of the nuclei, crystallising the remaining glass.

Nucleation requires a critical number of atoms converging to form a nucleus. When the nucleus reaches critical size, nucleation occurs. In many glass compositions, nucleation is hampered by the fact the material is silica-based and highly viscous, making it difficult for the required atoms to come together. The crystal compositions can also be complex making nucleation difficult. These factors aid glass forming and cooling without crystallisation.

The devitrification heat treatment must be carefully controlled to ensure the maximum number of nuclei are formed and that these nuclei grow into a uniform fine crystal structure. In order to obtain a high concentration of nuclei throughout the structure, it is common to add a nucleating agent to the glass composition.

Nucleating Agents

The most common nucleating agents are TiO₂ and ZrO₂. Other materials that have been used for nucleating agents include P₂O₅, platinum group and noble metals and some fluorides.

Glass Ceramic Compositions

While many different glass ceramic compositions exist, there are 3 main families:

• LAS – A mixture of lithium, aluminium and silicon oxides (Li₂O-Al₂O₃-SiO₂), with other glass forming agents (e.g. Na₂O, K₂O and CaO).
• MAS – A mixture of magnesium, aluminium and silicon oxides (MgO- Al₂O₃-SiO₂) with glass forming agents
• ZAS - A mixture of zinc, aluminium and silicon oxides (ZnO- Al₂O₃-SiO₂) with glass forming agents

Applications of Glass Ceramics

Some applications of glass ceramics include:

• Radomes – made from Corning 9606 (2MgO.2 Al₂O₃, cordierite system)
• Cookware, bakeware and cooktops – made from Corning 9608 (ß-spodumene system)
• Telescopic mirrors – made from Owens-Illinois Cer-vit (ß-quartz system)
• Insulators – made from General Electric Re-X (Li₂O.2SiO₂ system)
• Bioactive glass for biomaterials – Bioglass 45S5 (46.1 mol% SiO₂, 26.9 mol% CaO, 24.4 mol% Na₂O and 2.5 mol% P₂O₅) has been FDA approved and many different variations on this composition have resulted.
• Engineering components – made from Macor, a machineable glass ceramic

References

• How are glass, ceramics and glass ceramics defined? - IWT Global
• Introduction to Ceramics, Second Edition, Kingery W.D, Bowen H.K. and Uhlmann D.R., John Wiley & Sons, 1976.
• The Science and Design of Engineering Materials, Schaffer J.P et al, Richard D. Irwin Inc, 1995
• Modern Ceramic Engineering, Richardson D.W., Marcel Dekker Inc., 1992.