...strength), creep resistance room temperature and high corrosion resistance. (1)

An insulator might reach the end of its working life for many reasons. Cracks in the body might be caused by a mechanical failure or a thermal mismatch between the ceramic part and the metal part. In addition, flashover might result from contamination on the glaze or weathering that leads to a small crack on the surface. (2) Thermal cycling causes material failure since it promotes the growth of micro-cracks on the surface. Thermal stress is generated by temperature differences between day and night, which may result in 80[degrees]C differences on the glaze surface. The heat generated may also come from the passage of fault-current arcs in the insulator. (3)

Generally, the mechanical strength of the porcelain body can be improved by reducing the internal thermal mismatch between crystalline particles and glassy phases. (4) An increase in mullite crystals can result in a change of the thermal expansion property of the body. The coefficient of thermal expansion is a thermal property related to chemical composition, firing temperature, particle size distribution of raw materials, particle packing of green and fired bodies, crystal structure, and the glassy phase of the body. Crazing of the glaze can be caused by a thermal mismatch between the body and glaze. Usually, the thermal expansion of the body should be higher than that of the glaze in order to generate compressive stress (instead of tension) in the glaze layer.

Many methods are available to determine the coefficient of thermal expansion, including calculation from the proportion and types of oxides in the body composition. However, this method is not accurate because many factors affect the calculated values. The most precise method is through measurement by a dilatometer.

Insulators are produced in many shapes, which are formed by different forming processes. For complicated shapes that cannot be formed by jiggering, slip casting is used. Other products, like a fused support with a simple shape, can be formed by pressing.

It is important to understand the physical, mechanical and thermal properties of porcelain bodies that are formed by slip casting, extruding and pressing. The effects of forming processes on fired properties can be explained by the phases and microstructures of the fired products. Understanding...

NOTE: All illustrations and photos have been removed from this article.



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