Chapter 14. Ceramics - Applications and Processing


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14.1 Introduction

Ceramics properties that are different from those of metals lead to different uses.  In structures, designs must be done for compressive loads.  The transparency to light of many ceramics leads to optical uses, like in windows, photographic cameras, telescopes and microscopes. Good thermal insulation leads to use in ovens, the exterior tiles of the Shuttle orbiter, etc.  Good electrical isolation are used to support conductors in electrical and electronic applications.  The good chemical inertness shows in the stability of the structures thousands of years old. 

14.2 Glass Properties

A special characteristic of glasses is that solidification is gradual, through a viscous stage, without a clear melting temperature.  The specific volume does not have an abrupt transition at a temperature but rather shows a change in slope at the glass-transition temperature (Fig. 14.3). 

The melting point,  working point, softening point and annealing point are defined in terms of viscosity, rather than temperature (Fig. 14.4), and depend on glass composition.. 

14.3 Glass Forming (not covered)

14.4 Heat Treating Glasses 

Similar to the case of metals, annealing is used at elevated temperatures is used to remove stresses, like those caused by inhomogeneous temperatures during cooling. 

Strengthening by glass tempering is done by heating the glass above the glass transition temperature but below the softening point and then quenched in an air jet or oil bath. The interior, which cools later than the outside, tries to contract while in a plastic state after the exterior has become rigid.  This causes residual compressive stresses on the surface and tensile stresses inside.  To fracture, a crack has first to overcome the residual compressive stress, making tempered glass less susceptible to fracture.  This improvement leads to use in automobile windshields, glass doors, eyeglass lenses, etc. 

14.5 -  14.12 – Not included. 

Terms:

Glass tempering
Glass transition temperature 
Melting point
Thermal tempering