Flexible glass for display screens such
as laptops - source Abrisa

Industrial applications of glass are pervasive and include fibre for reinforcing plastics and for insulation, glass lined steel vessels, safety spectacles, x-ray tubes, furnace windows and laminated car safety windscreens. Modern glasses can have remarkable properties, even flexible enough to bend easily.

15.3 Glass Ceramics

In 1957, Corning discovered (re-discovered) that glasses could be converted into the crystalline state by annealing (reheating) at a moderate temperature with added nucleating agents to control the crystallization, producing the important class of materials called “glass ceramics”. If the glass is of a suitable composition, a large fraction (30 to 90%) becomes crystalline and the crystals can be uniform and very small, typically less than 500 microns, making them pore free and potentially transparent. This can produce strong materials, considerably stronger than the original glass, typically able to scratch carbon steel, with good thermal shock resistance. One example is the important glass ceramic, lithium aluminium silicate. This material was originally developed for astronomical mirrors, but is now is used for cooker tops, bakeware and high performance projector mirrors.

Indesit hobs

Corning’s product called “Pyroceram” is also used for nosecones and other aerospace applications in very high velocity (Mach 4) applications. Other uses include erosion resistant glass-ceramic linings for pipes. Glass ceramics can be readily tailored by changing the composition of the glass or the annealing process. In particular its coefficient of expansion can be varied to match metals making it easier to achieve good metal/ceramic bonds.

15.4 Composites

More obvious types of composite of ceramics and other materials are formed from layers of different materials sandwiched together, their combination forming a solution to a particular problem. For example, high-grade military and civil body armour is typically made up of two layers of optimised material, such as Kevlar, sandwiching a hot-pressed boron and/or silicon carbide ceramic plate, one inch (2.5cm) thick overall, which has high resistance to knives and projectiles. Also a composite of hexagonal ceramic segments 1cm across combined with fibreglass and polymer can be used. On a much larger scale the armour plating of military tanks is also a structure containing ceramic plates and elastic layers within a metal skin. Military applications of ceramics are not only for external armour but also to protect sensitive control systems in missiles.

Sheffield Hallam body armour, Craig
ballistic plate and Ceradyne body armour

On the other hand, “cermet” composites are derived from metals and ceramics that are bonded together at the chemical level so they are a “solid solution” that has some of the characteristics of both. The ceramic is usually in the range from 15% to 85% by volume, but an even smaller amount may be added to a metal to strengthen it. As an example, a copper matrix may contain a small amount of alumina as particles. The composite behaves as copper at room temperature, but the alumina significantly improves its high temperature and high radiation performance. Development work on cermets was initially focussed on improving the refractory strength of metals, to achieve a material with the refractory characteristics of a ceramic and the ductility and thermal shock resistance of a metal. Chromium-bonded, alumina-based cermets have very good high temperature strength, good erosion and wear resistance, and excellent resistance to molten metals and oxidation. Titanium and tungsten carbide-based cermets have been used to contain molten sodium and sodium/potassium at temperatures exceeding 550 ºC, used for advanced nuclear reactors.