Ceramics, glasses and high temperature alloys at TWI

Ceramics

All ceramics are inorganic non-metallic materials made up of the same basic ingredients, i.e. carbon, nitrogen, oxygen or boron, in combination with a metal, e.g. aluminium or silicon. They combine in relatively simple forms, such as Al 2 O 3 , or SiO 2 .

In general, ceramics have the following inherent properties:

Ceramics
  • Hard
  • Resistant to plastic deformation
  • Resistant to high temperatures
  • Good corrosion resistance
  • Low thermal conductivity
  • Low electrical conductivity

However, some ceramics exhibit high thermal conductivity and/or high electrical conductivity.

The combination of these properties means that ceramics can provide:

  • High wear resistance with low density
  • Wear resistance in corrosive environments
  • Corrosion resistance at high temperatures

The choice of ceramic is based on the final application of the component to be manufactured, as well as the manufacturing route.

TWI can provide advice on materials selection and processing and manufacturing routes for most applications of ceramics.

For more information on Ceramics at TWI, see below:

Knowledge summary

FAQs

Glass ceramics

Glass-ceramics are polycrystalline solids prepared by controlled crystallisation (or devitrification) of a parent glass. Usually, the glass is quenched from a melt then formed to shape; it is then converted to a ceramic (a glass-ceramic) by a suitable heat treatment. Glass and glass-ceramics are usually, but not necessarily, based on the oxides of silicon, aluminium and/or boron with additions of various other metal oxides.

A glass-ceramic can be engineered, by controlling the composition and microstructure, to meet a range of properties - most notably the coefficient of thermal expansion (CTE). Many glass-ceramics possess desirable properties such as high hardness, excellent corrosion and erosion properties and good thermal shock resistance; these properties are also retained to high temperatures. As such, glass-ceramics exhibit the best of both worlds; the formability of glass with the properties of ceramics.

Numerous glass-ceramics have been developed for diverse applications such as substrates for electronic devices, radomes, mirror blanks and cooker tops (ceramic hobs). They are also used to make hermetic seals in many glass-ceramic/metal systems. Among the most common of the glass-ceramic systems are lithium-alumino-silicate (LAS), zinc-alumino-silicate (ZAS) and magnesium-alumino-silicate (MAS). These systems have been studied in detail and many formulations are commercially available.

For more information on Glass-Ceramics at TWI, see below:

Best practice guides

Knowledge summaries

FAQs

Staff papers

Glasses

The term glass describes a state of matter where the atoms/molecules are randomly arranged. An ASTM definition of glass describes the inorganic, amorphous, product of a rapidly cooled melt. Using this definition, rapidly cooled metals can also be said to be glassy. Terms such as glassy, amorphous and vitreous all describe the same thing; a material with a randomly arranged crystal structure.

Glasses can be thought of as very viscous liquids. Glasses also exhibit a glass transition temperature, above which they flow much more readily. These demonstrate some of the differences between a glass and a ceramic, even though they can have exactly the same composition. For example, silica glass has the same composition as quartz (crystallised silica). The difference in behaviour lies in their atomic structure. In glass, the building blocks (SiO4 tetrahedra) are arranged randomly; whereas silica crystals have a very ordered structure.

For more information on Glasses at TWI, see below:

Best practice guides

Knowledge summaries

FAQs

High temperature alloys

High temperature alloys are generally recognised as being those alloys used above temperatures of 600°C. These alloys exhibit high strength at temperature, resistance to environmental attack (including nitridation, carbonisation, oxidation and sulphidation), excellent creep resistance, creep rupture strength, toughness, and metallurgical stability. The maximum operating temperature of various materials including superalloys and oxide dispersion strengthened (ODS) alloys are given in the figure below.

Maximum operating temperature of various materials graph

The main constituent of high temperature alloys is one of nickel, iron, chromium or cobalt. Other alloying additions such as aluminium, molybdenum and tungsten may also be used to give the alloy certain desired properties.

FAQs

Copyright © 2005 TWI Ltd
Information and advice from TWI and its partners are provided in good faith and based, where appropriate, on the best engineering knowledge available at the time and incorporated into TWI's website in accordance with TWI's ISO 9001:2000 accredited status. No warranty expressed or implied is given regarding the results or effects of applying information or advice obtained from the website, nor is any responsibility accepted for any consequential loss or damage.