Ceramics and Glasses

What is a ceramic material and different types of it?

A non-metallic inorganic material

Types: crystalline, amorphous, glass-ceramics and bonded ceramics

What is a crystalline ceramic?

based on the regular arrangement of atoms/ions

what is an amorphous ceramic?

atoms/ions aren’t arranged in a regular long range structure - may be some longer or shorter bond lengths or distorted bond angles

what is a glass-ceramic?

ceramics with both crystalline and amorphous regions

What is a bonded ceramic?

includes the clay materials

What are some general properties of a ceramic?

hard, brittle, low toughness, high melting point, high young’s modulus, poor thermal conductor, electrical insulator

(poor in tension, better in compression)

what are some examples of crystalline ceramics?

Aluminium Oxide - used as insulators and abrasives

Aluminium Nitride - used as a substrate in high power electronic devices

Silicon carbide - used as abrasives and protective coatings

Silicon nitride - used in bearings

Why can’t ceramics be casted?

high melting point

Why are ceramics often used as coatings?

they provide wear resistance and protection

What is the structure of Silica (SiO2)

each Si atom bonds to 4 O atoms in a tetrahedral shape

When can metals form amorphous structures?

If they are cooled rapidly from the molten state and processed carefully

What is Viscosity?

a measure of resistance of a fluid to flow

viscosity = shear stress / shear strain rate

How do glass (amorphous) materials depend on temperature?

an amorphous material doesn’t have a definite temperature where it transforms from a solid to a liquid

What is the glass transition temperature?

below it the material becomes “glassy” (brittle and relatively hard)

What is a strain point?

temperature at which internal stress is relieved within several hours

What is an annealing point?

temperature at which internal stress is relieved within a few minutes

what is a softening point?

glass slumps under its own weight (viscosity of roughly 4 × 10E6)

what is the working point?

soft enough to be formed into shape

what are dislocations?

an extra half-plane of atoms disrupting the structure and ability for atomic rows to slip

what do defects in ceramic structures lead to?

stress raisers that cause cracks to initiate

How do glass-ceramics behave compared to glass?

can be processed like glass but crystalline regions have better properties

How can thermal shock cause failure to ceramics?

uneven rapid heating or cooling can create stresses due to differences in the expansion of the interior and exterior of the ceramic

as a ceramic is cooled, the exterior cools alot faster than the interior - placing the exterior under tension which can cause cracks

what does annealing do to glass?

heating around annealing point allows stresses to be relieved

what does thermal tempering do to glass?

introduces compressive residual surface stresses

How to thermally temper glass?

1. Glass heated > Tg and below softening point

2. exterior cooled rapidly in air or oil

3. exterior cools and becomes “rigid” – interior still at high temperature flows to fit the space

4. As interior cools, it shrinks, placing the exterior in compression

What is the structure of clay materials?

layered structures containing strong covalent and ionic bonds, but bonded together by weaker van der waals forces forming larger platelets

What can the surface of the platelets do?

attract water providing a lubricant that enables them to slide easily over each other, the clay can be formed into shapes (the clay is hydroplastic)

What is the greens state of clay?

upon drying, water is driven off and the particles of clay come together with large pores between them - the object can be handled without significant damage

what happens when you fire clay?

at high temperatures, this causes complex reactions to take place (vitrification) between the clay particles

this is often followed by glaze which seals the pores on the outside

what is the type of bonding in ceramics?

ionic and/ or covalent