Ceramics and Glasses

Facing materials: • Non-load bearing • Used for appearance Load bearing products (in compression only): • e.g. bricks • Low cost, good insulation, fireproof Paving units: • Abrasion resistant • Roofing tiles Chemically resistant products: • Sewers and piping • Industrial chimneys

• Brittle solids • No means for plastic deformation • Can be high strength • High stiffness • Low ductility and toughness

• The bonds in covalent ceramics are directional meaning dislocation motion is hard. • Alternating charges in ionic ceramics also prevent dislocation motion. • Therefore dislocations are unable to relieve stress.

• Amorphous (non-crystalline) • No period packing • Complex structures • Most commonly silica based

• Impurities interfere with the formation of a crystalline structure making it amorphous. • They also reduce the melting point.

• Rapid cooling • Brittle • Limited dislocation motion as there is no periodic structure • Compressive strength = 1000MPa • Modulus of rupture = 50MPa • Elastic modulus = 75MPa

• Transparency for windows • Corrosion resistant • Electrical and thermal insulators • Ease of fabrication allowing you to manufacture objects easily and cheaply

• Glasses do not crystallise and become extremely viscous on melting. • They are formed by heating raw materials above their melting point.

• Decreases the viscosity and number of impurities. • This allows you to work the molten glass at lower temperatures thus using less energy.

Pressing: • Pressure is applied to the materials in a heated mould in order to produce the desired shape. Blowing: • A temporary shape is formed by pressing and blowing compressed air into a mould in order to get the glass to expand and conform to the desired shape. Drawing: • Used to produce glasses with a constant cross-section. Fibre-forming: • Glass is drawn through a small hope to form tiny fibres. Float glass: • Molten glass floats on the surface of molten tin producing a high quality, flat finish. The glass is then passed into an annealing lehr which controls cooling.

• Tempering • Chemical strengthening • Reinforcement

• Glass is heated above the state transition temperature before being cooled by jets of air. • This causes the surface to cool more rapidly and become rigid. • The interior continues to cool and contract, drawing in the rigid surface. • This produces surface compressive stresses which must be overcome to cause tensile failure.

• Na+ ions on the glass surface are replaced with larger K+ ions which puts the surface into compression. • These compressive stresses must be overcome to cause tensile failure.

Annealing Tempering

The strength of glass decreases over time, because from the moment of manufacture, slow crack growth occurs by: • Internal stresses • Chemical interaction • Mechanical interaction

Poor: • Ductility and toughness • Difficult to manufacture • High density Good: • Thermal stability • Chemical resistance • High compressive strength • High stiffness • Excellent wear properties

Natural: • Sandstone • Granite Traditional: • Generally formed from local clays which are cheap, abundant and easily formed. • Used in structural clay products such as bricks and tiles. • Whitewares such as crockery and plumbing fixtures.

• Water causes weak Van der Waals bonds to shear allowing the layers to slide over each other, becoming more plastic allowing effective shaping. • Complex crystal structures allowing hypoplastic formation. • Contains significant liquid and so have a low strength. • As you dry clay, water is removed and layer size and spacing decreases for the clay platelets.

Slip casting: • Slip is formed with a high water content and poured into a porous mould. • The water is then absorbed away into the mould leaving a solid layer. Complete casting: • Produces a solid cast. Incomplete drying: • Produces a drain cast. Firing: • Density and mechanical properties are increased as the green body is transformed into a fired solid. Vitrification: • Formation of liquid glass flows into pores to produce dense ceramics.

• They are normally ‘hydroplastic‘. • Water is mixed in to make a plastic and pliable solid. • This can then be moulded or extruded. • The component is then dried and fired.

• Large stiffness due to ionic bonding. • Low density as generally made from light elements. • Wear resistant and hard as it is difficult to plastically deform. • Brittle. • Low toughness/ductility. • Poor strength in tension.

• Used in engines to increase fuel efficiency. • Useful in bearing surfaces or abrasives. • Corrosion resistance. • Electrical or thermal insulators.

• High purity powder • Controlling powder diameter • Adding polymer binders and plasticisers

Uniaxial stress: • Ceramic powder and binder are placed into die. • Pressure is applied by a ram. • Simple cheap and fast. • Automated. • Limited to simple shapes. Isostatic press: • Ceramic powder and binder are placed into rubber preform. • Pressure is applied by a fluid to provide aqual pressure in all directions. • Time consuming and expensive. • Labour intensive. • Able to produce complex shapes. • Slip casts