Chapter Outline: Applications and Processing of Ceramics Chapter 13:Applications and Processing of Ceramics • Short review of glass/ceramics applications and processing (13.1-13.3,13.8,13.10)
Formation and processing of Ceramics • Ceramic materials have relatively high melting temperature and are brittle strain hardening cannot be applied • Some ceramics formed by powder pressing. involve drying and firing,… Sintering: powder pressing + firing below melting T • Cements formed from a fluid paste that hardens by chemical reactions • Glasses produced by complete melting of raw ingredients
Applications of Ceramics • Compressive strength is typically tentimestensile strength. • Transparency to light optical applications (windows, photographic cameras, telescopes, etc) • Good thermal insulation ovens, exterior tiles of the Shuttle orbiter, etc. • Good electrical isolation used to support conductors in electrical and electronic applications. • Good chemical inertness applications in reactive environments.
Silicate Glasses Non-crystalline silicates (SiO2) containing other oxides (CaO, NaO2, K2O, Al2O3) Containers, windows, lenses, fiberglass, etc. Example: Container/window glasses contain ~ 30 wt% oxides (CaO, Na2O) whose cations are incorporated within SiO4 network: network modifiers. Quartz sand + soda ash or limestone
Properties of Glasses (I) • Solidification is gradual, through a viscous stage (viscosity increasing with decreasing T) without a clear melting temperature • Specific volume (1/density) does not have abrupt transition at fixed temperature. Shows a change in slope at the glass-transition temperature
Viscosity (from Chapter 12) Viscosity: a measure of non-crystalline (glass or liquid) material’s resistance to deformation High-viscosity fluids resist flow; low-viscosity fluids flow easily. Viscosity: How readily a moving layer of fluid drags adjacent layers along with it. Units are Pa-s: Poises (P) 1 P = 0.1 Pa-s Viscosity of water at room temp is ~ 10-3 P Viscosity of typical glass at room temp >> 1016 P
Properties of Glasses (II) • Important temperatures(viscosity) in glass • Melting point: viscosity = 100 P, below this viscosity (higher T) glass is liquid • Working point: viscosity = 104 P, glass is easily deformed • Softening point: viscosity = 4107 P, maximum T at which a glass piece maintains shape for a long time • Annealing point: viscosity = 1013 P, relax internal stresses (diffusion) • Strain point: viscosity = 3x1014 P, above this viscosity, fracture occurs before plastic deformation • Glass forming operations occur between softening and working points
Properties of Glasses (II) Important temperatures in glasses can be defined by viscosity
Heat treatment of Glasses Annealing: elevate temperature to remove thermal stresses resulting from inhomogeneous temperatures during cooling (similar to annealing of metals) Tempering:heating glass above glass transition temperature but below softening point; then quench in an air jet or oil bath. The interior: cools later than outside, tries to contract while in a plastic state after exterior has already become rigid. Causes residual compressive stresses on surface and tensile stresses inside. In fracture: crack has to overcome residual compressive stress, making tempered glass less susceptible to fracture. Used in automobile windshields, glass doors, eyeglass lenses, etc.
Summary Make sure you understand language and concepts: • Glass tempering • Glass transition temperature • Melting point (glass)