Fabrication and Processing of Ceramics

Chapter 13.11 - 13.14; Week 11, Lecture 3

Glass properties

• Noncrystalline glasses do not cool like crystalline materials.

• Glass temperature: supercooled liquid transforms into solid glass upon cooling past Tg.

• Supercooled glass liquid: very viscous - upon cooling, glass becomes more and more viscous in a continuous manner with decreasing temperature.

Upon heating:

 Strain point (3 x 1013 Pa·s): fracture below this, Tg above this

 Annealing point (1012 Pa·s): relieves stresses - atomic diffusion is rapid

 Softening point (4 x 106 Pa·s): handling causes dimensional changes beyond this

 Working point (1000 Pa·s): easily deformed at this viscosity

 Melting point (10 Pa·s): molten liquid

 Working range: between the softening and working temperatures – forming processes

Glass forming

• Heating raw materials to an elevated temp. above which melting occurs

• Made form silica soda lime, Na2O and CaO added, limestone too

• Essential: homogeneous and pore free

• Homogeneity achieved by complete melting and mixing raw ingredients

• Porosity results from small gas bubbles produced - must be eliminated by methods:

1. Pressing: Fabrication of thick walled pieces (dishes); formed by pressure application in a graphite coated cast iron mold having the desired shape; heated to ensure an even surface

2. Blowing: mechanical pressing mold; inserted into a finishing blow mold and forced to confirm mold contours by pressure from blast air

3. Drawing: Form long glass pieces that have constant cross section area

4. Sheet

5. Fiber forming

Heat treatments of glass

Annealing: Relieves thermal stresses - faster surface cooling vs slower interior cooling

• ceramic cooled from high temp. - internal thermal stresses introduced as a result of difference in cooling rate between outside and inside

• thermal shock - weaken material, fracture

• avoid thermal stress - cooking slowly

• eliminate/reduce - annealing and slowly cooled

Glass tempering:

• Thermal tempering: increases glass strength

• Compressive residual stress on surface

Clay products: properties

• Aluminosilicates: alumina (Al2O3), silica (SiO2) and chemically-bound H2O

• Layered crystal structure

• Water molecules allow sliding (free to move) of clay particles: hydroplasticity (very plastic when water is added - mol. fit between layered sheets and form a thin film around clay particles)

• Fuses/melts over range of temperatures: retains shape during firing

Hydroplastic forming

• Mixed with water: plastic and pliable

• Low yield strength but enough to maintain shape during handling and drying

• Extruded into shape

• Brick, pipe and tiles

Slip casting

• Slip: Suspension of clay and/or nonplastic materials in water

• Porous mold – Plaster of paris

• Slip water absorbed into mold until:

  • Whole cavity is solid (solid casting)

  • Wall of cast shell reaches desired thickness by pouring out excess slip (drain casting)

Drying

• Liquid removed → green body (a body that has been formed and dried out but not fired)

• Shrinkage occurs → cracking/warpage

  • Surface evaporation should be <= interior diffusion

  • Reduce thickness

  • Keep water content in slip low

  • Increase particle size/add large nonplastic particles

Firing

• Green body heated at 900 – 1 400 °C

• Improves density and strength of green body

• Vitrification: formation of a glassy liquid phase that fills pore volumes through capillary action

• Cooling forms a glassy matrix

• Higher temperature → higher degree of vitrification (fluxing agent)

  • Bricks fired at 900 °C → porous

  • Optically translucent porcelain fired at 1 200 °C → very dense

Powder pressing

Like powder metallurgy of metals

• Powder binded by water is compacted into desired shape

• Particles are various sizes to fill voids

• Binder (e.g. water) lubricates powder for compaction

• Sintering: coalescence of powder particles to improve density of isostatic and uniaxially pressed products

Advantages:

• Used for materials that form liquid phases at too high temperatures

• High densities without grain growth

Disadvantages:

• Expensive, longer production times, stricter controls needed, limited mold life

Tape casting

Slip: ceramic particles in an organic liquid with binders and plasticisers