Ceramics
• Compounds of metallic and nonmetallic
elements
• Predominately ionic bonding
• Hard, wear resistant, stiff, brittle
• Can be: Insulators (electrical and thermal),
Semiconductor, Superconductors, Magnetic
• Glasses
ENGR45 Materials Engineering
Spring 04
Ceramic Grouping
Ceramic
Materials
Glasses
Clay
Products
Refractories
Glasses
Structural
Clay
Products
Fireclay
GlassCeramics
Whitewares
Silica
Abrasives
Cerments
Advanced
Ceramics
Basic
Special
ENGR45 Materials Engineering
Spring 04
Crystalline Structure
• Similar to interstitials
• Coordination Number is the
number of nearest neighbors
for a cation
• Charge Balance
ENGR45 Materials Engineering
Spring 04
Defects
• Point Defect - Frenkel, Shottky, Impurities
• Linear Defects – Dislocations
• Surface Defects – Grain Size, Cracks
• Porosity
ENGR45 Materials Engineering
Spring 04
Point Defects
• Frenkel defect – Cation
moved from normal position
to an interstitial site
• Schottky defect –
Missing cation & anion pair
• Impurity – Both
substitutional and interstitial
ENGR45 Materials Engineering
Spring 04
Linear Defects
Dislocations are present in the ceramic
crystalline structure but are unable to move
due to the high strength of the ionic bonds
holding the atoms together.
Before such a large force could be applied to
cause dislocation movement, other defect
would be overloaded and cause the ceramic
to fail.
ENGR45 Materials Engineering
Spring 04
Surface Defects
• Improper / Missing bonding
• Cracks – K IC = Yσ πa
• Geometric Shape – Y is a geometric
factor due to stress concentration of
different shapes (e.g.. sharp corners,
rounded fillets)
ENGR45 Materials Engineering
Spring 04
Porosity
• Porosity are voids in the bulk ceramic
material usually introduced during
fabrication
• Modulus of Elasticity – E = EO (1 − 1.9P + 0.9P2 )
• Flexural Strength – σ fs = σ O exp(− nP )
ENGR45 Materials Engineering
Spring 04
Mechanical Properties
• Failure Modes (Brittle)
• Flexural Strength
Ø σ = 3FL = FL
2bd 2 πR3
• Toughening Methods
Ø Cermet (tough matrix / hard ceramic particle)
Ø Ceramic Matrix Composite (interface block cracks)
Ø Transformation-Toughened
Ø Improving Processing (fine grain, pure, dense)
ENGR45 Materials Engineering
Spring 04
Glasses
• Glasses vs Crystalline
Structures
• Glass Transition Temp
• Glass Modifiers (Na2O,
CaO)
• Heat Treatment – Annealing,
Tempering
ENGR45 Materials Engineering
Spring 04
Glass Properties
• Melting Point – Liquid
• Working Point – Easily deformed
• Softening Point – No significant
deformation
• Annealing Point – Removal of
residual stresses
• Strain Point – Will fracture before
plastic deformation
ENGR45 Materials Engineering
Spring 04
Sintering
• High Temperature and Pressure
• Diffusion across grain boundaries
• Decrease porosity with increase
in sintering
ENGR45 Materials Engineering
Spring 04