Ceramics

CeramicsThermal Coatings

History and Background

• Applications date into antiquity - earthenware, pottery, clay product, bricks, etc

• More modern uses: Transparent glass, structural glass, refractories

• Advanced uses: Thermal barrier coatings, structural ceramics, composite armor, electronics, glass-ceramics

• Ceramics can be Amorphous or Crystalline

• Atomic structure contains strong Ionic Bonds

What are they?• A compound of metallic and nonmetallic

elements, for which the inter atomic bonding is predominantly ionic.• They tend to be oxides, carbides, etc of

metallic elements.• The mechanical properties are usually good:

high strength, especially at elevated temperature.

• However, they exhibit low to nil-ductility, and have low fracture toughness.

Crystalline Ceramics

• As with plastics, the amorphous ceramics tend to be transparent

• The structural ceramics tend to be crystalline and show greater strength, as well as stability at high temperature

Structure Anion Packing Examples

AX FCC NaCl, MgO, FeO

AX Simple Cubic CsCl

AX FCC ZnS, SiCAX2 Simple Cubic CaF2, UO2, ThO2ABX3 FCC BaTiO3, SrZrO3AB2X4 FCC MgAl2O4, FeAl2O4

AX Structure - CsCl

Simple Cubic Crystal

Cs+

• Cl-

Note: This is nota BCC structure.

AX Structure - NaCl

2- FCCinterpenetratinglattices.

Try it!

a

a

Cl-

Na+

For this NaCl structure, the crystal lattice parameter is a= 2 ( r Na+ + r Cl -), where r is ionic radius.

Compute the theoretical density of Rock Salt based on its crystal structure.

)2.16g/cm(actual

g/cm142

)ions/mol(6.023x10)]cm0.181x1002[(0.102x1

g/mol 35.45)(22.99ions4

Na

)AA(4

V

M

3

3

23377

A3

ClNa

.

AX structure - Si C

Summary of most common ceramic crystal structures

Silicate Ceramics

• Silica, SiO2

• Many polymorphs:Quartz CrystobaliteTridymite

• Low density: Quartz: 2.65g/cm3

Crystalline Crystabolite

Carbon

• Pure carbon has many polymorphs with vastly varying properties. It also exists in the amorphous state.

• Diamond: Is similar to ZnS in structure

• Graphite is considered to be a crystalline ceramic

• Fullerenes, C60, are a newly discovered polymorph - with interesting properties.

Diamond

• AX type crystal structure similar to that of ZnS.

• Each carbon atom is covalently bonded to four other C atoms in a diamond-cubic crystal structure.

• The material is optically transparent and extremely hard (hardest natural material known) and durable.

• In engineering applications, cruder or industrial forms of diamond, that are much less expensive than the gemstone forms, are used as abrasives, indentors, and coatings (especially thin films) for a variety of applications.

Graphite

• Layers of hexagonally arranged and

covalently bonded C atoms. • Between layers, weaker Van der Walls

bonds are active, giving easy slip

on the {0001} crystallographic planes. • Excellent as a dry lubricant, relatively high strength at

elevated temperatures, high thermal and electrical conductivity, low thermal expansion, resistance to thermal shock, and good machinability.

• Usage: electrodes, heating elements, crucibles, casting molds, rocket nozzles, and other applications.

Fullerenes, C60

• Molecular form of carbon with a

hollow spherical structure resembling

a geodesic dome (soccer ball.) • Called buckyballs after R. Buckminister

Fuller, who pioneered the geodesic dome.Discovered in 1985 and have since been found to occur naturally in several sources.

• In the solid crystalline state, C60 molecules pack together in a FCC unit cell arrangement with a lattice parameter a=1.41 nm.

• The pure solid material density is about 1.65 g/cm3 and

it is relatively soft and is non-conducting since it has no free electrons.

Properties of Buckyballs• When alkali metal anions, most notably K+, are in the

structure (usually 3 per C60 molecule), the resulting molecular material (K3C60) displays the characteristics of a metal. In fact, K3C60 is considered to be the first molecular metal ever encountered.

• K3C60 buckyballs and similar molecular materials become super conducting (practically no electrical resistance) at about 18K (relatively high temperature for this phenomenon)

• Applications in low-power consumption, low-pollution, magnetic-levitation and propulsion devices for mass transit systems.

• Other synthetic ceramic materials have been developed that display superconductivity at even higher temperatures (up to 100K) above the temperature of liquid nitrogen, a relatively inexpensive and abundant coolant.

Try It! • Calcualte the theoretical density of

pure C60 based on a FCC unit cell as shown:

a=1.41 nm

)1.65g/cm(actual

1.71g/cm

N)(1.41x10

11)4(60)(12.0

V

M

3

3

A37

Defects in Crystalline Ceramics

• Vacancy• Interstitial• Dislocation• Grain Boundary

Cation InterstitialAnion VacancyCation Vacancy

Schotky DefectFrenkel Defect

Electro-neutrality

Mechanical Properties

• Brittle Materials, hard to perform a Tension Test.• Flexural Test (Bend) is often substituted.• Obtain Flexural Strength (Modulus of Rupture),

Stiffness (Modulus of Elasticity), and Ductility.• Strength is often good, Stiffness my be high, but

Ductility and affected properties are poor.• In crystalline ceramics, dislocation motion is

difficult because of the need to maintain electro-neutrality. Consequently plastic deformation is restricted.

Flexural Test Configuration

a

L

a

P P

bd

Four point bend fixture

D

X-Section

3D

Pa32mr

2d b

a P6mr

Rectangular: Circular:

Stress-Strain Behaviour

0.00100.00080.00060.00040.00020.00000

100

200

300

Bending Strain

Ben

ding

Str

ess,

MP

a

Aluminum Oxide

Soda-Lime Glass

Mechanical Properties of Various Ceramics

a Sintered with about 5% porosity

Hardness of Ceramics

Relative Hardness

B4C, SiCWC, Al2O3

Glass

Effect of Porosity on Stiffness

)0.9P1.9P(1EE 2o

Where Eo is the theoretical modulus of elasticity with no porosity, and P is the volume fraction of porosity.

Effect of Porosity on Strength

Where o is the theoretical modulus of rupture with no porosity, P is the volume fraction of porosity, and n is an empirical material constant

nPe omr

Fracture Toughness

Fracture Toughness (MPam)

Amorphous Ceramics - Glasses (Na20, Ca0, K2O, etc)

• The viscosity of the material at ambient temperature is relatively high, but as the temperature increases there is a continuous decrease in viscosity.

• When the viscosity has decreased to the point that the ceramic is a fluid, it is considered to have melted.

• At ambient temperature while it is still solid, it is said to be in the “glassy” condition.

• There is no distinct melting temperature (Tm) for these materials as there is with the crystalline materials.

• The glass transition temperature, Tg, is used to define the temperature below which the material is a “solid” and

defines a practical upper limit on service temperature.

Liquid

supercooled liquid

glass

Crystaline solid

Tg Tm

Crystalization

TEMPERATURE

Specific volume of amorphous and crystalline

ceramics.

Viscous Behaviour in Amorphous Ceramics

• Plastic deformation does not occur by dislocation motion in amorphous or non-crystalline ceramics, such as glass.•Deformation is by viscous flow: rate of deformation proportional to applied stress.

dydvA

F

/;

= shear stress = viscosity of material

Ceramic Phase Diagrams

• Note: They are similar to metal alloy systems - except the temperatures are generally higher.

Binary Eutectic Ceramic Alloy

Spinel