Ceramic MaterialsMaterials Science and Engineering

an IntroductionW.D. Callister. Jr.

1. Introduction

1.1. Definition: inorganic, nonmetallic, consist of metallic and non metallic elements, primarily ionic and/or covalent bond, simple compound to complex phases structure.

1.2. Properties: hard and brittle, low toughness and ductility, good electrical and thermal insulators, high melting temperature and high chemical stability.

2. Ceramic Structures

Ionic and Covalent Bonding

• Mixture of ionic and covalent bonding.

• Percentages of ionic and covalent bonds depend on the electro-negativity differences between the components.

Ionic arrangements in ceramic• Packing of the ions is determined by the relative

size of the ions and electrostatic charge balance.• Cations are normally smaller than anions.• Coordination number (CN) is the number of

nearest anions surrounding the central cation.• For stability:1as many CN as possible. 2anions

contact with cation. 3 maintain charge neutrality.• Critical (minimum) radius ratio: the anions just

touch each other and contact the central cation.

2.1 AX-Type Crystal Structures

Rock salt (NaCl) Crystal Structure

• CN = 6• Anions at FCC sites,

cations at the interstitial sites between the FCC atoms.

Cesium Chloride (CsCl) Crystal Structure• CN = 8 • Anions at the corners, cation at the center.

Zinc Blend (ZnS) Structure

• CN = 4• One type of ion at FCC

sites, the other type at half of tetrahedral sites.

2.2 AmXp-Type Crystal Structures

Fluorite Structure (CaF2)• Cations at FCC sites, anions

at eight of the tetrahedralsites.

Antifluorite Structure(Na2O)

• Anions at FCC sites, Cations at eight of the tetrahedral sites.

2.3 AmBnXp-Type Crystal StructuresPerovskite Structure (BaTiO3), (CaTiO3)Ba and O at FCC sites: Ba at corner, O at face center.Ti at center of octahedral site.Spinel (AB2O4) StructureO at FCC site, A and B at octahedral and tetrahedral sites depending on the type of spinel

2.4 Silicate Structure

4 Oxygen ions surround 1 Silicon ion

Silica Structure• Most simple silicate

material is silicon dioxide, or silica (SiO2)

Silica Glass• Silica can be made to

exist as a non-crystalline solid or glass. Usually other oxides such as Na2Oand CaO are added to silica.

Simple Silicate• One, two or three of

oxygen in SiO4tetrahedral are share by other tetrahedral. Other cation such as Ca2+ ,Mg2+ , and Al 3+

compensate the negative charge. Ex. Mg2SiO4,Ca2MgSi2O7

Layer Silicate• Kaolinite clay

Al2(Si2O5)(OH)4

• Talc Mg3(Si2O5)2(OH)2

• Mica KAl3Si3O10(OH)2

3. Ceramic Applications

3.1 Clay Products: 3 basic components1. Clay: hydro-plasticity, harden after firing.2. Silica (flint): filler, refractory component.3. Feldspar (Aluminosilicate with K, Na or Ca, ex. K2O·Al2O3·6SiO2) :flux, low melting pt., bond other components together.

• Structural clay products: building brick, tile.• White ware: electrical porcelain, china,

sanitary ware

3.2. Glass : non-crystalline silicates containing other oxide, notably CaO, Na2O, K2O and Al2O3

3.3 Refractory ceramics: withstand high temperature, inert and thermal insulation.

1. fireclay: temp. < 1587 oC for structural parts of the furnace.2. Silica or acid refractory.: temp. < 1650 oC for acid slag3.Basic refractory.: high periclase or MgO for basic slag.4. Special refractory.: alumina, silica, magnesia, zirconia(ZrO2), mullite(3Al2O3-2SiO2), graphite, SiC.

3.4 Cement • grinding and mixing of clay and lime stone (CaO).

Calcination or heating in rotary kiln at about 1400 oC.Add gypsum(CaSO4-2H2O) to retard the setting process.

• Several constituents of portland cement, the principal ones are tricalcium silicate (3CaO-SiO2) and dicalcium silicate (2CaO-SiO2)

• Hydration reaction to form gel2CaO·SiO2 + xH2O = 2CaO·SiO2·xH2O

Harden not from drying but from chemical bonding reaction by water.

• Concrete = cement + sand + gravel

3.5 Abrasive ceramics• Use for grinding or cutting other materials.• Abrasive materials in order of hardness:

diamond, CBN (cubic boron nitride), SiC, WC, Al2O3, Silica sand.

• 3 forms of abrasive products: 1.bonded to grinding wheel (glass ceramic, resin, and metal bond) 2. coated abrasive 3. loose grains.

3.6 Engineering ceramics• Alumina (Al2O3): refractory or chemical

tubing or lining, crucible, spark plug insulator.• Silicon nitride(Si3N4): Useful combination of

mechanical engineering properties, strength, fracture toughness and refractoriness. Used for cutting tool, antifriction roller, ball bearing and components for spark-ignited and automobile engine.

• Silicon Carbide (SiC): high hardness, chemical inertness and creep resistant but low fracture toughness. Used for chemical process industry such as seals and valves, lens molds, nozzle., wear and erosion parts, creep resistant parts such as heat exchanger tubes, furnace.

• Zirconia (ZrO2) : wear resistant and cutting devices. Relatively high thermal expansion for coating on metal.

3.7 Electronic ceramics• Insulator: Electrical porcelain (~ 50%clay, 25%

silica, 25% feldspar), steatite (~90% talc, 10% clay), fosterite (Mg2SiO4), alumina.

• Dielectric materials for capacitors: Barium titanate(BaTiO3) has high dielectric constant of 1200-1500.

• Semiconductors: silicon, SiC, GaAs.• Superconductors: YBa2CuO7-x (Y is most of rare

earth elements)• Ionic conductors: "-Al2O3• Electronic packaging: Electrical insulator but high

thermal conductivity for substrate, AlN, Al2O3