Introduction to Ceramics

Metals High density High density Medium to high melting point Medium to high melting point Medium to high elastic modulus Medium to high elastic modulus Reactive Reactive Ductile Ductile Polymers Very low density Low melting point Low elastic modulus Very reactive Ductile and brittle types Ceramics Low density High melting point Very high elastic modulus Unreactive Brittle Types of synthetic materials

Types of Ceramic Glasses Glasses Traditional ceramics -clay based Traditional ceramics -clay based Engineering ceramics Engineering ceramics Cement and concrete Cement and concrete Rocks and Minerals Rocks and Minerals Ceramic Composites Ceramic Composites Covalent or ionic, interatomic bonding, Often compounds; usually oxides New “engineering ceramics” can also be carbides, nitrides and borides

Ceramics data Hard brittle solids – no unique failure strength because it depends on crack size Hard brittle solids – no unique failure strength because it depends on crack size Data can vary markedly from manufacturer to manufacturer Data can vary markedly from manufacturer to manufacturer Strength may depend on history after manufacture (surface damage) Strength may depend on history after manufacture (surface damage) Some data are invariant - structure insensitive e.g. Melting point, Density, Elastic Modulus Some data are invariant - structure insensitive e.g. Melting point, Density, Elastic Modulus Others are highly structure sensitive e.g. Tensile Strength,Fracture Toughness,Thermal conductivity,Thermal Expansion Coefficient

Natural Ceramic Materials Stone is one of the oldest construction materials Very durable Stone is one of the oldest construction materials Very durable (The Pyramids and Stonehenge!) Very cheap (The Pyramids and Stonehenge!) Very cheap Limestone (CaCO3) Limestone (CaCO3) Sandstone (SiO2) Sandstone (SiO2) Granite Granite (aluminosilicates) (aluminosilicates) Behaviour similar to all brittle ceramic materials Behaviour similar to all brittle ceramic materials

Cement and Concrete Used on an enormous scale in the construction industry Only brick and timber rival in volume (then steel) Used on an enormous scale in the construction industry Only brick and timber rival in volume (then steel) Very cheap - about one tenth the cost per volume of steel Very cheap - about one tenth the cost per volume of steel Mixtures of lime (CaO), silica (SiO2) and alumina (Al2O3) which hydrate (react with water) to form solids. Mixtures of lime (CaO), silica (SiO2) and alumina (Al2O3) which hydrate (react with water) to form solids. Can be cast to shape. Can be cast to shape. Relatively easy to manufacture from raw materials Relatively easy to manufacture from raw materials

Glass Enormous tonnages used - about the same as aluminium. Up to 80% of the surface area of a modern building may be glass (not load bearing) Enormous tonnages used - about the same as aluminium. Up to 80% of the surface area of a modern building may be glass (not load bearing) Load bearing applications in vehicle windows, pressure vessels, vacuum chambers Load bearing applications in vehicle windows, pressure vessels, vacuum chambers Inert glass coatings used in chemical & food Inert glass coatings used in chemical & food industries (glazes) industries (glazes)

Typical Glasses and Applications Soda-lime Glass Soda-lime Glass 70% SiO2, 10% CaO, 15%Na2O, 5% MgO / Al2O3: Windows, bottles etc. Low melting/softening point, easily formed 70% SiO2, 10% CaO, 15%Na2O, 5% MgO / Al2O3: Windows, bottles etc. Low melting/softening point, easily formed Borosilicate Glass (Pyrex) Borosilicate Glass (Pyrex) 80% SiO2, 13% B2O3, 4% Na2O, 3% Al2O3: Cooking and chemical glassware. High temperature strength, low coefficient of thermal expansion (CTE), good thermal shock resistance 80% SiO2, 13% B2O3, 4% Na2O, 3% Al2O3: Cooking and chemical glassware. High temperature strength, low coefficient of thermal expansion (CTE), good thermal shock resistance LAS Glass-Ceramic LAS Glass-Ceramic 60% SiO2, 20% Al2O3, 20% Li2O, + TiO2 (nucleating agent): cooker tops, ceramic composites. Heat treatment causes glass to crystallise to form crystal/amorphous composite with greater creep resistance and very low CTE – hence excellent thermal shock resistance 60% SiO2, 20% Al2O3, 20% Li2O, + TiO2 (nucleating agent): cooker tops, ceramic composites. Heat treatment causes glass to crystallise to form crystal/amorphous composite with greater creep resistance and very low CTE – hence excellent thermal shock resistance

Traditional Ceramics (“whitewares”) Pottery, porcelain, tiles, structural and refractory bricks are still made by processes very similar to those of 2000 years ago Pottery, porcelain, tiles, structural and refractory bricks are still made by processes very similar to those of 2000 years ago Made from clays which are moulded in a plastic state and then fired Made from clays which are moulded in a plastic state and then fired Consist of a glassy phase which melts and “glues” together a complex polycrystalline multiphase body Consist of a glassy phase which melts and “glues” together a complex polycrystalline multiphase body

Trad. Ceramics: Raw Materials Clays: complex hydrous aluminosilicates e.g. Clays: complex hydrous aluminosilicates e.g. Kaolinite: Al2(Si2O5)(OH)4 Kaolinite: Al2(Si2O5)(OH)4 Montmorrilonite Al5 (Na,Mg) (Si2O5)6(OH)4 Montmorrilonite Al5 (Na,Mg) (Si2O5)6(OH)4 Feldspars (low melting point): K2O.Al2O3.8SiO2 Feldspars (low melting point): K2O.Al2O3.8SiO2 Quartz sand / “Flint” (cheap, high m.p.): SiO2 Quartz sand / “Flint” (cheap, high m.p.): SiO2

Engineering Ceramics Traditional ceramics are weak because they contain many pores and cracks. Their elastic moduli are low because of the glassy phases present. Traditional ceramics are weak because they contain many pores and cracks. Their elastic moduli are low because of the glassy phases present. “Engineering ceramics” have been developed: are pure, fully dense ceramics with many fewer cracks and higher intrinsic elastic modulus. “Engineering ceramics” have been developed: are pure, fully dense ceramics with many fewer cracks and higher intrinsic elastic modulus.

Advanced ceramics Electronic ceramics Electronic ceramics Magnetic ceramics Magnetic ceramics Superconducting ceramics Superconducting ceramics Structural or engineering ceramics Structural or engineering ceramics Bioceramics Bioceramics Ceramic – ceramic composites Ceramic – ceramic composites Other ceramic composites Other ceramic composites

Fabrication of ceramic shapes Because of their high melting point, hardness and brittleness, ceramic components cannot be made by the manufacturing routes used with metals and polymers. Because of their high melting point, hardness and brittleness, ceramic components cannot be made by the manufacturing routes used with metals and polymers. Incongruent melting Incongruent melting Main method is Sintering or firing Main method is Sintering or firing Starts with powder. Starts with powder. Powder handling and powder processing are required. Powder handling and powder processing are required.

How ceramics are made powder processing Green body sintering shaping Dense or porous body machining

Major steps Powder Synthesis Powder Synthesis Powder Handling Powder Handling Green Body Formation Green Body Formation Sintering of Green Body Sintering of Green Body Final Machining and Assembly Final Machining and Assembly

Ceramic Powder typically in the size range µm typically in the size range µm Natural materials such as clays are weathered mineral powders of this size mixed with water Natural materials such as clays are weathered mineral powders of this size mixed with water Traditional ceramics are made from treated mixtures of clays Traditional ceramics are made from treated mixtures of clays Engineering ceramic powders are synthesized( different techniques) Engineering ceramic powders are synthesized( different techniques) Eg. Al2O3 and ZrO2 are precipitated from ore sands (bauxite and zircon) Eg. Al2O3 and ZrO2 are precipitated from ore sands (bauxite and zircon) SiC and Si3N4 are made by reaction e.g. the reduction of sand by coke SiC and Si3N4 are made by reaction e.g. the reduction of sand by coke

Chemical Methods of powder preparation Precipitation Precipitation Sol –gel Sol –gel Hydrothermal Hydrothermal heating heating Evaporation and oxidation Evaporation and oxidation

Powder Handling Often a liquid suspension stage followed by drying Often a liquid suspension stage followed by drying to give solid for compaction. to give solid for compaction. Need to control the forces between particles in suspension so that they repel each other until adhesion is required Need to control the forces between particles in suspension so that they repel each other until adhesion is required Premature particle bonding leads to agglomeration Premature particle bonding leads to agglomeration Dried powders are then compacted to form a “greenbody” before firing. This must have some interparticle strength to hold shape Dried powders are then compacted to form a “greenbody” before firing. This must have some interparticle strength to hold shape

Agglomeration Ceramic powders agglomerate because of Van der Waals surface forces Ceramic powders agglomerate because of Van der Waals surface forces Agglomerated powders do not fill space efficiently Agglomerated powders do not fill space efficiently May get voids in final product May get voids in final product Control by forming emulsion in fluid, usually water - “ceramic slips” Control by forming emulsion in fluid, usually water - “ceramic slips” In “engineering ceramics”, surfactants added, pH controlled. In “engineering ceramics”, surfactants added, pH controlled. Process ceramic slips and then remove fluid Process ceramic slips and then remove fluid Evaporate, Air dry, Spray dry, Freeze dry Evaporate, Air dry, Spray dry, Freeze dry

Sintering Sintering is the conversion of a ceramic green body into a solid by heating. Sintering is the conversion of a ceramic green body into a solid by heating. Process consists of mass transfer deforming the ceramic powder, filling interparticle voids and causing overall shrinkage of the compact Process consists of mass transfer deforming the ceramic powder, filling interparticle voids and causing overall shrinkage of the compact Process is thermally activated and controlled by diffusion. Process is thermally activated and controlled by diffusion.

Sintering stages

Sintering Driving Forces Sintering is driven by reduction in surface energy Sintering is driven by reduction in surface energy Two surfaces (green body) replaced by one (lower energy) grain boundary (sintered solid). Two surfaces (green body) replaced by one (lower energy) grain boundary (sintered solid).

Driving Force for sintering Driving force is approximately surface energy /volume of particle Driving force is approximately surface energy /volume of particle Ε/V = γ(4πr2)/(4πr3/3) = 3γ/r Ε/V = γ(4πr2)/(4πr3/3) = 3γ/r A typical ceramic has a surface energy of 1Jm -2 A typical ceramic has a surface energy of 1Jm -2 Thus driving force for a 1µm diameter ceramic powder is = 3MJm -3 Thus driving force for a 1µm diameter ceramic powder is = 3MJm -3

microstructure Progress of Sintering and microstructure development

Structure and properties of ceramics Polymeric materials covalent Polymeric materials covalent Metallic “ sea of electrons” Metallic “ sea of electrons” Ceramicsionic Ceramicsionic

Solids and Crystal structure Crystalline – arrangement of atoms in a crystalline solid is represented by a three dimensional space lattice which is described by unit cell. Depending on the axial length and angles, there are seven crystal systems such as cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic Crystalline – arrangement of atoms in a crystalline solid is represented by a three dimensional space lattice which is described by unit cell. Depending on the axial length and angles, there are seven crystal systems such as cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic Non crystalline or amorphous Non crystalline or amorphous

Phase Allotropy elements or compounds can exhibit two or more phases in the solid state Allotropy elements or compounds can exhibit two or more phases in the solid state Eg. Alumina, α, γ etc.

Phase change and its implication Zirconia exists in 3 different crystal structures a) monoclinic at low temperature a) monoclinic at low temperature b) tetragonal at intermediate temperature b) tetragonal at intermediate temperature c) cubic at high temperature c) cubic at high temperature High MgO or CaO: can get metastable cubic form at room temp ~2.5% Y2O3 : can get metastable tetragonal form at room temp

Imperfections in crystals Point ( Schottky and Fenkel defects) Point ( Schottky and Fenkel defects) line line or plane defects ( Grain Boundary) or plane defects ( Grain Boundary)

Line dislocation

Plane dislocation, grain boundary evolution and microstructure Ceramics are Polycrystalline Single crystal

Strength of ceramics Tensile fracture stress σF is controlled by the defects Tensile fracture stress σF is controlled by the defects present either from fabrication or from damage present either from fabrication or from damage σF = KIc/ α √ aπ σF = KIc/ α √ aπ KIc - Fracture toughness KIc - Fracture toughness α – geometrical factor (~1) α – geometrical factor (~1) a – size of biggest crack under stress a – size of biggest crack under stress

Toughness, Crack Size and Strength

toughness stress strain Hard- brittle Dectile - tough Dectile - soft

Why poor mechanical properties !!! Plastic flow by dislocation motion is very difficult in covalent and ionic materials Plastic flow by dislocation motion is very difficult in covalent and ionic materials High yield stress and hardness High yield stress and hardness Very limited plastic flow at crack tips – low fracture Very limited plastic flow at crack tips – low fracture toughness toughness Compression: strong (high yield stress); may flow or Compression: strong (high yield stress); may flow or propagate shear cracks (crushing). propagate shear cracks (crushing). Tension: weak (low toughness); always fail by brittle Tension: weak (low toughness); always fail by brittle fracture fracture

One material different Applications A simple example Aluminium Oxide Preparation : aluminium salt aluminium hydroxide aluminium hydroxide aluminiu oxide, Al 2 O 3 aluminiu oxide, Al 2 O 3

Spark plug The spark plug is connected to thousands of volts generated by the ignition coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes The spark plug is connected to thousands of volts generated by the ignition coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes

High temperature furnace

Cutting tool

Integrated Circuit, ICs

Alumina-Transparent Window

One material different Applications

reading Introduction to the class of ceramic materials: traditional ceramics, engineering Introduction to the class of ceramic materials: traditional ceramics, engineering ceramics, glasses and ceramic composites. ceramics, glasses and ceramic composites. Interatomic bonding and crystal structures found in ceramics. Interatomic bonding and crystal structures found in ceramics. Structure of glasses - random network model. Structure of glasses - random network model. Brittle nature of ceramics. Brittle nature of ceramics. Fabrication of ceramics: powder synthesis, powder processing, sintering and reaction Fabrication of ceramics: powder synthesis, powder processing, sintering and reaction sintering. sintering. Microstructures, mechanical properties and applicatio.ns Microstructures, mechanical properties and applicatio.ns Reading List Reading List “Engineering Materials 2”, M.F. Ashby and D.r.H. Jones, Chapters “Engineering Materials 2”, M.F. Ashby and D.r.H. Jones, Chapters “Introduction to Ceramics”, W.D. Kingery, H.K. Bowen and D.R. Uhlmann. “Introduction to Ceramics”, W.D. Kingery, H.K. Bowen and D.R. Uhlmann. “Ceramic Science for Materials Technologists”, I.J. McColm. “Ceramic Science for Materials Technologists”, I.J. McColm. “Ceramic Microstructures”, W.E. Lee and W.M. Rainforth “Ceramic Microstructures”, W.E. Lee and W.M. Rainforth “Materials Science and Technology – volume 11 - Structure and Properties of “Materials Science and Technology – volume 11 - Structure and Properties of Ceramics”, edited by M.V. Swain Ceramics”, edited by M.V. Swain “Mechanical Behaviour of Ceramics”, R.W. Davidge “Mechanical Behaviour of Ceramics”, R.W. Davidge "An Introduction to the Mechanical Properties of Ceramics", D.J. Green "An Introduction to the Mechanical Properties of Ceramics", D.J. Green