Introduction to Ceramics

1. Introduction to Ceramics

2. Types of synthetic materials Metals • High density • Medium to high melting point • Medium to high elastic modulus • Reactive • 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

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

4. Ceramics data • Hard brittle solids – no unique failure strength because it depends on crack size • Data can vary markedly from manufacturer to manufacturer • Strength may depend on history after manufacture (surface damage) 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

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

6. Cement and Concrete • 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 • Mixtures of lime (CaO), silica (SiO2) and alumina (Al2O3) which hydrate (react with water) to form solids. • Can be cast to shape. • Relatively easy to manufacture from raw materials

7. 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) • Load bearing applications in vehicle windows, pressure vessels, vacuum chambers • Inert glass coatings used in chemical & food industries (glazes)

8. Typical Glasses and Applications • Soda-lime Glass 70% SiO2, 10% CaO, 15%Na2O, 5% MgO / Al2O3: Windows, bottles etc. Low melting/softening point, easily formed • 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 • 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

9. Traditional Ceramics (“whitewares”) • 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 • Consist of a glassy phase which melts and “glues” together a complex polycrystalline multiphase body

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

11. Engineering Ceramics • 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.

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

13. 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. • Incongruent melting • Main method is Sintering or firing • Starts with powder. • Powder handling and powder processing are required.

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

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

16. Ceramic Powder • typically in the size range 0.5 - 5.0 µm • Natural materials such as clays are weathered mineral powders of this size mixed with water • Traditional ceramics are made from treated mixtures of clays • Engineering ceramic powders are synthesized( different techniques) • 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

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

18. Powder Handling • Often a liquid suspension stage followed by drying to give solid for compaction. • 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 • Dried powders are then compacted to form a “greenbody” before firing. This must have some interparticle strength to hold shape

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

20. Sintering • 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 is thermally activated and controlled by diffusion.

21. Sintering stages

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

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

24. microstructure Progress of Sintering and microstructure development

25. Structure and properties of ceramics • Polymeric materials covalent • Metallic “ sea of electrons” • Ceramics ionic

26. 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 • Non crystalline or amorphous

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

28. Phase change and its implication Zirconia exists in 3 different crystal structures • a) monoclinic at low temperature • b) tetragonal at intermediate 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

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

30. Line dislocation

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

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

33. Toughness, Crack Size and Strength

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

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

36. One material different Applications A simple example Aluminium Oxide Preparation : aluminium salt aluminium hydroxide aluminiu oxide, Al2 O3

37. 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

38. High temperature furnace

39. Cutting tool

40. Integrated Circuit, ICs

41. Alumina-Transparent Window

42. One material different Applications

43. reading • • Introduction to the class of ceramic materials: traditional ceramics, engineering • ceramics, glasses and ceramic composites. • • Interatomic bonding and crystal structures found in ceramics. • • Structure of glasses - random network model. • • Brittle nature of ceramics. • • Fabrication of ceramics: powder synthesis, powder processing, sintering and reaction • sintering. • • Microstructures, mechanical properties and applicatio.ns • Reading List • • “Engineering Materials 2”, M.F. Ashby and D.r.H. Jones, Chapters 15-20. • • “Introduction to Ceramics”, W.D. Kingery, H.K. Bowen and D.R. Uhlmann. • • “Ceramic Science for Materials Technologists”, I.J. McColm. • • “Ceramic Microstructures”, W.E. Lee and W.M. Rainforth • • “Materials Science and Technology – volume 11 - Structure and Properties of • Ceramics”, edited by M.V. Swain • • “Mechanical Behaviour of Ceramics”, R.W. Davidge • • "An Introduction to the Mechanical Properties of Ceramics", D.J. Green