Ceramics are inorganic, non-metallic. Ceramics. Ceramics exhibit ionic, covalent bonding or a combination of the two (like in Al 2 O 3 )

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1 What are Ceramics? CERAMIC MATERIALS Ceramics are inorganic, non-metallic and crystalline materials that are typically produced using clays and other minerals from the earth or chemically processed powders Ceramics are crystalline and are compounds formed between metallic and non-metallic elements such as aluminium and oxygen (alumina- Al 2 O 3 ), silicon and nitrogen (silicon nitride- Si 3 N 4 ) and silicon and carbon (silicon carbide-sic). Glasses are non-metallic, inorganic but amorphous.. They are often considered as belonging to ceramics. Characteristics of Ceramics Structure of Ceramics Ceramics Low density High T m High elastic modulus Brittle Non-reactive Goff electrical and thermal insulators High hardness and wear resistance Metals High density Medium to high T m Medium to high elastic modulus Ductile Reactive (corrode) Good electrical and thermal conductors Polymers Very low density Low Tm Low elastic modulus Ductile and brittle Ceramics exhibit ionic, covalent bonding or a combination of the two (like in Al 2 O 3 ) Type of bonding strongly influences the crystal structure of ceramics lceramics crystallise in two main groups: 1. Ceramics with simple crystal structure (e.g; SiC, MgO) 2. Ceramics with complex crystal structures based on silicate SiO 4 (known as silicates)

2 Ionic bonding: metallic ions + nonmetallic ions Cations Anions Stable structure Coordination Number: R C /R A Ceramic Bonding Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. Large vs small ionic bond character: CaF 2 : large SiC: small R C /R A = , Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. 6 Ceramic crystal structure considerations Charge Neutrality The bulk ceramic must remain electrically neutral For example, the compound MgO 2 does not exist Mg +2 & O - 2 : net charge / molecule = 1(+2) + 2(-2) 2) = -2 must MgO Coordination Number (CN) : The number of atomic or ionic nearest neighbors. Depends on atomic size ratio CN increases as the R C /R A increases CN determines the possible crystal structure, Thus, CN determines the physical properties

3 Examples of AX type structure MgO and FeO Cs + Cl - MgO and FeO also have the NaCl structure O 2- Mg 2+ r O = nm r Mg = nm Rock Salt Structure = Na + = Cl , Callister 7e. Each oxygen has 6 neighboring Mg AX Crystal Structures AX Type Crystal Structures include NaCl, CsCl, and zinc blende Zinc Blende structure AX Crystal Structures Cesium Chloride structure: Ex: ZnO, ZnS, SiC r + Cs = = r Cl 12.3, Callister 7e. Each Cs + has 8 neighboring Cl , Callister 7e

4 AX 2 Crystal Structures ABX 3 Crystal Structures Fluoride structure Perovskite Calcium Fluoride (CaF 2 ) cations in cubic sites Ex: complex oxide BaTiO 3 UO 2, ThO 2, ZrO 2, CeO , Callister 7e. 12.5, Callister 7e Mechanical Properties We know that ceramics are more brittle than metals. Why? Consider method of deformation slippage along slip planes in ionic solids this slippage is very difficult too much energy needed to move one anion past another anion Glasses Glasses Glass- ceramics Clay Products ceramics Refractories Abrasives Cements Eng. Ceramics Our focus is HERE!!! 15

5 Engineering ceramics are generally classified into the following: Structural ceramics, Industrial wear parts, bioceramics, cutting tools, engine components Electrical and Electronic ceramics, Capacitors, insulators, substrates, IC packages, piezoelectrics, magnets, superconductors Ceramic coatings, Industrial wear parts, cutting tools, engine components Bioceramics Cutting tools Engine parts Chemical processing & environmental ceramics Filters, membranes, catalysts Coating Silicate Ceramics Most common elements on earth are Si & O Si-O O Tetrahedron O Si Silicates Combine SiO 4 4- tetrahedra by having them share corners, edges, or faces The strong Si-O bond leads to a strong, high melting material (1710ºC) Mg 2 SiO 4 Ca 2 MgSi 2 O , Callister 7e. Cations such as Ca 2+, Mg 2+, & Al 3+ act to neutralize & provide ionic bonding 20

6 Si-O O Tetrahedron O Silicate Ceramics O Two most common silicate ceramics are: Silica and silica glasses Si 2. Silica Glasses If the tetrahedra are randomly arranged, a non- crystalline structure, known as Glass is formed. 1. Silica (SiO 2 ) If the tetrahedra are arranged in a regular and ordered manner, a crystalline structure is formed. Silica have 3 different types: quartz, crystobalite and tridymite Silica Silica glasses is a dense form of amorphous silica - Charge imbalance corrected with counter cations such as Na + -Borosilicate glass is the pyrex glass used in labs -better temperature stability & less brittle than sodium glass Amorphous Silica Silica gels - amorphous SiO 2 Si 4+ and O 2- not in well-ordered lattice Charge balanced by H + (to form OH - ) at dangling bonds very high surface area > 200 m 2 /g SiO 2 is quite stable, therefore unreactive makes good catalyst support 12.11, Callister 7e. Other oxides may also be incorporated into a glassy SiO 2 network in different ways: 1. Network formers: form glassy structures (B 2 O 3 ) 2. Network modifiers: added to terminate (break up) the network (CaO, Na 2 O). These are added to silica glass to lower its viscosity (so that forming is easier) 3. Network intermediates: these oxides cannot form glass network but join into the silica network and substitute for Si. 23

7 Carbon Forms Carbon Forms Carbon black amorphous surface area ca m 2 /g layer structure aromatic layers Diamond tetrahedral carbon ❿hard no good slip planes ❿brittle can cut it large diamonds jewellery small diamonds 12.17, Callister 7e. ❿often man made - used for cutting tools and polishing diamond films ❿hard surface coat tools, medical devices, etc , Callister 7e. 25 weak van der Waal s forces between layers planes slide easily, good lubricant 26 Carbon Forms Fullerenes or carbon nanotubes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes ❿Like a soccer ball C 60 - also C 70 + others Defects in Ceramic Structures Frenkel Defect -a cation is out of place. Shottky Defect --a paired set of cation and anion vacancies. Shottky Defect: Frenkel Defect 12.21, Callister 7e. (Fig is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) Adapted from Figs & 12.19, Callister 7e. Equilibrium concentration of defects ~ e Q D / kt 27 28

8 Mechanical Properties of Ceramics Mechanical Properties of Ceramics Ceramics have inferior mechanical properties compared to metals, and this has limited their applications The main limitation is that ceramics fail in brittle manner with little or no plastic deformation. Fracture strength of ceramics are significantly lower than predicted by theory because of the presence of very small cracks in the material (stress concentrators). Lack of ductility in ceramics is due to their strong ionic and covalent c bonds. Ceramics have excellent compressive strength (used in cement and concrete in foundations for structures and equipment) The principles source of fracture in ceramics is surface cracks, porosity, inclusions and large grains produced during processing. Testing ceramics using the usual tensile testing is not possible,, so a transverse bending test is used and a modulus of rupture (MOR) is determined. Strength of ceramics can only be described by statistical methods s and it is dependent on specimen size. Flexural 3F f L strength, σ fs = 2 2bd 3F f = 3 πr L Rectangular cross section Circular cross section Transverse rupture Compressive Elastic strength strength modulus Hardness Poisson s Density Material Symbol (MPa) (MPa) (GPa) (HK) ratio (n) (kg/m 3 ) Aluminum Al 2O oxide Cubic boron CBN nitride Diamond Silica, fused SiO Silicon SiC carbide Silicon Si3 N nitride Titanium TiC carbide Tungsten WC ,000 15,000 carbide Partially PSZ stabilized zirconia The properties vary widely depending on the condition of the material (crack size)

9 Factors Affecting Strength of Ceramics Failure of ceramics occurs mainly from structural defects; surface cracks, porosity, inclusions and large grains during processing. Toughening Mechanisms of Ceramics Porosity in ceramics acts as stress concentrators: crack forms and propagates leading to failure. Once cracks start to propagate, they will continue to grow until fracture occurs. Porosity also decrease the cross-sectional sectional area over which a load in applied: lower the stress a material can support. Strength of ceramics is thus determined by many factors: 1. Chemical composition 2. Microstructure - In dense ceramics materials, no large pores, the flaw is related to grain size. Finer grain size ceramics, smaller flaws size at the boundaries, hence stronger than large grain size. 3. Surface condition 4. Temperature and environment (failure at RT, usually due to large e flaws). Fracture strength or toughness of ceramics can be improved only by mechanisms that influence the crack propagation (ceramics always contain cracks). There are various methods used to improve the toughness of ceramics: 1. Transformation toughening 2. Microcrack induced toughening 3. Crack deflection 4. Crack bridging 1. Transformation Toughening: e.g. Partially Stabilised Zirconia (PSZ) Zirconia (ZrO 2 ) exists on 3 different crystal structures: Melt Cubic Tetragonal Monoclinic 2680 o C 2370 o C Transformation toughening is achieved by stabilising the tetragonal structure at room temperature by adding other oxides such as: MgO, CaO, and Y 2 O 3 to zirconia. If cubic ZrO 2 is stabilised, so it retains cubic structure at RT called fully stabilised zirconia. If tetragonal ZrO 2 is stabilised, it called as PSZ o C Mixture of ZrO 2-99 mol %MgO is sintered at 1800 o C, then rapidly cooled to RT become metastable cubic structure. The materials is reheated at 1400 o C for sufficient time, a fine metastable precipitate with tetragonal structure known as PSZ formed. This transformation is accompanied by a volume expansion, causing g a compressive stress locally and in turn a squeezing effect on the crack and enhancing the fracture toughness also significantly extends the reliability and lifetime of products made with stabilized zirconia. Matrix is cubic ZrO 2 - MgO Precipitate is tetragonal ZrO 2 -MgO Precipitate around crack is monoclinic ZrO 2 -MgO As the crack propagates, it creates a local stress field that induces transformation of the tetragonal structure to the monolithic (or monoclinic) structure in that region.

10 Single crystals of the cubic phase of zirconia are commonly used as diamond simulant in jewelery. 2. Micro-crack crack Induced Toughening: Microcracks are purposely introduced by internal stresses during processing of the ceramics tend to blunt the tip of the propagating crack and thus reduce the stress concentration at the crack tip. This micro-crack crack will interfere the crack tip propagation. The cubic phase of zirconia also has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC in jet and diesel engines to allow operation at higher temperatures. 3. Crack Deflection and Crack Bridging This is achieved by reinforcing the ceramics: produce ceramic based composites (CMC) Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures. Advanced ceramics Ceramic as abrasive materials The high hardness of some ceramic materials makes them useful as abrasive materials for cutting, grinding, polishing e.g. Al 2 O 3 and SiC, diamonds MEMS mechanical devices that integrated with large number of electrical elements on a substrate of Silicon e.g. for microsensors Current research on ceramic materials to replace silicon, because ceramic are tougher, more refractory and more inert e.g. silicon carbonitrides (silicon carbide-silicon nitrides alloys)

11 Properties of Glasses Glasses posses properties not found in other engineering materials. Combination of transparency, ability to transfer light, hardness at room temperature, a sufficient strength and corrosion resistance to most m environments. These make glasses important for many applications: : vehicle glazing, lamps, electronic industry, laboratory apparatus. Deformation of glass varies with temperature: At high temperatures: viscous flow At low temperatures: elastic and brittle At intermediate temperatures: visco-elastic Heat Treatment of Glasses Glasses can be rendered more fracture resistance by introducing compressive stresses on the glass surface. This is followed by glass tempering 1. Glass Annealing Used to reduce internal residual stresses, which weaken the glass s and may lead to fracture. The glass is heated to the annealing temperature, then slowly cooled oled to RT 2. Glass Tempering Used to strengthen glass by inducing compressive stresses at the surface. Tempering is achieved by heating the glass to a temperature > T g, then rapidly cooled to room temperature. The surface of the glass cools first and contracts; later the centre cools and attempts to contract but is prevented from doing so by the rigid and strong surface. This produces high tensile stresses in the centre but compressive e stresses at the surface. This tempering treatment increases the strength of the glass because applied tensile stresses must surpass the compressive stresses on surface before fracture occurs. Tempered glass has higher impact resistance than annealed glass and about 4x stronger than annealed glass.