What are Ceramics? Ceramics are inorganic, non -metallic and crystalline materials that are typically produced using clays and other minerals from the earth or CERAMIC chemically processed powders MATERIALS Ceramics are crystalline and are compounds formed between metall ic MATERIALS and non -metallic elements such as aluminium and oxygen (alumina - Al 2O3 ), silicon and nitrogen (silicon nitride - Si 3N4) 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 Characteristics of Ceramics Structure of Ceramics Ceramics Metals Polymers Ceramics exhibit ionic, covalent bonding or a combination of the two (like in Al 2O3) Low density High density Very low density High T m Medium to high T m Low Tm Type of bonding strongly influences the crystal structure of cer amics High elastic modulus Medium to high elastic Low elastic modulus Brittle modulus Brittle modulus Ductile and brittle lCeramics crystallise in two main groups: Non -reactive Ductile Goff electrical and Reactive (corrode) thermal insulators Good electrical and 1. Ceramics with simple crystal structure (e.g; SiC, MgO) High hardness and thermal conductors wear resistance 2. Ceramics with complex crystal structures based on silicate SiO 4 (known as silicates) Ceramic Bonding • Bonding: Ionic bonding : metallic ions + nonmetallic ions -- Mostly ionic, some covalent. Cations Anions -- % ionic character increases with difference in electronegativity. Stable structure • Large vs small ionic bond character: Coordination Number: RC/R A CaF 2: large SiC: small RC/R A = 0.155 Adapted from Fig. 2.7, 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 must MgO Coordination Number (CN) : The number of atomic or ionic nearest neighbors. Depends on atomic size ratio CN increases as the RC/R A increases CN determines the possible crystal structure, Thus, CN determines the physical properties MgO and FeO Examples of AX type structure MgO and FeO also have the NaCl structure Cs + 2- O rO = 0.140 nm Cl - 2+ Mg rMg = 0.072 nm Rock Salt Structure = Na + Adapted from Fig. = Cl - 12.2, Callister 7e. Each oxygen has 6 neighboring Mg 2+ 10 AX Crystal Structures AX Crystal Structures AX–Type Crystal Structures include NaCl, CsCl, and zinc blende Zinc Blende structure Cesium Chloride structure: Ex: ZnO, ZnS, SiC r + .0 170 Cs = = .0 939 r − .0 181 Cl + - Each Cs has 8 neighboring Cl Adapted from Fig. 12.4, Callister 7e. Adapted from Fig. 12.3, Callister 7e. 11 12 ABX Crystal Structures AX 2 Crystal Structures 3 Fluoride structure • Perovskite • Calcium Fluoride (CaF ) 2 Ex: complex oxide BaTiO • cations in cubic sites 3 • UO 2, ThO 2, ZrO 2, CeO 2 Adapted from Fig. 12.6, Callister 7e. Adapted from Fig. 12.5, Callister 7e. 13 14 Mechanical Properties We know that ceramics are more brittle than metals. ceramics Why? • Consider method of deformation slippage along slip planes Glasses Refractories Cements Eng. Ceramics in ionic solids this slippage is very difficult Clay Products Abrasives too much energy needed to move one anion past another anion Glasses Glass - ceramics Our focus is HERE !!! 15 Engineering ceramics are generally Bioceramics classified into the following: Cutting tools Structural ceramics, Industrial wear parts, bioceramics, cutting tools, engine components Electrical and Electronic ceramics, Capacitors, insulators, substrates, IC packages, piezoelectrics, magnets, superconductors Engine parts Ceramic coatings, Industrial wear parts, cutting tools, engine components Chemical processing & environmental Coating ceramics Filters, membranes, catalysts Silicates Silicate Ceramics Si -O Tetrahedron O 4- Combine SiO 4 tetrahedra by having them share Most common elements on Si corners, edges, or faces earth are Si & O The strong Si-O bond leads to a strong, high melting material Adapted from Fig. (1710ºC) 12.12, Callister 7e. Mg 2SiO 4 Ca 2MgSi 2O7 Cations such as Ca 2+ , Mg 2+ , & Al 3+ act to neutralize & provide ionic bonding 20 Si -O Tetrahedron O Silicate Ceramics O Two most common silicate ceramics are: 2. Silica Glasses Si Silica and silica glasses If the tetrahedra are randomly arranged, a non - crystalline structure, known as Glass is formed. Silica glasses is a dense form of amorphous Silica 1. Silica (SiO ) Silica (SiO 2) silica - Charge imbalance corrected with + If the tetrahedra are arranged in a “counter cations” such as Na regular and ordered manner, a -Borosilicate glass is the pyrex glass crystalline structure is formed. Silica used in labs have 3 different types: quartz, -better temperature stability & less crystobalite and tridymite brittle than sodium glass Amorphous Silica Other oxides may also be incorporated into a • Silica gels - amorphous SiO 2 glassy SiO 2 network in different ways: Si 4+ and O 2- not in well-ordered lattice 1. Network formers: form glassy structures (B O ) Charge balanced by H + (to form 2 3 OH -) at “dangling” bonds 2. Network modifiers: added to terminate (break 2 very high surface area > 200 m /g up) the network (CaO, Na 2O). These are added to silica glass to lower its viscosity (so SiO 2 is quite stable, therefore unreactive that forming is easier) makes good catalyst support 3. Network intermediates: these oxides cannot form glass network but join into the silica network and substitute for Si. Adapted from Fig. 12.11, Callister 7e. 23 Carbon Forms Carbon Forms • Carbon black – amorphous – • layer structure – aromatic layers surface area ca. 1000 m 2/g • Diamond tetrahedral carbon hard – no good slip planes brittle – can cut it Adapted from Fig. large diamonds – jewellery 12.17, Callister 7e. small diamonds often man made - used for cutting tools and polishing Adapted from Fig. 12.15, Callister 7e. weak van der Waal’s forces between layers diamond films hard surface coat – tools, planes slide easily, good lubricant medical devices, etc. 25 26 Carbon Forms Defects in Ceramic Structures • Frenkel Defect • Fullerenes or carbon nanotubes -a cation is out of place. wrap the graphite sheet by curving into ball or tube • Shottky Defect -- a paired set of cation and anion vacancies. Buckminister fullerenes Shottky Like a soccer ball C 60 - also C 70 + others Defect: Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 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.) Frenkel Defect −Q / kT Adapted from Figs. • Equilibrium concentration of defects D 12.18 & 12.19, ~ e Callister 7e. 27 28 Mechanical Properties of Ceramics Mechanical Properties of Ceramics Ceramics have inferior mechanical properties compared to metals, and this Ceramics have excellent compressive strength (used in cement and has limited their applications concrete in foundations for structures and equipment) The main limitation is that ceramics fail in “brittle ” manner with little or no The principles source of fracture in ceramics is surface cracks, porosity, plastic deformation. inclusions and large grains produced during processing. Fracture strength of ceramics are significantly lower than predi cted by Testing ceramics using the usual tensile testing is not possible , so a theory because of the presence of very small cracks in the mater ial (stress transverse bending test is used and a modulus of rupture (MOR) is concentrators). determined. Lack of ductility in ceramics is due to their strong ionic and c ovalent bonds. Strength of ceramics can only be described by statistical method s and it is dependent on specimen size. Transverse rupture Compressive Elastic strength strength modulus Hardness Poisson’s Density Material Symbol (MPa) (MPa) (GPa) (HK) ratio ( n) (kg/m 3) Aluminum Al 2O3 140–240 1000–2900 310–410 2000–3000 0.26 4000–4500 oxide Cubic boron CBN 725 7000 850 4000–5000 — 3480 nitride Diamond — 1400 7000 830–1000 7000–8000 — 3500 Silica, fused SiO 2 — 1300 70 550 0.25 — Silicon SiC 100–750 700–3500 240–480 2100–3000 0.14 3100 carbide Silicon Si 3 N4 480–600 — 300–310 2000–2500 0.24 3300 nitride Titanium TiC 1400–1900 3100–3850 310–410 1800–3200 — 5500–5800 carbide Flexural 3F L Tungsten WC 1030–2600 4100–5900 520–700 1800–2400 — 10,000–15,000 f carbide 2 Rectangular cross section strength, σfs = 2 Partially PSZ 620 — 200 1100 0.30 5800 bd stabilized zirconia 3 Ff L The properties vary widely depending on the condition of the material (crack size) = 3 Circular cross section πR 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 Fracture strength or toughness of ceramics can be improved only by concentrators: crack forms and propagates mechanisms that influence the crack propagation (ceramics always leading to failure. contain cracks). Once cracks start to propagate, they will continue to grow until fracture occurs. There are various methods used to improve the toughness of Porosity also decrease the cross -sectional ceramics: area over which a load in applied: lower the stress a material can support. 1. Transformation toughening Strength of ceramics is thus determined by many factors: 2. Microcrack induced toughening 1. Chemical composition 2. Microstructure - In dense ceramics materials, no large pores, the flaw is 3. Crack deflection related to grain size. Finer grain size ceramics, smaller flaws size at the boundaries, hence stronger than large grain size. 4. Crack bridging 3. Surface condition 4. Temperature and environment (failure at RT, usually due to larg e flaws).