CERAMICS MATERIALS

CERAMICS MATERIALS

Ceramic materials are inorganic materials consisting of metallic and non-metallic elements chemically bonded together to form complex compounds.

Ceramic materials are usually ionic or covalently-bonded materials, and can be crystalline or amorphous.

Important examples:

Silica - silicon dioxide (SiO2), the main ingredient in most glass productsAlumina - aluminum oxide (Al2O3), used in various applications from abrasives to artificial bonesMore complex compounds such as hydrous aluminum silicate (Al2Si2O5(OH)4), the main ingredient in most clay products

Properties and applications of Ceramics Traditional Ceramics New Ceramics Glass Some Important Elements Related to Ceramics Guide to Processing Ceramics

1. Traditional ceramics ‑ clay products such as pottery and bricks, common abrasives, and cement

2. New ceramics ‑ more recently developed ceramics based on oxides, carbides, etc., and generally possessing mechanical or physical properties superior or unique compared to traditional ceramics

3. Glasses ‑ based primarily on silica and distinguished by their noncrystalline structure

» In addition, glass ceramics ‑ glasses transformed into a largely crystalline structure by heat treatment

CERAMICS MATERIALS

Ceramics properties High hardness, (high strength, stiffness, wear resistance) Brittle, Low ductility or malleability i.e. low plasticity, Electrical and thermal insulating, Chemical stability, and high melting temperatures Some ceramics are translucent, window glass (based on silica). Lower density than most metals, Low resistance to fracture, highly resistant to compressive loads. Corrosion resistance Ceramics are hard, brittle, totally elastic and are heat resistant.At extremely low temperature, exhibit superconductivity. Due to high resistance to heat, application in furnace linings. Ceramics are often used as protective coatings to other materials.

CERAMICS MATERIALS

Electrical Ceramics, insulators, electrical devices, Superconductors. Coatings, Biocompatible coatings (fusion to bone), Self-lubricating bearings Abrasives Piezoelectric materials are lead zirconate titanate and barium titanate. Design of high-frequency loudspeakers, Transducers for sonar, and Actuators for atomic force and scanning tunneling microscopes. Semiconducting ceramics are also employed as gas sensors. Corrosion resistant applications, Windows, Television screens, Magnetic materials (audio/video tapes, disks, etc.), Magnets. Ceramic fibers, graphite and aluminum oxide, fiber-reinforced composites Pottery, clay, glasses, vitreous enamels, and Cutting tools. Chemically Bonded Ceramics (e.g. cement and concrete) Structural Ceramics, Whitewares (e.g. porcelains), Engineering ceramics

1- Oxides (SiO2, Al2O3, Fe 2O3, MgO, SrTiO3, MgAl2O4, YBa2Cu3O7-x)2- Carbides (SiC, WC, TiC), Borides, Nitrides (Si3N4, TiN, AlN, GaN, BN),3- Composites: Particulate reinforced

CERAMICS MATERIALS

Applications of Ceramics

Clay construction products - bricks, clay pipe, and building tile Refractory ceramics ‑ ceramics capable of high temperature applications

such as furnace walls, crucibles, and molds Cement used in concrete - used for construction and roads Whiteware products - pottery, stoneware, fine china, porcelain, and other

tableware, based on mixtures of clay and other minerals Glass ‑ bottles, glasses, lenses, window pane, and light bulbs Glass fibers - thermal insulating wool, reinforced plastics (fiberglass), and

fiber optics communications lines Abrasives - aluminum oxide and silicon carbide Cutting tool materials - tungsten carbide, aluminum oxide, and cubic boron

nitride Ceramic insulators ‑ applications include electrical transmission

components, spark plugs, and microelectronic chip substrates Magnetic ceramics – example: computer memories Nuclear fuels based on uranium oxide (UO2)

Bioceramics - artificial teeth and bones

CERAMICS MATERIALS

Advanced Ceramics: Sensors, Valves, Dielectrics, Space shuttle, Spark plugs, Magnetic recording media,

Glasses: Optical Composite, Porous ceramic tiles

Cements: Composites structural Clay: Whiteware Bricks

Refractories: Bricks for high T

Abrasives: Sandpaper Polishing

Heat EnginesExcellent wear &corrosion resistanceLow frictional lossesHeat resistantLow densityBrittle, Difficult toMachineSi3N4, SiC, & ZrO2

Ceramic ArmorAl2O3, B4C, SiC & TiB2Hard materials

Electronic PackagingBoron nitride (BN)Silicon Carbide (SiC)Aluminum nitride (AlNGood expansionGood heat transfer coefficient Poor electricalConductivity

CERAMICS MATERIALS

Traditional Ceramics Primary products are fired clay (pottery, tableware, brick,

and tile), cement, and natural abrasives such as alumina Glass is also a silicate ceramic material and is sometimes

included among traditional ceramics

Raw Materials for Traditional Ceramics Mineral silicates, such as clays of various compositions, and

silica, such as quartz, are among the most abundant substances in nature and constitute the principal raw materials for traditional ceramics

Another important raw material for traditional ceramics is alumina

CERAMICS MATERIALS

Clay and Silica as a Ceramic Raw Material Clays consist of fine particles of hydrous aluminum silicate,

Most common clays are based on the mineral kaolinite, (Al2Si2O5(OH)4)

When mixed with water, clay becomes a plastic substance that is formable and moldable. When heated to a sufficiently elevated temperature (firing ), clay fuses into a dense, strong material. Thus, clay can be shaped while wet and soft, and then fired to obtain the final hard product

Silica is available naturally in various forms, most important is quartz, the main source of quartz is sandstone, Low in cost; also hard and chemically stable

Principal component in glass, and an important ingredient in other ceramic products including whiteware, refractories, and abrasives

CERAMICS MATERIALS

Alumina as a Ceramic Raw Material Bauxite - most alumina is processed from this mineral, which is

an impure mixture of hydrous aluminum oxide and aluminum hydroxide plus similar compounds of iron or manganese. Bauxite is also the principal source of metallic aluminum

Corundum - a more pure but less common form of Al2O3, which contains alumina in massive amounts

Alumina ceramic is used as an abrasive in grinding wheels and as a refractory brick in furnaces

Traditional Ceramic Products Pottery and Tableware Brick and tile Refractories Abrasives

CERAMICS MATERIALS

New Ceramics

Ceramic materials developed synthetically over the last several decades

The term also refers to improvements in processing techniques that provide greater control over structures and properties of ceramic materials

In general, new ceramics are based on compounds other than variations of aluminum silicate, which form most of the traditional ceramic materials

New ceramics are usually simpler chemically than traditional ceramics; for example, oxides, carbides, nitrides, and borides

Thin films of many complex and multi-component ceramics are produced using different techniques such as sputtering, sol-gel, and chemical-vapor deposition (CVD).

Fibers are produced from ceramic materials for several uses: as a reinforcement in composite materials, for weaving into fabrics, or for use in fiber-optic systems.

CERAMICS MATERIALS

Oxides Ex. Properties

Aluminum oxide Al2O3 high strength and hardness, high stiffness, high thermal stability

magnesium oxide MgO high thermal stability

Mullite Al6Si2O13 Low coefficient of thermal expansion, high thermal stability

silicon dioxide SiO2 Low density, transparency

Zirconium dioxide

ZrO2 high toughness when transformation toughened

Carbides Ex. Properties

Diamond C high strength, stiffness, low coefficient of thermal expansion,

Graphite C high strength, stiffness, low coefficient of thermal expansion

silicon carbide SiC high strength and hardness, high stiffness

tungsten carbide WC high strength and hardness

Nitrides Ex. Properties

Boron nitride BN very high strength and hardness, very high stiffness

silicon nitride Si3N4 high strength, hardness, stiffness and high thermal stability

CERAMICS MATERIALS

Oxide Ceramics Most important oxide new ceramic is alumina Although also included as a traditional ceramic, alumina is today produced

synthetically from bauxite, using an electric furnace method Through control of particle size and impurities, refinements in processing

methods, and blending with small amounts of other ceramic ingredients, strength and toughness of alumina are improved substantially compared to its natural counterpart

Alumina also has good hot hardness, low thermal conductivity, and good corrosion resistance

Products of Oxide Ceramics

Abrasives (grinding wheel grit) Bioceramics (artificial bones and teeth) Electrical insulators and electronic components Refractory brick Cutting tool inserts Spark plug barrels Engineering components

CERAMICS MATERIALS

Carbides Silicon carbide (SiC), tungsten carbide (WC), titanium carbide (TiC),

tantalum carbide (TaC), and chromium carbide (Cr3C2) Although SiC is a man‑made ceramic, its production methods were

developed a century ago, and it is generally included in traditional ceramics group

WC, TiC, and TaC are valued for their hardness and wear resistance in cutting tools and other applications requiring these properties

WC, TiC, and TaC must be combined with a metallic binder such as cobalt or nickel in order to fabricate a useful solid product

Nitrides

The important nitride ceramics are silicon nitride (Si3N4), boron nitride (BN), and titanium nitride (TiN)

Properties: hard, brittle, high melting temperatures, usually electrically insulating, TiN being an exception

Applications: – Silicon nitride: components for gas turbines, rocket engines, and

melting crucibles– Boron nitride and titanium nitride: cutting tool material and coatings

CERAMICS MATERIALS

Functional Classification of Ceramics

CERAMICS MATERIALS

Functional Classification of Ceramics

CERAMICS MATERIALS

Strength Properties of Ceramics Theoretically, the strength of ceramics should be higher than metals because

their covalent and ionic bonding types are stronger than metallic bonding However, metallic bonding allows for slip, the basic mechanism by which

metals deform plastically when subjected to high stresses Bonding in ceramics is more rigid and does not permit slip under stress The inability to slip makes it much more difficult for ceramics to absorb

stresses

Imperfections in Crystal Structure of Ceramics Ceramics contain the same imperfections in their crystal structure as metals

‑ vacancies, displaced atoms, interstitialcies, and microscopic cracks Internal flaws tend to concentrate stresses, especially tensile, bending, or

impact

– Hence, ceramics fail by brittle fracture much more readily than metals

– Performance is much less predictable due to random imperfections and processing variations

CERAMICS MATERIALS

Compressive Strength of Ceramics Ceramics are substantially stronger in compression than in

tension For engineering and structural applications, designers have

learned to use ceramic components so that they are loaded in compression rather than tension or bending

Methods to Strengthen Ceramic Materials Make starting materials more uniform Decrease grain size in polycrystalline ceramic products Minimize porosity Introduce compressive surface stresses Use fiber reinforcement Heat treat

CERAMICS MATERIALS

Physical Properties of Ceramics

Density – in general, ceramics are lighter than metals and heavier than polymers

Melting temperatures - higher than for most metals

– Some ceramics decompose rather than melt Electrical and thermal conductivities - lower than for metals;

but the range of values is greater, so some ceramics are insulators while others are conductors

Thermal expansion - somewhat less than for metals, but effects are more damaging because of brittleness

CERAMICS MATERIALS

CERAMICS MATERIALS

Properties of Ceramics

Single crystals of appreciable size (e.g. ruby laser crystals).

Glass (non-crystalline) of appreciable size (e.g. sheets of float glass).

Crystalline or glassy filaments.

Polycrystalline aggregates bonded by a glassy matrix (e.g. porcelain pottery).

Glass free polycrystalline aggregates (e.g. ultra pure, fine grained, zero porosity forms of alumina, magnesia, and beryllia).

Polycrystalline aggregates produced by heating glasses of special composition (e.g. glass-ceramics).

Composites (e.g. silicon carbide or carbon filaments in a matrix of glass or glass-ceramic; magnesia graphite refractories, concrete).

CERAMICS MATERIALS

Classification of ceramics into micro-structural terms

Glass As a state of matter, the term refers to an amorphous (noncrystalline)

structure of a solid material

– The glassy state occurs in a material when insufficient time is allowed during cooling from the molten state for the crystalline structure to form

As a type of ceramic, glass is an inorganic, nonmetallic compound (or mixture of compounds) that cools to a rigid condition without crystallizing

Because SiO2 is the best glass former

– Silica is the main component in glass products, usually comprising 50% to 75% of total chemistry

– It naturally transforms into a glassy state upon cooling from the liquid, whereas most ceramics crystallize upon solidification

CERAMICS MATERIALS

Glass temperature - The temperature below which an undercooled liquid becomes a glass.

Glass formers - Oxides with a high-bond strength that easily produce a glass during processing.

Intermediates - Oxides that, when added to a glass, help to extend the glassy network; although the oxides normally do not form a glass themselves.

Glass modifier –

CERAMICS MATERIALS

Crystalline materials: -- crystallize at melting temp, Tm -- have abrupt change in spec. vol. at Tm

• Glasses: -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, Tg

-- transparent

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When silica crystallizes on cooling, an abrupt change in the density is observed. For glassy silica, however, the change in slope at the glass temperature indicates the formation of a glass from the undercooled liquid. Glass does not have a fixed Tm or Tg. Crystalline materials have a fixed Tm and they do not have a Tg.

CERAMICS MATERIALS

Adapted from Fig. 13.6, Callister, 7e.

T

Specific volume

Supercooled Liquid

solid

T m

Liquid(disordered)

Crystalline (i.e., ordered)

T g

Glass (amorphous solid)

Specific volume (1/r) vs Temperature (T)

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The effect of temperature and composition on the viscosity of glass.

CERAMICS MATERIALS

CERAMICS MATERIALS

Sheet forming – continuous draw– originally sheet glass was made by “floating” glass on a pool of

mercury

Adapted from Fig. 13.9, Callister 7e.

CERAMICS MATERIALS

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.Techniques for forming lass products: (a) pressing, (b) press and blow process, and (c)

drawing of fibers.

CERAMICS MATERIALS

Glass ProductsWindow glass, Containers – cups, jars, bottles, Light bulbs, Laboratory glassware – flasks, beakers, glass tubing, Glass fibers – insulation, fiber optics, Optical glasses - lenses

Glass‑Ceramics is a ceramic material produced by conversion of glass into a polycrystalline structure through heat treatment

Proportion of crystalline phase range = 90% to 98%, remainder being unconverted vitreous material

Grain size - usually between 0.1 ‑ 1.0 m (4 and 40 -in), significantly smaller than the grain size of conventional ceramics – This fine crystal structure makes glass‑ceramics much stronger than the

glasses from which they are derived Also, due to their crystal structure, glass‑ceramics are opaque (usually grey

or white) rather than clear

Processing of Glass Ceramics Heating and forming operations used in glass working create product shape Product is cooled and then reheated to cause a dense network of crystal

nuclei to form throughout – High density of nucleation sites inhibits grain growth, leading to fine

grain size Nucleation results from small amounts of nucleating agents in the glass

composition, such as TiO2, P2O5, and ZrO2 Once nucleation is started, heat treatment is continued at a higher

temperature to cause growth of crystalline phases

CERAMICS MATERIALS

Advantages of Glass‑Ceramics Efficiency of processing in the glassy state Close dimensional control over final product shape Good mechanical and physical properties

– High strength (stronger than glass)

– Absence of porosity; low thermal expansion

– High resistance to thermal shock Applications:

– Cooking ware

– Heat exchangers

– Missile radomes

CERAMICS MATERIALS

Elements Related to Ceramics Carbon

– Two alternative forms of engineering and commercial importance: graphite and diamond

Silicon Boron Carbon, silicon, and boron are not ceramic materials, but they sometimes

– Compete for applications with ceramics – Have important applications of their own

GraphiteForm of carbon with a high content of crystalline C in the form of layers Bonding between atoms in the layers is covalent and therefore strong, but

the parallel layers are bonded to each other by weak van der Waals forces This structure makes graphite anisotropic; strength and other properties

vary significantly with direction – As a powder it is a lubricant, but in traditional solid form it is a

refractory– When formed into graphite fibers, it is a high strength structural

material

CERAMICS MATERIALS

Diamond is a carbon with a cubic crystalline structure with covalent bonding between atoms– This accounts for high hardness

Industrial applications: cutting tools and grinding wheels for machining hard, brittle materials, or materials that are very abrasive; also used in dressing tools to sharpen grinding wheels that consist of other abrasives

Industrial or synthetic diamonds date back to 1950s and are fabricated by heating graphite to around 3000C (5400F) under very high pressures

Boron is semi-metallic element in same periodic group as aluminum Comprises only about 0.001% of Earth's crust by weight, commonly

occurring as minerals borax (Na2B4O7‑ 10H2O) and kernite (Na2B4O7‑4H2O)

Properties: lightweight, semiconducting properties, and very stiff (high modulus of elasticity) in fiber form

Applications: B2O3 used in certain glasses, as a nitride (cBN) for cutting tools, and in nearly pure form as a fiber in polymer matrix composites

CERAMICS MATERIALS

Silicon is a semi-metallic element in the same periodic table group as carbon One of the most abundant elements in Earth's crust, comprising 26% by

weight Occurs naturally only as chemical compound ‑ in rocks, sand, clay, and soil

‑ either as silicon dioxide or as more complex silicate compounds Properties: hard, brittle, lightweight, chemically inactive at room

temperature, and classified as a semiconductor

Applications and Importance of Silicon Greatest amounts in manufacturing are in ceramic compounds (SiO2 in

glass and silicates in clays) and alloying elements in steel, aluminum, and copper

Also used as a reducing agent in certain metallurgical processes Of significant technological importance is pure silicon as the base material

in semiconductor manufacturing in electronics The vast majority of integrated circuits produced today are made from

silicon

CERAMICS MATERIALS

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Different techniques for processing of advanced ceramics.

CERAMICS MATERIALS

Processing of Advanced Ceramics

Schematic of the jaw, rotary, crushing rollers, and hammermill crushing equipment and ball mill (grinding) equipment. (Jaw, rotary, crushing, and hammermill: Source: From Principles of Ceramics Processing, Second Edition, by J.S. Reed, p. 314, Figs. 17-1 and 17-2. Copyright © 1995 John Wiley & Sons, Inc. Reprinted by permission. Ball mill grinding: Source: From Modern Ceramic Engineering, by D.W. Richerson, p. 387, Fig. 9-3. Copyright © 1992 Marcel Dekker, Inc.)

CERAMICS MATERIALS

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Processes for shaping crystalline ceramics: (a) pressing, (b) isostatic pressing, (c) extrusion, (d) jiggering, and (e) slip casting.

CERAMICS MATERIALS

Guide to Processing Ceramics

Processing of ceramics can be divided into two basic categories:

1. Molten ceramics - major category of molten ceramics is glassworking (solidification processes)

2. Particulate ceramics - traditional and new ceramics (particulate processing)

CERAMICS MATERIALS

Tape Casting

Thin sheets of green ceramic cast as flexible tape used for integrated circuits and capacitors cast from liquid slip (ceramic + organic solvent)

Adapted from Fig. 13.18, Callister 7e.

CERAMICS MATERIALS

Diffusion processes during sintering and powder metallurgy. Atoms diffuse to points of contact, creating bridges and reducing the pore size

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

1 2 43 5

Hot pressing: Diffusion Processes during Sintering

CERAMICS MATERIALS

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

The steps in diffusion bonding: (a) Initially the contact area is small; (b) application of pressure deforms the surface, increasing the bonded area; (c) grain boundary diffusion permits voids to shrink; and (d) final elimination of the voids requires volume diffusion

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Hot pressing: Diffusion Processes during Sintering

CERAMICS MATERIALS

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The change in the volume of a ceramic body as moisture is removed during drying. Dimensional changes cease after the interparticle water is gone.

CERAMICS MATERIALS

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

During firing, clay and other fluxing materials react with coarser particles to produce a glassy bond and reduce porosity.

CERAMICS MATERIALS