Ceramics

An Introduction to Ceramic Materials

Content What are ceramics? Types of ceramics Structure and bonding Properties of ceramics Processing of ceramics  Applications Modern trends

The Word Ceramics  Greek term = Keramos = Pottery  Going back from Greek. The Greek took this word from older Sanskrit root meaning “to burn”  Thus the Greeks used this word to mean “burned stuff” or “burned earth”

Defining Ceramics Materials  Ceramics encompass such a vast array of materials that a concise definition is almost impossible.  One workable definition of ceramics is a refractory, inorganic, and nonmetallic material. It can also be defined as products made from inorganic materials having non-metallic properties, usually processed at a high temperature at some time during their manufacture.  The American Society for Testing and Materials (ASTM) defines a ceramic article as “an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matured by the action of the heat.”

Examples  So generally speaking almost all the carbides, borides, oxides and nitrides are ceramic materials. Carbides: SiC, WC etc Nitrides: Si3N4, TiN etc Oxides: SiO2, Al2O3, MgO etc Borides: TiB2, MgB2, MgB4 etc

Types of Ceramics Ceramics can be divided into two classes: a. Traditional Ceramics b. Engineering Ceramics  Traditional Ceramics: Traditional ceramics include products, silicate glass and cement

clay

 Engineering Ceramics: They consist of carbides (SiC), pure oxides (Al2O3), nitrides (Si3N4), non-silicate glasses and many others

Structure and Bonding  Ceramic materials are inorganic compounds consisting of metallic and non-metallic elements which are held together with ionic and/or covalent bonds.  So this means that the bond between ceramics could be ionic, could be covalent and could be both ionic and covalent. To elaborate this consider this chart: Compound

M.P

Covalent

Ionic

MgO Al2O3 SiO2 Si3N4 SiC 2500

2800 2050 1700 1900

27% 37% 49% 70%

73% 63% 51% 30%

89%

11%

Calculation of Ionic Character The percentage of ionic character of a ceramic material/compound can be calculated by: Percentage of ionic character = {1 – exp[-0.25(XAXB)2]}x100 Where: XA = Electro-negativity of A element XB = Electro-negativity of B element

Example  Calculate ionic and covalent characters of CaF2 and SiC CaF 2: Large

H 2.1 Li

Be

1.0

1.5

Na

He -

C

F

Ne

2.5

4.0

-

Mg

Si

Cl

Ar

0.9

1.2

1.8

3.0

-

K

Ca

Ti

Cr

Fe

Ni

Zn

As

Br

Kr

0.8

1.0

1.5

1.6

1.8

1.8

1.8

2.0

2.8

-

Rb

Sr

I

Xe

0.8

1.0

2.5

-

Cs

Ba

At

Rn

0.7

0.9

2.2

-

Fr

Ra

0.7

0.9

SiC: Small

Table of Electronegativities

For CaF2 Using formula: Percentage of ionic character = {1 – exp[-0.25(XAXB)2]}x100

= {1 – exp[-0.25(1-4)2]}x100 = {1 – exp[-0.25(3)2]}x100 = {1 – exp[-2.25]}x100 = {1 – 0.105}x100 = 89.5% So Percentage of covalent character = 10.5%

For SiC Using formula: Percentage of ionic character = {1 – exp[-0.25(XAXB)2]}x100

= {1 – exp[-0.25(2.5-1.8)2]}x100 = {1 – exp[-0.25(0.7)2]}x100 = {1 – exp[-0.1225]}x100 = {1 – 0.88}x100 = 12% So Percentage of covalent character = 88%

Properties of Ceramics Of oxide ceramics Oxidation resistant Chemically inert Electrically insulating Generally low thermal conductivity

Of other compounds ceramics Low oxidation resistance Extreme hardness Chemically inert High thermal and electrical conductivity

High melting points of ceramics Ceramic

Melting point

Si3N4 Al2O3 SiC ZrO2 WC ThO2 HfO2

1750-1900ºC 2050ºC 2300-2500ºC 2500-2600ºC 2775ºC 3300ºC 3890ºC

High hardness of ceramics Ceramic Al2O3 SiC ZrO2 (+ CaO) NaCl Fused SiO2 Diamond

Vicker hardness 3360 Kpsi 4680 Kpsi 1980 Kpsi 30 Kpsi 780 Kpsi 13,780 Kpsi

Most ceramics are usually crystalline

ZrO2

NaCl

Grain boundary structure

Typical grain size of ZrO2

Processing of Ceramics  Raw materials selection criteria  Powder sizing  Pre-consolidation  Shape forming processes – Pressing – Casting – Plastic forming – Other forming processes  Sintering  Final machining  Quality control  Non-destructive testing

Raw material selection criteria  Purity – Effect of any impurity depends upon chemistry of both matrix material and the impurity, distribution of impurity and service conditions. Example of Ca in Si3N4. – It effects high temperature properties of the ceramic material.

 Particle size and reactivity – Consolidation/shaping depends upon particle size and its distribution. Explanation. – Reactivity of ceramic powder play an important role during sintering (of the compacted shape).

 Polymorphic form – Polymorphic transformations can play an important role in the sintering operations. Example of Si3N4 and SiC

Powder Sizing             

Screening Air classification Elutriation Ball milling Attrition milling Vibratory milling Fluid energy milling Hammer milling Precipitation Freeze drying Laser Plasma Calcining

Pre-consolidation  These are special treatments compacting/consolidation

done

before

 Three types of consolidations are there and preconsolidation of each is different. These are: – Pressing – Slip casting – Injection moulding A comparison of three is shown

Pressing – Binder addition – Lubricant addition – Sintering aid addition

Slip casting – – – – –

Slurry preparation Binder addition pH control Viscosity control De-airing

Injection moulding – – – – –

Thermoplastic addition Plasticizer addition Wetting agent addition Lubricant addition De-airing

Functions of additives to ceramics  Binder – Green strength  Lubricant – Mold release, inter-particle sliding  Plasticizer – Improving flexibility of binder film, allowing plastic deformation of granules  Deflocculant – pH control, particle-surface charge control, dispersion  Wetting agent – Reduction of surface tension

Functions of additives to ceramics (cont.)  Water retention agent – Retain water during pressure application  Fungicide and bactericide – Stabilize against degradation with aging  Sintering aid – Aid in densification  Antistatic agent – Charge control  Antifoam agent – Prevent foam  Foam stabilizer – Strengthen desired foam

Example of additives used Organic

Inorganic

PVA Waxes Cellulose Thermoplastic & thermosetting resins Lignins Rubbers Proteins Bitumens Chlorinated hydrocarbons Gelatins

Mg-Al silicates Soluble silicates Colloidal silica Colloidal alumina Clays Bentonites Aluminates Phosphates Borophosphates

Shape forming processes  Pressing – Uniaxial pressing – Isostatic pressing – Hot pressing – Hot isostatic pressing  Casting – Slip casting – Thixotropic casting – Soluble mold casting

Shape forming processes (Cont)  Plastic forming – Extrusion – Injection moulding – Transfer moulding – Compression moulding  Others – Tape forming – Flame spray – Green machining

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

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

Sintering  The densification of a particulate ceramic compact is technically referred to as sintering  Sintering is essentially a removal of the pores between the starting particles, combined with growth together and strong bonding between adjacent particles  The following criteria must be met before sintering can occur: – A mechanism for material transport must be present – A source of energy to activate and sustain this material transport must be present

Sintering Mechanism  Sintering can occur by a variety of mechanisms. The main mechanisms are: – Vapour phase sintering • Material transport mechanism is evaporation-condensation and the driving energy is difference in the vapour pressure

– Solid state sintering • Material transport mechanism is diffusion and the driving force is difference in free energy/chemical potential

– Liquid state sintering • Material transport mechanism is viscous flow, diffusion and the driving force is capillary pressure, surface tension

Sintering furnace

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

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

Effect of sintering temperature on density

Effect of sintering time on density

Joining Ceramics

Final Machining  Difficuilt to machine due to their high hardness and brittle nature  The tool must have higher hardness than the ceramic being machined  Ceramic material can be mechanisms – Mechanical machining – Chemical machining – Thermal machining

machined

by

following

Mechanical Machining  Mounted abrasion machining – Small, hard, abrasive particles bonded to or immersed in a softer matrix – These abrasive particles can be SiC, Al2O3, Al2O3-ZrO2, or other hard ceramics and matrix could be rubber, organic resin, glass etc – For hard ceramic materials diamond is the most efficient abrasive, mounted in a matrix of soft metal/organic resin

 Free abrasion machining – In it we use loose abrasive material along with coolant like water, oil etc – Used for final polishing

 Impact abrasion machining – Al2O3 and SiO2 are used frequently – They are fired/blasted by high velocity gases – Abrasion depends upon particle size, material nature and angle

Chemical Machining  Photo-etching – Some glass compositions can be chemically machined into very complex geometries using this photo-etching technique  Electrical discharge machining – It is done only on electrical conductive materials – Its advantage is no mechanical load and disadvantage is limited to conducting ceramic materials only  Laser machining – Very few work is done on laser machining of ceramic materials – The mechanism of material removal appeared to be localized thermal shock spalling

Quality control  Quality control is required throughout processing of any material/product, and ceramics are no exception.  The degree of QC is determined by the criticality of the application.  Critical/demanding applications may require destructive sampling, proof testing, or non-destructive inspection (NDI).

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

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

Glass  A state of matter as well as a type of ceramic  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

Why So Much SiO2 in Glass?  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

Other Ingredients in Glass  Sodium oxide (Na2O), calcium oxide (CaO), aluminum oxide (Al2O3), magnesium oxide (MgO), potassium oxide (K2O), lead oxide (PbO), and boron oxide (B2O3)  Functions: – Act as flux (promoting fusion) during heating – Increase fluidity in molten glass for processing – Improve chemical resistance against attack by acids, basic substances, or water – Add color to the glass – Alter index of refraction for optical applications

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

Glass‑Ceramics 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 µ m (4 and 40 µ 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 glassworking 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

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

Applications  Barium Titanate (often mixed with strontium titanate) displays Ferro-electricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements.  Bi-Strontium calcium copper oxide, a high temperature superconductor  Boron carbide, which is used in some personal, helicopter and tank armor.  Boron nitride is structurally iso-electronic to carbon and takes on similar physical forms: a graphite-like one used as a lubricant, and a diamond-like one used as an abrasive.

Applications (Cont)  Bricks (mostly aluminium silicates), used for construction.  Earthenware , which is often made from clay, quartz and feldspar.  Ferrite, which is ferrimagnetic and is used in the core of electrical transformers and magnetic core memory.  Lead zirconate titanate is another ferroelectric material.  Magnisium diboride, is an unconventional superconductor  Silicon carbide, which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material.

Applications (Cont)  Silicon nitride, is used as an abrasive powder.  Steatite is used as an electrical insulator.  Uranium oxide, used as fuel in nuclear reactors.  Yttrium barium copper oxide (YBa2Cu3O7), another high temperature superconductor.  Zinc oxide, which is a semiconductor, and used in the construction of varistors.  Zirconium dioxide (zirconia), is used in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

Modern Trends  In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, due to Carnot’s theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts.

Modern Trends (Cont)  Work is being done in developing ceramic parts for gas turbine blades. Currently, even blades made of advanced metal alloys used in the engine’s hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.  Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.

Modern Trends (Cont)  Since the late 1990s, highly specialized ceramics, usually based on boron carbide, formed into plates and lined with Spectra, have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Very similar technology is used to protect cockpits of some military airplanes, because of the low weight of the material.

Modern Trends (Cont)  Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, Ca10(PO4)6(OH)2 (Ca/P = 1.67) the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Most hydroxyapatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. Work is being done to make strong-fully dense nano crystalline hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic natural bone mineral.

Modern Trends (Cont)  The space shuttle makes use of ~25,000 reusable, lightweight, highly porous ceramic tiles that protect the aluminum frame from the heat generated during re-entry into the Earth’s atmosphere.

Turbo Charger Ceramic Rotor

Candidate Materials for Turbocharger

CONCLUSION In the past few decades Ceramics materials have over powered many metallic and polymeric materials due to their superior and wide range of properties. With times to come they will gain more and more importance and the coming time will bring a revolutionary era in this fascinating material. The last shocking words

Example of Ca  Ca severely decreases the creep resistance of Si3N4 hot-pressed with MgO as a densification (sintering) aid, but appears to have little effect on Si3N4 hot pressed with Y2O3 as the densification aid. In the former case, the Ca is concentrated at the grain boundaries and depresses the softening temperature of the grain boundary glass phase. In the later case, the Ca is apparently absorbed into solid solution by the crystalline structure and does not significantly reduces the refractoriness of the system. BACK

Particle size effect  A single particle size does not produce good packing. Optimum packing for particles all the same size results in over 30% void spaces. Adding particles of a size equivalent to the largest voids reduces the void contents to 26%. Adding a third, still smaller particle size can reduce the pore volume to 23%. Therefore, to achieve maximum particle packing, a range of particle size is required. BACK

Example of Si3N4 & SiC  Alpha Si3N4 is superior to beta Si3N4 as the starting powder for hot pressing. A similar case is present in SiC. The stable form at high temperature is hexagonal alpha SiC, so it can be pressed to a greater range as compared to beta SiC which is cubic and is stable at relatively lower temperature. BACK

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Schematic of a tape casting machine. (Source: From Principles of Ceramics Processing, Second Edition, by J.S. Reed, p. 532, Fig. 26-6. Copyright © 1995 John Wiley & Sons, Inc. Reprinted by permission.) BACK

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Ceramic - Composite Armor  Ceramic armor systems are used to protect military personnel and equipment.  Advantage: low density of the material can lead to weight efficient armor systems.  Typical ceramic materials used in armor systems include alumina, boron carbide, silicon carbide, and titanium diboride.  The ceramic material is discontinuous and is sandwiched between a more ductile outer and inner skin.  The outer skin must be hard enough to shatter the projectile.  Most of the impact energy is absorbed by the fracturing of the ceramic and any remaining kinetic energy is absorbed by the inner skin, that also serves to contain the fragments of the ceramic and the projectile preventing severe impact with the personnel/equipment being protected. (as shown in diagram)

Ceramic - Composite Armor Outer hard skin

CeramicDiscontinuous

Projectile Personnel and Equipment

Inner ductile skin

Ceramic Armor System BACK

Cementless fixation: Hydroxyapatite (HA)

Plasma Spray HA coating on Dental Implants (Sun et al., 2001)

Zimmer HA APR® Hip Stem

coated

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Ceramic in medical (Bio-ceramics)

Ceramic Crowns

Zimmer PureForm Ceramic Copings

Zinc Phosphate Dental Cements

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All the material presented in this presentation is included in the course. Thank you