An Introduction to Ceramic Materials

Content What are ceramics?Types of ceramicsStructure and bondingProperties of ceramicsProcessing of ceramics ApplicationsModern trends

The Word CeramicsGreek term = Keramos = PotteryGoing back from Greek. The Greek took this word from older Sanskrit root meaning to burnThus 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.

ExamplesSo generally speaking almost all the carbides, borides, oxides and nitrides are ceramic materials.

Carbides: SiC, WC etcNitrides: Si3N4, TiN etcOxides: SiO2, Al2O3, MgO etcBorides: TiB2, MgB2, MgB4 etc

Types of Ceramics Ceramics can be divided into two classes:

Traditional CeramicsEngineering Ceramics

Traditional Ceramics:Traditional ceramics include clay products, silicate glass and cement

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

Structure and BondingCeramic 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 2800 27% 73%Al2O3 2050 37% 63%SiO2 1700 49% 51%Si3N4 1900 70% 30%SiC 2500 89% 11%

Calculation of Ionic CharacterThe percentage of ionic character of a ceramic material/compound can be calculated by:

Percentage of ionic character = {1 exp[-0.25(XA-XB)2]}x100

Where:

XA = Electro-negativity of A elementXB = Electro-negativity of B element

Example Calculate ionic and covalent characters of CaF2 and SiC

For CaF2Using formula:Percentage of ionic character = {1 exp[-0.25(XA-XB)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 SiCUsing formula:Percentage of ionic character = {1 exp[-0.25(XA-XB)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 CeramicsOf oxide ceramicsOxidation resistantChemically inertElectrically insulatingGenerally low thermal conductivityOf other compounds ceramicsLow oxidation resistanceExtreme hardnessChemically inert High thermal and electrical conductivity

High melting points of ceramicsCeramicMelting point

Si3N41750-1900CAl2O3 2050CSiC2300-2500CZrO22500-2600CWC 2775CThO2 3300CHfO2 3890C

High hardness of ceramicsCeramic Vicker hardness

Al2O3 3360 KpsiSiC 4680 KpsiZrO2 (+ CaO) 1980 KpsiNaCl 30 KpsiFused SiO2 780 KpsiDiamond 13,780 Kpsi

Most ceramics are usually crystallineZrO2NaCl

Grain boundary structure

Typical grain size of ZrO2

Processing of CeramicsRaw materials selection criteriaPowder sizingPre-consolidationShape forming processesPressingCastingPlastic formingOther forming processesSintering Final machiningQuality control Non-destructive testing

Raw material selection criteriaPurityEffect 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 reactivityConsolidation/shaping depends upon particle size and its distribution. Explanation.Reactivity of ceramic powder play an important role during sintering (of the compacted shape).Polymorphic formPolymorphic transformations can play an important role in the sintering operations. Example of Si3N4 and SiC

Powder SizingScreeningAir classificationElutriationBall millingAttrition millingVibratory millingFluid energy millingHammer millingPrecipitationFreeze dryingLaserPlasmaCalcining

Pre-consolidation These are special treatments done before compacting/consolidation

Three types of consolidations are there and pre-consolidation of each is different. These are:PressingSlip castingInjection moulding

A comparison of three is shown

PressingBinder additionLubricant additionSintering aid additionSlip castingSlurry preparationBinder additionpH controlViscosity controlDe-airing Injection mouldingThermoplastic additionPlasticizer additionWetting agent additionLubricant additionDe-airing

Functions of additives to ceramicsBinder Green strengthLubricantMold release, inter-particle slidingPlasticizerImproving flexibility of binder film, allowing plastic deformation of granulesDeflocculantpH control, particle-surface charge control, dispersionWetting agentReduction of surface tension

Functions of additives to ceramics (cont.)Water retention agentRetain water during pressure applicationFungicide and bactericideStabilize against degradation with agingSintering aidAid in densificationAntistatic agentCharge controlAntifoam agentPrevent foamFoam stabilizerStrengthen desired foam

Example of additives usedOrganicInorganic

PVAMg-Al silicatesWaxesSoluble silicatesCellulose Colloidal silica Thermoplastic & thermosetting resinsColloidal aluminaLigninsClaysRubbersBentonitesProteinsAluminates BitumensPhosphatesChlorinated hydrocarbonsBorophosphatesGelatins

Shape forming processesPressingUniaxial pressingIsostatic pressingHot pressingHot isostatic pressing

CastingSlip castingThixotropic castingSoluble mold casting

Shape forming processes (Cont)Plastic formingExtrusionInjection mouldingTransfer mouldingCompression moulding

Others Tape formingFlame sprayGreen machining

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

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

Sintering MechanismSintering can occur by a variety of mechanisms. The main mechanisms are:

Vapour phase sinteringMaterial transport mechanism is evaporation-condensation and the driving energy is difference in the vapour pressureSolid state sinteringMaterial transport mechanism is diffusion and the driving force is difference in free energy/chemical potentialLiquid state sinteringMaterial 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

Effect of sintering temperature on density

Effect of sintering time on density

Joining Ceramics

Final MachiningDifficuilt 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 machined by following mechanismsMechanical machiningChemical machiningThermal machining

Mechanical MachiningMounted abrasion machiningSmall, hard, abrasive particles bonded to or immersed in a softer matrixThese abrasive particles can be SiC, Al2O3, Al2O3-ZrO2, or other hard ceramics and matrix could be rubber, organic resin, glass etcFor hard ceramic materials diamond is the most efficient abrasive, mounted in a matrix of soft metal/organic resinFree abrasion machiningIn it we use loose abrasive material along with coolant like water, oil etcUsed for final polishingImpact abrasion machiningAl2O3 and SiO2 are used frequentlyThey are fired/blasted by high velocity gasesAbrasion depends upon particle size, material nature and angle

Chemical MachiningPhoto-etchingSome glass compositions can be chemically machined into very complex geometries using this photo-etching techniqueElectrical discharge machiningIt is done only on electrical conductive materialsIts advantage is no mechanical load and disadvantage is limited to conducting ceramic materials onlyLaser machiningVery few work is done on laser machining of ceramic materialsThe mechanism of material removal appeared to be localized thermal shock spalling

Quality controlQuality 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).

GlassA state of matter as well as a type of ceramicAs a state of matter, the term refers to an amorphous (noncrystalline) structure of a solid materialThe 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 GlassSodium 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 heatingIncrease fluidity in molten glass for processingImprove chemical resistance against attack by acids, basic substances, or waterAdd color to the glassAlter index of refraction for optical applications

Glass ProductsWindow glassContainers cups, jars, bottlesLight bulbsLaboratory glassware flasks, beakers, glass tubingGlass fibers insulation, fiber opticsOptical glasses - lenses

GlassCeramics 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 materialGrain 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 glassceramics much stronger than the glasses from which they are derived Also, due to their crystal structure, glassceramics are opaque (usually grey or white) rather than clear

Processing of Glass CeramicsHeating and forming operations used in glassworking create product shapeProduct 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 sizeNucleation 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 GlassCeramicsEfficiency of processing in the glassy stateClose dimensional control over final product shapeGood mechanical and physical propertiesHigh strength (stronger than glass)Absence of porosity; low thermal expansionHigh resistance to thermal shock Applications: Cooking wareHeat exchangersMissile radomes

ApplicationsBarium 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 superconductorBoron 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 superconductorSilicon 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 TrendsIn 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 Carnots 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 engines 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 Earths atmosphere.

Turbo ChargerCeramic Rotor

Candidate Materials for Turbocharger

CONCLUSIONIn 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.

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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 ArmorCeramic 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 ArmorBACK

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Projectile

Outer hard

skin

Ceramic-Discontinuous

Inner

ductile

skin

Personnel

and

Equipment

Ceramic Armor System

Cementless fixation: Hydroxyapatite (HA)Plasma Spray HA coating on Dental Implants (Sun et al., 2001)Zimmer HA coated APR Hip StemBACK

Ceramic in medical (Bio-ceramics)BACK

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

Thank you