Ceramics

DefinitionsInorganic (metal) & non-metallicBonds: ionic or covalentSimply: Clay, SiO2 and feldspar

Complex: Al2O3SiO2H2O clay with small amounts of other oxide such as

TiO2, Fe2O3, MgO, CaO, Na2O and K2O.

Feldspar: K2OAl2O3 6SiO2

Many are a mixture of compounds, present in a variety of

phases

Compounds: homogeneous substance consisting of atoms/ ions of 2 or more different elements in definite proportions

Classification of ceramic materials

Traditional ceramics

Clay-workability

Silica/fint/quarts-high temperature resistance

Feldspar/potash-low melting temp-firing

Examples: Building bricks,

sewer pipe, drain tile, roofing tile and floor tile, white ware products such as electrical porcelain, table ware, and sanitary ware

Engineering/Technical ceramics

Pure compoundsOxides: Al2O3 (electrical applications, refractory

components, integrated circuit chips, automotive gas turbine engine),

ZrO2 (heat engine components)Carbides: WC (dies and cutting tools), SiC (reinforcements)Nitrides: Si3N4 (structural components in heat exchangers)

TiN (coating), CBN (hard cutting tools)Cermets: ceramics + metalDiamond, graphite and asbestosUse: Machine tools and agricultural machinery, increasing

life by up to 10 times. inert and biocompatible so they are potentially suitable for artificial joints where wear is a large consideration

StructureMost ceramics phases are crystalline Material is made of a collection of phasesCrystal of bulk phase importantExtent of crystallinity of other phasesMost ceramics are composed of light atoms O, C, N, Al and Si

therefore have low densities. Because of this, their specific moduli (E/) are high.

Properties depends onBonding in each phasePhase distribution and sizePhase boundary properties

GlassesBased on SiO2, heated to fusion (about 1200oC), high viscosity, network modifiers (Na2O and CaO) to reduce viscosity .

Ceramics processing

Additives can act as - binder- lubricant- wetting agent- plasticizer- deflocculant1

Problems - high costs- dimensional stability- control of porosity- 20 % shrinkage

1. changes the electrical charges on the particles of ceramic so that they repel instead of attract each

other while in a liquid suspension. Typical deflocculants are Na2CO3 and Na2SiO3 in amounts of <1%

Classification scheme for ceramics-forming techniques

Fig: Basic PM processing route

Fig: Steps in

(a) solid and

(b) drain slip casting

Ceramics processing route

Mixing (powder + additives)

SinteringHot isostatic Pressing

Fig: Batch attritorFig: Irregular movement

Fig: Several stages in the removal of water from between clay particles during (a) Wet body (b) Partially dry body ( C) completely dry body

Fig: Liquid phase sintering

Ceramics - parts

Properties of ceramics - mechanical

In general

- hard and brittle, low toughness and ductility (strain 0.1% - 1%)

- good electrical and thermal insulators

- high melting points- high chemical stability

But there is a wide variation

Strength

Strength: in tension - one order of magnitude less than in compression

UTS UTS0 e-nP

where P is the volume fraction of porosity (50%), e.g.; Common earthware has porosity in the range of 10 and 15 %, with porcelain having approx. 3%

UTS0 is the ultimate tensile strength at zero porosity, and exponent n ranges from 4 to 7.

Young’s modulus• Usually linear elastic behaviour • Specific Stiffness is high• Identifiable Young’s modulus depends on

temperature > metal• E depends on porosity and temperature E E0 (1-1.9P+0.92)

where E0 is Young's Modulus at absolute zero, and b and T0 are empirical constants, 1% E change per 100K.

T

TbTEE 0

0 exp

Density & hardness

Density: Most ceramics are composed of light atoms O, C, N, Al and Si therefore have low densities. Because of this, their specific moduli (E/) are high

Hardness: they are the hardest of solids

Ductility and toughness

• Typically on 0.1% strain before failure• Processing results in many initiations sites

for cracking• Surface flows often key

Aim is to detect, branch or arrest crack growthScatter is important in analysis data – Weibull statistics

Thermal shock

• Need low thermal expansion• Good thermal conductivity• Conductivity depends on porosity

K = K0 (1-P)

Where K0 is the conductivity at zero porosity and P is the fraction porosity

There is a wide range

Research in ceramics

• Improve toughness

- tailor grain boundary phases and bulk phases

- use of reinforcements

• Improve processability

- lower pressure, lower temperature, consolidation, near- net shape processing,

- improve machinability

- joining

• Understand structure/property and relationship

Ceramic die materials Non-oxide ceramics• Metal carbides: B4C, SiC, TiC

• Metal nitrates: Si3N4, BN, TiN– High strength and thermal shock resistance but can corrode due to excessive oxidation.

Oxide ceramics• Metal oxides: Al2O3, ZrO2

– Usable up to 1200 deg C and corrosion resistant against oxidisation but lower

mechanical properties and thermal strength.

Dense versus porous ceramics

Dense - high temp strength, corrosion and wear resistance

- poor thermal fatigue

Porous – better thermal fatigue and worse strength

1. Munstermann, S. and Telle, R., Ceramic tool concepts for semi-solid processing of steel, Mat. – wiss, u. Werkstofftech, 37, 4, 2006

Case Study

• What die materials and fabricated structures could be investigated to allow for semi-solid steel forming?

Ceramic die materialsCharacteristics required

• Mechanical – Strength, Wear, Fatigue

• Thermal shock

• Thermal fatigue

• Corrosion

• Oxidation

1. She, J., and Ohji, T., Thermal shock behavior of porous silicon carbide ceramics, J. Am. Ceram. Soc., 85, 8, 2002

2. Meyer-Rau, S. and Telle, R., Testing strategies for corrosive interaction of ceramics with semi-solid and molten metal alloys, J. of European Ceramic Society, 25, 2005

3. Evans, H., and Taylor, M., Oxidation of high temperature coatings, J. Aerospace Engineering, IMechE, 220, Part G, 2006

4. Velay, V., et al., A continuum damage model applied to high temperature fatigue lifetime prediction of a martensitic tool steel, Fatigue Fract Engng Struct, 28, 2005.

Ceramic die materials

• Partially stabilised zirconia – US4279655 (’81) and by Mg, Ca or Y additions CA1053709 (’79)

• Cermet casting mould – FR1258926 (Renault)– alumina in Fe, Cr, Al mix; pressed between 7000 and 70,000 psi

and sintered between 2190 and 2640 deg F; TBC / lubricants applied: alumina, silica, silico-aluminate, graphite, lamp or acetylene black

Metal die materials• H13 tool steel – reduce heat checking by substituting with material with higher elevated mechanical

properties– reduce magnitude of thermal stress, for example, by increasing the bulk metal die

temperature

• Ni based super alloys – e.g. INCONEL, Cr-Ni-Mo

• Electroslag cast steel– good ductility and toughness as well as high tensile strength at elevated temperatures -

important for high thermal shock resistance

1. Sakhuja, A., and Brevick, J., Prediction of thermal fatigue life in tooling for die-casting copper via finite element analysis, Am. Inst. Phys., 2005.

2. Fang, J., et al., The characteristics of fatigue under isothermal and thermo-mechanical load in Cr-Ni-Mo cast hot die steel, Fatigue Fract Engng Struct, 25, 2005.

3. Moon, Y., Kim, J., and Tyne, C., Thermal shock resistance of electroslag cast steel for hot working tools, J. Mat. Proc. Tech., 115-156, 2004.

Die fabtrication methods

• Machining from bulk feedstock – conventional

• Sintering – metal and ceramic feedstock

• Cladding by shrink fitting – GKN Patent US3664411 (’73)

• Multiple layer ceramic – similar to LOM principle – EP1042803 (2000)– glass & alumina powder sintered into 0.2 mm thick sheets; applicable to metal carbides: B4C, SiC, & TiC and

metal nitrates: Si3N4, BN, & TiN

• Sialon ceramic with metal coating – JP1011046 (’89) – and sialon with metal compound coating film e.g. super hardened Ti, Si, … film, by ion planting

• Clamped ceramic columns of polygon cross section – JP63171239 (’88)– applicable with metal oxide or metal nitride (e.g. Si3N4 and ZrO2)

• Mould insert used as cladding – WO2006004713 (’06)– from developer of melt away core process

Fabricated die structure• Bulk metal

• Bulk ceramic

• Cermet

• Layered structure / Hybrid structure

• Channels for heating/cooling cartridges / heat transfer fluid

• Inserts / shapes that reduce heat build up