heat resistant ceramics

7/31/2019 Heat Resistant Ceramics

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MS. IVY ELSIE OFORI


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What are heat resistant ceramics?

High strength high temperature ceramics

Formation of high temperature ceramics

Silicon nitride

Silicon carbide

Basic characteristics of various ceramics

Factors controlling ceramic strength


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GROWTH OF CERAMIC

APPLICATIONS


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HEAT RESISTANT CERAMICS

Heat resistant ceramics are ceramics which maintaintheir bonding strength at high temperatures andhave low thermal expansion coefficients andexcellent corrosion resistance.

Such materials include Silicon Carbide, SiliconNitride,(which are good materials for high

temperature structural materials) AIN amongstothers.


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When the theoretical strength is equal to E/10, forSiN4, SiC, AIN etc, where E is large; it provides highbonding strength.

Thermal stress caused by internal heat distribution athigh temperature is low.


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The strength of a material is determined by thecracks that exist within the structure. Thus exceptwhisker and filament structures which are close tothe ideal, only a strength of the order of 1/100 of thetheoretical value can be obtained.

REASONS: this is because the local stress at the

point of the crack just before fracture has the samevalue as the bonding strength of the material.


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Si3N4 and SiC are difficult to sinter because they arestable and show little loss strength at hightemperatures.


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Five main conditions for ceramics to maintain

high strength at high temperature are;

The compound must have strong covalent bonds(Si3N4, SiC and AIN etc)

The actual density must be near the theoretical

density Grain size must be small and have uniform

distribution

The shape of the particles must be anisotropic

(plate-like, needle-like, etc)

The grain boundary phase between the particlesmust have high heat resistance.


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Best hightemperature structural uses ceramics:

Silicon carbide, silicon nitride, SIALONS (alloy ofSi3N4 and Al2O3)

REASONS:

Creep resistance is up to 1300C

Low expansion

High conductivity

gives resistance to thermal creep


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FORMATION OF HEAT RESISTANT CERAMICS

Hot pressing fine powders

Vapour deposition

Nitriding Silicon already pressed to shape


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High temperature high strength ceramics

Silicon based ceramics are high temperature highstrength materials.

In gas turbine engines etc structural materials withhigh temperature durability's are needed.


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Uses of HIGH-TEMPERTAURE STRUCTURAL

MATERIALS

For heat engines such as gas turbines and dieselengines the general requirements to the high-tempertaure structural materials are:

High fracture strength from ambient to hightemperatures, especially high strength per density.

High fracture strength from ambient to high temperature

High creep resistance to high temperatures

High oxidation and corrosion resistance

High wear resistance

High impact resistance


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Si3N4

STRUCTURE Has composite microstructure

Rod-like large grains

Equiaxial small grains Grain size and morphology are affected by the sintering additives used.

SINTERING ADDITIVES:

The most remain in the grain boundary after sintering in a glassy phase

This strongly affects thermal and mechanical properties of the sinteredbody.

LOCATION OF GLASSY PHASE;

Located at the triple points; called glassy pockets

Along grain boundary, between grains.

Glass pockets can be changed by post-sintering heat treatment.


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SiC

Has covalent bond But can be densified by solid state sintering

Sintering additives; combination of boron and carbon

Carbon; reacts with SiO2 on the surface of the silicon carbideparticles

Boron; Increase grain boundary diffusion rate Structure; has 2 crystal structures; alpha and beta. Where

SiC is the low temperature phase.

SiC produced by solid state sintering has;

Lower toughness Lower thermal shock resistance compared to SiC or Si3N4

and has heterogeneous phase at grain boundary.

Application of Si-based ceramics to heat engines

SiC is also known as Carborundum


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Uses of Si3N4


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Silicon carbide

First applications; automotiveengine turbochargers, Siliconnitride turbochargers for disel andgas engines.

Silicon carbide key properties;

Low density High strength Low thermal expansion

High thermal conductivity

High hardness

High elastic modulus Excellent thermal shock

resistance

Superior chemical inertness

Typical usesFixed nd moving turbineomonetsSuction box coversSeals, bearingsBall value partsHat gas linersHeat exchangers

Semiconductor processequipment.


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BASIC CHARACTERISTICS OF VARIOUS CERAMIC

MATERIALS

Material Density(g/cm3)

Elasticity(kg/mm2)

Meltingpoint/decompositiontemperature(C)

Heat expansioncoefficient (10-8 degC-1)

AIN 3.26 3.4 x 104 2,450 4.9Al2O3 3.99 3.6x 104 2,050 8

BeO 3.02 3.8x 104 2,530 10

SiC 3.25 5.7x 104 2,600 4.3

Si3N4 3.2 3.8x 104 1,900 2.5-3

QUARTZ

GLASS

~2.2 0.7x 104 _ 0.6


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P ti f t t l i


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Properties of structural ceramics

Structural ceramics have hardness, stiffness and elasticmodulus, wear resistance, high strength retention at elevatedtemperatures and corrosion resistance associated withchemical inertness.

Compared to traditional ceramics advanced ceramics have50-fold increase in specific strength.

Some nitride and oxide ceramics have 50-fold increase inspecific strength.

Some nitride and oxide ceramics have operating temperature

of 1500C. Ceramics; such as SiC and Si3N4 can exhibit high-

temperature strength in the temperature range, where metallicalloys soften and can not be used as structural materials.


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Disadvantages of ceramics for hih structuaral

applications

Poor fracture toughness,


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To address this issue ceramic aomposites are beingdeveloped.

High strength

High operating temperature (SiC, Alumina) Have high elastic modulus or hardness.

Al2O3-- 19GPa which is three times the hardnessof fully hardened martensitic steel (~7GPa)


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CERAMICS COMPOSITES

Ceramics can further be reinforced to form ceramic composites whichimproves upon their properties.

Reiforcements includes; whiskers, platelets, particulates and fibers.

Two major classes of ceramics composites are Fiber-reinforced particulats

Whiskers reinforced ceramic composites Eg Silicon carbide fiber-reinforced glass ceramics.

Drawbacks of using reinforced ceramics.

High cost of ceramic fibers

Expensive composite production route. Chemical compatibility of fiber with matrix

Oxidation of SiC fibers at high temperature.


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heat resistant ceramics

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