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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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HEAT RESISTANT CERAMICS
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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HEAT RESISTANT CERAMICS
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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HEAT RESISTANT CERAMICS
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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HEAT RESISTANT CERAMICS
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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