Lesson 6 2014
Dec 16, 2015
Lesson 62014
Lesson 62014
Our goal is, that after this lesson, students are able to recognize the key criteria for selecting ceramics and are able to use this knowledge to support the systematic material selection process.
What are ceramics for engineering applications?
?
Typical ceramics used in engineering applicationsAluminium oxide (Alumina) Al2O3
Aluminium nitride AlNSilicon carbide SiCSilicon nitride Si3N4
SiAlON (Si-Al-O-N)Zirconium oxide (Zirconia) ZrO2
Boron carbide B4CBoron nitride BN
Some general properties of ceramicsDensity
In general the density of ceramics is between metals and polymers.
Most light weight ceramics are Boron compounds and Silicon compounds.
For ceramics two types of density values are used: Density to describe weight Density to discribe the functional porosity (e.g. in
filters porosity could be 40-80% of total volume) Functional density can be tuned according to
requirements.
Melting pointThe melting point of ceramics is remarkably
higher compared to metals.Heat conductivity
Heat conductivity of ceramics is between metals and polymers.
Can be tuned according to requirements.Heat expansion
Depends a lot about the compound. Can be tuned according to requirements.
Modulus of elasticityThe modulus of elasticity is near or above the
values of metals.Modulus of elasticity can be tuned by utilizing
composite compounds: E.g. WC+Co: E= 600GN/ mm2, Al2O3+SiO2-particles added + Al: E= 200 GN/mm2
Al: E=70 GN/mm2
Strength Brittle behaviour is typical for ceramics.Instead of yeld strength only ultimate
tensile/compression strengths are given.Compression strength is even 10-times higher
than tensile strength. Porosity affects greatly the strength.Ceramics have higher strength in extreme
(high) temperatures compared to metals, but due to so-called “glass deformation phase” also the strength of ceramics usually decreases in elevated temperatures (could be avoided with reaction sintering).
HardnessThe highest values of hardness are found
among the ceramics (Boron compounds).High hardness remains also in high
temperatures (even at 1000 °C).The final hardness is achieved during the
sintering process.
Electrical propertiesCeramics can function as electric insulators
(the most typical option), semi-conductors , conductors or even superconducters
Also piezo-electric properties can be producedSome ceramics are good electric insulators and
have also good heat and corrosion resistance.Magnetic properties
Possible to produce permanent magnets.
DETAILED SELECTION OF
CERAMICS
SINTERING METHOD
SINTERING METHOD
LEVEL OF PURITY
LEVEL OF PURITY
POROSITYPOROSITY
ALLOYINGALLOYING
GRAIN SIZEGRAIN SIZE
DIRECTION OF COMPRESSION
DUCTILITY EFFECT (ZIRCONIUM)
PROPETIES OF THE RAW MATERIALPROPETIES OF
THE COMPOUND
ASPECTS OF THE POWDER
METALLURGICAL PROCESS
MANUFACTURE OF THE MOLD AND THE
COMPRESSION TOOLS
MANUFACTURING (GRINDING) PROCESSES OF
THE POWDER
MIXING PROCESSES OF THE POWDER
SHAPING OF THE POWDER
SINTERING PROCESS
FINISHING PROCESSES
ATOMIZATION
REDUCTION PROCESSES
ELECTROLYSIS
MgO, CeO2, Al2O3, SiO2, Y2O3, ZrO2, CaO, MoSi2
Pressureless sinteringNitride bonding
Reaction bondingLiquid-phase sintering
Recrystallization Hot isostatic pressing
Hot pressing
POWDER METALLURGIC MANUFACTURING PROCESS
During the properly made systematic material selection process it is necessary to recognize::Limitations and possibilities of powder metallurgical manufacturing process
Guidelines of designing the suitable geometry of the product for the powder metallurgical manufacturing process
The optional materials (typically constructional ceramics) which can be applied to powder metallurgical manufacturing process
Temperature TPressure p
Time t
Density
Compression pressure
The most important process parameters in powder metallurgical process
OPTIONAL MANUFACTURING TECHNOLOGY 1
Component made of Silicon carbide and nitridewith Gelcasting.
Powder layer to be sintered
Motion path of the sintering laser beam
Product made of sintered powder layers
Utilized space for powder mass
Laser beam
OPTIONAL MANUFACTURING TECHNOLOGY 2
Example of expressing the purity grade
Ceramics(alloying)
Bending strength (N/mm²)
Al2O3 350-380
Al2O3 + ZrO2 350-550
ZrO2 + MgO 650-800
ZrO2 + Y2O3 1000-1500
How alloying affects the bending strength?
EFFECT OF ALLOYING ON HEAT CONDUCTIVITY
SiC (BeO alloying)
SiC (B4C alloying)
TEMPERATURE [°C]
HE
AT
CO
ND
UC
TIV
ITY
[W
/mK
]
CeramicsE
[GPa]Hot pressed silicon nitride
Alloying with 8% Y2O3 335
Alloying with 1% MgO 325
Alloying with 10% CeO2 327
Alloying with 4% Y2O3 + 3% Al2O3 305
Alloying with 4% Y2O3 + SiO2 305
How alloying affects the modulus of elasticity?
10% improvement
Zirconium oxide itself suffers from unbalanced change of length and volume expansion between the tetragonal and monoclinic phase. This causes huge internal stresses and pure Zirconium oxide can not be used for constructional purposes.
The phase change can be fully stabilized by Calcium oxide (CaO) alloying, but the strength and heat resistance properties will be so poor that neither the fully stabilized Zirconium oxide can’t be used in mechanical constructions.
How Zirconium oxide (ZrO2) alloying affects the mechanical properties of other ceramic compounds?
MOLTEN
CUBIC PHASE
TETRAGONAL PHASE
MONOCLINIC PHASE
CRYSTAL STRUCTURE
Partially stabilized Zirconium oxide (PSZ =Partially Stabilized Zirkonia) can be manufactured by alloying Yttrium oxide (Y2O3) or cerium oxide (CeO2). This decreases the risk of failure due to internal stresses and increases the strength and ductility.
Partially stabilized Zirconium oxide can be utilized as an alloying component in other ceramic compounds to improve their ductility (e.g Zirkonia Toughened Alumina, ZTA).
Different powder particles(Compound)
Different sizes
Same powder particlesDifferent sizes mixed
Same powder particlesOptional (same) sizes selected
Ceramics(commercial grades)
Strength Grain size Grain sizeGrain size
ratioDensity
Modulus of elasticity
MPa min μm max μm max/min g/cm³ GPa
Sintered Silicon carbide
General Electric β-SiC 439 0,5-2 100 50 - 200 3,04 376
Carborundum α-SiC 325 2-5 15-18 3 – 9 3,09 428
Kyocera α-SiC 386 1,5-5 10 2 – 7 3,14 403
When the grain size ratio decreases, the strength decreases and the modulus of elasticity increases. However, the selected sintering process gives a new viewpoint to this conclusion!
How grain size affects strength and modulus of elasticity?
Same powder particlesDifferent sizes mixed
MPa
STRENGTH
STRENGTH OF SILICON CARBIDES / GRAIN SIZE
Note!Selected sinteringprocess affects together with the grain size!
Note!Smaller grainsize referresto higherstrength
Same powder particlesOptional (same) sizes selected
GPa
MODULUS OF ELASTICITY
MODULUS OF ELASTICITY OF SILICON CARBIDES / GRAIN SIZE
Note!Smaller grainsize referresto lower modulus ofelasticity
Note!Selected sinteringprocess affects together with the grain size!
Same powder particlesOptional (same) sizes selected
Powder metallurgy/Focus of review articles
Changes of the grain structure due to sintering and compression.
What happens during the compression and sintering?
Examples of the overall shrinkage of some ceramics during the powder metallurgical manufacturing process.
Ceramics Shrinkage %
Silicon carbide 18-20
Aluminium oxide 17-20
Zirconium oxide 25-32
SILICON CARBIDE
SiC LPSIC(Liquid-phase sintered)
SSIC(Pressureless sintered)
HPSIC(Hot pressed)
HIPSIC(Hot isostatic pressed)
SSIC(Pressure sintered)
NSIC(Nitride bonded)
RSSIC(Reaction-bonded)
RCSIC(Recrystallized)
SINTERING PROCESSES / SILICON CARBIDE
BONDING REACTION
PRESSING T
ECHNOLOGY
SILICON NITRIDE
Si3N4
HPSN (Hot pressed)RBSN
(Reaction-bonded)
HIPSN(Hot isostatic pressed)
SSN(Pressure sintered)
SINTERING PROCESSES / SILICON NITRIDE
Examples of typical sintering temperatures
Ceramics Sintering
temperature
[°C]
Al2O3 1800
SiC 2500
Si3N4 1700
Ceramics andsintering process
Bending strength (N/mm²)
Reaction sinteredSiC
200-450
SinteredSiC
350-550
Reaction sinteredSi3N4
200-400
SinteredSi3N4
500-750
How sintering process affects bending strength?
Note!If reaction sinteringis used, the strengthof ceramics will NOTdecrease in elevated temperatures!
Ceramics andSintering process
Modulus of
elasticity
GPa
Hot pressed Silicon carbide
450
Sintered Silicon carbide
400
Reaction sintered Silicon Carbide
360
How sintering process affects modulus of elasticity?
Sintering process of Silicon nitride
Hardness
Pressure sintered 1400-1800 HV
Hot pressed 1500-1800 HV
Reaction bonded 400-700 HV
How sintering process affects hardness of ceramics?
MPa
STRENGTH
STRENGTH OF SILICON CARBIDES / SINTERING PROCESSES
Note!Selected sinteringprocess affects together with the grain size!
GPa
MODULUS OF ELASTICITY
MODULUS OF ELASTICITY OF SILICON CARBIDES / SINTERING PROCESSES
Note!Selected sinteringprocess affects together with the grain size!
COMPRESSION STRENGTH / COMPRESSION DIRECTION
Note!Bonding reactionaffects together with the compressionangle!
BENDING STRENGTH / COMPRESSION DIRECTION
Note!Bonding reactionaffects together with the compressionangle!
THERMAL EXPANDING / COMPRESSION DIRECTION
Note!Bonding reactionaffects together with the compressionangle!
Strength!
Heatresistance!
Compressionangle
BO, CA, XP!
Justification of ceramic applicationsCeramics The most important property for industrial applications
ALUMINIUM OXIDES Cost-effectiveness compared to other ceramics with Good chemical resistance.
ALUMINIUM NITRIDES Excellent thermal conductor but at the same time excellent electric insulator.
SILICON CARBIDES Good heat resistance.
SILICON NITRIDES Good heat resistance combined with excellent resistance against heat shocks.
Si-Al-O-N (one type of silicon nittide)
Mechanical properties close Silicon nitride combined with chemical resistance close to properties of Aluminium oxide.
ZIRCONIUM OXIDE Could be utilized to improve the toughness/ ductility of other ceramic materials. Use in oxygen sensors.
BORON CARBIDE Extremely hard (place 4. in the list of constructional materials)
BORON NITRIDE Extremely hard (place 3. in the list of constructional materials)
A ball valve made of aluminium oxide.
Applications of Aluminium oxide
Aluminium nitride is used in waveguide amplifiers and angular waveguides (MW-mechanics or high power electronics applications).
Applications of Aluminium nitride
Turbine (blades) made of Silicon carbide
Applications of Silicon carbide
Chemically resistant seals made of Silicon carbide
Applications of Silicon nitride
Applications of Si-Al-O-NSi-Al-O-N based cutting tools Si-Al-O-N based seals and sliding bearings
Ceramic foam filtersUnit porosity(percentage ): 80…90 %Density (g /cm3): 1.0Approximate use temperature 1700 °C.Thermal shock resistance: in 1110° C above 7 times
Applications of Zirconium oxide
Ceramic Zirconia based oxygen sensors
Applications of Boron Carbide
Applications of Silicon nitrideThe boron-nitride coatings combine the
strength and durability with the lubrication and anti-frictional properties e.g. of pistons and cylinders in an engine.
HARDNESS
HEAT RESISTANCE
(COMPRESSION)STRENGTH
ELECTRICAL OR
HEAT CONDUCTIVITY
FOUR-FIELD ANALYSIS FOR MECHANICAL/ELECTRICAL ENGINEERING
HEAT RESISTANCE
COSTS
FUNCTIONALPOROSITY
CHEMICALSTABILITY
FOUR-FIELD ANALYSIS FOR PROCESS ENGINEERING
How to name properly the selected ceramics?In addition to the ceramics type (e.g.
Al2O3, SiC…) the following data is required:Purity level [%]Alloying [%], at least Zirconium content
if it usedGrain size [either grain size ratio or
grain size limit]Porosity level [vol-% and/or density]Sintering method [HPSN, RSSC…]Direction of compression
Remember the ”problems” with commercial names of different grades
Hot pressed Silicon carbides
Norton NC-203
Ceradyne 146A
Ceradyne 146I
Sintered Silicon carbides
General Electric β-SiC
Carborundum α-SiC
Kyocera α-SiC
Reaction sinteredSilicon carbides
Norton NC-435
Norton NC-430
UKAEA BNF Refel
Coors SC-1