-
Werkstofftechnik 3 – L7
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Werkstofftechnik 3
Lecture 7
Sintering and Microstructure
Prof.Dr.-Ing.
Kurosch Rezwan
[email protected]
Keramische Werkstoffe und Bauteile - Advanced Ceramics
Universität Bremen
Am Biologischen Garten 2, IW3
D - 28359 Bremen
Tel: +49 421 218 4507
Fax: +49 421 218 7404
http://www.ceramics.uni-bremen.de/
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Outline Course „Werkstofftechnik 3”
1. Introduction: Applications, Goals and Challenges
2. Atomic Bonding
3. Crystal – and Glass Structures
4. Microstructure and Property Relations
5. Fracture Mechanics of Brittle Materials
6. Powder Conditioning and Processing
7. Sintering and Microstructure
8. Structural Ceramics
9. Functional Ceramics
10. Bioceramics
11. Glass and Glass Ceramics
12. Ceramic Matrix Composites
13. Selected Applications of Advanced Ceramics
14. Summary
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From Powder to Advanced Ceramics
Colloid Crystals
Surface Micro Patterning
Bulk Materials
Surface
Coatings
Porous Materials
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The Importance of the Microstructure (“Gefüge”)
Raw Material Processing
Microstructure
Properties
Different Microstructure
=
Different Materials Properties!
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Flow Chart of Advanced Ceramic Processing
High Quality
Powder
Shaping(Green Body / Grünkörper)
Sintering(1000 – 1800 °C)
Finishing(Cutting / Polishing)
?
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Sinter Processes
Sintering
Solid Phase
Sintering
Festphasensintern
Liquid Phase
Sintering
Flüssigphasensintern
Pressure
Sintering
Drucksintern
Multi
Phase
Single
Phase
Liquid
15 Vol.%
Hot
Pressing
Hot
Isostatic
Pressing
(HIP)
no chemical
reaction
with chemical
reaction
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Sintering: Macro- and Microscopic Processes
On the Macroscale?
• Densification of the Ceramic Body
• Increase of mechanical strength
• Decrease of Porosity
• Shrinkage of Ceramic Body
And on the Microscale?
• Rearrangement of Particles
• Increase of Coordination
• Neck Formation
• Pores get smaller and isolated
• Increase of grain boundary interface
• Grain growth and coarsening
• Decrease of grain boundary/volume ratio
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I.) Initial Stage
- rearrangement of particles
- formation of sintering necks
- hardly any shrinkage
II.) Intermediate Stage
- particles stop moving
- growth of sintering necks
- strong decrease of porosity
- highest shrinkage rate, approx.
65-95% TD
III.) Final Stage
- decrease of porosity (< 5%)
- grain growth
- closed porosity disappears
The three Sintering Stagesre
l. D
ensi
ty(%
rel
. T
D)
Temperature
green
100 %
I. Initial
II. Intermediate
III. Final
RT / Time
TD = Theoretical Density
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Illustration of the Sintering Stages I. – III.
Neck
Formation
I. Initial Stage
Neck
Growth
II. Intermediate Stage
III. Final Stage
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Characteristic Sintering Curves
Isothermal Sintering (T=const.)Sintering with a constant heating
rate
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Microstructure of a-Al2O3 between Sinter Stage I./Stage II.
1450 °C, 0.5 h, relative Density 67 % TD
Neck
Formation
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Driving Energy for (Solid Phase) Sintering and Grain Growth
ΔGSPS =(GS
Surface Energy+GS
Grain Boundary Energy)–(G0
Surface Energy+G0
Grain Boundary Energy)
G0 >> GS
high surface area significantly reduced surface area
high grain boundaries/grain volume ratio after grain coarsening:
reduced
grain boundary to grain volume ratio
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Transport Mechanisms during Sintering
Surface Diffusion
& Evaporation/Condensation
& Volume. Diffusion from Surface
no shrinkage
Grain boundary and volume diffusion
(Korngrenzen- und Volumendiffusion)
with shrinkage
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Diffusion Paths & RatesIncreasing Temperature
Incre
asin
g D
iffu
sio
n R
ate
[Gjostein, in Diffusion, ASM, 1973] Tm/T(K)
Log D
(m2/s
ec)
surface
grain
boundary
volume
surface
>>
>
grain boundary
volume
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Stage II.: Microstructure Development
End of Stage I. End of Stage II.
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Intermediate Stage II. (II < 90 - 95 %TD)
• Particles stop rearranging: High Particle Coordination
• Material diffuses from grain boundary to neck region
• Pores form a three-dimensional network
• strong growth of sintering necks
• strong decrease of porosity
• highest shrinkage rate, approx. 65 - 95% TD
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III. Final Sinter Stage (ρIII ≈ 95 - 99.9 %TD)
Microstructure of a-Al2O3 after
the final sintering stage
(1500°C / 2h in air)
• decrease of inner porosity (< 5%)
• closed porosity disappears
• from now on:
grain growth and coarsening !re
l. D
ensi
ty(%
rel
. T
D)
Temperature
green
100 %
I. Initial
II. Intermediate
III. Final
RT / Time
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SnO2-agglomerate with different sinter temperatures on a gold
sputtered substrate
800 °C 900 °C 1000 °C
1100 °C 1200 °C 1250 °C
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Grain Coarsening (Kornvergröberung)1. Big grains grow
bigger at the
expense of small
grains
2. Straightening the
grain boundary:
- Grains with
concave grain
boundaries grow
- Grains with convex
grain boundaries
shrink
- 120° grain
boundary angles are
preferable
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III. Final Stage: Grain Growth and Coarsening
Schematic microstructure
with moving directions of
grain boundaries during
sintering:
Small grains disappear -
big grains grow!
(Numbers give surrounding
grains)
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Grain Size as a Function of Sintering Time
Grain Size Distribution of
a MgO microstructure
after different sintering
times.
t4 > t3 > t2 > t1
Grain Size [µm]
n
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Impact of Grain Size on the Dielectric Constant εr
Temperature dependency of
the dielectric constant εr as a
function of the average
microstructure grain size of
(Ba0,87Ca0,13)(Ti0,88Zr0,12)O3
[Waser, R. et al. 1994]
Temperature [°C]
ε r
How to control grain growth
and coarsening ?
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The Solution: Sintering Additives
Sintering Additives are used in order to
• decrease sintering times
• control grain growth and
to prevent grain coarsening
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Grain Growth Inhibition by Soluble Additives
Dopings (Dotierungen) with little solubilities
and with small diffusion coefficients may effect
the microstructure in the following ways:
• Dopings precipitate in grain boundary regions
• Uniformisation of interface energy
• Introduction of a space charge
• Introduction of mechanical stresses
• Slower diffusion than the atoms of
the host crystal
• decrease of grain boundary energy
• increase of surface energy
-> All these factors hamper the migration of
grain boundaries and thus grain growth.
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Grain Growth Inhibition by Insoluble Additives
Additives with no solubility may effect the
microstructure in the following ways:
• Additives move with the grain boundary and
feature a low resistance
• Additives move with the grain boundary but
determine the speed of movements
• Additives are so immobile that the grain
boundary needs to overcome them
-> All these factors hamper with an
increasing impact the migration of grain
boundaries and thus grain growth.
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Sintering AdditivesMaterial Verdichtungshilfen
Wachstumshemmer
Al2O3 LiF, TiO2 Mg, Zn, Ni, W, BN, ZrB2
MgO LiF, NaF MgFe, Fe, Cr, Mo, Ni,
BN
BeO LiO Graphit
Si3N4 MgO, Y2O3, BeSiN2 -
SiC B, Al2O3, Al -
TaC, TiC, WC Fe, Ni, Co, Mn -
ZrB2, TiB2 Ni, Cr -
ThO2 F Ca
ZrO2 H2, Cr, Ti, Ni, Mn
BaTiO3 Ti, Ta, Al/Si/Ti
Y2O3 Th
Pb(ZrTi)O3 Al, Fe, TA, La
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Sinter Processes
Sintering
Solid Phase
Sintering
Festphasensintern
Liquid Phase
Sintering
Flüssigphasensintern
Pressure
Sintering
Drucksintern
Multi
Phase
Single
Phase
Liquid
15 Vol.%
Hot
Pressing
Hot
Isostatic
Pressing
(HIP)
no chemical
reaction
with chemical
reaction
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Microstructure of Zirconia Toughened Alumina (ZTA)
ZTA with 4 weight-% ZrO2.
In the SEM picture the ZrO2grains show up bright due to
the atomic weight difference.
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Properties of ZTA (15% ZrO2-85% Al2O3) vs Al2O3
The addition of zirconia to the
alumina matrix increases
fracture toughness easily by two
times and can be improved by
as high as four times, while
strength is more than doubled.
Key Properties
• high wear resistance
• high temperature stability
• corrosion resistance
• slow crack growth
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Sinter Processes
Sintering
Solid Phase
Sintering
Festphasensintern
Liquid Phase
Sintering
Flüssigphasensintern
Pressure
Sintering
Drucksintern
Multi
Phase
Single
Phase
Liquid
15 Vol.%
Hot
Pressing
Hot
Isostatic
Pressing
(HIP)
no chemical
reaction
with chemical
reaction
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Reaction Sintering: Si3N4
Reaction:
3 Si + 2 N2 -> Si3N4T ≈ 1400 °C
Si Powder porous Si3N4 body
Increase of body density by the reaction with
Nitrogen.
Advantage: No shrinkage!
Disadvantage: Pores, weak mech. properties
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Sinter ProcessesSintering
solid phase
sintering
Festphasensintern
liquid phase
sintering
Flüssigphasensintern
pressure
sintering
Drucksintern
Multi
Phase
Single
Phase
Liquid
15 Vol.%
Hot
Pressing
Hot
Isostatic
Pressing
(HIP)
no chemical
reaction
with chemical
reaction
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Liquid Phase Sintering Stages%
rel. T
D
Sintering Time [min]
I. Particle
Rearrangement
II. Solution/
Precipitation
III. Framework
Sintering
Skelettsintern
Principle:
One component becomes
liquid during the sintering
process and enhances
the particle
rearrangement. Wetting
and capillary forces are
additional driving forces
to the sinter process.
I. II. III.
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Additional Force between two Particles bridged by a Liquid
F : Force between two particles N
: Wetting angle °
pK : Capillary pressure Pa
r1 : Radius of contact circle m
γlV : specific interface energy liquid gas [J/m2]
uckKapillardrBenetzung
KlV prrF2
11 cos2
Shrinkage Swelling
Wetting Capillary Pressure
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Particle Wetting
uckKapillardrBenetzung
KlV prrF2
11 cos2
Shrinkage Swelling
Wetting Capillary Pressure
Advantage
- Increased body density
Disadvantage
- More Complex Composition
needed
- Good Wetting is required!
Good Wetting
Poor Wetting
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Examples for Solid and Liquid Phase Sintering
Solid Phase Sintering
• Al2O3• MgO
• ZrO2• Perowskites (ABO3)
• Mullit
• Spinells
Liquid Phase Sintering
• Si3N4 (MgO or (Y2O3 + Al2O3 + SiO2) as melt) < 15 Vol.%
• ZnO (Bi2O3 + MeO-Additives as melt < 15 Vol.%)
• WC/Co (Co as melt > 15 Vol.%)
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Sinter Processes
Sintering
Solid Phase
Sintering
Festphasensintern
Liquid Phase
Sintering
Flüssigphasensintern
Pressure
Sintering
Drucksintern
Multi
Phase
Single
Phase
Liquid
15 Vol.%
Hot
Pressing
Hot
Isostatic
Pressing
(HIP)
no chemical
reaction
with chemical
reaction
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Hot Pressing
Advantage
- Simpler than HIP
Disadvantage
- Uniaxial pressure
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Hot Isostatic Pressing (HIP)
Advantage
- isostatic pressure (around 2000 bar)
ensures an even compaction/sintering
- high sinter density achievable
Disadvantage
- encapsulation necessary if ceramic
porosity is not closed
- high costs
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Mechanical Properties of differently processed Si3N4
Gas Pressure Sintered
Silicon Nitride (GPSSN)
Hot Isostatic
Pressed Silicon
Nitride (HIPSN)
Reaction Bonded
Silicon Nitride
(RBSN)
Density min. [g/cm3] 3.2-3.3 3.2-3.3 1.9-2.5
4-point-bending-strength
[MPa]
700 – 1000 800 – 1100 200 - 330
Elastic Modulus [GPa] 290 – 330 290 – 330 80 – 180
Hardness Vickers [GPa] 14 – 16 15 – 17 8 - 10
Stress Intensity Factor
[MPam-0.5]
5 – 8.5 8.5 1.8 – 4.0
Weibull Modulus [-] 10 – 15 12 – 20 14 - 16
[http://www.keramverband.de/]
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Summary: Driving Energies for Sintering
ΔGSintering = (GS
Surface Energy+GS
Grain Boundary Energy)
–(G0Surface Energy+G0
Grain Boundary Energy)
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Outline Course „Werkstofftechnik 3”
1. Introduction: Applications, Goals and Challenges
2. Atomic Bonding
3. Crystal – and Glass Structures
4. Microstructure and Property Relations
5. Fracture Mechanics of Brittle Materials
6. Powder Conditioning and Processing
7. Sintering and Microstructure
8. Structural Ceramics
9. Functional Ceramics
10. Bioceramics
11. Glass and Glass Ceramics
12. Ceramic Matrix Composites
13. Selected Applications of Advanced Ceramics
14. Summary