Werkstofftechnik 3 Lecture 7 Sintering and Microstructure€¦ · Werkstofftechnik 3 –L7...

Werkstofftechnik 3 – L7

www.ceramics.uni-bremen.deUniversität Bremen

krezwan@uni-bremen.de1

Werkstofftechnik 3

Lecture 7

Sintering and Microstructure

Prof.Dr.-Ing.

Kurosch Rezwan

krezwan@uni-bremen.de

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/




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




From Powder to Advanced Ceramics

Colloid Crystals

Surface Micro Patterning

Bulk Materials

Surface

Coatings

Porous Materials




The Importance of the Microstructure (“Gefüge”)

Raw Material Processing

Microstructure

Properties

Different Microstructure

=

Different Materials Properties!




Flow Chart of Advanced Ceramic Processing

High Quality

Powder

Shaping(Green Body / Grünkörper)

Sintering(1000 – 1800 °C)

Finishing(Cutting / Polishing)

?




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




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




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




Illustration of the Sintering Stages I. – III.

Neck

Formation

I. Initial Stage

Neck

Growth

II. Intermediate Stage

III. Final Stage




Characteristic Sintering Curves

Isothermal Sintering (T=const.)Sintering with a constant heating rate




Microstructure of a-Al2O3 between Sinter Stage I./Stage II.

1450 °C, 0.5 h, relative Density 67 % TD

Neck

Formation




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




Transport Mechanisms during Sintering

Surface Diffusion

& Evaporation/Condensation

& Volume. Diffusion from Surface

no shrinkage

Grain boundary and volume diffusion

(Korngrenzen- und Volumendiffusion)

with shrinkage




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




Stage II.: Microstructure Development

End of Stage I. End of Stage II.




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




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




SnO2-agglomerate with different sinter temperatures on a gold sputtered substrate

800 °C 900 °C 1000 °C

1100 °C 1200 °C 1250 °C




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




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)




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




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 ?




The Solution: Sintering Additives

Sintering Additives are used in order to

• decrease sintering times

• control grain growth and

to prevent grain coarsening




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.




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.




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




Sinter Processes

Sintering

Solid Phase

Sintering

Festphasensintern

Liquid Phase

Sintering


Pressure

Sintering

Drucksintern

Multi

Phase

Single

Phase

Liquid

15 Vol.%

Hot

Pressing

Hot

Isostatic

Pressing

(HIP)

no chemical

reaction

with chemical

reaction




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.




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




Sinter Processes

Sintering

Solid Phase

Sintering

Festphasensintern

Liquid Phase

Sintering


Pressure

Sintering

Drucksintern

Multi

Phase

Single

Phase

Liquid

15 Vol.%

Hot

Pressing

Hot

Isostatic

Pressing

(HIP)

no chemical

reaction

with chemical

reaction




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




Sinter ProcessesSintering

solid phase

sintering

Festphasensintern

liquid phase

sintering


pressure

sintering

Drucksintern

Multi

Phase

Single

Phase

Liquid

15 Vol.%

Hot

Pressing

Hot

Isostatic

Pressing

(HIP)

no chemical

reaction

with chemical

reaction




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.




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




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




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.%)




Sinter Processes

Sintering

Solid Phase

Sintering

Festphasensintern

Liquid Phase

Sintering


Pressure

Sintering

Drucksintern

Multi

Phase

Single

Phase

Liquid

15 Vol.%

Hot

Pressing

Hot

Isostatic

Pressing

(HIP)

no chemical

reaction

with chemical

reaction




Hot Pressing

Advantage

- Simpler than HIP

Disadvantage

- Uniaxial pressure




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




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/]




Summary: Driving Energies for Sintering

ΔGSintering = (GS

Surface Energy+GS


–(G0Surface Energy+G0





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

Werkstofftechnik 3 Lecture 7 Sintering and Microstructure€¦ · Werkstofftechnik 3 –L7...

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