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Mechanical properties!Structural ceramics

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Flexural strength!

alumina

f =KIC

c

KIC

$ G

influence of grain size

liquid phase sintering (SiO2)(purity 80-99%)

solid state sintering(purity 99-100%)

hot-pressing

Mechanical properties of ceramics, J. B. Watchman, J. Wiley & sons, 1996!

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alumina at high temperature

glassy phase softening, creep, plastic deformation

Mechanical properties of ceramics, J. B. Watchman, J. Wiley & sons, 1996!

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silicon nitride

flexuralstrength

0 400 800 1200 1600

silicon nitride!

silicon carbide!

alumina!

T (C)

maximum temperature for structural uses

T (C)

Si3N4 reaction bonded 1300sintered 900hot pressed 1200

SiC reaction bonded 1300

sintered 1200hot pressed 1400

Al2O3 99% 120090% 800

liquid phase sintering (SiO2, Y2O3, Al2O3 )(purity 90-99%) - SSN

hot-pressing(purity 99-100%) - HPSN

reaction bonding (Si + NH3) - RBSN

liquid phase sintering (SiO2, Al2O3 )(purity 90-99%) - SSC

hot-pressing(purity 99-100%) - HPSC

reaction bonding (Si + CO / C +Si) - RBSC

silicon carbide

Ceramics and Glasses, Engineered Materials Handbook, vol. 4, ASM international, USA, 2000!

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Reaction bonding!preform (Si powder)

NH3

T1400C

Si3N4 (RBSN)

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preform (Si powder)

CO

T1400C

SiC (RBSC)

preform (C powder)

liquid Si

T1500C

SiC (RBSC)

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mechanical strength

defects (c)

microstructure (KIC)

f=KIC

c

fabrication and finishing!

material and processing!

interaction between defects and microstructure!

Ceramic materials:! theoretical strength (10 GPa)E

10

mechanical strength

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KC1

K

c0.5c00.5

f1

KC2

f2

KC constant! KC increasing with c

!

K

c0.5c00.5

f1

KC(c)

f

stable growth!

f depending on c f independent from c

Toughening mechanical strength

R- or T-curve effect

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Thoughening mechanisms

dislocations

microcracks

phase transformation

ductile particles

grain interlocking

fibres

whiskers

ductile particles

(a) (b)

(a) process zone (frontal wake)

weakening of the material in front of the crack tip

s

eeT

sCGC 2CTh

(b) bridging zone (bridged interface)

crack closure stress (t)

GC 2 t(u)0

u*

du

volume fraction of toughening agent

surface fraction of the bridges

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Process zone mechanisms

Transformation toughening!

zirconia (ZrO2)

allotropic phases: cubic (c), tetragonal (t), monoclinic (m)

martensitic trasformation (MS1200C - 600C)

V4%, ij1-7%

MS decreases with:

presence of stabilizing oxides (MgO, CaO, Y2O3, CeO2) (grain size)-1 compression stresses (matrix) tphase metastable at room T!

temperature

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System ZrO2 - Y2O3

tetragonal zirconia polycrystals (TZP)(only t, g 0.5 - 2 m)

partially stabilized zirconia (PSZ)

(tin c, g 30 - 60 m)

c

t

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t m

Thoughening mechanism

c

Tr

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KC= 0.22E

1 T h

asymptotic increment of fracture toughness:

fraction of grains t

Typical systems (Kc 20 MPa m0.5):!

partially stabilized zirconia (PSZ)!

zirconia toughened alumina (ZTA)!

**

Journal of the American Ceramic Society, 1990!

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KC = Cy

yR

asymptotic increment of fracture toughness:

yielding

Sistems:!

alumina with Ni or Al particles (Kc 25 MPa m0.5)!

Bridging mechanisms

1. Bridging with ductile particles!

particles radius

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weak interface (if)

high friction coefficient

2. Bridging with fibres or whiskers!

Journal of the American Ceramic Society, 1990!

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matrix: aluminafibres: alumina-mullite

interfacce: monazite (LaPO4)

matrix: SiCfibres: SiC

interface: porous SiC

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matrix: C

fibres: SiCinterface: C

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asymptotic increment of fracture toughness:

(for long fibres)

fibre radius

fibres strengthinterface fracture toughness

differential thermal deformation

friction between fibre-matrix

pull-out length

Sistems:!

alumina with SiC whiskers (Kc 9 MPa m0.5)!

silicon nitride with SiC whiskers (Kc 11 MPa m0.5)!

SiC-SiCfor C-SiCfcomposites (Kc 30 MPa m0.5)

!

GC = df

2

EET( )

2

+

4iR1 ( )

'

(

))

*

+

,,+

2 hp2

R

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requirement: intergranular fracture Sistems:!

alumina (Kc 8 MPa m0.5)!

silicon nitride (Kc 10 MPa m0.5)!

silicon carbide (Kc 8 MPa m0.5)!

3. Bridging grains!

KC > 0 (200 - 300%)

Journal of the American Ceramic Society, 1990!

KC=

E

6(12)2wa

r

2

grain boundary friction

volumetric fraction of effective grains

length of effective grains

geometric factor of effective grains