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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