Transcript Diffusion in multiphase systems

Tetragonal Zirconia Polycrystals
Structure and properties
MSc Eng Marta Gajewska
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Zirconia - introduction
950°C
Melt
2680°C
Cubic
c
Engineering applications
2370°C
Tetragonal
t
1150°C
Monoclinic
m
tetragonal phases (mechanical properties)
cubic phases (electric properties)
Doping with oxides (Y2O3, CaO, Mg2O, CeO2
and others) allows to stabilize
the high-temperature phases at room
temperature
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
t - zirconia structure
Tetragonal zirconia unit cell in both the
body-centered tetragonal and
pseudofluorite description
Space group: P42/nmc
Coordination number: 8
Z: 2
Lattice parameters:
a = b = 5,1023 Å
c = 5,1817 Å
α=β=γ=90°
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
TZP - introduction
Tetragonal Zirconia Polycrystals with metastable tetragonal structure of very fine
zirconia grains sintered at low temperature (e.g. with 2-4 mol% Y2O3)
950°C
Tetragonal
t
1150°C
Monoclinic
m
Stabilization of the high-temperature tetragonal (t) form as metastable at room
temperature  technique of transformation - toughening
Metastable condition: surrounding structure opposes the expansive transition from t- to m-forms
Propagating crack 
concentrated stress field at
t-crystals transform into

the crack tip
stable but less dense m-ZrO2
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP structure
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Properties of TZP ceramics
High density – up to 6,1*10³ kg/m³
Low thermal conductivity – 20% of that of alumina ceramics
High fracture toughness
Very high flexural strength and hardness (11 GPa for 1.5 mol% yttria)
Coefficient of thermal expansion similar to that of cast iron
Modulus of elasticity similar to steel (150–200 GPa)
High chemical resistance
Good wear resistance
Low coefficient of friction
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
References
• P. Boch, J.-C. Niepce „Ceramic Materials: Processes, Properties and Applications”,
Hermes 2001, 219-228
•J. F. Shackelford, R. H. Doremus “Ceramic and Glass Materials: Structure,
Properties and Processing”, Springer 2008, 169-197
•R. E. Smallman, R. J. Bishop, “Modern Physical Metallurgy and Materials
Engineering”, Elsevier 1999, 330-331
• http://www.azom.com
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Tetragonal Zirconia Polycrystals
Why do we add yttria or other oxides to TZP?
MSc Eng Honorata Kazimierczak
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Introduction
Zirconia (ZrO2) is an important ceramic material having a wide range of applications in
engineering:
-catalysis,
-sensors,
-gas turbines,
-magnetic hydrodynamics process of power generation,
-thermal barrier coatings,
-high temperature nozzles in air engines, etc.
Zirconia exist in three different crystalline forms:
• cubic (c) (stable at 2680 -2370°C)
• tetragonal (t) (stable at 2370 -1170°C)
• monoclinic (m)
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Zirconia exist in three different crystalline forms:
• cubic (c) (stable at 2680 -2370°C)
• tetragonal (t) (stable at 2370 -1170°C)
• monoclinic (m)
t-m transformation:
•3-5% volume increase => extensive cracking in the material.
To stabilize the high temperaure t-phase at room temperature ,
CeO2, CaO, Y2O3 or MgO
are usually added to zirconia in appropriate proportions.
Ca-TZP
3Y-TZP
Mg-TZP
Ce-TZP
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
t-m transformation:
•3-5% volume increase
It is known that the metastable tetragonal zirconia
inclusions in a ceramic matrix transform to the stable
monoclinic modification on application of external tensile
stress around a crack tip.
This martensitic transformation
is associated with a volume
expansion from the tetragonal to
a larger monoclinic lattice which
reduces and eventually stops the
propagation of cracks, thus
improve the resistance to
mechanical failure.
rys.1 Representation of stress-induced transformation toughening
process.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
•In order to retain the tetragonal phase at room temperature the
grain size must be kept below a critical value.
Rys.2. Retention of tetragonal phase. Critical grain size against oxide content in
tetragonal zirconia.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Rys.3. Fracture toughness vs. yttria content.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
References:
1) C.Piconi, G.Maccauro „Zirconia as a ceramic biomaterial”, Biomaterials 20 (1999) 1-25
2) M.M.R.Boutz, A.J.A. Winnubst, A.J. Burggraaf „Yttria-Ceria Stabilized Tetragonal
Zirconia Polycrystals: Sintering, Grain Growth and Grain Boundary Segregation”,
Journal of European Ceramic Society 13 (1994) 89-102
3) Marek Faryna „Analiza zależności krystalograficznych faz składowych w kompozytach
z osnową ceramiczną”, IMIM PAN, Kraków 2003
4) H. El Attaoui , M. Saadaoui , J. Chevalier , G. Fantozzi „Static and cyclic crack
propagation in Ce-TZP ceramics with different amounts of transformation toughening”,
Journal of the European Ceramic Society 27 (2007) 483–486
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Tetragonal Zirconia Polycrystals
Mechanical properties of Y-TZP
MSc Eng Katarzyna Stan
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Why Y2O3?
Strength v. toughness curves for four types of transformationtoughened zirconia. Dashed line represents the critical stress
for the tm transformation.
Stabilized zirconia as a structural ceramic: An overview
J. Robert Kellya, Isabelle Denryb
Dental materials 24 ( 2008 ) 289–298
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
TZP materials with 2-3% mol Y2O3
Completely constituted by
tetragonal grains with sizes of
the order of hundreds of
nanometers
Amount of the T-phase fraction retained at
room temperature
Retention of tertagonal phase. Critical grain size against Yttria
content in tetragonal zirconia
Influence on mechanical properties of
TZP ceramics
C. Piconi, G. Maccauro; Zirconia as a ceramic biomaterial;
Biomaterials 20 (1999) 1 -25  J Mater Sci 1982;17:240-6
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Properties
• High strength
Aplications
Industry
• High fracture toughness
• High hardness
• Wear resistance
• Good frictional behavior
• Non-magnetic
• Electrical insulation
• Low thermal conductivity (20% that of
alumina)
• Corrosion resistance in acids and alkalis
• Modulus of elasticity similar to steel
• Coefficient of thermal expansion similar to iron
• Chemical inertness
• Use temperatures up to 2400°C
Precision ball valve balls and seats
High density ball and pebble mill grinding media
Rollers and guides for metal tube forming
Thread and wire guides
Hot metal extrusion dies
Deep well down-hole valves and seats
Powder compacting dies
Marine pump seals and shaft guides
Oxygen sensors
High temperature induction furnace susceptors
Fuel cell membranes
Electric furnace heaters over 2000°C in oxidizing
atmospheres
http://www.azom.com/article.aspx?ArticleID=
3299#_Mechanical_and_Physical_Properties
http://accuratus.com/zirc.html
Biomaterial – dental aplications, THR (Total Hip
Replacements) ball heads
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Good chemical and dimensional stability,
mechanical strength and toughness, coupled
with a Young’s modulus in the same order
of magnitude of stainless steel alloys was
the origin of the interest in using zirconia
as a ceramic biomaterial
P. F. Manicone, P. R. Iommetti, L. Raffaelli; An overview of zirconia ceramics: Basic properties and clinical applications; Journal of Dentistry 35 (2007) 819 – 826
C. Piconi, G. Maccauro; Zirconia as a ceramic biomaterial; Biomaterials 20 (1999) 1 -25
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Good mechanical properties due to refined grain sizes
Y-TZP with small grain size 0.3–0.4 μm,
High flexural strength 1000–1500 MPa and
High fracture toughness 8–10 MPa
Effects of material properties and testing parameters on wear properties
of fine-grain zirconia TZP/ Chih-Chung T. Yang, Wen-Cheng J. Wei;
Wear 242 2000. 97–104
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
.
Concerning industrial applications – there is a need to investigate
wear resistance of such material
Mechanical property degradation in zirconia, known as „ageing”, due to the progressive spontaneous
transformation of the metastable tetragonal phase into the monoclinic phase. This behavior is well
known in the temperature range above 200°C in the presence of water vapor
Without the occurrence
of monoclinic phase,
a better wear
resistance of the Y-TZP
with a high fracture
toughness is
expected
The wear resistance and amount of m-phase of Y-TZP
as a function of grain size
Effects of material properties and testing parameters on wear properties
of fine-grain zirconia TZP/ Chih-Chung T. Yang, Wen-Cheng J. Wei;
Wear 242 2000. 97–104
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Parameters of the ceramic material are strongly effected
by the density of the material.
Correlation between Microstructure, Phase Transformation during Fracture and the Mechanical Properties of Y-TZP
Ceramics; J. L. Shi, B. S. Li, Z. L.Lu and X. X. Huang
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Tetragonal Zirconia Polycrystals
Y-TZP composites
Reactions in material
MSc Eng Piotr Bobrowski
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP composites
Y-TZP properties:
- good bending strength
- good fracture toughness
- median hardness
- poor wear resistance
composites with Y-TZP as matrix:
- carbides: WC, TiC, SiC, NbC, CrxCy
- oxides: Al2O3, TiO2
- nitrides: TiN
- other: TiB2, LiNbO3, LiTaO3
Y-TZP composites are investigated in purpose of improving
hardness and wear resistance of pure ZrO2.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP/TiB2, TiN and TiC composites
Ti ceramics:
- excellent hardness
- poor bending strenght
- poor fracture toughness
Exerimental:
- powders diameter: 0.2-2µm
Vleugels , van der Biest: J.Am.Ceram.Soc. 82 (1999) 2717-2720
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP/WC composites
WC properties:
- boiling temperature: 6000OC
- excellent hardness
- excellent wear resistance
Tensile strength
Y-TZP/WC preparation:
- hot pressing needed to obtain dense ceramics
- oxygen free atmosphere
Solid state reactions in 1400-1500OC:
ZrO2 + 3C → ZrC + 2CO – leads to stabilization
of high symmetry phases
ZrO2 + 6WC → ZrC + 3W2C + 2CO – porous
structure
1400OC
180 MPa
355 MPa
520 MPa
1500OC
195 MPa
380 MPa
560 MPa
bending strength [MPa]
material / sintering temperature
TZP + 10% WC
TZP + 20% WC
TZP + 30% WC
WC mol% amount
Stresses caused by thermal expansion coefficient
mismatch during cooling: αWC=5.2*10-6 K-1, αTZP=11.0*106 K-1 favours t→m transformation
Pędzich, Haberko: Inżynieria Materiałowa 2 (1996) 40-45
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP/SiC composites
SiC properties:
- excellent hardness (Mohs: 9,5)
- excellent wear resistance
- very brittle
SiC inclusions shapes:
-whiskers
-platelets
-particles
Solid state reactions above 1400OC:
ZrO2 + 3/2SiC → ZrC + 3/2SiO +
1/2CO
– gaseous CO generates pores
SiC + CO → SiO + 2C – decomposition
of carbide
Composite properties
material
Y-TZP
Y-TZP/5% SiC
E
220 GPa
231 GPa
σ
959 MPa
730 MPa
KIC
9.0 MPa m1/2
10.8 MPa m1/2
Hv
12 GPa
13 GPa
-stresses caused by thermal expansion
coefficient mismatch during cooling
(αWC=4.9*10-6 K-1, αTZP=11.0*10-6 K-1 play
secondary role.
- other toughening mechanisms appear:
- crack deflection
- crack branching
- microcracking
Ding, Oberacker, Thuemmler: Journal of the European Ceramic Society 12 (1993) 377-385
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Y-TZP/Al2O3 composites
Al2O3 properties:
- twice as stiff as ZrO2
- chemically compatible with ZrO2, can be mixed
in a wide range of concentrations
Langa: Journal of Materials Science 17 (1982) 247-254
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Tetragonal Zirconia Polycrystals
Thermal etching, hot pressing,
pressureless sintering
MSc Eng Grażyna Kulesza
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Thermal etching
The thermal etching is performed in a furnace or
kiln under a controlled atmosphere chosen to the
character of etched phase (phases), sometimes it
may be vacuum (but also in specific cases) and
comprises the following successive stages: rapid
rise in the temperature of the furnace to a
temperature plateau, maintaining the temperature
at the plateau for few minutes, lowering the
temperature to the final temperature.
Thermally etched 99.9% alumina
After this treatment the grain boundaries, pores
and other microstructures become distinct due to
reconstruction by surface diffusion which tends to
minimize the total surface energy of the crystals.
93.1% Al2O3, 2.9% ZrO2
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
In the ceramic industry many methods of forming are used e.x.:
• pressing
• forming at elevated temperatures
• slip casting
• thermoplastic forming
• vibrating
To choose proper forming method is determined by:
• shape
• size
• the required dimensional accuracy
Hot pressing requires moisture powder to a few percent
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
Hot pressing has many advantages:
• possibility to obtain high density compacts
• forming of non-plastic materials
• high strength
• dimensional accuracy
• sharp edges
• high efficiency, low waste
• introducing of automation and mechanization
but also disadvantages:
• forming limited shapes
• cutouts and holes in the same direction like the pressing direction
• inhomogeneous densification along the direction of the applied pressure
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
Pressing is carried out in dense, rigid metal or graphite molds with smooth walls. It is
high-pressure compression mostly 30 MPa (sometimes even till 100 MPa)
pc
H
pc
ρ
κ
ρ – relative density
κ – pressing coefficient
p – pressing pressure
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
The decisive point is size and shape of grains.
Very hard to form is fine powder, this situation leads to the inhomogenity of the
texture*
Use of thicker grains (granules) reduces the risk of cracks.
For this purpose, the granulation is needed.
*Texture - the spatial distribution of elements of the structure
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
An important parameter characterizing if the element is properly pressed is bulk
density.
Bulk density is defined as weight ratio to the volume of powder poured into the
form.
where:

ρ – bulk density
m – weight ratio
V – volume of poured powder
alignment
a) regular loose, b) single chessboard,
c) double chessboard, d) pyramidal,
e) tetraedrical
m
V
coordination
number
open
porosity
regular loose
6
47,68
single
chessboard
8
39,55
double
chessboard
10
30,20
pyramidal
12
25,95
tetraedrical
12
25,95
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
Hot pressing is a high-pressure, low-strain-rate dense polycrystals synthesis process
for forming of a powder or powder compact at a temperature high enough to
induce sintering and creep processes.
This is achieved by the simultaneous application of heat and pressure.
gas
radiator
sample
400 MPa
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Hot pressing
Hot pressing vs. free sintering:
• intensification and acceleration of the process leads to denser samples at lower
temperatures and limits the growth of grains
• elimination of porosity
• better mechanical properties
Range of applied pressure depends on the matrix material and temperature
Material
Tmax [°C]
pmax [MPa]
Graphit
2500
70
Al2O3
1200
210
SiC
1500
280
W
1400
25
Cermet (WC 65%,
TiC 10%, Co 25%)
700
300
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Pressureless sintering
Mass transport mechanism:
1) volume diffusion (Nabarro-Herring)
2) diffusion along grain boundaries (Coble)
3) diffusion on the surface of grains
4) vapor pressure
4
3
2
2
1
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Pressureless sintering
F
R2
2
 1  1 
p   '      
 R1  R2  
R1  R2
p  0
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
R1
Pressureless sintering
1) and 2) mechanisms that cause contraction of the whole system, close-up of centers of each
grains, leads to loss of porosity
3) and 4) without contraction, only mass transport
3) at low temperatures from long time, activated as a first process, prevent elimination of
porosity, increase in neck
2) easier than 1) because diffusion along grain boundaries (as an area with a lot of defects)
1) at higher temperature, atoms exhaustion
4) at the highest temperatures, near melting temperature
4
3
2
2
1
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Pressureless sintering
Neck growth mechanism:
n
F (T )  t
x

 
Rm
R
x – neck radius
R – grain radius
t – time
n, m – powers identifying the mechanism of sintering
l.p.
Way of mass transport
Mass source
Wieght loss place
n
m
1
volume diffusion
grain boundary
neck
5
3
2
diffusion along grain boundaries
grain boundary
neck
6
4
3
diffusion on the surface of grains
grain surface
neck
7
4
4
vapor pressure
grain surface
neck
3
2
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Pressureless sintering
Pressureless sintering is the sintering of a powder compact (sometimes at very high
temperatures or relatively low temperatures, depending on the powder) without applied
pressure.
The powder compact (if ceramic) can be created by slip casting into a plaster mould, then the
final green compact can be machined if necessary to final shape before being heated to sinter.
Particular advantages of this powder technology include:
1. Very high levels of purity and uniformity in starting materials
2. Preservation of purity, due to the simpler subsequent fabrication process (fewer steps) that
it makes possible
3. Stabilization of the details of repetitive operations, by control of grain size during the input
stages
4. Absence of binding contact between segregated powder particles – or "inclusions" (called
stringering) – as often occurs in melt processes
5. No deformation needed to produce directional elongation of grains
6. Capability to produce materials of controlled, uniform porosity.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
References
1.
2.
R. Pampuch, K. Haberko, „Nauka o procesach ceramicznych”, PWN,
Warszawa 1982
Wykłady: Prof. dr hab. inż. K. Haberko, Dr. inż. Z. Pędzich, „Procesy i technologie
ceramiczne”
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Tetragonal Zirconia Polycrystals
Mechanisms inproveing fracture toughness
in TZP based composites
MSc Eng Jagoda Poplewska
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Fracture toughness
Fracture Toughness is ability of material to resist fracture when a crack is present
(The more energy is needed to grow a crack, the higher the toughness of the material).
General factors affecting the fracture toughness of material are:
temperature,
strain rate,
presence of structure defects,
presence of stress concentration (notch) on the specimen surface.
Stress intensity factor:
K = σ(πa)1/2f(r,θ)
where:
σ – normal stress
2a- size of the crack
Three types of stress intensity factors:
a) the opening mode KI
b) the sliding mode KII
c) the tearning mode KIII
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Toughening mechanisms in ceramics
Crack-Tip interactions – obstacles in the crack path to impede crack motion (second-phase
particles, whiskers, fibers, etc.):
a. Crack Bowing
b. Crack Deflection
Crack-Tip Shielding – eg. transformation toughening, microcrack toughening;
Crack Bridging – frictionally bonded fiber composites.
Comparison of crack fully bridged by
frictionally bonded fibers with the case
where fibers break during matrix cracking
forming a bridging zone behind the moving
crack front
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Crack Deflection
Crack Deflection – tilt and twist out of the crack plane
around grains and second-phase additions.
SEM image showing crack
propagation around
a sapphikon (Al2O3) fiber
in a calcium aluminosilicate
(CAS) glass-ceramic
SEM image showing fiber
pullout on the fracture surface
of AlPO4-coated alumina/
mullite fiber /Al2O3 CMC, hot
pressed at 1250°C for 1h
Ceramic materials: science and engineering, C. Barry Carter, M. Grant Norton
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Crack Bridging
•Ligaments can be formed by mechanical
interlocking of the grains;
•These ligaments will make it more difficult to
open the crack at a given applied stress and will
increase fracture toughness;
•This mechanism is important in frictionally
bonded fiber composites;
•In these materials the final failure is not the
result of propagation of a single crack.
Illustration of crack bridging mechanisms with
debonding and fiber pullout
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Transformation Toughening
•Some materials can transform from one crystal structure to another;
•Commonly this transformation is thermal, but in particular cases it is stress-induced;
•One uses the tetragonal to monoclinic phase transformation. The monoclinic structure is less
compacted than tetragonal structure, and the theory says that increment of this volume closes
the crack tips. This causes the toughening effect.
•Zirconia is the most important material due to transformation toughening behavior.
Illustration of transformation
toughening in a ceramics matrix
containing ZrO2 particles
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Transformation Toughening in ZrO2
•Unstabilized ZrO2: t ⌫ m =>
3vol% increase => cracks;
•Y2O3, CaO, MgO => Cubic “stabilized” ZrO2 (CSZ);
•Add smaller quantities of oxides and heat treat =>
c + t particles.
Cool to RT => metastable t-phase;
•Under stress: t- ZrO2 transforms martensitically to
m- ZrO2 => toughening effect.
Partially Stabilized ZrO2 (PSZ).
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
Transformation Toughening
Why does the stress-induced
transformation give rise to an increse in
fracture toughness?
The transformation zone can be thought of
as a large transformed inclusion that is
restricted by surrounding material;
The transformation within the zone tries
to enlarge the zone, but this is
counteracted by surrounding
untransformed material;
Thus, the latter material opposes the
dilatation of the transformation zone and
presses back with residual stresses.
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim
References
• „A survey on the mechanisms and mechanics of toughening in structural ceramics”, G. Th.M. Stam,
E. van der Giessen, P. Meijers, TU Delft, 1990
• „Transformation toughening of ceramics”, D.J. Green, R.H.J. Hannink, M.V. Swain, CRC Press 1989
• „Ceramic materials: science and engineering”, C. Barry Carter, M. Grant Norton, Springer, 2007
Interdyscyplinarne studia doktoranckie z zakresu inżynierii materiałowej z wykładowym językiem angielskim