Structural Ceramics

1
Structural Ceramics
2
Mechanical properties
Structural ceramics I

• Structural ceramics encompass all ceramic
  materials that fulfil mechanical functions.
• Advantages of ceramic materials over metals and
  polymers
• excellent temperature resistance
• high hardness
• high corrosion resistance
• low density
• Disadvantages
• lower fracture toughness
• higher price
• mechanical properties can only be indicated
  statistically

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Mechanical properties
Structural oxide ceramics
Aluminum oxide, alumina Al2O3
Zircon oxide, zirconia ZrO2
Partially stabilized zirconia (with CeO, CaO, MgO or Y2O3) Zr0.9Mg0.1O1.9
Aluminum titanate Al2TiO5 (ATi, AlTi)
Cordierite Mg2Al4Si5O18
Mullite Al6Si2O11
Spinel MgAl2O4
Lithium-aluminum-silicate Li2O-Al2O3-SiO2 - Basis (LAS)
SIALON Si3N4-Al2O3-Al-SiO2 - Basis
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Mechanical properties
Structural carbide and nitride ceramics
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Mechanical properties
Elastic deformation
?
simple shear deformation stress strain
relationship
uniaxial compression in 2-D of a isotropic
body strain
G Shear modulus
Microscopically the elastic deformation is due to
the reversible stretching of atomic bonds.
E Youngs modulus ? Poissons ratio
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Mechanical properties
Material strength
The theoretical strength of a material (for a
flawless single crystal) is related to the
elastic modulus
ceramic
ceramic composite
? surface energy a0 av. atomic distance
?
metal
Ceramics have much larger elastic moduli than
metals, e.g. they are much less elastically
deformed than metals The theoretical
strength of alumina should therefore be between
40 and 50GPa, the measured values however are
only 0.27GPa! Why?

elastic deformation
plastic deformation
?
Material
Youngs modulus (GPa)
Stress-strain relationships for different
materials at ambient temperature
diamond 1210, 970 Al2 O3 (s,p) 460,
390 MgO(s) 250 SiC (p)
560 Glasses 70 - 80 Aluminum
60 -75 Steel 190
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Mechanical properties
Fracture strength I
The maximum strength is based on the assumption
that a body fails by simultaneous separation of
all bonds, actual fracture in brittle material
however occur by enlargement of preexisting flaws
(cracks).
s
l
2c
Energy promoting crack growth Energy
resisting crack growth elastic energy release
stored at surface energy the crack tip
For a stable crack of length c e.g. one which
does not open more the elastic energy release
must be equal to the surface energy or less e.g.
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Mechanical properties
Fracture strength II
Above a certain size the crack will start to
selfpropagate for constant or even decreasing
stress. The critical stress for a certain crack
size c already present in the material is given
by
The much lower than theoretically predicted
strength of ceramic materials is due to the fact,
that it is impossible to manufacture perfect
ceramic parts which contain no cracks. A second
problem is, that the number and the size of
cracks present in a ceramic part are usually not
known.
Material
Fracture toughness (MPa /m2)
KICis called the fracture toughness for opening
mode loading, e.g.tensile stresses perpendicular
to the crack axis. Fracture propagation
prevention toughening through microstructural
adjustments - Transformation toughening -
Multiphase ceramics - Fiber reinforcement
Al2 O3 (s,p) 4.5, 3.5 - 4 MgO(s)
1 SiC(p) 4 - 6 Glasses
0.7 - 2 Aluminum 35 -45 Steel
40 - 60
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Mechanical properties
Plasticity of metals
Although metals have a lower Young modulus than
most ceramic materials, their actual strenght is
much larger. Moreover, at a certain strenghth the
deformation of metals becomes partly
irreversible. The higher strength and the plastic
behaviour is due to the dissipation of stress at
the crack tips by the creation and movement of
defects called dislocation. A linear disruption
of the periodicity of a crystal structure is a
linear defect, also called a dislocation. Three
types of dislocations are known pure edge, pure
screw and mixed dislocations. Edge
dislocations An edge dislocation is the boundary
of an extra half plane of atoms (unit cells)
inserted into a perfect crystal
lower boundary of half plane edge dislocation
(dislocation line) running perpendicular to the
paper foil
extra half plane of atoms
perfect structure
disturbed structure
perfect structure
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Mechanical properties
Burger vector
Characterization of dislocations Burgers vector
loop
6 lattice translations to the left
Closing gap Burgers vector
If such a loop does not close, one or more line
defects are present in the interior of the loop.
The line defect is characterized by the closure
failure. For edge dislocations, the Burgers
vector is perpendicular to the dislocation line.
If the Burgers vector has the direction and the
size of a lattice translation, the dislocation is
perfect.
start point
6 lattice translations up
6 lattice translations down
6 lattice translations to the right
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Mechanical properties
Screw dislocation
Screw dislocations An screw dislocation has a
Burgers vector parallel to the dislocation line.
dislocation line
start of Burgers vector loop
Burgers vector
Mixed dislocations Dislocations with Burgers
vector orientations oblique to the dislocation
line are called mixed.
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Mechanical properties
Movement of dislocations
Dislocation glide
t1
t2
t3
t4
Shear stress may initiate dislocations. Under
continuous stress the dislocation will move
through the crystal. Edge dislocations glide
plane is always parallel to dislocation line and
burgers vector. Screw dislocations glide plane
can have different orientations, because Burgers
vector and dislocationline are parallel.
t5
t6
t7
t8
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Mechanical properties
Energy of dislocations
The elastic energy of dislocations are
proportional to the square of the Burgers
vector Eel ?Gb2 ? const. G material
elastic property, shear modulus b Bugers vector
The most frequent Burgers vectors in a deformed
material are, therefore, usually equal to the
smallest lattice vectors of the phase.
Shortest lattice vectors of Metals Ceramic
Materials Fe 0.248 nm Al2O3 0.479nm Ag 0.288
nm ZrO2 0.363nm Ni 0.248 nm BaTiO3 0.399nm
The stress necessary to activate dislocations in
ceramic materials is thus much higher in ceramics
than in metals. Glide activation in ceramics is
only possible at high temperatures.
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Mechanical properties
Dislocation examples
High resolution electron transmission microscopy
(HRTEM) image of a edge dislocation in Si
(arrow). The vertical lines correspond to
lattice planes.
Conventional TEM images of dislocation lines in
MgO deformed under different stress. Straight
dislocation lines have either pure screw or edge
character. Curved lines and loops have mixed
character.
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Mechanical properties
Hardness I
Hardness is the property of a material to
withstand indentation and surface abrasion by
another hard object. It is an indication of the
wear resistance of a material. Alumina is very
hard, metals however have a lower hardness,
despite having a higher fracture toughness. When
a sharp tip is imprinted on a metal, the surface
will be deformed by the creation and glide of
dislocations, not so ceramic surfaces.
The Vickers hardness test method consists of
indenting the test material with a diamond
indenter, in the form of a right pyramid with a
square base and an angle of 136 degrees between
opposite faces subjected to a load F of 1 to 100
kg. The two diagonals d of the indentation left
in the surface of the material after removal of
the load are measured using a microscope and
their average calculated. The Vickers hardness is
the quotient obtained by dividing the kgf load by
the square mm area of indentation.
c
cracks
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Mechanical properties
Hardness II
Mohs Material Vickers Hardness Hardness 1
Talc 1 2 Gypsum 3 3
Calcite 9 4 Fluorite
21 5 Apatite 48 6 Orthoclase
72 7 Quartz 100 8 Topaze 200 9
Corundum 400 SiC 600 TiC
600 10 Diamond 1500
finger nail (2.5) coin (3.5) steel (5.5)
glass (6)
http//www.gordonengland.co.uk/hardness/vickers.ht
m
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Mechanical properties
Hardness III
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Mechanical properties
Structure of alpha-Al2O3
c0
c0
Al site empty site
a2
a1
Corundum structure, hexagonal unit cell setting,
only the cation sublattice is shown. The oxygen
form an hexagonal dense packed array.
(210) projection of the corundum structure.
Aluminum ions in adjacent face-sharing octahedra
mutually repell each other.
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Mechanical properties
Alumina as structural ceramic
Properties of reactive grade alumina impurities
Na2O 0.08wt melting temperature 2050C surface
area 6.8m2g-1 sintering temperature 1550
- 1600C sintered density 3.92 (2h
1650C) fracture toughness 4
-4.5MPam1/2 bend strength 500 - 600 MPa
Applications hip protheses, cutting tools
(zirconia-toughened)
Triangular alumina-based cutting element used to
machine metallic parts
Cutting elements made of alumina
http//www.cncmagazine.com/
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Mechanical properties
Alumina microstructure and strength I
Controlling the microstructure of alumina
ceramics to enhance mechanical properties
Dense hot pressed alumina without (top) and
with addition of MgO (bottom) Grain growth is
detrimental to the fracture strength of
ceramics d grain diam.
5mm
Doping alumina with MgO leads to the formation of
precipitates of spinell along the grain
boundaries, which lowers the grain boundary
mobility. (Bennison et al., 1983)
5mm
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Mechanical properties
Alumina microstructure and strength II
Porosity is detrimental to the mechanical
strength Doping alumina with periclase
reduces also the internal residual porosity. The
picture (Geskovich et al.500x) shows an alumina
body sintered without dopant. There is a large
number of entrapped pores.When sintered with a
dopant, the reduced grain boundary mobility
allows the filling of the pores when they are at
the grain boundaries, whereas fast grain growth
encloses the pores quickly into the interior of
the grain, where it is difficult to eliminate
them.
?0 strength at zero porosity b constant.
Pure alumina has a low fracture toughness. Mixing
ca. 10 of zirconia (BSE image, zirconia white)
into the alumina doubles the fracture toughness.
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Mechanical properties
Example Hip implants

• The articulation of hip implants require
• Mechanical strength. Typical maximal loads within
  the human body are 10 to 15 kN.
• Wear resistance e.g. high hardness
• Biocompatibility
• Alumina is the material of choice. It is
  biocompatible e.g. no rejection reaction nor
  degradation in physiological liquids. The
  mechanical strength, though not very high, is 10
  to 20 times higher than required for the maximum
  loads expected. The high hardness of alumina
  results in average wear rates for alumina-alumina
  coupling that are up to 50 times lower than for
  alumina - polyethylene or alumina - chrome cobalt
  alloys.

http//www.wmt.com/ceramic http//www.ceramic-hip.
com/healthcare/index.php
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Mechanical properties
Pepper / Salt Grinder

• Processing
• Net-shape Injection Molding
• Properties
• High Hardness
• Resistance against NaCl
• Advantages
• No Corrosion
• Long lifetime
• Cheaper

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Mechanical properties
Zirconia as structural ceramic
Properties of partially stabilized
zirconia dopant Y2O3, CaO 3 - 10wt melting
temperature 2500C sintered density 6.05 gcm -3
sintering temperature 1800C Youngs
modulus 170 -210 GPa fracture toughness 6
- 20 MPam1/2 Bend strength 400 - 700 MPa
Applications die material in the metall
industry, thermal barrier coatings, piston caps,
cutting tools
valve
sealing
Piston parts (valves, sealings etc. made of
stabilized zirconia.
Schematic drawing of a piston.
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Mechanical properties
Polymorphs of ZrO2
Schematic structures of the three zirconia
polymorphs
c
c
a
a
cubic c-phase 2370C - 2680C
tetragonal t-phase c/a 1.02! 1240C - 2370C
monoclinic c-phase lt 1240C
- The cubic phase can be stabilized by doping
with MgO, CaO or Y2O3 - The tetragonal -
monoclinic phase transformation involves a 4.7
volume increase. - This volume increase is the
basis for transformation toughening.

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Mechanical properties

• Partially stabilized zirconia (PSZ)

• Manufacturing of partially stabilized zirconia
• Add about 10 MgO
• Sinter in the cubic phase
• Lower temperature and heat treat (age) to
  nucleate small precipitates of t-phase
• These are growing below the critical size for t-m
  transformation
• Cool to room temperature
• Remaining c-phase has no time to transform

• ZrO2-MgO phase diagram

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Mechanical properties
Mg-PSZ Microstructures

• After sintering at 1800C an annealing stage at
  1400C is introduced
• -After 4-5 hours tetragonal precipitates, grow by
  conventional diffusion processes as coherent
  spheroids along 001 cube planes
• Below a well defined critical size of about 200
  nm the t-particles remain tetragonal down to room
  temperature
• - Optimum microstructures contains about 25 -
  30 by volume of tetragonal phase

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Mechanical properties
Transformation toughening I
1. The stresses concentrated at the crack tip
transform the surrounding tetragonal ZrO2
inclusions to the monoclinic polymorph. The
transformation absorbs fracture energy and slows
down crack propagation.
crack
tetragonal ZrO2 inclusion
transformed to monoclinic structure
stress orientation around the crack tip
transformation zone
Lense-shaped tetragonal inclusions in a matrix
(black) of cubic zirconia (A. Heuer).
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Mechanical properties
Transformation toughening II
2. Microcracking around the transformed
inclusions The volume stresses resulting from
the tetragona- monoclinic transformation
delocalize also the stresses from the crack tip
volume of the tetragonal zirconia inclusion
volume after transformation to monoclinic
stresses due to the volume increase
microfracture due to the volume stresses
3. Crack deflection due to volume stresses The
deflection of cracks increases the crack
surface.The stress releave per unit penetration
is, therefore, larger then for an inclusion free
zirconia.
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Mechanical properties
Transformation toughening III
100nm
Initially tetragonal zirconia inclusion in a
cubic zirconia matrix, which are completely
transformed to the monoclinic structure. The
bands within the inclusions are twin lamellae.