1. Structural Ceramics

2. Structural ceramics I Mechanical properties
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

3. Structural oxide ceramics
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

4. Structural carbide and nitride ceramics
Mechanical properties
Structural carbide and nitride ceramics
GPSSN
- gas pressured sintered BN
Boron nitride (hexagonal)
CBN (cubic boron-nitride)
- cubic
SSN
- sintered
RBSN (reaction-bonded SN)
- reaction bonded
HPSN
- hot pressed
HIPSN
- hipped
Si3N4 (SN)
Silicon nitride
PKD (polycristalline diamond)
Diamant
TiC
Titanium carbide
B4C
Boron carbide
CSiC-Si
- silizierte Kohle, reaktionsgebunden
HIPSiC
HPSiC
SiSiC
- silicon infiltration, reaction bonded
SiC
- pressureless sintered
Siliconcarbide

5.  Elastic deformation d0 l l0 d simple shear deformation
Mechanical properties
Elastic deformation l d l0 d0 
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: Young‘s modulus : Poisson‘s ratio

6.   Material strength The theoretical strength of a material
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
Young‘s modulus
(GPa)
Stress-strain relationships for different
materials at ambient temperature
diamond , 970
Al2 O3 (s,p) , 390
MgO(s)
SiC (p)
Glasses
Aluminum
Steel

7. Fracture strength I s l 2c
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.

8. 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) ,
MgO(s)
SiC(p)
Glasses
Aluminum
Steel

9. 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:
extra half plane of atoms
lower boundary of half plane = edge dislocation (dislocation line) running perpendicular
to the paper foil
perfect structure
disturbed
structure
perfect structure

10. Burger vector Characterization of dislocations: Burgers vector loop
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
down
6 lattice
translations up
6 lattice
translations
to the right

11. Screw dislocation Screw dislocations
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.

12. Movement of dislocations
Mechanical properties
Movement of dislocations
Dislocation glide
glide direction
glide or slip plane
The full slip system is indicated as:
[u v w] (h k l)
Burgers vector glide plane 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

13. Energy of dislocations
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 nm Al2O nm
Ag nm ZrO nm
Ni nm BaTiO nm
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.

14. Dislocation examples High resolution electron transmission
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.

15. 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

16. Hardness II Mohs Material Vickers Hardness Hardness 1 Talc 1
Mechanical properties
Hardness II
Mohs Material Vickers
Hardness Hardness
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaze
9 Corundum
SiC
TiC
10 Diamond
finger nail (2.5)
coin (3.5)
steel (5.5)
glass (6)

17. Mechanical properties
Hardness III

18. Structure of alpha-Al2O3
Mechanical properties
Structure of alpha-Al2O3
face-sharing
edge-sharing c0 c0
Al site
empty site a1 a2
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.

19. Alumina as structural ceramic
Mechanical properties
Alumina as structural ceramic
Properties of reactive grade alumina:
impurities Na2O 0.08wt% melting temperature 2050°C
surface area 6.8m2g-1 sintering temperature °C
sintered density (2h 1650°C) fracture toughness MPam1/2
bend strength MPa
Applications: hip protheses, cutting tools (zirconia-toughened)
Triangular alumina-based cutting element used to machine metallic parts
Cutting elements made of alumina

20. Alumina: microstructure and strength I
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

21. Alumina: microstructure and strength II
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.
5mm
Pure alumina has a low fracture toughness. Mixing ca. 10% of zirconia (BSE image, zirconia: white) into the alumina doubles the fracture toughness.

22. Example: Hip implants The articulation of hip implants require:
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.

23. Pepper / Salt Grinder Processing Net-shape Injection Molding
Mechanical properties
Pepper / Salt Grinder
Processing
Net-shape Injection Molding
Properties
High Hardness
Resistance against NaCl
Advantages
No Corrosion
Long lifetime
Cheaper

24. Zirconia as structural ceramic
Mechanical properties
Zirconia as structural ceramic
Properties of partially stabilized zirconia:
dopant Y2O3, CaO wt% melting temperature 2500°C
sintered density gcm -3 sintering temperature 1800°C
Young‘s modulus GPa fracture toughness MPam1/2
Bend strength 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.

25. Mechanical properties
Polymorphs of ZrO2
Schematic structures of the three zirconia polymorphs c c a a
cubic c-phase
2370°C °C
tetragonal t-phase
c/a = 1.02!
1240°C °C
monoclinic c-phase
< 1240°C
- 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.

26. Partially stabilized zirconia (PSZ)
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

27. Mg-PSZ Microstructures
Mechanical properties
Mg-PSZ Microstructures
After sintering at 1800°C an annealing stage at 1400°C 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

28. Transformation toughening I
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).
200 nm

29. volume of the tetragonal zirconia inclusion
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
crack
volume of the tetragonal zirconia inclusion
volume after transformation to monoclinic
stresses due to the volume increase
microfracture due to the volume stresses
crack
Penetration depth
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.

30. Transformation toughening III
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.