Eskisehir Technical University
Materials Science and Engineering
MLZ 453
Advanced Materials
and
Composites
TOUGHENING MECHANISMS
FOR
CERAMIC MATRIX COMPOSITES
Prof.Dr. ALPAGUT KARA
Tuğçegül İDİNAK
50065434388
CONTENTS
1. INTRODUCTION
1.1 CERAMICS
1.2 COMPOSITES
1.2.1 MATRIX MATERIALS
1.2.2 REINFORCEMENT MATERIALS
1.2.3. CLASSIFICATION OF COMPOSITE MATERIALS
2. CERAMIC MATRIX COMPOSITES
2.1. TOUGHENING MECHANISMS in CMC
2.1.1 CRACK DEFLECTION
2.1.2 CRACK BRIDGING AND FIBER PULL OUT
2.1.3 MICROCRACKING
2.1.4 TRANSFORMATION TOUGHENING
3. REFERENCES
1
1. INTRODUCTION
1.1 CERAMICS
A compound of metallic and non-metallic elements prepared by the action of heat and
subsequent cooling. There are two general categories of ceramic;
 Traditional ceramics  tiles, brick, sewer pipe, pottery
 Industrial ceramics  turbine, semiconductors, cutting tools
The structure of the ceramics may be crystalline, partially crystalline or amorphous. In
general, the atoms in the ceramic are covalently or ionically bonded and the more strongly
metallic bonds. The hardness and thermal and electrical resistance in ceramics are better than
in metals.1
1.2 COMPOSITES
The word composite usually signifies that two or more separate materials are
combined on a macroscopic scale to form a structural unit for various engineering
applications. Each of the material components may have different thermal, mechanical,
electrical, magnetic, optical and chemical properties. It is noted that a composite composed of
an assemblage of these different materials gives us a useful new material whose performance
characteristics are superior to those of the constituent materials acting independently.
One or more of the material
components is usually
discontinuous, stiffer, and stronger
and known as the reinforcement;
the less stiff and weaker material is
continuous and called the matrix.
Sometimes, due to chemical
interactions or other processing
effects, there is an additional phase
matrix
which is called as interphase
between the reinforcement and the
reinforcement
fibres
matrix.2
Figure-13
2
matrix
reinforcement
composites
+
=
Figure-23
1.2.1 MATRIX MATERIALS
They are the constituents that are continuously distributed in a composite. The
functions of the matrix are:
 To conducting forces between fibers,
 Hold fibers in proper orientations,
 Protect fibers from the environment,
 Stop cracks from spreading between fibers
In order to effectively realize those functions, a desired matrix material should have
good ductility, high toughness and interlaminar shear strength, stable temperature properties,
and high moisture/environmental resistance.2 Examples of matrix materials are:
 Polymers:
epoxies,
Thermoset plastic, which is the material
that can be melted and shaped only once
polyesters,
phenolics,
silicone
nylon,
Thermoplastic, which is, in contrast, a
material that can be melted and shaped
over and over again
polyethelene,
polystyrene,
polycarbonate
 Metals:
steel, iron, aluminum, zinc, carbon, copper, nickel, silver, titanium,
and magnesium
 Ceramics:
alumina, silicon carbide, aluminum nitride, silicon nitride, zirconia
3
1.2.2 REINFORCEMENT MATERIALS
Reinforcing materials generally impart hardness and greatly prevent crack
propagation. In particular, they force the mechanical properties of the matrix, and in many
cases they are harder, stronger and stiffer than the matrix.2
Reinforcement can be divided into four categories and examples are shown below:
Flakes
mica, aluminum, and silver
Fillers
calcium carbonate, aluminum oxide, lime, fumed silica
Particulates
Fibers
lead, copper, tungsten, molybdenum, and chromium
glass, carbon, boron, aramid, silicon carbide fiber
Scanning electron microscope (SEM) images of composites reinforced with different
types of fibers: (a) carbon; (b) glass; and (c) steel are shown Figure-3.
Figure-34
1.2.3. CLASSIFICATION OF COMPOSITE MATERIALS
I. Based on the
type of matrix material
II. Based on the geometry of
reinforcing material
MMC
Fiber
PMC
Whisker
CCC
Flake
CMC
Particulate
4
2. CERAMIC MATRIX COMPOSITES
CMC consists of a ceramic matrix
and embedded fibers of other ceramic
material.
Advantages of CMC include high
strength and hardness at very high
temperature, high service temperature
limits for ceramics, low density, and
chemical inertness.
Figure-46
CMC materials the major disadvantages of conventional technical ceramics, namely brittle
failure and low fracture toughness, and limited thermal shock resistance. Therefore, the
applications of CMCs are in fields requiring reliability at high temperatures and resistance to
corrosion and wear.5
The purpose of developing the ceramic matrix composites is to improve the desirable
properties of ceramics with adding reinforcements and limiting their inherent weaknesses.
The development of CMCs imparts various improvements over ceramics such as:
Degree of anisotropy on incorporation of fibers
Increased fracture toughness
Elongation to rupture up to 1%
Ceramic matrix composites may be classified into two categories:
One is a group of toughened ceramics reinforced with particulates and whiskers. These
materials exhibit brittle behavior in spite of considerable improvements in fracture toughness
and strength. The maximum in fracture toughness is around 10 MPa.m1/2 or more.
The second consists of continuous-fiber composites exhibiting quasi-ductile fracture
behavior accompanied by extensive fiber pull out. The fracture toughness of this class of
materials can be higher than 20 MPa.m1/2 when produced with weak interfaces between the
fibers and matrix.
The increase in toughness in CMCs can be explained by energy dissipation mechanism
where fiber matrix debonding, crack deflection, fiber bridging and fiber pull-out are the
common failure mechanisms.6
5
2.1. TOUGHENING MECHANISMS in CMC
As fracture toughness of ceramic matrix composites is higher as compared to
monolithic ceramics, it is important to look at different toughening mechanisms to why and
how toughening is taking place due to fiber reinforcement.7
Toughening mechanisms in ceramic-matrix composites are:
 Crack Deflection
 Crack Bridging and Fiber Pull Out
 Microcracking
 Transformation Toughening
2.1.1 CRACK DEFLECTION
Crack deflection is the first
interface, which helps a crack deflect away
from its principal direction, such that a
large stress concentration on the fiber is
avoided. Whether a crack deflects at the
inter face depends on the conditions at the
interface, the mechanical properties of the
matrix and fiber, and the criteria for matrix
crack deflection. Crack deflection as it
reaches carbon fibers is shown Figure-5.
Figure-59
Deflection occurs when the interaction of the crack with the residual stress fields to
differences in the thermal expansion coefficients or elastic moduli between the matrix and
reinforcement.
Deflection toughening increase with increasing volume fraction of reinforcement and
increasing with increasing aspect ratio of reinforcement. Also deflection toughening increase
dominated by twisting rather than tilting of crack. Crack deflects and becomes non-planar,
due to interaction between the reinforcement and crack front.8
6
2.1.2 CRACK BRIDGING AND FIBER PULL OUT
Crack bridging is an important toughening mechanism in CMCs having a frictional
bonding at the interface. In ductile particle
reinforced CMC, there are several physical
scenarios for crack extension: the crack
may avoid the particles and propagate in
the matrix only, stress concentration can
induce plastic deformation of the particles
as well as partial or complete debonding at
the interfaces, plastically deforming
particles may form a bridging zone and the
crack be advancing by failure of the
stretched particles.
Figure-611
For the toughening by crack bridging to be effective it is necessary that the following
conditions be met:
 The elastic stiffness of the particle must be less than that of the matrix for the
crack to be attracted by the particle; otherwise the crack will avoid the particle and grow only
in the matrix
 There must be a sufficient number of particles for the toughening effect to occur
 There must be satisfactory bonding between the matrix and the particle to utilize
the ductility of the particles in the toughening process.
The reinforcement can be in terms of fibers or whiskers and these may be pulled out of
the matrix. Fibers have high transverse fracture toughness which causes failure along the
fiber/matrix interface due to pulling out of fibers from the matrix. After matrix cracks deflect
along the fiber/matrix interface and there is interfacial debonding, the fibers eventually
fracture at some point in the debonding region. Fibers are then pulled out from the matrix.
The most significant source of work in fracturing for most fiber composites is interfacial
frictional sliding. Depending on the interfacial roughness, contact pressure and the sliding
distance, this process can absorb large quantities of energy.10
7
2.1.3 MICROCRACKING
In order to be effective the
microcracking must occur only in response
to the stress field around the crack tip in
conjunction with residual stresses and be
restricted to small well-dispersed sites (to
avoid microcrack linkage). Thus
microcracks formed throughout the micro
structure.13 In Figure-7 SEM image
microcracks formed is shown.
Figure-712
2.1.4 TRANSFORMATION TOUGHENING
In simple terms, transformation toughening is the increase in fracture toughness of a
material that is the direct result of a phase transformation occurring at the tip of an advancing
crack. There are a number of essential requirements for successful transformation toughening:
First, there must be a metastable phase present in the material and the transformation
of this phase to more stable state must be capable of being stress-induced in the crack-tip
stress field. Second, the transformation must be virtually instantaneous and not require timedependent processes such as long-range diffusion. Third, it must be change shape and/or
volume. It is this latter feature the deviatoric character of the transformation that allows it to
be stress-induced. It also provides the source of the toughening because the work done by the
interaction of the crack-tip stresses and the transformation strains dissipates a portion of the
energy that would normally be available for crack extension.13
Transformation Toughening by ZrO2
ZrO2 is used commonly as a
biomaterial due to its mechanical strength,
as well as its chemical and dimensional
stability and elastic modulus similar to
stainless steel.
Figure-814
8
Depending on the temperature, zirconia can exist in three forms: Pure zirconia has a
cubic structure at temperatures greater than 2,370° C. The cubic phase has a cubic form with
square sides and moderate mechanical properties with a density of 6.27 g/cm2 . The tetragonal
phase exists at temperatures ranging from 1,170° C to 2,370° C. The tetragonal structure has a
straight prism with rectangular sides and the most satisfactory mechanical properties with a
density of 6.1 g/cm2 . The monoclinic phase occurs at temperatures below 1,170° C and has a
deformed parallelepipedonal (ie, a prism with six faces) shape, as well as the weakest
mechanical properties with a density of 5.6 g/cm2 13
In terms of strength, it is essential to limit the amount of the monoclinic phase because
of its lower density. To stabilize zirconia at room temperature and control phase
transformations, metal oxides, such as
yttria (Y2O3 ) or ceria (CeO2 ), are added to
the crystal structure. The addition of
"stabilizing oxides" yields multiphase
materials called partially stabilized
zirconia. Technically, if yttria is added for
stabilization, then it is referred to as yttriastabilized tetragonal polycrystals (Y-TZP).
Figure-915
A unique characteristic of zirconia is its ability to stop crack growth, which is
"transformation toughening ”. Stress-induced transformation toughening is shown Figure-10.
Figure-1016
The ZrO2 addition to Al2O3 increases its toughness via the tetragonal to monoclinic
ZrO2 transformation, so called transformation toughening and microcracking. The bright
zirconia can be inter- or intra-granular. Oxide powder mixtures are hot pressed or pressureless
sintered at <1600ºC.16
9
3. REFERENCES
1- https://slideplayer.com/slide/10815874/
2- 1. Qin, Q. And Ye, J. (2015). Toughening Mechanisms İn Composite Materials.
Cambridge, Uk: Woodhead Publishing, P.1.
3- https://www.amp-composite.com/le-composite/
4- https://www.researchgate.net/publication/263115824_microscale_finite_element_analys
is_of_stress_concentrations_in_steel_fiber_composites_under_transverse_loading
5- V. A. Lavrenko: Corrosion Of High-Performance Ceramics, Springer-Verlag, 1992 Isbn
3-540-55316-9
6- M. Balasubramanian(2013), Composite Materials And Processing, Crc Press, Isbn
1439880549, 9781439880548
7- Marcel Dekker,: R. H. Jones, Pp.391-418,Environmental Effects On Engineered
Materials, Corrosion Technology Series, Ed: (Corrosion Technology Series), Environmental
Effects On Engineered Materials,
8- I M Low (2018), Advances İn Ceramic Matrix Composite,Woodhead Publishing Series
İn Composites Science And Engineering, Woodhead Publishing Isbn 0081021674,
9780081021675
9- https://www.researchgate.net/figure/crack-deflection-as-it-reaches-carbonfibers_fig14_275234990
10- Michał Basista Witold Węglewski(2006), Modellıng Of Damage And Fracture In
Ceramıc Matrıx Composıtes, Ournal Of Theoretıcal And Applıed Mechanıcs 44, 3, Pp. 455484, Warsaw 2
11- https://www.researchgate.net/figure/dispersion-of-a-01-b-05-multi-walled-carbonnanotubes-mwcnt-in-glass-epoxy_fig7_323454974
12- R. Warren (1991), Ceramic-Matrix Composites, Springer Science & Business
Media,Isbn 0216926823, 9780216926820
13- Garvie Rc, Hannink Rhj, Pascoe Rt. Nature 1975:258:703.
14- https://www.slideshare.net/mohamedmahmoud443/zirconia-overview
15- T. R. Cooke (1991). "Inorganic Fibres- A Literature Review". Journal Of The American
Ceramic Society. 74: 2959–2978. Doi:10.1111/J1151-2916.1991.Tb04289.X
16- Kumagawa; H. Yamaoka; M Shibuysa; T. Ymamura (1998). "Fabrication And
Mechanical Properties Of New İmproved Si-M-C-(O) Tyranno Fiber". Ceramic Engineering
And Science Proceedings. 19a: 65–72. Doi:10.1002/9780470294482.Ch8.
10