Subject: Composite Materials
Science and Engineering
Subject code: 0210080060
Prof C. H. XU
School of Materials Science and Engineering
Henan University of Science and Technology
Chapter 8:
Ceramic Matrix Composites (CMCs)
1
Ceramic Matrix Composites (CMCs)
This chapter will cover
 Introduction to CMCs
 Fabrication of CMCs
 Review of selected CMCs
 Toughening mechanisms
2
Introduction to Ceramic Matrix
Composites (CMCs)
 Ceramics: high
strength, stiffness,
and brittle
 Objective for CMCs is
to increase in the
toughness
 Use and fabricate
CMCs at high
temperature
 Less reinforcements
are available
Schematic force-displacement
curves for a monolithic and
CMCs, illustrating the greater
energy of fracture of the CMCs
3
Introduction to Ceramic Matrix
Composites (CMCs)
materials
Diamond (carbon)
Knoop
hardness
7000
Boron carbide (B4C)
2800
Silican carbide (SiC)
2500
Reinforcement
materials
Tungsten carbide (WC)
2100
 SiC
Aluminum oxide (Al2O3)
2100
Quartz (SiO2)
800
Glass
550
Matrix materials
 Alumina
 Glass
 Carbon
 B4C
 Carbon
4
Processing
Ceramic Matrix Composites (CMCs)
 Conventional mixing and pressing
 (a) A powder of the matrix is mixed with
reinforcement (particles or whiskers) together
with a binder
 (b) Pressure
 (c) Fire or hot pressure
 Difficulty during fabrication
 Difficult to obtain uniform mixture
 Damage to whiskers during mixing and
pressing operations
5
Processing
Ceramic Matrix Composites (CMCs)
Slurries (泥浆）
Simplified flow sheet (流程
图) for mixing (whiskers or
chopped fibers) as a slurry
prior to shaping
Mix reinforcement
and powdered matrix
in an appropriate solution
Improve dispersion by
ultrasonic or agitation mixer
Dry
Heat to evaporate water
Hot press
The properties of CMCs produced
by slurries is not good because of
more porosity in materials
Slip cast
Cold press
sinter
6
Processing
Ceramic Matrix Composites (CMCs)
Slurries: for continuous fibre reinforced composite
1）Fibers (glass fibers),
impregnated with slurry
(powder glass (1-50mm) in
water and water soluble resin
binder), are wound on to a
mandrel to form a tape.
2) The tape is cut into pies.
3) The types are stacked
(lay-up).
4) Burnout of the binder
5) Heat pressure
e.g. glass fiber reinforced
glass-ceramic matrix)
7
Processing
Ceramic Matrix Composites (CMCs)
Liquid State Processing
 Matrix transfer molding: glass matrix composite
production CMCs with tube
shape
1): SiC cloth (reinforcement)
and glass slug (matrix)
plunge in a cylinder
2) Heat to melt glass, press
liquid and inject in SiC cloth
3) Eject the mandrel and
cylinder
e.g. SiC reinforced glassceramic (polycrystalline
8
structure) matrix
Processing
Ceramic Matrix Composites (CMCs)
 Sol-gel (溶胶-凝胶) processing


sol: dispersion of small particles of less than 100 nm, obtained
by precipitation (沉淀) resulting from a reaction solution
Gel: sol lost some liquid to increase viscosity
Pour sol over perform
(reinforcement)
Repeat infiltration
and dry until
required density
Dry sol
Mix sol or gel with
reinforcement
dry
heat to produce
required ceramic
Hot press
Fire
Infiltration of a preform
e.g. ZrOCl2+NH3+3H2O=2NH4Cl+Zr(OH)4
Zr(OH)4 → ZrO2 at 550℃
Mixing reinforcement in a sol
or a gel
9
Processing
Ceramic Matrix Composites (CMCs)
 Vapor deposition techniques
e.g. TiCl4(g)+2BCl3(g)+5H2(g) = TiB2(s) + 10 HCl(g)
SiCl4(g) + CH4(g) = SiC (s) + 4HCl(g)
10
Processing
Ceramic Matrix Composites (CMCs)
Lanxide process
 Formation of a ceramic matrix by
the reaction between a molten
metal and a gas (e.g. molten
aluminum reacting with oxygen to
form alumina)
 growth rate is
 parabolic when the diffusion of
liquid metal controls the
process.
 linear when chemical reaction
at preform and infiltrated
preform controls process; In
this case, liquid metal diffuses
rapidly by a wicking (灯芯的)
process along grain
boundaries in ceramic matrix
when gsv> 2gsL.
11
Review of selected CMCs
- SiC reinforcement alumina
Usually made by slurry method (SiC whisker
and polycrystalline a-alumina)
12
Review of selected CMCs
- SiC reinforcement alumina
 left fig. showing: Improvement in
toughness due to SiC whiskers in
alumina matrix at various temperature
 Right fig. showing: Log-log plot of strain
rate versus stress showing that the creep
rate at a given stress is less for the SiC
reinforced alumina
13
Review of selected CMCs
- SiC reinforcement alumina
SiC whisker reinforced
alumina has good
thermal shock (热冲
击) resistance. The
reasons are
 lowers the coefficient
of thermal expansion;
 Increase the thermal
conductivity;
 Improves the
toughness;
Thermal shock behaviors of an alumina-20vol%SiC whisker composite
and alumina; cooling materials from high T to room T in water 14
Review of selected CMCs
- Zirconia-toughened alumina
 Zirconia, ZrO2, -toughened alumina (ZTA) contains
reinforcement (10-20vol% of fine Zirconia) and matrix
(alumina).
 ZrO2 Crystal:
Tetragonal (T) at
high temperature
 Monoclinic (M) at
low temperature
 T→M transformation
during cooling causes
an increase in 3%
volume, producing
microcrack in Al2O3
matrix.
 Microcracks absorb
energy to improve
toughness of composite

15
Review of selected CMCs
- Zirconia-toughened alumina
 Add stabilizing oxide,
such as 3mol.% Y2O3
to ZrO2 suppress t→m
transformation during
cooing.
 Fine metastable
tetragonal-ZrO2 at
room temperature in
ZTA
 ZrO2 particles at a
crack tip will transfer to
monoclinic-ZrO2 under
stress, which is called
as transformation
toughing.
16
Review of selected CMCs
- Glass-ceramic matrix composites
 Glass-ceramics: some glass with crystal structure
 E.g. lithium aluminosilicate (LAS) system
 Working temperature:
LAS-I 1000℃;
LAS-II 1100℃;
LAS-III 1200℃;
17
Review of selected CMCs
- Glass-ceramic matrix composites
Young’s modulus of SiC-LAS composites is larger than monolithic LAS
18
Review of selected CMCs
- Glass-ceramic matrix composites
Composites have
higher strength
than that of
monolithic LAS
 Elastic
deformation at
beginning (linear
curves)
 Matrix plastic
deformation and
reinforcement
elastic
deformation.
 Reinforcements
break from point
F
19
Review of selected CMCs
- Glass-SiC reinforcements
Room temperature Toughness of LAS-SiC composite
Vol% SiC
0
K1C (MPam1/2)
1.5
LSA-I
50 (unidirectional)
17
LSA-II
50 (Cross-plied)
10
LAS
20
Review of selected CMCs
- Glass-SiC reinforcements
 The properties
of composite
maintained to
1000℃ in inert
atmosphere.
 The properties
of composite
reduced from
800 ℃ in air.
Oxygen
diffuses along
microcracks in
the matrix and
reacts with
SiC.
21
 Unidirectional
reinforcement glass matrix
composite has
better fatigue
properties
 Cross-plied
reinforcement
glass matrix
composite has
less fatigue
properties.
22
Review of selected CMCs
- Carbon – Carbon Composites
Dense carbon-carbon composites
 Continuous fiber materials


the good mechanical properties of the better quality of
fiber
Produce a materials with a desired degree of
anisotropy (各向异性)
 Discontinuous fiber materials
 Being used to fabricate large components
 produce isotropic materials and improve inter-laminar
strength
 Applications: disc brakes for racing car and aircraft,
gas turbine components, nose cones and leading
edges for missiles
23
Review of selected CMCs
- processing dense carbon-carbon composites
Manufacture
a Preform
Continuous discontinues
carbon fibers, mats
Preform
Manufacture Matrix (dense treatment)
• Liquid phase process
• Chemical vapour infiltration
Thermosetting resins, Pitch
hydrocarbon
C-C composite
(mesophase carbon matrix)
Graphitization
C-C composite (graphite matrix)
Oxidation resistance
treatment
C-C composite with
a protective layer
24
Dense carbon-carbon composites
-Manufacture a reinforcement preform
Continuous and discontinues carbon fibers, mat
Reinforcement preform
25
Dense carbon-carbon composites
-Manufacture matrix
 Liquid phase processing

Raw materials - thermosetting resins (phenolic, furan, polymide,
polyenylene) :




Raw materials - Pitch:





Impregnation （注入） thermosetting resins in a reinforcement
preform
~ Polymerize at 250℃ to form cross-link polymer
Pyrolysis （高温分解）and carbonization at 600~1000℃ to form
amorphous, isotropic carbon (carbon yield about 45~80%)
Impregnation pitch in a reinforcement preform
thermoplastic polymer in nature
Pyrolysis and carbonize at 600~1000℃ to form a highly orientated
mesophase carbon; carbon yield about 50% under normal pressure
and up to 90% under high pressure
Each cycle needs about 3 days.
multiple impregnation and carbonization to obtain high density;
26
Dense carbon-carbon composites
-Manufacture matrix
 Chemical vapor infiltration (CVI) , also called as chemical
vapor deposition: thermal decomposition of hydrocarbon,
such as methane CH4(g) = C(s) + 2H2(g) under suitable
temperature and pressure
• Laminar aromatic
(芬芳的)
• Layered pyrolitic
carbon
• Isotropic sooty (乌黑
的)
• surface nucleated
dense pyrolitic
graphite
• continuously
nucleated graphite
27
Dense carbon-carbon composites
-Manufacture matrix
 Isothermal method:
The infiltration (渗透): under low pressure of 0.6 ~ 6 MPa at a
constant temperature of 1100℃.
 Problem: form an impermeable crust (外壳)
 The crust must be removed by a machine to remain continuous
infiltration.
 Thermal gradient method:
 The infiltration carried
out under atmosphere
pressure at a inner
temperature of 1100℃.
 The inner of sample
was heated by induction
coil.
 Pressure gradient method:
 Gas is forced into the
interior of samples

28
Dense carbon-carbon composites
- graphitization and coating
 Graphitization: heat treatment at high temperature up to 1500~2800℃ to
obtain graphite matrix
 coating
 In order to Improve oxidation resistance of composite


Coating must be satisfy





A coating system capable of offering protection up to 1400℃ currently;
Mechanically, chemically and thermally compatible with the composite
Adhere to the composite
Prevent diffusion of oxygen from the environment through to the
composite
Prevent diffusion of carbon from the composite to the environment
Complex protective systems



Large differences in the coefficient of thermal expansion (CTE) between
coating layer and composite during cooling lead to cracking of coating
and loss of oxidation protection.
SiC and Si3N4 as primary oxidation barrier coat, based on CTE.
Second protective system: add a glass former particles in to matrix to
form glass phase or having an additional glass coating.
29
Dense carbon-carbon composites
- Properties
The effects of different carbon matrix on the properties of C-C composite
30
Dense carbon-carbon composites
- Properties
1-D (one dimensional woven carbon fibre
reinforced composite) is strong but brittle.
2-D (two dimensional woven carbon fibre
reinforced composite) has properties
intermediate to those of the 1-D and 3-D
3-D (three dimensional woven carbon fibre
reinforced composite) has better toughness
and less strength
The low toughness of 1-D composite is
attributed to the poor interlaminar
properties
Schematic stress-strain curves illustrating the effects of the form of
reinforcement on strength and toughness
31
Dense carbon-carbon composites
- Properties
Comparison of the
fatigue
performance of
carbon fiber
reinforced carbon
composite and
carbon fiber
reinforced polymer
composite: (a)
torsion; (b) flexural
• Fatigue property
of CFRC is similar
to CFRP
32
Dense carbon-carbon composites
- Properties
Specific strength
versus
temperature for
ACC: made using
woven carbon cloth;
RCC: produced
from low modulus
fiber;
High strength C-C:
made with
unidirectional
carbon fibers
interplied with
woven cloth
33
Review of selected CMCs
- Porous carbon – carbon Composites
Porous carbon-carbon composites, also called as carbon
bonded carbon fibres (CBCF)
 Processing:






A mixture including carbon fiber, phenolic resin (binder),
and water;
The mixture pumped into a mould;
Water extracted under vacuum and dry
Carbonization at ~950℃, carbon yield about 50%,
Graphite at high temperature to obtain 99.9% carbon.
porosity contents are in the range 70-90%
 Application of CBCF as insulation at high temperature
under vacuum (no oxygen) or at the temperature less
than 400℃
34
Review of selected CMCs
- Porous carbon – carbon Composites
• Strength related to
the density
• The properties are
anisotropic.
• Fiber orientation
takes place under
vacuum during
processing
Strength of carbon bonded carbon fiber
as a function of density and orientation.
Z and X/Y denote the direction of the
tensile stress in the bend test
35
Toughening mechanisms
- Introduction
 There are many different toughening mechanisms.
 One or more toughening mechanisms may operative
in a composite.
 The effectiveness of the toughening mechanisms
depends on:





Size, morphology and volume fraction of the
reinforcement;
Interfacial bond;
Properties (e. g. mechanical, thermal expansion) of the
matrix and the reinforcement;
Phase transformation
……
36
Toughening mechanisms
- crack bowing (弓)
Crack bowing
 (a) Crack approaches to
reinforcements.
 (b) the crack bowed under
stress to form a nonlinear crack
front.
 Decrease in the stress
intensity K along the bowed
section in the matrix
 Increase in the stress
intensity K at the
reinforcement
 K reached to the fracture
toughness of the
reinforcement → the
reinforcement breaks
 Bowing needs more energy to
increase toughness
37
Toughening mechanisms
- crack bowing
Crack bowing
toughing ↑
• with ↑
the volume
fraction of
reinforcement (more
reinforcements)
• with ↑ aspect ratio of
the reinforcement
• with ↑ the properties
of reinforcement
38
Toughening mechanisms
- Crack deflection (偏斜, 偏转)
 Crack deflects and becomes
non-planar, due to interaction
between the reinforcement and
crack front.
 (a) Tilt (倾斜)of crack front
 (b) Twist (扭) of crack front
 There are 3 crack modes
 Flat crack propagates in
mode I.
 Tilt crack in modes I and II
 Twist crack in modes I and III
39
Toughening mechanisms
- Crack deflection
 Deflection occurs when the
interaction of the crack with
the residual stress fields
due to differences in the
thermal expansion
coefficients or elastic
moduli between the matrix
and reinforcement.
 Deflection toughening ↑:
 with↑volume fraction


of reinforcement
With ↑ aspect ratio of
reinforcement
Dominated by
twisting rather than
tilting of the crack
40
Toughening mechanisms
- Debonding toughening
 Debonding: Reinforcement fibre separates from
matrix.
 Debonding toughening: New surface in the
composite require energy in debonding.
 Debonding toughening ↑



Weak interface of matrix and reinforcement
Strong reinforcement
Large volume fraction of reinforcement.
41
Toughening mechanisms
- Pull-out toughening
 Pull out a fibre
 Pull-out
 Debonding
 Fibre fracture for long fiber
 The normal (法线）frictional
forces have to be overcome
during pull-out.
 The maximum pull-out
length of a fibre is ½ the
critical length (lc).
 If embedded length is
greater than lc. fibre will
break.
42
Toughening mechanisms
- Pull-out toughening
 Maximum work to pull out a
fibre is
W pull out (max)
D 2s Tf lc
16
per fibre
Where D, lc and sTf are
diameter, critical length and
fracture strength of the fibre,
respectively.
 The energy of pull-out is
greater than that of
debonding.
Pulling a fibre out of the matrix
43
Toughening mechanisms
- Fibre bridging toughening
 Fibre bridging: some fibres
debonds but not break.
 Fibres carry out stresses
under load.
 Reduce the stresses at crack
tip and hinder crack
propagation.
 Toughness-crack extension
curve:


Toughness increase with
crack extension at initial
cracking
Constant toughness
maintains when crack
reaches to critical value.
44
Toughening mechanisms
- Microcrack toughening
 Thermal stress forms between
matrix and reinforcement
during cooling, due to
difference in coefficient of
thermal expansion (a).
 af>am


Tangential compressive and a
radial tensile stresses in matrix
Circumferential crack forms
under high tensile stress.
 af>am
Tangential tensile stress in matrix
cause radial crack under high
tensile stress.
Stress distribution and microcrack
formation around spherical
particles when (a) af>am, (b)
af<am,C and T for compressive
and tensile stresses
45
Toughening mechanisms
- microcrack toughening
 The toughness of a materials can be
enhanced by the presence of microcracks,
due to crack blunting, branching and
deflection.
 The microcrack toughening is effective on the
limited density and size of cracks.
 Toughness of materials increases and
strength decreases in the microcrack
toughening.
46
Toughening mechanisms
- Transformation toughening
 Metastable tetragonal-ZrO2 at
room temperature in ZTA
 transformation toughing: ZrO2
particles at a crack tip will transfer
to monoclinic-ZrO2 under stress.
Energy is absorbed ahead of the
primary crack owing to the
transformation.
 Giving an increase in toughness
△KTT = 0.3vzirc △eEmro1/2
Where vzirc is the volume fraction of
metastable particles; △e is
unconstrained strain accompany the
transformation; Em is young’s alumina
Transformation toughening:
matrix and ro is the width of zone in
transformation of metastable
the crack.
 Strength and toughness of materials particles at the crack tip gives a Zone,
increase at same time.
of width ro, of transformed particles
47
Further Reading:
Text Book:
Composite Materials: Engineering and Science
(pages118-160, 326-356).
Reference book:
Introduction to Materials (page 241-283)
Other reference:
Lecture note 8
48