Prof C. H. XU

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Subject Composite MaterialsScience 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)

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Ceramic Matrix Composites (CMCs)

• This chapter will cover
• Introduction to CMCs
• Fabrication of CMCs
• Review of selected CMCs
• Toughening mechanisms

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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
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Introduction to Ceramic Matrix Composites (CMCs)
materials Knoop hardness
Diamond (carbon) 7000
Boron carbide (B4C) 2800
Silican carbide (SiC) 2500
Tungsten carbide (WC) 2100
Aluminum oxide (Al2O3) 2100
Quartz (SiO2) 800
Glass 550

• Matrix materials
• Alumina
• Glass
• Carbon
• Reinforcement materials
• SiC
• B4C
• Carbon

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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

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Processing Ceramic Matrix Composites (CMCs)
Slurries (??) Simplified flow sheet (???) for
mixing (whiskers or chopped fibers) as a slurry
prior to shaping
The properties of CMCs produced by slurries is
not good because of more porosity in materials
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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)
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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 glass-ceramic
(polycrystalline structure) matrix
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Processing Ceramic Matrix Composites (CMCs)

• Sol-gel (??-??) processing
• sol dispersion of small particles of less than
  100 nm, obtained by precipitation (??) resulting
  from a reaction solution
• Gel sol lost some liquid to increase viscosity

Pour sol over perform (reinforcement)
Mix sol or gel with reinforcement
Repeat infiltration and dry until required density
dry
heat to produce required ceramic
Dry sol
Hot press
Fire
Mixing reinforcement in a sol or a gel
Infiltration of a preform
e.g. ZrOCl2NH33H2O2NH4ClZr(OH)4 Zr(OH)4 ?
ZrO2 at 550?
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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)
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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
  (???) process along grain boundaries in ceramic
  matrix when gsvgt 2gsL.

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Review of selected CMCs- SiC reinforcement
alumina

• Usually made by slurry method (SiC whisker and
  polycrystalline a-alumina)

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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

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Review of selected CMCs- SiC reinforcement
alumina

• SiC whisker reinforced alumina has good thermal
  shock (???) resistance. The reasons are
• lowers the coefficient of thermal expansion
• Increase the thermal conductivity
• Improves the toughness

Thermal shock behaviors of an alumina-20volSiC
whisker composite and alumina cooling materials
from high T to room T in water
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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

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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.

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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?

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Review of selected CMCs- Glass-ceramic matrix
composites
Youngs modulus of SiC-LAS composites is larger
than monolithic LAS
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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

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Review of selected CMCs- Glass-SiC reinforcements

• Room temperature Toughness of LAS-SiC composite

Vol SiC K1C (MPam1/2)
LAS 0 1.5
LSA-I 50 (unidirectional) 17
LSA-II 50 (Cross-plied) 10
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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.

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• Unidirectional reinforcement -glass matrix
  composite has better fatigue properties
• Cross-plied reinforcement glass matrix composite
  has less fatigue properties.

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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 (????)
• 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

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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
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Dense carbon-carbon composites -Manufacture a
reinforcement preform
Continuous and discontinues carbon fibers, mat
Reinforcement preform
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Dense carbon-carbon composites -Manufacture
matrix

• Liquid phase processing
• Raw materials - thermosetting resins (phenolic,
  furan, polymide, polyenylene)
• Impregnation (??) thermosetting resins in a
  reinforcement preform
• Polymerize at 250? to form cross-link polymer
• Pyrolysis (????)and carbonization at 6001000? to
  form amorphous, isotropic carbon (carbon yield
  about 4580)
• Raw materials - Pitch
• Impregnation pitch in a reinforcement preform
• thermoplastic polymer in nature
• Pyrolysis and carbonize at 6001000? 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

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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 (???)
• Layered pyrolitic carbon
• Isotropic sooty (???)
• surface nucleated dense pyrolitic graphite
• continuously nucleated graphite

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Dense carbon-carbon composites -Manufacture
matrix

• Isothermal method
• The infiltration (??) under low pressure of 0.6
  6 MPa at a constant temperature of 1100?.
• Problem form an impermeable crust (??)
• 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

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Dense carbon-carbon composites - graphitization
and coating

• Graphitization heat treatment at high
  temperature up to 15002800? to obtain graphite
  matrix
• coating
• In order to Improve oxidation resistance of
  composite
• A coating system capable of offering protection
  up to 1400? currently
• Coating must be satisfy
• 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.

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Dense carbon-carbon composites - Properties
The effects of different carbon matrix on the
properties of C-C composite
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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
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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

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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
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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?

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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
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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

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Toughening mechanisms- crack bowing (?)

• 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

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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

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Toughening mechanisms- Crack deflection (??, ??)

• Crack deflects and becomes non-planar, due to
  interaction between the reinforcement and crack
  front.
• (a) Tilt (??)of crack front
• (b) Twist (?) 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

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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

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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.

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Toughening mechanisms- Pull-out toughening

• Pull out a fibre
• Pull-out
• Debonding
• Fibre fracture for long fiber
• The normal (??)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.

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Toughening mechanisms- Pull-out toughening

• Maximum work to pull out a fibre is
• 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
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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.

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Toughening mechanisms- Microcrack toughening

• Thermal stress forms between matrix and
  reinforcement during cooling, due to difference
  in coefficient of thermal expansion (a).
• afgtam
• Tangential compressive and a radial tensile
  stresses in matrix
• Circumferential crack forms under high tensile
  stress.
• afgtam
• Tangential tensile stress in matrix cause radial
  crack under high tensile stress.

Stress distribution and microcrack formation
around spherical particles when (a) afgtam, (b)
afltam,C and T for compressive and tensile
stresses
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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.

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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 youngs alumina matrix
  and ro is the width of zone in the crack.
• Strength and toughness of materials increase at
  same time.

Transformation toughening transformation of
metastable particles at the crack tip gives a
Zone, of width ro, of transformed particles
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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