Subject: Composite Materials Science and Engineering Subject code: 0210080060

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Prof C. H. XU

School of Materials Science and EngineeringHenan University of Science and Technology

Chapter 8:Ceramic Matrix Composites (CMCs)

Subject: Composite MaterialsScience and Engineering

Subject code: 0210080060

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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)

Matrix materials Alumina Glass Carbon

Reinforcement materials

SiC B4C

Carbon

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

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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)

Mix reinforcement and powdered matrix

in an appropriate solution

Improve dispersion by ultrasonic or agitation mixer

DryHeat to evaporate water

Slip cast

Hot press Cold press

sinter

Slurries (泥浆）Simplified flow sheet (流程图 ) 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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Slurries: for continuous fibre reinforced composite1 ） Fibers (glass fibers), impregnated with slurry (powder glass (1-50m) 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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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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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

Mix sol or gel with reinforcement

dry

heat to produce required ceramic Hot press

Pour sol over perform (reinforcement)

Dry sol

Repeat infiltration and dry until

required density

Fire

Infiltration of a preformMixing reinforcement in a sol

or a gele.g. ZrOCl2+NH3+3H2O=2NH4Cl+Zr(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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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 sv> 2sL.

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Review of selected CMCs- SiC reinforcement alumina

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

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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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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

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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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Young’s modulus of SiC-LAS composites is larger than monolithic LAS

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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 reinforcementsRoom 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 CompositesDense 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

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Review of selected CMCs- processing dense carbon-carbon composites

Manufacture a Preform

Manufacture Matrix (dense treatment)• Liquid phase process• Chemical vapour infiltration

Graphitization

Oxidation resistance treatment

Thermosetting resins, Pitchhydrocarbon

Continuous discontinues carbon fibers, mats

Preform

C-C composite (mesophase carbon matrix)

C-C composite (graphite matrix)

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 （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%) Raw materials - Pitch:

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;

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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

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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

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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

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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Schematic stress-strain curves illustrating the effects of the form of reinforcement on strength and toughness

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

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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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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 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

• Strength related to the density

• The properties are anisotropic.

• Fiber orientation takes place under vacuum during processing

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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 (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

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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 (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

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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 (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.

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Toughening mechanisms- Pull-out toughening

Maximum work to pull out a fibre is

Where D, lc and Tf are diameter, critical length and fracture strength of the fibre, respectively.

The energy of pull-out is greater than that of debonding.

fibreperlD

W cTfoutpull 16

(max)2

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

Stress distribution and microcrack formation around spherical

particles when (a) fmbfm,C and T for

compressive

and tensile stresses

Thermal stress forms between matrix and reinforcement during cooling, due to difference in coefficient of thermal expansion ().

f>m

Tangential compressive and a radial tensile stresses in matrix

Circumferential crack forms under high tensile stress.

f>m

Tangential tensile stress in matrix cause radial crack under high tensile stress.

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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 △Emro

1/2

Where vzirc is the volume fraction of metastable particles; △is unconstrained strain accompany the transformation; Em is young’s 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

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Subject: Composite Materials Science and Engineering Subject code: 0210080060