Reinforcement-Matrix InterfaceReinforcement-Matrix Interface The load acting on the matrix has to be transferred The load acting on the matrix has to be transferred

to the reinforcement via. Interfaceto the reinforcement via. Interface The reinforcement must be strongly bonded to the The reinforcement must be strongly bonded to the

matrix if high stiffness and strength are desired in matrix if high stiffness and strength are desired in the composite materialsthe composite materials

A weak interface results in low stiffness and A weak interface results in low stiffness and strength but high resistance to fracturestrength but high resistance to fracture

A strong interface produces high stiffness and A strong interface produces high stiffness and strength but often low resistance to fracture, i.e. strength but often low resistance to fracture, i.e. brittle behaviorbrittle behavior

WettabilityWettability Is defined the extent where a liquid will spread Is defined the extent where a liquid will spread

over a solid surfaceover a solid surface During the manufacturing process, the matrix is During the manufacturing process, the matrix is

often in the condition where it is capable of flowing often in the condition where it is capable of flowing or its behavior is like a liquidor its behavior is like a liquid

Good wettability means that the liquid (matrix) will Good wettability means that the liquid (matrix) will flow over the reinforcement, covering every ‘bump’ flow over the reinforcement, covering every ‘bump’ and ‘dip’ of the rough surface of reinforcement and and ‘dip’ of the rough surface of reinforcement and displacing all air.displacing all air.

Wetting will only occur if the viscosity of the Wetting will only occur if the viscosity of the matrix is not too high.matrix is not too high.

Interfacial bonding exists due to the Interfacial bonding exists due to the adhesion between the reinforcement and adhesion between the reinforcement and the matrix (wetting is good)the matrix (wetting is good)

WettabilityWettability

Drops of water on a hydrophobic surface

WettabilityWettability

Good or poor wettability?

WettabilityWettability

Let us consider a thin film of liquid (matrix) Let us consider a thin film of liquid (matrix) spreading over a solid (reinforcement) surfacespreading over a solid (reinforcement) surface

Figure Figure

WettabilityWettability

All surfaces have an associated energy All surfaces have an associated energy and the free energy per unit area of the and the free energy per unit area of the solid-gas, liquid-gas and solid-liquid are solid-gas, liquid-gas and solid-liquid are γSG, γLG dan γSL, respectively.γSG, γLG dan γSL, respectively.

γSG = γLG cos θ + γSLγSG = γLG cos θ + γSL θ is called the contact angle. May be used θ is called the contact angle. May be used

as a measure of the degree of the as a measure of the degree of the wettabilitywettability

WettabilityWettability

cos θ = (γSG – γSL)/ γLGcos θ = (γSG – γSL)/ γLG

If θ = 180º, the drop is spherical, no wetting takes If θ = 180º, the drop is spherical, no wetting takes placeplace

θ = 0, perfect wettingθ = 0, perfect wetting 0º<θ<180º, the degree of wetting increases as θ 0º<θ<180º, the degree of wetting increases as θ

decreases.decreases. Often it is considered that the liquid does not wet Often it is considered that the liquid does not wet

the solid if θ>90º the solid if θ>90º

These three quantities determine whether the liquid These three quantities determine whether the liquid spreads over the solid, or not; whether it "wets" it.spreads over the solid, or not; whether it "wets" it.

This is judged by the contact angle, .This is judged by the contact angle, .

  

Drops of water on a textile surfacebefore and after addition of wetting agent

                                 

Soalan 2002/2003Soalan 2002/2003 Kenalpasti dengan menggunakan kaedah Kenalpasti dengan menggunakan kaedah

pengiraan untuk menentukan samada gentian pengiraan untuk menentukan samada gentian alumina boleh digunakan sebagai bahan alumina boleh digunakan sebagai bahan tetulang dalam resin epoksi dan polietilena. Di tetulang dalam resin epoksi dan polietilena. Di dapati tenaga antara muka bagi resin epoksi dapati tenaga antara muka bagi resin epoksi ialah 40 mJ/m2 dan polietilena ialah 30 mJ/m2, ialah 40 mJ/m2 dan polietilena ialah 30 mJ/m2, sementara bagi gentian alumina ialah 1100 sementara bagi gentian alumina ialah 1100 mJ/m2. Andaikan tenaga permukaan bagi mJ/m2. Andaikan tenaga permukaan bagi alumina dengan epoksi ialah 1071.7 mJ/m2 alumina dengan epoksi ialah 1071.7 mJ/m2 manakala bagi alumina dengan polietilena ialah manakala bagi alumina dengan polietilena ialah 1105.21 mJ/m21105.21 mJ/m2

2 types of failure at interface2 types of failure at interface

Difficult to measure the strength of interface, Difficult to measure the strength of interface, this is because sometimes failure occur this is because sometimes failure occur interface, and sometimes notinterface, and sometimes not

2 types of failure at interface2 types of failure at interface

1) Adhesive failure - failure occur at 1) Adhesive failure - failure occur at interfaceinterface

2) Cohesive failure – failure occur close to 2) Cohesive failure – failure occur close to the interface (either at the fiber or matrix)the interface (either at the fiber or matrix)

Factors leading to good polymer-Factors leading to good polymer-filler bondingfiller bonding

Once the matrix has wet the Once the matrix has wet the reinforcement, bonding will occurreinforcement, bonding will occur

For a given system, more than one For a given system, more than one bonding mechanism may exist at the bonding mechanism may exist at the same timesame time

The bondings may change during The bondings may change during various production stages or during various production stages or during servicesservices

Interfacial bondingInterfacial bonding

Types of interfacial bonding at Types of interfacial bonding at interfaceinterface

1)1) Mechanical bondingMechanical bonding

2)2) Electrostatic bondingElectrostatic bonding

3)3) Chemical bondingChemical bonding

4)4) Reaction or interdiffusion bondingReaction or interdiffusion bonding

Mechanical bondingMechanical bonding-Mechanical interlocking or keying of two interfaces can leads to reasonable bond-The rougher the interface, the interlocking is Greater, hence the mechanical bonding is effective

Mechanical bonding is effective when the Mechanical bonding is effective when the force is applied parallel to the interfaceforce is applied parallel to the interface

If the interface is being pulled apart by If the interface is being pulled apart by tensile forces, the strength is likely to be low tensile forces, the strength is likely to be low unless there is a high density of features unless there is a high density of features (designated A)(designated A)

Electrostatic BondingElectrostatic Bonding

-Occur when one surface is positively charged and the other is negatively charge (refer to the above figure)-Interactions are short range and only effective over small distances of the order of atomic dimensions-Surface contamination and entrapped gases will decrease the effectiveness of this bonding

Chemical bondingChemical bonding

The bond formed between chemical groups The bond formed between chemical groups on the reinforcement surfaces (marked X) on the reinforcement surfaces (marked X) and compatibleand compatible groupsgroups (marked R) in the (marked R) in the matrixmatrix

Strength of chemical bonding depends on Strength of chemical bonding depends on the number of bonds per unit area and the the number of bonds per unit area and the type of bondtype of bond

Chemical bonding normally exist due to the Chemical bonding normally exist due to the application of coupling agentsapplication of coupling agents

For example, silanes are commonly For example, silanes are commonly employed for coupling the oxide group employed for coupling the oxide group groups on a glass surfaces to the molecules groups on a glass surfaces to the molecules of the polymer matrixof the polymer matrix

At one end (A) of the silane molecule, a At one end (A) of the silane molecule, a hydrogen bond forms between the oxide hydrogen bond forms between the oxide (silanol) groups on the glass and the (silanol) groups on the glass and the partially hydrolyzed silane, whereas at the partially hydrolyzed silane, whereas at the other end (B) it reacts with a compatible other end (B) it reacts with a compatible group in polymer.group in polymer.

Effect of Silane Coupling Agents on Effect of Silane Coupling Agents on the properties of Silver (Ag)-epoxy the properties of Silver (Ag)-epoxy

compositescomposites

To improve interaction between filler and To improve interaction between filler and polymer, by modifying filler surfacespolymer, by modifying filler surfaces

Used in low concentration (e.g. 0.1%), Used in low concentration (e.g. 0.1%), silane coupling agent- give rise to significant silane coupling agent- give rise to significant improvements in mechanical propertiesimprovements in mechanical properties

Silver (Ag) filled epoxy composites; Silver (Ag) filled epoxy composites; with the addition of silane coupling with the addition of silane coupling

agent (3APTES)agent (3APTES)

Flexural Properties of Treated and Untreated Ag/Epoxy CompositesFlexural Properties of Treated and Untreated Ag/Epoxy Composites

0

1000

2000

3000

4000

5000

6000

4 5Concentrantion of Ag Nanoparticles, V f (vol.%)

Flex

ural

Mod

ulus

, EB

(MPa

)

Untreated Ag Filled Epoxy Treated Ag Filled Epoxy

0

20

40

60

80

100

120

140

160

180

4 5Concentration of Ag Nanoparticles, V f (vol.%)

Flex

ural

Stre

ss,

Sf (

MPa

)

Untreated Ag Filled Epoxy Treated Ag Filled Epoxy

Silver (Ag) filled epoxy composites; with the addition Silver (Ag) filled epoxy composites; with the addition of silane coupling agent (3APTES)of silane coupling agent (3APTES)

After surface treatment of Ag, the After surface treatment of Ag, the dispersivity of Ag nanoparticles in epoxy dispersivity of Ag nanoparticles in epoxy system is remarkably improved.system is remarkably improved.

(a). 5 vol.% of untreated system

(b). 5 vol.% of treated system

155× 155×

Light microscopy micrographs reveal the degree of dispersivity Ag in epoxy matrixbefore and after chemical treatment of Ag

Silver (Ag) filled epoxy composites; with the addition Silver (Ag) filled epoxy composites; with the addition of silane coupling agent (3APTES)of silane coupling agent (3APTES)

Reaction or interdiffusion bondingReaction or interdiffusion bonding

-The atoms or molecules of the two components may interdiffuse at the interface- For interfaces involving polymer, this type of bonding can be considered as due to the intertwining of molecules

For system involving metals & ceramics, the interdiffusion of For system involving metals & ceramics, the interdiffusion of species from the two components can produce an interfacial species from the two components can produce an interfacial layer of different composition and structure from either of the layer of different composition and structure from either of the componentcomponent

The interfacial layers also will have different mechanical The interfacial layers also will have different mechanical properties from either matrix or reinforcementproperties from either matrix or reinforcement

In MMC, the interfacial layer is often a brittle In MMC, the interfacial layer is often a brittle intermetallic intermetallic compoundcompound

One of the main reason why interfacial layers are formed is in One of the main reason why interfacial layers are formed is in ceramic and metal matrices is due to the processing at high ceramic and metal matrices is due to the processing at high temperature- diffusion is rapid at high temp; according to the temperature- diffusion is rapid at high temp; according to the Arrhenius equation)Arrhenius equation)

Methods for measuring bond Methods for measuring bond strengthstrength

Single fiber testSingle fiber test Fiber pull-out test (a)Fiber pull-out test (a) Involves pulling a partially embedded Involves pulling a partially embedded

single fiber out of a block of matrix single fiber out of a block of matrix materialmaterial

Difficult to be carried out especially for Difficult to be carried out especially for thin brittle fiberthin brittle fiber

Fiber pull-out test (a)Fiber pull-out test (a)

Fiber pull-out test (a)Fiber pull-out test (a)

From the resulting tensile stress vs. strain From the resulting tensile stress vs. strain plot, the shear strength of the interface and plot, the shear strength of the interface and the energy of debonding and pull-out may the energy of debonding and pull-out may be obtainedbe obtained

Compression test fot interfacial shear strength Compression test fot interfacial shear strength (b)(b)

The interfacial shear strength (The interfacial shear strength (ζζ11) may be ) may be

evaluated using a specimen consisting of a block evaluated using a specimen consisting of a block of matrix materials with a single, embedded short of matrix materials with a single, embedded short fiber with accurately aligned longitudinal in a fiber with accurately aligned longitudinal in a center of the specimen (b)center of the specimen (b)

On testing in compression, shear stresses are set On testing in compression, shear stresses are set up at the ends of the fibers as a consequence of up at the ends of the fibers as a consequence of the difference in elastic properties of the fiber and the difference in elastic properties of the fiber and matrixmatrix

The shear stress eventually leads to debonding at The shear stress eventually leads to debonding at the fiber ends and the fiber ends and ζζ1 1 may be evaluated based on; may be evaluated based on;

ζζ11 ~ 2.5 ~ 2.5 σσc c ((σσc c is the compressive stress at which is the compressive stress at which

debonding occurs- difficult to be determined)debonding occurs- difficult to be determined)

Compression test for interfacial tensile Compression test for interfacial tensile strength (c)strength (c)

Debonding induced by tensile stresses if a Debonding induced by tensile stresses if a curves, neck specimen with a continuous curves, neck specimen with a continuous fiber is tested in compression (c)fiber is tested in compression (c)

At a compressive stress of At a compressive stress of σσc c , the tensile , the tensile

strength strength σσ11of the interface is reached and of the interface is reached and

tensile debonding occurs, tensile debonding occurs, σσ1 1 = C= C σσc c , C is a , C is a

constant which depends on Poisson’s ratio constant which depends on Poisson’s ratio and Young’s Modulus of fiber & matrixand Young’s Modulus of fiber & matrix

Bulk specimen testsBulk specimen tests

The simplest method and most widely employed

The tensile strength and shear strength obtained from the 3-point bending test are found to depend on the volume of fibers- not a true values for the bond strength

At a given load P, the max. stress At a given load P, the max. stress σσ is given is given as;as;

σ = 3PS/2D2B………(1)P= Load, S=span length, D= thickness

B=width

Micro-indentation testMicro-indentation test Employs a standard micro-indentation Employs a standard micro-indentation

hardness testerhardness tester The indentor is loaded with a force, P on to The indentor is loaded with a force, P on to

a center of a fiber, whose axis is normal to a center of a fiber, whose axis is normal to the surface, and caused the fiber to slide the surface, and caused the fiber to slide along the matrix-fiber interfacealong the matrix-fiber interface

Suitable for CMCSuitable for CMC

Composite PropertiesComposite Properties

Heat Capacity and densityHeat Capacity and density Can be predicted using Rule of Mixture.Can be predicted using Rule of Mixture. Density, Density, ρρcc = = ρρmmVVmm + + ρρffVVff

Heat Capacity,CHeat Capacity,Ccc =(C =(CmmρρmmVVmm + C + Cff ρρffVVff )/ )/ ρρcc

V= volume fraction, m=matrix, c=composite, V= volume fraction, m=matrix, c=composite, f= fiber, C= heat capacityf= fiber, C= heat capacity

Modulus of ElasticityModulus of Elasticity 2 Models can be used to predict the elastic 2 Models can be used to predict the elastic

modulus of the compositesmodulus of the composites (1) (1) Isostrain conditionIsostrain condition

- Load is applied parallel to the fiber - Load is applied parallel to the fiber alignment, assume equal deformation in the alignment, assume equal deformation in the componentscomponents

(2) (2) Isostress conditionIsostress condition

- Load is applied perpendicular to the fiber - Load is applied perpendicular to the fiber alignmentalignment

Tensile elastic modulus vs. volume Tensile elastic modulus vs. volume fraction of fiber.fraction of fiber.

StrengthStrength

Difficult to predict the strength by using Difficult to predict the strength by using the rule of mixture, this is due to the the rule of mixture, this is due to the sensitivity of strength toward the matrix sensitivity of strength toward the matrix and fiber structureand fiber structure

- For example, matrix and fiber structure - For example, matrix and fiber structure will be changed during the fabrication will be changed during the fabrication processprocess

ToughnessToughness

Depends on few factors:Depends on few factors:

1)1) Composition and microstructure of the Composition and microstructure of the matrixmatrix

2)2) Type, size and orientaion of fiberType, size and orientaion of fiber

3)3) Processing of composite; effect the Processing of composite; effect the microstructure, i.e. voids, distribution of microstructure, i.e. voids, distribution of fiber, etc.fiber, etc.

Common structural defects in Common structural defects in compositescomposites

Matrix-rich (fiber-poor) regionsMatrix-rich (fiber-poor) regions VoidsVoids Micro-cracks (may be due to thermal Micro-cracks (may be due to thermal

mismatch between the components, curing mismatch between the components, curing stresses, or absorption of moisture during stresses, or absorption of moisture during processing)processing)

Debonded regionsDebonded regions Delamination regionsDelamination regions Variation in fiber alignmentVariation in fiber alignment