Reinforcement-Matrix Interface

PRESENTED BYMehboob Elahi 09-MS-MME-10

SubjectEngineering Ceramics and Composites

Outlines of PresentationIntroduction

Why Interfaces are Important

Interface and coatings

Wettability

Interfacial bonding

Particle –Matrix Compatibility

Methods for Bond strength Measurement

Interfacial strength

Interfaces in PMC,MMC and CMC

Interface Failure

Importance of adhesion

Reinforcement

Introduction

Interface InterfaceIt is the boundary demarcating the distinct phase of reinforcement and matrix

Zone across which matrix and reinforcing phases interact(chemical, physical, mechanical)

For the composite to operate effectively, the phases must bond where they join at the interface

(a) direct bonding between primary and secondary phases

InterphaseIn some cases, a third ingredient must be added to achieve bonding of primary and secondary phases Called an interphase, this third ingredient can be thought of as an adhesive /coatings

(b) addition of a third ingredient to bond the primary phases and form an interphase

Why are Reinforcement matrix interfaces important?

1. Ef & Em quite different

Such large differences are shared through the interface.Stresses acting on the matrix are transmitted to the fiber across the interface.

2. The interfacial bond can influence

• Composite strength• Modes of failure• Young’s modulus• Interlaminar shear strength• Compressive strength• Environmental resistance• Structural stability at elevate temperatures• Fracture and fatigue behavior

Interface and CoatingsInterface To transfer the stress from matrix to reinforcement

CoatingSometimes surface treatment is carried out to achieve the required bonding to the matrix

Sizing – protect reinforcing material from mechanical damage

Finishes – Enhance bonding of reinforcement to matrix (Polyvinyle acetate or organosilane coupling agent)

The reinforcement must be strongly bonded to the matrix if high stiffness and strength are desired in the composite materialsThe interface between fibre and matrix is crucial to the performance of the composite - in particular fracture toughness; corrosion; moisture resistance

WettabilityIs defined the extent where a liquid will spread over a solid surface

During the manufacturing process, the matrix is often in the condition where it is capable of flowing or its behavior is like a liquid

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

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

Interfacial bonding exists due to the adhesion between the reinforcement and the matrix (wetting is good) Drops of water on a hydrophobic surface

Good or poor wettability?

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

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

γSG = γLG cos θ + γSL

θ is called the contact angle. May be used as a measure of the degree of the wettability

cos θ = (γSG – γSL)/ γLGIf θ = 180º, the drop is spherical, no wetting takes placeθ = 0, perfect wetting0º<θ<180º, the degree of wetting increases as θ decreases.Often it is considered that the liquid does not wet the solid if θ>90º

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

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

This is judged by the contact angle, .

Criteria for Better Wetting:Surface must be free of foreign particles. This removes weak boundary layer or contaminants (H2O, organic vapor, nitrates, ketones, alcohols, amines)

A large interfacial area of intimate contact

Thermodynamically, a high surface-energy surface is the most conductive to good wetting, particularly if adhesive contains polar functional group.

Surface energy of the adherent (reinforcement) should be greater than the adhesive surface energy (matrix).

Creation or addition of chemical group

Variation in surface topography (mechanical interlocking)

Improper wetting may cause voids at the interface that may lead to cracking.

Interfacial bonding

Once the matrix has wet the reinforcement, bonding will occur For a given system, more than one bonding mechanism may exist at the same

time The bondings may change during various production stages or during services

Types of interfacial bonding at interface Mechanical bonding Electrostatic bonding Chemical bonding Reaction or interdiffusion bonding

Mechanical 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 force is applied parallel to the interface

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

Electrostatic Bonding

Occur when one surface is positively charged and the other is negatively charge (refer to the 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 bonding

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

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

Chemical bonding normally exist due to the application of coupling agents

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

Reaction or interdiffusion bondingThe 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 species from the two components can produce an interfacial layer of different composition and structure from either of the component

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

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

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

5 vol.% of untreated system 5 vol.% of treated system

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

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

Particle-Matrix Compatibility

Regardless of filler size and shape, intimate contact between the matrix andreinforcing particles is essential, since air gaps represent points of zerostrength. Thus, compound strength is improved by good “wetting” of thereinforcement by the matrix and further enhanced when the matrix is adhered tothe reinforcing particle surface via chemical bonding.

Methods for measuring bond strength

1.Fiber pull-out test Involves pulling a partially embedded single reinforcing particle out of a block of matrix material

Difficult to be carried out especially for thin brittle fiber

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

2. Micro-indentation test

Employs a standard micro-indentation hardness testerThe indentor is loaded with a force, P on to a center of a reinforcing particle, whose axis is normal to the surface, and caused the particle to slide along the matrix-particle interfaceSuitable for CMC

Weak interface:

Composites provide low strength and stiffness Promotes fiber debonding and pull-out which

provide higher fracture toughness Weak interfaces provide a good energy

absorption mechanism

Strong Interface: Provides high strength but low fracture

toughness Strong interface leads to brittle composites

Interfacial strength

The utility of a reinforcing phase in composite matrix depends on the strength of the interfacial bond between the reinforcement and the matrix

poor bonding well-bonded

Strong interface leads to brittlecomposites

Weak interface leads to toughcomposites

Two fundamentally different approaches for composites

1. For PMC and MMC failure originates in or along the reinforcement

A high interfacial strength is desirable to maximize the overall composite strength

2. For CMCfailure originates in the matrix phase

To maximize the fracture toughness, it is desirable to have a relatively weak interfacial bond that allow the fiber to pull out

Crack is deflected along the fiber-matrix interface or bridged

Increased crack path significantly improves fracture toughness

Interfaces in PMC,MMC and CMC

Reinforcement–matrix interface failure

Matrix crack approaching fibre

Deflected along fiber-matrix interface

Increased crack path length due to fibre pull-out significantly improves fracture toughness

The micrographs of fracture surfaceof carbon fibers/epoxy resincomposites. A, untreated; B,treated.

effect of the normal stress effect of the shear stress

Simple example: Unidirectional carbon/epoxy composite

Importance of adhesion

The adhesion of the A-4 carbon fibers to the epoxy matrix, as quantified throughsingle-fiber fragmentation tests. The fiber-matrix adhesion increases in the order AU-4 >AS-4 > AS-4C. AU-4 has the lowest level of adhesion and fails by a frictional debonding mode; AS- 4 has an intermediate level of adhesion and fails by an interfacial crack growth mode; AS-4C has the highest level of adhesion and fails by a matrix-cracking mode perpendicular to the fiber axis

Fracture surface of A-4/epoxy [±45]3S composites, illustrating the different natureof the failure mode and interphase properties. The fiber-matrix adhesion decreases in theorder AS-4C > AS-4 > AU-4. AU-4 and AS-4 exhibit interfacial failure modes; AS-4C fails in a matrix-dominated mode. The presence of the fiber sizing on the AS-4C fiber has created a brittle interphase

Comparison between the tensile and compressive properties of the three types of [0]12 A-4 carbon-fiber-epoxy composites. The modulus values are similar in both the loading modes. The compression test yields much smaller strength than tensile strength.Also, the compressive strength is more sensitive than the tensile strength to fiber-matrix adhesion. The fiber-matrix adhesion decreases in the order AS-4C > AS-4 > AU-4. AU-4 and AS- 4 exhibit interfacial failure modes; AS-4C fails in a matrix- dominated mode

Comparison between the transverse tensile and flexural properties for [90]12 and the short beam shear strength of A-4 carbon-fiber-epoxy composites. The flexural strength is much higher than the tensile strength. The interlaminar shear strength and transverse tensile and flexural strengths all show the same trends. The fiber-matrix adhesion decreases in the order AS-4C > AS-4 > AU-4. AU-4 and AS-4 exhibit interfacialfailure modes; AS-4C fails in a matrix-dominated mode

Comparison between the mode I and mode II fracture toughness of the three composite materials. The mode II fracture toughness is about three times higher than the mode I fracture toughness. The fiber-matrix adhesion decreases in the order AS-4C > AS-4 > AU-4. AU-4 and AS-4 exhibit interfacial failure modes; AS-4C fails in a matrixdominated mode

Heard enough from me…….Any questions?

Mehboob Elahi

09-MS-MME-10