Reinforcement-Matrix Interface
PRESENTED BY
Mehboob Elahi
09-MS-MME-10
Subject
Engineering Ceramics and Composites
Outlines of Presentation
Introduction
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
Introduction
Reinforcement
Interface Interface
It 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
Interphase
In 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
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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 Coatings
Interface
To transfer the stress from matrix to
reinforcement
The reinforcement must be strongly bonded to the
matrix if high stiffness and strength are desired in the
composite materials
The interface between fibre and matrix is crucial to the
performance of the composite - in particular fracture toughness;
corrosion; moisture resistance
Coating
Sometimes 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)
Wettability
Is 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
Drops of water on a hydrophobic surface
the matrix (wetting is good)
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
Drops of water on a textile surface
before and after addition of wetting
agent
cos θ = (γSG – γSL)/ γLG
If θ = 180º, the drop is spherical, no wetting takes place
θ = 0, perfect wetting
0º<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
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
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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
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Mechanical bonding
Electrostatic bonding
Chemical bonding
Reaction or interdiffusion bonding
Mechanical bonding
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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
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Strength of chemical bonding depends on the number of bonds per unit area
and the type of bond
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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
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Reaction 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 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)
Silver (Ag) filled epoxy composites; with the
addition of silane coupling agent (3APTES)
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
Particle-Matrix Compatibility
Regardless of filler size and shape, intimate contact between the matrix
and
reinforcing particles is essential, since air gaps represent points of zero
strength. Thus, compound strength is improved by good “wetting” of the
reinforcement by the matrix and further enhanced when the matrix is
adhered to
the 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 tester
The 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 interface
Suitable for CMC
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
Weak interface:
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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
Weak interface leads to tough
composites
Strong Interface:
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Provides high strength but low fracture
toughness
Strong interface leads to brittle composites
Strong interface leads to brittle
composites
Interfaces in PMC,MMC and CMC
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 CMC
failure 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
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
effect of the normal stress
effect of the shear stress
The micrographs of fracture surface
of carbon fibers/epoxy resin
composites. A, untreated; B,
treated.
Importance of adhesion
Simple example: Unidirectional carbon/epoxy composite
The adhesion of the A-4 carbon fibers to the epoxy matrix, as quantified through
single-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 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. 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 interfacial
failure 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