ReinforcementMatrix Interface

1
Reinforcement-Matrix Interface

• The load acting on the matrix has to be
  transferred to the reinforcement via. Interface
• The reinforcement must be strongly bonded to the
  matrix if high stiffness and strength are desired
  in the composite materials
• A weak interface results in low stiffness and
  strength but high resistance to fracture
• A strong interface produces high stiffness and
  strength but often low resistance to fracture,
  i.e. brittle behavior

2
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.

3
Wettability

• 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)

4
Wettability
Drops of water on a hydrophobic surface
Good or poor wettability?
5
Wettability

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

6
Wettability

• All surfaces have an associated energy and the
  free energy per unit area of the solid-gas,
  liquid-gas and solid-liquid are ?SG, ?LG dan ?SL,
  respectively.
• ?SG ?LG cos ? ?SL
• ? is called the contact angle. May be used as a
  measure of the degree of the wettability

7
Wettability

• cos ? (?SG ?SL)/ ?LG
• If ? 180º, the drop is spherical, no wetting
  takes place
• ? 0, perfect wetting
• 0ºlt?lt180º, the degree of wetting increases as ?
  decreases.
• Often it is considered that the liquid does not
  wet the solid if ?gt90º

8

• These three quantities determine whether the
  liquid spreads over the solid, or not whether it
  "wets" it.
• This is judged by the contact angle, . 

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

9
Soalan 2002/2003

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

10
2 types of failure at interface

• Difficult to measure the strength of interface,
  this is because sometimes failure occur
  interface, and sometimes not
• 2 types of failure at interface
• 1) Adhesive failure - failure occur at interface
• 2) Cohesive failure failure occur close to the
  interface (either at the fiber or matrix)

11
(No Transcript)
12
Factors leading to good polymer-filler bonding
13
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

14
(No Transcript)
15
Types of interfacial bonding at interface

• Mechanical bonding
• Electrostatic bonding
• Chemical bonding
• Reaction or interdiffusion bonding

16
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

17

• 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)

18
Electrostatic 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

19
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

20

• 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

21

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

22
Effect of Silane Coupling Agents on the
properties of Silver (Ag)-epoxy composites

• To improve interaction between filler and
  polymer, by modifying filler surfaces
• Used in low concentration (e.g. 0.1), silane
  coupling agent- give rise to significant
  improvements in mechanical properties

23
Silver (Ag) filled epoxy composites with the
addition of silane coupling agent (3APTES)
24
Silver (Ag) filled epoxy composites with the
addition of silane coupling agent (3APTES)

• Flexural Properties of Treated and Untreated
  Ag/Epoxy Composites

25
Silver (Ag) filled epoxy composites with the
addition of silane coupling agent (3APTES)

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

155
155
(a). 5 vol. of untreated system
(b). 5 vol. of treated system
Light microscopy micrographs reveal the degree of
dispersivity Ag in epoxy matrix before and after
chemical treatment of Ag
26
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
27

• 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)

28
Methods for measuring bond strength

• Single fiber test
• Fiber pull-out test (a)
• Involves pulling a partially embedded single
  fiber out of a block of matrix material
• Difficult to be carried out especially for thin
  brittle fiber

29
Fiber pull-out test (a)
30
Fiber pull-out test (a)

• 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

31

• Compression test fot interfacial shear strength
  (b)
• The interfacial shear strength (?1) may be
  evaluated using a specimen consisting of a block
  of matrix materials with a single, embedded short
  fiber with accurately aligned longitudinal in a
  center of the specimen (b)
• On testing in compression, shear stresses are set
  up at the ends of the fibers as a consequence of
  the difference in elastic properties of the fiber
  and matrix
• The shear stress eventually leads to debonding at
  the fiber ends and ?1 may be evaluated based on
• ?1 2.5 sc (sc is the compressive stress at
  which debonding occurs- difficult to be
  determined)

32
(No Transcript)
33

• Compression test for interfacial tensile strength
  (c)
• Debonding induced by tensile stresses if a
  curves, neck specimen with a continuous fiber is
  tested in compression (c)
• At a compressive stress of sc , the tensile
  strength s1of the interface is reached and
  tensile debonding occurs, s1 C sc , C is a
  constant which depends on Poissons ratio and
  Youngs Modulus of fiber matrix

34
Bulk 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
35

• At a given load P, the max. stress s is given as
• s 3PS/2D2B(1)
• P Load, Sspan length, D thickness
• Bwidth

36
Micro-indentation test

• Employs a standard micro-indentation hardness
  tester
• The indentor is loaded with a force, P on to a
  center of a fiber, whose axis is normal to the
  surface, and caused the fiber to slide along the
  matrix-fiber interface
• Suitable for CMC

37
Composite Properties

• Heat Capacity and density
• Can be predicted using Rule of Mixture.
• Density, ?c ?mVm ?fVf
• Heat Capacity,Cc (Cm?mVm Cf ?fVf )/ ?c
• V volume fraction, mmatrix, ccomposite, f
  fiber, C heat capacity

38

• Modulus of Elasticity
• 2 Models can be used to predict the elastic
  modulus of the composites
• (1) Isostrain condition
• - Load is applied parallel to the fiber
  alignment, assume equal deformation in the
  components
• (2) Isostress condition
• - Load is applied perpendicular to the fiber
  alignment

39
Tensile elastic modulus vs. volume fraction of
fiber.
40

• Strength
• Difficult to predict the strength by using the
  rule of mixture, this is due to the sensitivity
  of strength toward the matrix and fiber structure
• - For example, matrix and fiber structure will be
  changed during the fabrication process

41

• Toughness
• Depends on few factors
• Composition and microstructure of the matrix
• Type, size and orientaion of fiber
• Processing of composite effect the
  microstructure, i.e. voids, distribution of
  fiber, etc.

42
Common structural defects in composites

• Matrix-rich (fiber-poor) regions
• Voids
• Micro-cracks (may be due to thermal mismatch
  between the components, curing stresses, or
  absorption of moisture during processing)
• Debonded regions
• Delamination regions
• Variation in fiber alignment