Transcript 2 Mode Kegagalan pada Antarafasa

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
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.
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)
Wettability
Drops of water on a hydrophobic surface
Good or poor wettability?
Wettability
 Let us consider a thin film of liquid (matrix)
spreading over a solid (reinforcement) surface
Figure
Wettability
 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
Wettability
 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, .
Drops of water on a textile surface
before and after addition of wetting
agent
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
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)
Factors leading to good polymerfiller bonding
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
1)
2)
3)
4)
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 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 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
 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.
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
Silver (Ag) filled epoxy composites;
with the addition of silane coupling
agent (3APTES)
Silver (Ag) filled epoxy composites; with the addition
of silane coupling agent (3APTES)
Flexural Properties of Treated and Untreated Ag/Epoxy Composites
180
6000
5000
Flexural Stress, Sf (MPa)
Flexural Modulus, E B (MPa)
160
4000
3000
2000
1000
140
120
100
80
60
40
20
0
0
4
5
Concentrantion of Ag Nanoparticles, V f (vol.%)
Untreated Ag Filled Epoxy Treated Ag Filled Epoxy
4
5
Concentration of Ag Nanoparticles, V f (vol.%)
Untreated Ag Filled Epoxy Treated Ag Filled Epoxy
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×
(a). 5 vol.% of untreated system
155×
(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
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)
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
Fiber pull-out test (a)
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
 Compression test fot interfacial shear strength
(b)
 The interfacial shear strength (ζ1) 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 ζ1 may be evaluated based on;
 ζ1 ~ 2.5 σc (σc is the compressive stress at which
debonding occurs- difficult to be determined)
 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 σc , the tensile
strength σ1of the interface is reached and
tensile debonding occurs, σ1 = C σc , C is a
constant which depends on Poisson’s ratio
and Young’s Modulus of fiber & matrix
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 fibersnot a true values for
the bond strength
 At a given load P, the max. stress σ is given
as;
σ = 3PS/2D2B………(1)
P= Load, S=span length, D=
thickness
B=width
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
Composite Properties
Heat Capacity and density
 Can be predicted using Rule of Mixture.
 Density, ρc = ρmVm + ρfVf
 Heat Capacity,Cc =(CmρmVm + Cf ρfVf )/ ρc
 V= volume fraction, m=matrix, c=composite,
f= fiber, C= heat capacity
 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
Tensile elastic modulus vs. volume
fraction of fiber.
 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
 Toughness
 Depends on few factors:
1) Composition and microstructure of the
matrix
2) Type, size and orientaion of fiber
3) Processing of composite; effect the
microstructure, i.e. voids, distribution of
fiber, etc.
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