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.

 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 Wettability 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,.  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 mJ/m2 manakala bagi alumina dengan polietilena ialah 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 polymer- filler 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 Interfacial bonding

Types of interfacial bonding at interface 1)Mechanical bonding 2)Electrostatic bonding 3)Chemical bonding 4)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)

Flexural Properties of Treated and Untreated Ag/Epoxy Composites 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. (a). 5 vol.% of untreated system(b). 5 vol.% of treated system 155× Light microscopy micrographs reveal the degree of dispersivity Ag in epoxy matrix before and after chemical treatment of Ag Silver (Ag) filled epoxy composites; with the addition of silane coupling agent (3APTES)

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)

 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 (ζ 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 ζ 1 may be evaluated based on;  ζ 1 ~ 2.5 σ c (σ c 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 σ c, the tensile strength σ 1 of the interface is reached and tensile debonding occurs, σ 1 = C σ c, 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 fibers- not a true values for the bond strength

 At a given load P, the max. stress σ is given as; σ = 3PS/2D 2 B………(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, ρ c = ρ m V m + ρ f V f  Heat Capacity,C c =(C m ρ m V m + C f ρ f V f )/ ρ c  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 - 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 - 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