Adhesive and reinforcing mechanism Interface Adhesive and reinforcing mechanism

The Interface There is always an interface between constituent phases in a composite material For the composite to operate effectively, the phases must bond where they join at the interface Figure 1 ‑ Interfaces between phases in a composite material: 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 Figure 2 ‑ Interfaces between phases: addition of a third ingredient to bond the primary phases and form an interphase

Another Interphase Interphase consisting of a solution of primary and secondary phases Figure 3 ‑ Interfaces and interphases between phases in a composite material: formation of an interphase by solution of the primary and secondary phases at their boundary

Role of Interface in Composite Materials Integration of Constituent Materials by Interfacial Bonding Ex: reduce the effects of coefficients of thermal expansion mismatch in the fibre/matrix system. Load Transfer between Matrix and Reinforcement crack resistance of a composite, transverse properties of a composite, etc.

Protection of Bulk Unit against Environments Preserve the fibre from environmental influences such as oxidation Preserve the fibre from degradation resulting from chemical interaction in the system during either composite fabrication or service at sufficiently high temperatures Endowing Functionality to the Composites

Adhesion Adhesion Cohesion

Adhesion Adhesion refers to the state in which two dissimilar bodies are held together by intimate interfacial such that mechanical force or work can be transferred across the interface.

Adhesion – The bonding of one material to another, namely an adhesive to a substrate, due to a variety of possible interactions Cohesion The internal strength of an adhesive as a result of a variety of interactions within the adhesive Figure 4: Forces in between substrates

Principal of interfacial adhesion Wetting Adhesion strength Interfacial strength Chemical bonding Van DerWaals Hydrogen bonding Mechanical bonding Cohesive strength

Mechanism of Interfacial Bonding Figure 5: Interfacial bonds : by molecular entanglement by electrostatic attraction by interdiffusion of elements by chemical reaction between groups A on one surface and group B on the other surface by chemical reaction following forming of a new compound(s) – MMC’s by mechanical interlocking

INTERDIFFUSION A bond between two surfaces may be formed by the interdiffusion of atoms or molecules across the interface Fundamental feature: must exist a thermodynamic equilibrium between the two constituents Promoted by: the presence of solvents and the amount of diffusion will depend on the molecular conformation, the constituents involved, and the ease of molecular motion

Example of interdiffusion Bonding between glass fibers and polymer resins through silane coupling agents by a process chemical bonding interdiffusion the interpenetrating network (IPN) formation in the interface region FIGURE 6: A schematic model for interdiffusion and IPN in a silane-treated glass fiber-polymer matrix composite. After Plueddemann (1988).

ELECTROSTATIC ATTRACTION Difference in electrostatic charge between constituents at the interface may contribute to the force of attraction bonding Strength of the interface will depend on the charge density Important when the fiber surface is treated with some coupling agent Ex: silane finishes are especially effective for certain acidic or neutral reinforcements like glass, silica, and alumina, but are less effective with alkaline surfaces like magnesium, asbestos, and calcium carbonate

CHEMICAL BONDING Chemical bonding mechanism is based on the primary bond at the interface A bond is formed between a chemical group on the fiber surface and another compatible chemical group in the matrix, the formation of which results from usual thermally activated chemical reactions Ex: A silane group in an aqueous solution of a silane coupling agent reacts with a hydroxyl group of the glass fiber surface, while a group like vinyl on the other end will react with the epoxide group in the matrix

REACTION BONDING Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled The atoms of the fiber surface diffuse through the reaction site In the center of the fiber or at the fiber-compound interface or the matrix atoms diffuse through the reaction product Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites

Special cases of reaction bonding include the exchange reaction bond and the oxide bond. The exchange reaction bond occurs when a second element in the constituents begins to exchange lattice sites with the elements in the reaction product in thermodynamic equilibrium

Example of Reaction Bonding Boride compound is initially formed at the interface region in an early stage of the process composed of both elements. Exchange reaction between the titanium in the matrix and the aluminum in the boride. Exchange reaction causes the composition of the matrix adjacent to the compound to suffer a loss of titanium, which is now embedded in the compound. Slows down the overall reaction rate Titanium-aluminum alloy reinforced with boron fibers

MECHANICAL BONDING Involve solely mechanical interlocking at the fiber surface Promoted by surface oxidation treatments, which produce a large number of pits corrugations large surface area of the carbon fiber Strength of this type of interface: unlikely to be very high in transverse tension unless there are a large number of re-entrant angles on the fiber surface strength in longitudinal shear may be significant depending on the degree of roughness

WETTING

Wetting Describes physical attraction between electrically neutral bodies Prerequisite to proper consolidation of composites, particularly for composites based on polymer resins and molten metals Involves very short-range interactions of electrons on an atomic scale which develop only when the atoms of the constituents approach within a few atomic diameters or are in contact with each other.

Quantitative expression of wetting Wetting can be quantitatively expressed in terms of the thermodynamic work of adhesion, WA, of a liquid to a solid using the Dupre equation WA = γ1 + γ2 – γ12 WA represents a physical bond resulting from highly localized intermolecular dispersion forces. equal to the sum of the surface free energies of the liquid and the solid, less the interfacial free energy

Young’s equation Resolution of forces in the horizontal direction at the point A where the three phases are in contact = surface free energy of solid-vapour = surface free energy of solid-liquid = surface free energy of liquid-vapour = contact angle must not be 0, or equation is not valid *termed spreading *COMPLETE WETTING FIGURE 7: Contact angle and surface energies of a liquid drop

Liquids that form contact angles greater than 90oC WETTING NON-WETTING Liquids that form contact angles greater than 90oC Liquids that form contact angles less than 90oC FIGURE 8: Wetting and non-wetting liquid

γSV (reinforcement) > γLV (matrix) The surface energy of a solid (i.e. reinforcement in composites), must be greater than a liquid (i.e. matrix resin) for proper wetting to take place γSV (reinforcement) > γLV (matrix) Example: Glass and carbon fibers can be readily wetted by thermoset resins like epoxy and polyester resins at room temperature unless the viscosity of the resin is too high

FIGURE 9: Surface energies properties of some fibres

Wetting Improvement Treatment of fibre surface Coating Metal coatings of carbon fibres allow them to be wetted by metals with a lower melting point, but they appear to be unstable in metal melts, i.e. fibre degradation, leading to a drop of composite properties, is still observed Example: method of coating carbon fibres with carbides Changing the chemical nature of the atmosphere before getting into contact with liquid Heat treatment of alumina, silicon carbide, and graphite particles is known to improve their wettability with aluminium melt

Oxidation treatments By gaseous, solution, electrochemical and plasma methods Plasma treatments: Oxidizing treatment and nitriding Ex: controlling the oxygen content of the environment can improve the wettability of aluminum alloys with carbon fibers This is because aluminum has a strong affinity toward atmospheric oxygen As the hot liquid metal comes in contact with air, a thin, adherent aluminum oxide layer is formed on its surface, which interferes with wetting Provide chemical modification of surface, removal of adherent surface layer & roughen fibre surface

Matrix modification Matrix alloy modification. Important from the point of view of liquid phase techniques of composite fabrication. To promote wetting it is necessary to decrease the surface energy of a fibre. Addition of dopant which react with fibre producing reaction products Form a thin surface layer (which is wetted by the molten matrix) Ex: Mg added to Al alloys disrupts the surface oxide and improves wettability with most reinforcements Add a wetting agent to the matrix or matrix precursor before infiltration

Experimental Measurement of Interface Adhesion

Single Fibre Pull-Out Test Widely used for an analysis of the behaviour of the interface in polymer matrix composites Three stages: Debonding Propagation of the debonding front Subsequent pull-out by frictional sliding FIGURE 10: Geometry of the fiber pull-out test: (a) disc –shaped specimen with restrained top loading (b) long matrix block specimen with fixed bottom loading (c) double pull-out multiple embedded fibers.

FIGURE 11: Schematic stress distributions and load- displacement plot during single fibre pull- out testing. The interfacial shear strength (*) is obtained from the pull-out stress (o) 1 2 3

Single Fibre Push-Out Test Conducted usually on a thin slice of the composite cut normally to the fibre direction More suitable for sufficiently rigid matrices which is a case of ceramic and metal matrices Specimen in form of thin slice with fibre axis normal to the plane of slice Fibre is displaced so that protrudes from bottom of specimen

FIGURE 12: Schematic stress distributions and load- displacement plot during the single fibre pushout test. One difference from the pull-out test is that the Poisson effect causes the fibre to expand (rather than contract), which augments (rather than offsets) the radial compressive stress across the interface due to differential thermal contraction

Fragmentation test Main concept is to embed a fibre within a transparent matrix that has a measurably higher strain prior to failure Under tension, tensile stress is transferred to the fiber through an interfacial shear the fibres inside the matrix will start to crack in different points where their failure strength (σf) has been exceeded the tensile strain in the fiber will eventually reach the failure strain of the fiber and the fiber will fracture This will continue to occur until the length of the all broken fibres reaches a critical length, lc This happens because the length of the fibres is so short (< lc), that not enough tensile stress can be transferred from the matrix to the fibre through the interface in order to break the fragment

FIGURE 13: Fibre fragmentation test specimen To calculate the average interfacial shear strength, i is used since matrix interface is placed under shear: d is the fiber diameter for a circular fiber cross-section

FIGURE 14: Schematic representation of the single-fiber fragmentation process