Properties of Composites Dependent on: Constituent phases Reinforcement Matrix Relative amounts Interface properties Geometry of reinforcement Processing Methods

Interface There is always an interface between constituent phases in a composite material Figure: Interfaces between phases in a composite material: (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. Interphase can be thought of as an adhesive Figure : Interfaces between phases: (b) addition of a third ingredient to bond the primary phases and form an interphase

Figure 9.4 ‑ Interfaces and interphases between phases in a composite material: (c) formation of an interphase by solution of the primary and secondary phases at their boundary. Another Interphase Interphase consisting of a solution of primary and secondary phases

Definitions Interface (2D interface) A surface formed by the common boundary of reinforcing fibre and matrix in contact which constitutes the bond in-between for the transfer of loads Interphase (3D interface region) Interphase includes the surface of the classical fibre-matrix contact, as well as the region of finite thickness extending between the fibre and matrix The chemical, physical and mechanical features vary continuously between the bulk fibre and matrix

Theories of Adhesion Adsorption and wetting Thermodynamic work of adhesion (physical bond resulting from highly localized intermolecular forces) W a =  SV +  LV -  SL =  LV (1+Cos  ) :Young-Duprè equation  SV =  SL +  LV Cos   > 90: non wetting;  < 90: wetting;  = 0: spreading The higher the work of adhesion the stronger the interactions between the liquid and solid phases.

Theories of Adhesion If the liquid does not form a droplet, i.e. ;  = 0, it is termed ‘spreading’ In this case, the equilibrium is expressed by an inequality:  SV -  SL =  LV  SV >  LV The surface energy of a solid (i.e. reinforcement in composites),  SV, must be greater than that of a liquid (i.e. matrix resin),  LV, for proper wetting to take place.

Examples If  Surface energy of glass fibres is 63 mJ/m 2  Surface energy of carbon fibers is 52 mJ/m 2  Surface energy of PE fibers is 31 mJ/m 2 )  Surface energy of epoxy is 39 mJ/m 2  Surface energy of polyester is 40 mJ/m 2 Then  Glass fibre and carbon fibre can be readily wetted by epoxy and polyester resins at room temperature unless the viscosity of the resin is too high.  In contrast, it is difficult to wet PE fibres with epoxy or polyester resins unless the fibers are surface treated.

Wetting in composite processing Contamination: the effective surface energy is much smaller than the base solid High viscosity: entrapped air at the solid surface Shrinkage during curing: displacements at the solid surface.

Fibre Surface Treatments Sizes Unsized fibres degrade rapidly and cannot be handled for processing. Sizes are applied as a very thin coating (less than 1  m) to glass and carbon fibres immediately after the fibres are drawn to avoid abrasion. Functions of Sizes To protect the fibre surface from damage To bind fibres together for ease of processing To lubricate the fibres so that they withstand abrasive tension during subsequent processing. To impart anti-electrostatic properties. To provide a chemical link between the fibre surface and the matrix

Typical components of a glass fibre size (after Dow Corning 1985) Film forming resin1.0 to 5 % Anti-static agent0.1 to 0.2 % Lubricant0.1 to 0.2 % Coupling agent1.0 to 0.5 % Solventbalance  Film former: a polymer, which binds the strand together. It must be compatible with the matrix. Fibre dispersion and wetting may be influenced by choice of film former, which dissolves or melts during downstream processing operations. Polyester, epoxy and PU resins are widely used.  Lubricant: usually a stearate. It reduces damage due to abrasion. Especially important for weaving.

Glass fibres and Coupling Agents Coupling agent, R-SiX 3, is a di-functional chemical. One end is designed to bond to the fibre, and the other to the resin. (  -aminopropyltriethoxysilane (APS), vinyltriethoxysilane (VS)) R=organo-functional group, to react with the resin X=siloxane group, to react with a hydroxy group of the glass surface M=Composition atom from the glass fibre

Two major bonding mechanisms: Chemical reaction between R-groups and functional groups in the resin Interpenetrating network (IPN): resin is interdiffused into the chemisorbed silane layers and the silane molecules are migrated into the matrix.

Carbon Fibre Surface Treatment As-manufactured surface of carbon fibre is essentially graphite with mainly the basal planes exposed. These have low chemical reactivity.

Carbon Fibre Surface Treatment To improve reactivity, surface treatments are necessary, which can be classified into the following. 1) Oxidative Treatments Dry Oxidation:Heating in air, oxygen, ozone or CO 2 at elevated temperatures (typically 500  C) Wet Oxidation: Mild oxidation in an electrolyte (e.g. nitric acid, acidic potassium permanganate, sodium hydrochloride, etc.). Effectiveness depends on acid concentration, exposure time, temperature. 2) Non-oxidative treatments Whiskerisation: involves a nucleation process and the growth of very thin and high strength single crystals of the chemical compounds (e.g. SiC, TiO 2, Si 3 N 4 ) on the fibre surface in a chemical vapour deposition (CVD) process. Plasma Treatment: etching or deposition can be performed by coupling energy into a high pressure gas (e.g. oxygen, chlorine, floride, Ar)

Effects of Carbon Fibre Surface Treatments To remove loose, weak material from the surface To increase the surface area by producing pits and rugosities To produce various functional groups on the surface (e.g. hydroxyl C-OH; carbonyl C=O; carboxyl OH-C=O groups). These groups may form primary or secondary chemical bonds with the resin matrix. To improve the interface bond strength and thus the interlaminar shear strength (ILSS), flexural and tensile strengths of the composite. Induces a loss in impact fracture toughness.

Effects of Carbon Fibre Surface Treatments Whiskerization: SiC whiskers grown on carbon fibre surface after CVD process

Interdiffusion A bond between two surfaces may be formed by interdiffusion of atoms of molecules across the interface (e.g. interpenetrating network, IPN) Electrostatic attraction A difference in electrostatic charge between constituents at the interface may constitute to the force of attraction bonding Chemical bonding A bond is formed between a chemical grouping on the fibre surface and a compatible functional group in the matrix (e.g. coupling agent coated glass fibres and surface treated carbon fibres with polymer reins) Reaction bonding Reaction occurs in MMCs and CMCs, forming new compounds at the interface region Reaction involves transfer of atoms from one or both of the constituent to the reaction site near the interface, and these transfer processes are diffusion controlled Types of bonding

Mechanical bond Mechanical bonds involves solely mechanical interlocking at the surface. The bond strength can be significant in shear depending on the degree of roughness and residual stresses present Other bonds Hydrogen bonding, van der Waals forces All these mechanisms take place at the interface region either in isolation, or, most likely, in combination to produce the final bond. Types of bonding

Interface bonding depends on Atomic arrangement, molecular conformation and chemical constitution of the fiber and matrix Morphological properties of the fiber Diffusivity of elements in the fibre and matrix. Interface is specific to each fiber-matrix system