INTERFACES IN COMPOSITES • Questions that need to be answered: –Are interfacial properties important at all? –What is the role/effect of interfaces with respect to overall mechanical properties of composites? –How can we measure interfacial adhesion in composites? –Can we develop analytical models that are useful for design purposes? –Is it possible to tailor interfacial adhesion? EXAMPLES OF PRACTICAL RELEVANCE (VARIOUS FIELDS) • Thin coatings on critical surfaces (reflective coatings on optical components, plating for corrosion protection, magnetic films for data storage…) • Interfaces in composites (carbon-epoxy, glass-PP, SiC-SiC, PE- PE…) • Interaction of biological systems or media with engineering materials (tissue bonding of prostheses, dental implants, artificial hips, cell adhesion [cells remain alive only if they adhere to a substrate]…) Wetting and adhesive strength • Adequate wetting is a necessary (but not sufficient!) condition for good adhesion between a liquid and a solid surface. • The contact angle between a liquid droplet and a surface is an indication of compatibility between these. • If q < 90°, the liquid ‘wets’the solid surface. • If q = 0°, the liquid ‘spreads’on the surface (complete wetting). • If q = 180°, the liquid ‘does not wet’the surface • In terms of surface energies: θ Surface energies (γ) θ solid vapor liquid γ SV γ LV γ SL (S = Solid, V = Vapor, L = Liquid) The equilibrium wetting or contact angle q is dictated by the Young equation, obtained by a balance of horizontal forces: γ SV = γ LV cosθ+ γ SL Thus, good wetting (θ = 0 or small) arises when the surface energy of the solid is equal to or greater than the sum of the liquid surface energy and the solid-liquid interface surface energy. γ SV ≥ γ LV + γ SL [Interface surface energies are difficult to measure (and may be influenced by chemical reactions) but are often smaller than thevalues for the phases in air] The lower the contact angle, the greater the wettability: Liquid is attracted to itself, not to solid Liquid is attracted to solid, less to itself Indirect measurement can be made by immersing the fiber into the liquid of interest and measuring the force of immersion or emersion (= Wilhelmy microbalance technique). A force balance permits the calculationof the contact angle if the fiber perimeter and surface energy of the liquid are known (see Bascom). Sessile drop method Capillary-rise method Wilhelmymethod Liquid Glass container Fibre Microbalance Krüss tensiometer Computer Drive system Fiber-matrix interfacial adhesion • Two general methodologies: –Indirect (or macromechanical) testing - focuses on the collective behavior of fibers in a polymer matrix [interfacial strength is interpreted via simplistic approximations; fast but questionable data collection] –Direct (or micromechanical) testing –probes interfacial behavior of individual fibers interacting with a polymer matrix [more fundamental information; variability within and between techniques; issue of relevance to real-life composites (scaling- up)] Indirect (or macromechanical) testing 1.The transverse tensile test 2. The short-beam shear (or interlaminarshear strength -ILSS) test 3. Asymetrical4-point (Iosipescu) bend test (And other macroscopic tests…) CONCLUSION – They are simple to perform, but yield ambiguous, inaccurate data. . methodologies: –Indirect (or macromechanical) testing - focuses on the collective behavior of fibers in a polymer matrix [interfacial strength is interpreted via simplistic approximations; fast but questionable data. micromechanical) testing –probes interfacial behavior of individual fibers interacting with a polymer matrix [more fundamental information; variability within and between techniques; issue