Page 230 - Handbook of Adhesives and Sealants
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Surfaces and Surface Preparation  199


              When a supposedly smooth solid surface is examined closely under
            a microscope, it is found to contain irregularities. It is not flat and
            smooth but contains many surface asperities, such as peaks and val-
            leys, with a certain degree of roughness. A rough surface provides
            more bonding area than a smooth one of the same gross dimensions.
            The greater effective surface area offers a larger area for the forces of
            adhesion to operate, thereby providing a stronger joint. However, a
            greater degree of surface roughness could also contribute to stress con-
            centrations in the adhesive joint, which reduces its strength, similar
            to a notch effect in metals. This effect depends on how well the ad-
            hesive wets the surface and penetrates into the surface roughness.
              Surfaces are full of surprises, and they are seldom what they seem.
            Often they contain constituents that are very different from the bulk
            material. For metals and alloys, these surfaces may consist of oxides
            and adsorbed gases. For many nonmetals, they may be moisture, mi-
            grating additives, or adsorbed films, such as shop contaminants. These
            outer layers can either be loosely bound or tightly adhered to the base
            material, and they may have high or low cohesive strength. Two sur-
            face characteristics that can hurt adhesion are when: (1) the chemical
            nature provides a low surface energy; and (2) the surface is either
            cohesively weak or weakly attached to the base substrate. When either
            one of these conditions are present, the substrate surface must be
            treated in some manner to either increase the surface energy or
            strengthen the surface layer.
              It does not take much contamination to affect adhesion. A single
            molecular layer of contaminant can prevent proper wetting of the sub-
            strate by the adhesive or sealant. The adhesive or sealant will try to
            wet the contaminant surface layer rather than the substrate itself.
            Since most contaminants (oils, greases, fingerprints, mold release,
            etc.) have a low surface energy, the adhesive will not wet the surface
            nor will it form a continuous film.
              Certain surfaces also may have weakly attached surface or bound-
            ary layers. Examples of these are contaminant films, oxide layers,
            rust, corrosion, scale, and loose surface particles. A weak substrate
            boundary layer can provide the ‘‘weak-link’’ for reduced bond strength
            or premature failure as shown in Fig. 6.1.
              In an ideal bonded assembly, the substrate should be the weakest
            link. In most assemblies that are properly bonded, the adhesive is the
            weak link because the forces of adhesion are greater than the forces
            holding the adhesive material together. Usually, the internal strength
            of the substrate and adhesive or sealant system is well understood
            and can be controlled. However, when the surface region becomes the
            weakest link, it may result in low failure strength and an inconsis-
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