Adherend surface roughness

Direct bonding. In many high-volume production applications (i.e., the automotive and appliance industries), elaborate surface preparation of steel ad-herends is undesirable or impossible. Thus, there has been widespread interest in bonding directly to steel coil surfaces that contain various protective oils [55,56,113-116], Debski et al. proposed that epoxy adhesives, particularly those curing at high temperatures, could form suitable bonds to oily steel surfaces by two mechanisms (1) thermodynamic displacement of the oil from the steel surface, and (2) absorption of the oil into the bulk adhesives [55,56]. The relative importance of these two mechanisms depends on the polarity of the oil and the surface area/volume ratio of the adhesive (which can be affected by adherend surface roughness). 

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

FIGURE 3.5 Effect of surface roughness on coplanarity of gas bubbles Upper adherend is smooth, and gas bubbles are in the same plane lower adherend has roughness, and gas bubbles are in several planes. 

Fig. 5. Tensile load bearing capacities of the co-cured lap joints with respect to the surface roughness between steel and composite adherends. (a) Co-cured single lap Joint and (h) cocured double lap Joint.

Adhesives, non-scientifically, are materials which provide adhesion as a result of mechanical interlocking with the basic roughness of the adherend surfaces. A precise definition of an adhesive describes it as a substance capable of holding materials together by surface attachment. 

Thus, such adhesive forces are the basis for the fact that a bond, consisting of adherends and adhesive layer, will last. Since they develop between the individual molecules or atoms, they are also called intermolecularforces. The distances over which adhesive forces can act are very short, they range in the order of 10 (one hundred thousandth) millimeters. So, it is to be explained that very smooth, finely polished surfaces can still have a certain adhesion to each other. Thus, part 1 in Figure 6.4 (left) is able to lift part 2 (without being connected to part 1 by an intermediate layer) from the base area. In the depiction on the right this is not possible due to the surface roughness. 

On the assumption of a clean and, respectively, pretreated surface, the next step will be the application of the adhesive. It has to be ensured, however, that in areas with adhesive forces emanating from the adherend surface, the adhesive molecules are really able to get closer. Only then can the adhesive distribute itself on the surface, that is, wet the surface despite a, more or less, existing roughness. Furthermore, sufficient flowability of the adhesive is important. A complete and equal wetting of the surface to be bonded is therefore an indispensible prerequisite for the production of a strong bonded joint. Figure 6.5 demonstrates the difference between a low-viscosity and high-viscosity adhesive. 

Real joints do not of course consist of simple, separate, elastic materials with a clear mathematical geometry. Metal adherend surfaces are micro-rough, possessing oxide layers, while concrete surfaces are macro-rough comprising aggregates and cement paste, and both surfaces readily adsorb air-bome contamination. The thickness and modulus of primer layers, if employed, is often unknown, and the thickness and properties of the adhesive layer are difficult to regulate and to determine. 

The use of Thermal spray processes to deposit thick layers, extending to typically 50 xm and with a surface roughness (Ra) of many micrometres, of Ti6A14V onto the same alloy has been demonstrated to be effective by a number of workers. Invariably, joints fabricated with PS pie-treated adherends perform very well in comparable durability trials, including those carried out at temperatures of 400 °C and greater. PS pre-treated adherends can give improved durability performance compared to CAA and similar results to the SHA process. 

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