Adhesive junctions

As is true for macroscopic adhesion and mechanical testing experiments, nanoscale measurements do not a priori sense the intrinsic properties of surfaces or adhesive junctions. Instead, the measurements reflect a combination of interfacial chemistry (surface energy, covalent bonding), mechanics (elastic modulus, Poisson s ratio), and contact geometry (probe shape, radius). Furthermore, the probe/sample interaction may not only consist of elastic deformations, but may also include energy dissipation at the surface and/or in the bulk of the sample (or even within the measurement apparatus). Study of rate-dependent adhesion and mechanical properties is possible with both nanoindentation and... 

An impressive progress in the fundamental study of friction was made more than half a century ago when Bowden and Tabor proposed that friction resulted from shear of adhesive junctions at the real contact area, which took up only a tiny portion of nominal contact zone and was proportional to the load [10], as schematically shown in Fig. 11. The model presents a satisfied explanation as to why friction is proportional to the load and independent of nominal contact area. 

The model proposed by Bowden and Tabor has been regarded as the most successful one for presenting a simple and logical theory capable of explaining the Amontons friction law. However, suspicions concerning the two fundamental assumptions in the model were gradually aroused over past years. Friction has been attributed, in Bowden and Tabor s model, to the adhesion between asperities in contact and torn-off of the adhesive junctions when the shear stress exceeds a critical value. This implies that plastic flow and surface destruction may occur at the moment of slip, and that friction is dominated by the shear strength of the adhesive conjunctions, which is material dependent. 

In static friction, the change of state from rest to motion is caused by the same mechanism as the stick-slip transition. The creation of static friction is in fact a matter of choice of system state for a more stable and favorable energy condition, and thus does not have to be interpreted in terms of plastic deformation and shear of materials at adhesive junctions. 

Surfaces of materials possess different degrees of roughness depending on the way they are produced. When two solid surfaces are pressed together with a force acting normal to them, they make contact at the tips of the asperities in the two surfaces. In the case of soft metals and polymers, because of yielding, these points of contact form adhesive junctions. The minimum force required to slide one surface over the other is the force of friction F) and is given by (37)... 

Figure 3. Schematic of the effect of a lubricant film on the formation of adhesive junctions at a solid-solid interface.

Vogl AW, Pfeiffer DC, Mulholland D, Kimel G, Guttman J. 2000. Unique and multifunctional adhesion junctions in the testis ectoplasmic specializations. Arch. Histol. Cytol. 63 1-15... 

Hemidesmosomes (HDs) are membrane-associated adhesive junctions linked to the filamentous networks of the epithelial cell cytoskeleton and the lamina lucida ( light green/ dark blue region in Fig. 5.1). The cytoskeleton of all mammalian cells is composed of three kinds of filaments microfilaments, intermediate filaments and microtubules. Microfilaments... 

Involved In strengthening pi 20 cadherin containing adhesion junctions... 

Steinberg, M., and McNutt, P. (1999). Cadherins and their connections adhesion junctions have broader functions. Curr. Opin. CeU Biol. 11,554—560. 

In physical terms, the behavior of a strong junction is governed by the dominant influence of the tensile phase of junction rupture. A weak junction is one in which translational motion of one asperity relative to the other can occur by sliding while the junction is still under compressive stress. In analytical terms, remains at a lower level and the peak of the curve for p. is broader than is found for a strongly adhesive junction. For either a strong or a weak junction, substitution of the results of the stress/strain analysis into Eqns 12-56a and 12-56b gives a steady-state, non-fluctuating value for the coefficient of friction. 

In functioning machinery the contacting parts repeatedly rub one another many times. The interaction of two surfaces on reiterated contact will in part depend on the condition in which the previous iteration left them. Under ordinary circumstances, with the machinery operating satisfactorily, each iteration is much like the one before and an analysis of steady-state wear or friction can be made on the basis of one cycle of surface interaction. Generally in such cases, but not necessarily always, asperity deformation is elastic rather than plastic. Whether an adhesive junction forms depends on the condition of the asperity surface. If the materials f>e.n. 4e are easily adhesive but the surfaces are covered by a film which inhibits adhesion, then to initiate adhesion obviously the film must first be removed, broken up or penetrated. The subsequent course of adhesive contact will then be governed by such factors as the size of the contact, the shape of the asperity, the impressed load, the strength of the material, etc., in accordance with the fundamental modes of behavior. 

The concept of adhesive interaction of contacting surfaces is already familiar to us from previous discussion of the adhesive mechanism of friction (Chapters 8 and 12). If the two bodies participating in the adhesive junction are in motion relative to each other, in particular tangential motion, the junction is ruptured shortly after it is established. Rupture of the junction at a location other than the original interface results in transfer of material from one body to the other. According to the broad definition of wear of Section 13.1, each body has been worn—one by loss of material, the other by gain—but there has been no net loss or gain in the system as a whole. 

In contrast, in the case of the contact deformation displacement of ball-to-flat counterformal contacts discussed above no effect of adhesion was found as compared with the influence of the (bulk) viscoelastic properties of the materials. (This may be due to elastic relief forces which may burst adhesive junctions during the loadingunloading contact deformation cycles.)... 

Adhesive Wear. Adhesive wear(5) is the transfer of material from one counterface to the other as a result of the formation of an adhesive junction under a normal load. Adhesion is facilitated by close contact, plastic deformation, or frictional heating.(lQ) Upon further sliding, some of the junctions can rupture, and the polymer can be transferred to the counterface of a metal or another polymer. 

When sliding occurred between high density polyethylene pin and glass surfaces, there was no transfer of polymer on the smooth surface but there was some transfer on the abraded surface. The large fluctuations in the coefficient of friction in sliding against the smooth surface (Figure 4) indicate intermittent fracture of adhesive junctions at the interface. It was accompanied by a wear rate of... 

Wear is a complex phenomenon, involving a wide range of possible processes (11) but invariably two of the most important governing factors are the normal load and sliding distance. At a constant normal load we have observed that both the non-stationarity and microslip increase with velocity. In terms of the adhesion model of friction, which provides a quantitative description of the frictional behaviour of PET fibres (2-3) a frictional event is the formation and subsequent rupture of an adhesive junction. It is reasonable to suppose that these events are the underlying cause of wear. Micro-... 

From a practical point of view we make a distinction between normal conditions of friction and wear and catastrophic conditions that cause a drastic increase of the friction coefficient and lead to rapid destruction of the contact. The described phenomenon called seizure normally appears only after some time because the contact gradually deteriorates until adhesive junctions cover a significant fraction of the surfaces and the contact abrupt fails when the surfaces stick together. 

Adhesive wear can be explained by a model first proposed by Archard [6]. The two surfaces in relative motion only touch at the asperities. When the normal force Fn is applied, the contact zones undergo plastic deformation and micro-welds, referred to as adhesive junctions, are formed. This is the same mechanism as that governing adhesive friction, but here we are interested not primarily in energy dissipation, but in the rate at which material is torn off from the adhesive junctions. If j is the number of adhesive junctions, the contact area is given by ... 

The four wear mechanisms described in this section all lead to Archard s law, but the physical interpretation of the wear coefficient differs. In the adhesive wear model the wear coefficient expresses the probability that an adhesive junction leads to formation of a wear particle. In the abrasive wear model the wear coefficient depends only on the geometry of the abrasive. The wear coefficient in delamination wear characterizes the critical number of cycles leading to fatigue fracture of microscopic subsurface cracks. Finally, the wear coefficient in oxidative wear includes the growth constant and the critical thickness of a surface oxide film. 

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