Contact zone

Fretting corrosion (36,37) can lead to high contact resistance of base metal contacts, such as tin plate in electronic connectors. Small cycHcal displacements of the connector halves occur because of external vibration or differential thermal expansion and contraction of the mating contacts. The wear debris that is formed remains in the contact zone. The accumulation of oxide debris in the contact region leads to increased contact resistance. Solutions to this problem are stmctures that do not permit movement of contact surfaces with respect to one another, the use of gold as a contact finish, and the appHcation of thick coatings of contact lubricants and greases, which reduce the rate of wear and restrict access of air to the contact surfaces. 

The quantitative computations were conducted using equilibrium thenuodynamic model. The proposed model for thermochemical processes divides layer of the sample into contacting and non-contacting zones with the material of the atomizer. The correlation of all initial components in thermodynamic system has been validated. Principles of results comparison with numerous experimental data to confirm the correctness of proposed mechanism have been validated as well. 

A sphere in contact with a flat surface under the action of an applied load P (P > 0 for compression and P < 0 for tension) deforms as shown in Fig. 3. Let a be the radius of the contact zone. The center of the sphere is displaced by a... 

The separation profile outside the contact zone, D versus x, is given by... 

The surfaces of all materials interact through van der Waals interactions and other interactions. These interfacial forces, which are attractive in most cases, result in the deformation of the solid bodies in contact. In practice, the radius of the contact zone is higher than the radius predicted by the Hertzian theory (Eq. 7). Johnson et al. [6] modified the Hertzian theory to account for the interfacial interactions, and developed a new theory of contact mechanics, widely known as the JKR theory. In the following section, we discuss the details of the JKR theory. The details of the derivation may be obtained elsewhere [6,20,21]. 

In an appropriately designed experiment, it is possible to measure the pull-off force (Ps), contact radius (a versus P, ao and aj, and the separation profile outside the contact zone (D versus j ). From these measurements, it is possible to determine the thermodynamic work of adhesion between two surfaces, if the contacting bodies are perfectly elastic. 

To account for some of the shortcomings of the JKR theory, Derjaguin and coworkers [19] developed an alternative theory, known as the DMT theory. According to the DMT theory, the attractive force between the surfaces has a finite range and acts outside the contact zone, where the surface shape is assumed to be Hertzian and not deformed by the effect of the interfacial forces. The predictions of the DMT theory are significantly different compared to the JKR theory. 

Fig. 16. Schematic of the JKR test specimen used by Brown et al. The crosslinked polyisoprene (PI) lens is first loaded in contact with the substrate with a load P to join the interface. The load is then removed and radius of the contact zone decreases with time. The contact radius a t) is measured using an optical microscope. Reproduced from ref. [45].

Means should be provided for measuring and varying the slurry concentration in the contacting zone up to 50 % by volume. 

The Nye Tray increases the area available for disengagement of this light froth. In addition to the normal perforated section, vapor can now flow into the Inlet area below the downcomer. Vapor enters the contact zone of the Nye Tray through the perforated face of the inlet panel, under the liquid coming out of the downcomer. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Komatsu [478] has put forward the hypothesis that reaction in many powder mixtures is initiated only at interparticle contact and that product formation occurs by diffusion through these contact zones. Here, one of the participating reactants is not covered with a coherent product layer. Quantitative consideration of this model leads to a modified Jander equation. 

The rate of polymerization, as well as the mobility of macromolecules at the contact zone, is affected by the presence of the filler near the matrix. This results in a reduction of the number of possible conformations of molecules in the boundary layer developed, causing the formation of a denser material in the boundary layer than in the bulk. 

As a result of asperity contact, the nominal contact zone is split into a number of discrete areas that can be cataloged either to the lubrication region or asperity contact area (Fig. 2). The mean hydrodynamic pressure in the lubrication regions, pi, can be calculated by the average flow model, while contact pressure is estimated via Eq (7). Consequently, the film thickness is determined through numerical iterations to... 

All the models of mixed lubrication developed previously were based on a traditional idea, as schematically shown in Fig. 2, that the nominal contact zone, O, has to be divided into two different t3q>es of areas the lubricated area, 0,1, where two surfaces are separated by a lubricant film and the asperity contact area, where two surfaces are assumed to be in direct contact. The present authors and Dr. Zhu [16,17] proposed a different strategy for modeling... 

Both numerical analyses and experiment studies show that the roughness experiences significant deformation in contact zone, which will certainly make a strong impact on the properties of mixed lubrication, but the behavior of the as-... 

In the experiments based on optical interferometer, Kaneta et al. [50] measured the film thickness variation and asperity deformation as an artificial circular bump or latticed asperities pass through the contact zone. They found... 

Contact ratio, a, is defined as the real contact area divided by the nominal contact zone, where the real contact area is referred to as the sum of all areas where film thickness is below a certain criterion in molecular scale. The contact area was measured by the technique of Relative Optical Interference Intensity (ROII) with a resolution of 0.5 nm in the vertical direction and 1 /xm in the horizontal direction [69]. 

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. 

Fig. 11 —Nominal contact zone and real contact areas between rough surfaces in contact, (a) film thickness profile along the central line of contact, (b) a contour plot of the contact geometry where the white circular area and gray spots inside the circle correspond to the nominal and real contact area, respectively.

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