Metal-Polymer Boundaries

Analysis of Metal-Polymer Boundaries using Ultrasonic Interface Waves... 

Surface modification of a polymer prior to metallization is widely used to improve adhesion. The most common surface modifications employed are electric discharge (corona and plasma) and, more recently, ion-beam treatments QJ- Several mechanisms have been proposed for the improved adhesion after such surface modifications (2). These include mechanical interlocking, the elimination of weak boundary layers, electrostatic attractions, and chemical bonding. All of these can play a role in adhesion depending on the surface modification used, metal/polymer system, type of metal deposition, and the extent of polymer preparation employed. However, for low power, short exposure modifications, the formation of new chemical species which can provide nucleation and chemical bonding sites for subsequent overlayers is considered to be of prime importance (3-51. 

Polymer melts adhere to metals, so there is no slip at the metal/polymer interface. When one metal boundary of the melt moves parallel to another at a velocity V, the drag flow causes a shear flow with a constant shear rate (Fig. 5.4). At the interfaces, the polymer and metal velocities are equal (0 at the stationary surface and Vm at the moving surface. The shear strain rate y is given by... 

The effect of the solid body surface on the structure of the polymer boundary layers has been considered in detail in a great number of publications and we will not consider this issue in detail. We point out only that a number of publications have reported correlation between the structure of the bound lIy layer and the adhesion strength for couples such as metal-polycapro lmide coating [33], fluoroplastic-steel [34], and epoxy rubber polymers-metals [35]. 

A gradual decrease in strength is noted for specimens cemented with Sprut-5M. The strength decreases under water over 24 x 10 h, by cohesion fracture pattern. The strength decrease practically stops then, but fracture occurs predominantly by mixed or adhesion pattern. This is probably connected with diffusion of water at the polymer-metal phase boundaries and with low water resistance, typical of polyester resin-based adhesives. 

The homogeneous models assttme three phases, i.e., metal, polymer film arrd an electrolyte solutiott. Electrorric, tttixed electrorric (electron or polaron) and iotric charge trarrsport processes are cotrsidered in the metal, within the polymer film and in the solutiott, respectively. The polymer phase itself consists of a polymer matrix with incorporated ions arrd solvent molecrrles. A one-dimensional model is used, i.e., the spatial changes of all qttantities (concerrtrations, potential) within the film are described as a function of a single coordirrate x, which is a good approach when an electrode of usual size is used. The metal Ipolymer and the polymer solution interfacial boundaries are taken as planes. Two intetfacial poterttial differences are considered at the two interfaces, and a potential drop inside the film when crrrrerrt flows. The thicknesses of the electric double layers at the irrterfaces are small in... 

In a broad sense a parallel combination of charge transfer resistance and CPE elements, in series with finite diffusion element typically represent the circuit. When potential modulation is introduced, charge-transfer-related impedances decrease with increases in electrochemical potential and capacitance for the metal-polymer interface. The capacitance is usually nonideal due to film or electrode porosity [13] and typically is represented by the CPE element. If the film is formed as a reflective boundary, the angle is sometimes different from -90° because of inhomogeneity of the film and distributed values for diffusion coefficients. If two films are formed on the electrode, two RI CPE semicircles are often observed. 

Sihcate solutions of equivalent composition may exhibit different physical properties and chemical reactivities because of differences in the distributions of polymer sihcate species. This effect is keenly observed in commercial alkah sihcate solutions with compositions that he in the metastable region near the solubihty limit of amorphous sihca. Experimental studies have shown that the precipitation boundaries of sodium sihcate solutions expand as a function of time, depending on the concentration of metal salts (29,58). Apparently, the high viscosity of concentrated alkah sihcate solutions contributes to the slow approach to equihbrium. 

This is a standard friction problem. A glance at Fig. 25.5 shows that, when polymers slide on metals and ceramics, x can be as low as 0.04. Among the polymers with the lowest coefficients are PTFE (Teflon ) and polyethylene. By coating the ski or sledge runners with these materials, the coefficient of friction stays low, even when the temperature is so low that frictional heating is unable to produce a boundary layer of water. Aircraft and sports skis now have polyethylene or Teflon undersurfaces the Olympic Committee has banned their use on bob-sleds, which already, some think, go fast enough. 

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

A further complication is that even if acceptable, simple, low-cost, but accurate methods of polymer detection existed, the level of product reserve in the bulk water often has little relevance to the reactions taking place at the metal-water interface and boundary layers. 

This velocity profile is commonly called drag flow. It is used to model the flow of lubricant between sliding metal surfaces or the flow of polymer in extruders. A pressure-driven flow—typically in the opposite direction—is sometimes superimposed on the drag flow, but we will avoid this complication. Equation (8.51) also represents a limiting case of Couette flow (which is flow between coaxial cylinders, one of which is rotating) when the gap width is small. Equation (8.38) continues to govern convective diffusion in the flat-plate geometry, but the boundary conditions are different. The zero-flux condition applies at both walls, but there is no line of symmetry. Calculations must be made over the entire channel width and not just the half-width. 

In summary, for metal surfaces in boundary lubrication, complex tribochemical reactions occur along with the physical/chemical adsorptions, which lead to the formation of surface hlms, consisting of reaction products, oxide layer, the mixture of particles and organometallic polymer, and perhaps a viscous layer. The surface hlms operate as a sacri-... 

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