Dissimilar Materials

Some of the most interesting and important chemical and physical interactions occur when dissimilar materials meet, i.e. at an interface. The understanding of the physics and chemistry at interfaces is one of the most challenging and important endeavors in modem science. 

Diffusion Theory. The diffusion theory of adhesion is mosdy appHed to polymers. It assumes mutual solubiUty of the adherend and adhesive to form a tme iaterphase. The solubiUty parameter, the square root of the cohesive eaergy deasity of a material, provides a measure of the iatermolecular iateractioas occurring within the material. ThermodyaamicaHy, solutioas of two materials are most likely to occur whea the solubiUty parameter of oae material is equal to that of the other. Thus, the observatioa that "like dissolves like." Ia other words, the adhesioa betweea two polymeric materials, oae an adherend, the other an adhesive, is maximized when the solubiUty parameters of the two are matched ie, the best practical adhesion is obtained when there is mutual solubiUty between adhesive and adherend. The diffusion theory is not appHcable to substantially dissimilar materials, such as polymers on metals, and is normally not appHcable to adhesion between substantially dissimilar polymers. 

Separation. In this function, the geotextile serves to separate two dissimilar materials (Fig. 3), eg, two different soils, landfill material and the native soil, stone material and subgrade sod, old and new pavement, foundation sods and various types of wads, or one of many other similar situations. In some instances, it is difficult to distinguish between the separation and stabilization functions because in both situations the geotextde is serving as a separator. However, in stabilization some additional phenomena occur. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

Thermocouples are composed of two dissimilar materials, usually ki the form of wkes, that accomplish a net conversion of thermal energy kito electrical energy with the occurrence of an electrical current. Unlike resistance thermometers, where the response is proportional to temperature, the response of thermocouples is proportional to the temperature difference between two junctions. Figure 5 illustrates such a ckcuit. 

Fig. 6. Rule 3 of thermocouples, where (a) represents a four-junction and (b) a two-junction thermocouple. A and B, the two legs of the thermocouple, are wires of dissimilar materials. Junctions are at temperatures T, and Ty Ey E, < ...

The necessary conditions for galvanic corrosion are (1) a corrosive interaction of electrochemically dissimilar materials that are (2) exposed to a common conductive fluid and are (3) physically linked so... 

In simple terms, galvanic potential is related to the magnitude of the current induced by coupling dissimilar materials exposed to a common conductive fluid. The magnitude of the potential depends on the materials that are coupled and on the characteristics of the fluid to which the metals are exposed. 

Can bond dissimilar materials together without special preparations or substrate incompatibility concerns. 

Thus, it is necessary to allow for the fact that two widely dissimilar materials have been combined into a single unit. In the basic design approach certain fundamental assumptions are made. The first, and most important assumption, is that the two materials act together. With a load applied (stretching, compression, twisting, etc.) the fibers and plastic under load is the same that is, the... 

The CLTE is an important consideration if dissimilar materials like one plastic to another or a plastic to metal and so forth that are to be assembled where material expansion or contraction is restricted. The CLTE is influenced by the type of plastic (liquid crystal, for example) and RP (particularly the glass fiber content and its orientation). It is especially important if the temperature range includes a thermal transition such as Tg. Normally, all this activity with dimensional changes is available from material suppliers. 

Sparks due to static electricity associated with the separation of two dissimilar materials (Table 5.5). The charges may be transported/conducted some distance after separation before there is sufficient accumulation to produce a spark, e.g. in the flow of liquids or... 

McGuiggan, P. M. (2008) Stick slip contact mechanics between dissimilar materials Effect of charging and large friction. Langmuir, 24, 3970-3976. 

Improve adhesion of dissimilar materials such as polymers to inorganic substrates. Also called primers. Primers generally contain a multifunctional chemically reactive species capable of acting as a chemical bridge. In theory, any polar functional group in a compound may contribute to improved bonding to mineral surfaces. However, only a few organofunc-tional silanes have the balance of characteristics required... 

Two dissimilar materials, with Hamaker constants, An and A22, interact as follows ... 

Debye phonon velocity) and lower in the case of very dissimilar materials. For example, the estimated Kapitza resistance is smaller by about an order of magnitude due to the great difference in the characteristics of helium and any solid. On the other hand, for a solid-solid interface, the estimated resistance is quite close (30%) to the value given by the mismatch model. The agreement with experimental data is not the best in many cases. This is probably due to many phenomena such as surface irregularities, presence of oxides and bulk disorder close to the surfaces. Since the physical condition of a contact is hardly reproducible, measurements give, in the best case, the temperature dependence of Rc. 

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