Metal friction joints

Another group of compositions of this type is used to protect metal friction joints during transportation and preservation. They represent compositions thickened by polymers of soap, silica gel and soap-hydrocarbon lubricants impregnated with anti-frictional fillers (molybdenum or graphite disulfide), oil-soluble Cl, anti-seizure additives and surfactants. To this type of Fluids belong such compositions as Tenox Film 30 (Sophes Co., France), Koral 530 (Hungary), SACI 500 and Shell Ensis Fluid 256 (Shell, GB). 

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

Parts of friction joints and seals are often made from inhibited structural plastics (see Chap. 4). Their main role in metal-polymer friction joints is to abate Mechano-chemical wearing of metal counterbodies. Application of inhibited antifrictional materials is considered to be a promising trend for extending the life of friction joints operating in hostile media. The structural polymers or their blends most often contain plasticizers. Cl, fillers and modifiers as additional components. The inhibited plastics are employed in plain bearings, sealing elements and other members of oil-extracting equipment and vehicles. 

Structural inhibited plastics have found application in the sealing of movable and stationary metal-polymer joints operating in hostile environment. They are most efficient in weakly and moderately loaded friction joints, plain bearings and seals. Their main role in metal-polymer friction joints is the reduction or suppression of mechanochemical wear. 

Despite the fact that the peculiarities of metal wearing in contact with polymers were determined more than 30 years ago [35], the serviceability of metal-polymer friction joints has resisted estimates for a long time. The origin of this corrosion wear mode was considered only for plastics processing equipment, in which metals are in contact with the moving polymer melt [36]. 

It should be emphasized that use of inhibited plastics in metal-polymer friction joints assists in a number of cases in solving the problem of fighting mechanochemical wear through the suppression of corrosion processes in the friction zone by Cl liberated by the polymer counterbody. Yet inhibited plastics are not the only means of corrosion inhibition in metal-polymer joints. 

Solid WI are used as triboengineering structural or lubricating materials. Plastics filled with solid lubricants are included in this class of materials since they lower the heat-induced strain in frictional joints through a reduction of the corrosion activity of the products of tribochemical reactions. Fusible metals and alloys of frictional parts may also ensure a protective effect... 

In its true sense, any component of a tribosystem promoting suppression of unfavorable tribochemical processes in the friction zone, irrespective of whether it was impregnated or formed internally, can be related to WI [34]. WI can be formed both as a result of physical-chemical interactions of polymers and metals, or physical fields and energy effects on a part or friction joint. This can be, e.g. thermochemical or radiation-thermal treatment, ion implantation, superposition of electrical and magnetic fields, shifts of electrode potentials, passage of electrical current, etc. 

Electrical polarization of friction joints is an effective means of fighting mechanochemical wear. Polarization parameters are chosen so as to transfer the metal into a stable passive state. In other words, the metal potential maintained during operation of the friction joint is different from a standard electrode one. The sources of such polarization can be metal-polymer parts and joints of machines and equipment containing M1-P-M2 pairs. 

Proceeding from the above, current generated in M1-P-M2 systems is able to ensure stable polarization of the friction pair. Polarization takes place when such materials as PA, PPl, PVC, polyolefines and some other thermoplastics are used as the lining material. The polarization sources may appear in seals containing polymer linings, in friction joints whose parts are covered by polymer coats, in reinforced polymer articles and so on. These sources may perceptibly affect friction and wear of metal parts operating in hostile media or found between facings in the electrical contact. 

Properties of engineering materials are known to vary under the action of electromagnetic fields. For instance, wear of metal-polymer friction joints was found to reduce substantially in response to exposure to a flow of charged particles in conditions of electrochemical reactions. Besides, methods of regulating the conductivity of materials and polarization of crystals by electrical fields, as well as by so-called training methods of semi-conducting substances and many others are known in the art. 

Let us examine some examples of improved service characteristics of metal-polymer friction joints with the help of electrical fields. A metal-polymer joint (MPJ) is a combination of metal and polymer parts operating in coordination [66]. The durability of an MPJ depends on its design, the properties of constituent materials and the operation conditions, including temperature, pressure, mutual displacement velocity, ambient media, physical fields, radiation etc. During operation, an MPJ undergoes certain changes in its material structure, wearing, etc. that impair its performances and life of the joint as a whole (Fig. 4.18, solid arrows). 

To lower the friction and mechanochemical wear in friction joints it is possible to use solid lubricants [71] containing fuming-oxides (FO) as an active modifier of nonferrous metals. The solid lubricants represent fine-dispersed sublimation products of lead smelting slag. 

Protective metal coats and films on the working surfaces of rubbing bodies are frequently used as protectors of friction joints. These films of more electronegative or more electropositive metals than the substrate can be ap>-plied on the contact surface. The former function as protectors, the latter passivate the substrate material operating as sacrificial anodes. 

Many researchers believe that the products of tribochemical transformations of polymers and metals used as corrosion inhibitors may enhance the wear resistance of metal-pol3mier friction joints operating in hush media. A friction joint can be theoretically presented as a reactor continuously generating products able to lower mechanochemical wear. For practical purposes we should know the regularities of these tribochemical transformations in macromolecules and their relation with the nature and surface parameters of the frictional metal counterbody. 

The brief information on polymer transformations under tribochemical effects cited above can be used for estimation of mechanochemical processes in the metal-inhibited plastic tribojoints only in the first approximation. A unified theory of tribochemical transformations of pol3Tuers has not yet been developed and the mechanisms of macromolecular conversions during friction are still to be studied comprehensively. Some investigation results of tribochemical phenomena in metal-polymer friction joints are discussed below. 

The composition of the products of tribochemical reactions depends on the service load and velocity regimes of the metal-polymer friction joint and the origin of the contacting materials. As has been indicated earlier, the... 

As has been indicated previously (see Sect. 4.2), polymer parts in friction joints add specific features to electrochemical processes in the friction zone. The polymer components acquire the properties of surfactants during rubbing against metals, which change the electrochemical activity of the metals. In this connection, the effect of the liberation of Cl from inhibited plastics on electrochemical processes in the friction zone has been studied [128]. 

Proceeding from the above data we can state that electrochemical processes in the metal-pol3mier friction zone can be regulated with the help of Cl, and hence friction and wear of metal-pol3mier joints can be monitored as well. 

It is quite apparent that Cl are able to function as efficient wear inhibitors and contribute to extending the lifespan of metal-polymer friction joints. 

Excellent high strength welds have been produced by inertia-welding, or friction-welding, which develops essentially no heat-affected zone. The actual interface is wrought molybdenum because the molten metal and the adjacent soHd metal that has been raised to very high temperatures have been expelled from the joint. 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

Clamped-insert joints (Fig. 10-167) are nsed for flexible plastic pipe np throngh the 2-in size. Friction between the pipe and the spnd is developed both by forcing the spud into the pipe and by tightening the clamp. For the larger sizes, which have thicker walls, these methods cannot develop adequate friction. Insert joints also have high pressure drop. Stainless-steel bands are available. Inserts are available in nylon, polypropylene, and a variety of metals. 

Polymers have found two specific applications in orthopedics. First, they are used for one of the articulating surface components in joint prostheses. Thus, they must have a low coefficient of friction and low wear rate when they are in contact with the opposing surface, which is usually made of metal or ceramic. Initially, poly(tetrafluoroethylene), PTFE, was used in this type of application, but its accelerated creep rate and poor stress corrosion caused it to fail in vivo. Ultimately, this material was replaced with... 

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