Fretting

Fretting generally occurs at contacting surfaces that are intended to be fixed in relation to each other but that actually undergo minute alternating 

Prevention. Fretting, because of the significant role of oxidation, is best combatted by oxidation-resistant coatings, for example, electroless nickel or softer self-lubricating coatings like silver or indium. Solid-film lubricants are also successfully employed. Additional information on prevention of fretting corrosion can be found in Chapter 2, Principles of Corrosion.  

Prevention of rolling-contact fatigue is accomplished through proper design and load ratings, lubrication, and the use of through-hardened or case-hardened premium quality (clean) alloy steels. 

One important means of reducing wear (as well as friction) is lubrication. Lubrication not only reduces the power consumption needed to overcome friction but also protects rolling and sliding contact surfaces from excessive wear. Even with lubrication, however, wear still occurs. 

There are several basic modes of lubrication. In all modes, contact surfaces are separated by a lubricating medium, which may be a solid, a semisolid, or a pressurized liquid or gaseous film. 

Probably the most controversial of all subjects associated with molybdenum disulphide has been that of corrosion. After the rapid increase in its application between about 1955 and 1962, there were widespread reports of corrosion arising from its use. In Europe thia was serious enough to result in a loss of confidence and a bad reputation from which it has probably still not completely recovered almost forty years later. 

In the United States it was already much more widely used than in Europe, including important applications in military aircraft and in the space programme. As a result there was a greater emphasis on analysing and overcoming the problem, but the association between molybdenum disulphide and corrosion still exists in many people s minds, in spite of the fact that the problem has been understood and controlled for well over twenty years. 

It is fair to say that some of the early reports were ill-founded, and magnified by rumour. There were also categorical reports from some users that molybdenum disulphide, even as free powder or burnished films, positively prevented or cured corrosion problems. These must in most cases have been equally ill-founded, although they provide an interesting parallel with the comments of Cramer in 1764, which were quoted in Chapter 1. 

One important factor in those early days may well have been the fact that designers were used to dealing with lubricating oils and greases which gave very 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

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H.Toda Quantiative evaluation of fretting fatigue cracks on the surface of the railroad axles by grazing SH Wave ultrasonic method. Journal of JSNDI, Vol. 40, March, pl58-164, (1991)... 

High temperature fatigue and fretting fatigue behavior has also been improved by implantation (113,114). This has been achieved by using species that inhibit oxidation or harden the surface. It is generally accepted that fretting behavior is closely coimected to oxidation resistance, perhaps due to third party effects of oxidation products. Oxidation resistance alone has also been improved by ion implantation (118—120). 

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

R. B. Waterhouse and A. Niku-Lari, Metal Treatments Mgainst Wear, Corrosion, Fretting, andFatigue, Vol. 6, Pergamon Press, New York, 1988. 

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. 

Fretting Corrosion This attack occurs when metals shde over each other and cause mechanical damage to one or both. In such a case, frictional heat oxidizes the metal and this oxide then wears away or the mechanical removal of protective oxides results in exposure of fresh surface for corrosive attack. Fretting corrosion is minimized by using harder materials, minimiziug friction (via lubrication), or designing equipment so that no relative movement of parts takes place. 

L - iiuluctor coil indtictance in henry = ground capacitance )er phase in farad atid / = system fret uency in Hz... 

Less Shaft Wear - Of course o-rings can damage and fret a shaft if the equipment is misaligned, but it takes much longer than with other designs. 

The seai rotary face moves to maintain aiignment with the stationary face. The o-ring slides and frets the shaft. 

The radial pressure is not eonstant over the length of the hub, but in faet peaks at the projeeting portions of the shaft whieh resist eompression resulting in an inereased pressure at the ends of the hub, or stress eoneentration. For this reason, fretting fatigue failure may be antieipated when the applied torque is alternating. 

Figure 13-9a shows the relative separation of the full-film, mixed-film, and boundary. If a full-film exists, the bearing life is almost infinite. The limitation in the case of full-film is due to lubricant breakdown, shock load, bearing surface erosion, and fretting of bearing components. Figures 13-9b and 13-9c are cross sections showing the various contamination types. Oil additives are contaminants that form beneficial surface films. 

Cold flow, wear, and fretting High vibration... 

ICPMS can be considered a high-sensitivity extension of mass spectrometry, as well as an increased-sensitivity detector replacing optical ICP (ICP-OES) analysis. In fret, both viewpoints are accurate, and the wide application of ICPMS analysis... 

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

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