Coating Selection for Parts in
Static Contact with Another with
Vibration (Fretting Situations)

If your part is in contact with another engineering component, but with no relative movements, then the main concern will be with corrosion. If the mating part is a dissimilar metal, then galvanic corrosion will be a significant risk. Both these eventualities are covered in the section on structural parts.

If the contact involves vibrational or impact motions then there will be a risk of fretting, fretting corrosion or even fretting fatigue. Before continuing with the analysis, you should read the section on Fretting in "Problem Solver".

This is particularly the case in splines and couplings where motion is transmitted from one part to another via a loaded contact. It can also occur in parts fastened or fitted together where there is a source of external vibration (e.g. heat-exchangers), as well as on bearing housings. Additionally, fretting-type failures are found on chains, pulleys and wire ropes.

The base material

Fretting is most prevalent with steel parts where the oxidative process creates a distinctive red oxide abrasive dust. Stainless steels are not immune, particularly ferritic types. Fretting of aluminium alloys produces a white oxide debris which is again very abrasive.

Contact conditions

A) Fretting and Fretting Corrosion.

With light loads or low cycles, fatigue will not be likely and the solution is usually one of reducing the tendency to oxidation by applying an inert coating. You can concider:

1. Heavy Electrolytic Nickel or Copper Plating will provide a low corrosion surface, but with a tendency to gall and to wear quickly
2. Electroless Nickel will provide good oxidation protection, with extra wear resistance if hardened. Additional improvements can be obtained by adding a further coating of MoS₂, ideally cured within an epoxy binder
3. Hard Chrome Plating for maximum wear prevention
4. Silver or Indium Plating, a soft, ductile interface with good oxidation resistance
5. Anodising for protection of aluminium alloys, preferably sealed with a self-lubricating polymer such as PTFE
6. Electro-Ceramic Coating, for protection of aluminium and magnesium alloys

B) Fretting Fatigue.

With high loads or prolonged operation, fretting may lead on to crack initiation and to fretting fatigue. Since electroplating can impair fatigue resistance of the substrate, the best solutions are usually thermally sprayed coatings with toughness and hardness. Try any of the following:

1. Ni/Cr for corrosion resistance and toughness in impact fretting
2. Tungsten Carbide/Co for maximum wear resistance
3. Ni/Cr/Chrome Carbide for higher temperature fretting situations

C) Oxidative Wear.

If there is small, slow speed relative sliding between the parts, this may also lead to a fretting-type wear situation (with oxidised wear debris trapped in the contact). It is common on chain links and wire ropes and sometimes occurs on pulleys. The only viable solution for wire ropes is by regular oil or grease soaking. For parts under high loading and which are traditionally made in high strength engineering steels there is no easy way of reducing the corrosive element of the wear process. The best approach is to increase the surface hardness so that it can at least resist the abrasion by the oxide.

1. Case hardening (Flame, Induction) for medium carbon steels
2. Carburising for low carbon steels
3. Nitriding or Nitrocarburising if the loads are not too high and the steel has some alloying elements such as Cr or Mo

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