• Wear

    Wear means the loss of solid material due to the effects of friction of contacting surfaces. It is generally harmful, although in some cases it can also be beneficial, for instance during the running-in of an engine. The deterioration of the surfaces in an engine is generally due to isolated or simultaneous mechanisms, among which we can distinguish the following.

    Adhesive wear
    This occurs as a result of metal-to-metal contact, due to overheating or insufficient lubrication. This in turn causes the formation of microwelds, with often a subsequent deposition of soft metal onto heavy metal (e.g., aluminum onto iron, lead onto steel). Consequently, there is a shearing of the junctions and a transfer of metal particles.

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Abrasive Wear

There are two types of adhesive wear – heavy, liberating relatively large metal particles (50 to 200 microns), called ‘scuffing’, and eventually leads to a failure of the engine; and moderate – the formation of very small metal oxides which is termed ‘soft’ or ‘normal adhesive’ wear (see Figure 1). Adhesive wear can be avoided by the use of an appropriate lubricant containing extreme pressure (EP) additives, and the choice of the correct viscosity oil.

Abrasive wear
This form of wear results from the grooving of a surface by hard asperities or by particles of rust or dust which have entered the oil. When these particles are very small, the phenomenon is known as ‘abrasive erosion’ (which is especially the case in hydraulic systems). Abrasive wear can be avoided by eliminating potentially abrasive particles through filtration.

Corrosive wear
This is chemical or galvanic attack, followed by the removal of the reaction products (chemical complexes) by mechanical action (friction). It can be avoided by the use of effective materials, also by the use of neutralizing additives in the oil. It may also be minimized by changing the oil in time.

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Fretting

Wear by fatigue
This means the removal of spalled away particles by fatigue resulting from contact, aided by vibration, high pressure, high temperature and other aggressive conditions. This type of wear may be reduced by re-equilibration of the system.

Contact corrosion (fretting corrosion)
Corrosion due to contact means the removal of material between two surfaces which are in almost static contact but subject to mechanical vibration and oscillation. Consequently, there is oxidation of certain particles. Thus, for iron materials, there is an accumulation of ‘red powder’. An example of this is the bearings of cars transported by railway (see Figure 2).

Erosion by cavitation
The formation of cavities by entrainment of air of gas bubbles present in the fluid in movement is a destructive phenomenon, which can provoke the removal of material particles (see Figure 3).

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Cavitation

Wear of electrical origin
This refers to the erosion by sparks, produced by inadequate electrical insulation in motors of alternators.

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