Plain bearing failure index

6.0 Chemical effects

6.1 Lubricant Oxidation Deposits
6.2 Lubricant Degradation Deposits
6.3 Copper Deposit on Thrust bearing
6.4 Tin Oxide Formation on White Metal
6.5 Sulphur Attack of White Metal
6.6 Sulphur Attack of Copper-Based Bearing Alloy
6.7 Sulphur Attack of Silver Bearing
6.8 Chemical Effects - Summary

A number of different chemical effects can cause problems in bearings. Although these effects may be different, there are similarities both in the appearance of the damage and the resulting failure. It is thus convenient to treat them together.

Three separate phenomena are described:

1. Chemical reactions taking place in the lubricant that result in the formation of deposits on the bearing surface. In most cases the rate of deposit formation tends to be temperature dependent so that deposit formation follows the temperature profile of the bearing surface. In general, the underlying bearing material is unaffected.
2. Chemical reactions between the bearing metal and the lubricant, lubricant degradation products or contaminants in the lubricant. This can have two effects: selective removal of different phases from the bearing metal or the formation of deposits on the surface of the bearing metal. Chemical reactions are temperature dependent and, once again, the corrosive removal of material or the formation of the reaction deposit reflects the temperature profile of the bearing surface.
3. Electrochemical reaction between materials in the lubricant, normally contaminants of some sort, and the bearing metal producing deposits on the bearing surface. The difference in this case is that electrochemical reactions are relatively independent of temperature so that the deposit forms uniformly over the bearing surface and is not restricted to the high temperature areas.

The problems created by these effects are similar: a modification of the bearing surface profile that leads to a degradation in its hydrodynamic film generating capacity and ultimately to a breakdown in the lubricant film. As thrust bearings are generally more sensitive in this respect than journal bearings, failure by these chemical mechanisms tend to be more frequent in the former, though the effects also manifest themselves in the latter. This can be an advantage in failure investigation, providing evidence that may be lost in the complete destruction of a thrust bearing.

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