Bearing Coatings

Thursday, January 03, 2013

Tags :  coatings

Rolling element bearings continue to develop to meet the needs of ever-increasing speeds and loads. We might expect that there would be few applications of engineering coatings to these components, but there is a growing number of applications for which we might consider various coating options.

A few months ago, in an article on plasma and thermally sprayed ceramic coatings, I wrote about the use of electrically insulating coatings being applied to the outer races of rolling element bearings in electric motors. With the increasing adoption of hybrid power units in race and roadcars, the effective electrical insulation of rolling bearings will be an important factor for reliability. There are several ways to achieve this, not all of which involve changes to a standard bearing, but electrically insulating coatings are one method which may be considered.

There are also reasons why one might consider the use of low-friction coatings on rolling element bearings. A rolling element bearing might seem an odd application for a low-friction coating, but these are in common use. There are two types of low-friction coating - those that are soft and which rely to some extent on shearing to give low-friction behaviour, and modern low-friction coatings which are extremely hard.

Soft metallic platings encompass the first group, of which silver plating is the most common. The application that you might be familiar with is the silver plating of steel bearing cages, which is especially common in needle roller bearings, as used widely for small-end bearings on two-stroke engines and for the support of camshaft drive gears on four-stroke race engines.

The modern, hard, low-friction engineering coatings also find use in rolling element bearings. Diamond-like carbon (DLC) is not a single coating but a ‘family’ of coatings of different composition and differing properties. Several bearing manufacturers supply rolling element bearings with the races coated with DLC.

When a bearing is functioning and lubricated properly, there might seem little reason to use a low-friction coating - after all, when adequate lubrication is available, there should be no contact between the rolling elements and the races. However, when the lubrication is marginal, or where loading conditions allow ’skidding’ of the rolling elements on the races, there is a case for hard coatings, especially where the races and rolling elements are made of the same or very similar material.

Where identical or similar materials are placed in moving and heavily loaded contacts, adhesive wear may occur. Minute high points on the contacting surfaces may become welded together, and material can be pulled off one surface. This damages both parts involved in the contact, and once roughened, the surfaces become much harder to separate satisfactorily through lubrication. By changing the surface so that we separate identical or similar materials, adhesive wear is very much less likely to occur.

Where loading is such that the rolling elements are allowed to skid across the races, and where lubrication is again marginal, the tangential component of the surface load can significantly increase the maximum subsurface stresses. As these are cyclic, wear may occur due to subsurface fatigue, shown as pitting. Having a low-friction coating helps in two ways: the tangential component of loading is minimised, as is the generation of heat in the contact. The temperature at the point of contact can be substantially higher than the surrounding material, and this can itself lead to increased stresses.

Written by Wayne Ward

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