Bearings roll with solid lube coatings

New breed of bearing coatings offer advanced solutions for extreme applications.

By By Dr. Xiao Bo Zhou

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In some difficult or extreme bearing operating environments, such as vacuum, high temperature and dry-running, conventional oil and grease fluid lubricants have limited effectiveness. Under such conditions, conventional fluid lubricants either fail early or never are considered as an option.

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Solid lubricant coatings, however, offer a solution for diverse applications exhibiting extreme and difficult running conditions. The coatings effectively reduce friction, have high hardness and wear resistance, and strongly bond to the bearing steel, providing long operating endurance. Because the coatings are only 1 to 2 microns thick, they are extremely smooth (steel mirror surface). The coatings are deposited on finished bearing components, including those made of hardened bearing steel, at a temperatures below 200ºC — a temperature that does not affect the supporting steel's mechanical properties. There is no need for post machining.

The solid lubricant coatings entered field testing in 1998 and are now in a full application and development stage, providing engineering, maintenance and supervisory personnel responsible for bearing reliability with solutions that were not possible with traditional lubrication technology.

Coating types

There are three prominent solid lubricant coatings; metal-alloyed diamond-like-carbon, hard graphite, and metal-doped molybdenum disulfide.

Metal-alloyed diamond-like-carbon

This metal-alloyed coating is deposited as a multi-layer coating in a low vacuum by means of physical vapor deposition. It consists of approximately half diamond structure and half graphite. The diamond structure provides hardness and wear resistance; graphite gives lubricity. A metallic interlayer between the coating and the steel substrate ensures strong bonding.

Hard graphite

Pure graphite offers optimum lubricity in wet or humid environments. A high energy beam under vacuum deposits graphite as a high density coating, which results in a hardness exceeding that of hardened bearing steel. A metallic interlayer between the coating and the steel substrate works in combination with the high hardness to provide long service life in water lubricated, highly loaded rolling contact conditions.

Metal doped molybdenum disulfide

Molybdenum disulfide is a conventional, well-known solid lubricant having a layered structure. A traditional molybdenum disulfide layer is rather soft and degrades in humid conditions.

Consequently, traditional molybdenum disulfide coatings do not function well as a solid lubricant for rolling bearings.

The new breed of molybdenum disulfide coatings — which benefit from metal doping — is three to five times harder than the pure molybdenum disulfide coating and provides high humidity resistance. Metal doped molybdenum disulfide coatings have a metallic interlayer and offer an extremely low coefficient of friction in dry running conditions and extended service life in highly loaded rolling bearings.

Coating features

Several characteristics make the solid lubricant coatings effective in rolling bearings:

  • The coatings can be deposited either on rolling elements such as rollers and balls or on a bearing's rings. For most applications, coating the rolling elements provides adequate lubrication.
  • The coatings have high thermal resistance and can be used in bearing environments up to 400º C.
  • The coating's dynamic coefficient of friction is less than one-quarter that of steel. Low friction reduces heat generation and wear, and provides long service life for rolling and sliding contacts.
  • The coatings are hard — 30 to 50 percent harder than hardened bearing steel. This higher hardness increases wear resistance and results in long service life.
  • The coatings can be used in combination with conventional fluid lubricants and environmentally friendly "green" lubricants for reliable and improved performance in applications calling for that type of multi-lube solution.
  • Rolling bearings coated with the solid lubricants demonstrate a significantly longer life and four times the load carrying capacity of rolling bearings with traditional solid lubricant coatings.

Bearing coatings at work

When conventional fluid lubricants are not an adequate solution, solid lubricant coatings enable bearing users to overcome technical challenges to reduce downtime and extend lube intervals (or, in some cases, eliminate them). The range of performance issues that lubricant coatings address extends to water lubrication, fuel lubrication, high contamination and clean conditions, among others.

The following examples from four industries address three equally demanding application challenges: marginal lubrication, dry running and vacuum conditions. Because the solid lubricant coatings are so new, some examples are still in the development stage.

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