Lubrication without grease or oil
Dry lubricants meet growing convenience and cleanliness requirements
Published in Der Zuliefermarkt, April 2004

Dry lubricants – and in particular bonded coatings, bearing compounds and electroplated coatings containing solid lubricants – serve primarily to prevent wear and optimise friction. They can be found in all kinds of applications and meet individual requirements over a very long time, e.g. corrosion protection, resistance to temperature, vacuum and radiation; they also help to prevent stick-slip and fretting corrosion. Since they firmly adhere to the component surface, problems of flow-off or similar phenomena that might occur during machine downtime or storage are avoided in the first place. The gluing together of lubricated parts, which is an issue in automatic assembly, is also eliminated. Other benefits include the versatility of dry lubricants allowing cost-saving designs in many industrial areas.

One thing can be assumed: Anybody who can avoid using a lubricating oil or grease will normally do so. Seals cannot reliably prevent leakage under all circumstances or keep the lubricant from flowing off due to gravity and its particular flow characteristics. It is therefore little surprising that dry lubricants are making their way into ever more applications. While a viscous or consistent lubricants requires motion at a certain speed to separate surfaces, a bonded coating retains this effect also when slowing down to a standstill. In applications where mixed friction prevails, bonded coatings fully separate the component surfaces and hence provide wear protection also under adverse operating conditions. Other drawbacks of oil or grease lubrication, like oxidation or cracking, are eliminated as well.


Bonded coatings

Bonded coatings adhere firmly to the component in form of a very thin layer. Wear protection is achieved not by extreme hardness, as is the case with hard material coatings, but by optimised friction and sliding characteristics. As the layer is very thin, shear stresses within the coating are low enough to be safely transferred from the coating across the transition to the substrate material, provided the coating has been properly applied.

Fig. 1: Surface with bonded coating, before and after running-in

In their structure and handling, bonded coatings are similar to other industrial lacquers (Fig. 1). Their composition is different, however:

Further components may be a filler (colouring agent) or additives (e.g. an anticorrosive).

When exposed to frictional load, some of the bonded coating is worn off, an effect which is referred to as "chalking off", leading to "transfer lubrication". It might be compared to a pencil, consuming its lead while writing. The resulting film of rubbed-off particles acts as a lubricating layer between the friction bodies.

The service life of a bonded coating is limited. It mainly depends on the binder's wear, its elasticity and adhesion to the component surface. The major performance features of bonded coatings are:

• Extremely wide service temperature range
• Effective wear protection
• Constant, optimised friction coefficient
• Lifetime lubrication
• Corrosion protection
• Prevention of stick-slip and fretting corrosion
• Resistance to vacuum and radiation

Bonded coatings normally reach their limits in applications involving long sliding distances, especially in combination with rough mating surfaces. This is due to the fact that relative motion against the opposing component causes some of the bonded coating to "chalk off" and be transferred to the mating surface. The extent to which this happens depends largely on the quality of the bonded coating (especially the binder and the solid lubricant contained) and the surface roughness of the opposing body.


Multiple uses

As long as the above described limitations are observed, bonded coatings offer excellent performance in a wide range of applications, especially in bearings, motor and gear components (normally in combination with oil or grease lubrication), screw connections and on elastomers.

In automotive factories, and in other sectors where components are assembled automatically, dry surfaces are of high value to prevent parts from sticking together. In the car industry, this has the added benefit of preventing annoying oil stains inside the car. Dry lubrication can be used on numerous car components: seat adjustment mechanisms, air nozzles, switches, doors, etc. Possible applications range from the engine and the brakes to the seat belt mechanisms, the steering column and locks. Bonded coatings have proven to be hardly affected by varying operating conditions.


Electroplated coatings with MoS₂

For vacuum applications, but also for extreme temperatures between –200 and 350 °C, electroplated coatings with MoS₂ might be suggested. Besides being "clean", this technique offers the following benefits:

• Consistency resistant to temperatures between –200 and 350 °C
• Lifetime and minimum-quantity lubrication
• Resistant to solvents, fuels and lubricants
• Suitable for bearing speed factors up to n x dm = 700,000 mm/min
• For high to ultra-high vacuum up to 10^-12
  

Depending on the application method used, the resulting layer thickness is 2 to 5 μm, possibly higher with special techniques. The inner and outer bearing ring as well as the cage are coated prior to bearing assembly. The bearing's internal clearance between rolling elements and rings should be C 4. Electroplating is possible on rolling bearings made of 100Cr6 steel and stainless steel, however not on aluminium and aluminium alloys. Bearings coated in this way are primarily used in precision engineering, optics, electronics as well as in aerospace applications, cryogenic and vacuum technology.


Self-lubricating compounds (dry reservoirs)

Bearing compounds are intended for special applications involving high loads and low speeds. They offer the following advantages:

• Clean, non-drip lubrication
• Consistency and viscosity resistant to temperatures up to 550 °C
• Lifetime lubrication
• Excellent corrosion protection
• Resistance to water and oil
• Water as solvent

A newly developed compound was tested in deep groove ball bearings and support roller bearings (Fig. 2). At temperatures up to 250 °C, excellent results of more than 1000 operating hours were obtained for deep groove ball bearings, and more than 10,000 operating hours for support roller bearings.

How the compound works: Upon treatment with a release agent, the bearing is completely filled with the compound paste. Once the compound has hardened and the bearing is made to rotate, a very small compound quantity will be caused to chalk off by the rolling motion. Dry lubricant is thus transferred to the friction surfaces, which ensures reliable, continuous dry lubrication. The quantity of rubbed-off compound must remain constant within a certain range: too much would jam the bearing, while an insufficient dry lubricant supply would cause pronounced wear due to lubricant starvation. Several tests have proven that the bearing compound is also suitable for high loads. All known applications were within a n x dm speed range up to 50,000 mm/min, as are often found in support and guide rollers in conveyors, automatic baking ovens and kiln carts.

A similar compound for the dry lubrication of plain bearings is also available. The compound's lubricating effect in a plain bearing has been thoroughly tested on a special test rig, which showed that excellent friction coefficients were attained with purely dry lubrication.

Fig. 2: 2" and 4" support roller bearings filled with self-lubricating bearing compound

How to save money

Besides their many technical benefits, bonded coatings have also proven on numerous occasions in practice that they can help to save costs. Diesel engines, for example, can be run in faster while yielding more power, which makes for better economy. In other applications, bonded coatings may replace expensive hard material layers. And it should not be forgotten that the use of bonded coatings always entails minimum lubricant quantities. For a layer thickness of approx. 10 μm that is to provide lifetime lubrication, less lubricant is required than with any other lubrication method. Lubricant quantities vary, of course, but the lubricant needed in a bonded coating is often less than 10 percent of that required with a wet lubricant. This goes along very well with the idea of using small quantities of high-performance lubricants and thus avoiding large quantities of used lubricant that need to be disposed of at considerable cost later on.

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