Fighting friction with solid-film coatings

By Brian Willis
Sr. Applications Engineer
Whitford Worldwide
West Chester, Pa.
www.whitfordww.com

Edited by Jean M. Hoffman

Low-friction coatings are used on bearings for two reasons. The first is as a backup lubricant if the primary (fluid) lubricant fails or is insufficient to prevent surface-to-surface contact. Lubricant failure occurs when new equipment is in its break-in period, during machinery start-up, or during reversing operations.

Occasionally, lubrication fails when a conventional hydrodynamic bearing is momentarily overloaded and the film of oil is partially pressed aside. In these conditions, there is mixed lubrication or, worse, boundary lubrication. Boundary lubrication refers to lubrication failure between two rubbing surfaces that don't have a full-fluid lubricating film between them.

The second reason for using low-friction coatings is to handle applications that can't use fluid as the primary lubricant. Examples include nonmetallic bearings made from fiber, plastic, composites, or paper in instrument and aircraft applications. The coating also reduces friction in the bearing, which in turn reduces the heat and wear on the coating.

Some bearing applications benefit more than others from the application of a coating. Here are some examples.

Length-to-diameter ratios in journal bearings with hydrodynamic lubrication should be greater than one. These types of bearings are found on line shafts, prop shafts, turbine shafts, pumps, machine tools, and some axle bearings. Side leakage from these bearings is not enough to cause total loss of the oil film. In many applications, notably engine bearings, the 1/d ratio of the bearings is 0.5 or less and side leakage can be great enough to cause metal-to-metal contact in the bearing, particularly under high loads.

A coating on the bearing surface can prevent momentary contact between the journal and the shaft. In bearing designs with a 1/d ratio below 0.4, experts recommend using a coating to help ensure fail-safe protection.

Slow-turning bearings such as journal bearings with shaft speeds below 600 rpm usually are built with medium fits. An accepted rule of thumb is to use a diametrical clearance that is 0.002 + (0.001 diameter) in. Under these conditions, overloads can result from momentary loss of lubrication, increased wear, and friction or heat. This also applies to bearings that frequently start and stop or reverse. Hydrodynamic lifts will help keep the journal floating during nonrotation and stem boundary lubrication problems.

Vibration damping of a rotating mechanism is influenced by lubrication fluids. A low-friction coating added to the bearing boosts damping but varies with coating thickness. However, to date, there have been no studies to determine the degree of damping dry-lubricant coatings provide. But where smoothness is paramount, a coating is worthwhile.

Infrequent operation of bearings causes fluid lubricants to migrate from the loaded area. And the bearings may experience severe wear during start-up. A low-friction coating can prevent metal-to-metal contact and wear during start-up.

SELECTING COATINGS FOR BEARINGS
Reduction of friction and support of bearing loads are both key for any coating used in a bearing. Coating formulators develop coatings based on tough resins, into which they add dry lubricants such as polytetrafluoroethylene (PTFE), molybdenum disulfide (MoS₂), and graphite.

PTFE is the premier low-friction constituent, with a coefficient of friction of 0.02. Coefficient of friction values for coatings with PTFE range from 0.02 to about 0.10, depending on PTFE loading. This corresponds favorably with the friction coefficients of bearings operating in the mixed and boundary layer conditions.

But coatings with low coefficients of friction have relatively high PTFE loadings which makes the coating soft and less wear resistant.

MoS₂ provides a coefficient of friction of approximately 0.12. The advantage of MoS₂ is its ability to bear high loads. Under low slip velocities, MoS₂ coatings can carry extreme loads.

Graphite is perhaps the oldest known dry lubricant. It has a coefficient of friction of about 0.12 to 0.16. In a coating, it's primarily used where ambient temperatures are high or where PTFE and MoS₂ can't be used. For example, graphite is the coating of choice on tobacco processing equipment. This is because it won't leave residue that would subsequently burn and be inhaled by humans. Graphite also performs well in applications where water is present.

APPLYING COATINGS TO BEARING SURFACES
Coatings containing PTFE, MoS₂, or graphite are available as one-coat products. They can be applied to almost any bearing material except high nickel alloys and polyolefin resins. Depending on the formulation, continuoususe temperatures are as high as 500°F, or 550°F for short periods of time.

Suitable metal substrates include steel (carbon and stainless), aluminum (wrought and cast), copper and its alloys, and titanium. Special precautions must be taken when coating powdered metal (P/M) parts. Many P/M parts are treated with resinous impregnates that get trapped in the porosity of the parts. P/M parts need prebaking at a temperature higher than the anticipated cure temperature for the coating. Contaminants that bleed to the surface during the prebake must be removed before coating.

Die-cast parts are another special case. These components are typically cast with alloys that can be porous. When coated parts are placed in an oven and heated, gases trapped internally expand and erupt. Cure temperatures over 400F may cause numerous eruptions on the coated surface. To avoid this, preheat the parts to the cure temperature to expel the trapped gases before coating. Another option is to cure parts below 400°F.

Engineering plastics that can be coated, include nylon, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), epoxy, polyester, and phenolic. The exceptions are families of polyolefins and fluoropolymers, which have natural release properties. Vinyl materials that contain a lot of plasticizer can also cause adhesion problems.

Coated plastic parts must cure at temperatures well below the softening temperature of the substrate to avoid distortion and polymer degradation. Most elastomeric parts that are not expected to elongate more than 30% in service can be coated. Woven and nonwoven materials are increasingly used as bearings. Coatings adhere well to the porosity of the fabric.

SURFACE CONSIDERATIONS
Here are some special considerations for improving the wear resistance of a coating in bearing applications:

In some cases, coating both surfaces of mating parts boosts lubricity and wear life slightly. But if only one surface can be coated, choose the surface with the greater swept area.

Many bearing designs use two dissimilar materials. In such situations, coat the softer of the two materials because it suffers the most damage from boundary lubrication.

The roughness of a mating surface also affects coating wear. The optimum surface has a roughness of 8 to 12- in. rms. Surprisingly, hyper-smooth surfaces of less than 4 in. produce higher wear rates than those from the optimum range to the machined surface range of 15 to 30 in. The reason is that hyper-smooth surfaces permit less PTFE transfer to mating surfaces, which increases friction. Surfaces that are rougher than 30 in. have higher wear rates.

Finally, curing temperature influences coating hardness. Coatings in bearing applications should be cured at or near their upper limit, typically 600 to 750°F.