Drive-line vibration and chatter are other problems of friction. In these cases, stick/slip motion is usually the cause. This unstable sliding motion occurs at very slow speeds, when movement starts and stops as dynamic friction rises above, then falls below static friction. (Interesting fact: only PTFE has lower static friction than dynamic friction.)

Deformation or destruction of delicate mechanisms such as lock components can be caused by excessive friction.

Friction coefficients (measured by steel rubbing against coating) typically vary from about 0.06 for PTFE materials to about 0.15 for moly coatings, although values as low as 0.02 have been measured for Xylan coatings.

Xylan is particularly useful when temperatures exceed the operating limits of conventional mineral oils or even synthetics. Because Xylan coatings are based on resin systems with wide temperature capabilities, they can be used from cryogenic levels to 260°C/500°F, with many being stable for brief periods at 320°C/600°F.

Where galling, abrasion, and high energy loss due to friction are anticipated, consider applying coatings of 25 microns/0.001 in. or more to minimize friction and wear.

Potential applications include rotors for compressors, air cylinder pistons, hinges and sliding bearings. The best coating choice is the one which provides the desired coefficient of friction and the maximum pressure/velocity capability.

Using a Xylan coating in a bearing cavity where a fluid lubricant is also used reduces friction losses in the bearing to the lowest possible level because Xylan is oleophobic (it sheds oil). During rotation, viscous shear forces within the bearing are reduced slightly. Thus, instrument bearings or other systems in which minimum bearing friction is critical can benefit from a thin coating (7.5 microns/0.0003 in.).

Excessive friction is also detrimental to bolted joints, in that much of the tightening torque is expended overcoming thread-to-thread and bearing-face friction. In these situations, the bolt is not properly tensioned (preloaded) and the joint can be unexpectedly weak in service. In addition, improperly fastened parts are subject to backout when vibration occurs. Coating the threads reduces the makeup torque by as much as 60 percent and enables users to set preloads more accurately.

Because of its toughness and corrosion resistance, the PTFE-matrix in a thermosetting binder is preferred for these applications. (For more information on friction, ask for a copy of "How to reduce friction with Xylan.")

The oil embargo
The oil embargo of 1974 increased fuel costs as much as 80 percent, catching America with cars that averaged 5.7 km per liter/13.3 mpg. The situation for trucks was even worse.

Previously, bonded dry-film lubricants had been used as insurance to back up fluid lubricants. However, the internal components of an engine operate in an environment that is hostile to most low-friction coatings. It is hot (>320°C/ >600°F), and many of the fluids encountered (fuel, combustion vapors, battery acid, brake fluid, glycol) attack many polymer coatings. Also, wear rates on pistons, bearings, gears, valve stems and fan drives are greater than most coatings can withstand.

Several formulations of Xylan coatings worked well in this environment. Because they were hard, wear resistant and stable at over 260°C/500°F, Xylan 1010, 1014 and 1052 were tried and selected for several applications.

In one early experiment, a trucking firm tested Xylan 1010 in the engine of a delivery unit, coating the pistons, bearings, connecting rods and crankshaft. Careful documentation proved that, after 200,000 miles/322,000 km, the engine used almost 15 percent less fuel.

Over the past fifteen years, engine manufacturers have found that friction reduction has resulted in increased engine output of as much as 16 percent.

In another example, a well known manufacturer of diesel engines replaced PTFE "buttons" on piston skirts with Xylan to reduce piston "slap." Other applications followed.

For viscous fan drives, Xylan proved to be the ideal coating to prevent the internal drive rotor from striking the drive housing. This eliminated the heat buildup that caused the drive fluid to gel.

Many of the parking brake actuators found on American vehicles are coated with Xylan — because it resists corrosion and the high thread loads (2,000 kg per cm²/28,000 psi).

Today, there are hundreds of different parts coated with Xylan in automobiles around the world, many of them in environments that would melt or degrade other coatings. From clutch actuators to air conditioning compressors, these coatings improve the mechanical performance of the products by reducing friction, resisting corrosion and withstanding wear.