Wear Resistant Coatings
Resin-bonded PTFE dry-film lubricant coatings can solve numerous problems, including friction/wear, corrosion, temperature extremes, sticking, vibration, galvanic activity, electrical insulation and conductivity. The selection of a coating depends on determining the problem of the application (wear, heat, corrosion, etc.) and matching it with the material that most effectively solves the problem.
Friction causes heat, wear and loss of energy in dynamic applications. In severe circumstances, friction can cause overheating and seizure. Friction also causes brinelling, galling, scoring, and underloading of fasteners. Friction coefficients (measured by mating surfaces rubbing against a coating) typically vary from about 0.06 for PTFE materials to about 0.15 for Moly coatings.
Xylan® or Teflon® coatings are particularly useful when temperatures exceed the operating limits of conventional mineral and synthetic oils. Because the coatings are based on resin systems with a wide range of temperature capabilities, they can be used from cryogenic levels to 260°C/500°F, with many being stable for brief periods at 315°C/600°F.
Where galling, abrasion, and high energy loss due to friction are anticipated, consider applying coatings of 25 microns/0.001 inch or more to minimize friction and wear. Potential applications include rotors for compressors, air-cylinder pistons, hinges, sliding bearings. The best coating choice is the one which provides the desired coefficient of friction and the maximum pressure/velocity.
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, if the bolt is not properly tensioned (preloaded), 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 65 percent. Because of its toughness and corrosion resistance, a PTFE-matrix in a thermosetting binder is preferred for these applications.
Initial contact between mating metal parts results in momentary welding of asperities (peaks) on each surface. As each part continues to move, the welded asperities are ripped off, leaving behind minute pits. Every bearing and wear surface, no matter how smooth the finish, has these asperities.
The problem is common to impellers and housings, air-cylinder pistons, machine slides, telescoping mechanisms, ball joints, plungers, gear-teeth, hinges, journal bearings, valves, power screws. Teflon® or Xylan® coatings provide a thin layer of lubrication to prevent the asperities on mating surfaces from making physical contact with each other. The selection of the best dry lubricant (PTFE, Moly, or graphite) for these applications depends primarily on the PV value (Pressure x Velocity), atmosphere, and temperature of the application.
Wear is often severe in bearing-type applications. Rods that slide through glands, rolling element bearings, slide assemblies, telescoping booms, ball reversers, rocker arms, ball joints, tracks, bushings, and thousands of other applications are configured so that one part rolls or slides over another part. In most cases, friction and wear of the parts are reduced when one or both are coated with a dry film lubricant. Also, the coatings serve as a thin cushion, spreading high point loads in bearings and reducing element fatigue.
The energy that is transmitted and dissipated in a bearing is a function of the PV of the application. As the PV increases, so do heat and wear on the bearing surfaces. Dry lubricants have a “limiting PV value” that they can withstand for a reasonable wear life. Typically, the highest limiting PV which a 25micron/0.001 inch coating of Xylan® can withstand is approximately 50,000 (PV). This limiting value varies from coating to coating. Two factors to bear in mind:
First: the ability of a coating to bear loads increases as thickness decreases. For instance, while a 25 micron/0.001 inch film may be able to bear PV of only 50,000, a 5 micron/0.0002 inch film (the practical lower limit using current application techniques) may be able to bear PV of 150,000. For this reason, the PV tolerance of a coating may be modified by the film thickness.
Second: the lubricants themselves. PV limits are not constant. They tend to increase with pressure and decrease with speed. This is particularly true with Moly coatings, which work better under high pressure and low speed, where galling is the principal reason for failure. For break-in, frequent starts, and marginal lubrication, remember: the period of greatest wear to a moving mechanism is when it is new.
When equipment is started and stopped frequently, lubricants are subjected to stress, which can diminish their ability to lubricate. This can bring sliding metal surfaces into virtual contact (a condition known as “boundary lubrication”). If metal-to-metal contact does occur, the boundary lubrication can convert into actual failure as the metal surfaces meet and begin to wear, which can, in turn, lead to seizure.
A thin, high-wear coating reduces the chance of failure and lengthens the life of such products as sprockets, seal plates for compressors, pump pistons, cams, ball joints, conveyor trolleys, gears, journal bearings. These coatings solved wear problems under start/stop conditions in reciprocating plungers in electrical solenoids. Typical plungers are nickel or chrome-plated (and extremely hard). But the starting and stopping at the end of each half-cycle put the plunger into a boundary lubrication condition, causing the plating to wear rapidly. When a matrix coating replaced chrome plating, the boundary lubrication condition was overcome and plunger life was extended by 90 percent.
PTFE-type coatings are recommended for applications where initial wear is anticipated to be light to moderate; Moly-type coatings are recommended for conditions of heavy wear, especially in high-load situations.