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| Think of a part in which friction, poor release characteristics, corrosion, wear, or noise creates a design impasse. Then rethink the same design using a fluoropolymer coating. Chances are that a properly applied coating will make those problems disappear. But there is more to using coatings than just knowing which one to specify. Designers must follow application guidelines as well as understand the problem at hand to get the most out of coatings. High-performance fluoropolymer coatings are low-friction, dry lubricant materials that combine the strengths of two engineering plastics: fluoropolymer, with the lowest coefficient of friction of any solid, and organic polymers that resist high temperatures. Together, they produce a coating that can operate successfully at temperature extremes. At high temperatures ordinary fluid lubricants char and turn to ash. At low temperatures the same fluids turn glassy and brittle. Even dry lubricants, such as molybdenum and graphite, can handle only a limited range of lubricating problems at high temperature and pressure. Fluoropolymer coatings work because they combine a slippery lubricant with a polymer binder or matrix. The soft fluoropolymer lubricant, which smears easily, is protected by the wear-resistant binder. Binders also help to hold the lubricating particles in place and allow them to adhere to a variety of substrates. |
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| Identifying the Problem |
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 Applying fluoropolymer coatings involves several considerations. Among the more critical design issues are friction, wear, release properties, corrosion, and noise. Friction causes automobile engines to operate inefficiently. Even parts that are well lubricated experience slight friction, especially when the engine is started or when it is cold. Low friction fluoropolymer coatings on internal engine parts can withstand over 500 ° F temperatures and improve engine performance and wear resistance. Friction causes heat, wear, and loss of energy in dynamic applications. In severe cases, friction can cause overheating and bearing seizure. Other effects include drive-line ally. This unstable sliding motion occurs at slow speeds, when movement starts and stops as dynamic friction rises above, then falls below, static friction. Excessive friction harms bearing applications by producing galling, abrasion, and high energy loss. To minimize friction in these cases, consider applying coatings to at least a thickness of 25 microns or 0.001 inch. Instrument bearings or other systems in which minimum bearing friction is critical can benefit from a thinner coating of 7.5 microns or 0.0003 inch. Bolted joints also suffer from too much friction since tightening torque must be used to overcome thread-to-thread and bearing face friction. If a bolt is not properly tensioned in these situations, the joint can become weak in service. Coating the threads reduces the makeup torque by up to 50% and allows tension to be done more accurately. Fluoropolymer coatings overcome friction in the internal components of automobile engines as well, where fluids and dry-film lubricants were used in the past. High temperatures often melted or degraded these materials. Although temperatures often surge over 600 ° F inside the engine, new fluoropolymer coating formulas handle the job without breaking down. Moisture builds up in the air flowing through a carburetor when the ambient temperature is near freezing. The problem is eliminated by coating the throttle shaft and butterfly with a 17.5 micron-thick film of high-release coating. Ice cannot adhere to the shaft or butterfly once they are coated, and it is swept into the engine where it becomes water vapor. Heat build up from friction at the tips and flanks of saw blades leads to rapid loss of sharpness. A 25 micron-thick fluoropolymer coating reduces both friction and heat, extending blade life by a factor of three or more while it reduces sap buildup. Experiments to reduce friction losses in diesel and spark-ignition engines confirm the problem of friction and prove that the environment is difficult for any lubricant to endure. But a high-temperature fluoropolymer coating, Xylan 1010, was able to lubricate these pistons. After operating for 250,000 miles, the coating shows some signs of scorching but is still operational. The pistons shows little wear. Wear is inevitable when two bearing surfaces rub against each other. The surfaces of mating metal parts are covered with minute peaks called asperity. Initial contact between the surfaces causes a momentary welding of the asperity. As each part continues to move, the welded asperity is ripped off leaving behind minute pits. No matter how smooth the finish, every bearing and wear surface has this problem. Coatings provide a thin layer of lubrication that prevents asperity on mating surfaces from making contact. The rate of wear is influenced by four factors: the pressure surfaces exert against each other, the velocity at which they move, the length of time they are in contact, and the wear-factor constant unique for each combination of surfaces. The amount of wear and service life is proportional to PV factors (P=pressure, V=velocity) below a certain limit. The limiting PV factor is the point at which increasing P or V converts normal wear into accelerated wear, often leading to catastrophic failure of a bearing. Several factors influence the limiting PV Factor, including temperature. Surfaces moving in contact generate heat which must be dissipated if the bearing is to survive. Coating and lubricants reduce the rate of heat generation and help preserve the bearing. All bearing materials, including coatings, must be used below their limiting PV factor to avoid failure. Release properties, or non-stick, should not be confused with low friction. Friction results from two surfaces sliding across each other. Release is a surface property that results in the ability of substances to adhere to it. It is a function of surface energy that can be measured by the angle of contact between the surface and a drop of liquid. The larger the contact angle the greater release property the coating has. Cookware is often coated to release food material. That release is equally vital to industrial processes such as thermal forming, rubber molding, automotive and adhesive assemblies, and copying machines. In these applications, build up of foreign particles is a far greater problem than high bearing loads or corrosion. Examples include carburetor shafts, choke plungers, butterfly spindles, conveyor parts, and instrument probes. Corrosion can result from single or multiple sources. This complex electrochemical process typically occurs when an oxidizing agent attacks metal. The agents range from salt water and wetting agents to fuels and synthetic lubricants. Joining dissimilar metals causes galvanic corrosion; vibration between tightly joined parts can cause a type of corrosion called fretting. Fluoropolymer coatings offer a simple solution to corrosion problems when applied at twenty-five microns or 0.001 inch thickness. But to cover microscopic pin holes that may occur in the coating, two thin layers can be applied. Even if corrosives eventually penetrate the coating and attack the substrate, little or no under-burrowing occurs, so the parts can be easily disassembled for refurbishing. For example, in spite of their appearance, heavily corroded fasteners remain functional if they are coated prior to placing them in service. There are several coating formulas developed specifically for fasteners that are used in the chemical processing and automotive industries. Noise comes from vibration sources such as high speed impeller blades, gear teeth, bearings, piston skirts, actuator plungers and reciprocating detents. Coatings dampen the vibration by absorbing energy before it is transmitted to resonant surfaces. In most cases, noise is effectively dampened by coating twenty-five to forty microns thick. When corrosion is not a consideration these films may be applied in one coat, although several thicker coats will absorb more energy. If noise damping is the primary application, multiple coats will achieve the best vibration reduction. Avoid excessive thickness to prevent delaminating or tearing. | ||||||||||||||
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