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V. G. FlTZSlMMONS and W. A. ZISMAN U. S. Naval Research Laboratory, Washington 25, D. C.
Thin Films of Polytetrafluoroethylene Resin as Lubricants and Preservative Coatings for Metals The experience outlined here shows that there is really no limit to the variety of lubrication problems Teflon is capable of solving in the laboratory, in industry-perhaps even in the home
THE remarkable low coefficient of dry friction of Teflon (polytetrafluoroethylene) was first recognized during cooperative research with the Du Pont c o . in 1943 (77). However, early tests on bearing materials of bulk Teflon or sintered mixtures of Teflon and metal powders exhibited two practical limitations (77): excessive flow at high unit loads, accelerated by frictional heating of the rubbing surfaces, and poor heat transfer through the composite bearing material. Thus, Teflon would be best used as a dry lubricant in the form of a thin coating on a hard backing, but this approach could not be pursued without a suitable technique for producing adherent thin films of Teflon on hard solids. Bowers, Clinton, and Zisman (5-8) investigated the effect of halogen substitution in polyethylene on the coefficient of friction and found that it increased with extent of chlorine substitution and decreased with fluorine substitution. Whenever Teflon was rubbed against the clean surface of a metal, a thin, shear-oriented film of Teflon was transferred, but this transfer ceased once the metal became coated. For this reason a t the commencement of sliding the coefficient of friction for steel against Teflon has a value of 0.10; it promptly drops during sliding to the value of 0.04 to 0.05, characteristic of Teflon sliding against Teflon. lubricant Coatings from Teflon Suspensoids
2
In 1950, after much research (3, 4, 72, 76), aqueous dispersions of Teflon resin
became available for coating metal surfaces. In practice, after the appropriate Teflon Suspensoid has been deposited on the solid surface, the film is allowed to dry in the air; evaporation of the water must not be so rdpid that Tefldn particles are forced apart, creating larger pores in the coating. When the dried surface is heated to approximately 725" F., the Teflon particles quicklysoften and sinter. After the surface has cooled,
a thin, continuous film of Teflon is left adhering to the surface. However, many small pores in the film cannot be eliminated because of the high melt viscosity of Teflon. Although pores do not limit lubrication applications, they offer points of attack for rusting or other types of corrosion. Therefore, when corrosion prevention is desired, one or more additional thin coatings are applied by repeating the operations. Two coats are usually sufficient for the applications described here. Each deposited Teflon layer should not exceed 0.0003 inch in thickness; otherwise many cracks in the film may develop, like the cracking of dried mud. When the film is to be used as a lubricant, tot41 film thickness should not exceed 0.0006 to 0.0007 inch. Teflon coating adheres best to steel, brass, and aluminum when the metal surface is cleaned and degreased without removing the surface oxide normally present. If excessively thick oxides have been removed-by acid pickling, for example-it is essential to heat the metal in air until a thin oxide layer has formed. Adhesion is also increased by increasing the real surface area; thus a sandblasted or mat finish is better than a mirror finish. The coefficient of friction of Tefloncoated surfaces can be decreased by roughening the surface of the metal substrate. However, durability under conditions of sliding friction is decreased when the metal roughness exceeds 32 microinches(rms). Roughness is produced most conveniently by sandblasting with No. 80 silica sand delivered by air to the nozzle a t pressures of 70 to 90 p This soft mat finish dimensionally raises the surface 0.0001 inch by the peening action of the silica; in fact, this dimensional change was used as the limiting value for terminating the sandblasting operation. Finishes up to 62 microinches were leveled by sandblasting to 32 microinches. This is an economic advantage in mass production. After being sandblasted, the metal surfaces are heated in air for oxidation prior to applying Teflon.
Several methods of fusing Teflon to metals were investigated because brass decreased in hardness nearly 50% after being briefly heated at 725" F. Highfrequency induction heating of brass cartridges (using 400 kc.) followed by a water quench resulted in excellent Teflon coatings without significant decrease in the hardness or tensile strength of the brass. The same approach was successful in coating aluminum. Mass production methods for producing Tefloncoated steel cartridges now include ionizing or electrostatic spray apparatus, and fusing is done by gas-fired infrared heaters. The process recommended for preparing metal surfaces and applying Teflon films, described in an instruction issued by the Bureau of Ordnance (70),is a useful supplement to the instruction brochure available from the producers of Teflon finishes (72). While Teflon is a soft plastic in bulk form and cold flows under loads of 2000 p.s.i., much greater loads could be carried without extruding the material when it was used in the form of a thin film on a hard substrate. Such films withstood cycling under a load of 50,000 p.s.i., and the initial coefficient of friction of 0.04 to 0.05 was nearly constant under 100 cycles of reciprocating sliding motion (5-8). The photomicrographs (Figures 1-4) show Teflon-coated steel panels used in friction and wear measurements using a Bowden-Leben "stick-slip" machine. The wear track was produced by sliding a 0.5-inch steel ball under a load of approximately 50,000 p s i . over the coated panels. Application to Ammunition
Thin coatings of Teflon are remarkable dry-film lubricants for cartridges (73). In 1951, gun firing tests using brass cartridges with one coat of clear Teflon Suspensoid revealed excellent performance and freedom from gun malfunctioning. After much cooperative experimentation with the Du Pont Co., VOL. 50,
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Figure 1. Wear track ( 9 0 X ) of 0.6mil Teflon coating shows slight film extrusion caused by low adhesion on smooth substrate
Figure 2. Smoother, narrower wear track ( 9 0 X ) in OC6-mil Teflon coating shows satisfactory behavior of properly applied coating
Figure 3. Rougher film applied to coarse sandblasted steel surface had lowest coefficient of friction (0.02) because of smaller real contact area
Steel surface had typical smooth finish of coldrolled steel
Greater adhesion to metal substrate resulted from sandblasting steel surface
Flattened areas in wear track (90x1 carried entire load, but rapid rupture on peaks i s undesirable
Teflon coatings were developed with excellent adhesion to brass and steel and greatly increased corrosion resistance. Coatings with satisfactory resistance to rain, humidity, and salt water spray had to comprise two or more coats having a total thickness of from 0.05 to 0.7 mil. T h e most suitable coatings consist of a 0.3-mil primer coat (Du Pont No, 851204) and a 0.3- to 0.4-mil second coat (Du Pont No. 851-204 or No. 852-201). Satisfactory resistance to a 207, salt water spray a t 120' F. for 150 hours can be obtained with such a film deposited on SAE No. 1020 cold-rolled steel. When corrosion of Teflon-coated steel occurs, the coating does not lift and the corrosion does not spread to areas under the coating. Exhaustive firing tests with coated steel and brass 20-mm. cartridges established the superiority and greater reliability of Teflon over other lubricants (73). Contract pilot-scale production studies by the Army and Navy have since developed satisfactory methods for the mass production of Teflon-coated steel cartridges.
were adequately lubricated; firing rates were identical with those obtained previously from the same guns lubricated with a good gun oil. When a synthetic diester lubricating oil was applied to a Teflon-coated M-3 gun, there was an approximately 10% increase in firing rate over the maximum value ever reported. This led to an investigation (73) of the practicability of lubricating and protecting the mechanisms of small arms with Teflon. Because many weapons are sensitive to dirt, all of the weapons are difficult to maintain in long-term storage without preservatives which are troublesome to apply and more troublesome to remove, and there is much need for a single practicable gun lubricant effective at ambient temperatures as extreme as -65' and 120' F. Thin coatings of Teflon similar to those developed for the 20-mm. steel cartridges were used on a total of 20 rifles, carbines, and 30-caliber machine guns. The two-coat system totaled 0.0005 to 0.0007 inch. A Teflon primer coat was used (Du Pont No. 851-204), and the top coat was Teflon black enamel (Du Pont No. 851. 205). At the Marine Corps Development Center, Quantico, Va., the Teflon-coated weapons or weapon components were subjected to severe comparative storage and exposure tests with counterparts lubricated or protected with conventional materials applied by standard methods. I n all tests the petroleum coatings gave
comparatively poor protection; in the low-temperature tests a t -80' F., these weapons became completely inoperable. When corrosive atmospheres were involved, the grease-coated parts could not be operated at all, and in most cases the parts could not be salvaged, whereas test components coated with Teflon always operated, even afrer slight corrosion occurred in the severest tests. At -80' F. there was no tendency for the weapons coated with Teflon to stick to the bare skin, as with conventionally protected weapons, an advantage when the mechanism must be repaired at subzero temperatures. Firing tests of Teflon-coated rifles, carbines, and machine guns were made under the most adverse conditions. At the end of the test period of 8 months, only the weapons coated with Teflon were functioning satisfactorily; not one had failed, and not one single malfunction occurred because of faulty lubrication or corrosion. All comparative firing tests with conventional lubricants produced malfunctions. Final recommendations (75) were that Teflon is suitable as a preservative for long-term storage of weapons and as a lubricant for small arms in troop use; its use as a lubricant for the moving parts of all Marine Corps weapons should be investigated.
Teflon-Coated Weapons
Successful applications of thin Teflon coatings were made to lubricate the widely used M-3 20-mm. gun and the new MK-12 20-mm. aircraft cannon, as well as the associated automatic ammunition feeders (73). Extensive gun fire tests revealed that all bearing surfaces
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Applications to Submarines
Although Teflon coatings on iron and
METAL LUBRICANTS steel can offer valuable protection against the corrosive attack of moist air or ordinary water, they are not always adequate for shipboard uses involving frequent or continuous contact with sea water. Their use for lubricating submerged mechanisms on board submarines and ships is not recommended unless the metal to be coated is itself highly resistant to sea water corrosion. All lubrication aboard a submarine is performed in an environment unfavorable for conventional lubricants. Atmospheric conditions inside submarines are deleterious to fluid or grease lubricants because of high humidity, contact with atmospheric condensate, and exposure to airborne acid. Lubrication with oils and greases on outboard mechanisms is performed in contact with sea water, leading to salt water-oil emulsification and contamination wiih solid deposits of salt and marine growths. In much of the manually operated and seldom-used gear, accumulations of deteriorated greases become hard or horny and impede rather than assist manual operation of the equipment. As a result, a nearly continuous effort is necessary to clean and relubricate these systems. I t was possible to select many lubrication sites aboard typical submarines which were suitable for Teflon coatings. Numerous different items were coated on three vessels at the Portsmouth Naval Shipyard, and no other lubricants were used on this equipment for 18 months. Commanders of these ships have reported progress every 6 months, and to date the reports continue to indicate superior lubrication performance (7,2, 73). Therefore, a project was established for applying and testing Teflon lubricant coatings in submarines and ships at the Mare Island Shipyard (74). At present over 3,000,000 square inches are in service. Teflon coatings were used to lubricate fuel meters of the swash-plate type requiring very low initial friction to guarantee a high degree of accuracy. Often fuel and sea water mixtures are passed through these meters; with the resulting corrosion, accuracy may drop to only 15% after several months of shipboard use. Teflon coatings for these fuel meters provided an accuracy of 99.95y0 even after large amounts of sea water were deliberately pulled through them. A submarine periscope bearing transmitter, which had training torque of 37 inch-pounds with conventional lubricants, required only 21 inch-pounds with Teflon coatings. Not only did these coatings provide unexcelled performance in lubricating the optical control mechanism, but their use on bearings and gears provided longer life and reduced machining costs by permitting the use of coarser surface finishes.
Figure 4. Cold flow, developed in one traverse of steel ball over 1 .O-mil Teflon film, shows how bulk cold flow properties make film thickness a critical factor Maximum thickness limits were 0.5 to 0.6 mil; for applications not requiring corrosion resistance to water, 0.1 -mil films were satisfactory
Also, the use of Teflon eliminated the often serious decrease in light transmission caused by condensation of fluid lubricants on prisms and lenses. Completely Teflon-coated periscope systems are now in use on submarines providing the smoothest action and least water leakage ever experienced. Other submarine mechanisms to which Teflon coatings have already been applied successfully include (73) : Hatch cover components-latches, gears, shafts, handwheels, hinges, and quick-acting water-tight doors. Valve components-flow controls, oxygen system plugs, reduction gears and shafts, operating linkages, and indicators. Ventilation system components-gagi ging gear, dampers, and closures, reduction gears and shafts, bearings, snorkel gear, and exhaust systems. Fuel and lubricating oil valves. Underwater mechanisms-gears, screwjacks, periscope hoists, and snorkel masts. Plumbing system mechanisms and plugcocks.
rubbing the clean, dry metal seat with a piece of Teflon shaped like a crayon; unidirectional rubbing is best. Laboratory tests have shown the advantages of coating with Teflon a variety of parts in hydraulic actuators and accumulators and also certain parts of hydraulic pumps. This has been confirmed by pump producers. Titanium has the unfortunate property of self-welding under conditions of boundary friction, and conventional lubricants are not helpful in preventing this action. Thin coatings of Teflon finish (Du Pont No. 851-204) were applied to a large number of titanium nuts and bolts of different sizes in the hope of providing a self-contained lubricant and an antigalling coating (73). These coatings eliminated thread galling or welding. In addition, true hold force (bolt tension) could be predicted from torque measurements because thread friction had been so nearly eliminated. Teflon-coated titanium fasteners are now used by naval aviation contractors (9).
Applications to Aircraft
Teflon coatings have proved advantageous to O-rings in aircraft hydraulic systems (73). For instance, the prolonged immersion of O-rings in oils or fuels produces a gummy surface which results in adhesion during periods of idleness. Then, when motion is initiated, the ring often continues to adhere causing spiral failure of the ring and rupture of the seal. I t has been common practice to exercise idle equipment 'occasionally to prevent such adhesion. Because Teflon-coated surfaces are nonadherent, their use minimizes spiral failures and other forms of damage between the elastomers and the mating surfaces. The coating needs to be only 0.1 mil thick, and sufficient protection may result by
Miscellaneous Applications
An unusual application of Teflon, coatings was to an MK-23 torpedo (73) which is steam-propelled through a turbine-driven gear train. The controlling gyroscope is operated by a compressedair turbine and a train of gears. Each of these gear trains, bearing systems, and associated journals was coated with Teflon. Operational tests demonstrated that Teflon coatings are very durable except when large amounts of power are transmitted through Teflon bearing surfaces without sufficient cooling. For example, Teflon coatings on dry-lubricated gears of the main drive exhibited thermal degradation after three successive torpedo firings. However, all other VOL. 50, NO. 5
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Teflon-coated gears which did not transmit so much power were adequately lubricated even after seven firing trials. Propellor shaft bushings were in good condition. A 20,000-r.p.m., air-operated, Teflon-coated plain bearing and shaft combination showed no wear after seven firings owing to the cooling effect of the exhaust air over the friction areas. Sea trials proved that a Teflon coating over the external body of the torpedo had not significantly decreased the friction with water; however, the external coatings did greatly improve ease of storing, handling and launching the torpedo. An unusual application of Teflon dry lubricants was to an actuating device containing a miniature electrical motor, a gear train, and a jackscrew. This device was designed to be watertight yet operable from -100’ to 300’ F. No petroleum or synthetic lubricant satisfied the requirements of a low starting torque, compatibility with elastomer seals, and adequate storage stability. Teflon coatings gave completely satisfactory results and are used in the production of this device. Gear pumps used in gasoline fuel systems have been successfully lubricated with Teflon. This was demonstrated by first operating the bronze gear pump system uncoated and then with the gears, shaft bushings, and mating surfaces coated with Teflon. Gasoline was pumped from an underground storage tank with a pump discharge pressure of 3 p s i . Whereas the uncoated pump required priming, exhibited overheating, and evidenced wear because of inadequate lubrication of gear journal bushings and packing gland, the Tefloncoated pump did not need priming even after one year of normal service, friction remained low, the pump remained cool during long periods of operation, and no difficulties were observed with the packing gland. Teflon coatings have been found practicable and advantageous when applied to numerous threaded units ranging from nuts and bolts to hoisting jacks. For example, Teflon coatings applied to pressure regulator screws have permitted easy turning of the manually operated handle-a significant advantage in oxygen regulators. As conventional lubricants create explosion hazards, and Teflon does not react with oxygen, it is recommended for such uses. An assortment of surgical and dental instruments from a naval dispensary were coated at this laboratory. Mechanical performance of these instruments was significantly improved; repeated sterilization had no effect on the Teflon coating. I t was suggested to the Bureau of Medicine and Surgery that such coatings would be valuable lubricants for prosthetic devices. Reports have since indicated successful use of Teflon in this field.
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Conclusions
Acknowledgment
Teflon coatings can be even more widely used to lubricate controls of optical instruments, controls of electromechanical systems and associated mechanisms, and gear trains and bearings for timing devices. Machines which are complex and difficult to clean and lubricate, such as gun control mechanisms, calculating machines, tabulating and assorting machines, and typewriters, offer a promising field for the application of Teflon coatings as lubricants and preservatives. I t should not be concluded that Teflon coatings can care for all lubrication problems, or that Teflon is a universally applicable dry lubricant. Many difficulties encountered in the past have been caused by failure to understand the limitations imposed by the softness of Teflon, the porosity of the coating, or the problem of heat transfer. Teflon’s low friction does not offer a panacea for all lubrication problems, nor is it always the best protection for metals just because it is the most chemically inert of all plastics. Owing to the inherent porosity of thin coatings of Teflon, this material is not always the best agent for preserving metal equipment. There are better corrosion-resistant coatings than Teflon from the standpoint of protection and economy, and corrosion prevention attributes should then be considered as an additional benefit. Equipment or mechanisms to be lubricated with Teflon films must satisfy one or more of the following conditions:
The authors wish to acknowledge the valuable cooperation of Harry R . Young. Stanley Detrick, and the late W. A. Calcott of the Du Pont Co. It is also a pleasure to acknowledge valuable cooperation from many associates a t the Kava1 Research Laboratory, Bureau of Ordnance, U. S. Marine Corps, Bureau of Ships, Mare Island Naval Shipyard, Office Chief of Ordnance of the Army, and Frankford Arsenal.
The surface sliding speed is low, and the applied load is light to moderate, or else the surface sliding speed is high, but the applied load is low. An effective coolant is present such as Tvater, oil, or air flow. The lubricated item is expendable or has a short term use before overhaul. Such rules of selection are necessary because a considerable quantity of frictional heat must be dissipated at the Teflon bearing surface when the amount of power handled by the bearing is high. Because of Teflon’s much lower thermal conductivity, heat dissipation may be too slow, and the resultant rapid rise in surface temperature will cause the Teflon film to soften excessively, be wiped off, or even degrade thermally. The designer, who is considering the use of a Teflon coating as a lubricating material, should know the maximum permissible conditions of lineal rubbing speed and bearing load. Although an effort has been made to develop quantitative guides, no success has been obtained to date. The necessary criteria are not simple for they depend greatly on the design of the system on the thickness of the coating, on thermal conductivity, and on the geometry of the metal substrate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
literature Cited
(1) Armstrong, D. R., “Antigalling Properties of Teflon Coated Surfaces on Similar Metals,” Portsmouth Saval Shipyard Tech. Rept. T-576 (March 1955). (2) Armstrong, D. R., “Anti-Seizing Properties of Teflon Coated Metals in Shaft-Bushing Applications,” Ibid.. T-577 (March 1955’). (3) Berry,’K. L., U. S. Patent 2,412,960 (Dec. 24, 1946). (4) Ibid., 2,478,229 (Aug. 9, 1949). (5) Bowers, R. C., Clinton, W. C., Zisman, W. A., “Frictional Behavior of Polyethyiene, Polytetrafluoroethylene, and Halogenated Derivatives,” NRL Rept. 4167 (May 19, 1953). (6) Bowers, R. C., Clinton, W. C., Zisman, 5%’. .4., J . Apfil. Phys. 24, 1066 (1953). (7) Bowers, R. C.? Clinton, W. C., Zisman, W. A., Lubrication Eng. 9, 204
(1953).
(8) Bowers, R. C., Clinton, W. C., Zisman, W. A., Modern Plastics 31,
No. 6, 131 (1954).
(9) Brenner, H. H., “Titanium Fastener Development Report,” Fasteners
(Industrial Fasteners Inst., Cleve-
land, Ohio), 9, No. 2, 7-10 (1953). (10) Bureau of Ordnance, U. S. S a v y , “Process of Applying Polytetrafluoroethylene Coatings on Steel Surfaces,” NAVORD OD 10362 (Jan. 11, 1956). (11) Calcott, W. A , , Detrick, S., Organic Chemicals Dept., Du Pont Co., personal communication, December 1943. (12) Du Pont Co., Fabrics and Finishes Division, “Teflon Finishes,” New Product Tech. Bull. 1 , 6th ed. (May 1956). (13) FitzSimmons: V. G., Zisman, W.A , , “Thin Films of Polytetrafluoroethylene, Resin (Teflon) as Lubricants and Preservative Coatings for Metals,” NRL Rept. 4753 (June 15, 1956), O.T.S., Dept. of Commerce, PB121161.
(14) Mare Island Naval Shipyard, “Evaluation of Teflon (Polytetrafluoroethylene) as a Protective Coating and Dry Lubricant for Ships Surfaces,’’ Test 346, Rept. NS-061-002 (Aug. 26, 1955). (15) Marine Corps Equipment Board, Marine Corps Development Center, Quantico, Va., “Teflon; Test of,” GMM:pf A9-T-1077 (April 19, 1955). (16) Osdahl, L. K., U. S.Patent 2,562,118 (July 24, 1951). (17) Zisman, W. A., Proc. Summer Conference on Mechanical Wear, Mass. Inst. of Technology, June 1948, p. 102, Am. SOC.Metals, 1950. RECEIVED for review January 4, 1957 ACCEPTEDJuly 29, 1957
