EP0076317A4

EP0076317A4 - Implantation of molybdenum disulfide into certain metallic surfaces by mechanical inclusion.

Info

Publication number: EP0076317A4
Application number: EP19820901482
Authority: EP
Inventors: Philip O Badger
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-04-16
Filing date: 1982-04-09
Publication date: 1985-04-23
Also published as: JPS58500513A; JPH0694044B2; EP0076317A1; WO1982003575A1

Abstract

MoS2 is applied to medium to high carbon and stainless steels after surface voids have been produced in the material, as by a final annealing operation, but before a final cold working operation reduces the material to finished size. The latter operation embeds or includes the MoS2 into the material and thereby provides a superior lubricated steel for piston ring, shaft, valve stem, spring, stranded wire and the like applications. The technique involved may also employ corrosion inhibiting agents to increase resistance to corrosion of the steel as well as to improve its lubricity.

Description

IMPLANTATION OF MOLYBDENUM DISULFIDE INTO CERTAIN METALLIC SURFACES BY MECHANICAL INCLUSION

Technical Field

The invention relates to improved techniques of treating certain steels with molybdenum disulfide to de¬ crease friction and to increase resistance to corrosion.

Background Art

Molybdenum disulfide (M0S2) is a well known, versatile dry film lubricant. Inherent basal cleavage occurring within its atomic structure results in low lamellar shear strength and contributes to its superior anti-friction or lubricious properties. There is evidence to show that this property is caused by Van der aals type bonding between two molecular unit cells on the basal plane connecting the six-fold symmetry of the respective layers comprising the crystal structure as a whole. It is pre¬ cisely these chemical bonds, relatively long in physical length, which cause low lamellar shear strength and there¬ fore low dry friction measurement on sliding and rotating contact. Conversely, the chemical bonding occurring in the crystal plane located at right angles to the basal plane is more typical of the mono-valent type, shorter in physical length and therefore many times stronger. Recent studies show the hardness for the basal plane as measured on the Vickers Scale to be 32 Kg./mm and that for the crystal plane to be 900 Kg./mm , the latter thus being harder by a multiple of nearly 29. Such a difference in physical measurements within a single unit crystal causes anisotropy, a condition which permits implantation of molybdenum disul- phide into metal surfaces to be practical and achievable.

In some motor spring applications an inorganic film containing M0S2 and bonded to the spring material has

ISUREA

OMPI been utilized to improve the torque consistency of such springs. Motor springs so treated have proved extremely useful in various mechanical time fuzes and in horological mechanisms of the classic escapement types. Indeed, clocks 5 with such springs have sometimes been operated 300% longer than those without coated springs before rewinding is re¬ quired. The increased performance in these instances is a product of the reduction in the kinetic coefficient of friction between the spring leaves during the unwinding 0 process, that is to say, a reduction in the "stick-slip" phenomenon historically associated with main spring appli¬ cations.

Application of films of 0S2 to various metallic substrates is usually by spraying or dipping methods and

__ subsequent drying and/or baking, or even by electroplating the molybdenum directly to the substrate and then heat treating in an atmosphere containing sulfur or sulfide gases. Whatever the technique, however, it is performed after the material has been processed to final or finished

20 size. It has been suggested that transfer of the^* M0S2 film to a metallic substrate in these instances is primarily a mechanical process resulting in (a) direct embedding of the solid M0S2 into a softer surface, (b) deposition of the solid MoS- into surface depressions generated in the sub-

25 strate by an "abrasive" action of the solid MoS„ itself during movement between the substrate and an opposing surface, and/or (c) deposition of the solid MoS^ into the depressions indigenous of the original surface finish and hardness of the substrate. But whatever the case may be,

30 and however satisfactory and lasting these techniques may be for certain applications, the results are not satisfactory in the case of items subject to high surface wear. The MoS^ film, essentially still only a surface film, simply does not endure in these environments but is rather quickly removed

35 or destroyed by abrasion. Such high wear items as piston rings, bearings, journals, valve stems, shafts, and the like, for instance, are composed of various grades of high

OMPI carbon or stainless steels and though obviously many of them could profit from the low friction characteristics and pro¬ tection afforded by oS₂, especially when other lubrication is at a minimum, no feasible way, so far as is known, has emerged for treating them with MoS^ such that the latter becomes a much more enduring part of the steel itself.

Cold drawn wire for use in stranded wire construc¬ tions, such as wire rope for lifting mechanisms, ship board cable systems, aircraft control cable and the like, and for use in coil springs such as compression, extension and torsion types, is also typically manufactured from various high carbon steel alloys. The finished wire is furnished to the spring manufacturer, for example, and the springs them¬ selves are formed by various operations including coiling, grinding, secondary forming, stress relieving, plating and, in many instances, special packing. Likewise, the wire for stranded wire constructions is also shipped to a separate place for manufacture of the rope itself. The wire for both springs and rope is produced by a combination of cold working, annealing and final ^' tempering operations to a predetermined diameter and shipped on spools, reels or loose wound coils.

Significant improvement in the performance of stranded wire cables ought also to be achieved by 0S2 applied to the wire strands in view of earlier work done by the Polymer Corporation of Reading, Pennsylvania, where crane sheaves made of nylon containing a fine dispersion of MoS^ were manufactured and tested. The lifetime of wire cable passing over the special sheaves was increased, in comparison to that when using standard sheaves. Eventual failure was caused by friction between individual strands in the wire cable.

Aircraft carrier arresting cables represent another area of utility for M0S2. Current practice is to reduce friction and corrosion of such cables by repeated applicatons of a barrier film, such as grease. But the grease is gradually removed from the cable during use and causes the flight deck to become slippery in critical places. In addition, the grease, being flammable, is a fire hazard, and its elimination would thus provide safer opera- ting conditions. Another carrier-related problem is corro¬ sion of internal aircraft control cables by salt air, and those too ought to be improved both in longevity and corro¬ sion resistance by 0S2.

In short, a wide range of products formed from medium to high carbon and stainless steels ought to benefit greatly from MoS« treatment if only the endurance of the treatment could be increased by making the M0S2 more a part of the steel itself. Not only would friction be reduced but corrosion resistance increased as well in those applications where corrosion is a problem that must be reckoned with. These are thus the chief objects of the present invention.

Disclosure of the Invention

The invention takes advantage of certain proper¬ ties of medium to high carbon and stainless steels when they ^are cold worked in order to bring them down to a predeter¬ mined finished size. Here and in the appended claims by "cold working" is meant operations such as drawing, forging, rolling and other standard methods of cold working metal, and by "medium to high carbon and stainless steels" are meant carbon steels, such as AISI-SAE Nos. 1035 to 1095 and AISI-SAE stainless Types 301, 302, 304, 316, 316L, 416, and the like, all of which are "interstitial" in nature or structure and typically used for high surface wear applica¬ tions. When such steels are cold worked they tend, as is well known, to "work hardened", whereupon they must be annealed in order to relieve the dislocation of their crystal lattice which the cold working has produced, the annealing being done at each metal's "recrystallization" temperature. After annealing the metal is then suitable for further cold working. Hence, in many applications the cold

^•"BURE

OMPI working process involves a succession of cold workings interspersed by a succession of annealings, the number of each and their arrangement depending upon the extent to which the metal must be reduced in cross sectional size, sometimes referred to as "breaking down" of the metal to finished size.

A byproduct of the annealing operation is a phenomenon called "carbide precipitation" owing to the interstitial nature of the metals concerned. During cold working the metal substrate forms macro-molecular carbon clusters between asperities on the metal's surface from which the heat of the annealing process removes the carbon and thereby creates decarbonized voids on the surface of the metal. These voids, not surprisingly, are more pronounced in size at the end of an annealing operation than they are at the end of a subsequent cold working operation.

The essence of the invention, therefore, is the application of the MoS₂ to the metal after the final anneal¬ ing operation but before the final cold working operation, the final annealing operation in this case in effect con¬ stituting a preparation of the metal for deposit of the MoS-. At that time the surface hardness of the metal is less than it is after final cold working and tempering. Hence the final cold working, owing to the extreme pressure under which it occurs, will effectively embed the 0S2 into the micro-structure of the metal substrate. Furthermore, the anisotrσphic properties of the MoS₂ allow the latter better to penetrate the voids than is possible when the MoS2 is applied to the metal substrate after final cold working when the substrate possesses a higher superficial hardness.

Typically, Mo is in micro-powder form which is sulfonated to obtain MoS^, also a powder. That is then sometimes mixed with graphite in a liquid carrier such as perchlorethylene, the graphite being added to improve lubricity at lower loads. When one form of the invention is practiced the M0S2 ⁱⁿ the foregoing state is further mixed with an inorganic binding agent such as sodium silicate. After the final annealing operation the steel is coated, in the manner hereafter described, with the foregoing mixture and the perchlorethylene "flash evaporated" before the final cold working operation. Where corrosion is not a problem this manner of practicing the invention is satisfactory. But in environments where the steel is open to corrosion when employed in various finished products, the graphite tends to accelerate corrosion rather than to decrease it even though the surface of the steel is somewhat decar- burized by the final annealing. In order to avoid that effect the invention is practiced in another form in which graphite is omitted and an organic based synthetic resin system is used as the binding agent in place of sodium silicate. The system includes a corrosion inhibiter in the form of a "redox" resin. The latter are synthetic polymers having a highly cross-linked hydro-carbon matrix with inherent reversible functional groups attached, such as quinone-hydro-quinone, which are alternately oxidized and reduced. In terms of corrosion inhibiting these resins can be referred to as "election exchangers", "redox ion ex¬ changers" or simply redox resins. In fact rapidly reacting M0S2 bonded films containing organic based binding systems including redox resins have only comparatively recently become commercially available. The corrosion inhibiting principle is based upon the slowing of the reaction rate in which corrosion occurs. In the oxidation phase, as metallic components ionize they attract anions and form a soluble salt or oxides, causing erosion of the base metal. In the reduction phase metallic cations are attracted toward cathodic cells that are created or become present owing to the ambient environment. The latter type of corrosion is typically termed galvanic corrosion. When redox resins are present and in intimate contact within the surface interface, they act as solid electrolytes facilitating an electrical balance, chemically speaking, and cause the rate at which the various reactions occur to be slowed. Corrosion protection is afforded the base alloy even in the absence of a sacrificial addition of anodi protection.

Best Modes For Carrying Out The Invention

The invention will first be set forth in terms o 5 a partially predicted application to the manufacture o piston ring material for internal combustion engines.

Piston rings for that purpose are typically fabri¬ cated from various high carbon steel alloys. Usually the material is furnished the ring manufacturer in the form of 0 flat strips of cross-sectional dimensions, for example, of .61 mm by 2.70 mm in the case of material for oil control rings, produced by a combination of cold working, annealing and final tempering operations. The invention, of course, would be practiced by the manufacturer of the strips before 15 the latter are shipped to the ring maker. Assume, for example, a strip of the above dimensions in finished size whose cross sectional area is thus 1.665 mm . Assume furthe that the interim cross-sectional area of the strip after the penultimate cold working operation and the ultimate anneal- on ing operation but before the final or ultimate cold working operation is 4.645 mm 2 whereby the latter operation accom¬ plishes about a 65% reduction in cross-sectional area.

Essentially the invention may be practiced in this instance by coiling an interim size strip, after preparation 5 by final annealing, on a take-off drum providing a low back tension on the strip and then recoiled on a suitable take-up drum spaced from the former drum in a sometimes called "coil-to-coil" operation. Between the two drums is disposed a tank, which may be open to the atmosphere, of perhaps 84 0 litre capacity and equipped with a suitable stiring device. Into the tank is placed a mixture of solid, micro-meter sized 0S2. graphite, a liquid carrier such as perchlor- ethylene and an inorganic bonding agent such as sodium silicate, all more or less at room temperature. Between the tank and the take-up drum is placed a thermostatically con¬ trolled oven or furnace capable of maintaining a temperature of about 260° C over a distance of 2 linear metres. The strip from the take-off drum is then led by suitable well known means through the mixture in the tank, which is agitated by the stirring device in order to keep the MoS2 in. suspension, from which it emerges "wet", and then through the oven where the perchlorethylene is "flash evaporated" in order to produce a dry bonded film of MoS₂ on the strip, the latter being finally recoiled on the takeup drum, all at the rate of about 3.65 linear metres per minute. A 34 litre mixture containing 3275 cc of solid M0S2, sodium silicate, and graphite in the proportions of 71, 22 and 8, respec¬ tively, the balance being perchlorethylene, should result in a dry bonded film of MoS« on the strip of between .0025 and .005 mm thickness. The strip from the take-up drum is next finally cold worked to its finished cross-sectional dimen¬ sion in the usual manner and then tempered at about 200° C. The temperatures of the strip during the deposit of the MoS-, the riddance of the carrier liquid and the final cold working and tempering operations, should be kept below the oxidation temperature of the MoS₂ which is about 425-480° C. Even so, the oxidation rate of MoS- to MoO- is quite slow and studies have shown that MoS₂ does not lose its lubricity at those temperatures until 30% or so has been oxidized.

As a result the oS^ is mechanically embedded or "included" in the working surfaces of the piston ring material to a greater extent than would be the case were the MoS2 applied in the usual manner as a film after cold work- ing to finished size. This is borne out by a quantity of piston ring high carbon compression ring steel, A1S1-SAE- 1075, in the form of cold rolled strip 1.78 mm thick and annealed, obtained from a mill. The material was divided into two specimens, the first being treated without change but the second being given extra decarburization before treatment in order to determine what effect that would have on depth penetration with respect to implantation of MoS₂

^'BU £ into the micro-structure of the substrate. A .005 mm thic film of MoS2 using the above mixture was applied to bot specimens. The latter were then returned to the mill an cold rolled to .60 mm thickness, a reduction in cross- sectional area of about 65%. Upon their return from the mill both specimens were then cut lengthwise at right angles to the direction of rolling or grain elongation to determine the amount of MoS- inclusion within the micro-structure of the substrate. Observation of the cut specimens showed partial and total mechanical incluson of 0S2 in unusually large deposits. The depth of the inclusion in the case of the first or raw specimen was .02 mm while that of the second or decarburized specimen was .028 mm. Hence it appeared that the depth of the inclusion is a function of the depth of the respective decarburized zones; that is to say, the degree of inclusion appears constant at about 55-60% of the starting depth of the zones concerned for a given amount of cold working. It was also concluded that annealing of the steel before treatment produces a rela- tively soft ferritic transformation near the surface compared with the underlying pearlite and therefore lends itself to substrate embodiment by the implantation process of the invention.

In another test, two plain carbon steel strips, A1S1-1078, were annealed in air. One was then coated with the foregoing mixture, dried, and reduced by cold working to 3.28 mm width and .61 mm thickness. The other strip was cold worked to the same dimension before being coated with the foregoing mixture. Both strips were then mounted in epoxy so that their cross-sections were visible and ground and polished using .05 micron alumina powder. Precautions were taken to minimize edge rounding during the polishing. Both samples were then viewed with incident light in a metallurgical microscope at magnifications up to l,000x. The first sample showed numerous inclusions of dark material into the surface to typical depths of .0025 mm, while the second sample showed no such inclusions. All the foregoing seemed to be accomplished with¬ out any essential change in the material itself from the standpoint of hardness, strength, and the like. The material can thereafter be fashioned into piston rings in the conventional manner but the typical operation of chrome plating of the finished ring would be omitted. The cus¬ tomary differential is superficial hardness between the chrome plated ring and the cylinder wall is thus reduced and thereby wear on the wall itself is also diminished. At the same time wear on the ring is reduced because its inherent lubrication supplements normal liquid or hydrodynamic lubrication between the ring and the cylinder wall. The lubricated ring better withstands high operating loads, better operates for short periods with boundary lubrication, better distributes local loads, and betters the wear charac¬ teristics of the mating surfaces. This is especially important adjacent top and bottom dead centers since the greatest wear of a cylinder wall in an internal combustion engine has been found to occur where piston direction changes and where the oil film is unable to accumulate sufficient thickness to separate the two opposing surfaces. This is because the zero velocity of the piston at those two points means that the hydrodynamic requirement of relative velocity between opposing surfaces is not met at these precise locations. The lubricated rings, on the other hand, provides supplemental lubrication at these points at these crucial times, something a chrome plated ring cannot do, as well as over the entire cylinder wall during the critical breakin period of a new engine.

The invention will next be set forth in terms of a predicted application to the manufacture of cold drawn wire for use in springs and stranded wire constructions.

Assume, for example, wire having a finished

2 diameter of .71 mm or a cross-sectional area of .40 mm , and that the interim diameter of the wire after the penulti¬ mate cold working operation and the ultimate annealing but

^"BU before the ultimate cold working operation is about 1.90 mm

2 or a cross-sectional area of 2.84 mm , whereby the latter operation accomplishes about an 85% reduction in cross- sectional area. Also assume that the foregoing annealing was accomplished in the usual manner in a non-oxidizing atmosphere to achieve recrystalization and that a subsequent annealing operation is performed in a strand type annealer at about 650° C for a flash type decarburization of between 40 and 60 seconds duration, the latter operation broadening the decarburized zone to about .045 mm and thus optimizing the surface for mechanical inclusion pursuant to the invention.

As before, the wire after the latter annealing operation, may be coiled on a pay-off reel with which is appropriately associated a take-up reel for a coil-to-coil coating operation. Between the two reels is disposed a refrigerated tank of perhaps 70 to 80 litre capacity into which is placed a mixture of solid, micro-meter sized MoS₂, organic based binding agents, such as phenolic-vinyl-resin or epoxy-vinyl-resin, together with redox resins to act as corrosion inhibiting agents, and a quick drying composite solvent or liquid carrier consisting of 60% cellulose acetate, 30% xylene, and 10% methyl ethyl ketone. The refrigeration of the tank is regulated to keep the mixture at a temperature between 0° and 5° C owing to the volatility of the solvent. Between the tank and the take-up reel is also disposed a thermostatically controlled oven or furnace capable of maintaining a temperature of 260° C over a dis¬ tance of 2 linear metres. The wire from the pay-off reel is then led by well-known means through the mixture in the tank which is agitated by a suitable stirring device in order to keep the MoS₂ in suspension. The wire then emerges from the tank "wet" and into the oven where the solvent is "flash evaporated" in order to leave a dry bonded film of MoS- on the wire. The latter is recoiled on the take-up reel at the rate of 3 to 3-1/2 linear metres per minute, the rapid rate of reaction of the resins concerned being essential because -12-

of the relatively immediate recoiling of the wire after it has passed through the tank. Finally, the wire is trans¬ ferred to a suitable wire drawing installation and reduced in cross-sectional area, that is to say, cold worked, to its 5 previously mentioned finished diameter. Such cold working in this case should be kept at temperatures below about 315° C because the resins involved tend to decompose above that point.

Wire so treated would have several advantages.

10 For instance, when used to manufacture coil springs, expen¬ sive post-plating for corrosion protection would become unnecessary, thus eliminating the inherent problems of hydrogen embrittlement, tangling of springs, and the absence of plating between the close-wound coils of extension

15 springs. Furthermore, the wire would retain improved ductility and toughness compared to hot dipped galvanized wire because the brittle alloy between the zinc and the steel interface would be eliminated. The same or similar advantages would also accrue to such wire when employed in

20 various stranded wire construction, as previously mentioned. To the extent the final cold working diminishes some of the corrosion inhibiting properties achieved by the invention, for added protection the wire can be recoated with oS₂ by the previously described prior art methods after final cold 25 working.

The invention is also believed applicable to other high wear components previously mentioned, such as bearings, journals, shafts and so forth, on which it would be prac¬ ticed in manners analogous to those described in the cases 30 of piston rings and wire. The invention is believed appli¬ cable as well to instances where the surface voids in the steel are achieved by preparatory means other than anneal¬ ing, such as by grinding, etching, sand blasting or pickling -of the material before the final cold working operation.

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Claims

What is claimed is:

1. A process of providing a lubricious surface for medium to high carbon and stainless steels for high wear applications wherein a length of the steel is reduced from an initial larger to a final smaller cross-sectional size by one or more cold working operations, characterized by: forming voids in the surface of the steel by a preparatory operation after a penultimate cold working operation; then covering the surface of the steel by a coating operation with a mixture containing micrometer sized solid molybdenum disulfide and a bonding agent in a liquid carrier; then removing the liquid carrier by a drying operation; and then performing an ultimate cold working operation upon the steel to reduce the same to said final crosssectional size.

2. The process of claim 1 further characterized by the preparatory operation including annealing the steel.

3. The process of claim 2 further characterized by again annealing the steel before said coating operation.

4. The process of claim 1, 2 or 3 further characterized by the liquid carrier being perchlorethylene and the bonding agent being sodium silicate.

5. The process of claim 1, 2 or 3 further characterized by the bonding agent being an organic based synthetic resin and the liquid carrier being a composite solvent.

6. The process of claim 5 further characterized by the bonding agent also including a redox resin.

7. The process of claim 6 further characterized by coating the steel with a mixture of micro-meter sized molybdenum disulfide and a bonding agent in a liquid carrier after said ultimate cold working operation.

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