1. Field of the Invention
The present invention relates to composite bearing materials which are self-lubricating and oxidation resistant over a broad temperature range up to and preferably in excess of about 930.degree. C. Composites of this invention are comprised of distinct metallic, glass and fluoride components.
These novel composites may be fabricated by infiltration of a porous, sintered metal with molten glass and fluorides as is well-known in the field of powder metallurgy. Optionally, the constituents of the composite may be co-deposited by plasma-spray techniques on a suitable substrate.
2. Description of the Prior Art
Lubrication of mating surfaces in frictional engagement has long posed problems of wear (e.g., abrasive, adhesive, chemical and fatigue) and overheating of machine parts which subsequently fail prematurely as the manifest result thereof. Typically, oils and greases have been employed to mitigate the deleterious effects of heat and abrasion by virtue of their very low coefficients of friction and relatively long-life properties. However, as technology progressed and thus imposed more severe operating conditions, most notably, higher operating temperatures, it was found that the lubrication properties of fluids were inherently limited, thus ultimately limiting the scope of the advanced design.
In response to the inherent deficiencies of fluid lubricants, solid lubricants emerged as clearly superior in extreme environmental conditions such as high temperatures at which fluids decompose or, on the other hand, extremely low temperatures at which fluids freeze. Additionally, it has been shown that many solid lubricants are extremely effective in chemically active environments which readily decompose fluid lubricants via chemical attack.
Moreover, solid lubricants effect overall savings in many systems insofar as vast weight reduction can be achieved through elimination of pumps, heat exchangers, recirculation systems, and the like, as well as the elimination of seals ofttimes necessary to isolate lubricating and working fluids. Similarly, replenishment of contaminated fluids is vitiated by the use of solid lubricants.
Perhaps the first and most widely employed solid lubricant was graphite. Graphite is formed as a covalently bonded carbon solid of hexagonal structure which may be viewed microscopically as two-dimensional planar molecules occupying basal plane positions, stacked one on top of another and held together by weak, secondary van der Waals forces. The facility with which these "sheets" may readily part along basal planes provides the well-known lubrication qualities of graphite.
However, graphite possesses severe deficiencies as a lubricant in extreme conditions. It has well been demonstrated that the lubricating qualities of graphite are predicated upon its ability to adsorb gas, moisture or hydrocarbon vapors before the property of low shear strength is attained. While the gases and water vapor present in a normal atmosphere are usually sufficient to ensure an adequate supply of adsorbable material, at high altitudes or under vacuum conditions, for example, desorption occurs with the subsequent loss of lubrication features. Additionally, at temperatures over approximately 95.degree. C., adsorption is significantly decreased with a concomitant decrease in lubrication properties.
To minimize these deficiencies, it has been shown that graphite may be reacted with fluorine gas to yield an improved solid lubricant of the form CF.sub.x where x may vary from approximately 0.3 to 1.1. While this intercalation compound of graphite is capable of providing lubrication without the need of an adsorbed vapor or impurity up to temperatures of approximately 500.degree. C., the chemical reaction must be carefully controlled to yield suitable properties. Then too, oxidation or dissociation at temperatures approaching 500.degree. C. remain persistent problems in graphite systems regardless of the fabrication techniques or chemical alteration thereof.
Similar to graphite are such solid lubricants as molybdenum disulfide and tungsten disulfide which are also hexagonal-layered and whose shear properties are anisotrophic, with preferred easy shear parallel to the basal planes of the crystallites. In contradistinction to graphite, neither of these disulfides requires the presence of an adsorbed layer to achieve lubrication properties; however, these disulfides too are temperature limited, albeit at higher temperatures approaching 400.degree. C. where decomposition by oxidation occurs.
Moreover, in highly oxidizing conditions it is well known that the presence of molybdenum greatly contributes to the catastrophic oxidation of many engineering alloys. This catastrophic or accelerated oxidation results as molybdenum oxidizes to MoO.sub.3 at temperatures greater than 400.degree. C. This oxide of molybdenum will melt at approximately 795.degree. C., and it has been suggested that this low-melting oxide phase may then act as a flux to dislodge or dissolve protective films. Additionally, eutectic combinations of the molybdenum oxide and other oxides present will further reduce the melting point thus aggravating the structural degradation attendant this high temperature oxidation.
While the effectiveness of the above-noted layer-lattice lubricants may be greatly enhanced through resin bonding techniques, severe limitations are yet presented as temperatures increase within the range of interest. Still too the problem of catastrophic oxidation are not overcome by such resin bonding of the lubricant.
The breakthrough in high temperature lubrication came with the discovery that various fluorides provide low friction surfaces under extremes of temperatures and ambient chemical environment. Note, for example, U.S. Pats. No. 3,157,529, No. 3,419,363 and No. 3,508,955 each to H. E. Sliney and assigned to the National Aeronautics and Space Administration. Each of these patents relates generally to solid lubricants comprised of, inter alia, fluorides.
More particularly, U.S. Pat. No. 3,157,529 discloses fluoride lubricant coatings applied as a film to the surface to be lubricated, which coatings are basically comprised of calcium fluoride and a suitable ceramic binder therefor. While such techniques have proved extremely effective in the field of high-temperature solid-lubricant coatings, the useful life of such coatings is limited. When the coating eventually is worn away, the lubricant cannot be readily replenished and lubrication ceases. A method whereby solid lubricant is replenished as wear takes place is clearly desirable. Self-lubricating composites are an approach to accomplish this replenishment.
To reduce the problems of film-type solid lubricants, composite bearing materials containing fluoride lubricants were subsequently developed by Sliney. For example U.S. Pat. No. 3,419,363 discloses such a composite comprised of a porous metal impregnated with Group I or Group II metallic fluorides with the eutectic composition of barium fluoride and calcium fluoride as the preferred lubricant. However, such composites have posed new problems insofar as the porous metal component provides a greatly increased surface area as opposed to that of solid substrates. Accordingly, high temperature oxidation of these porous, sintered metals poses significant problems at temperatures exceeding about 700.degree. C.
Accordingly, a need exists for improved high-temperature, self-lubricating materials which exhibit the excellent lubricating qualities of fluoride-containing composites and yet are not susceptible to high temperature oxidation. The need for such materials is presently becoming critical in advanced aircraft where aerodynamic heating at speeds of Mach 3 and higher can result in vehicle skin temperatures well above the temperature limitations of presently available air frame bearings. As an extreme case, it is predicted that the maximum skin temperature for the Space Shuttle Orbitor during re-entry will approach or exceed 1100.degree. C. Air frame bearings and control surface seals for the orbitor proximate these heated surfaces must be capable of high temperature operation without degradation. Other areas in which high temperature lubrication has become increasingly more important include sliding contact seals for automotive turbine regenerators, shaft seals for turbo pumps, piston rings for high performance reciprocating compressors, hot glass processing machinery, and the like.