LUBRICANT FOR OXYGEN - ENRICHED ENVIRONMENTS

Abstract

A lubricating grease containing about 75 to 85% by weight of a liquid perfluoropolyether and about 15 to 25% by weight of a particulate boron nitride, preferably monomodal boron nitride having a particle size of about 7 to 10 microns; use of the lubricating grease for lubricating structures subjected to an oxygen-enriched environment and devices containing such lubricated structures.

Description

BACKGROUND OF THE INVENTION

The present invention relates to improvements in the lubrication of structures located within an oxygen-enriched environment, the formulation of greases based on high molecular weight perfluoropolyether oils for use in oxygen-enriched environments, and structures to be subjected to an enriched oxygen environment which have been lubricated by the perfluropolyether grease described herein.

The prior art describes a number of lubrication formulations based on fluorinated oils, including liquid perfluoropolyethers. When compounded as grease compositions, these fluorinated oils, including perfluoropolyethers, can be thickened with organic thickeners such as poly(tetrafluoroethylene) and with inorganic thickeners such as aluminum nitride, boron nitride and silicon carbide. Whether as oils or greases, compositions based on perfluoropolyethers are characterized high temperature stability, excellent resistance to chemicals and oxidizers, good lubricating properties and wide compatibility with materials of construction such as plastics, metals and elastomers.

U.S. Pat. No. 7,838,475 discloses a composition that comprises an aryl perfluoropolyether, optionally a halogenated oil, and further optionally a thickening agent. The composition can be used as a lubricant itself or as an additive to an oil or grease lubricant and can withstand high temperatures without decomposition. The halogenated oil can be a perfluoropolyether, a fluorosilicone, a polytrifluorochloroethylene, or combinations of two or more thereof The thickening agent can be finely divided silica, boron nitride, clay, soap, poly(tetrafluoroethylene), talc, silica, titanium dioxide, polyurea, polyurethane, or combinations of two or more thereof.

U.S. Pat. No. 7,709,424 discloses a lubricating oil composition, which comprises a perfluoropolyether base oil and a fluorine-containing polyether diphosphonic acid ester, and a grease composition which further contains a thickening agent in addition to the lubricating oil composition. The composition may include solid lubricants such as molybdenum disulfide, graphite, boron nitride, silane nitrides, etc.

U.S. Pat. No. 7,544,646 discloses a method of lubricating a sootblower comprising use of an oil or grease that is resistant to thermal oxidation. By “resistant to thermal oxidation” is meant that the oil or grease can withstand temperatures less than 300 degrees centigrade for an indefinite time, that is, without thermally or oxidatively degradating. This characteristic is distinct from use within the highly oxygen enriched environments contemplated by the present invention. A sootblower employs air, steam or water under high pressure. The oil or grease of the '646 patent can be an ester, silicone, or halogenated oil or grease including perfluoropolyether or perfluoroalkyl ether, a fluorosilicone, a polytrifluorochloroethylene, derivatized perfluoropolyethers, such as fluoroether triazines, and mixtures of two or more thereof. The composition may comprise a thickening agent such as poly(tetrafluoroethylene), boron nitride, talc, silica, metal soaps, polyurea and so on.

U.S. Pat. No. 7,265,080 discloses a rolling bearing to be incorporated in the compressor for a fuel cell system including a fluorine-based grease containing a fluororesin and a fluorine-based oil, a urea grease containing a urea compound and a synthetic oil, or a lithium complex grease containing a lithium complex and a synthetic oil encapsulated therein. Perfluoroalkylether may be the base oil, polytertafluroethylene may be the thickening agent and various extreme pressure agents, such as organic metal compounds including organic molybdenum compounds, organic zinc compounds, organic antimony compounds and so on, or halogen-based extreme pressure agents, or solid lubricants including molybdenum disulfide, tungsten disulfide, graphite, boron compounds, e.g. boron nitride, etc. are used in the lubricant compositions of the '080 patent.

U.S. Pat. No. 7,232,932 discloses a perfluoropolyether possessing improved heat stability and which comprises perfluoroalkyl radical end groups. Greases containing the new perfluropolyether can include standard thickeners such as poly(tetrafluoroethylene), fumed silica and boron nitride and combinations thereof.

U.S. Pat. No. 6,632,780 discloses a highly thermal conductive grease composition which comprises from 70 to 90% by volume of an inorganic powder which is a mixture containing at least two kinds of inorganic powders different from each other in each average particle size, and from 10 to 30% by volume of a base oil containing a mineral oil or a synthetic oil. The mixture of inorganic powders includes zinc oxide, magnesium oxide, titanium oxide, aluminum nitride, aluminum oxide, silicon carbide, boron nitride, etc. The base oils can be numerous hydrocarbon oils such as polyol esters, polyphenyl ethers, alkyl phenylethers or trimellitic acid esters, or at times can be liquid silicone oils or fluorohydrocarbon oils such as chlorofluorocarbon and perfluoropolyethers.

U.S. Pat. No. 6,528,457 discloses a composition that can be used as lubricant for a high electric field including spark plug boots. The composition comprises, or is produced by combining, a halogenated oil such as a perfluoropolyether, a polytrichlorofluoroethylene, a fluorosilicone, or combinations of two or more thereof; a basic thickener; and optionally an additional thickener in which the basic thickener is a metal hydroxide, a metal salt, an ammonium salt, or combinations of two or more thereof. The optional additional thickener is polytetrafluoroethylene, talc, silica, clay, boron nitride, metal soaps, titanium dioxide, polydimethylsiloxane, polyurea, polyurethane, or combinations of two or more thereof.

U.S. Pat. No. 6,432,887 discloses a rolling device utilizing as a lubricant, among others, a composition containing a mixture of a liquid fluorinated polymer oil having a specific kinetic viscosity as a base oil and a thickening agent, wherein the liquid fluorinated polymer oil lubricant may be, for example, perfluoropolyether, telomer of trifluoroethylene and fluorosilicone polymer, and in which the thickening agent can be a solid fluoroinated polymer, layered mineral powder (such as powders of mica series mineral, a vermiculite mineral, and a montmorillonite series mineral each having a layered crystal structure or a hexagonal system boron nitride), or ultra fine particles and white nonmetal powder.

U.S. Pat. No. 6,040,277 discloses grease compositions that are resistant to high temperatures and oxidative or chemically aggressive environments, containing a liquid fluorinated polymer (representative examples of which include telomers of chlorofluoroethylene, fluorosilicone polymers and perfluoropolyethers) and a thickening agent containing hexagonal lattice boron nitride powder preferably having a bimodal particle size distribution (between 25 and 75 weight percent having an average particle size of 2 to 50 micrometers and between 75 and 25 weight percent having an average particle size of 0.01 to 1 micrometers) and optionally containing a solid fluorinated polymer thickening agent (such as polytetrafluoroethylene). In an Example and in Comparative Examples, when the optional thickening agent is present, the boron nitride powder is described as an aggregate powder having an average particle size of 5 to 15 microns. As with U.S. Pat. No. 7,544,646, discussed above, the skilled artisan would understand oxidative environment herein to mean high temperatures and/or pressures which can cause oxidative degradation of the liquid fluoroinated polymer as opposed to the enriched oxygen environment involved with the invention of the present application.

US 2008/0028969 discloses an image forming device comprising two components capable of relative movement and a lubricant disposed between such components. The lubricant may include any polymeric type carrier medium such as fluorinated oils (perfluoropolyether) and silicone oils, and hexagonal boron nitride with average particle size between 0.1 and 10 microns, for example an average particle size of 0.3 to 0.7 microns, capable of reducing friction between the components.

US 2010/0222244 discloses lubricating compositions based on perfluoropolyethers. More specifically, this publication discloses a solid polymer containing at least one aromatic ring that is used as an additive to stabilize fluorinated lubricants, preferably perfluoropolyether oils, at high temperatures, which compositions also include known thickening agents such as poly(tetrafluoroethylene), talc, silica and boron nitride.

EP 0982392 discloses a grease composition for use in, e.g., a semiconductor device to remove heat from electronic parts, having high thermal conductivity and excellent heat dissipation ability, comprising (A) 100 parts by weight of a base oil such as silicone oils and fluoroinated hydrocarbon oils (such as perfluoropolyethers and others) and (B) 500 to 1,200 parts by weight of metallic aluminum powder, and as an additional component, zinc oxide and/or boron nitride.

EP 0939115 discloses a thermally conductive grease composition comprising (A) 100 weight parts of at least one base oil selected from liquid silicones, liquid hydrocarbons or fluorohydrocarbon oils such as perfluoropolyethers and 500-1,000 weight parts of a thermally conductive filler mixture which is constituted of (B) an inorganic filler having Mohs hardness of at least 6 and thermal conductivity of at least 100 W/m ° K such as aluminum nitride powder and (C) an inorganic filler having Mohs hardness of at most 5 and thermal conductivity of at least 20 W/m ° K such as hexagonal boron nitride powder or zinc oxide powder.

Even with the state of the art as now exists with perfluoropolyether lubricants, a need still exists for lubricants tailored to withstand the severe operating conditions imposed by high oxygen-containing fluids used in many specialized devices, such as hyperbaric medical devices and military devices, and processes for lubricating the internal parts of such devices. Also, a need exists for providing devices internally pressurized with a high oxygen-containing fluid, and containing improved internal lubrication of surfaces and structures.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for lubricating structures, such as bearings, gears, seals, chains, shafts, etc., subjected to a continuous or intermittent oxygen-enriched environment.

Another object of the present invention is to provide grease formulations usable in processes for lubricating structures subjected to a continuous or intermittent oxygen-enriched environment.

Still another object of the invention is to provide grease formulations which can be used in processes for lubricating internal structures within a pressurized oxygen environment.

A further object of this invention is to provide devices comprising a partially or fully enclosed area subjected to an oxygen enriched gas and containing internal structures such as bearings, gears, seals, chains, shafts, etc., lubricated with the grease disclosed herein.

Other objects of the present invention will be apparent to the skilled artisan from the detailed description of the invention hereinafter.

In accordance with the present invention, a process has now been found for lubricating structures subjected to an oxygen enriched fluid which comprises lubricating such structures with a grease comprising a high molecular weight perfluoropolyether and a particulate boron nitride of defined particle size. More particularly, it has now been found that structures internal to a device subjected to a continuous or an intermittent oxygen-enriched fluid can be lubricated without breakdown of the lubricant by using a lubricating grease comprising a high molecular weight perfluoropolyether and a boron nitride having a particle size of 1 to 20 microns, especially a 7 to 10 micron particle size. The lubricating grease used in the present invention comprises about 70 to 90 percent by weight of the high molecular weight perfluoropolyether and about 10 to 30 percent by weight of the particulate boron nitride, more particularly, about 75 to 85 percent by weight of the perfluoropolyether and about 15 to 25 percent by weight of the boron nitride. The devices lubricated in accordance with the present invention, even when exposed to an oxygen enriched environment or atmosphere approaching or at essentially 100% oxygen content, are characterized by an extended useful life, all other factors being constant, and especially as compared with conventional lubrication as carried out in the prior art.

In preferred embodiments of the present invention, novel devices to be subjected to a continuous or intermittent oxygen enriched atmosphere, or internally pressurized by an oxygen enriched atmosphere, are provided. These devices include internal structures lubricated with a grease comprising about 75 to about 85 percent by weight high molecular weight perfluoropolyether and about 15 to 25 percent by weight of particulate boron nitride having a particle size of about 7 to about 10 microns. In a more preferred embodiment of the present invention, the particulate boron nitride is monomodal boron nitride having a particle size of 7 to 10 microns.

In preferred embodiments of the present invention, the inventive grease consists essentially of the high molecular weight perfluoropolyether and the particulate boron nitride, or consists of the perfluoropolyether and the boron nitride. Other preferred and non-preferred embodiments of the present invention will be obvious to the skilled artisan from the detailed description of the invention hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to improving the internal lubrication of devices which in use are subjected to an oxygen enriched environment. Such devices are well known and include gaskets, actuators, regulators, valves and valve packings. They range in use from ventilators, oxygen concentrators, certain types of scuba and other diving equipment and medical devices such as hyperbaric chambers to military equipment such as internal sections of rockets through which high oxygen content gases flow under high pressure. A representative listing of such devices, which is not exhaustive by any means, would include:

Oxygen Cylinder or Distribution Manifold Service:

Cylinder valves, pressure regulators, valve integrated pressure regulators, relief valves and check valves

Oxygen Hospital Distribution Systems (LOX and GOX):

Master/station valves, flow meters and quick connect fittings

Oxygen Ventilator/Respiratory Care Systems:

All aspects of respiratory care systems, oxygen ventilators, air/oxygen sleep apnea units, oxygen conservers and oxygen concentrators

Hyperbaric and Hypobaric Chambers

By “oxygen-enriched environment” is meant a gaseous or liquid phase in which the oxygen content is more than 25 mol % oxygen, and is meant to include an oxygen content of up to essentially 100 mol % oxygen.

When less than 100 percent oxygen is present within a particular device, the remainder would be whatever content is normally employed in the particular device. In one aspect of the present invention, where desired, the oxygen content within the device may be increased above its normal level without concern for the device's lubricated surfaces and structures.

The grease of this invention is usable with materials and applications in connection with the production, storage, transportation or distribution of oxygen, in addition to materials and applications involved with the use of oxygen

The grease involved in the present invention would have an apparent viscosity of at least about 250 poise at 20 degrees centigrade, preferably at least about 200 poise at 20 degrees centigrade, more preferably at least about 150 poise at 20 degrees centigrade.

The first required ingredient of the grease composition with which the present invention is involved is a high molecular weight liquid perfluoropolyether, also known as a perfluoroalkylether. These products are sold under the trademarks “KRYTOX” by E.I. du Pont de Nemours and Company of Wilmington, Del., “FOMBLIN” by Solvay of Brussels and “DEMNUM” by Daikin Industries of Japan. Of course, perfluoropolyethers available from other sources or produced synthetically by processes known in the art are equally applicable for the practice of the present invention. For use in the present invention, the perfluoropolyether has a molecular weight of at least 1000, preferably about 1500 to 12,500, and most preferably about 3,000 to 8,000.

Thus, one group of preferred fluoroinated oils for use in the present invention comprises certain of the poly(hexafluoropropyleneoxide) compounds sold under the “KRYTOX” trademark. These compounds have the formula: F—(CF(CF₃)—CF₂—O)_k—CF₂CF₃ in which k is about 10 to 60, preferably about 40 to 60.

Another group of preferred fluorinated oils for use in the present invention comprises certain of the “DEMNUM” fluids which are also polyfluoroethercompounds, but where the central propylene moieties are of a straight chain, which compounds are of the formula: F—(CF₂CF₂CF₂—O)_d—CF₂CF₃ in which d is about 15 to 60, preferably about 30 to 60.

A third group of fluorinated oils for use in the present invention comprises certain of the “FOMBLIN” fluids, such as the “FOMBLIN Y” and “FOMBLIN M” fluids. “FOMBLIN Y” has the formula: CF₃O—[CF(CF₃)CF₂—O]_m—[CF₂—O]_n—CF₃ in which m+n is about 8 to 45, preferably about 20 to 45 and m/n is about 20 to 1,000, preferably about 100 to 1000 and “FOMNLIN M” has the formula: CF₃O—(CF₂CF₂—O)_p—(CF₂—O)_q—CF₃ in which p+q is 40 to 180, preferably 100 to 180 and p/q is 0.5 to 2, preferably 1 to 2.

The particulate boron nitride powder must be of a defined particle size for the grease to possess its excellent lubricity and possess its high oxygen stability. That is, the boron nitride powder preferably has an average particle size of between 7 and 11 microns, preferably between 7 and 10 microns. At least 95 percent, preferably at least 99 percent of the boron nitride particles fall within the above-stated particle size ranges, and preferably at least 99.9 percent of the boron nitride particles are within a size range of 7 to 10 microns. Further, the boron nitride is composed, at least primarily, of single crystal hexagonal platelets, which is known as hexagonal lattice boron nitride. This type of boron nitride is known in the art and is commercially available from, among others, Momentive Performance Materials, Inc. Preferably, the boron nitride used herein consists of monomodal boron nitride having a single crystal hexagonal platelet purity of at least 99.9 percent.

Optionally, minor amounts of conventional lubricating oil and grease components may be included in the formulations of the present invention, as long as any such ingredient(s) do not adversely affect the lubricating and oxygen compatibility characteristics of the inventive grease. Commonly found grease ingredients include dispersing agents, wetting agents, antiwear agents, anticorrosive agents and metal protectants. Specific examples of these and other conventional grease formulation additives are well known in the art and thus are not listed herein.

The inventive greases can be formulated by adding the selected fluids, or base oils, to a mixer followed by insertion of the mixer impeller. The impeller is fully submerged and does not touch the bottom of the container. Mixing is begun by turning on the impeller at the minimum speed while not causing splashing. Solids are periodically added with increasing mixer speed as the blend thickens. The speed is maintained as high as possible while not exceeding a certain Hz. When all ingredients have been added, mixing is allowed to continue until a uniform mix is obtained, which appears homogenous and free of visible particles. The mixer is then stopped and the impeller is removed and scrapped of excess grease. The mixture is then blended to a grease consistency by passing it through a 3-roll mill at about 25 degrees Centigrade with the rollers set at an opening of about 0.0015 to 0.002 inch.

The final grease compositions are tested by various ASTM tests to determine penetration, evaporation and other tests where appropriate.

The following formulation examples are illustrative of grease compositions of this invention.

EXAMPLE 1

Several greases are prepared using the basic procedure described above. One base oil used is a linear perfluoropolyether with a viscosity of about 150 cSt at 20 degrees Centigrade. This product is commercially available from Solvay as Fomblin M-15. The thickener used is ultra fine powders of single-crystal hexagonal BN platelets with very high purity. In this instance, 77.6 weight percent of the fluorinated oil is added to the mixing container, then with the mixer running, 22.4 weight percent of the boron nitride (grade HCP available from Momentive Performance Materials) is added to make a product which is then milled to the consistency of a grease. For small batches, a planetary Kitchen Aid mixer can be used; for large batches, a stationary Lightnin mixer can be used prior to the milling.

EXAMPLE 2

In other examples, the types of base oils, that is, linear and branched perfluoropolyether oils, are used with different viscosities and blended at different ratios with the thickener to produce greases with a range of penetration values.

The excellent degree of compatibility of the lubricating greases of the present invention with gaseous and liquid oxygen-enriched environments is shown by employing standard testing protocols. Gaseous oxygen compatibility of the inventive formulations has been evaluated using test methodologies indicated by ASTM G63, Standard Guide for Evaluating Nonmetallic Materials for Oxygen Service. The following tests have been carried out on the inventive greases, with the test results for two compositions prepared in accordance with the formulations of the present invention being indicated.

Autogeneous Ignition Temperature (AIT)

AIT is a relative indicator of a material's propensity for ignition, and is defined as the temperature at which the tested material spontaneously self-ignites in oxygen, and was tested according to ASTM G72-86 (reapproved 2009). AIT is tested in a gaseous oxygen environment of >99.5% O₂ at an initial pressure of approximately 1500 psig, with graduated temperature increase of the test vessel. Three samples of two test compositions were tested with the following results. AIT is indicated as above the temperature just below the test temperature at which ignition occurs, with a temperature increase of 5±1° C. per minute at that stage of the test.

Composition 1:

Sample 1: AIT of >493° C.

Sample 2: AIT of >492° C. Sample 3: AIT of >490° C.

Composition 2:

Sample 1: AIT of >493° C.

Sample 2: AIT of >486° C.

Sample 3: AIT of >489° C.

The greases of the present invention are characterized by an AIT in >99.5% oxygen concentration of at least above 400° C., preferably of at least above 450° C. and most preferably at least above 480° C. AIT of about 480 degrees Centigrade is a preferred characteristic in the inventive greases.

Oxygen Index Testing (OI)

The OI is the minimum oxygen concentration in a mixture of oxygen and nitrogen that will just support candle-like combustion at ambient pressure and initially at ambient temperature. The OI is a relative indication of a material's flammability or propensity for fire propagation and sustained burning. A modified version of the test method of ASTM G125 was used. The ASTM standard does not include specifications for testing non-rigid materials such as the greases of the present invention. The modification consisted of using a horizontal cylinder and gas flow configuration, instead of the vertical configuration of the ASTM standard. Other test parameters were consistent with the ASTM standard. The specified mixture of oxygen and nitrogen was supplied to one end of the glass cylinder at a flow rate that produced a gas velocity of 40±4 mm/s within the tube. The material was deemed flammable at a supplied oxygen concentration if 1) the flame propagated greater than 2 in. against the flow from the initial point of igniter flame impingement or, 2) if the sample burned for more than 3 minutes. At an oxygen concentration of 100% (>99.5% oxygen), there was no sign of ignition after several attempts with the grease contained horizontally in a ceramic boat within the cylinder.

Heat of Combustion Testing (ΔHc)

The ΔHc is an intrinsic property of a material and does not change substantially as pressure increases. The ΔHc of the greases of the present invention indicates high oxygen compatibility and low damage potential should the grease ignite under service conditions. The ΔHc is the quantity of heat liberated when a unit mass of material is burned completely in oxygen and is measured according to ASTM D4809. It is expressed as calories/gram. The ΔHc is an absolute value of a material's energy release upon burning, which is an indication of its damage potential. An automated isoperibol calorimeter is used to accurately measure temperature rise and calculate the ΔHc. An average ΔHc±Standard deviation for a grease Composition 1 as formulated according to the present invention 1 was 1280±30 while for a grease composition of Composition 2, it was 1280±58. These measurements are at 99.5% oxygen content.

The greases of the present invention are characterized by a ΔHc of no more than about 1010 cal/g, preferably no more than about 1338 cal/g, and most preferably about 1280 cal/g, all within 99.5% oxygen concentration.

Variations of the invention will be apparent to the skilled artisan from the above detailed description.

Claims

1. A process for lubricating a structure exposed to an oxygen enriched fluid which comprises lubricating the structure with a lubricating grease comprising about 75 to 85 weight percent of a high molecular weight liquid perfluoropolyether and about 15 to 25 weight percent of a particulate boron nitride having a particle size of about 7 to 11 microns.

2. The process of claim 1 wherein the perfluoropolyether has a molecular weight of at least 1,000 and the boron nitride has a particle size of 7 to 10 microns.

3. The process of claim 2 wherein the boron nitride is composed primarily of single crystal hexagonal platelets.

4. The process of claim 3 wherein the boron nitride is monomodal boron nitride.

5. The process of claim 4 wherein the lubricated structure is subjected to a gaseous or liquid fluid containing at least about 99.5 mol percent oxygen.

6. The process of claim 1 wherein the perfluoropolyether comprises a perfluoropolyether of the formula: F—(CF(CF3)—CF2—O)k—CF2CF3 in which k is about 10 to 60.

7. The process of claim 1 wherein the perfluoropolyether comprises a perflluoropolyether of the formula: F—(CF2CF2CF2—O)d—CF2CF3 in which d is about 15 to 60.

8. The process of claim 1 wherein the perfluoropolyether comprises a perfluoropolyether of the formula CF3O—[CF(CF3)CF2—O]m—[CF2—O]n—CF3 in which m+n is about 8 to 45 and m/n is about 20 to 1,000 or of the formula: CF3O—(CF2CF2—O)p—(CF2—O)q—CF3 in which p+q is 40 to 180 and p/q is 0.5 to 2.

9. The process of claim 1 wherein the grease has an autogenous ignition temperature of at least about 400 degrees centigrade in >99.5% oxygen concentration.

10. The process of claim 1 wherein the grease has an average heat of combustion of about 1,280 calories/gram in >99.5% oxygen concentration.

11. A device of the type which will contain a continuously or intermittently supplied oxygen enriched atmosphere, said device comprising an internal lubricated structure lubricated with the grease of claim 1.

12. A device of the type internally pressurized with an oxygen enriched atmosphere, said device comprising an internal structure lubricated with the grease of claim 1.

13. A device of the type which will contain a continuously or intermittently supplied oxygen enriched atmosphere, said device comprising an internal lubricated structure lubricated with the grease of claim 2.

14. A device of the type internally pressurized with an oxygen enriched atmosphere, said device comprising an internal structure lubricated with the grease of claim 2.

15. A device of the type which will contain a continuously or intermittently supplied oxygen enriched atmosphere, said device comprising an internal lubricated structure lubricated with the grease of claim 4.

16. A device of the type internally pressurized with an oxygen enriched atmosphere, said device comprising an internal structure lubricated with the grease of claim 4.

17. A lubricating grease comprising about 75 to 85 percent by weight of a liquid perfluoropolyether having a molecular weight of at least 1000 and about 15 to 25 weight percent of a particulate monomodal boron nitride having a particle size of about 7 to 10 microns.

18. The lubricating grease of claim 17 wherein the boron nitride is composed of single crystal hexagonal platelets.

19. A lubricating grease consisting essentially of about 75 to 85 percent by weight of a liquid perfluoropolyether having a molecular weight of at least 1000 and about 15 to 25 weight percent of a particulate monomodal boron nitride having a particle size of about 7 to 10 microns.

20. The lubricating grease of claim 19 wherein the boron nitride is composed of single crystal hexagonal platelets.

21. The lubricating grease of claim 19 wherein the perfluoropolyether comprises a perfluoropolyether of the formula: F—(CF(CF3)—CF2—O)k—CF2CF3: in which k is about 10 to 60.

22. The lubricating grease of claim 21 wherein the perfluoropolyether comprises a perflluoropolyether of the formula: F—(CF2CF2CF2—O)d—CF2CF3 in which d is about 15 to 60.

23. The lubricating grease of claim 19 wherein the perfluoropolyether comprises a perfluoropolyether of the formula CF3O—[CF(CF3)CF2—O]m—[CF2—O]n—CF3 in which m+n is about 8 to 45 and m/n is about 20 to 1,000 or of the formula: CF3O—(CF2CF2—O)p—(CF2—O)q—CF3 in which p+q is 40 to 180 and p/q is 0.5 to 2.

24. A lubricating grease consisting of about 75 to 85 percent by weight of a liquid perfluoropolyether having a molecular weight of at least 1000 and about 15 to 25 weight percent of a particulate monomodal boron nitride having a particle size of about 7 to 10 microns.

25. The lubricating grease of claim 24 wherein the boron nitride is composed of single crystal hexagonal platelets.

26. The lubricating grease of claim 24 wherein the perfluoropolyether comprises a perfluoropolyether of the formula F—(CF(CF3)—CF2—O)k—CF2CF3 in which k is about 10 to 60.

27. The lubricating grease of claim 24 wherein the perfluoropolyether comprises a perflluoropolyether of the formula F—(CF2CF2CF2—O)d—CF2CF3 in which d is about 15 to 60.

28. The lubricating grease of claim 24 wherein the perfluoropolyether comprises a perfluoropolyether of the formula CF3O—[CF(CF3)CF2—O]m—[CF2—O]n—CF3 in which m+n is about 8 to 45 and m/n is about 20 to 1,000 or of the formula: CF3O—(CF2CF2—O)p—(CF2—O)q—CF3 in which p+q is 40 to 180 and p/q is 0.5 to 2.