60/460,703, entitled"Corrosion Tolerant Rolling Element Bearing"which was filed on April 3,2003, and which is incorporated herein by reference.

Technical Field The invention is concerned with reducing the effects of mild aqueous corrosion on rolling elements and bearing raceways which are susceptible to aqueous corrosion damage by coating the rolling elements with a thin nanocomposite (NC) coatingThe NC coated rollers do not protect the rolling element surfaces from mild aqueous corrosion damage, but rather function to prolong bearing life by preventing adhesive wear and minimizing the effect of debris damage on bearing life caused by micropits, etch marks and rust formation on the raceways.

Background Art It is well known that in a humid or moist environment, water-based corrosion damage may occur on static bearing inner and outer ring functional surfaces ("raceways"). This may occur in a variety of moist environments, and to varying degrees. Most commonly, general aqueous corrosion on raceways due to bearing contact with water results in etching, pitting and rusting. These forms of damage may cause premature bearing failure. Corroded bearing components are often detected due to noisy bearing operation, but often the corrosion may go undetected before component failure.

This invention is most effective in a"mild"aqueous corrosion situation during which the bearing is operational with no gross macroscale raceway or non-raceway damage. The effects of mild aqueous corrosion include the formation of micropits, etch damage, and/or surface oxide formation (e. g., light reddish-brown"rust") on the raceways. After the bearing is restarted after static mild aqueous corrosion damage, the rolling element/raceway contact may dislodge brittle surface oxides, forming debris that are detrimental to bearing life. The raceway micropits and nascent

surfaces remaining after oxide dislodgement may become initiation sites for spalling failures due to adhesive interactions with uncoated rolling element functional surfaces.

Mechanisms that describe the detrimental effects of aqueous corrosion on bearing life have been proposed. For example, brittle oxide corrosion products may be removed during bearing operation, thus resulting in fine abrasive debris contamination. The debris may cause denting and abrasive wear of uncoated raceways (1). The dent shoulders, raised damaged areas, or the debris particle itself may also experience adhesive wear with uncoated rolling element functional surfaces that leads to scuffing or surface origin spalling failure. Certain topographical features assocjated with corrosion micropits may interact with the rolling element surfaces in a likewise manner.

The typical approach for achieving extended rolling element bearing life in moist corrosive environments is to treat the surfaces that are most susceptible to corrosion attack. Examples of anti-corrosion raceway and rolling element treatments include ion implantation of chromium, coating with electroplated thin dense chromium (2), the application of other types of zinc-alloy"sacrificial"coatings (3), and the use of stainless steel, yellow metal, or ceramic bearing components. These approaches have been used in the airframe, off-highway vehicle, food processing, rolling mill, and hydrostatics applications areas.

For example, bearing raceways have been coated with AquaspexxM (a zinc alloy coating available from The Timken Company of Canton, Ohio) or chrome to prolong the life of rolling mill bearings that are subject to water- based corrosion. However, such coatings offer a less than an ideal situation, because sacrificial coatings wear away and chrome coatings delaminate and crack.

Summary of the Invention The present invention comprises the use of a thin nanocomposite (NC) coating on rolling elements in combination with uncoated raceways.

To use our invention, a designer would call for NC-coated rolling elements in

the bearings. There may not be a compelling reason for the designer to specify NC-coated rolling elements absent the aqueous corrosion problems.

A central premise of this invention is that the reduction of life due to corrosion of bearings with NC-coated rolling elements is much lower than the expected reduction of life due to wear caused by oxide debris and adhesive interactions with mildly corroded raceways. The use of NC-coated rolling elements increases bearing life in mildly corroded bearings beyond that which is attainable using uncoated rolling elements.

Brief Description of Drawings The objects of the invention are achieved as set forth in the illustrative embodiments shown in the drawings which form a part of the specification.

Figure 1 is a bar graph showing a comparison of bearing life with and without the presence of debris particles using coated and uncoated bearings; and Figures 2A-2C show coated roller elements of this invention.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Best Mode for Carrying Out the Invention The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what we presently believe is the best mode of carrying out the invention. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We propose that nanocomposite (NC) coated rolling element surfaces mitigate the detrimental effects of aqueous corrosion on rolling element bearing life in airframe, marine and on and off-highway land

vehicles, stationary process industries applications including but not limited to, food processing and steel rolling and hydrostatics applications as compared to the same bearings without NC coated rolling element surfaces.

Such corrosion-tolerant rolling element bearings must have the following key characteristics: An"NC"-type coating on the rolling element's functional surfaces.

The coating comprises a metallic interlayer upon which is deposited additional layers containing at least carbon and any combination of additional elements such as: metal, silicon, boron, nitrogen, oxygen, and hydrogen.

The"NC"-type coatings must be applied to the functional surfaces of the rolling elements. FIGS. 2A-C show a tapered roller bearing, a ball bearing, and a cylindrical roller bearing, respectively, in which their respective functional surfaces are coated with the NC coating of the present invention. The functional surfaces of the tapered roller bearing (FIG. 2A) include its tapered surface and its large end face.

Depending on the application in which the bearing will be used, the NC coating can be applied only to the tapered sides, only on the large end face, or to the tapered sides and large end face of the tapered roller bearing element. The functional surface of the ball bearing (FIG. 2B) include the complete surface of the ball bearing.

The functional surface of the cylindrical roller bearing (FIG. 2C) includes the outer cylindrical surface of the rolling element.

'The inner/outer bearing raceways must be made of steel and must be susceptible to aqueous corrosion damage An"NC"-type coating must have a low solubility relative to the inner/outer bearing ring functional surface (e. g. , raceway) materials.

An"NC"-type coating must have higher surface hardness than the inner/outer bearing ring functional surface (e. g. , raceway) material.

These corrosion-tolerant bearings with"NC"-type coatings on the rolling element functional surfaces may be used alone or in

conjunction with other anti-corrosion measures including greases, oils, and inner/outer raceway sacrificial coatings.

Liquid or gas phase water-based corrosion damage may occur in airframe, marine and on-and off-highway land vehicles, stationary process industries applications including but not limited to, food processing and steel rolling, and hydrostatic rolling element bearing applications due to a humid environment or water ingress due to leaky seals. Thus, the present invention is specified for rolling element bearings present in these applications.

Candidate NC-type coatings are defined to have a carbonaceous matrix, but may contain incorporated metal, hydrogen, silicon, boron, and other elements. The coatings may be multi-layered, duplex, or functionally gradient. One or more adhesive interlayers that promote adhesion of the coating to the substrate may support the NC top layer."NC,"as used in this invention, refers to a coating material that has any or all of the characteristics previously listed. For example, an NC coating for rolling element functional surfaces that is within the scope of this invention is a composite film comprised of nanocrystalline metal carbides and an amorphous hydrocarbon matrix with a thin metallic adhesive interlayer (-100-500 nm thick). This coating has the following desirable properties for use in this invention. The coating is between 1 and 5 micrometers thick.

The coating is very hard, having a hardness approximately 1.5x the hardness of HRC 60 bearing steel as determined by indentation method using force displacement measurement (that is, the depth of the indentation in the coating is less than 10% of the coating thickness). The coating is also chemically dissimilar to the material from which the bearing is made. NC coatings may be deposited onto the functional surfaces of rolling elements by a variety of techniques, including but not limited to: physical vapor deposition, chemical vapor deposition, plasma-enhanced chemical vapor deposition, ion beam deposition, laser or electron beam ablation deposition, and reactive sputtering.

The presence of a"nanocomposite" (NC) coating on rolling element surfaces minimizes the detrimental effects of raceway water-based corrosion on bearing life. Lab tests have shown that NC coatings on rolling elements help to minimize the injurious effects of metallic debris damage on tapered roller bearing life. The data displayed in the chart of FIG.

1compares the relative life of bearings with a nanocomposite coating (ES300) and bearings made from a high performance steel, but without a nanocomposite coating (P900) both with and without debris. The debris used in the test was a metallic debris having a particle size of 25-53ßm. As seen in the chart of FIG. 1, the nanocomposite coating dramatically improved the life of the bearing.

Although the debris used in our lab tests were not generated by corrosion reactions, NC-coated steel rolling elements are expected to prolong life in the presence of corrosion-generated debris in the same manner. NC coatings have high hardnesses that protect the rolling element surfaces from abrasive wear.. As a result, debris particles do not become embedded in the coated rolling element surfaces. NC coatings have a low surface energy that is similar to that of polymeric materials, allowing the rolling element surfaces to resist solid-solid adhesion with uncoated steel bearing raceway counterfaces. In a like manner, NC-coated rolling elements resist adhesive interactions with raceway debris dents, asperities, and raised features associated with micropitting. NC coatings are insoluble with and chemically dissimilar to steel. By resisting abrasive wear due to oxide debris and adhesive interactions with mildly corroded raceways, the use of NC-coated rolling elements increases bearing life in mildly corroded bearings beyond that which is attainable using uncoated rolling elements.

Our novel approach for designing a bearing to be used in water- based corrosive environments is to reduce the effect that the detrimental symptoms of corrosion have on bearing life rather than trying to eliminate corrosion damage altogether. Ours is a corrosion-tolerant rather than a corrosion-avoidant approach. Our invention is implemented by utilizing hard, wear resistant NC coatings on the functional surfaces of rolling

elements to contact the mildly corroded raceway surfaces. NC coatings are not expected to protect the rolling element surfaces from mild aqueous corrosion damage, but rather they will prolong bearing life by preventing adhesive wear and minimizing the effect of debris damage on life caused by micropits, etch marks, and rust formation on the raceways.

Specific Example A 2 micrometer thick coating of WC/a-C: H, available from The Timken Company under the product name TIMKEN ES300, is applied to the bodies and ends (functional surfaces) of every steel roller in a tapered roller bearing having steel raceways. As noted above, TIMKEN ES300 is a nanocomposite coating. The rolling element can be made, for example, from a case-carburized SAE 4340 steel alloy, but could be made from other steel alloys, or even other metals. This bearing may then be used in an environment susceptible to mild aqueous corrosion, such as those found in airframe, marine and on and off-highway land vehicles, stationary process industries applications including but not limited to, food processing and steel rolling and hydrostatic applications. The treated bearing has a longer useful life than the same bearing with uncoated rollers.

While this example describes the NC coating as being applied to steel bearing components, the coating can be applied to bearing components made from other materials as well. For example, the NC coating could be applied to metal matrix composites, aluminum alloys and titanium alloys. The coating could be applied to other metals as well.

References (1) M. R. Hoeprich, R. L. Widner,"Environmental factors and bearing damage, "SAE Paper 800678, Society of Automotive Engineers, Warrendale, PA (April 1980). <BR> <BR> <P>(2) R. E. Maurer, "Friction wear, and corrosion control in rolling bearings through coatings and surface modification: A review,"J. Vac.. Sci.

Technol. A 4 [6] (1986) 3002-3006.

(3) J. W. Smitek, T. E. Springer, R. C. Schrama, A. S. Morrone, C. R.

Ribaudo, L. J. Guist Jr. ,"Corrosion-resistant antifriction bearings," AISE Steel Tech. 76 [12] (1999) 19-23.