GB2278865A - Earth-boring bit with improved rigid face seal - Google Patents

Abstract

An earth-boring bit has a bit body, at least one cantilevered bearing shaft 31, including a base and a cylindrical journal bearing surface extending inwardly and downwardly from the bit body, and at least one cutter 33 mounted for rotation on the cylindrical journal bearing surface of the bearing shaft. A seal assembly 42 is disposed between the cylindrical journal bearing surface and the cutter proximally to the base of the cantilevered bearing shaft. The seal assembly includes at least one rigid seal ring 52, 60 having a seal face 56, 58 in contact with a second seal face. At least one of the seal faces 56, 58 is at least partially formed of a super-hard, abrasion-resistant material having wear-resistance greater than, and a coefficient of sliding friction less than, that of the rigid seal ring material. The super-hard material may be formed from a coating of synthetic diamond-like material. <IMAGE>

Description

EARTH-BORING BIT WITH IMPROVED RIGID FACE SEAL BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates generally to earthboring bits, especially the seal and lubrication systems for earth-boring bits of the rolling cutter variety. More particularly, the present invention relates to improving the wear-resistance and to reducing friction in the seal systems of such earth-boring bits.

2. Background Information: The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit, but the two-cone rock bit, invented by Howard R. Hughes, U.S.

Patent No. 930,759, drilled the hard cap rock at the Spindletop Field, near Beaumont, Texas with relative ease.

That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of earth-boring bits of the rolling cutter variety.

In drilling boreholes in earthen formations by the rotary method, earth-boring bits typically employ at least one rolling cone cutter, rotatably mounted thereon. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material. The rolling cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring.

As the cutters roll and slide along the bottom of the borehole, the cutters, and the shafts on which they are rotatably mounted, are subjected to large static loads from the weight on the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, most earthboring bits are provided with precision-formed journal bearings and bearing surfaces, as well as sealed lubrication systems to increase drilling life of bits. The lubrication systems typically are sealed to avoid lubricant loss and to prevent contamination of the bearings by foreign matter such as abrasive particles encountered in the borehole.A pressure compensator system minimizes pressure differential across the seal so that lubricant pressure is equal to or slightly greater than the hydrostatic pressure in the annular space between the bit and the sidewall of the borehole.

Early Hughes bits had no seals or rudimentary seals with relatively short life, and, if lubricated at all, necessitated large quantities of lubricant and large lubricant reservoirs. Typically, upon exhaustion of the lubricant, journal bearing and bit failure soon followed.

An advance in seal technology occurred with the "Belleville" seal, as disclosed in U.S. Patent No.

3,075,781, to Atkinson et al. The Belleville seal minimized lubricant leakage and permitted smaller lubricant reservoirs to obtain acceptable bit life.

During the quest for improved journal bearing seals, bits employing anti-friction ball or roller bearing elements rose to prominence in bit technology. Roller bearing elements reduce the importance of lubricants and lubrication systems, but introduce a number of other disadvantages. A principal disadvantage is that a failure of any one of the numerous elements likely would permit metallic particles to enter the bearing with almost certain damaging results. Additionally, the necessity of a hole bored into the bearing race for insertion of the elements, and retention of the elements with a welded plug, adds complexity to the manufacture of bits employing antifriction bearings.

An adequately sealed journal-bearing bit should have greater strength and load-bearing capacity than an antifriction bearing bit. The seal disclosed by Atkinson would not seal lubricant inside a journal-bearing bit for greater than about 50-60 hours of drilling, on average. This was partially due to rapid movement of the cutter on its bearing shaft (cutter wobble), necessitated by bearing and assembly tolerances, which causes dynamic pressure surges in the lubricant, forcing lubricant past the seal, resulting in premature lubricant loss and bit failure.

The O-ring, journal bearing combination disclosed in U.S. Patent No. 3,397,928, to Galle unlocked the potential of the journal-bearing bit. Galle's O-ring-sealed, journal-bearing bit could drill one hundred hours or more in the hard, slow drilling of West Texas. The success of Galle's design was in part attributable to the ability of the O-ring design to help minimize the aforementioned dynamic pressure surges.

A major advance in earth-boring bit seal technology occurred with the introduction of a successful rigid face seal. The rigid face seals used in earth-boring bits are improvements upon a seal design known as the "Duo-Cone" seal, developed by Caterpillar Tractor Co. of Peoria, Illinois. Rigid face seals are known in several configurations, but typically comprise at least one rigid ring, having a precision seal face ground or lapped thereon, confined in a groove near the base of the shaft on which the cutter is rotated, and an energizer member, which urges the seal face of the rigid ring into sealing engagement with a second seal face. Thus, the seal faces mate and rotate relative to each other to provide a sealing interface between the rolling cutter and the shaft on which it is mounted.The combination of the energizer ring and rigid ring permits the seal assembly to move slightly to minimize pressure fluctuations in the lubricant, and to prevent extrusion of the energizer past the cutter and bearing shaft, which can result in sudden and almost total lubricant loss. U.S. Patent Nos. 4,516,641, to Burr; 4,666,001, to Burr; 4,753,304, to Kelly; and 4,923,020 to Kelly, are examples of rigid face seals for use in earthboring bits. Rigid face seals substantially improve the drilling life of earth-boring bits of the rolling cutter variety. Earth-boring bits with rigid face seals frequently retain lubricant and thus operate efficiently longer than prior-art bits.

Because the seal faces of rigid face seals are in constant contact and slide relative to each other, the dominant mode of failure of the seals is wear. Eventually, the seal faces become pitted and the coefficient of friction between the seal faces increases, leading to increased operating temperatures, reduction in seal efficiency, and eventual seal failure, which ultimately result in bit failure. In an effort to minimize seal wear, seal rings of prior-art rigid face seals are constructed of tool steels such as 440C stainless, or hardenable alloys such as Stellite. Use of these materials in rigid face seals lengthens the drilling life of bits, but leaves room for improvement of the drilling longevity of rigid face seals, and thus earth-boring bits.

A need exists, therefore, for a rigid face seal for use in earth-boring bits having improved wear-resistance and reduced coefficients of sliding friction between the seal faces.

SUMMARY OF THE INVENTION It is a general object of the present invention to provide an improved rigid face seal for use in an earthboring bit, the rigid face seal having improved wearresistance and reduced coefficients of sliding friction between the seal faces thereof.

This and other objects of the present invention are accomplished by providing an earth-boring bit having a bit body, at least one cantilevered bearing shaft, including a cylindrical journal bearing surface extending inwardly and downwardly from the bit body, and at least one cutter mounted for rotation on the cylindrical journal bearing surface of the bearing shaft. A seal assembly is disposed between the cylindrical journal bearing surface and the cutter proximally to the base of the cantilevered bearing shaft. The seal assembly includes at least one rigid seal ring having a seal face in contact with a second seal face.

At least one of the seal faces is at least partially formed of a super-hard, abrasion-resistant material having wearresistance greater than, and a coefficient of sliding friction less than, the material of the rigid seal ring.

According to the preferred embodiment of the present invention, the second seal face is a radial seal face on a second rigid seal ring and at least the second seal face portion of the second rigid seal ring is at least partially formed of a super-hard, abrasion-resistant material.

According to one embodiment of the present invention, the second seal face is formed on the cutter of the earth boring bit and the second seal face is formed of a superhard, abrasion-resistant material.

The preferred super-hard, abrasion-resistant material is AMORPHIC DIAMOND&commat;, which has wear-resistance greater than, and a coefficient of sliding friction less than, that of the material of the rigid seal ring.

Other objects, features, and advantages of the present invention will be apparent to those skilled in the art with reference to the figures and detailed description, which follow.

DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary section view of a section of an earth-boring bit according to the present invention.

Figure 2 is an enlarged, fragmentary section view of the preferred seal assembly for use with earth-boring bits according to the present invention.

Figure 3 is an enlarged, fragmentary section view of an alternative seal assembly contemplated for use with the present invention.

Figure 4 is a graphical comparison of the results of a test of friction pairs of material coated according to the present invention versus conventional material.

DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 depicts, in a fragmentary section view, one section of an earth-boring bit 11 according to the present invention. Earth-boring bit 11 is provided with a body 13, which is threaded at its upper extent 15 for connection into a drillstring (not shown).

Earth-boring bit 11 is provided with a pressure compensating lubrication system 23. Pressure compensating lubrication system 23 is vacuum pressure filled with lubricant at assembly. The vacuum pressure lubrication process also ensures that the journal bearing cavity generally designated as 29 is filled with lubricant through passage 27. Ambient borehole pressure acts through diaphragm 25 to cause lubricant pressure to be substantially the same as ambient borehole pressure.

A cantilevered bearing shaft 31 depends inwardly and downwardly from body 13 of earth-boring bit 11. A generally frusto-conical cutter 33 is rotatably mounted on cantilevered bearing shaft 31. Cutter 33 is provided with a plurality of generally circumferential rows of inserts or teeth 35, which engage and disintegrate formation material as earth-boring bit 11 is rotated and cutters 33 roll and slide along the bottom of the borehole.

Cantilevered bearing shaft 31 is provided with a cylindrical bearing surface 37, a thrust bearing surface 38, and a pilot pin bearing surface 39. These surfaces 37, 38, 39 cooperate with mating bearing surfaces on cutter 33 to form a journal bearing on cantilevered bearing shaft 31 on which cutter 33 may rotate freely. Lubricant is supplied to journal bearing through passage 27 by pressurecompensating lubricant system 23. Cutter 33 is retained on bearing shaft 31 by means of a plurality of precisionground ball locking members 41.

A seal assembly 42 according to the present invention is disposed proximally to a base 43 of cantilevered bearing shaft 31 and generally intermediate cutter 33 and bearing shaft 31. This seal assembly is provided to retain the lubricant within bearing cavity 29, and to prevent contamination of lubricant by foreign matter from the exterior of bit 11. The seal assembly may cooperate with pressure-compensating lubricant system 23 to minimize pressure differentials across seal 42, which can result in rapid extrusion of and loss of the lubricant, as disclosed in U.S. Patent No. 4,516,641, to Burr. Thus, pressure compensator 23 compensates the lubricant pressure for hydrostatic pressure changes encountered by bit 11, while seal assembly 42 compensates for dynamic pressure changes in the lubricant caused by movement of cutter 33 on shaft 31.

Figure 2 depicts, an enlarged section view, a preferred seal configuration 42 contemplated for use with the present invention. Seal assembly 42 illustrated is known as a "dual" rigid face seal because it employs two rigid seal rings, as opposed to the single-ring configuration illustrated in Figure 3. Dual rigid face seal assembly 42 is disposed proximally to base 43 of bearing shaft 31 and is generally intermediate cutter 33 and shaft 31. Seal assembly 42 is disposed in a seal groove defined by shaft groove 47 and cutter groove 49.

Dual rigid face seal assembly 42 comprises a cutter rigid ring 52, a cutter resilient energizer ring 54, shaft rigid ring seal ring 60, and shaft resilient energizer ring 62.

Cutter rigid seal ring 52 and shaft rigid seal ring 60 are provided with precision-formed radial seal faces 56, 58, respectively. Resilient energizer rings 54, 62 cooperate with seal grooves 47, 49 and rigid seal rings 52, 60 to urge and maintain radial seal faces 56, 58 in sealing engagement. The seal interface formed by seal faces 56, 58 provides a barrier that prevents lubricant from exiting the journal bearing, and prevents contamination of the lubricant by foreign matter from exterior of bit 11.

According to the preferred embodiment of the present invention, at least a portion of seal faces 56, 58 of rigid seal rings 52, 60 is formed of a super-hard, abrasionresistant material having a coefficient of sliding friction lower than the material of rigid seal rings 52, 60.

Preferably, the entirety of both seal faces 56, 58 is formed of super-hard, abrasion-resistant material. This super-hard, abrasion-resistant material reduces wear on seal faces 56, 58, thereby enhancing the life of seal assembly 42 and reducing friction between seal faces 56, 58, which can degrade seal function. Exemplary dimensions for the seal illustrated in Figure 2 may be found in U.S.

Patent No. 4,516,641 to Burr.

Figure 3 illustrates, in enlarged section view, an alternative seal configuration 142. Seal assembly 142 comprises shaft seal groove 147, cutter seal groove 149, rigid seal ring 152, and resilient energizer ring 154. A precision-formed radial seal face 156 is formed on rigid seal ring 152, and mates with a corresponding precisionformed seal face 158 formed in cutter 33. Resilient energizer ring 154 cooperates with shaft seal groove 147 and rigid seal ring 152 to urge and maintain seal faces 156, 158 in sealing engagement.

At least a portion, and preferably the entirety, of seal faces 156, 158 of seal assembly 142 is formed of super-hard, abrasion-resistant material having a coefficient sliding friction less than that of the material of rigid seal ring 152. Exemplary dimensions for the seal assembly depicted in Figure 3 may be found in U.S. Patent No. 4,753,304 to Kelly.

The seal assemblies depicted in Figures 1, 2, and 3 are somewhat representative of rigid face seal technology and are shown for illustrative purposes only. The utility of the present invention is not limited to the seal assemblies illustrated, but is useful in all manner of rigid face seals.

The super-hard, abrasion-resistant materials contemplated for use with the seal assemblies of the present invention are typically known as "thin-film diamond" or 1,thin-film diamond-like carbon." These materials are formed primarily of carbon, but are not easily classified because they share characteristics with various forms of carbon, including the crystalline structure of diamond and the amorphous properties of graphitic materials. These materials tend to possess the properties of generally high hardness and wear-resistance, and have low coefficients of sliding friction.These materials are to be distinguished from other low-friction materials such as polytetrafluoroethylene and other fluoroplastics in that they have generally superior wear resistance over such materials. "Thin-film is generally understood to denote coatings having a thickness of 1 micron or less. Thicker-film coatings often do not adhere well to substrate materials.

One drawback to the use of thin-film diamond-like carbon materials is that these materials are difficult to coat or form onto metallic substrates such as the rigid seal rings disclosed herein. The processes for coating such substrates generally involve high temperatures, expensive coating apparatus, and generally low rates of deposition of diamond-like carbon material.

However, one particular type of diamond-like carbon has proved to be successful in adhering to metallic substrates. This material is available under the tradename AMORPHIC DIAMOND, a trademark of SI Diamond Technology, Inc., of Houston, Texas. This material, and the formation process thereof, is fully described in U.S. Patent Nos.

4,987,007, January 22, 1991, to Wagal et al., and 5,098,737, March 24, 1992, to Collins et al. The process for formation for AMORPHIC DIAMONDs coatings involves extracting ions from a laser ablation plume in a vacuum environment and accelerating the ions through a grid for deposit on the substrate. Although the apparatus for formation of AMORPHIC DIAMONDs is expensive, it provides for the formation of a coating on a substrate material that occurs at a relatively high and economical rate, and produces a coating that adheres well to the substrate material and possesses generally good and uniform mechanical properties.

Figure 4 is a graph comparing operating temperature (T), coefficient of sliding friction (pzGd ), and friction force (F=6m) for a friction pair of conventional material versus a friction pair coated with super-hard, abrasionresistant material according to the present invention. The test forming the basis for the graph of Figure 4 was conducted pursuant to A.S.T.M. D-2714, and comprised rotating both a conventional, uncoated test ring and a test ring having a coating according to the present invention on a test block of the same respective material (see below) at 196 revolutions per minute for 60 minutes, resulting in 11,760 cycles.

The conventional test ring and block were formed of 440C stainless steel hardened to approximately 52 or higher on the Rockwell C scale. The test ring and block according to the present invention were similarly formed, but were provided with a thin-film (S 1 micron thickness) coating of the AMORPHIC DIAMONDS super-hard, abrasion-resistant material.

The test was conducted with 100 milliliters of test lubrication fluid prescribed by the aforementioned A.S.T.M.

D-2714 test parameter. The following data was obtained by measuring the aforementioned properties at various time intervals during the test: Prior Art

Time Temp Coefficient Friction (Min) ( C) of Friction Force(lbf) 0 25.0 0 0 10 27.0 .140 0.7 20 28.0 .140 1.4 30 29.7 .133 2.0 40 31.0 .120 2.4 50 32.6 .116 2.9 60 34.0 .110 3.3 Present Invention

Time Temp Coefficient Friction (Min) (C) of Friction Force (lbf) 0 24.5 0 0 10 25.5 .05 .25 20 26.0 .045 .45 30 27.0 .037 .55 40 27.6 .033 .65 50 28.0 .030 .85 60 29.0 .028 .85 Figure 4 is a graphical representation of this data for comparison purposes. For this graphical representation, the coefficient of friction values were multiplied by a factor of 100 and the frictional force values (F=) were multiplied by a factor of 10. Graphed lines 100 and 101 represent the operating temperatures of the conventional friction pair and the friction pair according to the present invention, respectively. Graphed lines 200 and 201 represent the measured frictional force (multiplied by a factor of 10) for the conventional friction pair and the friction pair according to the present invention, respectively.Graphed lines 300 and 301 represent the measured coefficient of sliding friction of the conventional friction pair and the friction pair according to the present invention, respectively. As is demonstrated in Figure 4, the friction pair according to the present invention operates at a lower temperature, with a lower frictional force, and with a lower coefficient of sliding friction than the conventional friction pair.

In operation, earth-boring bit 11 is attached to a drillstring (not shown) and run into a borehole for drilling operation. The drillstring and earth-boring bit 11 are rotated, permitting cutters 33 to roll and slide along the bottom of the borehole, wherein inserts or teeth 35 engage and disintegrate formation material. While cutters 33 rotate relative to body 13 of earth-boring bit 11, seal assemblies retain lubricant in bearing cavities 29, promoting the free rotatability of cutters 33 on bearing shafts 31.

Resilient energizer rings 54, 62, 154 maintain rigid seal rings 52, 60, 152 and seal faces 56, 58, 156, 158 in sealing engagement. Seal faces 56, 158 associated with cutter 33 rotate relative to seal faces 58, 156 associated with bearing shaft 31, which remain essentially stationary.

Thus, seal faces 56, 58, 156, 158 are in constant sliding contact, and are subject to abrasive and frictional wear.

Rigid face seals having seal faces formed according to the present invention provide increased wear-resistance, lower coefficients of sliding friction therebetween, and a lower operating temperature over prior-art rigid face seals. These factors combined provide a seal assembly, and thus an earth-boring bit, having longer operational life.

The ability of the seal assembly to withstand wear and operate longer than prior-art seals permits retention of lubricant in the bearing surfaces for longer periods of time, thus resulting in an earth-boring bit having increased life and therefore more economical operation.

The present invention has been described with reference to a preferred embodiment thereof. Those skilled in the art will appreciate that the invention is thus not limited, but is susceptible to variation and modification without departure from the scope and spirit thereof.

Claims (13)

CLAIMS:

1. An earth-boring bit with an improved mechanical face seal assembly, the earth-boring bit comprising: a bit body; at least one cantilevered bearing shaft, including a base and a bearing surface, extending inwardly and downwardly from the bit body; at least one cutter mounted for rotation on the cantilevered bearing shaft; a seal assembly disposed between the bearing shaft and the cutter and proximally to the base of the cantilevered bearing shaft, the seal assembly including at least one rigid seal ring having a seal face in contact with a second seal face, at least one of the seal faces being at least partially formed of a super-hard, abrasion-resistant material.

2. The earth-boring bit according to claim 1 wherein the super-hard, abrasion-resistant material is AMORPHIC DIAMOND.

3. The earth-boring bit according to claim 1 wherein the second seal face is a radial seal face on a second rigid seal ring, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

4. The earth-boring bit according to claim 1 wherein the second seal face is on the cutter of the earth-boring bit, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

5. The earth-boring bit according to claim 1 wherein at least the seal face of the rigid seal ring and the second seal face are formed entirely of super-hard, abrasionresistant material.

6. An earth-boring bit with an improved mechanical face seal assembly, the earth-boring bit comprising: a bit body; at least one cantilevered bearing shaft, including a base and a bearing surface, extending inwardly and downwardly from the base; at least one cutter mounted for rotation on the cantilevered bearing shaft; a seal assembly disposed between the bearing shaft and the cutter and proximally to the base of the cantilevered bearing shaft, the seal assembly including at least one rigid seal ring having a seal face in contact with a second seal face, at least one of the seal faces being at least partially formed of a wear-resistant material having a coefficient of sliding friction less than that of the rigid seal ring material.

7. The earth-boring bit according to claim 6 wherein the second seal face is a seal face on a second rigid seal ring, the second seal face being at least partially formed of a material having a coefficient of sliding friction less than that of the second rigid seal ring material.

8. The earth-boring bit according to claim 6 wherein the second seal face is on the cutter of the earth-boring bit, the second seal face being at least partially formed of a wear-resistant material having a coefficient of sliding friction less than that of the cutter cone material.

9. The earth-boring bit according to claim 6 wherein the wear-resistant material is AMORPHIC DIAMONDS.

10. The earth-boring bit according to claim 6 wherein at least the seal face of the rigid seal ring and the second seal face are formed entirely of the wear-resistant material.

11. An improved rigid face seal for use in a downhole wellbore tool of the type having a bearing disposed between a first member and a second member, the first member being rotatable relative to the second member, the improved rigid face seal comprising: a seal receptacle formed generally intermediate the first member and the second member; a seal assembly disposed in the seal receptacle including at least one rigid seal ring having a seal face in contact with second seal face, at least one of the seal faces being at least partially formed of a super-hard, abrasion-resistant material.

12. The improved rigid face seal according to claim 11 wherein the second seal face is a seal face on a second rigid seal ring, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

13. A rigid face seal substantially as herein described with reference to the accompanying drawings.

13. The improved rigid face seal according to claim 11 wherein the second seal face is integral with the first member, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

14. The improved rigid face seal according to claim 11 wherein the super-hard, abrasion-resistant is AMORPHIC

DIAMOND.

15. An improved rigid face seal for use in a downhole wellbore tool of the type' having a bearing disposed between a first member and a second member, the first member being rotatable relative to the second member, the improved rigid face seal comprising: a seal receptacle formed generally intermediate the first member and the second member; a seal assembly disposed in the seal receptacle including at least one rigid seal ring having a seal face in contact with a second seal face, at least one of the seal faces being at least partially formed of a wearresistant material having a coefficient of sliding friction less than that of the rigid seal ring material.

16. The improved rigid face seal according to claim 15 wherein the second seal face is a seal face on a second rigid seal ring, the second seal face being at least partially formed of the wear-resistant material.

17. The improved rigid face seal according to claim 15 wherein the second seal face is integral with the first member, at least a portion of the second seal face being at least partially formed of the wear-resistant material.

18. The improved rigid face seal according to claim 15 wherein the wear-resistant material is AMORPHIC DIAMOND.

19. An earth-boring bit substantially as herein described with reference to the accompanying drawings.

20. A rigid face seal substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. An earth boring bit comprising: a bit body; at least one cantilevered bearing shaft, including a base and a bearing surface, extending inwardly and downwardly from the bit body; at least one cutter mounted for rotation on the cantilevered bearing shaft; a seal assembly disposed between the bearing shaft and the cutter and proximally to the base of the cantilevered bearing shaft, the seal assembly including at least one rigid metallic seal ring having a seal face in contact with a second seal face, at least one of the seal faces being at least partially formed of a super-hard, abrasion-resistant material.

2. An earth-boring bit as claimed in claim 1 wherein the super-hard, abrasion-resistant material is anamorphic diamond.

3. An earth-boring bit as claimed in claim 1 or claim 2 wherein the second seal face is a radial seal face on a second rigid seal ring, the second seal face being at least partially formed of the super hard, abrasion-resistant material.

4. An earth-boring bit as claimed in claim 1 or claim 2 wherein the second seal face is on the cutter of the earthboring bit, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

5. An earth-boring bit as claimed in any one of claims 1 to 4 wherein at least the seal face of the rigid seal ring and the second seal face are formed entirely of super-hard, abrasion-resistant material.

6. An earth-boring bit as claimed in any one of the preceding claims wherein the coefficient of sliding friction of the super-hard wear-resistant material is less than that of the rigid metallic seal ring material.

7. A rigid face seal for use in a downhole wellbore tool of the type having a bearing disposed between a first member and a second member, the first member being rotatable relative to the second member, the rigid face seal comprising: a seal receptacle formed generally intermediate the first member and the second member; a seal assembly disposed in the seal receptacle including at least one rigid metallic seal ring having a seal face in contact with second seal face, at least one of the seal faces being at least partially formed of a superhard, abrasion-resistant material.

8. A seal as claimed in claim 7 wherein the super-hard, abrasion-resistant material is anamorphic diamond.

9. A seal as claimed in claim 7 or claim 8 wherein the second seal face is a seal face on a second rigid seal ring, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

10. A seal as claimed in claim 7 or claim 8 wherein the second seal face is integral with the first member, the second seal face being at least partially formed of the super-hard, abrasion-resistant material.

11. A seal as claimed in any one of claims 7 to 10 wherein the coefficient of sliding friction of the superhard wear-resistant material is less than that of the rigid metallic seal ring material.

12. An earth-boring bit substantially as herein described with reference to the accompanying drawings.