Drilling tool with elastomer seal having graded properties

1. A subterranean drilling tool, comprising:

a seal gland defined by a rotating surface and a static surface;

a lubricant reservoir containing a lubricant;

a seal formed of homogenous material composition throughout, the material composition comprising an elastomeric compound, the seal being located in the seal gland, the seal having an annular dynamic portion in sliding engagement with the static surface and an annular static portion in static engagement with the rotating surface, the seal sealing the lubricant from borehole fluid;

a pressure compensator in communication with the lubricant reservoir and exposed to an exterior of the tool for equalizing pressure of the lubricant with pressure of borehole fluid;

the elastomeric compound on the dynamic portion of the seal having greater cross-linking than the elastomeric compound within a remaining portion of the seal, defining a property that changes from a first level at an exterior of the dynamic portion to a second level within a core portion of the seal;

the entire static portion being free of a property gradient; and

wherein the dynamic portion is formed of the same material composition and has the same constituents as the remaining portion of the seal.

2. The tool according to claim 1 , wherein the property comprises a selected one of the group consisting of hardness, elastic modulus, and bulk modulus.

3. The tool according to claim 1 , wherein:

the property is at the second level within an outer diameter portion of the seal.

4. The tool according to claim 1 , wherein:

the seal has a cross-sectional thickness extending from an inner diameter portion to an outer diameter portion; and

the property gradient has a depth that is less than about one-half the cross-sectional thickness.

5. The tool according to claim 1 , wherein:

the seal has a cross-sectional thickness extending from an inner diameter portion to an outer diameter portion; and

the property gradient has a depth that is not greater than about one-tenth the cross-sectional thickness.

6. The tool according to claim 1 , wherein:

in its natural state, the seal has a transverse cross-section that is rounded, the transverse cross-section having a first dimension along an x-axis and a second dimension along a y-axis; and

the property gradient in transverse cross-section spans less than the second dimension.

7. The tool according to claim 1 , wherein the property throughout the seal, other than in the dynamic portion, is substantially uniform and at the second level.

8. The tool according to claim 1 , wherein the property comprises an IRHD (International Rubber Hardness Degree) value, and the second level differs from the first level by approximately at least ten percent.

9. The tool according to claim 1 , wherein:

the seal has a cross-sectional thickness extending from an inner diameter portion to an outer diameter portion; and

the property gradient has a depth that is at least about one percent of the cross-sectional thickness.

10. A subterranean drilling tool, comprising:

a rotating component located adjacent a stationary component, each having an exterior portion adapted to be exposed to drilling fluid and an interior portion that defines a bearing cavity containing a lubricant;

a seal groove formed on at least one of the components;

a seal having a uniform material composition throughout the seal, the material composition comprising an elastomeric polymer compound, the seal being located in the seal groove in contact with the lubricant for sealing between the components to prevent drilling fluid from entry into the bearing cavity, the seal having an annular inner diameter portion in sliding and sealing contact with the stationary component, the seal having an annular outer diameter portion in static and sealing contact with the rotating component;

a pressure compensator in communication with the bearing cavity and exposed to an exterior of the tool for equalizing pressure of the lubricant with pressure of borehole fluid;

the polymer compound on the inner diameter portion having greater cross-linking than the polymer compound within a remaining portion of the seal, defining a gradient portion on the inner diameter portion that has a decreasing IRHD value;

wherein the polymer compound throughout the seal, including the gradient portion, is chemically homogeneous; and

wherein the outer diameter portion is free of a gradient portion and has an IRHD value on its exterior that is less than the IRHD value on an exterior of the gradient portion on the inner diameter portion.

11. The tool according to claim 10 , wherein:

the hardness in the gradient portion decreases gradually to a minimum level that is about equal to an IRHD value of the seal within a central area equidistant between the inner and outer diameters.

12. The tool according to claim 11 , wherein:

the IRHD value of the outer diameter portion is substantially equal to the IRHD value of the seal within the central area.

13. The tool according to claim 10 , wherein:

the seal has a cross-sectional thickness extending from the inner diameter portion to the outer diameter portion; and

the gradient portion has a depth that is less than about one-half the cross-sectional thickness.

14. The tool according to claim 10 , wherein:

the seal has a cross-sectional thickness extending from the inner diameter portion to the outer diameter portion; and

the gradient portion has a depth that is not greater about than one-tenth the cross-sectional thickness.

15. The tool according to claim 10 , wherein:

the seal has a cross-sectional thickness extending from the inner diameter portion to the outer diameter portion; and

the gradient portion has a depth that is at least about one percent of the cross-sectional thickness.

16. The tool according to claim 10 , wherein the hardness value decreases by about at least ten percent.