Method for manufacturing an ignition electrode for spark plugs and spark plug manufactured therewith

1. A spark plug, comprising:

a metallic shell;

an insulator at least partially surrounded by the metallic shell;

a plurality of ignition electrodes including a center electrode at least partially surrounded by the insulator and a ground electrode, wherein at least one of the center electrode or the ground electrode includes a metal injection molded (MIM) composite part with a base metal or a base metal alloy and a precious metal or a precious metal alloy; and

a spark gap defined between the center electrode and the ground electrode;

wherein the metal injection molded (MIM) composite part has an area bordering the spark gap that includes a plurality of islands made of the precious metal or the precious metal alloy between which the base metal or the base metal alloy comes to a surface, the plurality of islands are located at least partially in a plurality of pores formed in the base metal or the base metal alloy and project above the base metal or the base metal alloy at the surface so as to promote ignition sparks according to a point effect.

2. The spark plug of claim 1 , wherein an intermetallic compound is formed in a bonding zone between each island of the plurality of islands and the base metal or the base metal alloy.

3. The spark plug of claim 2 , wherein the precious metal or the precious metal alloy is an iridium-based alloy and the base metal or the base metal alloy is a nickel-based alloy, and the intermetallic compound contains both iridium and nickel.

4. The spark plug of claim 1 , wherein the metal injection molded (MIM) composite part includes a nickel-based electrode core as the base metal or the base metal alloy and a thin precious metal or precious metal alloy coating is selectively applied to a portion of the nickel-based electrode core to at least partially form the plurality of islands.

5. The spark plug of claim 1 , wherein the area bordering the spark gap is a circumferential surface and the metal injection molded (MIM) composite part is a cylindrical piece that is welded to the center electrode.

6. The spark plug of claim 1 , wherein a plurality of sparks generated at the spark gap cause the plurality of islands, over time, to project further above the base metal or the base metal alloy at the surface.

7. A spark plug, comprising:

a metallic shell;

an insulator at least partially surrounded by the metallic shell;

a plurality of ignition electrodes including a center electrode at least partially surrounded by the insulator and a ground electrode, wherein at least one of the center electrode or the ground electrode includes a metal injection molded (MIM) composite part with a nickel-based electrode core and a thin precious metal-based coating selectively applied to a portion of the nickel-based electrode core; and

a spark gap defined between the center electrode and the ground electrode;

wherein the metal injection molded (MIM) composite part has an area bordering the spark gap and an uncoated end surface, the area bordering the spark gap includes the precious metal-based coating connected to the nickel-based electrode core by both a positive mechanical interlock and a metallurgical bond, and the uncoated end surface includes the nickel-based electrode core without the precious metal-based coating and is welded to the at least one of the center electrode or the ground electrode.

8. The spark plug of claim 7 , wherein an intermetallic compound is formed in a bonding zone between the nickel-based electrode core and the thin precious metal-based coating.

9. The spark plug of claim 7 , wherein the area bordering the spark gap is a circumferential surface and the metal injection molded (MIM) composite part is a cylindrical piece that is welded to the center electrode.

10. A method for manufacturing an ignition electrode for a spark plug, comprising the steps of:

metal injection molding (MIM) a core using a base metal or a base metal alloy;

selectively coating a spark gap facing surface of the core with a precious metal or a precious metal alloy, while leaving an end surface of the core uncoated, so as to form a coated core with a precious metal or precious metal alloy coating layer on the spark gap facing surface;

debinding the core either before or after selectively coating the spark gap facing surface of the core with the precious metal or the precious metal alloy; and

sintering the debound and coated core to form a composite part.

11. The method of claim 10 , wherein the debinding step occurs before selectively coating the spark gap facing surface of the core with the precious metal or the precious metal alloy.

12. The method of claim 10 , wherein the spark gap facing surface is a circumferential surface and the method further comprises the step of welding the end surface to a center electrode.

13. The method of claim 10 , wherein the spark gap facing surface is an inner circumferential surface of an annular ground electrode.

14. The method of claim 10 , wherein the spark gap facing surface is a radially inward-facing end face of a ground electrode.

15. The method of claim 10 , wherein an iridium-based alloy is used as the precious metal or the precious metal alloy.

16. The method of claim 15 , wherein the iridium-based alloy is mixed with a binder before selectively coating the spark gap facing surface of the core.

17. The method of claim 16 , wherein the sintering step removes the binder in the iridium-based coating layer.

18. The method of claim 15 , wherein the debinding step occurs before the selectively coating step, and the iridium-based coating layer shrinks more than the core to create a positive mechanical interlock between the core and the coating layer.