A SPARK PLUG

Abstract: A spark plug for providing an ignition spark has a center electrode (19). At least a portion of the center electrode (19) contains an alloy material from an alloy family of M-CrAlY wherein M is nickel, cobalt, iron or a combination of these elements and Cr is chromium, Al is aluminum, Y is an element from the group yttrium, zirconium, hafnium or titanium.

Description: This invention relates to a spark plug and, in particular, to materials for a spark plug. Prolonging the life of spark plugs is desirable for a variety of reasons including meeting government regulations, reducing cost, minimizing maintenance and improving reliability. Typically, current production spark plugs have an expected life of about 48280 km (30,000 miles). One of the major limiting factors of spark plug live is the deterioration of the center electrode. Attempts to overcome this limitation have included developing platinum-tipped center electrodes. For example, a thin platinum pin can be inserted in the ceramic insulator before sintering. Alternatively, a small platinum disk can be recessed into the turned down tip of a conventional electrode and held in place by spot welding. However, using noble metals, such as platinum, is costly. The most common materials used for production of center electrodes are nickel alloys such as Inconel (nickel-iron-chromium). These alloys have proven to be adequate the meet durability requirements of 48280 km (30,000 miles). A modest degree of durability improvement can be achieved with copper cored center electrodes using a nickel alloy as the cladding material. There are believed to be two basic mechanisms that contribute to the operational deterioration of the center electrode: (1) spark erosion and (2) chemical corrosion. The relationship of each particular type of deterioration, i.e. spark erosion or chemical attack, in relation to temperature is shown in Fig. 2. Although spark erosion appears to be operative at all temperatures, chemical corrosion appears to be a major contributor at normal operating temperatures. A study of nickel alloy central electrodes indicates that both chromium oxide and chromium sulfide are formed on the surface of the electrode during normal engine operation. These materials are only weakly bonded to the surface and easily removed. This produces fresh surfaces on the nickel alloy electrode that are again susceptible to further chemical corrosion. Also known are various coatings used to protect gas turbine super alloy components against oxidation and sulfuration. In particular, a family of alloys called M-CrAlB has been developed, where M = Ni (Nickel), or Co (Cobalt), or Fe (Iron) or combinations of Nickel, Cobalt and Iron, such as NiCo, FeCo, etc. Cr = 15 - 30 Wt.% Chromium Al = 5 - 15 Wt.% Aluminum B = 0 - 2 Wt.% Yttrium or other active elements e.g. Zr (Zirconium) Hf (Hafnium) Ti (Titanium) A discussion of the relationship of these alloys to protecting gas turbine super alloy components is in: (1) Boone, D.H. Strangman, T.E. Wilson, L.W. "Some Effects of Structure and Composition on the Properties of Electron Beam Vapor Deposited Coatings for Gas Turbine Superalloy", J.Vac. Sci. Technol., 11 (4) 641, 1974 and (2) Strangmen, T.E. Hopkins, S.W. "Thermal Fatigue of coated Superalloys", Bul. Am. Ceram. Soc. 55, (3) 305, 1976. In this literature, it is taught that the aluminum in the alloys forms a protective scale of A1₂O₃ and that the Yttrium provides a strong bond between the Al₂O₃ and the metal electrode substrate. GB-A-2,044,849 discloses a spark plug having a central electrode made of M-CrAlB, wherein M is selected from the group comprising nickel, cobalt, iron and mixtures thereof and Cr is chromium, Al is aluminium, and B is an element selected from the group consisting of yttrium, zirconium, hafnium and titanium. According to the present invention there is provided a spark plug for use in providing an ignition spark, the spark plug including a center electrode (19) for providing a conductive path and a grounding electrode (12), spaced from said center electrode (19), for providing a conductive path so that a spark can be generated between said center and grounding electrodes (19,21) characterised in that said center electrode (19) has a copper core and an alloy material covering at least a portion of said center electrode, said alloy material being M-CrA1B wherein M is selected from the group comprising nickel, cobalt, iron and mixtures thereof and Cr is chromium, A1 is aluminium, and B is an element selected from the group consisting of yttrium, zirconium, hafnium and titanium. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a cross section view of a spark plug in accordance with an embodiment of this invention. Figure 2 is a graphical representation relating deterioration for spark erosion and chemical attacks to the temperature operation of a spark plug, Figure 3 is a cross section of a center electrode,formed entirely of M-CrA1B, Figure 4 is a cross section of a center electrode in accordance with one embodiment of this invention including M-CrA1B cladding on a copper core, and Figure 5 is a cross section of another embodiment of this invention wherein a M-CrA1Y coating is put on a nickel alloy cladding which is on a copper core. Referring to Figure 1, a center electrode 19 is positioned coaxially with the main axis of a spark plug 10 and is spaced from an earth or ground electrode 12. A spark gas 13 is positioned between earth electrode 12 and center electrode 19. Center electrode 19 is electrically connected to a conductive seal 14, terminal and stud 15 and terminal 16 to couple center electrode 19 to an external circuit. A tubular insulator 17 surrounds terminal stud 15, conductive seal 14 and center electrode 19. Surrounding the central portion of insulator 17 is a body rollover 18. An alloy from the family of alloys called M-Cra1B is used for the fabrication in the center electrode. M is chosen from the group of nickel, cobalt and iron or combinations of these materials. Advantageously, the chromium is 15-30% of the weight of the alloy. The aluminum is 5-15% of the weight of the alloy. The aluminum is 5-15% of the weight of the alloy and B is the yttrium or other active elements such as zirconium, hafnium or titanium is about 0.1% of the weight of the alloy. Referring to Figure 4, a center electrode 40 in accordance with an embodiment of this invention has a copper core electrode 41 and uses a M-CrA1B type alloy for a cladding material 42 on copper core electrode 41. Referring to Figure 5, a center electrode 50 has a standard or conventional copper core electrode 51 as a substrate with an Inconel cladding 53 and an overlay coating 52 of M-CrA1B covering the tip portion of the Inconel clad substrate electrode 51. There are a number of techniques by which coating 52 may be applied including, for example, electron beam vapor deposition, ion plating, sputtering, plasma spraying or arc source evaporation. Of the embodiment shown, it is believed that the one shown in Figure 5 where there is a coating of M-CrA1B on an Inconel clad copper core has the best low cost potential and lends itself to larger volume, batching operations. Conventional copper core center electrodes with overlay coatings of M-CrA1Y of .0125 cm (0.005 inches) thickness have been fabricated and tested. For example, a sample of electrodes have been exposed to 1000°C in air for a total of nine hours without any observable deterioration of the coating. Additional samples of the coated electrodes have been assembled into spark plugs and tested on an engine dynamometer. After a period of fifty hours at 4500 rpm using leaded fuel, scanning electron micrographs show that coatings are still intact. After a 140 hour test with unleaded fuel, equivalent in sparks to 27358 km (17,000 miles), scanning electron microscopy revealed much less rounding of the coated center electrodes compared to an uncoated Inconel.

Claims: A spark plug for use in providing an ignition spark, the spark plug including a center electrode (19) for providing a conductive path and a grounding electrode (12), spaced from said center electrode (19), for providing a conductive path so that a spark can be generated between said center and grounding electrodes (19,21) characterised in that said center electrode (19;33;40;50) has a copper core and an alloy material covering at least a portion of said center electrode, said alloy material being M-CrAlB wherein M is selected from the group comprising nickel, cobalt, iron and mixtures thereof and Cr is chromium, Al is aluminum, and B is an element selected from the group consisting of yttrium, zirconium, hafnium and titanium. A spark plug as claimed in Claim 1, wherein said center electrode (19) has a cladding (42;53) of said alloy material covering the whole of said copper. A spark plug as claimed in Claim 1, wherein said center electrode has a nickel alloy cladding (53) and a coating (52) of said alloy material on a portion of said center electrode adjacent said grounding electrode. A spark plug as claimed in claim 3, wherein said alloy material coating is applied by electron beam vapour deposition, ion plating, sputtering, plasma spraying or arc source evaporation. A spark plug as claimed in any one of the preceding claims, wherein said center electrode is a generally cylindrical elongated, electrical terminal. A spark plug as claimed in any one of the preceding claims, wherein said chromium is between 15-30% by weight of said alloy material, said aluminum is between 5-15% by weight of said alloy material and the element chosen from the group yttrium, zirconium, hafnium or titanium, is between 0-1% by weight of said alloy material.

Inventors: GIACHINO, JOSEPH MARIO, HOFFMAN, DAVID WYETH, HORN, WILLIAM FREDERICK, KAZMER, GERALD PAUL
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1987
Main classification: H01T13
IPC codes: H01T13