Spark electrode and method of manufacturing same

Abstract: A spark discharge tip, such as for a spark plug for an internal combustion engine in which the life of the tip is prolonged. A composite material is formed by bonding a material for a discharge layer, mainly composed of platinum, to a material for a thermal stress relieving layer, which is an alloy mainly composed of platinum, and having a coefficient of thermal expansion between that of the electrode and that of the discharge layer. A composite tip is formed by stamping the composite material into a columnar shape from the discharge layer side so that the periphery of the interface between the discharge layer and stress relieving layer is covered by the discharge layer. The composite tip is then resistance welded to the electrode.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an improvement of a noble metal tip provided at a spark discharge gap. Such a tip may be used with a spark plug for an internal combustion engine. The present invention also relates to a method of manufacturing the same which improves heat resistance and durability. Description of the Art A spark plug for an internal combustion engine has a center electrode and an earth electrode which face each other and produce a spark discharge when a high voltage is applied between the electrodes. Discharge tips composed of noble metals are mounted respectively on sections of the pair of electrode members facing each other to define a gap for producing spark discharge between the tips. Conventionally, in order to prolong the life of such a spark plug, the tip structure has included a thermal stress relieving layer joined between a layer made of a discharge member and each electrode as disclosed, for example, in Japanese Patent Laid-open No. 60-262374. However, the life of the product cannot be effectively prolonged simply by including the stress relieving layer if it is joined to the electrode by resistance welding. When such a composite tip is resistance-welded to the electrode, welding current generates heat at the interface between the discharge layer and the stress relieving layer and thermal deformation is caused due to the heat and welding pressure. This deformation appears as an expansion in the radial direction of the tip, particularly at the interface between the discharge layer and stress relieving layer. FIGURE 5 shows a sectional structure of an earth electrode 14 of a spark plug having a composite tip 13 including a discharge layer 11 and a thermal stress relieving layer 12. In the Figure, composite tip 13, after it has been joined to earth electrode 14 by resistance welding, becomes generally trapezoidal in sectional shape due to the expansion of stress relieving layer 12. Alternatively, earth electrode 14 contacts the periphery of the interface between discharge layer 11 and stress relieving layer 12, due to a remarkable expansion of stress relieving layer 12. If the composite tip has a tapered sectional shape even before welding, as shown in the aforementioned Laid-open document, the deformation becomes even more pronounced after welding. Further, even if the sectional tapered shape of the composite member is turned up side down as compared to the above example, the difference of size between the discharge layer and stress relieving layer is about 0.05 mm, which hardly compensates for the thermal deformation in the radial direction during resistance welding of the tip. When a tip with the trapezoidal shape is used, discharge layer 11 becomes thin as a result of spark consumption over a long period of time and spark discharge is then generated from the periphery of stress relieving layer 12. Therefore, stress relieving layer 12 is consumed. Also, stress relieving layer 12 is directly exposed to high temperatures and the oxidizing atmosphere of the combustion chamber of the internal combustion engine, thereby advancing oxidation and corrosion thereof. Spark consumption, oxidation and corrosion of stress relieving layer 12 damage its thermal stress relieving function and causes discharge layer 11 to fail, shortening the life of the spark plug. In the document US-A-4 670 684, there is disclosed a spark plug electrode which comprises a base, a thermal stress relieving layer which is bonded to said base and a corrosion resistant layer which is bonded to the surface of said thermal stress relieving layer opposite said base. However, the general structure of said composed electrode is similar to the structure of the earth electrode 14 of a spark plug mentioned above and shown in fig. 5. Therefore, the same problems occur since, when the spark plug is subjected to a certain consumption due to the high temperatures, etc, both the thermal stress relieving layer and the corrosion resistant layer are exposed directly to the high temperatures and the oxidizing atmosphere of the combustion chamber. Then, the stress relieving layer will be damaged by oxidation and corrosion. Accordingly, it is an object of the present invention to overcome the aforementioned problems by providing a spark electrode according to claim 1 such as for a spark plug for an internal combustion engine and a method of manufacturing the same according to claim 13 having a stress relieving layer joined to a discharge layer and also resistance welded to an electrode, yet which prolongs the life of the tip and is reliable. SUMMARY OF THE INVENTION In order to achieve the aforementioned goal, according to the present invention, the tip includes a composite structure having a discharge layer and thermal stress relieving layer, interposed between the discharge layer and the electrode base to relieve thermal stress generated at the interface of the junction with the discharge layer. The discharge layer is composed of a material having an excellent resistance to spark consumption. The periphery of the stress relieving layer including the periphery of the interface between the discharge layer and the stress relieving layer is covered by the discharge member. The discharge layer may be made of a material including platinum. The stress relieving layer may be made of material including platinum and having a hardness equal to or more than that of the discharge layer. The composite tip is manufactured by stamping a plate, in which the material for the discharge layer and stress relieving layer are laminated together, from the direction of the stress relieving layer in a shape corresponding to a discharge tip. Then the stress relieving layer is resistance welded to an electrode member. The composite tip described above obtains the stress reducing advantages of a stress relieving layer. At the same time, the periphery of the stress relieving layer, including the interface between the discharge layer and stress relieving layer, is covered by the discharge layer, so that the stress relieving layer is not exposed. Accordingly, the goal for prolonging the life of the tip may be achieved and reliability is improved. BRIEF DESCRIPTION OF THE DRAWINGS The manner in which the foregoing and other objects of this invention are accomplished will be apparent from the accompanying specification and claims considered together with the drawings wherein: FIGURE 1 is a section view illustrating a structure of a spark plug for an internal combustion engine according to one embodiment of the present invention; FIGURE 2 is a section view illustrating a state of a tip to be joined to an earth electrode of the spark plug in FIGURE 1 by welding; FIGURE 3 is an explanatory drawing illustrating how a discharge layer and stress relieving layer are joined; FIGURE 4 is a section view illustrating the tip in FIGURE 2 joined to the earth electrode; and FIGURE 5 is a section view illustrating a tip junction section of a prior art example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, one embodiment of the present invention will be explained in detail. FIGURE 1 shows a sectional structure of a spark plug used for an internal combustion engine. A cylindrical housing 21 made of a metallic material has a thread groove 22 formed on an outer, lower peripheral section. Housing 21 is mounted to a cylinder head section (not shown) of the internal combustion engine by means of thread groove 22. An air tight seal is maintained by a gasket 23. The lower end portion of a cylindrical insulator 24 is fitted coaxially in housing 21 and a center electrode 25 is inserted and fixed at the center hole section of insulator 24 in correspondence to the lower end portion of insulator 24. Electrode 25 is a column whose inner member is composed of copper and whose outer member is composed of Ni base alloy and whose tip portion is exposed out of the lower end of the insulator 24. A center conductor 26 is inserted in the upper portion of the hollow section of insulator 24. An end of center conductor 26 extends above insulator 24 to provide terminal 27 through which an ignition voltage signal is supplied. A conductive glass sealing material 28 is interposed between center conductor 26 and center electrode 25. Sealing material 28 is heated to weld center conductor 26 and center electrode 25 to electrically connect them. A first discharge electrode tip 29 is composed of a noble metal and is welded and mounted to the surface of center electrode 25. An earth electrode 30, extending from and integral with housing 21 faces tip 29. A second tip 31 is welded and mounted on earth electrode 30 at a position facing tip 29 to form a gap 32 for generating a spark discharge. FIGURE 2 shows an initial sectional structure of second tip 31 before being welded on earth electrode 30. As shown in FIGURE 2, tip 31 is a composite of a discharge layer 40 joined to a thermal stress relieving layer 41. Discharge layer 40 is composed of an alloy whose main component is platinum and has an excellent resistance to spark consumption. Stress relieving layer 41 is composed of an alloy whose main component is a noble metal such as platinum. Stress relieving layer 41 is interposed between discharge layer 40 and earth electrode 30 in order to reduce thermal stress produced at the interface between discharge layer 40 and earth electrode 30. A coefficient of thermal expansion of stress relieving layer 41 is set so that its value is between the coefficients of thermal expansion of discharge layer 40 and the material of earth electrode 30. FIGURE 3 is an explanatory drawing illustrating a method for forming discharge layer 40 and stress relieving layer 41. First, a plate 50, in which materials for discharge layer 40 and stress relieving layer 41 are laminated, is placed on a base plate 51 with the material of discharge layer 40 facing downward. Base plate 51 is provided with a round hole 51a having a desired tip diameter. Plate 50 is placed so that it covers hole 51a. Then a press 52 is driven down toward hole 51a of base plate 51 to obtain a round composite tip 43 having the desired diameter from plate material 50. Thus the composite material having a double layer structure in which the discharge layer 40 and thermal stress relieving layer member 41 are joined, is stamped in a columnar shape from the discharge layer 40 side. By stamping as described above, a shear drop 40a is created at the periphery section of the discharge layer 40. A shear drop 41a is also created at the periphery of stress relieving layer 41 due to the shear drop 40a. Then a portion 401 of the discharge layer fills shear drop 41a of stress relieving layer 41. As a result, a composite tip 43 is formed having a sectional shape covered by discharge layer portion 401. The stress relieving layer 41 side of composite tip 43 is resistance welded to earth electrode 30 as shown in FIGURE 4. The material of discharge layer 40 is selected to be no harder than the material of stress relieving layer 41. Therefore, during welding, thermal deformation of discharge layer 40 is greater than that of stress relieving layer 41. Accordingly, a skirt section 402 is created in discharge layer 40 about the periphery of stress relieving layer 41. Thus, the periphery of stress relieving layer 41 is covered by skirt section 402. Since the periphery of stress relieving layer 41 is covered by skirt section 402, stress relieving layer 41 remains protected after spark consumption of the base material of earth electrode 30 and is protected from high temperature oxidizing due to combustion near interface 42 of stress relieving layer 41 and discharge layer 40 when the spark plug is used for a long period of time. Accordingly, stress relieving layer 41 can reduce thermal stress caused by the difference in the coefficient of thermal expansion of discharge layer 40 and that of earth electrode 30 and the life of the spark plug may be achieved as targeted. Although FIGURE 4 illustrates skirt section 402 covering the entire periphery of stress relieving layer 41, the advantageous effects of the present invention can also be achieved if skirt 402 covers only a portion of the periphery of stress relieving layer 41. In fact, depending on the hardness of discharge layer 40 and stress relieving layer 41 and the pressure and temperature used in resistance welding, discharge layer 40 need not extend beyond the diameter of stress relieving layer 41. Portion 401 itself is sufficient to achieve the results of the present invention. In this case, discharge layer 40 and stress relieving layer 41 have the same diameter. Next, sectional shapes of the discharge layer and relaxation layer after welding are studied when the kind of the alloy of the discharge layer and that of the stress relieving layer are altered. The following Table 1 shows the result. Table 1 shows respective study results of the composite tips of each combination when discharge layer 40 is composed of "Pt-Ir" and stress relieving layer 41 is composed of various alloys including platinum. The composite tips were stamped into a columnar shape from the discharge layer 40 side with a diameter of 0.9 mm and a height of 0.6 mm. The thicknesses of discharge layer 40 and relaxation layer 41 were set, respectively, to 0.4 mm and 0.2 mm. Then, the specimens were welded to electrodes with a force of 25 Kg. The resistance welding was performed with 10 cycles of resistance welding current in a range from 650 A to 800 A. Hardnesses Hv of the discharge layers and the relaxation layers after annealing are listed at the right of Table 1. It can be seen that the elongation deformation of composite tip 43 caused by Joule heat generated on the surface of stress relieving layer 41 and earth electrode 30 and by the welding force during the resistance welding corresponds to the hardness of the materials. In Table 1, an "O" in the column labeled "Sectional Shape After Welding" indicates an acceptable shape and an "X" indicates an unacceptable shape. That is, to assure that composite tip 43 has the sectional shape shown in FIGURE 2, the hardness of stress relieving layer 41 needs to be equal to or more than that of discharge layer 40. Although the above discussion relates to second tip 32 which was joined to earth electrode 30, the life of the spark plug may be prolonged and its reliability can be improved by structuring first tip 29, joined to the tip of center electrode 25, in the same way. As described above, according to the present invention, the relaxation layer may be protected for a long period of time, the life of the spark plug may be prolonged and the reliability thereof may be improved utilizing thermal deformation produced during resistance welding of the composite tip by structuring the tip so that the hardness of the stress relieving layer is equal to or more than that of the discharge layer. In this case, a more rigid junction shape may be obtained by stamping the composite material from the discharge layer side.

Claims: A spark electrode comprising: a base (30), a thermal stress relieving layer (41) bonded to said base (30), and a corrosion resistant discharge layer (40) bonded to the surface of said thermal stress relieving layer (41) opposite to said base (30) characterized in that said discharge layer (40) is covering at least a portion of adjacent peripheral edges of said thermal stress relieving layer (41). A spark electrode according to claim 1, wherein a hardness of said stress relieving layer (41) is equal to or more than that of said discharge layer. A spark electrode according to claim 1, wherein the coefficient of thermal expansion of said stress relieving layer (41) is intermediate of that of said base (30) and that of said discharge layer (40). A spark electrode according to claim 1, wherein said discharge layer (40) includes platinum. A spark electrode according to claim 1, wherein said stress relieving layer (41) includes an alloy of platinum. A spark electrode according to claim 2, wherein said stress relieving layer (41) includes an alloy of the primary material of said discharge layer (41). A spark electrode according to claim 1, wherein the diameter of said stress relieving layer (41) is the same as that of said discharge layer (40). A spark plug wherein at least one spark electrode according to any of claims 1 to 7, is used in at least one of a first and a second electrode. A spark plug according to claim 8, wherein said thermal stress relieving layer (41) is provided to relieve thermal stress generated between said discharge layer (40) and the base of said at least one of said electrodes. A spark plug according to claim 8, wherein the surface of said stress relieving layer (41) contacting said discharge layer (40) has an inclined peripheral edge (41a). A spark plug according to claim 8, wherein both said first and second electrodes have said stress relieving layer (41) and said discharge layer (40). A spark plug according to claims 8 to 11, characterized in that it is used in a combustion engine. A method for manufacturing a spark electrode according to any of claims 1 to 7 comprising the steps of stamping a composite material (50) having a corrosion resistant discharge layer (40) bonded to a thermal stress relieving layer (41) having a hardness equal to or more than that of said discharge layer (40) from the direction of said thermal stress relieving layer (41) to form a chip, attaching said chip to a base (30). A method for manufacturing a spark electrode according to claim 13 further comprising the step of forming a composite material by bonding said corrosion resistant discharge layer (40) to a thermal stress relieving layer (41) having a hardness equal to of more than that of said discharge layer (40). A method for manufacturing a spark electrode according to claim 13 or claim 14, wherein said attaching step includes the step of resistance welding said chip to said base (30). A method for manufacturing a spark electrode according to claim 15, wherein said attaching step further includes the step of applying pressure to said chip and said base (30) during said resistance welding step. A method for manufacturing a spark electrode according to any of the claims 13 to 15, wherein said stamping step further includes the step of inclining a peripheral edge of an interface between said discharge layer (40) and said stress relieving layer (41) toward said stress relieving layer. A method for manufacturing a spark electrode according to any of claims 13 to 17, further comprising the step of repeating said forming step to form a second chip, said attaching step including the step of attaching of said second chip to the other of said first and second electrode. A method for manufacturing a spark electrode for a spark plug for an internal combustion engine according to any of claims 13 to 18, comprising the steps of creating a composite material by bonding material for a discharge layer (40) composed of a material including platinum having excellent resistance to a spark consumption and a material for a relaxation layer for relaxing thermal stress composed of material including platinum having a hardness equal to or more than that of said material for said discharge layer, creating a composite tip in which the periphery of said relaxation layer including the periphery of the interface of said discharge layer and said relaxation layer is covered by said discharge layer by stamping said composite material into a columnar shape from the direction of said material of said discharge layer in accordance to a desired shape, and resistance welding said relaxation layer side of said composite tip to at least one of two electrode members forming a spark discharge gap.

Inventors: TAKAMURA KOZO, TAKAMURA, KOZO
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1992
Main classification: H01T13
IPC codes: H01T13, H01T21