Spark plug

Abstract

The present invention provides a spark plug (10) configured so that the wear resistance of the tip can be increased. This spark plug comprises: a ground electrode (16) that includes a base material (17) and a tip (18) that is joined to the base material through a weld (19); and a center electrode (13) that is electrically insulated from the ground electrode. An element having a thermal conductivity higher than that of the base material is present in an interface (30) between the weld and the tip, and the content of the element in the interface is greater than the content of the element in the base material.

Description

Spark plug

[0001] The present invention relates to a spark plug having a ground electrode that includes a tip.

[0002] In a spark plug that includes a ground electrode including a tip joined to a base material via a weld and a center electrode electrically insulated from the ground electrode, discharge occurs mainly between the tip of the ground electrode and the center electrode (Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2019-125569

[0004] There is a need for a technique to reduce tip wear caused by electrical discharges.

[0005] The present invention has been made to meet this demand, and has as its object to provide a spark plug that can improve the wear resistance of the tip.

[0006] A first aspect of a spark plug for achieving this object includes a ground electrode including a base material and a tip joined to the base material via a weld, and a center electrode electrically insulated from the ground electrode, wherein an element having a higher thermal conductivity than that of the base material is present at the interface between the weld and the tip, and the content of the element at the interface is greater than the content of the element in the base material.

[0007] In a second embodiment, in the first embodiment, the element is at least one of Zn, Cr, Fe, Cu, and Ag.

[0008] In a third aspect, in the first or second aspect, the tip includes a discharge surface facing the center electrode, a bottom portion facing the base material, and a side portion connecting the bottom portion and the discharge surface, and the interface portion where the element is present is present at least between the side portion and the weld portion.

[0009] In a fourth aspect, in any of the first to third aspects, the length of the portion of the interface where an element having a content higher than the content of the element in the base material is present is 6% or more of the length of the interface.

[0010] According to the present invention, an element with a higher thermal conductivity than the base material is present at the interface between the tip and the welded portion that joins the tip to the base material of the ground electrode, and the content of the element at the interface is greater than the content of the element in the base material. This makes it easier for the heat generated in the tip by discharge to be transferred to the base material through the interface, thereby reducing overheating of the tip and improving its wear resistance.

[0011] 2 is a cross-sectional view of a portion of a ground electrode of a spark plug according to an embodiment of the present invention; FIG. 3 is a cross-sectional view of a portion of a ground electrode of a spark plug according to an embodiment of the present invention; FIG.

[0012] A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a half-sectional view of a spark plug 10 according to one embodiment, taken along an axis X. In Fig. 1, the lower side of the page is referred to as the leading end side of the spark plug 10, and the upper side is referred to as the trailing end side of the spark plug 10 (the same applies to Figs. 2 and 3).

[0013] 1, a spark plug 10 includes an insulator 11, a center electrode 13 held by the insulator 11, a metal shell 15 disposed on the outer periphery of the insulator 11, and a ground electrode 16 connected to the metal shell 15. The insulator 11 is a substantially cylindrical member made of ceramic such as alumina, which has excellent mechanical properties and insulating properties at high temperatures. The insulator 11 has an axial hole 12 formed along an axis X.

[0014] The center electrode 13 is a rod-shaped conductor disposed in the axial hole 12 of the insulator 11 and extending along the axis X. The center electrode 13 has a copper-based core covered by a cylindrical metal with a bottom. The core can be omitted. An example of the metal constituting the center electrode 13 is a Ni-based alloy. The center electrode 13 has a tip that protrudes from the insulator 11. In this embodiment, a tip containing a precious metal is disposed at the tip of the center electrode 13, but the tip can of course be omitted.

[0015] The center electrode 13 is electrically connected to a metal terminal 14 in the axial hole 12. The metal terminal 14 is a rod-shaped member to which an ignition device (not shown) is connected, and is made of a conductive metal material (e.g., low-carbon steel). The metal terminal 14 is fixed to the rear end of the insulator 11 with its front end inserted into the axial hole 12 and its rear end protruding from the insulator 11.

[0016] A metal shell 15 is fixed to the outer periphery of the insulator 11. The metal shell 15 is provided with a male thread for coupling to a spark plug hole of an engine (not shown). A ground electrode 16 is connected to the metal shell 15. The ground electrode 16 is a conductor that extends from the metal shell 15 toward the axis X. A core material mainly composed of copper is embedded in the ground electrode 16. The core material may be omitted.

[0017] Fig. 2 is a cross-sectional view of the ground electrode 16. Fig. 2 illustrates a portion of the ground electrode 16 centered around the tip 18 provided near the tip surface 22, and omits portions away from the tip surface 22. The ground electrode 16 includes a metal base material 17, a metal tip 18 made of a different type of metal than the metal constituting the base material 17, and a weld 19 that joins the tip 18 to the base material 17. In this embodiment, the base material 17 is a curved rod with a rectangular cross section, and the tip 18 is disk-shaped.

[0018] The first element having the highest content rate among the elements contained in the base material 17 is different in type from the second element having the highest content rate among the elements contained in the tip 18. The first element contained in the base material 17 is, for example, Ni, and the second element contained in the tip 18 is, for example, one of precious metals such as Pt, Ir, and Ru. The weld 19 formed by melting the base material 17 and the tip 18 contains the first element and the second element.

[0019] The base material 17 includes an opposing surface 20 facing the center electrode 13 (see FIG. 1 ), a back surface 21 located behind the opposing surface 20, and a tip surface 22 connecting the opposing surface 20 and the back surface 21. The tip 18 is disposed on the opposing surface 20 of the base material 17. The tip 18 includes a discharge surface 23 facing the center electrode 13, a bottom portion 24 facing the base material 17, and a side portion 25 connecting the bottom portion 24 and the discharge surface 23. The bottom portion 24 is a portion located behind the discharge surface 23.

[0020] The analysis of the first element contained in the base material 17 is performed using a scanning electron microscope (SEM-EDS) equipped with an energy dispersive X-ray analyzer. The elements of a portion of the base material 17 away from the weld 19 are analyzed. The analysis position of the base material 17 is, for example, the intersection 29 of the center line 26 and a perpendicular line 28 drawn from the center of gravity 27 of the tip 18 to the center line 26 between the opposing surface 20 and the back surface 21. The center line 26 is a straight line in which the distance between the opposing surface 20 and the center line 26 is equal to the distance between the back surface 21 and the center line 26. The center of gravity 27 is the geometric center calculated by known means when the outline of the tip 18 is represented as a planar figure. The type of element contained in the base material 17 is identified by qualitative analysis of the intersection 29 using point analysis by SEM-EDS, and the content of each element is determined by quantitative calculation, and the first element with the highest content is determined.

[0021] Figure 3 is an enlarged cross-sectional view of the ground electrode 16 showing the portion III in Figure 2. The ground electrode 16 includes an interface 30, which is the boundary between the tip 18 and the weld 19, and an interface 33, which is the boundary between the base material 17 and the weld 19. The thickness of the interface 30 from the tip 18 is 4 µm. The thickness of the interface 30 is equal to the measurable area of ​​the SEM-EDS.

[0022] The interface 30 contains an element (hereinafter referred to as a "good conductive element") whose thermal conductivity is higher than that of the base material 17. The good conductive element is of a different type from the second element, which has the highest content among the elements contained in the tip 18. The good conductive element may be present only in the interface 30, or may be widely distributed in the weld 19 other than the interface 30 in addition to the interface 30.

[0023] The thermal conductivity of the base material 17 is usually lower than that of the first element having the highest content per unit amount of the base material 17. Therefore, the thermal conductivity of the first element is estimated as the upper limit of the thermal conductivity of the base material 17. The thermal conductivity of the elements is determined by reference to values ​​at 20°C (literature values) published in literature. The thermal conductivities of representative elements are: Zn: 113 W / mK, Cr: 90 W / mK, Fe: 72 W / mK, Cu: 398 W / mK, Ag: 428 W / mK, and Ni: 67 W / mK. When Ni is the first element, Zn, Cr, Fe, Cu, and Ag all have higher thermal conductivities than Ni and are good conductive elements. One or more good conductive elements are contained in the interface portion 30.

[0024] Qualitative and quantitative analyses of the elements contained in the interface 30 are performed by SEM-EDS. The content of the good conductive elements in the interface 30 is higher than the content of the good conductive elements in the base material 17. The content (wt %) of the good conductive elements in the base material 17 is determined from the element distribution results at the intersection 29 (see FIG. 2). The content (wt %) of the good conductive elements in the interface 30 is determined from the element distribution results obtained by area analysis of the welded portion 19 including the interface 30. The type of good conductive element may be identified by point analysis of a portion of the interface 30 where the good conductive element is present, and the content of the good conductive element may be determined by quantitative calculation of that portion.

[0025] In the spark plug 10, the content of the highly conductive element in the interface 30 is greater than the content of the highly conductive element in the base material 17. Therefore, heat generated in the tip 18 of the ground electrode 16 by discharge between the tip 18 and the center electrode 13 is easily transferred to the base material 17 through the interface 30 containing the highly conductive element. The heat transferred to the base material 17 is transferred to the engine (not shown) via the metallic shell 15 (see FIG. 1), thereby cooling the tip 18. Since overheating of the tip 18 can be reduced, the wear resistance of the tip 18 can be improved.

[0026] The ground electrode 16 is obtained by providing a film containing a highly conductive element on the surface of a base material 17 by, for example, thermal spraying or plating, and then joining the tip 18 to the base material 17 via a weld 19 by laser welding, resistance welding, or the like. When the weld 19 is formed, the film melts into the weld 19, so that the highly conductive element contained in the film appears at the interface 30. This improves the thermal conductivity of the interface 30.

[0027] The interface portion 30 includes a first portion 31 having a content of the conductive element greater than that of the base material 17, and a second portion 32 having a content of the conductive element equal to or less than that of the base material 17. The length of the first portion 31 is 6% or more of the length of the interface portion 30. The first portion 31, which contains a large amount of the conductive element, improves thermal conductivity from the tip 18 to the welded portion 19, thereby further reducing overheating of the tip 18.

[0028] At least a portion of first portion 31 is located in a direction away from base material 17 relative to the position of opposing surface 20. Because first portion 31 is located close to discharge surface 23 of tip 18, thermal conductivity from tip 18 to base material 17 via welded portion 19 is improved compared to when first portion 31 is not located in this position.

[0029] The ratio of the length of the first portion 31 to the length of the interface portion 30 is obtained by analyzing, using image processing software, an element distribution image obtained by SEM-EDS area analysis of a cross section of the ground electrode 16 including the welded portion 19, enlarged 500 times, to determine the lengths of the first portion 31 and the second portion 32, and then dividing the length of the first portion 31 by the combined length of the first portion 31 and the second portion 32. An example of the image processing software is ImageJ.

[0030] The first portion 31 is present between the side portion 25 (see FIG. 2 ) of the tip 18 and the welded portion 19. Compared to the case where the first portion 31 is present between the bottom portion 24 of the tip 18 and the welded portion, the amount of elements that interfere with the weld quality that are present between the bottom portion 24 of the tip 18 and the welded portion can be reduced, thereby ensuring the peeling resistance of the tip 18.

[0031] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0032] (Preparation of Sample 1) The tester applied various platings to a rod-shaped base material made of nickel-based alloy NCF601, covered the base material with films containing Zn, Ag, Cr, Fe, and Cu, and then joined tips to the base material by resistance welding to obtain multiple ground electrodes of various types. The main component (second element) of the tips was Pt.

[0033] The cross sections of the ground electrodes were analyzed by SEM-EDS. The SEM-EDS analysis conditions were: acceleration voltage: 20 kV, irradiation current: 90 nA, working distance: 11 mm, measurement time: 30 seconds, selected elements: O, Zn, Ag, Cr, Cu, Ni, Fe, Al, Pt, magnification: 2000 times. As a result of the analysis, Zn, Ag, Cr, Fe, and Cu elements contained in the film were detected at the interface at 1 wt% or more. The first element with the highest content among the elements contained in the base material was Ni. Each of the elements Zn, Ag, Cr, Fe, and Cu was a good conductive element with a thermal conductivity higher than that of the first element (Ni).

[0034] Various spark plugs including a ground electrode were obtained using an uncut ground electrode in the examples. For comparison, spark plugs including a ground electrode in the comparative examples were also produced, each having a tip joined by resistance welding to a base material not provided with a plating film including Zn, Ag, Cr, Fe, or Cu.

[0035] (Wear Resistance Test) The tester applied a voltage of 10±5 kV between the center electrode and the ground electrode of the spark plugs of the example and the comparative example, and generated 9×10 sparks between the center electrode and the ground electrode. ７ After the spark plugs were spun, the wear amount (mm) of the tip was measured using a three-dimensional shape measuring machine. The wear amount of the tip of the spark plug in the example was less than that of the spark plug in the comparative example. It was revealed that the wear resistance of the tip could be improved by the example in which Zn, Ag, Cr, Fe, or Cu (a good conductive element) was present at the interface of the ground electrode.

[0036] (Preparation of Sample 2) The tester plated a rod-shaped base material made of nickel-based alloy NCF601 with Zn, covered the base material with a Zn-containing film, and then irradiated the film with a laser beam to remove it. Various base materials with different film thicknesses were obtained by varying the conditions for irradiating the laser beam. For comparison, base materials in which the Zn plating had been completely removed using a stripping agent were also prepared. A tip was joined to the base material by resistance welding to obtain multiple ground electrodes of various types. The main component of the tip was Pt.

[0037] The cross sections of the ground electrodes were analyzed using SEM-EDS, and the location of Zn in the interface was investigated by area analysis. The SEM-EDS analysis conditions were the same as those for Sample 1. Furthermore, the area analysis images were processed using ImageJ to measure the length of the first portion of the interface where the Zn content was 0.1 wt% or more and the length of the second portion of the interface where the Zn content was below the detection limit. The ratio (length (%)) of the length of the first portion divided by the combined length of the first and second portions was calculated. The length (%) was calculated as the average of four samples analyzed for each condition.

[0038] Eight spark plugs including a ground electrode were obtained for each of Examples 1 to 4 using the uncut ground electrode. Eight spark plugs for the comparative example were also produced using a base material from which the Zn plating had been completely removed. By adjusting the energy during resistance welding, etc., the spark plugs for Examples 1 to 3 had Zn present at the interface between the side of the tip and the base material, and the spark plug for Example 4 had Zn present at the interface between the bottom of the tip and the base material. The spark plugs for the comparative example had no Zn present at the interface.

[0039]

[0040] (Wear Resistance Test) After carrying out the above wear resistance test, the tester measured the amount of wear (mm) of the tip using a three-dimensional shape measuring machine (n=4). The amount of wear shown in Table 1 is the average of n=4. Table 1 shows that the amount of wear decreases as the length (%) increases. In particular, it was revealed that when the length (%) was 6% or more, the amount of wear could be reduced by 15% or more compared to the comparative example.

[0041] (Peeling Resistance Test) The tester subjected the spark plugs to 1,000 thermal shock cycles (n=4), each cycle consisting of heating the tip provided on the ground electrode of the prepared spark plug with a burner for one minute, and then moving the burner away from the spark plug for two minutes to cool the ground electrode to room temperature. The temperature of the tip during heating was 1,050°C. The tip temperature was measured before the peeling resistance test by using a spark plug in which a hole was drilled in the base metal of the ground electrode that reached close to the tip, with a thermocouple hot junction positioned near the leading edge of the base metal near the tip.

[0042] After the test, a cross section including the axis of the spark plug was made, and the interface between the welded portion of the ground electrode and the base metal, and the interface between the tip and the welded portion were observed using an optical microscope. The lengths of cracks that had propagated to these interfaces were measured, and those whose crack length was 50% or less of the length of the interface were rated as A, and those whose crack length exceeded 50% were rated as B.

[0043] According to Table 1, it is clear that the spark plugs of Examples 1 to 3 in which Zn is present on the side of the tip can improve the peeling resistance of the tip compared to the spark plug of Example 4 in which Zn is present on the bottom of the tip.

[0044] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0045] In the embodiment, the tip 18 is described as being disk-shaped, but this is not necessarily limited to this. The shape of the tip 18 may be set appropriately, such as a square prism, a polygonal prism other than a square prism, or a truncated cone.

[0046] In the embodiment, the base material 17 of the ground electrode 16 is described as a curved rod with a rectangular cross section, but this is not necessarily limited to this. The shape of the base material 17 can be a curved one or a linear one. The cross section of the base material 17 can be a square one or a circle, an ellipse, a semicircle, or the like.

[0047] In the embodiment, the tip 18 is provided at a position away from the tip surface 22 of the base material 17, but this is not limiting. It is of course possible to arrange the tip 18 on the base material 17 so that the tip surface 22 of the base material 17 and the end of the tip 18 are aligned.

[0048] In the embodiment, a case has been described in which a weld 19 is provided between the side 25 of the tip 18 and the base material 17, and the bottom 24 of the tip 18 is in contact with the base material 17, but this is not necessarily limited to this. It is of course possible to provide the weld 19 from the side 25 to the bottom 24 of the tip 18 by laser welding. In this case, an interface 30 extends from the side 25 to the bottom 24, so that a highly conductive element can be present between the bottom 24 and the weld 19, between the side 25 and the bottom 24 and the weld 19, or between the side 25 and the weld 19. When the weld 19 is formed by resistance welding, the side 25 of the tip 18 is open and the interface 30 can be provided at the bottom 24, so that a highly conductive element can be present between the bottom 24 of the tip 18 and the weld 19.

[0049] REFERENCE SIGNS LIST 10 Spark plug 13 Center electrode 16 Ground electrode 17 Base material 18 Tip 19 Welded portion 23 Discharge surface 24 Bottom portion 25 Side portion 30 Interface portion 31 First portion (portion with high element content)

Claims

1. A spark plug comprising a ground electrode including a base material and a tip joined to the base material via a welded portion, and a center electrode electrically insulated from the ground electrode, wherein an element having a higher thermal conductivity than the thermal conductivity of the base material is present at an interface portion between the welded portion and the tip, and a content rate of the element in the interface portion is larger than a content rate of the element in the base material.

2. The spark plug according to claim 1, wherein the element is at least one of Zn, Cr, Fe, Cu, and Ag.

3. The tip includes a discharge surface facing the center electrode, a bottom surface facing the base material, and a side portion connecting the bottom surface and the discharge surface, and the interface portion where the element is present is present at least between the side portion and the welded portion according to claim 1 or 2.

4. The spark plug according to claim 3, wherein a length of a portion where the element having a larger content rate than the content rate of the element in the base material is present in the interface portion is 6% or more with respect to the length of the interface portion.

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