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

Technical Field

[0001] The present invention relates to a spark plug including a ground electrode including a tip.

Background Art

[0002] A spark plug including a ground electrode joined to a base material via a welded portion and a center electrode electrically insulated from the ground electrode mainly generates a discharge between the tip of the ground electrode and the center electrode (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] There is a demand for a technique for reducing the consumption of the tip caused by discharge.

[0005] The present invention has been made to meet this demand, and an object thereof is to provide a spark plug capable of improving the wear resistance of the tip.

Means for Solving the Problems

[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 welded portion, and a center electrode electrically insulated from the ground electrode, and an element having a higher thermal conductivity than the thermal conductivity of the base material exists at an interface portion between the welded portion and the tip, and the content rate of the element at the interface portion is larger than the content rate of the element in the base material.

[0007] A second aspect is that, in the first aspect, the element is at least one of Zn, Cr, Fe, Cu, and Ag.

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

[0009] The fourth aspect is that, in any of the first to third aspects, the length of the portion of the interface in which elements with a higher content than the elemental content in the base material are present is 6% or more of the length of the interface. [Effects of the Invention]

[0010] According to the present invention, at the interface between the welded portion that joins the tip to the base material of the ground electrode and the tip, there is an element with a higher thermal conductivity than the base material, and the element content at the interface is greater than the element content in the base material. Since the heat generated by the discharge from the tip is easily transferred to the base material through the interface, overheating of the tip can be reduced and the wear resistance of the tip can be improved. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view of one side of a spark plug in one embodiment. [Figure 2] This is a cross-sectional view of the ground electrode. [Figure 3] This is a cross-sectional view of the ground electrode, enlarged, showing the portion indicated by III in Figure 2. [Modes for carrying out the invention]

[0012] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 is a one-sided cross-sectional view of a spark plug 10 in one embodiment, with axis X as the boundary. In Figure 1, the lower side of the paper is the tip side of the spark plug 10, and the upper side of the paper is the rear end side of the spark plug 10 (the same applies to Figures 2 and 3).

[0013] As shown in Figure 1, the spark plug 10 comprises an insulator 11, a central electrode 13 held by the insulator 11, a main body fitting 15 arranged on the outer circumference of the insulator 11, and a ground electrode 16 connected to the main body fitting 15. The insulator 11 is a substantially cylindrical member made of ceramic such as alumina, which has excellent mechanical properties and insulation properties at high temperatures. The insulator 11 has an axial hole 12 along its axis X.

[0014] The central electrode 13 is a rod-shaped conductor extending along the axis X, positioned within the axial hole 12 of the insulator 11. The central electrode 13 consists of a core material mainly composed of copper, covered with a bottomed cylindrical metal. The core material can be omitted. A nickel-based alloy is an example of the metal constituting the central electrode 13. The tip of the central electrode 13 protrudes from the insulator 11. In this embodiment, a tip containing a precious metal is placed at the tip of the central electrode 13, but it is of course possible to omit the tip.

[0015] The central electrode 13 is electrically connected to the terminal fitting 14 within the shaft hole 12. The terminal fitting 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 terminal fitting 14 is fixed to the rear end of the insulator 11 with its tip inserted into the shaft hole 12 and its rear end protruding from the insulator 11.

[0016] A main fitting 15 is fixed to the outer circumference of the insulator 11. The main fitting 15 is provided with a male thread for connecting to the spark plug hole of an engine (not shown). A ground electrode 16 is connected to the main fitting 15. The ground electrode 16 is a conductor extending from the main fitting 15 toward axis X. The ground electrode 16 has a core material mainly composed of copper embedded in it. The core material can be omitted.

[0017] Figure 2 is a cross-sectional view of the ground electrode 16. Figure 2 shows the portion of the ground electrode 16 centered on the tip 18 located near the tip surface 22, and the portion further away from the tip surface 22 is not shown. The ground electrode 16 includes a metal base material 17, a metal tip 18 made of a different type of metal than that which makes up the base material 17, and a welded portion 19 that joins the tip 18 to the base material 17. In this embodiment, the base material 17 is a curved rod with a square cross-section, and the tip 18 is disc-shaped.

[0018] The first element, which has the highest concentration among the elements in the base material 17, is of a different type from the second element, which has the highest concentration among the elements in the tip 18. The first element in the base material 17 is, for example, Ni, and the second element in the tip 18 is exemplified by one of the precious metals such as Pt, Ir, and Ru. The welded joint 19 formed by the melting of the base material 17 and the tip 18 contains both the first and second elements.

[0019] The base material 17 includes an opposing surface 20 facing the central electrode 13 (see Figure 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 positioned on the opposing surface 20 of the base material 17. The tip 18 includes a discharge surface 23 facing the central 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 the part located behind the discharge surface 23.

[0020] The analysis of the first element contained in the base material 17 is performed by a scanning electron microscope (SEM-EDS) equipped with an energy dispersive X-ray analyzer. The elements of the part of the base material 17 that is away from the welded part 19 are analyzed. The analysis position of the base material 17 is, for example, the intersection point 29 of the perpendicular line 28 dropped from the centroid 27 of the chip 18 to the center line 26 between the opposing surface 20 and the back surface 21 and the center line 26. The center line 26 is a straight line where 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 centroid 27 is the geometric center calculated by well-known means when the outer shape of the chip 18 is regarded as a planar figure. The type of element contained in the base material 17 is specified by qualitative analysis of the intersection point 29 by point analysis of SEM-EDS, the content rate of each element is specified by quantitative calculation, and the first element with the largest content rate is determined.

[0021] FIG. 3 is a cross-sectional view of the ground electrode 16 obtained by enlarging the part shown by III in FIG. 2. The ground electrode 16 includes an interface part 30 that is the boundary between the chip 18 and the welded part 19, and an interface part 33 that is the boundary between the base material 17 and the welded part 19. The thickness of the interface part 30 from the chip 18 is 4 μm. The thickness of the interface part 30 is equal to the measurable region of SEM-EDS.

[0022] In the interface part 30, there is an element (hereinafter referred to as "good conduction element") having a higher thermal conductivity than the thermal conductivity of the base material 17. The good conduction element has a different type from the second element having the largest content rate among the elements contained in the chip 18. The good conduction element may exist only in the interface part 30, or may be widely distributed in the welded part 19 other than the interface part 30 in addition to the interface part 30.

[0023] The thermal conductivity of the base material 17 is usually lower than that of the first element, which has the highest concentration in 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 based on the values ​​at 20°C (literature values) listed in the literature. Typical thermal conductivity values ​​for 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, which have higher thermal conductivity than Ni, are all good conductive elements. One or more good conductive elements are contained in the interface 30.

[0024] Qualitative and quantitative analysis of elements contained in the interface 30 is performed by SEM-EDS. The content of good conductive elements in the interface 30 is greater than the content of good conductive elements in the base material 17. The content of good conductive elements (wt%) in the base material 17 is determined from the elemental distribution results at intersection 29 (see Figure 2). The content of good conductive elements (wt%) in the interface 30 is determined from the elemental distribution results obtained by surface analysis of the welded part 19 including the interface 30. Alternatively, the type of good conductive element may be identified by point analysis of the part 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 part.

[0025] In the spark plug 10, the content of good conductive elements in the interface 30 is greater than the content of good conductive elements in the base material 17. Therefore, the heat from the tip 18, which is heated by the discharge between the tip 18 of the ground electrode 16 and the center electrode 13, is easily transferred to the base material 17 through the interface 30 containing good conductive elements. The heat transferred to the base material 17 is then transmitted to the engine (not shown) via the main metal fitting 15 (see Figure 1), thus cooling the tip 18. This reduces overheating of the tip 18, thereby improving the wear resistance of the tip 18.

[0026] The ground electrode 16 is obtained by, for example, applying a film containing good conductive elements to the surface of the base material 17 by thermal spraying or plating, and then joining the tip 18 to the base material 17 via a weld 19 by laser welding or resistance welding. As the film melts into the weld 19 when it is formed, the good conductive elements contained in the film become visible at the interface 30. This improves the thermal conductivity of the interface 30.

[0027] The interface portion 30 includes a first portion 31 in which the content of good conductive elements is greater than that of good conductive elements in the base material 17, and a second portion 32 in which the content of good conductive elements is less than or equal to that of good conductive elements in the base material 17. The length of the first portion 31 is 6% or more of the length of the interface portion 30. Because the first portion 31, which contains a large amount of good conductive elements, improves the thermal conductivity from the tip 18 to the weld portion 19, overheating of the tip 18 can be further reduced.

[0028] The first part 31 is positioned such that at least a portion of it is away from the base material 17 with respect to the position of the opposing surface 20. Because the first part 31 is located close to the discharge surface 23 of the tip 18, the heat conductivity from the tip 18 to the base material 17 via the weld 19 is improved compared to the case where the first part 31 is not located in this position.

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

[0030] Part 1 31 is located between the side portion 25 of the tip 18 (see Figure 2) and the weld portion 19. Compared to the case where Part 1 31 is located between the bottom portion 24 of the tip 18 and the weld portion, the amount of elements that hinder welding quality present between the bottom portion 24 of the tip 18 and the weld portion can be reduced, thereby ensuring the peel resistance of the tip 18. [Examples]

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

[0032] (Preparation of Sample 1) The testers applied various platings to a rod-shaped base material made of nickel-based alloy NCF601, and then coated the base material with films containing Zn, Ag, Cr, Fe, and Cu. After that, they joined the tips to the base material by resistance welding, obtaining 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 using SEM-EDS. The SEM-EDS analysis conditions were: acceleration voltage: 20kV, irradiation current: 90nA, working distance: 11mm, measurement time: 30 seconds, selected elements: O, Zn, Ag, Cr, Cu, Ni, Fe, Al, Pt, magnification: 2000x. The analysis results showed that each of the elements Zn, Ag, Cr, Fe, and Cu contained in the film was detected at the interface at a concentration of 1wt% or more. Among the elements contained in the base material, the first element with the highest content was Ni. Each of the elements Zn, Ag, Cr, Fe, and Cu was a good conductive element with a higher thermal conductivity than the first element (Ni).

[0034] Using an uncut ground electrode, various spark plugs were obtained in the examples including the ground electrode. For comparison, spark plugs in comparative examples were also fabricated, which included a ground electrode in which the tip was joined by resistance welding to a base material without a plating film containing 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 in the examples and comparative examples, and generated a spark of 9 × 10 between the center electrode and the ground electrode. 7After firing several times, the wear amount (mm) of the tip was measured using a 3D shape measuring machine. The wear amount of the spark plug tip in the example was less than that of the spark plug tip in the comparative example. It was found that the wear resistance of the tip could be improved in 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 testers plated a rod-shaped base material made of nickel-based alloy NCF601 with zinc, covered the base material with a zinc-containing film, and then removed the film by irradiating it with a laser beam. By varying the laser beam irradiation conditions, various base materials with different film thicknesses were obtained. For comparison, base materials with the zinc plating completely removed using a stripping agent were also prepared. Tips were joined to the base material by resistance welding to obtain multiple ground electrodes of various types. The main component of the tips was platinum (Pt).

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

[0038] Using an uncut ground electrode, eight spark plugs were obtained for each of Examples 1-4, including the ground electrode. Eight spark plugs were also fabricated for the comparative example, using a base material from which the Zn plating had been completely removed. By adjusting the energy during resistance welding, Zn was present at the interface between the side of the tip and the base material in the spark plugs of Examples 1-3, and at the interface between the bottom of the tip and the base material in the spark plugs of Example 4. Zn was not present at the interface of the spark plugs in the comparative example.

[0039] [Table 1]

[0040] (Wear resistance test) After conducting the above wear resistance test, the testers measured the wear amount (mm) of the chip using a 3D shape measuring machine (n=4). The wear amounts shown in Table 1 are the average of n=4. According to Table 1, it was found that the wear amount decreased as the length (%) increased. In particular, it was found that when the length (%) was 6% or more, the wear amount could be reduced by more than 15% compared to the comparative example.

[0041] (Peel resistance test) The testers subjected the spark plug to 1000 thermal shock cycles (n=4), consisting of heating the tip on the ground electrode of the fabricated spark plug with a burner for 1 minute, then removing the burner from the spark plug for 2 minutes to allow the ground electrode to cool to room temperature. The tip temperature during heating was 1050°C. The tip temperature was measured before starting the peel resistance test by using a spark plug with a hole drilled in the base material of the ground electrode that reached near the tip, and placing the thermocouple's measuring junction near the tip of the base material.

[0042] After the test, a cross-section including the axis of the spark plug was prepared, and the interface between the weld of the ground electrode and the base material, and the interface between the tip and the weld were observed using an optical microscope. The length of the cracks that had grown at these interfaces was measured, and those where the crack length was 50% or less of the interface length were classified as A, and those where it exceeded 50% were classified as B.

[0043] Table 1 shows that the spark plugs in Examples 1-3, where Zn is present on the side of the chip, exhibit improved chip peel resistance compared to the spark plug in Example 4, where Zn is present on the bottom of the chip.

[0044] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0045] In this embodiment, the case where the chip 18 is disc-shaped was described, but it is not necessarily limited to this. The shape of the chip 18 can be set as appropriate, such as a rectangular prism, a polygonal prism other than a rectangular prism, or a frustum of a cone.

[0046] In the embodiment, the base material 17 of the ground electrode 16 was described as a curved rod with a square cross-section, but it is not necessarily limited to this. Examples of base material 17 shapes include straight ones as well as curved ones. Examples of cross-sections of base material 17 include circles, ellipses, and semicircles as well as squares.

[0047] In the embodiment, a case was described in which the tip 18 is provided at a position away from the tip surface 22 of the base material 17, but it is not limited to this. It is of course possible to position 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 described, a welded portion 19 is provided between the side portion 25 of the tip 18 and the base material 17, and the case in which the bottom portion 24 of the tip 18 is in contact with the base material 17 has been explained, but it is not necessarily limited to this. It is of course possible to provide a welded portion 19 from the side portion 25 to the bottom portion 24 of the tip 18 by laser welding. In this case, since the interface portion 30 extends from the side portion 25 to the bottom portion 24, a good conductive element can be present between the bottom portion 24 and the welded portion 19, or between the side portion 25 and the bottom portion 24 and the welded portion 19, or between the side portion 25 and the welded portion 19. When the welded portion 19 is made by resistance welding, the side portion 25 of the tip 18 is left open and the interface portion 30 can be provided at the bottom portion 24, so it is of course possible to have a good conductive element between the bottom portion 24 of the tip 18 and the welded portion 19. [Explanation of Symbols]

[0049] 10 Spark plugs 13 Center electrode 16 Ground electrode 17 Base material 18 chips 19 Welded section 23 Discharge surface 24 Bottom 25 Side 30 Interface part 31 Part 1 (The part with a high concentration of elements)

Claims

1. A ground electrode including a base material and a tip joined to the base material via a welded portion, A spark plug comprising a central electrode electrically insulated from the ground electrode, At the interface between the welded portion and the tip, there is an element with a higher thermal conductivity than the base material. A spark plug in which the content of the element at the interface is greater than the content 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 chip includes a discharge surface facing the central electrode, a bottom facing the base material, and a side portion connecting the bottom and the discharge surface. The spark plug according to claim 1 or 2, wherein the interface containing the element is located at least between the side portion and the welded portion.

4. The spark plug according to claim 3, wherein the length of the portion of the interface in which the element exists in a proportion greater than the proportion of the element in the base material is 6% or more of the length of the interface.

Images