Spark plug

Abstract

To provide a spark plug capable of reducing wear of a chip.SOLUTION: A spark plug comprises an insulator in which an axis hole extending along an axis line is provided, a center electrode which is disposed in the axis hole, a main body fitting which is disposed in an outer periphery of the insulator, and a ground electrode which is connected to the main body fitting. At least one of the center electrode and the ground electrode includes a chip containing Ru as a main component, and the chip includes a discharge surface which is opposed to the other of the center electrode and the ground electrode in an axial direction. An average of numbers of crystal grain boundaries per unit length of a first test line, a distance between the first test line and the discharge surface being 10 μm, with which a crystal grain boundary of the chip crosses the first test line when the first test line is drawn on a cross section of the chip which passes the center of gravity of the discharge surface and is in parallel with the axis line is from 20 / mm or more to 400 / mm or less, and an average of lengths of circumferences of crystal grain boundaries which the first test line crosses is from 6.5 μm or more to 320 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spark plug having a tip containing Ru. [Background technology]

[0002] Patent Document 1 discloses a prior art in which at least one of the center electrode and the ground electrode includes a tip made of simple Ru or a Ru alloy. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 5-54955 Summary of the Invention [Problem to be solved by the invention]

[0004] Ru oxidizes and volatilizes significantly at high temperatures, so prior art spark plugs are prone to tip wear, potentially shortening the life of the spark plug.

[0005] The present invention has been made to solve this problem, and has as its object to provide a spark plug that can reduce wear of the tip. [Means for solving the problem]

[0006] A first aspect to achieve this object includes an insulator having an axial hole extending along an axis, a center electrode disposed in the axial hole, a metallic shell disposed on the outer periphery of the insulator, and a ground electrode connected to the metallic shell, wherein at least one of the center electrode and the ground electrode includes a tip primarily composed of Ru, and the tip includes a discharge surface axially facing the other of the center electrode and the ground electrode. When a first test line is drawn on a cross section of the tip that passes through the center of gravity of the discharge surface and is parallel to the axis, the average number A of grain boundaries per unit length of the first test line, where the grain boundaries of the tip intersect with the first test line, is 20 / mm or more and 400 / mm or less, and the average perimeter of the grains intersecting the first test line is 6.5 / mm or more and 320 / mm or less.

[0007] A second aspect of the present invention includes an insulator having an axial hole extending along an axis, a center electrode disposed in the axial hole, a metallic shell disposed on the outer periphery of the insulator, and a ground electrode connected to the metallic shell, wherein at least one of the center electrode and the ground electrode includes a tip primarily composed of Ru, and the tip includes a discharge surface facing the other of the center electrode and the ground electrode in a direction perpendicular to the axis. When a first test line is drawn on a cross section of the tip parallel to the axis and passing through the center of gravity of the discharge surface and parallel to the axis, the distance from the discharge surface to the first test line is 10 μm. The average number A of grain boundaries per unit length of the first test line where the grain boundaries of the tip intersect with the first test line is 20 / mm or more and 400 / mm or less, and the average perimeter of the grains intersecting the first test line is 6.5 μm or more and 320 μm or less.

[0008] In the third aspect, in the first or second aspect, when a second test line is drawn on the cross section with a distance of 10 μm between the side connected to the discharge surface and the second test line, the average number of grain boundaries per unit length of the second test line where the grain boundaries of the chip intersect is 20 or more / mm and 400 or less / mm.

[0009] In a fourth aspect, in any of the first to third aspects, when a third test line perpendicular to the first test line is drawn on a cross section, the value A / B obtained by dividing the average number A by the average number B of grain boundaries per unit length of the third test line where the grain boundaries of the chip intersect with the third test line is 0.5 or more and 2.0 or less.

[0010] A fifth aspect is any of the first to fourth aspects, in which, when a straight line perpendicular to the first test line is drawn on the cross section and the length of the chip is defined as the length of the shortest line segment formed by cutting the straight line by the boundary of the chip, a fourth test line parallel to the first test line is drawn within a range of half the length of the chip, including the first test line, and the average number of grain boundaries per unit length of the fourth test line where the grain boundaries of the chip intersect with the fourth test line is 20 / mm or more and 400 / mm or less, and the average circumference of the crystal grains intersected by the fourth test line is 6.5 μm or more and 320 μm or less.

[0011] In a sixth aspect, in any of the first to fifth aspects, when a fifth test line parallel to the first test line is drawn over half the length of the chip excluding the first test line, the average number of grain boundaries per unit length of the fifth test line where the grain boundaries of the chip intersect with the fifth test line is 20 / mm or more and 400 / mm or less, and the average circumference of the grains intersected by the fifth test line is 6.5 μm or more and 320 μm or less.

[0012] In a seventh aspect, in any one of the first to sixth aspects, the chip has a porosity of 1% or more and 7% or less. [Effects of the Invention]

[0013] According to the present invention, the average number A of grain boundaries per unit length of the first test line, where the first test line, which is 10 μm away from the discharge surface of the chip, intersects with the grain boundaries of the chip, is set to 20 / mm or more and 400 / mm or less, and the average perimeter of the grains intersecting the first test line is set to 6.5 μm or more and 320 μm or less, thereby reducing the loss of grains near the discharge surface due to oxidation of the grain boundaries and reducing wear on the chip. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a half-sectional view of a spark plug according to a first embodiment. [Figure 2] FIG. 3 is a cross-sectional view of a portion where a center electrode and a ground electrode face each other. [Figure 3] FIG. 2 is a cross-sectional view of the tip of the center electrode. [Figure 4] FIG. 1 is a cross-sectional view of a grain intersecting a first test line. [Figure 5] FIG. 6 is a cross-sectional view of a spark plug according to a second embodiment. [Figure 6] FIG. 6 is an enlarged cross-sectional view of the spark plug showing a portion VI in FIG. 5. DETAILED DESCRIPTION OF THE INVENTION

[0015] Preferred embodiments 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 a first embodiment, taken along an axis X. In Fig. 1, the upper side of the drawing is the leading end side of the spark plug 10, and the lower side is the trailing end side of the spark plug 10.

[0016] As shown in Fig. 1, a spark plug 10 includes an insulator 11, a center electrode 13, a metallic shell 15, and a ground electrode 16. The insulator 11 is a substantially cylindrical member made of ceramics such as alumina, which has excellent mechanical properties and insulating properties at high temperatures. The insulator 11 has an axial hole 12 extending therethrough along the axis X. The center electrode 13 is a rod-shaped electrode disposed in the axial hole 12 along the axis X.

[0017] The terminal fitting 14 is a rod-shaped member to which an ignition device (not shown) is connected, and its tip side is disposed in the axial hole 12 of the insulator 11. The terminal fitting 14 is electrically connected to the center electrode 13 in the axial hole 12.

[0018] The metal shell 15 is a substantially cylindrical metal member that is fixed to a screw hole (not shown) of an internal combustion engine. The metal shell 15 is made of a conductive metal material (such as low-carbon steel). The metal shell 15 is fixed to the outer periphery of the insulator 11. A ground electrode 16 is connected to the metal shell 15.

[0019] 2 is a cross-sectional view of the portion of the spark plug 10 where the center electrode 13 and the ground electrode 16 face each other. The center electrode 13 includes a base material 17 and a tip 19 provided at the tip of the base material 17.

[0020] A core material (not shown) with excellent thermal conductivity is embedded in the base material 17. The material of the base material 17 is, for example, Ni or an alloy mainly containing Ni, and the material of the core material is, for example, Cu or an alloy mainly containing Cu. The core material can be omitted.

[0021] The tip 19 is joined to the base material 17 by a fusion zone 18. The tip 19 and the base material 17 are melted together in the fusion zone 18. The fusion zone 18 is formed by laser welding, resistance welding, diffusion bonding, or the like. The tip 19 includes a discharge surface 20 facing the ground electrode 16 and a side surface 21 connected to the discharge surface 20.

[0022] The ground electrode 16 includes a base material 22 connected to the metallic shell 15 and a tip 24 provided on the base material 22. A core material (not shown) with excellent thermal conductivity is embedded in the base material 22. The base material 22 is made of an alloy containing Ni as its main component, and the core material is made of Cu or an alloy containing Cu as its main component. The core material can be omitted. An intermediate member protruding toward the center electrode 13 may be provided on the base material 22, and the tip 24 may be joined to the intermediate member. The intermediate member is a part of the base material 22.

[0023] The tip 24 is joined to the base material 22 by a fusion zone 23. The tip 24 and the base material 22 are melted together in the fusion zone 23. The fusion zone 23 is formed by laser welding, resistance welding, diffusion bonding, or the like. The tip 24 includes a discharge surface 25 facing the center electrode 13 and a side surface 26 connected to the discharge surface 25.

[0024] At least one of the chips 19, 24 contains Ru as a main component. "Ru is the main component" means that Ru is the element with the largest content among the elements constituting the chips 19, 24. The Ru content is preferably 50% by mass or more, more preferably 60% by mass or more or 70% by mass or more, of the total amount of components constituting the chips 19, 24.

[0025] When the tip 19 of the center electrode 13 is primarily composed of Ru, or when the tip 24 of the ground electrode 16 is primarily composed of Ru, the elements other than Ru that make up the tips 19, 24 can be one or more selected from Rh, Pd, Os, Ir, Pt, Ta, W, Mo, Nb, Re, Cr, Mn, Fe, Co, Ni, V, Ti, Zr, Hf, Al, and Sc.

[0026] When the tip 19 of the center electrode 13 is mainly composed of Ru, the ground electrode 16 is either one that includes a tip 24 mainly composed of Ru, one that includes a tip 24 mainly composed of one or more platinum group elements other than Ru (Rh, Pd, Os, Ir, Pt), or one in which the fusion zone 23 and the tip 24 are not provided in the base material 22.

[0027] When the tip 24 of the ground electrode 16 is primarily composed of Ru, the center electrode 13 is either one that includes a tip 19 primarily composed of Ru, one that includes a tip 19 primarily composed of one or more platinum group elements other than Ru (Rh, Pd, Os, Ir, Pt), or one in which the fusion zone 18 and the tip 19 are not provided in the base material 17.

[0028] The spark plug 10 is manufactured, for example, by the following method. First, the center electrode 13 is inserted into the axial hole 12 of the insulator 11. Next, the metal terminal 14 is inserted into the axial hole 12 to ensure electrical continuity between the metal terminal 14 and the center electrode 13, and then the metal shell 15, to which the ground electrode 16 is previously connected, is assembled to the outer periphery of the insulator 11. The ground electrode 16 is bent to form a spark gap between the center electrode 13 and the ground electrode 16, and the spark plug 10 is obtained.

[0029] The chips 19, 24, which are primarily composed of Ru, are made by sintering a compact of metal powder containing Ru, punching a metal plate containing Ru, cutting a metal wire containing Ru, etc. The shape of the chips 19, 24 is not limited and may be a disk, a truncated cone, an elliptical cylinder, or a polygonal prism such as a triangular prism or a square prism.

[0030] Figure 3 is a cross-sectional view of the tip 19 of the center electrode 13. The cross-sectional view in Figure 3 is an example of a scanning electron microscope (SEM) image of the polished surface of the tip 19 cut parallel to the axis X at the position of the center of gravity 27 of the discharge surface 20. The center of gravity 27 of the discharge surface 20 is the geometric center when the discharge surface 20 is considered as a planar figure. An interface 28 between the base material 17 and the molten zone 18, and an interface 29 between the molten zone 18 and the tip 19 appear in the cross section.

[0031] To observe the cross-sectional structure near the discharge surface 20 of the tip 19, a first test line 30 (straight line) is drawn parallel to the discharge surface 20 at a distance of 10 μm from the discharge surface 20. Because the cross section of the tip 19 sags (curls) on the discharge surface 20, reducing the accuracy of cross-sectional structure observation, a test line is not drawn on the discharge surface 20, and a distance of 10 μm is left between the discharge surface 20 and the first test line 30.

[0032] 4 is a cross-sectional view of crystal grains intersecting with first test line 30. First test line 30, drawn on the cross-section of chip 19, intersects with crystal grains 38, 39, 40, 41, 42, and 43. Both ends of first test line 30 terminate within crystal grains 37 and 44, respectively. In this embodiment, the number of intersections between grain boundary 45 and first test line 30 is seven.

[0033] The length of the first test line 30 is set so that it intersects with 10 or more crystal grains. The first test line 30 is drawn at various positions randomly within a range that satisfies the condition that the distance from the discharge surface 20 is 10 μm, and the number of intersections between the crystal grain boundaries 45 and the first test line 30 is measured multiple times to obtain an average value. The length of the first test line 30 may be changed for each measurement. The average value is divided by the length of the first test line 30 to obtain the average number A (numbers / mm) of intersections of crystal grain boundaries per unit length. The average A is between 20 and 400 per mm.

[0034] The average perimeter of crystal grains 38, 39, 40, 41, 42, and 43 that intersect with first test line 30 used to determine average A is calculated. When calculating the average perimeter, the perimeters of crystal grains 37 and 44, whose ends terminate internally on first test line 30, are not included. The average perimeter of the crystal grains that intersect with first test line 30 is 6.5 μm or more and 320 μm or less.

[0035] When the average A is low and there are few grain boundaries, the grains are large, so large thermal stress acts on the grains due to temperature changes in the tip 19, and grains tend to fall off. When the average A is high and there are many grain boundaries, oxidation progresses along the grain boundaries, and grains tend to fall off due to destruction of the grain boundaries. When the average A is 20 particles / mm or more and 400 particles / mm or less, destruction of grain boundaries due to oxidation near the discharge surface 20 is reduced, and further, the thermal stress acting on the grains can be reduced, so that grains near the discharge surface 20 can be reduced.

[0036] If the average perimeter of the crystal grains (length of the crystal grain boundaries) is short, oxidation along the crystal grain boundaries tends to be completed early, leading to early crystal grain shedding due to the destruction of the crystal grain boundaries. If the average perimeter of the crystal grains is long, the crystal grain boundaries have a complex shape, which increases the thermal stress acting on the crystal grains due to temperature changes in the tip 19, making it more likely for the crystal grains to shedding. If the average A is 20 particles / mm or more and 400 particles / mm or less, and the average perimeter of the crystal grains is 6.5 μm or more and 320 μm or less, shedding of crystal grains near the discharge surface 20 can be reduced, thereby reducing wear on the tip 19.

[0037] Returning to Figure 3, in order to observe the cross-sectional structure near the side surface 21 of the chip 19, a second test line 31 (straight line) is drawn parallel to the side surface 21 at a distance of 10 µm from the side surface 21. Because the cross section of the chip 19 sags (curls) on the side surface 21, reducing the accuracy of cross-sectional structure observation, a test line is not drawn on the side surface 21, and instead a distance of 10 µm is left between the side surface 21 and the second test line 31.

[0038] The length of the second test line 31 is set so that it intersects with at least 10 crystal grains. The second test line 31 is drawn randomly at various positions within a range that satisfies the condition that the distance from the side surface 21 is 10 μm. As with the first test line 30, the number of intersections between the crystal grain boundaries and the second test line 31 is measured multiple times and the average value is calculated. The length of the second test line 31 may be changed for each measurement. The average value is divided by the length of the second test line 31 to calculate the average number of crystal grain boundary intersections per unit length (numbers / mm). The average is preferably 20 or more / mm and 400 or less / mm. This reduces the damage to the crystal grain boundaries near the side surface 21 and further reduces the thermal stress acting on the crystal grains, thereby reducing wear on the tip 19 due to the shedding of crystal grains near the side surface 21.

[0039] In order to observe the cross-sectional structure of the chip 19 over a wide range, multiple third test lines 32 (straight lines) perpendicular to the first test lines 30 are drawn. The number of third test lines 32 is preferably between four and eight. The positions at which the multiple third test lines 32 are drawn are equally spaced along the length of the discharge surface 20. This is to ensure a thorough observation of the cross-sectional structure of the chip 19. The third test lines 32 are preferably drawn in the range between the discharge surface 20 and a position 0.2 mm away from the discharge surface 20 toward the interface 29. This is to control the cross-sectional structure up to 0.2 mm from the discharge surface 20.

[0040] The length of the third test line 32 is set so that it intersects with 10 or more crystal grains. As with the first test line 30, the number of intersections between the grain boundaries and the third test line 32 is measured multiple times to determine the average value. The length of the third test line 32 may be changed for each measurement. The average value is divided by the length of the third test line 32 to determine the average number of grain boundary intersections per unit length, B (numbers / mm). The value A / B, obtained by dividing the average A by the average B, is preferably 0.5 or more and 2.0 or less.

[0041] A value A / B less than 1 indicates that the crystal grains are long in the direction perpendicular to the discharge surface 20, and a value A / B greater than 1 indicates that the crystal grains are long in the direction parallel to the discharge surface 20. If the value A / B is 0.5 or greater and 2.0 or less, this indicates that the length of the crystal grains in the direction perpendicular to the discharge surface 20 and the length in the direction parallel to the discharge surface 20 are approximately equal, which reduces the thermal stress acting on the crystal grains due to temperature changes in the tip 19, further reducing wear on the tip 19 due to falling off of crystal grains.

[0042] A straight line perpendicular to the first test line 30 is drawn, and the length of the shortest line segment formed when that line is cut by the boundaries of the tip 19 (the discharge surface 20 and the interface 29) is taken as the length L of the tip 19. In this embodiment, the distance between the interface 29 and the left end of the discharge surface 20 (see FIG. 3) is the shortest, so the distance between the left end of the discharge surface 20 and the interface 29 is the length L of the tip 19.

[0043] The range of the length L of the chip 19 is divided into a range 33 that is half the length L of the chip 19 and includes the first test line 30, and a range 34 that is half the length L of the chip 19 and does not include the first test line 30. A fourth test line 35 parallel to the first test line 30 is drawn in the range 33. The length of the fourth test line 35 is set so that the fourth test line 35 intersects with 10 or more crystal grains. As with the first test line 30, the fourth test line 35 is drawn randomly in the range 33, and the number of intersections between the grain boundaries and the fourth test line 35 is measured multiple times to obtain an average value. The length of the fourth test line 35 may be changed for each measurement. The average value is divided by the length of the fourth test line 35 to obtain the average number of intersections of the grain boundaries per unit length (numbers / mm).

[0044] When wear progresses from discharge surface 20, area 33 is worn first. The average number of grain boundary intersections in area 33 is preferably 20 / mm or more and 400 / mm or less, and the average perimeter of the grains intersecting fourth test line 35 is preferably 6.5 μm or more and 320 μm or less. This is to reduce damage to the grain boundaries in area 33 and further reduce the thermal stress acting on the grains, thereby reducing wear of tip 19 due to grain shedding.

[0045] A third test line 32 may be drawn in the area 33, and the average number of grain boundary intersections per unit length, B (numbers / mm), may be calculated so that the value A / B is 0.5 or more and 2.0 or less. This is to reduce the thermal stress acting on the grains in the area 33 due to temperature changes in the chip 19, and reduce wear in the area 33.

[0046] A fifth test line 36 parallel to the first test line 30 is drawn in the area 34. The length of the fifth test line 36 is set so that the fifth test line 36 intersects with 10 or more crystal grains. As with the first test line 30, the fifth test line 36 is drawn randomly in the area 34, and the number of intersections between the crystal grain boundaries and the fifth test line 36 is measured multiple times to obtain an average value. The length of the fifth test line 36 may be changed for each measurement. The average value is divided by the length of the fifth test line 36 to obtain the average number of intersections of the crystal grain boundaries per unit length (numbers / mm).

[0047] Area 34 appears after area 33 has worn away when wear progresses from discharge surface 20. The average number of grain boundary intersections in area 34 is preferably 20 or more / mm and 400 or less / mm, and the average perimeter of the grains intersecting fifth test line 36 is preferably 6.5 μm or more and 320 μm or less. This is to reduce damage to the grain boundaries in area 34 and further reduce the thermal stress acting on the grains, thereby extending the life of tip 19.

[0048] The porosity of the chip 19 is preferably 1% or more and 7% or less. The porosity of the chip 19 is the ratio of the area of ​​the pores to the area of ​​the SEM image. If the porosity of the chip 19 is 1% or more, the pores can buffer the thermal stress caused by temperature changes in the chip 19. As the porosity increases, there is a tendency for the oxidative wear of the chip 19 to increase, so the porosity is preferably 1% or more and 7% or less.

[0049] When the tip 24 of the ground electrode 16 is mainly composed of Ru, the average number of grain boundaries and the average circumferential length of the grains when the first test line 30, the second test line 31, the third test line 32, the fourth test line 35, and the fifth test line 36 are drawn on the cross section of the tip 24 fall within the same range as the numerical range described for the tip 19. This reduces wear on the tip 24.

[0050] When the chips 19, 24 are produced by sintering a compact of metal powder containing Ru, the cross-sectional structure of the chips 19, 24 can be controlled by the particle size distribution of the metal powder and the sintering temperature and time.When the chips 19, 24 are produced by punching a metal plate containing Ru or cutting a metal wire containing Ru, the cross-sectional structure of the chips 19, 24 can be controlled by the temperature and time of heat treatment of the plate or wire.

[0051] A second embodiment will be described with reference to Figures 5 and 6. In the first embodiment, the discharge surface 20 of the tip 19 of the center electrode 13 faces the leading end side in a direction parallel to the axis X, and the discharge surface 25 of the tip 24 of the ground electrode 16 faces the rear end side in a direction parallel to the axis X. In contrast, in the second embodiment, the discharge surface 63 of the tip 62 of the center electrode 53 faces in a direction perpendicular to the axis X, and the discharge surface 58 of the tip 57 of the ground electrode 56 faces in a direction perpendicular to the axis X.

[0052] Figure 5 is a cross-sectional view of a spark plug 50 according to the second embodiment. In Figure 5, the lower side of the page refers to the leading end side of the spark plug 50, and the upper side of the page refers to the trailing end side of the spark plug 50. The cross section of the trailing end side of the spark plug 50 is omitted from Figure 5.

[0053] As shown in Fig. 5, a spark plug 50 includes an insulator 51, a center electrode 53, a metallic shell 55, and a ground electrode 56. The insulator 51 is a substantially cylindrical member made of ceramics such as alumina, which has excellent mechanical properties and insulating properties at high temperatures. The insulator 51 has an axial hole 52 penetrating along the axis X. The center electrode 53 is a rod-shaped electrode disposed in the axial hole 52 along the axis X.

[0054] The metal terminal 54 is a rod-shaped member to which an ignition device (not shown) is connected, and is electrically connected to the center electrode 53 in the axial hole 52. The metal shell 55 is a substantially cylindrical member made of metal (for example, low-carbon steel) that is fixed to a screw hole (not shown) of the internal combustion engine. The metal shell 55 is fixed to the outer periphery of the insulator 51. A ground electrode 56 is connected to the metal shell 55.

[0055] Figure 6 is an enlarged cross-sectional view of the spark plug 50 showing a portion VI in Figure 5. The ground electrode 56 includes a rod-shaped base material (not shown) and a tip 57 joined to the tip of the base material. The tip 57 includes a discharge surface 58 facing the center electrode 53 and a side surface 59 connected to the discharge surface 58.

[0056] The center electrode 53 includes a base material 60 and a tip 62 provided at the tip of the base material 60. The tip 62 is mainly composed of Ru. The tip 62 is joined to the base material 60 by a fusion zone 61. The fusion zone 61 is formed by the tip 62 and the base material 60 melting together. The tip 62 includes a discharge surface 63 facing the ground electrode 56 and a side surface 64 connected to the discharge surface 63. The discharge surface 63 of the tip 62 and the discharge surface 58 of the tip 57 face each other in a direction perpendicular to the axis X (see FIG. 5).

[0057] The cross-sectional view of Figure 6 illustrates an SEM image of the polished surface of the tip 62 cut perpendicular to the axis X at the position of the center of gravity 63a of the discharge surface 63. There are no restrictions on the shape of the tip 62, but if the tip 62 is cylindrical extending in the direction of the axis X, the discharge surface 63 is a part of the cylindrical surface (a curved surface). The center of gravity 63a of the discharge surface 63 is the geometric center of the plane figure (rectangle in this embodiment) when the discharge surface 63 is projected in a direction perpendicular to the discharge surface 58 of the tip 57. An interface 65 between the base material 60 and the molten zone 61 and an interface 66 between the molten zone 61 and the tip 62 appear in the cross section.

[0058] To observe the cross-sectional structure near the discharge surface 63 of the tip 62, a first test line 67 is drawn parallel to the discharge surface 63 at a distance of 10 μm from the discharge surface 63. To observe the cross-sectional structure near the side surface 64 of the tip 62, a second test line 68 is drawn parallel to the side surface 64 at a distance of 10 μm from the side surface 64. In addition, to observe the cross-sectional structure over a wide area of ​​the tip 62, multiple third test lines 69 perpendicular to the first test lines 67 are drawn.

[0059] A straight line perpendicular to the first test line 67 is drawn, and the length of the shortest line segment formed when that line is cut by the boundary of the chip 62 is defined as the length L of the chip 62. In this embodiment, the length of the side surface 64 is the length L of the chip 62. The chip 62 is divided into an area 70 that is half the length L of the chip 62 and includes the first test line 67, and an area 371 that is half the length L of the chip 62 and does not include the first test line 67. A fourth test line 72 parallel to the first test line 67 is drawn in the area 70, and a fifth test line 73 parallel to the first test line 67 is drawn in the area 71.

[0060] When the first test line 67, the second test line 68, the third test line 69, the fourth test line 72, and the fifth test line 73 are drawn on the cross section of the tip 62, the average number of grain boundaries and the average circumferential length of the crystal grains fall within the same ranges as those described in the first embodiment. This reduces wear on the tip 62. The same applies when the tip 57 of the ground electrode 56 is mainly composed of Ru. [Example]

[0061] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0062] (Test 1) The tester fabricated cylindrical tips made of a Ru-Pt alloy containing 15% by mass of Pt and the remainder Ru using a powder metallurgy method. The tips had a diameter of 0.4 mm and a height of 0.4 mm. By varying the powder particle size distribution and sintering temperature, various tips with different structures were obtained. The tester then fabricated center electrodes by bonding each tip to a base material, and produced multiple spark plug samples No. 1-17 similar to the first embodiment, each with a spark gap between the center electrode tip and the ground electrode.

[0063] The tester obtained SEM images of the cross section of the chip for Sample No. 1-17, passing through the center of gravity of the chip's discharge surface and parallel to the axis. The tester then calculated the average number of grain boundaries per unit length of the first test line (A, counts / mm), the average circumference of the grains intersecting the first test line (μm), the average number of grain boundaries per unit length of the second test line (counts / mm), the average number of grain boundaries per unit length of the third test line (counts / mm), and the value A / B obtained by dividing the average A by the average B. The third test line was drawn in the area between the discharge surface and a position 0.2 mm away from the chip's discharge surface. The results of these calculations are listed in Table 1. The porosity of the chip determined from the SEM images was in the range of 1% to 7%.

[0064] The tester attached each of Samples No. 1-17, except for those for which SEM images were taken, to a gasoline-fueled engine (displacement 1.3L), and ran the engine at 3500 rpm for one minute with the intake throttle valve fully open, and then at 760 rpm for one minute, repeating this cycle, for a total of 100 hours of operation.

[0065] After the test, a 3D shape measuring machine was used to measure the total wear (mm) of the discharge surface and side surfaces of the tip of each sample, and samples No. 1-17 were divided into four ranks, A to D, based on the wear. A was a wear of less than 0.08 mm, B was a wear of 0.08 mm to less than 0.09 mm, C was a wear of 0.09 mm to less than 0.10 mm, and D was a wear of 0.10 mm or more. The results are shown in the judgment column in Table 1.

[0066] [Table 1]

[0067] As shown in Table 1, sample Nos. 4-14, in which the average number A of grain boundaries per unit length of the first test line was 20 or more / mm and 400 or less / mm, and the average perimeter of the grains intersecting the first test line was 6.5 μm or more and 320 μm or less, were evaluated as A, B, or C, whereas samples Nos. 1-3 and 15-17, in which the average A was outside this range or the average perimeter of the grains intersecting the first test line was outside this range, were evaluated as B. It was found that tip wear could be reduced by setting the average A to 20 or more / mm and 400 or less / mm, and the average perimeter of the grains intersecting the first test line to 6.5 μm or more and 320 μm or less.

[0068] Samples No. 5-13, in which the average number of grain boundaries per unit length of the second test line was between 20 / mm and 400 / mm, were rated A or B, whereas samples No. 4 and No. 14, in which the average number of grain boundaries per unit length of the second test line was outside this range, were rated C. It became clear that by setting the average number of grain boundaries per unit length of the second test line to between 20 / mm and 400 / mm, tip wear could be further reduced.

[0069] Samples No. 6-12, for which the A / B value was between 0.5 and 2.0, were judged as A, whereas samples No. 5 and 13, for which the A / B value was outside this range, were judged as B. It became clear that tip wear could be further reduced by setting the A / B value between 0.5 and 2.0.

[0070] (Test 2) The tester fabricated cylindrical tips made of a Ru-Pt alloy containing 15% by mass of Pt and the remainder of Ru by powder metallurgy. The tips had a diameter of 0.4 mm and a height of 0.4 mm. Learning from the relationship between the manufacturing conditions of the tips and the structure of the tips in Test 1, the tester fabricated spark plug samples Nos. 18-20 similar to the first embodiment, each including a center electrode with a tip having an average number of grain boundaries per unit length of the first test line of 20 / mm, by varying the powder particle size distribution and sintering temperature. The tester also fabricated spark plug samples Nos. 21-23 similar to the first embodiment, each including a center electrode with a tip having an average number of grain boundaries per unit length of the first test line of 400 / mm.

[0071] Samples 20 and 23 include chips that satisfy "Condition 1," in which the average number of grain boundaries per unit length of the fourth test line is 20 to 400 per mm and the average perimeter of the grains intersecting the fourth test line is 6.5 to 320 μm. Also, "Condition 2," in which the average number of grain boundaries per unit length of the fifth test line is 20 to 400 per mm and the average perimeter of the grains intersecting the fifth test line is 6.5 to 320 μm. Samples 19 and 22 include chips that are cylindrical and laser-hardened on one side to enlarge the grains near the laser-hardened surface, thereby satisfying Condition 1 but not Condition 2. Samples 18 and 21 include chips that do not satisfy Condition 1 or Condition 2.

[0072] The tester conducted the same test as Test 1 for samples No. 18-23 other than those for which SEM images were taken, and measured the amount of wear in the same manner as Test 1. The lifespan of each sample was estimated based on the amount of wear of sample No. 18, and samples that were less than 110% of the lifespan of sample No. 18 were rated A, samples that were 110% to 120% of the lifespan of sample No. 18 were rated G, and samples that were 120% or more of the lifespan of sample No. 18 were rated E. The results are shown in Table 2. Samples that met the conditions were marked M in Table 2.

[0073] [Table 2]

[0074] As shown in Table 2, it was revealed that samples Nos. 19, 20, 22, and 23, which satisfy condition 1, can have a lifespan that is approximately 10% longer than that of samples Nos. 18 and 21, which do not satisfy condition 1. It was revealed that samples Nos. 20 and 23, which satisfy conditions 1 and 2, can have a lifespan that is approximately 20% longer than that of samples Nos. 18 and 21, which do not satisfy conditions 1 and 2.

[0075] 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.

[0076] In the first embodiment, the ground electrode 16 is bent, but this is not limiting. Naturally, a straight ground electrode 16 can be used instead of the bent ground electrode 16. In this case, the tip end side of the metallic shell 15 is extended in the axial direction, and the straight ground electrode 16 is joined to the metallic shell 15. The number of ground electrodes 16 is also set appropriately.

[0077] In the second embodiment, a case where a straight ground electrode 56 is used has been described, but the present invention is not limited to this. Naturally, it is possible to use a curved ground electrode 56 instead of the straight ground electrode 56. The number of ground electrodes 56 is also set appropriately. Naturally, it is possible to cover the tip end side of the metallic shell 55 with a cap having a hole penetrating in the thickness direction. [Explanation of symbols]

[0078] 10.50 spark plug 11,51 Insulator 12,52 shaft hole 13,53 Center electrode 15,55 Metal body 16,56 ground electrode 17,60 Base material 19.62 chips 20,63 Discharge surface 21,64 Side 27,63a Center of gravity 29,66 interface 30,67 First test line 31,68 Second test line 32,69 Third Test Line 33, 34, 70, 71 range 35,72 Fourth Test Line 36,73 5th Test Line X axis

Claims

1. an insulator provided with an axial hole extending along an axis; a center electrode disposed in the axial hole; a metallic shell disposed on the outer periphery of the insulator; a ground electrode connected to the metallic shell, At least one of the center electrode and the ground electrode includes a tip containing Ru as a main component, the tip includes a discharge surface facing the other of the center electrode and the ground electrode in the axial direction, a spark plug in which, when a first test line is drawn on a cross section of the tip that passes through the center of gravity of the discharge surface and is parallel to the axis, the distance from the discharge surface is 10 μm, and the grain boundaries of the tip and the first test line intersect with the first test line, the average number A of the grain boundaries per unit length of the first test line is 20 / mm or more and 400 / mm or less, and the average perimeter of the grains that intersect with the first test line is 6.5 μm or more and 320 μm or less.

2. an insulator provided with an axial hole extending along an axis; a center electrode disposed in the axial hole; a metallic shell disposed on the outer periphery of the insulator; a ground electrode connected to the metallic shell, At least one of the center electrode and the ground electrode includes a tip containing Ru as a main component, the tip includes a discharge surface facing the other of the center electrode and the ground electrode in a direction perpendicular to the axis, a spark plug in which, when a first test line is drawn on a cross section of the tip that passes through the center of gravity of the discharge surface and is parallel to the axis and in the direction in which the other faces, the distance from the discharge surface is 10 μm, and the grain boundaries of the tip and the first test line intersect with the first test line, the average number A of the grain boundaries per unit length of the first test line is 20 / mm or more and 400 / mm or less, and the average perimeter of the grains intersecting with the first test line is 6.5 μm or more and 320 μm or less.

3. 3. The spark plug according to claim 1, wherein when a second test line is drawn on the cross section, the distance between the second test line and a side surface connected to the discharge surface is 10 μm, the average number of grain boundaries per unit length of the second test line where the grain boundaries of the tip intersect with the second test line is 20 grains / mm or more and 400 grains / mm or less.

4. 3. The spark plug according to claim 1, wherein a third test line perpendicular to the first test line is drawn on the cross section, and a value A / B obtained by dividing the average number A by an average number B of the grain boundaries per unit length of the third test line where the grain boundaries of the tip intersect with the third test line is 0.5 or more and 2.0 or less.

5. When a straight line perpendicular to the first test line is drawn on the cross section, and the length of the shortest line segment obtained by cutting the straight line by the boundary of the chip is defined as the length of the chip, 3. The spark plug according to claim 1, wherein when a fourth test line parallel to the first test line is drawn on the tip over a range of half the length of the tip including the first test line, the average number of grain boundaries per unit length of the fourth test line where the grain boundaries of the tip intersect with the fourth test line is 20 grains / mm or more and 400 grains / mm or less, and the average perimeter of the grains intersecting with the fourth test line is 6.5 μm or more and 320 μm or less.

6. 6. The spark plug according to claim 5, wherein when a fifth test line parallel to the first test line is drawn in a range of half the length of the tip excluding the first test line, the grain boundaries of the tip intersect with the fifth test line, and the average number of the grain boundaries per unit length of the fifth test line is 20 grains / mm or more and 400 grains / mm or less, and the average perimeter of the grains intersecting with the fifth test line is 6.5 μm or more and 320 μm or less.

7. 3. The spark plug according to claim 1, wherein the tip has a porosity of 1% or more and 7% or less.

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