spark plug

Abstract

Spark plug (100) comprising a center electrode (20) and a ground electrode (30) and in which a space (g) is provided between the center electrode (20) and the ground electrode (30), wherein at least one of the center electrode (20) and the ground electrode (30) is provided with a base material (31) containing nickel (Ni) as a major component and with a tip (300) connected to the base material (31) and containing platinum (Pt) as a major component, wherein the tip (300) contains one or more elements selected from the group comprising rhodium (Rh), rhenium (Re), ruthenium (Ru) and tungsten (W), wherein the total content of the elements is not less than 10% by weight, and furthermore contains 5% or more by weight nickel (Ni), wherein the tip (300) has a discharge surface (310) which forms the space (g), wherein an opposite surface (320), which is a surface of the tip (300) on one side opposite the discharge surface (310), is connected to the base material (31), and wherein a connecting surface (Sz) between the opposite surface (320) of the tip (300) and the base material (31) is not less than 0.6 mm 2 is, whereby: the relationship Hb / Ha ≤ 2.3 is satisfied, where Hb represents a Vickers hardness in the cross-section perpendicular to the discharge surface (310) of the tip (300) and Ha represents a Vickers hardness in the cross-section of the tip (300), where the Vickers hardness Ha is measured after a process in which the tip (300) is held for ten hours at 1200°C in an argon (Ar) atmosphere.

Description

[Technical field]

[0001] The present invention relates to a spark plug provided with an electrode comprising a base material and a tip connected to the base material. [State of the art]

[0002] A spark plug is used to ignite a device (e.g., an internal combustion engine) that burns a fuel. For example, a spark plug is used that is provided with an electrode comprising a base material and a tip bonded to the base material.

[0003] Further relevant prior art is disclosed in the following documents: DE 11 2015 002 643 T5; DE 699 04 938 T2 and JP 2010 - 238 498 A. [Prior art document][Patent document]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open (kokai) JP 2010 - 238 498 A [Summary of the invention][Problem of the invention]

[0005] A tip wears down with repeated spark discharges. As the tip wears, the distance between the points increases. This increase in the distance between points due to tip wear is suppressed by increasing the tip's volume. Furthermore, the tip's temperature varies with repeated firing. When the temperature varies, the tip repeatedly undergoes thermal expansion and contraction, causing peeling of the bonding element between the tip and the base material. The larger the tip's volume, the more likely this peeling is to progress. Therefore, it is difficult to improve both the wear and peel resistance of the tip.

[0006] The present description describes a technique that can improve the wear resistance and peel resistance of a tip. [Problem solving]

[0007] To solve the problem described above, a spark plug with the feature of claim 1 is specified. Another embodiment has the features of claim 2. The application example 3 described below is an embodiment of the present invention. The technology of the present invention can be embodied in the following application examples. [Application example 1]

[0008] Spark plug comprising a center electrode and a ground electrode and in which a space is provided between the center electrode and the ground electrode, wherein at least one of the center electrode and the ground electrode is provided with a base material containing nickel (Ni) as a major component and with a tip connected to the base material and containing platinum (Pt) as a major component, wherein the tip contains one or more elements selected from the group comprising rhodium (Rh), rhenium (Re), ruthenium (Ru) and tungsten (W), the total content of the elements being not less than 10% by weight, and further containing 5% by weight or more of nickel (Ni), wherein the tip has a discharge surface forming the gap, wherein an opposite surface, which is a surface of the tip on one side opposite the discharge surface, is connected to the base material, and wherein a contact area between the opposite surface of the tip and the base material is not less than 0.6 mm 2 is.

[0009] According to this configuration, if the tip contains 5% or more nickel by weight, the contact area between the opposite surface of the tip and the base material is not less than 0.6 mm. 2 and contains the tip of one or more elements selected from a group including rhodium, rhenium, ruthenium, and tungsten, with the total element content being not less than 10% by weight. This improves the tip's peel resistance and wear resistance. [Application example 2]

[0010] Spark plug according to application example 1, wherein: the average grain size of crystal grains in a cross-section perpendicular to the discharge surface of the tip is not greater than 150 µm.

[0011] According to this configuration, large cracks are suppressed compared to a case where the average grain size of crystal grains at the tip is large. [Application example 3]

[0012] Spark plug according to application example 1 or 2, wherein: the relationship Hb / Ha ≤ 2.3 is satisfied, where Hb represents a Vickers hardness in the cross-section perpendicular to the discharge surface of the tip and Ha represents a Vickers hardness in the cross-section of the tip, where the Vickers hardness Ha is measured after a process in which the tip is held for ten hours at 1200°C in an argon (Ar) atmosphere.

[0013] According to this configuration, deformation of the tip due to a variation in temperature is suppressed.

[0014] The technology of the present invention can be embodied in various embodiments. For example, the present invention can be embodied in embodiments such as a spark plug, an ignition device that uses the spark plug, an internal combustion engine equipped with the spark plug, and an internal combustion engine equipped with the ignition device that uses the spark plug. [Brief description of the drawings] [ Fig. 1] Cross-sectional view of a spark plug 100 according to one embodiment. [ Fig. 2] Schematic view showing a configuration of an earth electrode 30. [ Fig. 3] Table showing sample configurations of spark plug 100 in association with test results. [ Fig. 4] Schematic view explaining measurement positions P1. [ Fig. 5A] Schematic view explaining a method for calculating a grain size Dz. [ Fig. 5B] Schematic view explaining the procedure for calculating the grain size Dz. [ Fig. 6] Schematic view explaining a cross section of the earth electrode 30. [ Fig. 7A] Schematic view showing an example of a cross section of a second tip 300 after a thermal test. [ Fig. 7B] Schematic view showing an example of a cross section of the second tip 300 after a thermal test. [ Fig. 7C] Schematic view showing an example of a cross section of the second tip 300 after a thermal test. [ Fig. 7D] Schematic view showing an example of a cross section of the second tip 300 after a thermal test. [ Fig. 8] Schematic view illustrating a cross-section of the earth electrode 30. [Embodiments of the invention] A. Embodiment A-1. Configuration of a spark plug

[0015] Fig. Figure 1 is a cross-sectional view of a spark plug 100 according to one embodiment. Fig. 1, a center axis CL (also referred to as "axis CL") of the spark plug 100 and a flat cross section including the center axis CL of the spark plug 100 are shown. Hereinafter, the direction parallel to the center axis CL is referred to as the "axis CL direction" or simply as the "axial direction" or "front-to-back direction." A radial direction of a circle centered on the center axis CL is referred to as the "radial direction." The radial direction is perpendicular to the axial direction CL. A circumferential direction of the circle centered on the center axis CL is referred to as the "circumferential direction." In the direction parallel to the center axis CL, the downward direction is referred to as Fig. 1 is referred to as the front end direction Df or forward direction Df and represents an upward direction in Fig. 1 is referred to as the rear end direction Dfr or reverse direction Dfr. The front end direction Df is a direction from a metal terminal 40 described below to a center electrode 20 described below. Furthermore, the side of the front end direction Df is Fig. 1 is designated as the front side of the spark plug 100 and is the rear end side Dfr in Fig. 1 is referred to as the rear side of the spark plug 100.

[0016] The spark plug 100 comprises: a tubular insulator 10 having a through-hole 12 extending along the axis CL (also referred to as the "axial hole 12"); a center electrode 20 held at the front of the through-hole 12; a metal terminal 40 held at the rear of the through-hole 12; a resistor 73 arranged between the center electrode 20 and the metal terminal 40 in the through-hole 12; a conductive first sealing element 72 in contact with the center electrode 20 and the resistor 73 to electrically connect elements 20 and 73; and a conductive second sealing element 74 in contact with the resistor 73 and the metal terminal 40 to electrically connect elements 73 and 40. a tubular metal sleeve 50 fixed to the outer peripheral side of the insulator 10;and an earth electrode 30 having opposite ends, one end being connected to an annular, front end surface 55 of the metal sleeve 50 and the other end being arranged such that it is opposite the center electrode 20 via a gap g.

[0017] A large-diameter section 14 with a largest outer diameter is formed essentially at the axial center of the insulator 10. A rear stem section 13 is formed on the rear side relative to the large-diameter section 14. A front stem section 15 with an outer diameter smaller than that of the rear stem section 13 is formed on the front side relative to the large-diameter section 14. On the front side relative to the front stem section 15, a reduced-outer-diameter section 16 and a leg section 19 are formed in that order. The outer diameter of the reduced-outer-diameter section 16 gradually decreases in the forward direction Df. Near the reduced-outer-diameter section 16 (in the example of Fig. 1 of the front stem part 15) is formed a reduced inner diameter part 11 whose inner diameter gradually decreases in the forward direction Df. The insulator 10 is preferably formed with consideration of mechanical strength, thermal strength, and electrical strength. For example, the insulator 10 is formed by sintering alumina (although other insulating materials may also be used).

[0018] The center electrode 20 is a metal element located at one end on the forward-direction Df side of the through-hole 12 of the insulator 10. The center electrode 20 comprises a substantially cylindrical, rod-shaped section 28 and a first tip 29, which is connected to a front end of the rod-shaped section 28 (for example, by laser welding). The rod-shaped section 28 comprises a head section 24 on the reverse-direction Dfr side and an axial section 27, which is connected to the forward-direction Df side of the head section 24. The axial section 27 extends in the forward-direction Df parallel to the axis CL. A portion on the forward-direction Df side of the head section 24 is a flange section 23 with an outer diameter larger than the outer diameter of the axial section 27. a surface on the forward direction Df side of the flange part 23 is held by the reduced inner diameter part 11 of the insulator 10.The axial part 27 is connected to the forward-direction Df side of the flange part 23. The first tip 29 is connected to a front end of the axial part 27. The rod-shaped part 28 is an example of a base material to which the first tip 29 is connected.

[0019] The rod-shaped part 28 comprises an outer layer 21 and a core part 22, which is arranged on an inner circumferential side of the outer layer 21. The outer layer 21 is made of a material (e.g., an alloy containing nickel as a major component) that has greater oxidation resistance than the core part 22. A "major component" here refers to a component that constitutes the largest proportion (weight percentage). The core part 22 is made of a material (e.g., pure copper, an alloy containing copper as a major component, etc.) that has a higher thermal conductivity than the outer layer 21. The first tip 29 is made of a material (e.g., precious metals such as iridium (Ir) and platinum (Pt)) that has greater resistance to discharge than the axial part 27.In the center electrode 20, a portion on the forward direction Df side, including the first tip 29 on the forward direction Df side from the axial hole 12 to the insulator 10, is exposed. A portion 20t on the reverse direction Dfr side of the center electrode 20 is located in the axial hole 12. The portion 20t of the center electrode 20 is located at a front end portion 10t of the insulator 10. The front end portion 10t of the insulator 10 contains the front end of the insulator 10. The first tip 29 can also be omitted. And the core portion 22 can also be omitted.

[0020] The metal connector 40 is a beam-shaped element extending parallel to the axis CL. The metal connector 40 is made of a conductive material (e.g., a metal containing iron as a major component). The metal connector 40 comprises a cap mounting part 49, a flange part 48, and an axial part 41, arranged in that order in the forward direction Df. The axial part 41 is inserted into a portion on the reverse-direction Dfr side of the axial hole 12 of the insulator 10. The cap mounting part 49 is exposed to the outside of the axial hole 12 at the rear end of the insulator 10.

[0021] The resistor 73, used to reduce electrical noise, is located between the metal terminal 40 and the center electrode 20 in the axial hole 12 of the insulator 10. The resistor 73 is made of a conductive material (e.g., a mixture of glass, carbon particles, and ceramic particles). The first sealing element 72 is located between the resistor 73 and the center electrode 20. The second sealing element 74 is located between the resistor 73 and the metal terminal 40. Both sealing elements 72 and 74 are made of a conductive material (e.g., a mixture of metal particles and the same glass as in the resistor 73). The center electrode 20 is electrically connected to the metal terminal 40 via the first sealing element 72, the resistor 73, and the second sealing element 74.

[0022] The metal sleeve 50 is a tubular element with a through-hole 59 extending along the axis CL. In this embodiment, the central axis of the metal sleeve 50 coincides with the axis CL. The insulator 10 is inserted into the through-hole 59 of the metal sleeve 50, and the metal sleeve 50 is fixed to the outer circumference of the insulator 10. The metal sleeve 50 is made of a conductive material (e.g., a metal such as carbon steel containing iron as a major component). A portion on the forward-direction Df side of the insulator 10 is exposed outside the through-hole 59. A part on the rearward direction Dfr side of the insulator 10 is exposed to outside the through hole 59. The metal shell 50 includes a tool engagement part 51 and a front stem part 52. The tool engagement part 51 is a part into which a spark plug wrench (not shown) is engaged.The front stem section 52 comprises the front end surface 55 of the metal sleeve 50. A screw-in part 57 is formed on the outer circumferential surface of the front stem section 52 such that it can be screwed into a mounting hole of an internal combustion engine (not shown). The screw-in part 57 has an external thread extending in the direction of the axis CL.

[0023] On an outer peripheral surface between the tool engagement part 51 of the metal sleeve 50 and the front-side stem part 52, a flange-shaped central stem part 54 is formed, projecting radially outward. The outer diameter of the central stem part 54 is larger than the maximum outer diameter of the screw part 57 (i.e., the outer diameter of the crest of the screw). A surface 54f on the forward direction Df side of the central stem part 54 is a seating surface (hereinafter referred to as "seating surface 54f"). The seating surface 54f forms a seal with a mounting part (e.g., an engine head) of the internal combustion engine, in which a mounting hole is formed.

[0024] An annular gasket 9 is disposed between the screw portion 57 of the front stem portion 52 and the seating surface 54f of the center stem portion 54. The gasket 9 is crushed and deformed when the spark plug 100 is mounted on the internal combustion engine, and seals a gap between the seating surface 54f of the metal shell 50 and the mounting portion (e.g., the engine head) of the internal combustion engine (not shown). The gasket 9 may be omitted. In this case, the seating surface 54f of the metal shell 50 is in direct contact with the mounting portion of the internal combustion engine to seal the gap between the seating surface 54f and the mounting portion of the internal combustion engine.

[0025] The front stem portion 52 of the metal sleeve 50 has a projecting portion 56 that extends radially inwards. The projecting portion 56 has an inner diameter that is smaller than the inner diameter of at least one portion on the reverse Dfr side of the projecting portion 56. In this embodiment, the inner diameter of a surface 56r (also referred to as the rear surface 56r) on the reverse Dfr side of the projecting portion 56 is gradually reduced in the forward Df direction. A front seal 8 is arranged between the rear surface 56r of the projecting portion 56 and the reduced outer diameter portion 16 of the insulator 10. The front seal 8 is, for example, a plate-shaped ring made of iron (however, another material (e.g., a metal such as copper) can also be used).The protruding part 56 (specifically, a part of the protruding part 56 forming the rear surface 56r) indirectly holds the reduced outer diameter part 16 of the insulator 10 from the forward direction Df side via the gasket 8. The gasket 8 may be omitted. In this case, the protruding part 56 (specifically, the rear surface 56r of the protruding part 56) may be in contact with the reduced outer diameter part 16 of the insulator 10. That is, the protruding part 56 can directly hold the insulator 10. The protruding part 56 corresponds to a holding part that directly or indirectly holds the reduced outer diameter part 16 of the insulator 10.

[0026] On the rear side, relative to the tool gripping part 51 of the metal sleeve 50, a rear end part 53 is formed, which forms a rear end of the metal sleeve 50 and is thinner than the tool gripping part 51. Furthermore, a connecting part 58, which connects the central stem part 54 to the tool gripping part 51, is formed between the central stem part 54 and the tool gripping part 51. The connecting part 58 is thinner than both the central stem part 54 and the tool gripping part 51. Ring-shaped ring members 61 and 62 are arranged between an inner circumferential surface extending from the tool gripping part 51 of the metal sleeve 50 to the rear end part 53 and an outer circumferential surface of the rear stem part 13 of the insulator 10. The space between the ring members 61 and 62 is filled with talc powder 70.When the rear end part 53 of the spark plug 100 is bent inwards and crimped during manufacturing, the connecting part 58 deforms outwards when a force is applied. This results in the metal sleeve 50 and the insulator 10 being fixed together. In this embodiment, the connecting part 58 is curved so that it bends outwards in the radial direction (hereinafter, the connecting part 58 is also referred to as the curved part 58). The talc 70 is compressed during the crimping process, thereby increasing the airtight seal between the metal shell 50 and the insulator 10. The seal 8 is pressed between the reduced outer diameter part 16 of the insulator 10 and the projecting part 56 of the metal sleeve 50, sealing the gap between the metal sleeve 50 and the insulator 10.

[0027] The ground electrode 30 is a metal component comprising a beam-shaped main body 37. An end portion 33 (also referred to as the "base end portion 33") of the main body 37 is connected to the front end face 55 of the metal sleeve 50 (for example, by resistance welding). The main body 37 extends in the front end direction Df from the base end portion 33 connected to the metal sleeve 50, is bent towards the central axis CL, extends in a direction intersecting the axis CL, and reaches a front end portion 34. A second tip 300 is connected to a surface on the reverse direction Dfr side of the front end portion 34 (for example, by resistance welding). The second tip 300 of the ground electrode 30 and the first tip 29 of the central electrode 20 form a gap g.This means that the second tip 300 of the earth electrode 30 is arranged on the forward direction Df side with respect to the first tip 29 of the center electrode 20 and is opposite the first tip 29 across the gap g.

[0028] The main body 37 comprises an outer layer 31 and an inner layer 32, which are arranged on an inner circumferential side of the outer layer 31. The outer layer 31 is made of a material (e.g., an alloy containing nickel as a major component) that has greater oxidation resistance than the inner layer 32. The inner layer 32 is made of a material (e.g., pure copper, an alloy containing copper as a major component, etc.) that has a higher thermal conductivity than the outer layer 31. The inner layer 32 can also be omitted.

[0029] The second tip 300 is connected to the outer layer 31 of the main body 37. The outer layer 31 is an example of a base material to which the second tip 300 is connected. A-2. Configuration of the earth electrode 30:

[0030] Fig. Figure 2 is a schematic view showing the configuration of the earth electrode 30. Fig. Figure 2 shows a cross-section including parts of the center electrode 20 and the ground electrode 30, which form the space g. In particular, it shows Fig. 2 a part on the forward direction Df side of the first tip 29 of the center electrode 20 and a part of the ground electrode 30, which includes the second tip 300 and the front end part 34 of the main body 37. This cross-section includes the axis CL. In Fig. 2 is a first direction D1, a direction in which a part, including the front end part 34 of the main body 37 of the earth electrode 30, extends. The first direction D1 extends from the outer circumferential side to the inner circumferential side. A second direction D2 is a direction opposite to the first direction D1.

[0031] A recess 400, extending in the forward direction Df, is formed on a portion of the front end 34 of the main body 37 of the ground electrode 30, on the reverse direction Dfr side. The second tip 300 fits into the recess 400. In this embodiment, the recess 400 has a substantially cylindrical shape around the axis CL. The second tip 300 also has a substantially cylindrical shape around the axis CL. The axis CL is also a central axis of the second tip 300. A surface 310 on the reverse direction Dfr side of the second tip 300 faces a surface 210 on the forward direction Df side of the first tip 29 of the center electrode 20. These surfaces 210 and 310 form the space g. A discharge occurs between these surfaces 210 and 310. In the following, areas 210 and 310 will also be referred to as discharge areas 210 and 310.In this embodiment, a cross-section containing the axis CL of the second tip 300 is a cross-section perpendicular to the discharge surface 310 of the second tip 300.

[0032] A surface 320 on the forward-direction Df side of the second tip 300 is connected to a base surface 420 on the forward-direction Df side of the recess 400. In this embodiment, the second tip 300 is connected to the main body 37 by resistance welding. The surface 320 of the second tip 300 opposite the discharge surface 310 is connected to the main body 37 (hereinafter, the surface 320 is also referred to as the "opposite surface 320"). A gap S34 is formed between a side surface 330 of the second tip 300 and a side surface 430 of the recess 400. This means that the outer diameter of the second tip 300 is slightly smaller than the inner diameter of the recess 400, so that the second tip 300 can easily fit into the recess 400. In this embodiment, the outer diameter of the second tip 300 is substantially equal to the inner diameter of the recess 400, so that the gap S34 is small.During resistance welding, a force in the forward direction Df is exerted on the second tip 300, causing the surface 320 of the second tip 300 to be pressed against the bottom surface 420 of the recess 400. Thus, the surface 320 of the second tip 300 is welded to the bottom surface 420 of the recess 400. A portion 340 on the forward-direction Df side of the second tip 300 can extend radially outwards. The side surface 330 of this portion 340 can be connected to the side surface 430 of the recess 400. The recess 400 is provided in the outer layer 31 of the main body 37. The second tip 300 is connected to the outer layer 31. The outer diameter of the second tip 300 can be equal to or greater than the inner diameter of the recess 400. In this case, the second tip 300 can be pressed into the recess 400 and then welded to the outer layer 31. A-3: Assessment test:

[0033] Fig. Figure 3 is a table showing configurations of samples of the spark plug 100 in association with test results. In this table, sample numbers, configurations of the second tip 300, test results, and summary evaluation results are associated with each other. Regarding the configuration of the second tip 300, the composition (the unit is weight percent), the bonding area Sz (the unit is mm 2 ), the grain size (the unit is µm) and the hardness ratio Hb / Ha.

[0034] Regarding the composition, the weight percentages of platinum (Pt), rhodium (Rh), rhenium (Re), tungsten (W), ruthenium (Ru), iridium (Ir), and nickel (Ni) are given. Any blank space indicates 0 weight percent. The second tip 300 of each sample is formed from one or more components selected from Pt, Rh, Re, W, Ru, Ir, and Ni. In particular, the second tips 300 of samples No. 3 through No. 32 contain Pt as a major component.

[0035] The composition of the second tip 300 (specifically the weight percent of the components) was determined as follows. The cross-section of the second tip 300 was mirror-polished, and the mirror-polished cross-section was analyzed by wavelength-dispersive X-ray spectrometry (WDS, accelerating voltage of 20 kV, spot diameter of 10 mm) using an electron probe microanalyzer (EPMA; JXA-8500F from JEOL Ltd.) to measure the weight composition.

[0036] Fig. 4 is a schematic view explaining measurement positions P1. In Fig. 4 shows a cross-section of the second tip 300. This cross-section contains the center axis CL of the second tip 300. In Fig. Figure 4 shows two reference lines Lp and Lq in the cross-section of the second tip 300. In this embodiment, the discharge area 310 is defined by an almost straight line in the cross-section of Fig. Figure 4 shows that each of the reference lines Lp and Lq is a straight line parallel to the discharge surface 310 of the second tip 300. The two reference lines Lp and Lq are arranged in sequence from the discharge surface 310 to the interior of the second tip 300 (in the forward direction Df). The first reference line Lp is a distance dp from the discharge surface 310, and the second reference line Lq is a distance dq from the first reference line Lp.

[0037] A plurality of measurement positions P1 are arranged on the reference lines Lp and Lq. Specifically, the plurality of measurement positions P1 are arranged at equal intervals di on the reference lines Lp and Lq with respect to a position P1 on the axis CL of the second tip 300. The composition of Fig. 3 is an arithmetic mean of the measured values ​​obtained at the plurality of measurement positions P1. In this test, the first distance dp is 0.05 mm, the second distance dq is 0.1 mm, and the interval di is 0.1 mm.

[0038] The connecting surface Sz ( Fig. 2, Fig. 3) is a connecting surface between the opposite surface 320 of the second tip 300 and the main body 37. As described above, in addition to the opposite surface 320 of the second tip 300, a portion of the side surface 330 can also be connected to the main body 37. The side surface 330 is excluded from the connecting surface Sz. That is, the connecting surface Sz is a surface of a portion of the opposite surface 320 connected to the main body 37, which is a surface on a side of the second tip opposite the discharge surface 310.

[0039] As described above, the second tip 300 of each sample has a substantially cylindrical shape around the axis CL. The connection area Sz can be determined based on the radius of the second tip 300 (Sz = π × radius). 2The radius of the second tip 300 can be calculated using the cross-section with the central axis CL of the second tip 300 contained therein.

[0040] The grain size Dz ( Fig. 3) is an average grain size of crystal grains at the cross-section of the second tip 300 (hereinafter also referred to as "average grain size Dz"). The grain size Dz is calculated using the number of crystal grains detected, which are specified based on JIS G0551 (2013).

[0041] Fig. 5A and Fig. Figure 5B shows schematic views illustrating a method for calculating the grain size Dz. Fig. Figure 5A shows a cross-section of the second tip 300. The cross-section contains the central axis CL of the second tip 300 and is perpendicular to the discharge surface 310. Fig. Figure 5A shows three test lines La, Lb, and Lc in the cross-section of the second tip 300. Each of the test lines La, Lb, and Lc is a straight line parallel to the discharge surface 310 of the second tip 300. The three test lines La, Lb, and Lc are arranged at equal intervals dk from the discharge surface 310 to the interior of the second tip 300 (in the forward direction Df). The first line La is a straight line located a distance dk from the discharge surface 310. The second line Lb is a straight line located a second distance dk from the first line La. The third line Lc is a straight line located a distance dk from the second line Lb. Each of the test lines La, Lb, and Lc extends from the side surface 330 on one side of the second tip 300 to the side surface 330 on the opposite side. Fig. 5A, the lengths of the test lines La, Lb and Lc are specified by Xa, Xb and Xc respectively.

[0042] Fig. 5B is an enlarged view of part Ps of the cross-section of Fig. 5A. This part Ps contains a part where the first line La is in contact with the side surface 330 of the second tip 300. Fig. Figure 5B schematically shows crystal particles of the metal (e.g., an alloy) of the second tip 300.

[0043] In Fig. 5B, the detected crystal grains, i.e., the crystal grains detected by the first line La, are represented by hatching. The detected crystal grains are crystal grains in contact with the first line La and consist of three types of crystal grains: Ga, Gb, and Gc. The first type of crystal grain Ga is a crystal grain through which the first line La passes. The second type of crystal grain Gb is a crystal grain in which the first line La ends. This means that the second type of crystal grain Gb contains an end Lae of the first line La. As shown in Fig. As shown in Figure 5B, the end Lae of the first line La is located at side face 330. The second type of crystal grain, Gb, includes a portion (i.e., the end Lae) where side face 330 is in contact with the first line La. The third type of crystal grain, Gc, is a crystal grain in which the first line La is in contact with a grain boundary. The crystal grains captured by the second line Lb and those captured by the third line Lc are determined similarly.

[0044] To calculate the grain size Dz, the numbers Na, Nb, and Nc of the crystal grains detected by the test lines La, Lb, and Lc are used. When counting the number of detected crystal grains, the numbers described below, which are previously determined according to the number of intersections of the test lines and the crystal grains, are applied to the corresponding crystal grains Ga, Gb, and Gc. Thus, for the first-type crystal grain Ga, a number of "1" is applied to one crystal grain. For the second-type crystal grain Gb and the third-type crystal grain Gc, a number of "0.5" is applied to one crystal grain. For example, one crystal grain Ga of the first type is counted as one crystal grain, one crystal grain Gb of the second type is counted as 0.5 crystal grains, and one crystal grain Gb of the third type is counted as 0.5 crystal grains.Based on these numbers, the numbers Na, Nb and Nc of the crystal grains are each calculated using the test lines La, Lb and Lc.

[0045] The grain size Dz is calculated according to the following formula: Dz=(Xa+Xb+Xc) / (Na+Nb+Nc)

[0046] The grain size Dz thus represents the average grain size of the multitude of crystal grains detected by the three test lines La, Lb, and Lc. The cross-section of the second tip 300 is mirror-polished to calculate the grain size Dz. An image showing the grain structure in the cross-section is acquired using a metallograph or a scanning electron microscope (SEM). The acquired image is then analyzed to calculate the grain size Dz. In this test, the distance dk is 0.05 mm.

[0047] In the evaluation test of Fig. The grain size Dz was determined using a metallograph. A metallograph can determine grain sizes no smaller than 50 µm. For samples 1 to 20 and 27 to 32, the grain size of the crystal particles is smaller than 50 µm and cannot be determined with a metallograph. Therefore, the grain size Dz is less than 50 µm in samples 1 to 20 and 27 to 32. Furthermore, a scanning electron microscope (SEM) can determine grain sizes (i.e., grain size Dz) no smaller than 50 µm. However, this evaluation test does not include determining the grain size Dz using an SEM.

[0048] The hardness ratio Hb / Ha ( Fig. 3) was determined as follows. At each of the plurality of measurement positions P1 in the cross section of the second tip 300 of Fig. 4. The Vickers hardness was measured using a Vickers hardness tester. The load was set to 200 gf and the holding time to 10 seconds. Then, an arithmetic mean of a multitude of measurements at the multitude of measurement positions P1 was used as the Vickers hardness of the corresponding sample. The hardness Ha is a hardness obtained after a heat treatment in which the sample is held at 1200°C in an argon atmosphere for 10 hours. The cross-section of the second tip 300 of the sample after heat treatment was exposed by polishing. The hardness Ha was then measured using the exposed cross-section. The hardness Hb is a hardness before heat treatment. The cross-section of the second tip 300 of the sample, which was not subjected to any heat treatment, was exposed by polishing. The hardness Hb was then measured using the exposed cross-section.In the following, the hardness Ha is also referred to as the post-heating hardness Ha, and the hardness Hb is referred to as the pre-heating hardness Hb. The hardness ratio is the ratio of the pre-heating hardness Hb to the post-heating hardness Ha.

[0049] As the temperature of the sample (especially tip 300) increases, the metal crystals of the second tip 300 grow and increase in size. As the crystals enlarge, their hardness decreases, and tip deformation becomes more likely. Typically, the post-heating hardness Ha after heat treatment is lower than the pre-heating hardness Hb. This means that the hardness ratio Hb / Ha is greater than 1. When the internal combustion engine is running, the spark plug 100 experiences repeated temperature rises due to fuel combustion and cooling due to air intake. If the hardness ratio Hb / Ha is low, even with repeated temperature rises and cooling cycles, variation in the hardness of the second tip 300 is suppressed, thus preventing deformation of the second tip 300.As a result, a variation in the distance of the gap g is suppressed.

[0050] The wear resistance ( Fig. 3) corresponds to the evaluation result of the resistance of every second tip 300 to wear due to spark discharge. The evaluation method for wear resistance is as follows. An engine with an exhaust turbine-type supercharging device was prepared. This engine is a four-cylinder direct injection engine and has a displacement of 2.0 L. Each spark plug sample 100 was mounted on the engine. The gap distance g of each sample was set to 0.75 mm. This engine was operated continuously for 300 hours under conditions of a speed of 4000 rpm, an air / fuel ratio of 12.0, and an average working pressure of 190 kPa (this operation is also called test operation). The gap distance g after the test operation was measured using a test pin. Then, the increase in the gap distance g during the test operation was calculated.The greater the magnification in the distance of the gap g, the greater the wear rate of the second tip 300. In the table of . Fig. 3 indicates a rating of “A” that the magnification at the distance of the space g is less than 0.15 m, and indicates a rating of “C” that the magnification at the distance of the space g is not less than 0.15 mm.

[0051] The peel resistance ( Fig. 3) corresponds to the evaluation result of the resistance to peeling of the second tip 300 from the main body 37. The following thermal test was performed to evaluate the peeling resistance. Specifically, a cycle of heating and cooling a part near the front end 34 of the main body 37 of the ground electrode 30 of each sample was repeated 1000 times. In a single cycle, the part near the front end 34 of the main body 37 was heated by a burner for two minutes and then cooled for one minute. The burner's heat output was adjusted such that the temperature of the front end 34 of the main body 37 reached 1000°C during the two-minute heating period.

[0052] Fig. Figure 6 is a schematic view showing the cross-section of the earth electrode 30 after the thermal test. This cross-section includes the axis CL of the second tip 300. Fig. Figure 6 shows a part containing the second tip 300. During the thermal test, the thermal expansion and contraction of the second tip 300 are repeated. As a result, the second tip 300 can peel away from the main body 37. In the example of Fig. 6, peeling occurs at an outer peripheral edge portion 500 between the opposing surface 320 of the second peak 300 and the bottom surface 420 of the recess 400.

[0053] In Fig. 6 is a length Du, a length of a connecting part between the opposite surface 320 and the base surface 420 in the cross-section. This length Du is a length of a connecting part that shows no peeling after the thermal test (hereinafter also referred to as the "post-test length Du"). The post-test length Du was measured as follows. After the thermal test, the earth electrode 30 was embedded in a synthetic resin. By polishing the earth electrode 30 embedded in the synthetic resin, a cross-section of the earth electrode 30 was exposed. The exposed cross-section contains the axis CL of the second tip 300. The exposed cross-section was then viewed under a microscope to measure the post-test length Du.

[0054] Accordingly, a Fig. The length Dt shown in Figure 2 represents a length of the connecting portion between the opposing surface 320 and the bottom surface 420 in the cross section. This length Dt corresponds to the length of the connecting portion before the thermal test (hereinafter also referred to as "pre-test length Dt"). The method for measuring the pre-test length Dt is the same as the method for measuring the post-test length Du. Thus, the ground electrode 30 of each sample not subjected to the thermal test was embedded in a resin. By polishing the ground electrode 30 embedded in the resin, a cross section of the ground electrode 30 was exposed. Then, the exposed cross section was observed with a microscope to measure the pre-test length Dt.

[0055] In this embodiment, the connecting element between surfaces 320 and 420 has a straight shape perpendicular to the axis CL in the cross-section. However, the connecting element in the cross-section can also have other shapes. In any case, a length in the direction perpendicular to the axis CL of the second point 300 can be used for the lengths Du and Dt.

[0056] In general, the post-test length Du is likely to be shorter than the pre-test length Dt. Regarding peel resistance, the degree of reduction in the post-test length Du was evaluated using a rating X as follows. The rating X is calculated according to the following formula: X=(Dt−Du) / Dt

[0057] Generally, the score X is neither less than 0 nor greater than 1. The smaller the score X, the longer the post-test length (Du) and, consequently, the smaller the exfoliated portion. (See table of...) Fig. 3 indicates a rating “A” that the rating value X is not greater than 0.5, and indicates a rating “C” that the rating value X is greater than 0.5.

[0058] The tip crack formation ( Fig. 3) corresponds to the evaluation result of the resistance of the second tip 300 to fine cracks that are generated in the second tip 300. One method for evaluating tip crack formation is as follows. After the aforementioned thermal test, the earth electrode 30 was embedded in synthetic resin. By polishing the earth electrode 30 embedded in the synthetic resin, a cross-section of the earth electrode 30 was exposed. The exposed cross-section contains the axis CL of the second tip 300.

[0059] Fig. Figures 7A to 7D are schematic views showing examples of cross-sections of the second tip 300 after the thermal test. Fig. Figure 7A shows an example of a cross-section without a crack. Fig. Figures 7B to 7D each show an example of a cross-section with cracks 390. Fig. 7B to 7D, the second peak 300 includes thin cracks 390 extending inward from the discharge surface 310. Such thin cracks 390 may be caused along grain boundaries of crystal grains of the metal. The size (area) of the cracks 390 is smaller in the order of Fig. 7B, Fig. 7C and Fig. 7D. In the example of Fig. 7D, which has the largest cracks 390, the cracks 390 are larger and thicker than those of the examples from Fig. 7B and Fig. 7C. Furthermore, in the example of Fig. 7D chip formation occurs in the areas 395 in contact with the multitude of cracks 390 (hereinafter also referred to as "chip formation areas 395"). In the chip formation areas 395, the metal of the second tip 300 is flake off. Such chip formation areas 395 are caused by polishing the second tip 300. If the second tip 300 contains a multitude of cracks, the metal flakes off in the areas where the multitude of cracks are formed, resulting in the chip formation areas 395. Various types of cracks can form, such as cracks extending inwards from the side surface 330, but these are not shown here.

[0060] When evaluating tip cracking, the ratio of a crack area to the cross-sectional area of ​​the second tip 300 (hereinafter also referred to as the "crack area percentage") was assessed. The cross-sectional area of ​​the second tip 300 includes the crack areas 390 and the chip formation areas 395. The ground electrode 30 may include a connected portion between the second tip 300 and the main body 37. The connected portion is a part between the second tip 300 and the main body 37 that was melted during welding and then cooled and solidified (hereinafter also referred to as the "melted portion"). The area of ​​the molten portion is excluded from the cross-sectional area of ​​the second tip 300. The crack area is an area in which the metal of the second tip 300 has been lost in the polished cross-section.The crack area includes the area of ​​the chip formation regions 395 in addition to the area of ​​the region indicating the cracks 390. If the chip formation regions 395 are caused by polishing, a multitude of cracks are formed in areas corresponding to the chip formation regions 395 of the second tip 300 before polishing. Therefore, the crack formation area, including the area of ​​the chip formation regions 395, can be used as an approximate index indicating the size of cracks in the second tip 300. The cross-sectional area of ​​the second tip 300 and the crack area are determined by microscopic observation. In the table of . Fig. 3 indicates a rating “A” that the crack area percentage is less than 1%, a rating “B” indicates that the crack area percentage is not less than 1% and less than 10%, and a rating “C” indicates that the crack area percentage is not less than 10%.

[0061] The deformation ( Fig. 3) indicates the evaluation result of resistance to deformation caused by a temperature increase of the second tip 300. The thermal test mentioned above was performed to evaluate the deformation. Fig. Figure 8 shows the cross-section of the earth electrode 30 after the thermal test. This cross-section includes the axis CL of the second tip 300, and Fig. 8 shows a part containing the second tip 300. During the thermal test, the thermal expansion and thermal contraction of the second tip 300 are repeated. A mechanical stress caused by a repetition of the thermal expansion and thermal contraction can deform the second tip 300. The stress indicated by a dashed line in Fig. The second peak 300 indicated in Figure 8 is the second peak 300 before the thermal test, and the second peak 300 indicated by a solid line is the second peak 300 after the thermal test. Fig. In Figure 8, the deformation of the second tip 300 due to the thermal test is shown enlarged for clarity. As shown in Fig. 8, the second tip 300 can be deformed such that the corners are rounded. Such a deformation of the second tip 300 causes the distance of the gap g ( Fig. 1 and Fig. 2) varies. To prevent variation in the distance of the gap g, the deformation of the second tip 300 should be small.

[0062] The deformation was evaluated using the change in the protrusion length of the second tip 300 from a surface 37r on the back-direction Dfr side of the main body 37. Fig. Figure 8 specifies the protrusion length Da, the protrusion length of the second tip 300 before the thermal test (hereinafter also referred to as the "pre-test protrusion length Da"). The protrusion length Db specifies the protrusion length of the second tip 300 after the thermal test (hereinafter also referred to as the "post-test protrusion length Db"). These protrusion lengths Da and Db are lengths in the direction parallel to the axis CL of the second tip 300. A cross-section for measuring the pre-test protrusion length Da and a cross-section for measuring the post-test protrusion length Db were prepared by embedding the ground electrode 30 in a resin, as in the case of the cross-section for the pre-test length Dt and the cross-section for the post-test length Du described above, and the protrusion lengths Da and Db were calculated based on the dimensions of the cross-sectional images. Then, the deformation was evaluated using a deformation quantity Dd (= Db-Da), which is a difference between the protrusion lengths.In the table of . Fig. 3 indicates a rating “A” that the deformation size Dd is less than 0.03 mm, a rating “B” indicates that the deformation size Dd is not less than 0.03 mm and not greater than 0.05 mm, and a rating “C” indicates that the deformation size Dd exceeds 0.05 mm.

[0063] The summary assessment result ( Fig. 3) indicates the result obtained by combining the four test results. A rating of "A" indicates that all four test results were rated "A". A rating of "B" indicates that either "tip cracking" or "deformation" was rated "B", while the remaining three test results were rated "A". A rating of "C" indicates that "tip cracking" and "deformation" were rated "B", while the remaining two test results were rated "A". A rating of "D" indicates that "wear resistance" and / or "peel resistance" were rated "C".

[0064] In samples No. 1 and No. 2, the second tip 300 did not contain Pt, and the wear resistance was rated "C". In the samples containing Pt as a major component (especially No. 6 and No. 32), the wear resistance was rated "A". Therefore, when the second tip 300 contained Pt as a major component, the wear resistance of the second tip 300 was improved.

[0065] For samples No. 1 to No. 3 and No. 5 to No. 8, the peel resistance was rated "C". For samples No. 4 and No. 9 to No. 32, the peel resistance was rated "A". A key difference between these two groups is the proportion of Ni in the second tip 300. In samples No. 1 to No. 3 and No. 5 to No. 8, the Ni content is less than 5% by weight. In samples No. 4 and No. 9 to No. 32, the Ni content is not less than 5% by weight. As described above, the second tip 300 is bonded to the outer layer 31 of the main body 37. The outer layer 31 contains Ni as a major component. Therefore, if the proportion of Ni in the second peak 300 is high, the affinity between the second peak 300 and the outer layer 31 of the main body 37 is improved compared to a case where the proportion of Ni in the second peak 300 is low.As a result, the peeling resistance of the second tip 300 from the main body 37 is improved. Specifically, when the Ni content is not less than 5 wt% (No. 4, No. 9 to No. 32), the peeling resistance under a severe condition such as the above-mentioned thermal test is improved compared to a case where the Ni content is less than 5 wt% (No. 1 to No. 3, No. 5 to No. 8).

[0066] The proportions of nickel (Ni) in samples No. 4 and No. 9 through No. 32, which was rated "A" for peel resistance, are 5, 10, and 25 wt%, respectively. A preferred range for the proportion of Ni can be determined using these three values. In particular, any one of these three values ​​can be used as a lower limit for the preferred range of the proportion of Ni. For example, the proportion of Ni can be 5 wt% or more. Any one of the three values ​​not less than the lower limit can be used as an upper limit for the preferred range of the proportion of Ni. For example, the proportion of Ni can be 20 wt% or less. It is expected that at a higher proportion of Ni, the affinity between the second tip 300 and the main body 37 will be greater. Therefore, the proportion of Ni can be greater than 20 wt%.

[0067] For samples No. 1 to No. 4, the wear resistance was rated "C". For the other samples (especially No. 6 to No. 32), the wear resistance was rated "A". A key difference between these two groups is that the contact area Sz of the second tip 300 is different. The contact areas Sz in samples No. 1 to No. 4 are less than 0.6 mm. 2 , while the connection surfaces Sz in samples No. 6 to No. 32 are not smaller than 0.6 mm 2 If the contact area Sz is large, heat is conducted more efficiently from the second tip 300 to the main body 37 than if the contact area Sz is small. This suppresses a temperature increase in the second tip 300. Therefore, it is assumed that wear on the second tip 300 is prevented when the contact area Sz is large.

[0068] The connection surfaces Sz in samples No. 6 to No. 32, which were rated “A” for wear resistance, are 0.6, 1 and 2 (mm²) respectively. 2 ) large. A preferred range of the bonding area Sz can be determined using these three values. In particular, any of the three values ​​can be used as a lower limit of the preferred range of the bonding area Sz. For example, the bonding area Sz can be 0.6 mm 2 or larger. In addition, any of the three values ​​that is not less than the lower limit can be used as an upper limit of the preferred range of the joint area Sz. For example, the joint area Sz can be 2 mm. 2 or smaller. The larger the connection area Sz is, the more the temperature rise in the second tip 300 is suppressed. Therefore, the connection area Sz can also be larger than 2 mm 2 be.

[0069] Various methods can be used to adjust the joint surface Sz. For example, the joint surface Sz can be adjusted by adjusting the outer diameter of the second tip 300.

[0070] In sample no. 5, the connection surface Sz was 0.6 mm. 2 large and the wear resistance was rated as "C". For samples No. 6 to No. 16 and No. 29 to No. 32, the joint area Sz was equal to that of sample No. 5, i.e., 0.6 mm 2large, and the wear resistance was rated "A". A major difference between Sample No. 5 and Samples No. 6 to No. 16 and No. 29 to No. 32 is that the total content of components other than Pt and Ni is different. In Sample No. 5, in the composition of the second tip 300, Rh is used as another component in addition to Pt and Ni, and the content of Rh is 5 wt%. In Samples No. 6 to No. 16 and No. 29 to No. 32, in the composition of the second tip 300, one or more of Rh, Re, W, and Ru are used as other components in addition to Pt and Ni, and the content of these components is not less than 10 wt%. Therefore, by optimizing the components other than Pt and Ni within the components in the second tip 300 and optimizing the total content of the components, the wear resistance of the second tip 300 is improved.

[0071] Specifically, the second tips 300 of samples Nos. 9 to 12 each contain Rh, Re, W, and Ru (and their proportion is 10% by weight). Each of these four samples achieved a summary evaluation result of "B" (specifically, wear resistance was rated "A," peeling resistance was rated "A," and tip cracking was rated "A"). Thus, each of the Rh, Re, W, and Ru components improved the wear resistance, peeling resistance, and cracking resistance of the second tip 300.

[0072] Furthermore, the second tip 300 of each of samples No. 29 to No. 32 contains any two of the components Rh, Re, W, and Ru, and the total proportion of these components is not less than 10% by weight. The combination of these two components differs between samples No. 29 and No. 32. Each of the four sample types achieved the overall evaluation result rated "B" (specifically, wear resistance, peel resistance, and cracking resistance were rated "A"). Thus, when the second tip 300 contained two components from the group Rh, Re, W, and Ru, and the total proportion of these components was not less than 10% by weight, the wear resistance, peel resistance, and cracking resistance of the second tip 300 were improved.

[0073] Based on the test results of samples Nos. 9 to 12 and Nos. 29 to 32, it is assumed that one or more types of specific elements, optionally selected from the group of Rh, Re, W, and Ru, can improve the wear resistance, peel resistance, and crack resistance of the second tip 300. In particular, it is assumed that the performance of the second tip 300 will be improved if the second tip 300 contains one or more types of specific elements and the total content of these elements is not less than 10% by weight.For example, the second tip 300 may contain: two types of elements optionally selected from the group of Rh, Re, W, and Ru, the total content of which is not less than 10 weight percent; three types of elements, the total content of which is not less than 10 weight percent; or four types of elements, the total content of which is not less than 10 weight percent.

[0074] For samples No. 21 to No. 24, tip cracking was rated "B." For the other samples (especially No. 9 to No. 20, No. 25 to No. 32), tip cracking was rated "A." A key difference between these two groups is the difference in their average grain size Dz. The average grain size Dz of samples No. 21 to No. 24 is 200 µm, while the average grain size Dz of samples No. 9 to No. 20 and No. 25 to No. 32 is no larger than 150 µm.

[0075] Generally, a crack in a metal propagates along a grain boundary between crystal grains. Furthermore, the direction in which a grain boundary propagates varies depending on the position where the grain boundary diverges. Therefore, a crack propagating along the grain boundary is likely to stop at a position where the grain boundary diverges. For example, if there is a first crystal grain, a second crystal grain, and a third crystal grain, and a crack occurs at a grain boundary between the first and second crystal grains, this crack is likely to stop at a position in contact with the third crystal grain.

[0076] If, as described above, a crack occurs at a grain boundary, the size of the crack can be as large as a crystal grain. A small crack is likely to occur at each of the grain boundaries of a multitude of crystal grains. If a multitude of small cracks from a multitude of crystal grains are linked together, a large crack can be formed. If the average grain size Dz is small, the size of a crack corresponding to a crystal grain is small, and therefore the size of a crack formed by a multitude of linked cracks is likely to be large. Therefore, cracks in the second peak 300 are thought to be suppressed when the average grain size Dz is small.

[0077] The average grain sizes in samples No. 9 to No. 20 and No. 25 to No. 32, which were rated "A" for peak cracking, were 150 µm or less than 50 µm. A preferred range of average grain size Dz can be determined using these two values. In particular, one of the two values ​​can be used as an upper limit of the preferred range of average grain size Dz. For example, the average grain size Dz can be 150 µm or less. The smaller the average grain size Dz, the more second-peak cracking 300 is suppressed. Therefore, the average grain size Dz can be any value less than 50 µm.

[0078] Tip cracking is evaluated under the severe thermal test mentioned above. The operating conditions for spark plug 100 in an actual internal combustion engine may be less stringent compared to the severe conditions of the aforementioned thermal test. In this case, the average grain size Dz may lie outside the preferred range mentioned above. For example, the average grain size Dz may be one of several values, not exceeding 200 µm. The average grain size Dz may also be greater than 200 µm.

[0079] Various methods can be used to adjust the grain size Dz. During the manufacturing of the second tip 300, a heat treatment can be applied to the second tip 300. Such heat treatment of the second tip 300 causes the crystal grains in the second tip 300 to grow, resulting in an increased grain size Dz. The grain size Dz can be kept small, for example, by reducing the heat treatment time during the manufacturing of the second tip 300, keeping the temperature of the second tip 300 low, or omitting the heat treatment.

[0080] For samples No. 19 to No. 22 and No. 25 to No. 28, the assessment result for deformation was "A". Furthermore, for samples No. 9 to No. 18, No. 23, No. 24, and No. 29 to No. 32, the assessment result for deformation was "B" or "C". A key difference between these two groups lies in the differing Hb / Ha hardness ratios. The Hb / Ha hardness ratios of samples No. 19 to No. 22 and No. 25 to No. 28 were no greater than 2.3, while the Hb / Ha hardness ratios of samples No. 9 to No. 18, No. 23, No. 24, and No. 29 to No. 32 were 2.5. If, as described above, the hardness ratio Hb / Ha is small, a variation in the hardness of the second tip 300 is suppressed even if a temperature rise and cooling of the spark plug 100 is repeated, thereby suppressing deformation of the second tip 300. This is especially true if the hardness ratio Hb / Ha was not greater than 2.3 (Nos. 19 to 22, Nos. 25 to 300).28), deformation of the second tip 300 was suppressed under the severe condition such as the thermal test mentioned above, compared to a case where the hardness ratio Hb / Ha was greater than 2.3 (Nos. 9 to 18, 23, 24, 29 to 32).

[0081] The Hb / Ha hardness ratios of samples Nos. 19 to 22 and Nos. 25 to 28, which were rated "A" for deformation, were 2.1 or 2.3. A preferred range of Hb / Ha hardness ratios can be determined using these two values. In particular, one of the two values ​​can be used as an upper limit of the preferred range of Hb / Ha hardness ratios. For example, the Hb / Ha hardness ratio can be 2.3 or less.

[0082] Furthermore, one of these values, not greater than the upper value, can be used as a lower limit of the preferred range for the Hb / Ha hardness ratio. For example, the Hb / Ha hardness ratio can be 2.1 or greater. The smaller the Hb / Ha hardness ratio, the more effectively variation in the hardness of the second tip 300 is suppressed. In this way, deformation of the second tip 300 is suppressed. Therefore, the Hb / Ha hardness ratio can be one of several values ​​less than 2.1. Usually, the Hb / Ha hardness ratio is not less than 1.

[0083] The deformation of the second tip 300 is evaluated under the aforementioned severe thermal test. The operating conditions for the spark plug 100 in an actual internal combustion engine may be less stringent than the severe conditions of the aforementioned thermal test. In this case, the hardness ratio Hb / Ha may lie outside the aforementioned preferred range. For example, the hardness ratio Hb / Ha may be greater than 2.3. The hardness ratio Hb / Ha may be one of several values, not greater than 2.5, or it may be greater than 2.5.

[0084] Various methods can be used to adjust the hardness ratio Hb / Ha. During the manufacturing of the second tip 300, a heat treatment may be applied to the second tip 300. Such heat treatment of the second tip 300 causes the crystal grains in the second tip 300 to grow. If the crystal grains in the second tip 300 have previously grown due to heating of the second tip 300, further growth of the crystal grains is suppressed during use of the spark plug 100. Thus, a small hardness ratio Hb / Ha is realized, and deformation of the second tip 300 during use of the spark plug 100 can be suppressed.

[0085] When the number of components contained in the second tip 300 is large, crystal grain growth is less likely than when the number of components is small. Therefore, when the second tip 300 contains a larger number of elements from Pt, Rh, Re, W, Ru, and Ni, crystal grain growth is suppressed during use of the spark plug 100. Thus, a small hardness ratio Hb / Ha is realized, and deformation of the second tip 300 during use of the spark plug 100 is suppressed.

[0086] Generally, the higher the Ni content in the second peak 300, the harder the second peak 300. For example, the second peak 300 is harder when the Ni content is 10 weight percent than when the Ni content is 5 weight percent. Therefore, it is assumed that increasing the Ni content will achieve a small Hb / Ha hardness ratio.

[0087] To suppress deformation of the second tip 300, the hardness ratio Hb / Ha is preferably low. One method for reducing the hardness ratio Hb / Ha is, for example, to induce prior growth of crystal grains in the second tip 300 by heating it during its manufacture. Furthermore, to suppress tip cracking, the average grain size Dz, as described above, is preferably low. To reduce the average grain size Dz, a temperature increase of the second tip 300 during its manufacture is preferably suppressed.If heat treatment is applied to the second tip 300 during its manufacture, a treatment condition is preferably determined taking into account a balance between suppressing deformation of the second tip 300 and suppressing tip cracking in the second tip 300. For example, conditions such as the time profile for starting the heating, the duration of the heating, the temperature of the second tip 300 during heating, etc., can be determined experimentally. B. Modifications

[0088] (1) Regarding the configuration of the second tip 300, various other configurations can be used instead of the configuration described above. For example, the discharge surface 310 ( Fig. 2) not be perpendicular to the axis CL of the second tip 300, but can be inclined diagonally with respect to the axis CL. Furthermore, the shape of the second tip 300 is not limited to a cylindrical shape and various other shapes (e.g., a square rod shape) can be used. In any case, the average grain size Dz ( Fig. 5A and Fig. 5B) and the hardness ratio Hb / Ha ( Fig. 4) the second tip is determined using a cross-section perpendicular to the discharge surface of the second tip. Such a cross-section can be one that includes the central axis of the second tip (e.g., the central axis extends from the discharge surface to an opposite surface on one side facing away from the discharge surface).

[0089] If the second tip has a square, rod-like shape, the connecting area Sz can be calculated as follows. On the outer surface of the second tip, the length of a segment equal to the length of one side of the opposite surface of the second tip (i.e., the surface connected to the base material) is measured. For example, the length of one side of the discharge surface of the second tip is measured. Then, a cross-section of the second tip is obtained, passing through a midpoint of the measured length and perpendicular to that length. In this cross-section, the width of the second tip is measured in a direction parallel to the connected surface. The measured width of the second tip is equal to the length of another side of the connected surface, perpendicular to the side corresponding to the previously measured length.The connecting area Sz can be calculated by multiplying the lengths of the two sides specified in this way.

[0090] If the second tip is small, it can be difficult to see the three test lines La, Lb and Lc ( Fig. 5) to arrange them on the cross-section of the second tip. In this case, the three test lines La, Lb, and Lc can be arranged on the cross-section of the second tip with a reduced distance dk. Alternatively, the three test lines La, Lb, and Lc can be arranged with a reduced distance between the discharge surface (e.g., discharge surface 310) and the first line La. (2) The composition of the second tip 300 is not related to the compositions of the samples of Fig. 3, and various other compositions may also be used. For example, the second tip 300 may contain: Pt as a main component; not less than 5 weight percent of Ni; and a total of 10 weight percent or more of one or more elements optionally selected from the group consisting of Rh, Re, Ru, and W. The composition of the second tip 300 may be a composition containing iridium (Ir). (3) The configuration of the earth electrode 30 is not dependent on the configuration of Fig. The method is limited to 2, and various other configurations can also be used. For example, the recess 400 can be omitted, and the second tip 300 can be connected to the flat outer surface of the main body 37 (in this case, to the outer layer 31). The method for connecting the second tip 300 to the main body 37 (in this case, to the outer layer 31) is not limited to resistance welding, and other methods can also be used. For example, the second tip 300 can be connected to the outer layer 31 by laser welding. In general, the second tip 300 and the main body 37 can be connected to each other by various other types of welding. (4) The above-mentioned various configurations of the second tip 300 of the ground electrode 30 can be applied to the first tip 29 of the center electrode 20. For example, the first tip 29 may contain: Pt as a main component; not less than 5 wt% of Ni; and a total of 10 wt% or more of one or more elements optionally selected from the group consisting of Rh, Re, Ru, and W. (5) The configuration of spark plug 100 is not dependent on the configuration of Fig. 1, and various other configurations may also be used. For example, the front seal 8 may be omitted. In this case, the protruding part 56 of the metal sleeve 50 directly holds the reduced outer diameter part 16 of the insulator 10. In addition, the resistor 73 may be omitted. A magnetic body may be arranged between the center electrode 20 and the metal terminal 40 in the through-hole 12 of the insulator 10. The first tip 29 on the center electrode 20 may be omitted. The second tip of the ground electrode 30 may be omitted. Instead of the front end surface of the center electrode (e.g., a surface on the forward direction Df side of the first tip 29 of Fig.1) The side surface of the center electrode (a surface of the spark plug 10 in the direction perpendicular to the axis CL) and the ground electrode can form a gap for spark discharge. Therefore, the center axis of the tip of the ground electrode can differ from the center axis of the spark plug. The total number of gaps for spark discharge can be two or more. The ground electrode 30 can be omitted. In this case, a spark discharge can be caused between the center electrode 20 of the spark plug and another component in the combustion chamber.

[0091] The present invention has been described above with reference to embodiments and modified embodiments, the embodiments of which described herein being intended to clarify the present invention and not limiting it in any way. The embodiments described herein can be changed and modified in various ways without thereby departing from the scope of the invention as defined by the claims and their equivalents. [List of reference symbols] 8 front seal 9 Seal 10 Insulator 10t front end section 11 Reduced inner diameter part 12 Axial hole (through hole) 13 rear trunk part 14 large diameter part 15 front trunk part 16 Reduced outer diameter part 19 Leg part 20 center electrode 20t part 21 outer layer 22 Core part 23 Flange part 24 headboard 27 Axial part 28 rod-shaped part 29 first peak 30 Earth electrode 31 outer layer 32 inner layer 33 Base end part 34 front end part 37 Main body 37r area 40 Metal connection 41 Axial part 48 Flange part 49 Cap mounting part 50 metal sleeves 51 Tool engagement part 52 front stem part 53 rear end section 54 middle stem section 54f Seat area 55 front end surface 56 preceding part 56r rear surface 57 Screw part 58 curved part (connecting part) 59 Through hole 61 Ring link 70 Talk 72 first sealing part 73 Resistance 74 second sealing part 100 spark plugs 210 discharge area 300 second peak 310 discharge area 320 opposite area 330 side area 340 parts 395 Chip formation area 400 In-depth study 420 floor space 430 side area 500 edge part g space Df front end direction (forward direction) Dfr rear end direction (reverse direction) P1 measuring position CL center axis (axis) La to Lc test line Ga to Gc crystal grain Da, Db protrusion length Dd deformation size the interval dk distance Lp, Lq reference line dp distance dq distance Ps part Dt Pre-test length You post-test length Dz grain size Sz connecting surface S34 space Lae End

Claims

[1] A spark plug (100) comprising a center electrode (20) and a ground electrode (30) and in which a gap (g) is provided between the center electrode (20) and the ground electrode (30), wherein at least one of the center electrode (20) and the ground electrode (30) is provided with a base material (31) containing nickel (Ni) as a main component and with a tip (300) bonded to the base material (31) and containing platinum (Pt) as a main component, wherein the tip (300) contains one or more elements selected from the group comprising rhodium (Rh), rhenium (Re), ruthenium (Ru) and tungsten (W), the total content of the elements being not less than 10% by weight, and further containing 5% by weight or more of nickel (Ni), wherein the tip (300) has a discharge surface (310) forming the gap (g), wherein an opposite surface (320), which is a surface of the tip (300) on a side opposite the discharge surface (310), is bonded to the base material (31), and wherein a connecting surface (Sz) between the opposite surface (320) of the tip (300) and the base material (31) is not less than 0.6 mm 2 where: the relationship Hb / Ha ≤ 2.3 is satisfied, where Hb represents a Vickers hardness in the cross section perpendicular to the discharge surface (310) of the tip (300) and Ha represents a Vickers hardness in the cross section of the tip (300), the Vickers hardness Ha being measured after a process in which the tip (300) is kept at 1200°C in an argon (Ar) atmosphere for ten hours. [2] Spark plug (100) according to claim 1, wherein: the average grain size (Dz) of crystal grains in a cross section perpendicular to the discharge surface (310) of the tip (300) is not larger than 150 µm.

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