Spark plug

Abstract

To provide a spark plug capable of reducing occurrence of cracking in 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 shaft 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 base material and a chip which is joined to the base material. The chip contains Ru as a main component, is opposed to the other of the center electrode and the ground electrode and includes a tip end face of a tip end in a thickness direction of the chip. An average of crystal grain diameters obtained by dividing a length of a test line which is drawn vertically to the thickness direction of the chip on a cross section which passes the center of gravity of the tip end face and is in parallel with the thickness direction of the chip with the number of crystal grains which the test line crosses is from 1 μm or more to 50 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] If the chip material contains Ru, its mechanical strength tends to decrease, and there is a risk that the chip may crack due to thermal stress.

[0005] The present invention has been made to solve this problem, and has as its object to provide a spark plug that can reduce the occurrence of cracks in 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 base material and a tip joined to the base material. The tip is primarily composed of Ru, faces the other of the center electrode and the ground electrode, includes a front end face at the front end in the thickness direction of the tip, and in a cross section passing through the center of gravity of the front end face and parallel to the thickness direction of the tip, the average crystal grain size calculated by dividing the length of a test line drawn perpendicular to the thickness direction of the tip by the number of crystal grains intersected by the test line is 1 μm or more and 50 μm or less.

[0007] In the second embodiment, in the first embodiment, the average crystal grain size is 1 μm or more and 30 μm or less.

[0008] In the third aspect, in the first or second aspect, the average aspect ratio, calculated by dividing the length in a direction parallel to the axis of the crystal grain by the length in a direction perpendicular to the axis of the crystal grain, in the cross section is 0.8 or more and 2.0 or less.

[0009] A fourth embodiment is the third embodiment, wherein the average aspect ratio is 1.0 or more and 2.0 or less.

[0010] In a fifth aspect, in any one of the first to fourth aspects, the linear expansion coefficient of the base material is 1.0 × 10 -6 K -1 Over 1.8 x 10 -6 K -1 The following is the result. [Effects of the Invention]

[0011] According to the present invention, a test line is drawn perpendicular to the thickness direction of the chip on a cross section parallel to the thickness direction of the chip, and the average crystal grain size obtained by dividing the length of the test line by the number of crystal grains that intersect with the test line is 1 μm or more and 50 μm or less. This ensures the mechanical strength of the chip, thereby reducing the occurrence of cracks in the chip. [Brief explanation of the drawings]

[0012] [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 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

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] A core material (not shown) having excellent thermal conductivity is embedded in the base material 17. The material of the base material 17 is, for example, Ni or an alloy containing Ni as a main component, and the material of the core material is, for example, Cu or an alloy containing Cu as a main component. The core material can be omitted. The linear expansion coefficient of the base material 17 is 1.0×10 -6 K -1Over 1.8 x 10 -6 K -1 The following is the result.

[0019] 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 tip surface 20 at the tip in the thickness direction of the tip 19 (the vertical direction in FIG. 2 ) and a side surface 21 connected to the tip surface 20.

[0020] 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. The linear expansion coefficient of the base material 17 is 1.0×10 -6 K -1 Over 1.8 x 10 -6 K -1 The following is the result.

[0021] 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 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 tip surface 25 at the tip in the thickness direction of the tip 24 (the vertical direction in FIG. 2 ) and a side surface 26 connected to the tip surface 25. In this embodiment, the tip surface 25 of the tip 24 and the tip surface 20 of the tip 19 face each other, and a spark gap is provided between the tip surface 20 and the tip surface 25.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] FIG. 3 is a cross-sectional view of the tip 19 of the center electrode 13. The cross-sectional view of FIG. 3 illustrates a scanning electron microscope (SEM) image of a polished surface of the tip 19 cut parallel to the thickness direction of the tip 19 at the position of the center of gravity 27 of the tip face 20. In this embodiment, the thickness direction of the tip 19 coincides with the direction in which the axis X extends. The center of gravity 27 of the tip face 20 is the geometric center when the tip face 20 is viewed as a plane 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.

[0029] To observe the cross-sectional structure of the tip 19, a test line 30 (straight line) is drawn perpendicular to the thickness direction of the tip 19 (parallel to the tip surface 20). Because the cross section of the tip 19 sags (curls) on the tip surface 20, reducing the accuracy of cross-sectional structure observation, the test line is not drawn on the tip surface 20, and the distance between the tip surface 20 and the test line 30 is kept at least 10 μm.

[0030] Figure 4 is a cross-sectional view of crystal grains intersecting with test line 30. The number of crystal grains intersecting with test line 30 is counted. In this embodiment, test line 30 drawn on the cross section of chip 19 intersects with crystal grains 32, 33, 34, 35, 36, and 37 (six grains). Both ends of test line 30 end within crystal grains 31 and 38, respectively. Each end of test line 30 is counted as intersecting half of crystal grains 31 and 38. Therefore, the number of crystal grains intersecting with test line 30 is (6 + 2 × 1 / 2) = 7.

[0031] The length of the test line 30 is set so that it intersects with at least 10 crystal grains. The test line 30 is drawn randomly at various positions within a range that satisfies the condition that it is perpendicular to the thickness direction of the chip 19, and the number of crystal grains that intersect with the test line 30 is counted multiple times to obtain the average value. The length of the test line 30 may be changed for each measurement. The average crystal grain size (μm) is obtained by dividing the length of the test line 30 used for the measurement by the average value (number of crystal grains). The average crystal grain size of the chip 19 is 1 μm or more and 50 μm or less, and preferably 1 μm or more and 30 μm or less.

[0032] When the average crystal grain size of the tip 19 is in the range of 1 μm to 50 μm, the proportion of the crystal grain boundaries between the crystal grains becomes relatively large compared to when the average crystal grain size exceeds 50 μm, making it difficult for dislocations to move, thereby improving the mechanical strength of the tip 19. Therefore, the occurrence of cracks in the tip 19 due to thermal stress can be reduced.

[0033] The average aspect ratio L1 / L2, obtained by dividing the length L1 of the crystal grains 39 of the tip 19 in a direction parallel to the axis X (see FIG. 1) by the length L2 of the crystal grains 39 in a direction perpendicular to the axis, is preferably 0.8 or more and 2.0 or less, and particularly preferably 1.0 or more and 2.0 or less. This is because the thermal strain that occurs in the tip 19 due to temperature changes during use of the spark plug 10 is easily absorbed by the deformation of the crystal grains, thereby reducing the occurrence of fracture at the crystal grain boundaries. This further reduces the occurrence of cracks in the tip 19.

[0034] The linear expansion coefficient of the base material 17 (see Figure 3) is 1.0 × 10 -6 K -1 Over 1.8 x 10 -6 K -1 It is preferable that the coefficient of linear expansion of tip 19 containing Ru is less than or equal to the coefficient of linear expansion of base material 17. This reduces the occurrence of damage to interface 28 between base material 17 and fusion zone 18 and interface 29 between fusion zone 18 and tip 19 due to temperature changes during use of spark plug 10.

[0035] When the tip 24 of the ground electrode 16 (see FIG. 2) is mainly composed of Ru, a test line is drawn perpendicular to the thickness direction of the tip 24 (parallel to the front end surface 25), and the length of the test line is divided by the number of crystal grains intersecting the test line to obtain an average grain size of preferably 1 μm to 50 μm, more preferably 1 μm to 30 μm. The conditions for the aspect ratio of the crystal grains in the tip 24 and the linear expansion coefficient of the base material 22 are the same as those described for the center electrode 13.

[0036] A second embodiment will be described with reference to Figures 5 and 6. In the first embodiment, the leading end surface 20 of the tip 19 of the center electrode 13 faces the leading end in a direction parallel to the axis X, and the leading end surface 25 of the tip 24 of the ground electrode 16 faces the rear end in a direction parallel to the axis X. In contrast, in the second embodiment, the leading end surface 63 of the tip 62 of the center electrode 53 faces the leading end in a direction parallel to the axis X, and the leading end surface 58 of the tip 57 of the ground electrode 56 faces in a direction perpendicular to the axis X.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 tip surface 58 that is the tip of the tip 57 in the thickness direction and faces the center electrode 53, and a side surface 59 that is connected to the tip surface 58.

[0041] 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 melting the tip 62 and the base material 60. The tip 62 includes a tip surface 63 at the tip in the thickness direction of the tip 62 and a side surface 64 connected to the tip surface 63. The side surface 64 of the tip 62 and the tip surface 58 of the tip 57 face each other in a direction perpendicular to the axis X (see FIG. 5).

[0042] The cross-sectional view in Figure 6 is an SEM image of the polished surface of the tip 62 cut parallel to the thickness direction of the tip 62 at the position of the center of gravity 65 of the front end surface 63. There are no restrictions on the shape of the tip 62. An interface 66 between the base material 60 and the molten zone 61, and an interface 67 between the molten zone 61 and the tip 62 appear in the cross section.

[0043] To observe the cross-sectional structure of the tip 62, a test line 68 (straight line) is drawn perpendicular to the thickness direction of the tip 62 (parallel to the tip surface 63). The average crystal grain size of the tip 62, calculated by dividing the length of the test line by the number of crystal grains intersecting the test line, is preferably 1 μm or more and 50 μm or less, and more preferably 1 μm or more and 30 μm or less. This reduces the occurrence of cracks in the tip 62. The conditions related to the aspect ratio of the crystal grains of the tip 62 and the linear expansion coefficient of the base material 60 are the same as those described in the first embodiment.

[0044] When the tip 57 of the ground electrode 56 is mainly composed of Ru, a test line is drawn perpendicular to the thickness direction of the tip 57 (parallel to the front end surface 58), and the average grain size calculated by dividing the length of the test line by the number of crystal grains intersecting the test line is preferably 1 μm to 50 μm, more preferably 1 μm to 30 μm. The conditions related to the aspect ratio of the crystal grains of the tip 57 and the linear expansion coefficient of the base material (not shown) are the same as those described in the first embodiment. [Example]

[0045] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0046] The researchers fabricated cylindrical chips using a powder metallurgy method using metal powder containing Ru. The chip dimensions were 0.6 mm in diameter and 0.5 mm in height. By varying the metal powder material, particle size distribution, and sintering temperature, they obtained a variety of chips with different compositions and structures. They also obtained a variety of chips with different aspect ratios by varying the pressure conditions during sintering.

[0047] The tester prepared various base materials with different linear expansion coefficients by varying the composition of the Ni-based alloy, and fabricated various center electrodes by joining the tip to the base material by laser welding. The tester manufactured multiple spark plug samples No. 1-42 similar to the first embodiment, each of which had a spark gap between the tip of the center electrode and the ground electrode.

[0048] The tester took SEM images of the cross section of the tip of sample No. 1-42, passing through the center of gravity of the tip surface and parallel to the axis, and then counted the number of crystal grains intersecting the test line parallel to the tip surface at multiple positions. The average crystal grain size (μm) was calculated by dividing the length of the test line by the number of crystal grains and rounding to the nearest tenth. The average aspect ratio L1 / L2 of the crystal grains was calculated from the SEM images. The magnification of the SEM images was appropriately set between 500x and 2000x depending on the size of the crystal grains in the sample.

[0049] The tester attached each of the No. 1-42 samples, except for the one for which SEM images were taken, to an engine (Model L13A), connected an ignition device to the sample, and conducted a test in which a spark discharge was generated between the center electrode and the ground electrode while the engine was running at 5,000 rpm for 100 hours. The energy supplied from the ignition device to the sample for each spark discharge was 100 mJ. The air-fuel ratio during the test was 10.5, the engine combustion chamber pressure was 60 kPa, and the tip temperature was 600°C. The tip temperature was measured before the test using a thermocouple hot junction placed near the tip. After the test, Sample No. 1-42 was removed from the engine and the tip and base material were cut parallel to the axis at the center of gravity of the tip's tip end face.

[0050] (Judgment 1) The cut surface of the tip was observed with a metallurgical microscope, and the length of the crack in the radial direction of the tip was measured. Tips with no cracks were rated A, those with a crack length less than 25% of the tip diameter were rated B, those with a crack length between 25% and 50% of the tip diameter were rated C, and those with a crack length 50% or more of the tip diameter were rated D.

[0051] (Judgment 2) The interface of the fusion zone was observed with a metallurgical microscope, and the length of the cracks that had entered the interface was measured. Cracks with a length of less than 10 μm were rated A, and cracks with a length of 10 μm or more were rated D. The chip composition, chip crystal grain size, crystal grain aspect ratio, linear expansion coefficient of the base material, and the results of Judgment 1 and Judgment 2 are shown in Table 1.

[0052] [Table 1]

[0053] According to Table 1, samples Nos. 1-6, 8-13, 15-20, 22-27, 29-34, and 36-42, which had an average grain size of 1 μm or more and 50 μm or less, were graded A, B, or C in Grade 1, while samples Nos. 7, 14, 21, 28, and 35, which had an average grain size greater than 50 μm, were graded D in Grade 1. Therefore, it was revealed that chips with an average grain size of 1 μm or more and 50 μm or less can reduce the occurrence of cracks.

[0054] Samples Nos. 1-3, 8-10, 15-17, 22-25, 29-32, and 36-40, which had an average grain size of 1 μm or more and 30 μm or less, were graded A or B in Grade 1, while samples Nos. 4-6, 11-13, 18-20, 26, 27, 33, 34, 41, and 42, which had an average grain size of more than 30 μm and 50 μm or less, were graded B or C in Grade 1. This demonstrates that chips with an average grain size of 1 μm or more and 30 μm or less can further reduce the occurrence of cracks.

[0055] Comparing samples No. 4 and No. 5, No. 4 had an aspect ratio of 1.1 and received a grade of B, while No. 5 had an aspect ratio of 0.7 and received a grade of C. Comparing samples No. 29-31, No. 30 had an aspect ratio of 1.2 and received a grade of A. On the other hand, No. 29 had an aspect ratio of 0.6 and received a grade of B, and No. 30 had an aspect ratio of 0.3 and received a grade of B. This shows that chip cracking can be further reduced when the aspect ratio is between 0.8 and 2.0.

[0056] The linear expansion coefficient of the base material is 1.0 x 10 -6 K -1 Over 1.8 x 10 -6 K -1 The following Nos. 1-6, 8-16, 18-20, 23-35, 37, 38, 40, and 41 were judged as A in judgment 2, but the linear expansion coefficient of the base material was 1.0 × 10 -6 K -1 Less than or 1.8 x 10 -6 K -1 The larger samples Nos. 7, 17, 21, 22, 36, 39, and 42 were judged D in judgment 2. Therefore, the linear expansion coefficient of the base material was 1.0 × 10 -6 K -1 Over 1.8 x 10 -6 K -1 It has been found that cracks at the interface of the fusion zone can be reduced if the temperature is below 1000°C.

[0057] 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.

[0058] 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.

[0059] 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]

[0060] 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 Tip surface 27,65 Center of gravity 30,68 Test Line L1, L2 length 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 base material and a tip joined to the base material, The tip is made of Ru as a main component and faces the other of the center electrode and the ground electrode. the tip includes a tip surface at a tip end in a thickness direction of the tip, a spark plug in which an average crystal grain size obtained by dividing the length of a test line drawn perpendicular to the thickness direction of the tip in a cross section that passes through the center of gravity of the front end surface and is parallel to the thickness direction of the tip by the number of crystal grains that intersect with the test line is 1 μm or more and 50 μm or less.

2. 2. The spark plug according to claim 1, wherein the average grain size is 1 μm or more and 30 μm or less.

3. 3. The spark plug according to claim 1, wherein an average aspect ratio, calculated by dividing the length of the crystal grains in a direction parallel to the axis by the length of the crystal grains in a direction perpendicular to the axis, in the cross section is equal to or greater than 0.8 and equal to or less than 2.

0.

4. 4. The spark plug according to claim 3, wherein the average aspect ratio is equal to or greater than 1.0 and equal to or less than 2.

0.

5. The linear expansion coefficient of the base material is 1.0 × 10 －６ K －１ 1.8 x 10 －６ K －１ 3. The spark plug according to claim 1, wherein:

Images