Spark plug

Abstract

The spark plug of the present application is one in which, in a mirror-polished surface obtained by mirror polishing a cut surface obtained by cutting the insulator in a direction perpendicular to the axial direction at a position 2 mm away from the maximum diameter portion of the enlarged diameter portion toward the rear end side, in a case where 20 first observation regions of 192 μm x 255 μm are set so as to overlap with a reference position, which is a position 0.2 mm away in the radial direction from the inner peripheral surface side of the insulator, and not to overlap with each other, the proportion of the area of all the air holes included in the mirror-polished surface with respect to the total area is 3.5% or less, and in a thermal-etched surface obtained by subjecting the mirror-polished surface to thermal etching, in a case where 20 second observation regions of 32 μm x 43 μm are set so as to overlap with the reference position and not to overlap with each other, when the particle size distribution of the alumina particles included therein is assumed to be a normal distribution and the average particle diameter is assumed to be A and the standard deviation of the particle diameter is assumed to be σ, A is 1.9 μm or more and 2.8 μm or less, and A + 3σ is 3.0 μm or less.

Description

Technical Field

[0001] This invention relates to spark plugs. Background Technology

[0002] Spark plugs used in internal combustion engines have a cylindrical insulator made of an alumina-based sintered body with alumina as the main component and a center electrode housed inside the insulator (e.g., Patent Document 1). The center electrode has a rod-shaped electrode body (electrode leg) with its front end protruding from the insulator and its rear end housed inside the insulator, and an enlarged diameter portion (electrode flange) connected to the rear end of the electrode body. The enlarged diameter portion is radially extended from the electrode body portion, and the center electrode is housed inside the insulator in a stepped bulge within the inner wall of the insulator with this enlarged diameter portion engaged. It should be noted that an electrode head with a smaller diameter than the enlarged diameter portion is connected to the rear end of the enlarged diameter portion.

[0003] With the central electrode housed inside the insulator, the rear end portion of the central electrode (i.e., the enlarged diameter portion and the electrode head) and the inner wall of the insulator are radially spaced apart and facing each other. Furthermore, a conductive sealing member is disposed inside the insulator, filling the space between them and covering the rear end of the central electrode. The sealing member is, for example, composed of a conductive material containing glass particles (such as B2O3-SiO2) and metal particles (Cu, Fe, etc.).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-57559

[0007] (The problem that the invention aims to solve)

[0008] In the aforementioned section where the rear end of the center electrode and the inner wall of the insulator face each other radially, heat tends to accumulate from the front end to the rear end of the center electrode during spark plug 1 operation. Furthermore, when a high voltage is applied to the center electrode, the electric field tends to concentrate. In particular, in the rear end of the center electrode, where the radially expanding portion faces the inner wall of the insulator, the gap is narrower, making heat and electric field concentration more likely. Therefore, it can be said that the portion of the insulator facing the radially expanding portion of the center electrode is subjected to the most severe environment.

[0009] Given these circumstances, there is a demand for spark plugs with excellent insulator properties, such as high voltage resistance. Summary of the Invention

[0010] The purpose of this invention is to provide spark plugs with excellent insulator properties such as voltage resistance.

[0011] (Methods used to solve problems)

[0012] The inventors have discovered that in a spark plug having an insulator made of an alumina-based sintered body and a center electrode housed inside the insulator, if alumina particles with abnormal particle growth of a certain size or greater exist inside a specific part of the main body of the insulator, when a high voltage is applied to the center electrode, the electric field tends to concentrate around the alumina particles, and the vicinity of the alumina particles is the starting point for the failure of the insulator.

[0013] Furthermore, through intensive research conducted by the inventors to achieve the aforementioned objective, they discovered that if the average particle size of the alumina particles constituting the sintered body is within a specified range that is not the particle size during abnormal particle growth, and the deviation in particle size of the alumina particles is suppressed, then the voltage withstand performance of the insulator can be ensured, thus completing the invention of this application.

[0014] The means for solving the aforementioned problem are as follows. That is,

[0015] <1> A spark plug comprises: an insulator, which is cylindrical and extends along an axial direction, and is made of an alumina-based sintered body; a center electrode having a rod-shaped electrode body and an expanded diameter portion, the electrode body being inserted into the insulator with its front end protruding from the insulator and its rear end housed inside the insulator, the expanded diameter portion being connected to the rear end of the electrode body and having a shape that expands radially from the electrode body and is engaged with the inner wall of the insulator; and a conductive seal, housed inside the insulator and disposed on the rear end side of the center electrode, wherein a mirror-polished surface is obtained by mirror polishing a cut surface obtained by cutting the insulator in a direction perpendicular to the axial direction at a position 2 mm away from the rear end side along the axial direction from the maximum diameter portion of the expanded diameter portion, and a surface that is radially aligned with the inner circumferential surface side of the insulator. When 20 first observation areas of 192μm×255μm are set in a manner that overlaps with but does not overlap with each other, with a position 0.2mm away from the reference position, the ratio (porosity) of the area of ​​all pores contained in the 20 first observation areas to the total area (100%) of the 20 first observation areas is 3.5% or less. In the thermally etched surface obtained by thermal etching the mirror polished surface, when 20 second observation areas of 32μm×43μm are set in a manner that overlaps with but does not overlap with each other, when the particle size distribution of alumina particles contained in the 20 second observation areas is regarded as a normal distribution and the average particle size of the alumina particles is set as A and the standard deviation of the particle size of the alumina particles is set as σ, A is 1.9μm or more and 2.8μm or less, and (A+3σ) is 3.0μm or less.

[0016] <2> According to the above <1> In the spark plug described above, when 60 representative alumina particles with large length and diameter are selected from the top three alumina particles with large length and diameter in each of the 20 second observation areas in the thermally etched surface, and the frequency distribution of the aspect ratio of the representative alumina particles is regarded as a normal distribution, and the average aspect ratio of the representative alumina particles is set as B, and the standard deviation of the aspect ratio of the representative alumina particles is set as σ, (B+3σ) is 4.8 or less.

[0017] <3> According to the above <2> In the spark plug, in the 20 second observation areas of the thermally etched surface, there are two or fewer representative alumina particles with an aspect ratio of 3.5 or higher.

[0018] <4> According to the above <1> ~ <3> In any of the spark plugs described, the number of pores in the 20 first observation areas of the mirror-polished surface is less than 600.

[0019] (Invention Effects)

[0020] According to the present invention, it is possible to provide a spark plug with an insulator having excellent voltage resistance and other properties. Attached Figure Description

[0021] Figure 1 This is a cross-sectional view along the axial direction of the spark plug according to Embodiment 1.

[0022] Figure 2 It is an enlarged cross-sectional view of the area near the electrode flange of the central electrode housed within the main body of the insulator.

[0023] Figure 3 This is a schematic diagram illustrating the mirror-polished surface obtained by mirror polishing the cut surface of the main body in an insulator.

[0024] Figure 4 This is an explanatory diagram showing the binarized image obtained by binarizing a SEM image.

[0025] Figure 5 This is an illustrative diagram schematically showing the thermally etched surface of the main body in an insulator.

[0026] Figure 6 This is an explanatory diagram showing the SEM image corresponding to the second observation area.

[0027] Figure 7 This is an illustrative diagram showing a cross-section of an insulator containing alumina particles with abnormal particle growth. Detailed Implementation

[0028] <Implementation Method 1>

[0029] Referring to the spark plug 1 according to Embodiment 1 of the present invention... Figures 1-6 While explaining. Figure 1 This is a cross-sectional view of the spark plug 1 according to Embodiment 1 along the AX direction. Figure 1 The single-dot dashed line extending vertically shown is the axis AX of spark plug 1. Figure 1 In the diagram, the length direction of spark plug 1 (axis AX direction) corresponds to... Figure 1 The up and down direction. In Figure 1 The lower side shows the front end of spark plug 1. Figure 1 The upper side shows the rear end side of spark plug 1.

[0030] Spark plug 1 is installed in the engine of a car (an example of an internal combustion engine) and is used to ignite the air-fuel mixture in the engine's combustion chamber. Spark plug 1 mainly includes an insulator 2, a center electrode 3, a ground electrode 4, terminal fittings 5, a main body fitting 6, a resistor 7, and sealing components 8 and 9.

[0031] Insulator 2 is a generally cylindrical component extending in the axial direction AX that contains a through hole 21 inside. Details of insulator 2 will be described later.

[0032] The main component 6 is a member used when installing the spark plug 1 onto the engine (specifically, the engine cover). It is a cylindrical component extending along the AX axis as a whole, and is made of a conductive metal material (e.g., low-carbon steel). A threaded portion 61 is formed on the outer peripheral surface of the front end side of the main component 6. Furthermore, an annular washer G is embedded at the rear end of the threaded portion 61 (the so-called threaded end). The washer G is annular and formed by bending a metal plate. This washer G is positioned between the rear end of the threaded portion 61 and a seat portion 62 positioned further back than the threaded portion 61, sealing the gap formed between the spark plug 1 and the engine (engine cover) when the spark plug 1 is installed in the engine.

[0033] A tool engaging portion 63 is provided at the rear end of the main body component 6 for engaging tools such as wrenches when installing the main body component 6 onto the engine. Furthermore, a thin-walled tightening portion 64 that bends radially inward is provided at the rear end of the main body component 6.

[0034] Furthermore, the main body component 6 has a through hole 65 extending along the axial direction AX, through which the insulator 2 is held within the main body component 6. The rear end of the insulator 2 extends outward from the rear end of the main body component 6. Figure 1 The upper side of the insulator 2 protrudes significantly. In contrast, the front end of the insulator 2 extends outward from the front end of the main body component 6. Figure 1 (The lower side) is slightly protruding.

[0035] An annular region is formed between the inner circumferential surface of the portion from the tool engaging part 63 to the tightening part 64 in the main body component 6 and the outer circumferential surface of the insulator 2 (the outer circumferential surface of the rear cylindrical part 25 described later). A first ring member R1 and a second ring member R2, arranged in annular form and separated from each other in the axial direction AX, are disposed in this region. Powdered talc 10 is filled between the first ring member R1 and the second ring member R2. The rear end of the tightening part 64 is bent radially inward and fixed to the outer circumferential surface of the insulator 2 (the outer circumferential surface of the rear cylindrical part 25 described later).

[0036] Furthermore, the main body component 6 includes a thin-walled compression deformation portion 66 disposed between the seat portion 62 and the tool engagement portion 63. During the manufacturing of the spark plug 1, the compression deformation portion 66 is compressed and deformed by pressing it towards the front end by the tightening portion 64 fixed to the outer peripheral surface of the insulator 2. Through this compression deformation by the compression deformation portion 66, the insulator 2 is pressed towards the front end within the main body component 6 via the first ring member R1, the second ring member R2, and the talc 10. At this time, the outer peripheral surface of a portion of the insulator 2 that extends outward in a ring shape (the first diameter expansion portion 26 described later) is pressed against the surface of the step portion 66 provided on the inner peripheral side of the main body component 6, with the gasket P1 placed in the middle. Therefore, even if gas from the combustion chamber of the engine enters the gap formed between the main body component 6 and the insulator 2, the gasket P1 disposed in the gap prevents the gas from leaking outward.

[0037] With the insulator 2 assembled inside the main body component 6, a center electrode 3 is disposed inside the insulator 2. The center electrode 3 has a rod-shaped center electrode body 31 extending along the AX direction and a generally cylindrical (generally circular plate-shaped) end (center electrode tip) 32 mounted at the front end of the center electrode body 31. The center electrode body 31 is a member whose length in the longitudinal direction is shorter than that of the insulator 2 and the main body component 6, and is held in the through hole 21 of the insulator 2 such that its front end is exposed laterally to the outside. The rear end of the center electrode body 31 is received inside the insulator 2 (through hole 21). The center electrode body 31 has an electrode base material 31A disposed on the outside and a core portion 31B embedded inside the electrode base material 31A. The electrode base material 31A is formed, for example, using nickel or an alloy with nickel as the main component (e.g., NCF600, NCF601). The core 31B is formed of copper, which has a higher thermal conductivity than the electrode substrate 31A, or a nickel-based alloy with copper as the main component.

[0038] Furthermore, the central electrode body 31 includes an electrode flange 31a mounted at a predetermined position along the AX axis, an electrode head 31b (a portion further rearward than the electrode flange 31a), and an electrode leg 31c (a portion further frontward than the electrode flange 31a). The electrode leg (an example of the electrode body portion) 31c is a rod-shaped member inserted into the through hole 21 of the insulator 2 with its front end protruding from the insulator 2 and its rear end housed inside the insulator. The electrode flange (an example of the expanded diameter portion) 31a is connected to the rear end of the electrode leg (electrode body portion) 31c and has a shape that extends radially from the electrode leg 31c. When the electrode flange 31a is housed within the insulator 2, it is engaged with a step portion 23a (described later) formed on the inner wall 21a of the insulator 2. The front end of the electrode leg 31c (that is, the front end of the central electrode body 31) protrudes further forward than the front end of the insulator 2. The electrode flange portion 31a is a rod-shaped portion that is shorter than the electrode leg portion 31c and has a smaller diameter than the electrode flange portion 31a.

[0039] The end 32 is generally cylindrical (generally circular plate-shaped) and is joined to the front end of the central electrode body 31 (the front end of the electrode leg 31c) by resistance welding, laser welding, or the like. The end 32 is made of a material with a high melting point noble metal as the main component (e.g., an iridium-based alloy with iridium (Ir) as the main component).

[0040] Terminal fitting 5 is a rod-shaped member extending in the AX direction, installed by insertion into the rear end of the through hole 21 of the insulator 2. Terminal fitting 5 is disposed within the insulator 2 (through hole 21) at a position further rear-end than the center electrode 3. Terminal fitting 5 is made of a conductive metallic material (e.g., low-carbon steel). It should be noted that the surface of terminal fitting 5 may also be plated with nickel or the like for corrosion protection purposes.

[0041] The terminal accessory 5 includes a rod-shaped terminal leg 51 disposed on the front end side, a terminal flange 52 disposed on the rear end side of the terminal leg 51, and a cap assembly portion 53 disposed further rearward than the terminal flange 52. The terminal leg 51 is inserted into the through hole 21 of the insulator 2. The terminal flange 52 is the portion that protrudes from the rear end of the insulator 2 and is locked toward the rear end. The cap assembly portion 53 is the portion for assembling a spark plug cap (not shown) connected to a high-voltage cable, through which a high voltage for generating a spark discharge is applied from the outside.

[0042] The resistor 7 is disposed within the through hole 21 of the insulator 2 between the front end of the terminal fitting 5 (the front end of the terminal leg 51) and the rear end of the center electrode 3 (the rear end of the center electrode body 31). The resistor 7 has a resistance value of 1kΩ or more (e.g., 5kΩ) and functions to reduce electromagnetic noise during spark generation. The resistor 7 is composed of a composition including glass particles as the main component, ceramic particles other than glass, and a conductive material.

[0043] A gap is provided between the front end of the resistor 7 and the rear end of the center electrode 3 within the through hole 21, and a conductive sealing member 8 is provided to fill this gap. Similarly, a gap is also provided between the rear end of the resistor 7 and the front end of the terminal fitting 5 within the through hole 21, and a conductive sealing member 9 is provided to fill this gap. Each sealing member 8 and 9 is, for example, composed of a conductive material containing glass particles (such as B2O3-SiO2) and metal particles (such as Cu and Fe).

[0044] The grounding electrode 4 comprises a grounding electrode body 41 joined to the front end of the main body accessory 6 and a prism-shaped grounding electrode tip 42. The grounding electrode body 41 is generally constructed of a plate bent into a roughly L-shape midway, and its rear end 41a is joined to the front end of the main body accessory 6 by resistance welding or the like. Thus, the main body accessory 6 and the grounding electrode body 41 are electrically connected. The grounding electrode body 41, for example, is formed using nickel or a nickel-based alloy (e.g., NCF600, NCF601) with nickel as its main component, similar to the main body accessory 6. The grounding electrode tip 42, like the end 32 of the center electrode 3, is constructed of an iridium-based alloy with iridium (Ir) as its main component. The grounding electrode tip 42 is joined to the front end of the grounding electrode body 41 by laser welding.

[0045] The ground electrode tip 42 at the front end of the ground electrode body 41 and the end 32 at the front end of the center electrode 3 are arranged in a spaced-apart and opposite manner. That is, there is a gap SP between the end 32 at the front end of the center electrode 3 and the ground electrode tip 42 at the front end of the ground electrode 4. If a high voltage is applied between the center electrode 3 and the ground electrode 4, a spark discharge is generated in the gap SP in a manner approximately along the axial direction AX.

[0046] Next, insulator 2 will be described in detail. Insulator 2 is generally a cylindrical shape extending slenderly along the AX axis, as shown below. Figure 1As shown, the insulator 2 contains a through hole 21 extending in the AX direction. The insulator 2 is composed of a cylindrical alumina-based sintered body with alumina as the main component. The insulator 2 includes: a long leg portion 22 disposed at the front end; a main body portion 23 disposed at the rear end of the long leg portion 22, with a larger diameter than the long leg portion 22; and a flange portion 24 disposed at the rear end of the main body portion 23, with a larger diameter than the main body portion 23. It should be noted that a first diameter-enlarging portion 26 is provided between the long leg portion 22 and the main body portion 23, and a second diameter-enlarging portion 27 is provided between the main body portion 23 and the flange portion 24.

[0047] The long leg portion 22 is generally a slender cylindrical shape (cylindrical shape) with its outer diameter gradually increasing from the front to the rear, and has an outer diameter smaller than that of the main body 23 and the first expansion portion 26. The long leg portion 22 is exposed to the combustion chamber when the spark plug 1 is installed in the engine (engine cover).

[0048] The flange portion 24 is located approximately at the center of the insulator 2 along the AX axis and is in the shape of a ring. A resistor 7 is disposed in the through hole 21 located inside the flange portion 24.

[0049] The first enlarged diameter portion 26 is the part connecting the long leg portion 22 and the main body 23, and is cylindrical (annular) in shape with the outer diameter gradually increasing from the front to the rear. When the insulator 2 is assembled to the main body accessory 6, the outer surface of the first enlarged diameter portion 26 in the insulator 2 is positioned relative to the surface of the stepped portion 66 provided on the inner circumference side of the main body accessory 6 so that the gasket P1 is placed in the middle.

[0050] The second diameter expansion section 27 is the part that connects the main body 23 and the flange section 24. It is cylindrical (ring-shaped) with an outer diameter that is larger than that of the first diameter expansion section 26 and gradually increases in size from the front to the rear.

[0051] The main stem 23 is cylindrical, with its outer diameter set to be approximately the same in the AX direction. When the insulator 2 is assembled to the main body fitting 6, a slight gap (space) exists between the outer surface (outer circumferential surface) of the main stem 23 and the inner surface (inner circumferential surface) of the main body fitting 6. An annular step portion 23a is provided on the inner side (inner circumferential surface side) near the front end of the main stem 23. When the central electrode body 31 of the central electrode 3 is housed in the through hole 21 of the insulator 2, the electrode flange portion (expanded diameter portion) 31a of the central electrode body 31 is secured by the surface of the step portion 23a. The wall thickness (radial thickness) of the main stem 23 is greater than the wall thickness of the long leg portion 22. Furthermore, the wall thickness of the main stem 23 from the front end to the portion where the step portion 23a is formed is greater than the wall thickness of the portion further rearward.

[0052] The outer periphery of the main body 23 is exposed to the atmosphere (air), meaning that the main body 23 is in an environment where it is more difficult for current to pass through compared to the long leg 22. Therefore, the wall thickness of the main body 23 is set to be greater than that of the long leg 22.

[0053] In this specification, unless otherwise specified, "thickness of the main body 23" refers to the thickness of the wall portion of the main body 23 where the wall thickness is approximately constant (that is, the portion further back than the step portion 23a). The thickness of the main body 23 is not particularly limited as long as it does not impair the purpose of the invention, but it can be set, for example, in the range of 2.0 mm or more and 3.0 mm or less.

[0054] It should be noted that the insulator 2 also includes a cylindrical rear section 25 that is connected to the rear end of the flange 24 and extends in the AX direction. The rear section 25 has an outer diameter smaller than that of the flange 24. A rod-shaped terminal leg 51, etc., of the terminal fitting 5 is provided in the through hole 21 inside the rear section 25.

[0055] Figure 2 This is an enlarged cross-sectional view near the electrode flange (expanded diameter portion) 31a of the central electrode 3 (central electrode body 31) housed within the main body 23 of the insulator 2. (See image below.) Figure 2 As shown, with the central electrode body 31 of the central electrode 3 housed inside the insulator 2, a gap exists between the rear end portion of the central electrode body 31, namely the electrode flange (expanded diameter portion) 31a and the electrode head 31b, and the inner wall 21a of the insulator 2. Furthermore, the aforementioned sealing member 8 is filled into the through hole 21 of the insulator 2 to fill this gap and cover the rear end of the central electrode body 31. This sealing member 8 contains an alkaline component derived from glass particles or the like.

[0056] The gap between the electrode flange (expanded diameter portion) 31a of the central electrode 3 and the inner wall 21a of the insulator 2 is narrower than the gap between the electrode head 31b and the inner wall 21a of the insulator 2. In this location, heat moving from the front end of the central electrode body 31 of the central electrode 3 via the electrode flange (expanded diameter portion) 31a tends to accumulate. Moreover, in this location, the electric field tends to concentrate when a high voltage is applied to the central electrode 3. Therefore, in the insulator 2, the portion of the main body 23 facing the electrode flange (expanded diameter portion) 31a in the radial direction is subjected to the most severe environment.

[0057] It should be noted that since the inner side of the cylindrical main body 23 is filled with a sealing member 8, the inner wall 21a of the main body 23 is in direct contact with the sealing member 8, and the alkaline components from the sealing member 8 can also come into contact with the inner wall 21a of the main body 23. The insulator 2 of this embodiment exhibits excellent voltage resistance and other properties because the internal structure of the alumina-based sintered body constituting the main body 23 at least satisfies conditions 1 and 2 as shown below.

[0058] <Condition 1>

[0059] In the mirror-polished surface 230a obtained by mirror polishing the cut surface 230 obtained by cutting the insulator 2 in a direction perpendicular to the AX direction at a position 2 mm away from the rear end of the spark plug 1 along the AX direction of the maximum diameter portion of the electrode flange portion (expanded diameter portion) 31a of the center electrode 3, when 20 first observation areas X of 192μm×255μm are set in such a way that they overlap with and do not overlap with the reference position m1 at a position 0.2 mm away from the inner peripheral surface 2a side of the insulator 2, the ratio of the area of ​​all the pores contained in the 20 first observation areas X to the total area (100%) of the 20 first observation areas X (porosity) is 3.5% or less.

[0060] Here, while referring to Figures 2-4 On the one hand, a detailed explanation of condition 1 is provided. For example... Figure 2 As shown, the "part with the largest diameter of the electrode flange portion (expanded diameter portion) 31a of the center electrode 3" indicated in condition 1 refers to the part of the electrode flange portion (expanded diameter portion) 31a in the center electrode body 31 of the center electrode 3 with the largest diameter D. Figure 2 In the diagram, the straight line L1 is shown as the portion of the electrode flange (expanded diameter portion) 31a that intersects perpendicularly to the axis AX and traverses the maximum diameter portion.

[0061] Furthermore, the insulator 2 is cut off at a position 2 mm away from the maximum diameter portion of the electrode flange (expanded diameter portion) 31a along the AX direction towards the rear end of the spark plug 1, as described later. It should be noted that the portion of the insulator 2 extending at least 2 mm away from the maximum diameter portion of the electrode flange (expanded diameter portion) 31a along the AX direction is the part where durability (voltage withstand performance, etc.) is most critical. Since the internal structure of the alumina-based sintered body constituting this range is substantially the same, in this embodiment, considering ease of cutting, the position 2 mm away from the rear end of the maximum diameter portion of the electrode flange (expanded diameter portion) 31a is set as the location where the insulator 2 is cut off.

[0062] It should be noted that when the maximum diameter portion of the electrode flange portion (expanded diameter portion) 31a is formed with a certain degree of amplitude from the front end side to the rear end side in the axial direction AX, the reference position (the position shown by the straight line L1) when it is set to be 2mm away from the rear end side is set to be the position closest to the front end side in the maximum diameter portion.

[0063] exist Figure 2 In the diagram, the section where insulator 2 is cut off is represented by straight line L2. This straight line L2 is positioned from straight line L1 towards the rear end side ( Figure 2 It is shown as intersecting perpendicularly to the axis AX at a position 2mm away from the upper side. (As shown) Figure 2 As shown, the straight line L2 extends radially across the main body 23 of the insulator 2. In condition 1, the internal structure of the cut surface 230 obtained by radially cutting the main body 23 along such a straight line L2 is specified.

[0064] Figure 3 This is a schematic diagram illustrating the mirror-polished surface 230a obtained by mirror polishing the cut surface 230 of the main body 23 in the insulator 2. Figure 3 In the middle, along Figure 2 The cut surface 230 obtained by cutting the main stem 23 into a ring shape by the straight line L2 shown is shown in a state of being polished into a mirror surface. It should be noted that the cut surface 230 that has become mirror-like after undergoing the mirror polishing treatment described later is called mirror polished surface 230a.

[0065] The mirror polishing treatment of the cut surface 230 is performed using known methods that utilize abrasives such as diamond grinding stones and diamond paste. The mirror polishing treatment continues until the surface roughness (Ra) of the cut surface 230 becomes, for example, about 0.001 μm.

[0066] The mirror-polished surface 230a was observed using a scanning electron microscope (SEM). Therefore, carbon vapor deposition for imparting conductivity can also be performed on the mirror-polished surface 230a as needed. In this embodiment, the accelerating voltage of the SEM for observing the mirror-polished surface 230a was set to 20 kV, and the SEM magnification was set to 500x.

[0067] like Figure 3As shown, the mirror-polished surface 230a is annular. In this mirror-polished surface 230a, a circular reference position m1 is set at a position radially away from the inner circumferential surface 2a of the insulator 2 by 0.2 mm. Furthermore, in condition 1, 20 first observation areas X are set in the mirror-polished surface 230a such that, when viewed from above, they overlap with the reference position m1 but do not overlap with each other.

[0068] The first observation area X is a rectangular area defined to observe the state of pores (voids) 11 in the internal structure of the mirror-polished surface 230a (cut surface 230). The first observation area X is a rectangular area with one side length of 192μm and the other side length of 255μm (that is, 192μm×255μm).

[0069] The first observation area X is set so that, when viewed from above, it overlaps with a circular reference position m1 located 0.2 mm radially away from the inner peripheral surface 2a side of the insulator 2. It should be noted that if the first observation area X is set on the mirror-polished surface 230a near the inner peripheral surface 2a of the insulator 2, it would be impossible to observe the original internal structure of the insulator 2 if the internal structure of the insulator 2 (main body 23) is eroded by alkaline components. Therefore, in this embodiment, as described above, the first observation area X is set to overlap with the reference position m1. A total of 20 such first observation areas X are set in the mirror-polished surface 230a without overlapping each other. In this embodiment, as... Figure 3 As shown, these first observation areas X are preferably set in a circular mirror-polished surface 230a in a manner that keeps them spaced apart from each other and arranged in a ring.

[0070] By using SEM to photograph the mirror-polished surface 230a within the range corresponding to such a first observation region X, an SEM image corresponding to the first observation region X is obtained. SEM images are obtained for each of the 20 first observation regions X. That is, a total of 20 SEM images are obtained corresponding to a total of 20 first observation regions X.

[0071] For a total of 20 SEM images, image analysis processing was performed using well-known image analysis software (e.g., WinROOF (registered trademark), manufactured by Mitani Corporation) that runs on a computer.

[0072] In image analysis and processing, firstly, for each SEM image, a size correction (calibration) is performed based on the scale bar attached to the SEM image.

[0073] Next, the corrected SEM image is binarized. Figure 5 This is an explanatory diagram illustrating the binarized image obtained by binarizing an SEM image. In the binarization process, the luminance (brightness) of each pixel in the SEM image is binary-coded using a predetermined threshold (e.g., threshold = 118). That is, pixels with luminance below the threshold are assigned a luminance of "0", and pixels with luminance exceeding the threshold are assigned a luminance of "255". This binary-coded process eliminates intermediate gray levels, resulting in the binarized image. Figure 4 In the binary image, the pores 11 are represented in black, and the rest (ceramic part) 12 is represented in white.

[0074] Subsequently, using the binarized image corresponding to the first observation region X, all pores (voids) 11 contained in the first observation region X are extracted using known image analysis methods. Furthermore, during the extraction of the pores 11, the area of ​​each pore 11 is also calculated using known image analysis methods.

[0075] Next, the total area of ​​all pores 11 extracted from the binarized image is calculated. Then, the ratio (porosity) (hereinafter sometimes referred to as "ratio V") of the total area of ​​all pores 11 contained in the 20 first observation regions X to the total area (100%) of the 20 first observation regions X is calculated.

[0076] In this embodiment, the internal structure of the insulator 2 (main body 23) is formed such that the ratio V (porosity) in condition 1 is 3.5% or less.

[0077] For example, in the forming process in the manufacturing direction of the insulator 2 described later, when the granulated powder is formed using a specified forming mold, the insulator 2 with pores that satisfy condition 1 is obtained by applying pressure under higher pressure conditions than before.

[0078] <Condition 2>

[0079] In the thermally etched surface 230b obtained by thermally etching the mirror-polished surface 230a, when 20 second observation areas Y of 32μm×43μm are set in a manner that overlaps with the reference position m1 but does not overlap with each other, when the particle size distribution of alumina particles contained in the 20 second observation areas is regarded as a normal distribution and the average particle size of alumina particles is set as A and the standard deviation of particle size of alumina particles is set as σ, A is 1.9μm or more and 2.8μm or less, and (A+3σ) is 3.0μm or less.

[0080] Here, while referring to Figure 5 and Figure 6Condition 2 will be explained in detail. In Condition 2, as in Condition 1, the state of the internal structure in the cut surface 230 of the same insulator 2 (main body 23) is specified. However, in Condition 2, the state of the internal structure is observed not in the mirror-polished surface 230a as described above, but in the state of the thermally etched surface 230b obtained by thermally etching the mirror-polished surface 230a.

[0081] Thermal etching is performed as follows: A sample (insulator 2) containing the mirror-polished surface 230a is placed in a specified electric furnace or similar environment and held at a temperature approximately 200°C lower than the firing temperature of the insulator 2 (e.g., 1400°C) for a specified time (e.g., 1 hour), followed by furnace cooling. This process creates depressions at the interfaces of the alumina particles present on the cut surface 230 (thermally etched surface 230b), allowing for individual observation of the alumina particles. The alumina-based sintered body constituting the insulator 2 is a liquid-phase sintered body; through thermal etching, the liquid phase (glass composition) surrounding the alumina particles in the cut surface 230 is removed.

[0082] Figure 5 This is an illustrative diagram schematically showing the thermally etched surface 230b of the main body 23 in the insulator 2. The thermally etched surface 230b is observed using a scanning electron microscope (SEM). Therefore, carbon evaporation for imparting conductivity can also be performed on the thermally etched surface 230b as needed. It should be noted that the accelerating voltage of the SEM used for observing the thermally etched surface 230b is set to 20 kV, and the SEM magnification is set to 3000x.

[0083] The thermally etched surface 230b is also annular, just like the mirror-polished surface 230a. In this thermally etched surface 230b, a circular reference position m1 is set at a position 0.2 mm away radially from the inner peripheral surface 2a side of the insulator 2.

[0084] The second observation area Y is designated to monitor the state of alumina particles within the internal structure of the thermally etched surface 230b (cut surface 230). The second observation area Y is smaller in size than the first observation area X, but like the first observation area X, it is rectangular in shape. The second observation area Y is a rectangular region with one side length of 32 μm and the other side length of 43 μm (that is, 132 μm × 43 μm).

[0085] The second observation area Y is set in a manner that overlaps with the reference position m1 when viewed from above. The reason for setting the second observation area Y to overlap with the reference position m1 is the same as the reason for setting the first observation area X to overlap with the reference position m1 in the mirror-polished surface 230a. Furthermore, a total of 20 second observation areas Y are set in the thermally etched surface 230b in a manner that they do not overlap when viewed from above. Figure 5 As shown, these second observation regions Y are preferably set in a ring-shaped thermal etched surface 230b in a manner that maintains equal intervals between each other and is arranged in a ring shape.

[0086] Condition 2 stipulates that when the particle size distribution of alumina particles contained in the 20 second observation regions Y set in this way is regarded as a normal distribution, and the average particle size of alumina particles is set as A, and the standard deviation of particle size of alumina particles is set as σ, A is 1.9 μm or more and 2.8 μm or less, and (A+3σ) is 3.0 μm or less.

[0087] The particle size of the alumina particles contained in the second observation region Y was determined based on the SEM image of the thermally etched surface 230b of the range corresponding to the second observation region Y. Figure 6 This is an explanatory diagram showing the SEM image corresponding to the second observation region Y. Figure 6 The image shows numerous alumina particles 28. The SEM images were obtained by capturing images of the thermally etched surface 230b corresponding to the second observation region Y using a SEM. SEM images were obtained for each of the 20 second observation regions Y. In other words, a total of 20 SEM images were obtained corresponding to a total of 20 second observation regions Y.

[0088] The particle size of the alumina particles was measured using a SEM image corresponding to the second observation area Y, according to JIS G0551 "Determination of Ferrite-Austenite Grain Size". Then, the average particle size A of the alumina particles was determined using the measured particle size results. In this embodiment, the average particle size A of the alumina particles was adjusted to be 1.9 μm or more and 2.8 μm or less.

[0089] When measuring the average particle size A of alumina particles, SEM images are binarized appropriately using known image analysis software (the same applies when measuring the major and minor diameters of alumina particles, as described later).

[0090] Furthermore, the internal structure of the insulator 2 (main body 23) was formed such that the particle size distribution (frequency distribution of particle size) of the alumina particles contained in the second observation region Y was regarded as a normal distribution and the standard deviation of the particle size of the alumina particles in such a normal distribution was set as σ, and (A+3σ) was less than 3.0 μm.

[0091] Insulator 2 that satisfies condition 2 is obtained, for example, by using Al compound powder (alumina powder, etc.) with a narrow (steep) particle size distribution after removing small Al compound powder that promotes abnormal particle growth during manufacturing.

[0092] In this embodiment of the spark plug 1, if the internal structure of the insulator 2 (especially the main body 23) at least satisfies the above conditions 1 and 2, then the alumina-based sintered body constituting the insulator 2 (main body 23) is dense, the particle size (average particle size A) of the alumina particles is relatively large, and the particle size of most of the alumina particles is controlled within a specified narrow range (A±3σ). Therefore, the presence of alumina particles that would be the starting point for the failure of the insulator is substantially eliminated. Consequently, the insulator 2 of the spark plug 1 of this embodiment has excellent voltage resistance, and in addition, there are fewer pores where alkaline components penetrate, resulting in excellent resistance to alkaline corrosion.

[0093] Furthermore, in the spark plug 1 of this embodiment, in addition to the conditions 1 and 2 described above, the internal structure of the main body 23 of the insulator 2 can also be formed in a manner that satisfies condition 3 described later.

[0094] <Condition 3>

[0095] In the case where 60 representative alumina particles with a large major diameter d1 are selected by selecting the top 3 alumina particles with the largest major diameter d1 in each of the 20 second observation areas Y in the thermally etched surface 230b, and the frequency distribution of the aspect ratio of the representative alumina particles is regarded as a normal distribution, and the average aspect ratio of the representative alumina particles is set as B, and the standard deviation of the aspect ratio of the representative alumina particles is set as σ, (B+3σ) is 4.8 or less.

[0096] Condition 3 specifies the state of 60 alumina particles with the largest major diameter d1 among the alumina particles in the internal structure of the thermally etched surface 230b (cut surface 230).

[0097] The major axis d1 and minor axis d2 of the alumina particles in the second observation region Y were determined using the intercept method. First, in the SEM image of the rectangular region corresponding to the second observation region Y, alumina grains intersecting at least one of the two diagonals were selected. For each selected grain, its maximum diameter was calculated and set as the major axis d1 of the alumina particle. The maximum diameter is the maximum value when the outer diameter of the grain is measured from all directions. Then, the outer diameter of the alumina grain on the straight line passing through the midpoint of the major axis d1 and orthogonal to the major axis d1 was set as the minor axis d2 of the alumina particle.

[0098] The major diameter d1 of the alumina particles was measured for all alumina particles contained in the 20 second observation regions Y. It should be noted that the minor diameter d2 of the alumina particles can also be measured for at least the representative alumina particles described later.

[0099] After measuring the major diameter d1 of the alumina particles, 60 alumina particles with the largest major diameter d1 were selected from 20 second observation regions Y. Specifically, for each second observation region Y, the top 3 alumina particles with the largest major diameter d1 were selected. These 60 alumina particles selected in this way are called "representative alumina particles".

[0100] In addition, the aspect ratio of the 60 representative alumina particles was calculated based on their respective major diameter d1 and minor diameter d2. The aspect ratio (d1 / d2) of the representative alumina particles is the ratio of the major diameter d1 to the minor diameter d2.

[0101] The internal structure of the insulator 2 (main body 23) can also be formed in such a way that the frequency distribution of the aspect ratios of the 60 representative alumina particles is regarded as a normal distribution, the average aspect ratio of the representative alumina particles is set as B, and the standard deviation of the aspect ratios of the representative alumina particles is set as σ, and (B+3σ) becomes 4.8 or less.

[0102] Generally, if alumina particles exhibit abnormal growth, their aspect ratio increases. If insulator 2 (main body 23) satisfies condition 3, it means that even if a representative alumina particle is selected as the largest among the alumina particles, the aspect ratio of this representative alumina particle is relatively small and falls within a specified narrow range (B±3σ). Such a representative alumina particle more reliably does not contain alumina particles with abnormal growth that would become the starting point of insulator failure.

[0103] Figure 7 This is an illustrative diagram showing a cross-section of an insulator containing alumina particles 280 with abnormal particle growth. Figure 7 The image shows a SEM image of the cross-section (thermal etched surface) of the main body in the insulator of the comparative example. Figure 7 The large alumina particles 280 shown are considerably larger than the surrounding alumina particles, and the vicinity of these alumina particles 280 is likely to become the starting point for the breakdown of the insulator.

[0104] <Condition 4>

[0105] In the 20 second observation areas Y in the thermally etched surface 230b, the number of alumina particles with an aspect ratio of 3.5 or higher is 2 or less.

[0106] Condition 4 specifies the state of representative alumina particles in the internal structure of the thermally etched surface 230b (cut surface 230). Alumina particles with an aspect ratio of 3.5 or higher are more likely to have abnormal particle growth, and preferably, such alumina particles are not included in the internal structure of the main body 23 of the insulator 2.

[0107] If insulator 2 (main body 23) satisfies condition 4, it means that even if the alumina particles are large alumina particles, their aspect ratio is further smaller, and they do not reliably include alumina particles that are the starting point of abnormal particle growth when the insulator is destroyed, as in condition 3.

[0108] <Condition 5>

[0109] In the 20 first observation areas X of the mirror-polished surface 230a, the number of pores is less than 600.

[0110] Condition 5, like Condition 1, specifies the state of pores (voids) 11 in the internal structure of the mirror-polished surface 230a (cut surface 230). In the 20 first observation areas X, the number of pores is preferably 600 or less.

[0111] If the insulator 2 (main body 23) meets condition 5, it is easy to control the porosity in condition 1 to the specified value, and it is easy to improve the resistance to alkali corrosion.

[0112] Next, the manufacturing method of insulator 2 will be explained. Insulator 2 is manufactured in a manner that satisfies conditions 1 and 2 as described above. As for the manufacturing method of insulator 2, there are no particular restrictions as long as the final obtained insulator 2 satisfies conditions 1 and 2. Here, an example of the manufacturing method of insulator 2 will be described.

[0113] The manufacturing method of insulator 2 mainly includes slurry preparation, degassing, granulation, forming, grinding and firing processes.

[0114] <Slurry preparation process>

[0115] The slurry preparation process is the process of mixing raw material powder, binder, and solvent to produce a slurry. The raw material powder is a compound powder that is converted into alumina through firing (hereinafter referred to as Al compound powder). For example, alumina powder is used as the Al compound powder.

[0116] In the slurry preparation process, a pulverization process is performed for the purpose of mixing and pulverizing the raw material powder. This pulverization process is carried out using a wet mill, such as a ball mill. The diameter of the grinding balls used in the wet mill is not particularly limited as long as it does not impair the purpose of this invention, but is preferably 3 mm or more and 20 mm or less, more preferably 3 mm or more and 10 mm or less, and even more preferably 3 mm or more and 6 mm or less. Furthermore, two or more types of grinding balls with different diameters can be combined. Through this pulverization process, the raw material powder becomes a raw material powder with small particle size deviation and a steep particle size distribution. If such a raw material powder is used, abnormal particle growth is suppressed in the alumina-based sintered body obtained after sintering, and the sintering density can be increased. Therefore, the alkali corrosion resistance of the insulator is improved.

[0117] The particle size (particle size after pulverization) of the Al compound powder (alumina powder, etc.) is not particularly limited as long as it does not impair the purpose of the present invention, but is preferably 1.5 μm or more, more preferably 1.7 μm or more, preferably 2.5 μm or less, and more preferably 2.0 μm or less. If the particle size of the Al compound powder (alumina powder, etc.) is within such a range, the number of defects in the insulator is suppressed, and a suitable sintering density can be obtained. It should be noted that the particle size is the median diameter (D50) of the volume reference, measured by laser diffraction (using a Microtrac particle size distribution measuring device manufactured by Nikkiso Co., Ltd., product name "MT-3000").

[0118] Al compound powder is preferably prepared such that, when the mass (oxide conversion) of the sintered alumina-based body is set to 100% by mass, it is 90% or more by oxide conversion. More preferably, it is 90% or more and 98% or less by mass, and even more preferably, it is 90% or more and 97% or less by mass. It should be noted that, without prejudice to the purpose of the present invention, powders other than Al compound powder may also be included in the raw material powder.

[0119] Binders are added to slurries to improve the formability of the raw material powder, etc. Examples of such binders include hydrophilic binders such as polyvinyl alcohol, waterborne acrylic resin, gum arabic, and dextrin. They can be used alone or in combination of two or more.

[0120] There are no particular limitations on the amount of adhesive to be mixed, as long as it does not impair the purpose of the present invention. However, for example, it can be mixed in a ratio of 1 to 10 parts by weight relative to 100 parts by weight of raw material powder, preferably in a ratio of 3 to 7 parts by weight.

[0121] Solvents are used for purposes such as dispersing raw material powders. Examples of solvents include water and alcohol. They can be used alone or in combination of two or more.

[0122] The amount of solvent added is not particularly limited as long as it does not impair the purpose of the present invention, but for example, it is preferably added at a ratio of 23 to 40 parts by weight relative to 100 parts by weight of the raw material powder, and more preferably at a ratio of 25 to 35 parts by weight. It should be noted that, as needed, other components besides the raw material powder, binder, and solvent may also be added to the slurry. Known stirring and mixing devices can be used for mixing the slurry.

[0123] <Defoaming process>

[0124] For slurries prepared after the slurry preparation process, a degassing process can also be performed as needed. In the degassing process, for example, a container holding the mixed (blended) slurry is placed in a vacuum degassing device, and the pressure is reduced to a low-pressure environment, thereby removing air bubbles contained in the slurry. By comparing the density of the slurry before and after degassing, the amount of air bubbles in the slurry can be determined.

[0125] <Granulation Process>

[0126] The granulation process is the process of producing spherical granulated powder from a slurry containing raw material powders, etc. There are no particular limitations on the method of producing granulated powder from the slurry, as long as it does not impair the purpose of this invention; however, spray drying is an example. In the spray drying method, the slurry is spray-dried using a prescribed spray drying device to obtain granulated powder with a prescribed particle size. It should be noted that there are no particular limitations on the particle size of the granulated powder, as long as it does not impair the purpose of this invention; however, for example, a particle size of 212 μm pass ≥ 95% or less is preferred, and more preferably 180 μm pass ≥ 95% or less is preferred.

[0127] <Forming Process>

[0128] The forming process is a process of obtaining a molded body by shaping granulated powder into a predetermined shape using a forming mold. The forming process is performed by rubber compression molding, metal mold compression molding, etc. In this embodiment, the pressure (compression rise rate) applied to the forming mold (e.g., the inner and outer rubber molds of a rubber compression molding machine) from the outer periphery is adjusted in a gradually increasing manner. Furthermore, it is preferable to adjust the pressure to a range higher than conventionally (e.g., 100 MPa or more). It should be noted that the upper limit of the pressure is not particularly limited as long as it does not impair the purpose of the present invention, but it can be adjusted to, for example, 200 MPa or less.

[0129] <Grinding Process>

[0130] The grinding process involves removing machining allowances from the molded body obtained after the molding process and polishing the surface of the molded body. In the grinding process, machining allowances are removed and the surface of the molded body is polished by grinding with a resin-bonded grinding stone. Through this grinding process, the shape of the molded body is refined.

[0131] <Firing Process>

[0132] The firing process is a process of firing a shaped body that has been shaped by the grinding process to obtain an insulator. In the firing process, for example, it is fired at 1450°C or higher and 1650°C or lower for 1 to 8 hours in an atmospheric atmosphere. By cooling the shaped body after firing, an insulator 2 composed of an alumina-based sintered body is obtained.

[0133] The spark plug 1 of this embodiment is manufactured using the insulator 2 obtained as described above. As mentioned above, the structure of the spark plug 1, except for the insulator 2, is the same as that of known structures.

[0134] The present invention will now be described in further detail based on embodiments. It should be noted that the present invention is not limited to these embodiments in any way.

[0135] [Example 1]

[0136] (Preparation of test samples)

[0137] Three insulators with the same basic structure as the spark plug insulator illustrated in Embodiment 1 were manufactured using the same manufacturing method as in Embodiment 1. It should be noted that the thickness of the main body of the insulator is 3 mm. Furthermore, during the slurry preparation process, when the raw material powder is pulverized using a wet grinder, 3 mm diameter pebbles are used... and 10mm diameter pebbles Use at 50% by mass and 50% by mass respectively.

[0138] (Determination of voltage withstand capability at room temperature)

[0139] A test specimen was fabricated by assembling a rod-shaped central electrode body inside the insulator into a main assembly. This test specimen was placed in a high-voltage chamber, and with carbon dioxide (CO2) supplied to the chamber at a pressure of approximately 5 MPa, a voltage was applied from the front end of the central electrode body at a rate of 0.1 kV / sec. Grounding was then performed from the main assembly. The breakdown voltage through the insulator was then measured. The results are shown in Table 1.

[0140] (Determination of Alkali Erosion Resistance Voltage)

[0141] To determine the withstand voltage against alkaline corrosion, a pre-processed insulator was prepared. Specifically, insulation was pre-processed around the long legs, ensuring that the front end of the central electrode body did not protrude from the long legs and that the thickness of the long legs remained approximately constant, with the central electrode body assembled inside the insulator. A test sample was then fabricated by assembling a structure obtained by assembling a rod-shaped central electrode body inside the insulator and sealing the opening at the front end of the insulator with a main body component. It should be noted that a chamfered radius (R-shaped chamfer) was applied to the front end of the central electrode body to suppress electric field concentration at the front end. The test sample was placed in a furnace maintained at approximately 200°C, and a voltage of 35 kV was applied from the front end of the central electrode body for 100 hours. Grounding was performed from the main body component during this time. By continuously applying voltage to the insulator of the test sample, an electric field concentration was generated at a designated location on the main body of the insulator (the radially opposite part to the electrode flange (expanded diameter part)) without external discharge, forcibly subjecting that designated location to alkaline corrosion. It should be noted that the presence or absence of alkaline corrosion can be determined by using an electron probe microanalyzer (EPMA) to measure the presence or absence of alkali metals such as Na and alkaline earth metals in the insulator.

[0142] Subsequently, using the alkali-etched insulator, the breakdown voltage through the insulator was measured using the same method as described above in the "Determination of Withstand Voltage at Room Temperature". The results are shown in Table 1.

[0143] (Observation of the cross-section (mirror-polished surface) of the main stem 1)

[0144] Regarding the obtained insulator, at a position 2 mm away from the rear end along the axial direction from the maximum diameter portion of the electrode flange (expanded diameter portion) of the central electrode, the insulator was cut in a direction perpendicular to the axial direction. Then, the cut surface of the obtained insulator was ground to a mirror finish, and the microstructure of this cut surface (mirror-polished surface) was observed using a SEM (model "JSM-IT300LA", manufactured by Nippon Electron Ltd.). The SEM accelerating voltage was set to 20 kV, and the SEM magnification was set to 500x. Then, 20 first observation regions X, each 192 μm × 255 μm, were set on this cut surface (mirror-polished surface) in a manner that overlapped with but did not overlap with the reference position located 0.2 mm radially away from the inner circumferential surface of the insulator, and a total of 20 SEM images corresponding to these 20 first observation regions were obtained. Then, image analysis processing based on image analysis software (WinROOF (registered trademark), manufactured by Mitani Shoji Co., Ltd.) was performed on these SEM images to determine the ratio (porosity) of the area of ​​all pores contained in the 20 first observation regions X to the total area (100%) of the 20 first observation regions X. The results are shown in Table 1.

[0145] Furthermore, the number of pores contained in 20 first observation areas of the cut surface (mirror-polished surface) was measured through image analysis processing. The results are shown in Table 1.

[0146] (Observation of the cut surface (thermal etched surface) of the main body 2)

[0147] The insulator, including the mirror-polished surface, was placed in a specified electric furnace and held at 1400°C for 1 hour, followed by cooling within the furnace. This thermal etching of the mirror-polished surface of the test sample was performed. The cut surface (thermally etched surface) of the resulting test sample was observed using SEM. The SEM accelerating voltage was set to 20 kV, and the SEM magnification was set to 3000x.

[0148] Then, within this cut surface (thermal etched surface), 20 second observation regions Y, each 32μm × 43μm, were established in a manner that overlapped with but did not overlap with the reference position located 0.2mm radially away from the inner circumferential surface of the insulator. A total of 20 SEM images corresponding to these 20 second observation regions Y were obtained. Then, by performing image analysis processing according to JIS G0551 "Determination of Ferrite-Austenite Grain Size" on these SEM images, the particle size of the alumina particles in the 20 second observation regions Y was measured. Then, using the measured particle size of the alumina particles, the average particle size A of the alumina particles was determined. The results are shown in Table 1.

[0149] In addition, the value of (A+3σ) [μm] was determined when the particle size distribution of alumina particles contained in the 20 second observation regions was considered to be a normal distribution and the standard deviation of the particle size was set to σ. The results are shown in Table 1.

[0150] In addition, the major and minor axes of each alumina particle in the 20 second observation areas were measured using the intercept method. Then, for each second observation area, the top three alumina particles with the largest major axes were selected from the measured major axes, resulting in a total of 60 alumina particles with the largest major axes selected as representative alumina particles.

[0151] For the 60 representative alumina particles selected, the aspect ratio (d1 / d2) was calculated based on their major and minor axes. Additionally, the average aspect ratio B was calculated for the 60 representative alumina particles. The results are shown in Table 1.

[0152] Furthermore, the value of (B+3σ) was determined when the frequency distribution of the aspect ratios of the 60 representative alumina particles was considered to be a normal distribution and the standard deviation of the aspect ratios of the representative alumina particles was set as σ. The results are shown in Table 1.

[0153] Regarding the 60 representative alumina particles, the number of representative alumina particles with an aspect ratio of 3.5 or higher was investigated. The results are shown in Table 1.

[0154] [Examples 2-9]

[0155] Except for appropriately changing the ratio of pebbles used when crushing raw material powder in the slurry preparation process, the insulators of Examples 2 to 9 were prepared in the same manner as in Example 1.

[0156] [Comparative Example 1]

[0157] In addition to using a wet grinder to crush the raw material powder during the slurry preparation process, 3mm diameter pebbles are also used. 10mm diameter pebbles and diameter Except for the use at proportions of 10%, 40%, and 50% by mass, respectively, the insulators of Comparative Example 1 were prepared in the same manner as in Example 1.

[0158] [Comparative Examples 2-4]

[0159] Except for appropriately changing the ratio of pebbles used when crushing raw material powder in the slurry preparation process, insulators of Comparative Examples 2 to 4 were prepared in the same manner as Comparative Example 1.

[0160] Regarding the obtained insulator, the above-mentioned "determination of room temperature withstand voltage", "determination of alkaline erosion withstand voltage", "observation 1 of the cross-section (mirror-polished surface) of the main body" and "observation 2 of the cross-section (thermal etched surface) of the main body" were performed in the same manner as in Example 1. The results are shown in Table 1.

[0161] [Table 1]

[0162]

[0163] As shown in Table 1, Examples 1-9 showed superior resistance to voltage at room temperature and resistance to voltage under alkaline corrosion compared to Comparative Examples 1-4.

[0164] Compared with Example 5, Examples 1-4 and Examples 6-9 in Examples 1-9 showed superior results in voltage resistance at room temperature and voltage resistance to alkali corrosion.

[0165] In addition, compared with Examples 5 and 6, Examples 1 to 4 and Examples 7 to 9 in Examples 1 to 9 show excellent results in terms of voltage resistance at room temperature and voltage resistance to alkaline corrosion.

[0166] In particular, Examples 2-4 and Example 7 in Examples 1-4 and Examples 7-9 showed excellent results in resisting alkali corrosion voltage.

[0167] Explanation of reference numerals in the attached figures

[0168] 1…Spark plug, 2…Insulator, 21…Through hole, 22…Long leg, 23…Main body, 230…Cut surface, 230a…Mirror-polished surface, 230b…Hot-etched surface, 24…Flange, 25…Rear cylinder, 26…First diameter expansion, 27…Second diameter expansion, 3…Center electrode, 31…Center electrode body, 31a…Electrode flange (diameter expansion), 31b…Electrode head, 31c…Electrode leg (electrode body), 4…Ground electrode, 5…Terminal fitting, 6…Main body fitting, 7…Resistor, 8…Sealing member, 9…Sealing member, 11…Vent, AX…Axis, X…First observation area, Y…Second observation area.

Claims

1. A spark plug, comprising: An insulator, in the form of a cylindrical shape extending along the axial direction, composed of an alumina-based sintered body; A central electrode has a rod-shaped electrode body and an expanded diameter portion. The electrode body is inserted into the insulator with its front end protruding from the insulator and its rear end housed within the insulator. The expanded diameter portion is connected to the rear end of the electrode body, extends radially from the electrode body, and is engaged with the inner wall of the insulator. A conductive seal is housed inside the insulator and disposed on the rear end side of the central electrode. in, In a mirror-polished surface obtained by mirror polishing a cut surface obtained by cutting the insulator in a direction perpendicular to the axial direction at a position 2 mm away from the maximum diameter of the expanded portion along the axial direction towards the rear end, when 20 first observation areas of 192 μm × 255 μm are set in such a way that they overlap with but do not overlap with each other at a reference position 0.2 mm away from the inner circumferential surface of the insulator, the ratio (porosity) of the area of ​​all pores contained in the 20 first observation areas to the total area (100%) of the 20 first observation areas is 3.5% or less. In the thermally etched surface obtained by thermal etching the mirror-polished surface, when 20 second observation areas of 32μm×43μm are set in a manner that overlaps with the reference position but does not overlap with each other, when the particle size distribution of the alumina particles contained in the 20 second observation areas is regarded as a normal distribution, and the average particle size of the alumina particles is set as A, and the standard deviation of the particle size of the alumina particles is set as σ, A is 1.9μm or more and 2.8μm or less, and (A+3σ) is 3.0μm or less.

2. The spark plug according to claim 1, In the case where 60 representative alumina particles with large length and diameter are selected by selecting the top 3 alumina particles with large length and diameter for each of the 20 second observation areas in the thermally etched surface, when the frequency distribution of the aspect ratio of the representative alumina particles is regarded as a normal distribution, and the average aspect ratio of the representative alumina particles is set as B, and the standard deviation of the aspect ratio of the representative alumina particles is set as σ, (B+3σ) is 4.8 or less.

3. The spark plug according to claim 2, In the 20 second observation areas of the thermally etched surface, there are two or fewer representative alumina particles with an aspect ratio of 3.5 or higher.

4. The spark plug according to any one of claims 1 to 3, In the 20 first observation areas of the mirror-polished surface, the number of pores is less than 600.

Images