Spark plug

Abstract

A spark plug 1 according to the present invention comprises: an insulator 2 comprising an alumina-based sintered body; a central electrode, which is a rod-shaped electrode that is inserted into the insulator 2 and which has an increasing-diameter portion that widens in a radial direction toward a rear end side thereof and that engages with an inner wall of the insulator; and an electrically conductive sealing material disposed inside the insulator, on the rear end side of the central electrode 3. In a mirror polished surface obtained by mirror-polishing a cut surface obtained by cutting the insulator in a direction perpendicular to an axial direction in a position 2 mm toward the rear end side, in the axial direction, from the maximum-diameter part of the increasing-diameter portion, if 20 observation regions of 192 μm × 255 μm are defined so as not to overlap one another, while each overlapping a reference position, being a central position between an inner circumferential surface and an outer circumferential surface of the insulator, the average proportion (porosity) of pores contained in the observation regions is at most equal to 3.5%, and with regard to the variability of the proportion (porosity), σ is at most equal to 0.36, where σ is the standard deviation.

Description

Technical Field

[0001] The present invention relates to a spark plug.

Background Art

[0002] A spark plug used in an internal combustion engine includes a cylindrical insulator made of an alumina-based sintered body mainly composed of alumina, and a center electrode housed inside the insulator (for example, Patent Document 1). The center electrode generally has a rod shape with a tip exposed from the insulator and a rear end housed inside the insulator, and has a diameter-expanded portion (electrode flange portion) that expands radially on the rear end side. The diameter-expanded portion is locked to a stepped raised portion on the inner wall of the insulator when the center electrode is housed inside the insulator. Note that an electrode head portion having a smaller diameter than the diameter-expanded portion is provided at the rear end of the diameter-expanded portion.

[0003] In a state where the center electrode is housed inside the insulator, the portion on the rear end side of the center electrode (that is, the diameter-expanded portion and the electrode head portion) and the inner wall of the insulator face each other while maintaining a distance in the radial direction. And a conductive seal member is provided inside the insulator in a form that fills the space between them and covers the rear end of the center electrode. The seal member is made of, for example, a conductive composition containing glass particles such as B2O3 - SiO2 - based and metal particles (Cu, Fe, etc.).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] (Problems to be Solved by the Invention) In the area where the rear end portion of the central electrode and the inner wall of the insulator face each other radially, heat tends to accumulate when the spark plug 1 is in use, as it moves from the front to the rear end of the central electrode. Furthermore, when a high voltage is applied to the central electrode, the electric field tends to concentrate there. In particular, within the rear end portion of the central electrode, the area where the radially widened, enlarged portion faces the inner wall of the insulator in the radial direction has a narrower gap, making it more susceptible to heat and electric field concentration. Therefore, the portion of the insulator facing the radially widened portion of the central electrode is subjected to the most severe conditions.

[0006] Furthermore, these insulating portions could be corroded by alkaline components originating from sealing materials, which could reduce the dielectric strength of the insulator. The portion of the insulator facing the enlarged diameter of the central electrode was in direct contact with the sealing material, and therefore, alkaline components contained in the sealing material could corrode the aforementioned portion of the insulator. [Overview of the project]

[0007] The object of the present invention is to provide a spark plug equipped with an insulator that has excellent alkali corrosion resistance and other properties.

[0008] (Means for solving the problem) The present inventors conducted diligent studies to achieve the above objective and discovered that if pores exist at a predetermined rate under predetermined variation conditions in the internal structure of the insulator at a position approximately 2 mm axially toward the rear end from the maximum diameter portion of the enlarged diameter part of the central electrode housed inside the insulator, the corrosion of the insulator by alkaline components originating from sealing members, etc., is suppressed, leading to the completion of the present invention.

[0009] The means to solve the aforementioned problem are as follows: <1> A spark plug comprising: an insulator made of an alumina-based sintered body and having a cylindrical shape extending along the axial direction; a rod-shaped electrode inserted into the insulator such that its tip is exposed from the insulator and its rear end is housed inside the insulator, the central electrode having an enlarged diameter portion that widens radially on its rear end side and engages with the inner wall of the insulator; and a conductive sealing material disposed on the rear end side of the central electrode inside the insulator, wherein a position 2 mm from the maximum diameter portion of the enlarged diameter portion toward the rear end along the axial direction. In a spark plug, the insulator is cut in a direction perpendicular to the axial direction, and the resulting cut surface is mirror-polished to obtain a mirror-polished surface, in which 20 observation areas of 192 μm × 255 μm are set such that each overlaps with a reference position which is the center position between the inner and outer surfaces of the insulator, but do not overlap with each other, the average of the percentage of pores (porosity) included in the observation areas is 3.5% or less, and the standard deviation of the variation in the percentage (porosity) is σ, where σ is 0.36 or less.

[0010] <2> In the aforementioned observation area, among the pores, the area of ​​0.05 μm 2 The average number of large stomata is between 200 and 600. <1> The spark plugs listed.

[0011] <3> Regarding the variation in the number of large pores in the observation area, when the standard deviation is σ, 3σ is 100 or less. <2> The spark plugs listed.

[0012] <4> The above 3σ is 50 or less. <3> The spark plugs listed.

[0013] <5> In the observation area, the average of the proportion of pores (porosity) is 1.0% or more, and the average number of large pores is 240 or more. <2> from <4> A spark plug as specified in one of the following.

[0014] <6> In the observation area, when the standard deviation of the variation in the number of large stomata is σ, the value of "average number + 3σ" is less than 330. <2> from <5> A spark plug as specified in one of the following.

[0015] <7> In the mirror-polished surface, when the region between the inner and outer surfaces of the insulator is divided into three equal radial lengths, 20 inner observation regions of 192 μm × 255 μm are set in the innermost region so as not to overlap, and 20 outer observation regions of 192 μm × 255 μm are set in the outermost region so as not to overlap, and the average porosity of the pores in the inner observation regions is 0.1 to 2% smaller than the average porosity of the pores in the outer observation regions. <2> from <6> A spark plug as specified in one of the following.

[0016] (Effects of the invention) According to the present invention, it is possible to provide a spark plug equipped with an insulator that has excellent alkali corrosion resistance and other properties. [Brief explanation of the drawing]

[0017] [Figure 1] Cross-sectional view of a spark plug according to Embodiment 1, along the axial direction. [Figure 2] Enlarged cross-sectional view of the enlarged diameter portion of the central electrode housed within the middle section of the insulator. [Figure 3] A schematic diagram illustrating the mirror-polished surface obtained by mirror-polishing the cut surface of the middle section of an insulator. [Figure 4] Diagram showing SEM images corresponding to the observation area. [Figure 5] This diagram shows a binarized image obtained by binarizing an SEM image. [Figure 6] A schematic diagram illustrating the inner and outer observation areas set on a mirror-polished surface. [Modes for carrying out the invention]

[0018] <Embodiment 1> The spark plug 1 according to Embodiment 1 of the present invention will be described while referring to FIGS. 1 to 6. FIG. 1 is a cross-sectional view along the axis AX direction of the spark plug 1 according to Embodiment 1. The dashed-dotted line extending in the vertical direction shown in FIG. 1 is the axis AX of the spark plug 1. In FIG. 1, the longitudinal direction (axis AX direction) of the spark plug 1 corresponds to the vertical direction in FIG. 1. The tip side of the spark plug 1 is shown on the lower side of FIG. 1, and the rear end side of the spark plug 1 is shown on the upper side of FIG. 1.

[0019] The spark plug 1 is attached to an engine (an example of an internal combustion engine) of an automobile and is used for igniting an air-fuel mixture in the combustion chamber of the engine. The spark plug 1 mainly includes an insulator 2, a center electrode 3, a ground electrode 4, a terminal fitting 5, a main body fitting 6, a resistor 7, and seal members 8 and 9.

[0020] The insulator 2 is a substantially cylindrical member extending in the direction of the axis AX including a through hole 21 inside. Details of the insulator 2 will be described later.

[0021] The main body fitting 6 is a member used when attaching the spark plug 1 to an engine (specifically, an engine head). As a whole, it has a cylindrical shape extending in the direction of the axis AX and is made of a conductive metal material (for example, low-carbon steel). A thread portion 61 is formed on the outer peripheral surface of the tip side of the main body fitting 6. Also, a ring-shaped gasket G is externally fitted to the rear end (so-called, thread neck) of the thread portion 61. The gasket G is annular and is formed by bending a metal plate. Such a gasket G is disposed between the rear end of the thread portion 61 and a seat portion 62 disposed on the rear end side of the thread portion 61, and seals the gap formed between the spark plug 1 and the engine (engine head) when the spark plug 1 is attached to the engine.

[0022] The rear end of the main fitting 6 is provided with a tool engagement portion 63 for engaging a tool such as a wrench when attaching the main fitting 6 to the engine. The rear end of the main fitting 6 is also provided with a thin-walled crimping portion 64 that is bent radially inward.

[0023] Furthermore, the main fitting 6 is equipped with a through hole 65 that penetrates through the axial direction AX, and the insulator 2 is held inside the main fitting 6 by being inserted through this through hole 65. The rear end of the insulator 2 protrudes significantly outward (upper side in Figure 1) from the rear end of the main fitting 6. In contrast, the front end of the insulator 2 protrudes slightly outward (lower side in Figure 1) from the front end of the main fitting 6.

[0024] An annular region is formed between the inner circumferential surface of the portion of the main fitting 6 from the tool engagement portion 63 to the crimping portion 64 and the outer circumferential surface of the insulator 2 (the outer circumferential surface of the rear cylindrical portion 25, which will be described later). In this region, annular first ring member R1 and second ring member R2 are arranged spaced apart from each other in the axial direction AX. Talc powder 10 is filled between the first ring member R1 and the second ring member R2. The rear end of the crimping portion 64 is bent radially inward and fixed to the outer circumferential surface of the insulator 2 (the outer circumferential surface of the rear cylindrical portion 25, which will be described later).

[0025] Furthermore, the main fitting 6 is provided with a thin-walled compression deformation portion 66 located between the seat portion 62 and the tool engagement portion 63. The compression deformation portion 66 is compressed and deformed during the manufacture of the spark plug 1 when the crimping portion 64 fixed to the outer circumferential surface of the insulator 2 is pressed toward the tip. As the compression deformation portion 66 is compressed and deformed in this way, the insulator 2 is pressed toward the tip within the main fitting 6 via the first ring member R1, the second ring member R2 and the talc 10. At that time, the outer circumferential surface of the annularly flared portion (the first enlarged diameter portion 26, described later), which is part of the insulator 2, is pressed against the surface of the stepped portion 66 provided on the inner circumferential side of the main fitting 6, with the packing P1 in between. Therefore, even if gas from the engine's combustion chamber enters the gap formed between the main fitting 6 and the insulator 2, the packing P1 provided in that gap prevents it from leaking to the outside.

[0026] With the insulator 2 mounted inside the main fitting 6, the central electrode 3 is disposed inside the insulator 2. The central electrode 3 comprises a rod-shaped central electrode body 31 extending along the axial direction AX, and a substantially cylindrical (substantially disc-shaped) tip (central electrode tip) 32 attached to the tip of the central electrode body 31. The central electrode body 31 of the central electrode 3 is, overall, a rod-shaped member whose longitudinal length is shorter than that of the insulator 2 and the main fitting 6. The central electrode body 31 is inserted into the through hole 21 of the insulator 2 such that its tip is exposed to the outside of the insulator 2 and its rear end is housed inside the insulator 2. The central electrode body 31 comprises 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 using, for example, nickel or an alloy mainly composed of nickel (for example, NCF600, NCF601). The core portion 31B is formed of copper or a nickel-based alloy mainly composed of copper, which has better thermal conductivity than the alloy forming the electrode base material 31A.

[0027] Furthermore, the central electrode body 31 is provided with an enlarged diameter portion (electrode flange portion) 31a at its rear end, which is radially widened. The central electrode body 31 also comprises an electrode head portion 31b, which is the portion at the rear end of the enlarged diameter portion 31a, and an electrode leg portion 31c, which is the portion at the front end of the enlarged diameter portion 31a. The electrode leg portion 31c is a rod-shaped member inserted into a through hole 21 of the insulator 2, such that its tip is exposed from the insulator 2 and its rear end is housed inside the insulator 2. The enlarged diameter portion 31a is connected to the rear end of the electrode leg portion 31c and is radially wider than the electrode leg portion 31c. When housed inside the insulator 2, the enlarged diameter portion 31a is locked to a stepped portion 23a (described later) formed on the inner wall 21a of the insulator 2. The tip of the electrode leg portion 31c (i.e., the tip of the central electrode body 31) protrudes toward the front end of the insulator 2. The enlarged diameter portion 31a is a rod-shaped part that is shorter than the electrode leg portion 31c and has a smaller diameter than the enlarged diameter portion 31a.

[0028] The tip 32 is roughly cylindrical (or roughly disc-shaped) and is joined to the tip of the central electrode body 31 (the tip of the electrode leg portion 31c) by resistance welding, laser welding, or the like. The tip 32 is made of a material mainly composed of a high-melting-point noble metal (for example, an iridium-based alloy mainly composed of iridium (Ir)).

[0029] The terminal fitting 5 is a rod-shaped member extending in the axial direction AX, and is installed by being inserted into the rear end of the through hole 21 of the insulator 2. The terminal fitting 5 is positioned at the rear end of the central electrode 3 within the insulator 2 (through hole 21). The terminal fitting 5 is made of a conductive metal material (for example, low-carbon steel). The surface of the terminal fitting 5 may be plated with nickel or the like for corrosion protection purposes.

[0030] The terminal fitting 5 comprises a rod-shaped terminal leg portion 51 positioned at the tip, a terminal flange portion 52 positioned at the rear end of the terminal leg portion 51, and a cap mounting portion 53 positioned further rear than the terminal flange portion 52. The terminal leg portion 51 is inserted into the through hole 21 of the insulator 2. The terminal flange portion 52 is the part that is exposed from the rear end of the insulator 2 and locks into that rear end. The cap mounting portion 53 is the part to which a plug cap (not shown) to which a high-voltage cable is connected is attached, and a high voltage for generating a spark discharge is applied from the outside via the cap mounting portion 53.

[0031] The resistor 7 is positioned within the through-hole 21 of the insulator 2, between the tip of the terminal fitting 5 (the tip of the terminal leg portion 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, for example, 1 kΩ or more (for example, 5 kΩ) and has a function to reduce radio wave noise when sparks are generated. The resistor 7 is composed of a composition that includes glass particles as the main component, ceramic particles other than glass, and a conductive material.

[0032] A gap is provided between the tip of the resistor 7 within the through-hole 21 and the rear end of the center electrode 3, and a conductive sealing member 8 is provided to fill this gap. Similarly, a gap is provided between the rear end of the resistor 7 within the through-hole 21 and the tip of the terminal fitting 5, and a conductive sealing member 9 is provided to fill this gap. Each sealing member 8 and 9 is made of a conductive composition containing, for example, glass particles such as B2O3-SiO2 and metal particles (Cu, Fe, etc.).

[0033] The grounding electrode 4 comprises a grounding electrode body 41 joined to the tip of the main fitting 6, and a rectangular prism-shaped grounding electrode tip 42. The grounding electrode body 41 is generally made of a plate piece that is bent into a roughly L-shape midway, and its rear end 41a is joined to the tip of the main fitting 6 by resistance welding or the like. This electrically connects the main fitting 6 and the grounding electrode body 41. The grounding electrode body 41 is formed using nickel or a nickel-based alloy with nickel as the main component (e.g., NCF600, NCF601), similar to the main fitting 6. The grounding electrode tip 42 is made of an iridium-based alloy with iridium (Ir) as the main component, similar to the tip 32 of the center electrode 3. The grounding electrode tip 42 is joined to the tip of the grounding electrode body 41 by laser welding.

[0034] The ground electrode tip 42 at the tip of the ground electrode body 41 and the tip 32 at the tip of the center electrode 3 are positioned opposite each other while maintaining a distance between them. In other words, there is a gap SP between the tip 32 at the tip of the center electrode 3 and the ground electrode tip 42 at the tip of the ground electrode 4, and when a high voltage is applied between the center electrode 3 and the ground electrode 4, a spark discharge is generated in this gap SP, generally along the axial direction AX.

[0035] Next, the insulator 2 will be described in detail. The insulator 2 is generally cylindrical in shape, elongated along the axis AX, and as shown in Figure 1, it contains a through hole 21 extending along the axis AX. The insulator 2 is made of a cylindrical alumina-based sintered body, with alumina as the main component. The insulator 2 comprises a leg portion 22 located at the tip, a middle section 23 located at the rear end of the leg portion 22 and having a larger diameter than the leg portion 22, and a flange portion 24 located at the rear end of the middle section 23 and having a larger diameter than the middle section 23. A first enlarged diameter portion 26 is provided between the leg portion 22 and the middle section 23, and a second enlarged diameter portion 27 is provided between the middle section 23 and the flange portion 24.

[0036] The leg portion 22 is generally elongated and cylindrical in shape, with its outer diameter gradually increasing from the front to the rear, and has a smaller outer diameter than the middle portion 23 and the first enlarged diameter portion 26. The leg portion 22 is exposed to the combustion chamber when the spark plug 1 is installed in the engine (engine head).

[0037] The flange portion 24 is positioned approximately in the center of the insulator 2 in the axial direction AX and has an annular shape. A resistor 7 is disposed in the through hole 21 located inside the flange portion 24.

[0038] The first enlarged diameter portion 26 is the part that connects the leg length portion 22 and the middle body portion 23, and has a cylindrical (annular) shape in which the outer diameter gradually increases from the front to the rear. Of the insulator 2, the outer surface of this first enlarged diameter portion 26 is placed on the surface of the stepped portion 66 provided on the inner circumference side of the main fitting 6, with the packing P1 in between, when the insulator 2 is attached to the main fitting 6.

[0039] The second enlarged diameter section 27 is the part that connects the middle section 23 and the flange section 24, and has a larger outer diameter than the first enlarged diameter section 26, and is cylindrical (annular) in shape with an outer diameter that gradually increases from the front to the rear.

[0040] The middle section 23 is cylindrical in shape, with an outer diameter that is approximately the same in the axial direction AX. When the insulator 2 is mounted on the main fitting 6, there is a small gap (space) between the outer surface (circumferential surface) of the middle section 23 and the inner surface (inner circumferential surface) of the main fitting 6. An annular stepped section 23a is provided on the inner side (inner circumferential surface side) of the middle section 23 near the tip, and when the central electrode body 31 of the central electrode 3 is housed in the through hole 21 of the insulator 2, the enlarged diameter portion 31a of the central electrode body 31 is locked in place by the surface of the stepped section 23a. The thickness of the wall portion of the middle section 23 (thickness in the radial direction) is greater than the thickness of the wall portion of the leg length portion 22. Furthermore, the thickness of the wall portion of the middle section 23 in the part where the stepped section 23a is formed from the tip side is greater than the thickness of the wall portion in the part further back.

[0041] The outer surface of the middle section 23 is exposed to the atmosphere (air), and can be said to be in an environment that conducts electricity more easily than the leg section 22. For this reason, the wall thickness of the middle section 23 is set to be larger than that of the leg section 22.

[0042] In this specification, "thickness of the middle section 23" refers to the thickness of the wall portion of the middle section 23 in the portion where the wall thickness is substantially constant (i.e., the portion on the rear end side of the stepped section 23a), unless otherwise specified. The thickness of the middle section 23 is not particularly limited as long as it does not impair the purpose of the present invention, but it is set to approximately 2.0 mm to 3.0 mm, for example.

[0043] Furthermore, the insulator 2 is connected to the rear end of the flange portion 24 and includes a cylindrical rear section 25 that extends in the axial direction AX. The rear section 25 has an outer diameter smaller than the outer diameter of the flange portion 24. A through hole 21 inside the rear section 25 is provided with a rod-shaped terminal leg 51 of the terminal fitting 5.

[0044] Figure 2 is an enlarged cross-sectional view of the area near the enlarged diameter portion 31a of the central electrode 3 (central electrode body 31) housed within the central body portion 23 of the insulator 2. As shown in Figure 2, when the central electrode body 31 of the central electrode 3 is housed inside the insulator 2, there is a gap between the enlarged diameter portion 31a and electrode head 31b, which are the rear end portions of the central electrode body 31, and the inner wall 21a of the insulator 2. The sealing member 8 described above 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 alkaline components derived from glass particles, etc.

[0045] The gap between the enlarged 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. Heat tends to accumulate in such areas, having moved from the tip side of the central electrode body 31 of the central electrode 3 through the enlarged diameter portion 31a. Moreover, when a high voltage is applied to the central electrode 3, the electric field tends to concentrate in these areas. Therefore, within the insulator 2, the portion of the middle section 23 that faces the enlarged diameter portion 31a in the radial direction is subjected to the most severe conditions.

[0046] Furthermore, since the inside of the cylindrical middle section 23 is filled with a sealing member 8, the inner wall 21a of the middle section 23 is in direct contact with the sealing member 8. As a result, alkaline components originating from the sealing member 8 can come into contact with the inner wall 21a of the middle section 22.

[0047] The insulator 2 of this embodiment has excellent alkali corrosion resistance and other properties because the internal structure of the alumina-based sintered body constituting the central body portion 23 satisfies at least condition 1 shown below.

[0048] <Condition 1> 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 direction of axis AX at a position 2 mm toward the rear end along the direction of axis AX from the widest diameter portion 31a, 20 observation areas X of 192 μm × 255 μm are set so that each overlaps with a reference position m1 which is the center position between the inner circumferential surface 2a and the outer circumferential surface 2b of the insulator 2, but do not overlap with each other, the average A of the proportion of pores 11 (porosity) contained in the observation area X is 3.5% or less, and the standard deviation σ of the variation in the proportion (porosity) is 0.36 or less.

[0049] Here, we will explain condition 1 in detail with reference to Figures 2 to 5. The "maximum diameter portion of the enlarged diameter section 31a" shown in condition 1 is the portion of the enlarged diameter section 31a in the central electrode body 31 of the central electrode 3 that has the largest diameter D, as shown in Figure 2. In Figure 2, a straight line L1 is shown that intersects perpendicularly with the axis AX and crosses the portion of the enlarged diameter section 31a with the largest diameter.

[0050] Then, the insulator 2 is cut in a ring shape at a position 2 mm away from the rear end of the spark plug 1 along the axis AX direction from the widest diameter portion 31a. Of the insulator 2, the area at least 2 mm away from the widest diameter portion 31a in the axis AX direction is the area where the greatest durability (voltage resistance performance, etc.) is required. Since the internal structure of the alumina-based sintered body constituting that area is basically the same, in this embodiment, the position 2 mm away from the widest diameter portion 31a towards the rear end was set as the cutting point for the insulator 2, taking into consideration ease of cutting, etc.

[0051] Furthermore, if the widest diameter portion of the enlarged diameter section 31a is formed with a certain width from the front end to the rear end in the axial direction AX, the reference position (indicated by the straight line L1) when setting a position 2 mm away from the rear end shall be the position closest to the front end within the widest diameter portion.

[0052] In Figure 2, the point where the insulator 2 is cut is indicated by a straight line L2. This straight line L2 is shown to intersect the axis AX perpendicularly at a position 2 mm away from the rear end side (upper side of Figure 2) of the straight line L1. As shown in Figure 2, the straight line L2 extends radially across the middle section 23 of the insulator 2. Condition 1 specifies the state of the internal structure of the cut surface 230 obtained by cutting the middle section 23 radially along such a straight line L2.

[0053] Figure 3 is a schematic explanatory diagram showing the mirror-polished surface 230a obtained by mirror-polishing the cut surface 230 of the middle section 23 of the insulator 2. Figure 3 shows the cut surface 230 obtained by cutting the middle section 23 in a ring shape along the straight line L2 shown in Figure 2, in a state where it has been polished to a mirror finish. The cut surface 230 that has been subjected to the mirror-polishing treatment described later and has become mirror-finished is referred to as the mirror-polished surface 230a.

[0054] The mirror polishing treatment of the cut surface 230 is carried out based on known methods using abrasives such as diamond grinding wheels and diamond paste. The mirror polishing treatment is carried out until the surface roughness (Ra) of the cut surface 230 becomes, for example, about 0.001 μm.

[0055] The mirror-polished surface 230a is observed using a scanning electron microscope (SEM). Therefore, carbon deposition for conductivity may be performed on the mirror-polished surface 230a as needed. In this embodiment, the acceleration voltage of the SEM when observing the mirror-polished surface 230a is set to 20kV, and the magnification of the SEM is set to 500x.

[0056] As shown in Figure 3, the mirror-polished surface 230a is annular in shape, and a circular reference position m1 is set on such a mirror-polished surface 230a, indicating the center position between the inner circumferential surface 2a and the outer circumferential surface 2b of the insulator 2. Under condition 1, 20 observation regions X, each measuring 192 μm × 255 μm, are set on the mirror-polished surface 230a, such that each overlaps with the reference position m1 but does not overlap with one another.

[0057] Observation area X is a region set up to understand the state of pores (voids) 11 in the internal structure of the mirror-polished surface 230a (cut surface 230), and is rectangular in shape. Observation area X is a rectangular region with one side length of 192 μm and the other side length of 255 μm (i.e., 192 μm × 255 μm).

[0058] Furthermore, if the observation area X is set to the mirror-polished surface 230a near the inner circumferential surface 2a of the insulator 2, it will not be possible to observe the original internal structure of the insulator 2 if the internal structure on the inner circumferential surface 2 side of the insulator 2 (middle section 23) has been eroded by alkaline components. Therefore, in this embodiment, as described above, the observation area X is set to overlap with the reference position m1. A total of 20 such observation areas X are set on the mirror-polished surface 230a so as not to overlap with each other. In this embodiment, it is preferable that these observation areas X are set to be arranged in a ring shape on the annular mirror-polished surface 230a, maintaining a distance from each other, as shown in Figure 3.

[0059] A mirror-polished surface 230a corresponding to the observation area X is imaged using a SEM, thereby obtaining an SEM image corresponding to the observation area X. An SEM image is obtained for each of the 20 observation areas X. In other words, a total of 20 SEM images are obtained corresponding to a total of 20 observation areas X. Figure 4 is an explanatory diagram showing an SEM image corresponding to an observation area X. As shown in Figure 4, multiple pores 11 are shown in the SEM image.

[0060] Image analysis processing is performed on a total of 20 SEM images using publicly known image analysis software (for example, WinROOF®, manufactured by Mitani Corporation) that runs on a computer.

[0061] In the image analysis process, first, each SEM image undergoes a size calibration process based on the scale bar attached to the SEM image.

[0062] Next, the SEM image after calibration is subjected to binarization. Figure 5 is an explanatory diagram showing a binarized image obtained by binarizing an SEM image. In binarization, the brightness (luminance) of each pixel in the SEM image is binarized using a predetermined threshold (for example, threshold = 118). That is, for pixels whose brightness is below the threshold, the brightness of that pixel is set to "0", and for pixels whose brightness exceeds the threshold, the brightness of that pixel is set to "255". By binarizing in this way and eliminating intermediate tones, a binarized image is obtained. In the binarized image of Figure 5, the pores 11 are shown in black, and the other parts (ceramic parts) 12 are shown in white.

[0063] Subsequently, using the binarized image corresponding to the observation region X, all stomata (voids) 11 contained in the observation region X are extracted using known image analysis techniques. In this embodiment, the extraction of stomata 11 is performed for each of the 20 observation regions X. Furthermore, during the extraction of the stomata 11, the area of ​​each stomata 11 is also determined using known image analysis techniques.

[0064] Next, for each observation region X, the total area of ​​all pores 11 extracted from the corresponding binarized image is calculated. Then, for each observation region X, the ratio of the total area of ​​all pores 11 contained in that observation region X to the area of ​​that observation region X (hereinafter referred to as porosity) is determined. Such porosity is calculated for each of the 20 observation regions.

[0065] Subsequently, the average porosity A is calculated using the 20 porosity values ​​obtained for each of the 20 observation areas.

[0066] In this embodiment, the internal structure of the insulator 2 (middle section 23) is formed such that the average porosity A under condition 1 is 3.5% or less.

[0067] Furthermore, Condition 1 specifies the variability of the porosity. Specifically, the frequency distribution of the 20 porosity values ​​corresponding to each observation area X is considered to be a normal distribution, and when the standard deviation of the porosity is denoted as σ, σ is set to be 0.36 or less.

[0068] An insulator 2 that satisfies condition 1 can be obtained, for example, by using an Al compound powder (such as alumina powder) with a narrow (sharp) particle size distribution during manufacturing, and by applying higher pressure than conventional methods when molding the granulated powder in a predetermined mold during the molding process in the manufacturing direction of the insulator 2, as described later.

[0069] In this embodiment, if the internal structure of the insulator 2 (particularly the central body portion 23) satisfies at least the above condition 1, corrosion by alkaline components is suppressed.

[0070] The alumina-based sintered body constituting the insulator 2 is a liquid-phase sintered body, in which a liquid phase (glass component) exists around the crystal grains of alumina particles. Pores 11 exist within this liquid phase. In the internal structure of the insulator 2 at a predetermined location, if the pores 11 exist in a manner that satisfies condition 1, the liquid phase containing the pores will exist in a state where it is uniformly dispersed while moving away from each other. Since the alkaline component originating from the sealing member 8 etc. moves by permeating into the liquid phase portion of the internal structure of the insulator 2, if the liquid phase exists in a state where it is uniformly dispersed while moving away from each other as described above, it becomes difficult for the alkaline component to penetrate and move into the internal structure of the insulator 2. Therefore, even if the insulator 2 of this embodiment is in direct contact with the sealing member 8, erosion by alkaline components is suppressed.

[0071] Furthermore, in the spark plug 1 of this embodiment, in addition to condition 1 described above, the internal structure of the middle section 23 of the insulator 2 may be formed to satisfy condition 2, which will be described later.

[0072] <Condition 2> In observation area X, among the stomata, those with an area of ​​0.05 μm 2The average number B of large stomata is between 200 and 600.

[0073] The average number of large stomata B under condition 2 can be calculated as follows: First, for 20 observation regions X, the area of ​​each observation region X is 0.05 μm². 2 The number of large stomata is measured. Then, based on the total number of large stomata measured for each of the 20 observation areas (number data), the average number of large stomata (average number) B is calculated.

[0074] In this embodiment, the internal structure of the insulator 2 (middle section 23) is formed such that the average number B of large pores under condition 2 is between 200 and 600.

[0075] An insulator 2 that satisfies condition 2 can be obtained, for example, by changing the size of the granules after spray drying during manufacturing.

[0076] When the insulator 2 of the spark plug 1 satisfies condition 2 in addition to condition 1, the number of large pores in the internal structure of the insulator 2 that are relatively susceptible to the penetration of alkaline components is kept within a certain range, thereby further improving alkali corrosion resistance.

[0077] Furthermore, in the spark plug 1 of this embodiment, in addition to conditions 1 and 2 described above, the internal structure of the middle section 23 of the insulator 2 may be formed to satisfy condition 3, which will be described later.

[0078] <Condition 3> Regarding the variability in the number of large stomata in the observation area X, when the standard deviation is σ, 3σ is less than or equal to 100.

[0079] Condition 3 specifies the variability in the number of large stomata. Specifically, the frequency distribution of the number values ​​(number data) of a total of 20 large stomata corresponding to each observation area X is assumed to be a normal distribution, and when the standard deviation of the number values ​​(number data) is denoted as σ, 3σ must be 100 or less.

[0080] An insulator 2 that satisfies condition 3 can be obtained, for example, by changing the size of the granules after spray drying during manufacturing.

[0081] In addition to conditions 1 and 2, if condition 3 is also met, the uneven distribution of the number of large pores in the internal structure of the insulator 2 (middle section 23) is reduced, localized strength deficiencies are suppressed, and the mechanical strength (impact resistance) of the insulator 2 is improved.

[0082] Furthermore, it is more preferable that 3σ in condition 3 is 50 or less. When 3σ is 50 or less, the alkali erosion resistance of the insulator 2 is further improved.

[0083] Furthermore, in the spark plug 1 of this embodiment, the internal structure of the middle section 23 of the insulator 2 may be formed in such a way that it satisfies condition 4, which will be described later.

[0084] <Condition 4> In observation area X, the average A of the aforementioned percentage of stomata (porosity) is 1.0% or more, and the average B of the aforementioned number of large stomata is 240 or more.

[0085] In the spark plug 1 of this embodiment, if condition 4 is met, the alkali corrosion resistance of the insulator 2 is further improved.

[0086] Furthermore, in the spark plug 1 of this embodiment, the internal structure of the middle section 23 of the insulator 2 may be formed in such a way that it satisfies condition 5, which will be described later.

[0087] <Condition 5> In observation area X, when the standard deviation of the variation in the number of large stomata is σ, the value of "average number + 3σ" is less than 330.

[0088] Condition 5 specifies the variability in the number of large stomata. Specifically, assuming that the frequency distribution of the total number of large stomata (number data) corresponding to each observation area X is a normal distribution, and with the standard deviation of the said number (number data) being σ, the value of "mean of the said number + 3σ" is less than 330.

[0089] In the spark plug 1 of this embodiment, when condition 5 is met, the alkali corrosion resistance of the insulator 2 is further improved.

[0090] Furthermore, in the spark plug 1 of this embodiment, the internal structure of the middle section 23 of the insulator 2 may be formed in such a way that it satisfies condition 6, which will be described later.

[0091] In the mirror-polished surface 230a, when the region S located between the inner circumferential surface 2a and the outer circumferential surface 2b of the insulator 2 is divided into three equal radial lengths, 20 inner observation regions Xa of 192 μm × 255 μm are set in the innermost inner region Sa so as not to overlap with each other, and 20 outer observation regions Xb of 192 μm × 255 μm are set in the outermost outer region Sb so as not to overlap with each other. The average porosity Aa contained in the inner observation region Xa is 0.1 to 2% smaller than the average porosity Ab contained in the outer observation region Xb. Furthermore, in condition 5, it is more preferable that the average porosity Aa contained in the inner observation region Xa is 1.8 to 2% smaller than the average porosity Ab contained in the outer observation region Xb.

[0092] Here, referring to Figure 6, we will explain how to determine the average porosity Aa of the pores in the inner observation area Xa and the porosity Ab of the pores in the outer observation area Xb, as defined in Condition 6. Figure 6 is a schematic diagram illustrating the inner observation area Xa and outer observation area Xb set on the mirror-polished surface 230a. In Condition 6, as in Condition 1, the state of the internal structure on the mirror-polished surface 230a (cut surface 230) of the insulator 2 is the subject of the definition. However, in Condition 6, the observation areas (inner observation area Xa, outer observation area Xb) set on the mirror-polished surface 230a for understanding the internal structure are different.

[0093] As shown in Figure 6, two circular reference lines m2 and m3 are set on the mirror-polished surface 230a such that the radial length of the annular region S (region S corresponding to the mirror-polished surface 230a) located between the inner surface 2a and the outer surface 2b of the insulator 2 is divided into three equal parts. By setting these two reference lines m2 and m3, the annular region S is divided into three concentric annular regions. Of these regions, the innermost region becomes the inner region Sa, and the outermost region becomes the outer region Sb.

[0094] Then, for the inner region Sa, 20 inner observation regions Xa, each measuring 192 μm × 255 μm, are set up so as not to overlap with each other. Similarly, for the outer region Sb, 20 outer observation regions Xb, each measuring 192 μm × 255 μm, are set up so as not to overlap with each other. Both the inner observation regions Xa and the outer observation regions Xb are rectangular regions with one side length of 192 μm and the other side length of 255 μm (i.e., 192 μm × 255 μm). Condition 6 defines the relationship between the internal structure of the mirror-polished surface 230a near the inner circumferential surface 2a and the internal structure of the mirror-polished surface 230a near the outer circumferential surface 2b.

[0095] It is preferable that the inner observation regions Xa are arranged in a ring shape within the annular inner region Sa, while maintaining distance from each other. Similarly, it is preferable that the outer observation regions Xb are arranged in a ring shape within the annular outer region Sb, while maintaining distance from each other.

[0096] Furthermore, it is preferable that the inner observation area Xa be set near the reference line m2 side of the inner area Sa, rather than on the inner circumferential surface 2a side.

[0097] Then, by photographing the mirror-polished surface 230a in the range corresponding to the inner observation area Xa using the SEM, an SEM image corresponding to the inner observation area Xa is obtained. Similarly, by photographing the mirror-polished surface 230a in the range corresponding to the outer observation area Xb using the SEM, an SEM image corresponding to the outer observation area Xb is obtained. In this embodiment, 20 corresponding SEM images are obtained for both the inner observation area Xa and the outer observation area Xb. The acceleration voltage of the SEM is set to 20kV, and the magnification of the SEM is set to 500x.

[0098] The same processing as described above for the SEM image corresponding to observation area X is performed on the 20 SEM images corresponding to the inner observation area Xa, thereby determining the average Aa of the porosity (percentage of pores) contained in the inner observation area Xa. In other words, for each inner observation area Xa, the ratio of the total area of ​​all pores contained in that inner observation area Xa to the area of ​​that inner observation area Xa (porosity) is determined. Then, based on the total of 20 such ratios (porosity) obtained for each of the 20 inner observation areas Xa, the average Aa of the porosity (percentage of pores) contained in the inner observation area Xa is determined.

[0099] Furthermore, the same processing as that performed on the SEM images corresponding to the outer observation region Xb is applied to the 20 SEM images corresponding to the outer observation region X, thereby determining the average Ab of the porosity (percentage of pores) contained in the outer observation region Xb. In other words, for each outer observation region Xb, the ratio of the total area of ​​all pores contained in that outer observation region Xb to the area of ​​that outer observation region Xb (porosity) is determined. Then, based on the total of 20 such ratios (porosity) obtained for each of the 20 outer observation regions Xb, the average Ab of the porosity (percentage of pores) contained in the outer observation region Xb is determined.

[0100] Subsequently, the difference between the average porosity Aa of the pores in the inner observation area Xa and the average porosity Ab of the pores in the outer observation area Xb (average Ab - average Aa) is calculated.

[0101] In this embodiment, the internal structure of the insulator 2 (middle section 23) may be formed such that the average porosity Aa of the proportion of pores in the inner observation area Xa is 0.1% to 2% smaller than the average porosity Ab of the proportion of pores in the outer observation area Xb.

[0102] In the spark plug 1 of this embodiment, when the above-described condition 6 is met, the porosity of the outer surface 2b of the insulator 2 is higher than that of the inner surface 2a of the insulator 2, thereby improving the mechanical strength (impact resistance) of the insulator 2.

[0103] Next, a method for manufacturing the insulator 2 will be described. The insulator 2 is manufactured to satisfy the conditions described above, such as condition 1. There are no particular restrictions on the method for manufacturing the insulator 2, as long as the final insulator 2 satisfies the conditions, such as condition 1. Here, an example of a method for manufacturing the insulator 2 will be described.

[0104] The method for manufacturing the insulator 2 mainly comprises a slurry preparation step, a defoaming step, a granulation step, a molding step, a grinding step, and a firing step.

[0105] <Slurry preparation process> The slurry preparation process involves mixing raw material powder, a binder, and a solvent to produce a slurry. The raw material powder primarily consists of a powder of a compound that converts to alumina upon calcination (hereinafter referred to as Al compound powder). For example, alumina powder is used as the Al compound powder.

[0106] In the slurry preparation process, a grinding process is carried out for the purpose of mixing and grinding the raw material powder. The grinding process is performed using a wet grinder such as a ball mill. The diameter of the pebbles used in the wet grinder is not particularly limited as long as it does not impair the objective of the present invention, but is preferably 3 mm to 20 mm, more preferably 3 mm to 10 mm, and even more preferably 3 mm to 6 mm. In addition, two or more types of pebbles with different diameters may be combined. Through such a grinding process, the raw material powder has a small variation in particle size and a sharp particle size distribution. When such raw material powder is used, abnormal grain growth is suppressed in the alumina-based sintered body obtained after sintering, and the sintering density can be increased. As a result, the alkali erosion resistance of the insulator is improved.

[0107] The particle size (particle size after grinding) of the Al compound powder (alumina powder, etc.) is not particularly limited as long as it does not impair the objective of the present invention, but for example, it is preferably 1.5 μm or larger, more preferably 1.7 μm or larger, preferably 2.5 μm or smaller, and more preferably 2.0 μm or smaller. When the particle size of the Al compound powder (alumina powder, etc.) is within this range, the number of defects in the insulator is suppressed and an appropriate sintering density is obtained. The particle size is the volume-based median diameter (D50) measured by laser diffraction (Microtrac particle size distribution analyzer, product name "MT-3000", manufactured by Nikkiso Co., Ltd.).

[0108] The Al compound powder is preferably prepared such that, when the mass (on an oxide basis) of the alumina-based sintered body after calcination is taken as 100% by mass, it accounts for 90% by mass or more on an oxide basis. More preferably, it is 90% by mass or more and 98% by mass or less, and even more preferably 90% by mass or more and 97% by mass or less. In addition, the raw material powder may contain powders other than Al compound powder as long as it does not impair the purpose of the present invention.

[0109] Binders are added to the slurry to improve the moldability of the raw material powder, etc. Examples of such binders include hydrophilic binders such as polyvinyl alcohol, aqueous acrylic resin, gum arabic, and dextrin. These may be used individually or in combination of two or more.

[0110] The amount of binder added is not particularly limited as long as it does not impair the purpose of the present invention, but for example, it is added in a ratio of 1 to 20 parts by mass, preferably 3 to 7 parts by mass, per 100 parts by mass of raw material powder.

[0111] The solvent is used for purposes such as dispersing raw material powders. Examples of solvents include water and alcohol. These may be used individually or in combination of two or more.

[0112] The amount of solvent added is not particularly limited as long as it does not impair the objective of the present invention, but for example, it is added in a ratio of 23 to 40 parts by mass, preferably 25 to 35 parts by mass, per 100 parts by mass of raw material powder. The slurry may also contain other components besides the raw material powder, binder, and solvent, as needed. Known stirring and mixing devices can be used to mix the slurry.

[0113] <Defoaming process> A defoaming process may be performed on the slurry after the slurry preparation process, if necessary. In the defoaming process, for example, the container containing the mixed (kneaded) slurry is placed in a vacuum defoaming apparatus and the pressure is reduced to create a low-pressure environment, thereby removing air bubbles contained in the slurry. The amount of air bubbles in the slurry can be determined by comparing the density of the slurry before and after defoaming.

[0114] <Granulation process> The granulation process is a process of producing spherical granulated powder from a slurry containing raw material powder, etc. There are no particular restrictions on the method of producing granulated powder from slurry as long as it does not impair the objective of the present invention, but for example, the spray drying method can be used. In the spray drying method, granulated powder with a predetermined particle size can be obtained by spray drying the slurry using a predetermined spray dryer device. The average particle size of the granulated powder is not particularly limited as long as it does not impair the objective of the present invention, but for example, 212 μm pass ≥ 95% or less is preferred, 180 μm pass ≥ 95% or less is more preferred, and 160 μm pass ≥ 95% or less is even more preferred.

[0115] <Forming process> The molding process is a process of obtaining a molded body by molding granulated powder into a predetermined shape using a mold. The molding process is carried out by rubber press molding, die press molding, etc. In this embodiment, the pressure (press pressure increase rate) applied to the mold (for example, the inner rubber mold and outer rubber mold of a rubber press molding machine) from the outer circumference is adjusted to increase in stages. Furthermore, it is preferable to adjust it to a higher pressure range than conventional methods (for example, 100 MPa or more). The upper limit of the pressure is not particularly limited as long as it does not impair the purpose of the present invention, but for example, it may be adjusted to 200 MPa or less.

[0116] <Grinding process> The grinding process is a process that removes excess material from the molded body obtained after the molding process and polishes the surface of the molded body. In the grinding process, excess material is removed and the surface of the molded body is polished by grinding with a resinoid grinding wheel or the like. Through this grinding process, the shape of the molded body is refined.

[0117] <Firing Process> The firing process involves firing the molded body, whose shape has been refined by the grinding process, to obtain an insulator. In the firing process, for example, the body is fired in an atmospheric environment at a temperature of 1450°C to 1650°C for 1 to 8 hours. After firing, the molded body is cooled to obtain an insulator 2 made of an alumina-based sintered body.

[0118] The spark plug 1 of this embodiment is manufactured using the insulator 2 obtained as described above. The configuration of the spark plug 1 other than the insulator 2 is the same as that of known configurations as described above.

[0119] The present invention will be described in more detail below based on the examples. However, the present invention is not limited in any way by these examples.

[0120] [Example 1] (Preparation of test samples) Three insulators (hereinafter referred to as "test samples") with the same basic structure as the spark plug insulator exemplified in Embodiment 1 were manufactured using the same manufacturing method as in Embodiment 1. The thickness of the middle section of the insulator was 3 mm. In the slurry preparation process, when grinding the raw material powder with a wet grinder, 3 mm diameter pebbles (φ3 mm) and 10 mm diameter pebbles (φ10 mm) were used in a ratio of 50% by mass and 50% by mass, respectively.

[0121] (Measurement of withstand voltage after alkaline erosion) To measure the dielectric strength after alkaline erosion, a pre-processed insulator was prepared. Specifically, when the central electrode body was mounted inside the insulator, the tip of the central electrode body was pre-insulated around the leg portion so that it would not be exposed from the leg portion and the thickness of the leg portion would be approximately constant. Then, a rod-shaped central electrode body was mounted inside such an insulator, with the tip rounded to prevent electric field concentration, and the opening at the tip of the insulator was closed. This was then assembled onto the main fitting to create a test sample. The test sample was placed in a heating furnace maintained at approximately 200°C, and a voltage of 35kV was applied from the tip of the central electrode body of the test sample for 100 hours. Grounding was performed from the main fitting. By continuously applying voltage to the insulator of the test sample in this way, electric field concentration occurred at a predetermined location in the middle section of the insulator (the part radially opposite the electrode flange (enlarged diameter section)) without external discharge, forcing alkaline erosion at that predetermined location. The presence or absence of alkaline erosion can be determined by measuring the presence of alkali metals such as sodium and alkaline earth metals on an insulator using an electron beam probe microalanizer (EPMA).

[0122] Subsequently, the test sample containing the alkali-eroded insulator was placed in a high-pressure chamber. While carbon dioxide (CO2) was supplied into the high-pressure chamber at a pressure of approximately 5 MPa, a voltage was applied from the tip of the central electrode body of the test sample at a voltage boosting rate of 0.1 kV / sec. Grounding was performed from the main metal fitting. The breakdown voltage upon penetration of the insulator was then measured. The results are shown in Table 1.

[0123] (Observation of the cross-section (mirror-polished surface) of the middle section 1) For the obtained test sample, the insulator was cut perpendicular to the axial direction at a position 2 mm away from the rear end along the axial direction from the maximum diameter of the enlarged portion of the central electrode. The cut surface of the obtained test sample was then polished to a mirror finish, and the structure of the cut surface (mirror-polished surface) was observed using a SEM (model "JSM-IT300LA", manufactured by JEOL Ltd.). The SEM acceleration voltage was set to 20 kV, and the SEM magnification was set to 500x. Then, on the cut surface (mirror-polished surface), 20 observation regions X of 192 μm × 255 μm were set, each overlapping with the reference position m1, which is the center position between the inner circumferential surface 2a and the outer circumferential surface 2b of the insulator 2, but without overlapping with each other. A total of 20 SEM images corresponding to these 20 observation regions X were acquired. Then, image analysis processing was performed on these SEM images using image analysis software (WinROOF®, manufactured by Mitani Corporation) to determine the average porosity (percentage of pores) A ​​contained in the observation area X. Furthermore, the frequency distribution of the 20 porosity values ​​corresponding to each observation area X was assumed to be a normal distribution, and the standard deviation σ of the porosity was calculated. The results are shown in Table 1.

[0124] Furthermore, for each of the 20 observation regions X, the area is 0.05 μm². 2 The number of large stomata was measured, and the average number of large stomata (average number) B was calculated based on the total number of these 20 large stomata (number data). The results are shown in Table 1.

[0125] Furthermore, assuming that the frequency distribution of the number of large stomata (count data) for a total of 20 stomata corresponding to each observation area X is a normal distribution, we calculated the value of "3σ" and the value of "the mean B + 3σ" (number of stomata) when the standard deviation of the aforementioned number values ​​(count data) is σ. The results are shown in Table 1.

[0126] [Examples 2-10, Examples 12-17] Insulators for Examples 2-10 and 12-17 were prepared in the same manner as in Example 1, except that the ratio of pebbles used when crushing the raw material powder in the slurry preparation process was appropriately changed.

[0127] [Comparative Example 1] In the slurry preparation process, the insulator of Comparative Example 1 was prepared in the same manner as in Example 1, except that when grinding the raw material powder with a wet grinder, pebbles with a diameter of 3 mm (φ3 mm), 10 mm (φ10 mm), and 30 mm (φ30 mm) were used in proportions of 10% by mass, 40% by mass, and 50% by mass, respectively.

[0128] [Comparative Example 2] The insulator of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the ratio of pebbles used when crushing the raw material powder in the slurry preparation process was appropriately changed.

[0129] For the insulators obtained in Examples 2-10, Examples 12-17, and Comparative Examples 1 and 2, the "measurement of withstand voltage after alkaline erosion" and "observation of the cross-section (mirror-polished surface) of the central section 1" were performed in the same manner as in Example 1.

[0130] Furthermore, for Examples 4, 9, 10, 12-14, and 16, 17, the "Observation of the Cross-Sectional Surface (Mirror Polished Surface) of the Middle Section 2" and "Evaluation of Impact Resistance" described below were performed. The results are shown in Table 1.

[0131] (Observation of the cross-section (mirror-polished surface) of the middle section 2) The mirror-polished surface of the insulator used in "Observation of the Cross-Sectional Surface (Mirror-Polished Surface) of the Middle Body 1" above was observed using a scanning electron microscope (SEM). The SEM acceleration voltage was set to 20kV, and the SEM magnification was set to 500x. Then, as shown in Figure 6, the region S located between the inner circumferential surface 2s and the outer circumferential surface 2b of the insulator was divided into three equal radial lengths. In the innermost region Sa, 20 inner observation regions Xa, each measuring 192μm × 255μm, were set so as not to overlap, and in the outermost region Sb, 20 outer observation regions Xb, each measuring 192μm × 255μm, were set so as not to overlap.

[0132] Subsequently, 20 SEM images corresponding to the inner observation area Xa and 20 SEM images corresponding to the outer observation area Xb were acquired. Based on these SEM images, the average porosity Aa in the inner observation area Xa and the average porosity Ab in the outer observation area Xb were determined. Then, the difference between the average Aa and the average Ab (average Ab - average Aa) was calculated. The results are shown in Table 1.

[0133] (Evaluation of impact resistance) The impact resistance of each insulator was evaluated by performing a Charpy test as specified in JIS B7733. The specific evaluation method is as follows: First, a spark plug (hereinafter referred to as the test spark plug) with the same configuration as illustrated in Embodiment 1 above was made using the insulator. The test spark plug was fixed with its axial direction set vertically and its tip facing downwards, by screwing the threaded portion of the main fitting of the test spark plug into a screw hole provided in the test stand. A hammer with a pivot point was rotatably mounted above the fixed test spark plug in the axial direction. The tip of the hammer was then lifted and released, allowing it to rotate by free fall, and the tip of the hammer struck a point approximately 1 mm from the rear end of the insulator. The lifting angle of the hammer (angle with respect to the axial direction) was set to 34 degrees, and the tip of the hammer was struck against the insulator of the test spark plug to check whether a crack occurred in the insulator. This hammer impact was performed a maximum of two times for each insulator. If the insulator cracked after the first impact, the test was terminated. Conversely, if the insulator did not crack after the first impact, a second impact was performed on the same insulator. The results are shown in Table 1. In Table 1, the symbol "×" indicates that the insulator cracked after the first impact, "〇" indicates that the insulator cracked after the second impact, and "◎" indicates that no crack occurred after the second impact.

[0134] [Table 1]

[0135] As shown in Table 1, Examples 1-10 and 12-17, which satisfy condition 1 described above, exhibit superior dielectric strength after alkaline erosion compared to Comparative Examples 1 and 2. In Examples 1-10 and 12-17, it was confirmed that alkaline erosion could be suppressed even when the treatment was carried out under conditions that forced alkaline erosion.

[0136] Of Examples 1-10 and Examples 12-17, Examples 1, 2, 4, 6-10, and 12-17, which also satisfy condition 2 described above, showed superior alkali erosion resistance compared to Examples 3 and 5.

[0137] Furthermore, it was confirmed that Examples 9, 10, 12-14, and 16-17, which satisfy condition 3 described above, exhibit superior impact resistance (Charpy strength) compared to Example 4.

[0138] Furthermore, among Examples 1, 2, 4, 6, 7, 9, 10, 12-15, and 17, it was confirmed that Examples 1, 7, 9-13, and 17, which also have the above-mentioned condition 3 where 3σ is 50 or less (i.e., 3σ ≤ 50), exhibit superior alkali erosion resistance compared to Examples 2, 4, 6, and 14-16.

[0139] Furthermore, Examples 1, 7, 8, 16, and 17, which satisfy condition 4 described above, exhibit excellent alkali erosion resistance. Of these, Examples 1, 7, and 17, where 3σ ≤ 50, exhibit particularly superior alkali erosion resistance compared to Examples 8 and 16, where 50 < 3σ ≤ 100.

[0140] Furthermore, among Examples 1, 2, 4, 6-10, and 12-17, Examples 1, 7-10, 12, 13, 15, and 17, which also satisfy condition 5 described above, were found to have superior alkali erosion resistance compared to Examples 2, 4, 6, 14, and 16.

[0141] Furthermore, among Examples 9, 10, 12-14, and 16-17, it was confirmed that Examples 10, 16, and 17, which also satisfy condition 6 described above, exhibit superior impact resistance (Charpy strength) compared to Examples 9 and 12-14. [Explanation of symbols]

[0142] 1...Spark plug, 2...Insulator, 21...Through hole, 22...Leg length, 23...Middle section, 230...Cut surface, 230a...Mirror polished surface, 24...Flange, 25...Rear cylinder, 26...First enlarged diameter section, 27...Second enlarged diameter section, 3...Center electrode, 31...Center electrode body, 31a...Enlarged diameter section (electrode flange), 31b...Electrode head, 31c...Electrode legs, 4...Ground electrode, 5...Terminal fitting, 6...Main fitting, 7...Resistor, 8...Sealing member, 9...Sealing member, 11...Pores, AX...Axis, Sa...Inner region, Sb...Outer region, X...Observation region, Xa...Inner observation region, Xb...Outer observation region

Claims

1. It has a cylindrical shape extending along the axial direction, and is an insulator made of an alumina-based sintered body, A rod-shaped electrode inserted into the insulator such that its tip is exposed from the insulator and its rear end is housed inside the insulator, comprising a central electrode having an enlarged diameter portion that widens radially on its rear end side and engages with the inner wall of the insulator, A spark plug comprising a conductive sealing material disposed on the rear end side of the central electrode inside the insulator, The insulator comprises a leg portion located at the tip, a middle portion located at the rear end of the leg portion and having a larger diameter than the leg portion, and a flange portion located at the rear end of the middle portion and having a larger diameter than the middle portion, wherein the enlarged diameter portion of the central electrode is housed in the middle portion. When the middle section of the insulator is cut perpendicular to the axial direction at a position 2 mm from the maximum diameter of the enlarged section toward the rear end along the axial direction, and the resulting cut surface is mirror-polished, 20 observation areas of 192 μm × 255 μm are set so that each overlaps with a reference position which is the center position between the inner and outer surfaces of the insulator, but do not overlap with each other, the average percentage of pores (porosity) included in the observation areas is 2.9% or less, and the standard deviation of the variation in the percentage (porosity) is σ is 0.33 or less. The surface roughness (Ra) of the aforementioned mirror-polished surface is 0.001 μm. The observation area is rectangular in shape, with a long side of 255 μm and a short side of 192 μm, and the observation area is arranged such that the reference position crosses the two short sides. The thickness of the middle section refers to the thickness of the wall portion in the middle section where the wall thickness is constant, and the thickness of the middle section is 3.0 mm. In the aforementioned observation area, the average number of large pores with an area of ​​0.05 μm² or more is between 200 and 600. A spark plug in which, when the standard deviation of the variation in the number of large pores in the observation area is denoted as σ, 3σ is 100 or less.

2. The spark plug according to claim 1, wherein the aforementioned 3σ is 50 or less.

3. A spark plug according to claim 1 or claim 2, wherein in the observation area, the average of the proportion of pores (porosity) is 1.0% or more, and the average number of large pores is 240 or more.

4. The spark plug according to claim 3, wherein, in the observation area, when the standard deviation of the variation in the number of large pores is σ, the value of "average number + 3σ" is less than 330.

5. The spark plug according to claim 4, wherein, in the mirror-polished surface, when the region between the inner and outer surfaces of the insulator is divided into three equal radial lengths, 20 inner observation regions of 192 μm × 255 μm are set in the innermost region so as not to overlap with each other, and 20 outer observation regions of 192 μm × 255 μm are set in the outermost region so as not to overlap with each other, and the average percentage of pores (porosity) in the inner observation regions is 0.1 to 2% smaller than the average percentage of pores (porosity) in the outer observation regions.

Images