Insulator for spark plug and spark plug

Abstract

To provide a spark plug insulator and a spark plug that can improve bending strength and thermal shock resistance.MEANS FOR SOLVING THE PROBLEM: When a bending force is applied to a spark plug insulator, causing it to break, the fracture surface created by the break is divided into two by a plane perpendicular to the direction of the force and including the axis, and within the range including the origin of the break, the average area of particles appearing in a planar image of the range is 4.4 μm2 or more and 8.0 μm2 or less. The maximum area of the particles is 600 μm2 or less, the particles include large particles with an area of 60 μm2 or more and 600 μm2 or less, and the large particles are present at 0.1 particles / mm2 or more per unit area of the planar image.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an insulator for a spark plug and a spark plug.

Background Art

[0002] In a spark plug including an insulator made of an alumina-based sintered body, in order to increase the mechanical strength of the insulator, a prior art is disclosed in Patent Document 1 in which the average grain size of the crystal grains of the insulator is 1.5 μm or less and the standard deviation of the particle size distribution of the crystal grains is 1.2 μm or less.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, there are requirements for improving the bending strength and thermal shock resistance of the insulator.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide an insulator for a spark plug and a spark plug that can improve the bending strength and thermal shock resistance.

Means for Solving the Problems

[0006] To achieve this object, a first aspect of the present invention is an insulator for a spark plug made of an alumina-based sintered body provided with an axial hole extending along an axis, wherein when a force for bending the insulator for a spark plug is applied to the insulator for a spark plug and it is broken, the range including the origin of the break among the ranges obtained by dividing the fracture surface formed by the break with a plane perpendicular to the direction of the force and including the axis into two parts has an average area of the particles appearing in the plane image of the range of 4.4 μm 2 or more and 8.0 μm 2The following, with the maximum area of the particles being 600 μm 2 or less. The particles include large particles with an area of 60 μm 2 or more and 600 μm 2 or less, and the large particles are present at 0.1 particles / mm per unit area of the planar image 2 or more.

[0007] In a second aspect, in the first aspect, the large particles are present at 6.2 particles / mm or less per unit area of the planar image 2 or less.

[0008] In a third aspect, in the first or second aspect, the particles include small particles with an area of 20 μm 2 or more and 59 μm 2 or less, and the small particles are present at 613 particles / mm or more and 2270 particles / mm 2 or less per unit area of the planar image 2 or less.

[0009] A fourth aspect is a spark plug, comprising an insulator for a spark plug according to any one of the first to third aspects.

Advantages of the Invention

[0010] Since the average area of the particles appearing in the planar image of the insulator for the spark plug is 4.4 μm 2 or more and 8.0 μm 2 or less, and the maximum area of the particles is 600 μm 2 or less, and large particles with an area of 60 μm 2 or more and 600 μm 2 or less are present at 0.1 particles / mm or more per unit area of the planar image, the flexural strength and thermal shock resistance can be improved. 2

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view combining an external view and a full cross-sectional view with respect to the axis C of the axial hole 12 of the spark plug 10 in one embodiment. The lower side of the paper surface of FIG. 1 is the tip side of the spark plug 10, and the upper side of the paper surface is the rear end side of the spark plug 10. The spark plug 10 includes an insulator 11 (insulator for spark plug). The insulator 11 is a cylindrical member provided with an axial hole 12 extending along the axis C, and is an alumina-based sintered body excellent in insulation and mechanical properties at high temperatures.

[0013] A rod-shaped metal center electrode 13 is disposed in the axial hole 12 of the insulator 11. The center electrode 13 has a core material excellent in thermal conductivity embedded in a base material. The base material is formed of an alloy mainly composed of Ni or a metal material made of Ni. The core material is formed of copper or an alloy mainly composed of copper. The core material can be omitted.

[0014] The terminal fitting 14 is a rod-shaped member to which an ignition device (not shown) is connected, and is formed of a conductive metal material (for example, low carbon steel, etc.). The tip side of the terminal fitting 14 is disposed in the axial hole 12 of the insulator 11, and the rear end side of the terminal fitting 14 protrudes from the insulator 11. The terminal fitting 14 is electrically connected to the center electrode 13 in the axial hole 12.

[0015] The main body fitting 15 is a substantially cylindrical member formed of a conductive metal material (for example, low carbon steel, etc.). The main body fitting 15 is disposed on the outer periphery of the insulator 11. A male thread 16 is provided on the tip side of the main body fitting 15.

[0016] The ground electrode 17 is a rod-shaped metal (for example, nickel-based alloy) member connected to the main body fitting 15. A spark gap is formed between the ground electrode 17 and the center electrode 13.

[0017] The spark plug 10 is manufactured, for example, by the following method. After inserting the center electrode 13 into the axial hole 12 of the insulator 11, the axial hole 12 is filled with a conductive powder containing a glass component. After inserting the terminal fitting 14 from the rear end side of the axial hole 12, the terminal fitting 14 is press-fitted while heating to a temperature higher than the softening point of the glass component contained in the powder, for example, to apply an axial load to the powder by the terminal fitting 14. The powder is compressed and sintered to electrically connect the center electrode 13 and the terminal fitting 14 in the axial hole 12. Next, after assembling the main body fitting 15 to which the ground electrode 17 is connected to the outer periphery of the insulator 11, the ground electrode 17 is bent to set a spark gap between the ground electrode 17 and the center electrode 13, thereby obtaining the spark plug 10.

[0018] An example of the manufacturing method of the insulator 11 will be described. The insulator 11 is manufactured through steps of slurry preparation, defoaming, granulation, molding, grinding, and firing. The following will be described in order.

[0019] The slurry preparation step is a step of mixing raw material powder, a binder, and a solvent to prepare a slurry. As the raw material powder, a powder of a compound (hereinafter referred to as "Al compound powder") that is converted into alumina by firing is used as the main component. As the Al compound powder, for example, alumina powder is used.

[0020] In the slurry preparation step, a grinding step for the purpose of mixing and grinding the raw material powder is performed. The grinding step is performed using a wet grinder such as a ball mill. The diameter of the grinding balls used in the wet grinder is not particularly limited as long as the object of the present invention is not impaired, but is preferably 2 mm or more and 20 mm or less, more preferably 2 mm or more and 10 mm or less, and still more preferably 2 mm or more and 6 mm or less. Two or more types of grinding balls having different diameters may be combined. By such a grinding step, the raw material powder has a small variation in particle size (particle diameter) and a sharp particle size distribution. When such raw material powder is used, in the alumina-based sintered body obtained after sintering, the particle size can be controlled and the sintering density can be increased.

[0021] The particle size (particle size after pulverization) of the Al compound powder (such as alumina powder) is not particularly limited as long as the object of the present invention is not impaired. For example, 1.5 μm or more is preferable, 1.7 μm or more is more preferable, 2.5 μm or less is preferable, and 2.0 μm or less is more preferable. When the particle size of the Al compound powder is within such a 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 the laser diffraction method (manufactured by Nikkiso Co., Ltd., Microtrac particle size distribution measuring device, product name "MT-3000").

[0022] The Al compound powder is preferably prepared so that it is 90% by mass or more in terms of oxide when the mass (in terms of oxide) of the alumina-based sintered body after firing is 100% by mass. More preferably, it is 90% by mass or more and 98% by mass or less, and still more preferably, it is 90% by mass or more and 97% by mass or less. As long as the object of the present invention is not impaired, the raw material powder may contain powders other than the Al compound powder.

[0023] The binder is added to the slurry for the purpose of improving the moldability of the raw material powder and the like. Examples of such binders include hydrophilic binders such as polyvinyl alcohol, aqueous acrylic resin, gum arabic, and dextrin. These may be used alone or in combination of two or more. The blending amount of the binder is not particularly limited as long as the object of the present invention is not impaired. For example, it is blended at a ratio of 1 to 10 parts by mass, preferably 3 to 7 parts by mass, per 100 parts by mass of the raw material powder.

[0024] The solvent is used for the purpose of dispersing the raw material powder and the like. Examples of the solvent include water, alcohol, and the like. These may be used alone or in combination of two or more. The blending amount of the solvent is not particularly limited as long as the object of the present invention is not impaired. For example, it is blended at a ratio of 23 to 40 parts by mass, preferably 25 to 35 parts by mass, per 100 parts by mass of the raw material powder. Other components other than the raw material powder, binder, and solvent may be blended in the slurry as necessary. Known stirring and mixing devices and the like can be used for mixing the slurry.

[0025] The prepared slurry may be defoamed as necessary. In the defoaming process, for example, the container containing the slurry after mixing (kneading) is placed in a vacuum defoaming device, depressurized, and placed in a low-pressure environment, so that the bubbles contained in the slurry are removed. By comparing the densities of the slurry before and after defoaming, the amount of bubbles in the slurry can be grasped.

[0026] The granulation process is a process of producing spherical granulated powder from a slurry containing raw material powder and the like. The method of producing granulated powder from the slurry is not particularly limited as long as the object of the present invention is not impaired. For example, the spray drying method can be mentioned. In the spray drying method, by using a predetermined spray dryer device to spray-dry the slurry, granulated powder having a predetermined particle size can be obtained. The particle size of the granulated powder is not particularly limited as long as the object of the present invention is not impaired. For example, 212μm pass ≧ 95% or less is preferable, and 180μm pass ≧ 95% or less is more preferable.

[0027] The molding process is a process of molding the granulated powder into a predetermined shape using a molding die to obtain a molded body. The molding process is performed by rubber press molding, die press molding, or the like. In the case of the present embodiment, the pressure (pressurization rate) applied to the molding die (for example, the inner rubber die and the outer rubber die of a rubber press molding machine) from the outer peripheral side is adjusted to increase step by step. Also, it is preferably adjusted to a pressure range higher than the conventional one (for example, 100 MPa or more). The upper limit value of the pressure is not particularly limited as long as the object of the present invention is not impaired. For example, it may be adjusted to 200 MPa or less.

[0028] The grinding process is a process of removing the machining allowance of the molded body obtained after the molding process and polishing the surface of the molded body. In the grinding process, by grinding a resinoid grinding wheel or the like, the removal of the machining allowance and the polishing of the surface of the molded body are performed. By such a grinding process, the shape of the molded body is adjusted.

[0029] The firing process is a process of firing a formed body whose shape has been adjusted by a 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 air atmosphere. After firing, the formed body is cooled to obtain an insulator 11 made of an alumina-based sintered body.

[0030] The insulator 11 is a brittle material and is weak against tensile stress. When a force is applied to bend the insulator 11, cracks develop from the pores and defects inherent in the insulator 11. By observing the structure of the fracture surface formed by breaking the insulator 11, defects and the like inherent in the insulator 11 can be clarified. Tensile force can be applied to the insulator 11 to break it and create a fracture surface using various means such as the three-point bending test and four-point bending test defined in JIS R1601:2008 and the insulator bending strength test defined in JIS B8031:2006.

[0031] With reference to FIG. 1, the insulator bending strength test defined in JIS B8031:2006 will be described. The male thread 16 of the spark plug 10 is tightened to the iron jig 18 with a specified maximum torque. After fixing the spark plug 10 to the jig 18, a force F perpendicular to the axis C is applied at a position within 5 mm from the rear end of the insulator 11. The insulator 11 is pushed at a speed of 10 mm / min or less with the force F without applying an impact to the insulator 11 to break the insulator 11 and create a fracture surface.

[0032] FIG. 2 is a schematic diagram of the fracture surface of the insulator 11. Among the ranges 21 and 22 obtained by dividing the fracture surface (the annulus around the shaft hole 12) into two by a plane 20 that is a plane perpendicular to the direction of the force F (see FIG. 1) applied to the insulator 11 and includes the axis C, the range 21 including the starting point of fracture is mainly a surface formed by the application of tensile force. The range 22 is a surface formed by the progression of cracks formed in the range 21. Abnormally grown particles during firing are one of the defects inherent in the insulator 11. Using a scanning electron microscope (SEM), the structure of the range 21 (the surface including the side where the force F is applied in this embodiment) including the starting point of fracture is observed to examine the size and distribution of the particles in the range 21.

[0033] Since the size of the particles cannot be confirmed even if the entire area 21 is captured in a single SEM image, the entire area 21 is divided into multiple parts, and an SEM image (planar image) is obtained for each part. An example of an SEM image is one in which the area 21 is divided into rectangular parts measuring 985 μm in length and 1231 μm in width, and enlarged at a low magnification (for example, 100 times). Since the area 21 has the shape of a ring cut in half, some SEM images show the shaft hole 12 inside the area 21 and the space outside the area 21 in part of the rectangular image, but an SEM image of the entire area 21 is obtained, including images in which there is blank space other than the area 21 in part of the image.

[0034] After acquiring a low-magnification SEM image of the entire area 21, image analysis is performed using known image analysis software (e.g., WinROOF (registered trademark), manufactured by Mitani Shoji Co., Ltd.). In the image analysis, the size of each SEM image is calibrated based on the scale bar attached to the SEM image, and then the SEM image is binarized to extract the edges of the image. In the binarization, the luminance (brightness) of each pixel of the SEM image is converted to two levels using a predetermined threshold (e.g., threshold 0-25). By converting the pixels to two levels and eliminating intermediate gradations, a binarized image in which grain boundaries are emphasized is obtained.

[0035] The area of ​​the particles is calculated by a known image analysis method using the binary image of the range 21, and among all the particles included in the range 21, the area of ​​the particles with an area of ​​60 μm 2 More than 600μm 2 The number of particles that are larger than or equal to 100 mm (hereinafter referred to as "large particles") is counted. Particles that have grown abnormally during firing are one of the defects that have a high possibility of becoming the starting point of destruction. In the insulator 11, the points where defects are concentrated become the starting point of destruction, so if there are particles that have grown abnormally during firing, they will appear in the range 21.

[0036] In order to reduce defects caused by abnormally grown grains and ensure the bending strength of the insulator 11, the grains present in the planar image of the range 21 have a maximum area of ​​600 μm 2 The particles in the planar image in range 21 have a maximum area of ​​60 μm 2It is desirable to be as described above. This is to ensure the toughness of the insulator 11 and the thermal shock resistance.

[0037] The insulator 11 has large particles of 0.1 pieces / mm or more per unit area in the planar image of the range 21. 2 The presence of large particles can ensure the toughness of the insulator 11. The presence of large particles is preferably 6.2 pieces / mm or less per unit area in the planar image of the range 21. This is to ensure the bending strength of the insulator 11. 2 It is preferable to be as described below. This is to ensure the bending strength of the insulator 11.

[0038] Separate from the low-magnification SEM image in which the number of large particles was examined, 10 high-magnification SEM images of a rectangular portion with a size of 100 μm in length and 163 μm in width of the range 21 are randomly acquired. The position for acquiring the images is set so that the entire range 21 is shown in the high-magnification SEM image and no blank outside the range 21 appears in a part of the image. After acquiring the high-magnification SEM image, image analysis similar to the process for the low-magnification SEM image is performed to obtain a binarized image in which grain boundaries are emphasized.

[0039] Using the binarized image of the range 21, by a known image analysis method, the area of all the particles appearing in the 10 images is obtained, and the area (average) per particle is obtained. Even if large particles are included in the 10 images, the large particles are also added to the area as one particle. When a particle is cut off at the edge of the image, the part appearing in the image is added to the area as one particle. The average area of the particles of the insulator 11 is 4.4 μm or more and 8.0 μm or less. This is to improve the bending strength and thermal shock resistance of the insulator 11. 2 and 8.0 μm 2 or less. This is to improve the bending strength and thermal shock resistance of the insulator 11.

[0040] Note that when the average area of the particles is less than 4.4 μm, the toughness of the insulator 11 tends to decrease and the thermal shock resistance tends to decrease. When the average area of the particles exceeds 8.0 μm, the pores between the particles become large and the bending strength tends to decrease. To ensure the bending strength, the porosity of the insulator 11 is preferably 5% or less. 2 When it is less than, the toughness of the insulator 11 tends to decrease and the thermal shock resistance tends to decrease. When the average area of the particles exceeds 8.0 μm, the pores between the particles become large and the bending strength tends to decrease. To ensure the bending strength, the porosity of the insulator 11 is preferably 5% or less. 2 When it exceeds, the pores between the particles become large and the bending strength tends to decrease. To ensure the bending strength, the porosity of the insulator 11 is preferably 5% or less.

[0041] Using a known image analysis method, among all the particles contained in 10 images, the number of particles with an area of 20 μm 2 or more and 59 μm 2 or less (hereinafter referred to as "small particles") is counted. It is preferable that the insulator 11 has 613 particles / mm 2 or more and 2270 particles / mm 2 or less of small particles per unit area of 10 images. This is to improve the thermal shock resistance and bending strength of the insulator 11 due to the presence of small particles.

Examples

[0042] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0043] (Fabrication of Insulator) Insulators having the same basic configuration as the insulator 11 of the spark plug 10 illustrated in the embodiment were fabricated in pairs in the same manner as in the embodiment for samples No. 1 to 12. For the insulator of sample No. 4, when pulverizing the raw material powder with a wet pulverizer in the slurry preparation step, jade stones with a diameter of 2 mm and jade stones with a diameter of 6 mm were mixed and used at a ratio of 50% by mass and 50% by mass, respectively. The insulators of samples No. 3, 5 - 8 were fabricated in the same manner as sample No. 4, except that the ratio of the jade stones used when pulverizing the raw material powder in the slurry preparation step was appropriately changed.

[0044] For the insulator of sample No. 1, when pulverizing the raw material powder with a wet pulverizer in the slurry preparation step, jade stones with a diameter of 8 mm and jade stones with a diameter of 12 mm were mixed and used at a ratio of 50% by mass and 50% by mass, respectively. The insulators of samples No. 2, 9 - 12 were fabricated in the same manner as sample No. 1, except that the ratio of the jade stones used when pulverizing the raw material powder in the slurry preparation step was appropriately changed.

[0045] (Insulator Bending Strength Test) Using each of the insulators of the samples No. 1 - 12 produced in pairs of two, one sample of the spark plug 10 described in the embodiment was produced one by one. The sample was prepared by tightening the spark plug 10 to the jig 18 with the specified maximum torque in accordance with the insulator bending strength test specified in JIS B8031:2006. Then, a force F perpendicular to the axis C was applied at a position within 5 mm from the rear end of the insulator 11, and the insulator 11 was pushed at a speed of 10 mm / min or less without applying an impact to the insulator 11 until the insulator 11 was broken. Samples with a bending strength (the magnitude of the force F when the insulator 11 was broken) of 7.5 kN or more were determined as A, and samples with a bending strength of less than 7.5 kN were determined as C.

[0046]

Table 1

[0047] (Observation of the fracture surface of the insulator) After dividing the fracture surface of the insulator 11 broken in the insulator bending strength test into two ranges 21 and 22 by a plane 20 that is perpendicular to the direction of the force F applied to the insulator 11 and includes the axis C, the structure of the range 21 including the fracture origin among the ranges 21 and 22 was observed by SEM (JEM-IT300LA, manufactured by JEOL Ltd.).

[0048] The entire range 21 was divided into a plurality of parts, and a plurality of SEM images magnified at a low magnification (100 times) of a rectangular part with a size of 985 μm in length and 1231 μm in width were obtained. After obtaining a binary image by performing processing with image processing software WinROOF2013 (WinROOF is a registered trademark), the area of the largest particle and the number per unit area of large particles (the number rounded to the second decimal place) with an area of 60 μm 2 or more and 600 μm 2 or less were determined by image analysis. The area of the largest particle was recorded in the "Maximum" column of Table 1, and the number of large particles per unit area was recorded in the "Number of large particles" column of Table 1.

[0049] Ten locations were randomly selected within the range 21, and ten high-magnification SEM images were obtained by magnifying rectangular portions with a size of 100 μm in length and 163 μm in width. After obtaining binary images through image processing, the area per particle (average rounded to the second decimal place) and the number per unit area of small particles with an area of 20 μm 2 or more and 59 μm 2 or less were determined. The area per particle (average) was recorded in the "Average" column of Table 1, and the number of small particles per unit area was recorded in the "Number of Small Particles" column of Table 1.

[0050] (Thermal Shock Test) The insulators 11 of Sample Nos. 1-12 were each stored in a thermostatic bath maintained at a predetermined temperature for 30 minutes and then immediately immersed in water at 20°C for rapid cooling. When immersing in water, the posture of the insulator 11 was set such that the axis C of the insulator 11 was parallel to the water surface. The presence or absence of cracks in the insulator 11 taken out of the water was visually confirmed by applying a penetrant. The temperature of the thermostatic bath in which the insulator 11 was stored was increased in 10°C increments from 150°C until cracks were found in the insulator 11. Samples with a temperature difference (critical temperature difference) between the temperature of the thermostatic bath when cracks were found in the insulator 11 and the water temperature (20°C) of 240°C or more were determined to have excellent thermal shock resistance (A), samples with a temperature difference of 230°C or more and less than 240°C were determined to have good thermal shock resistance (B), and samples with a temperature difference of less than 220°C were determined to have poor thermal shock resistance (C).

[0051] As shown in Table 1, samples with an average particle area of 4.4 μm 2 or more and 8.0 μm 2 or less, a maximum area of 600 μm 2 or less, and a large particle number of 0.1 particles / mm 2 or more, such as Sample Nos. 3-8, had a bending strength determination of A and a thermal shock resistance determination of A or B.

[0052] On the other hand, even if the average particle area is 4.4 μm 2 or more and 8.0 μm 2 or less, and the maximum area is 600 μm 2 or less, if the large particle number is 0.1 particles / mm 2Sample No. 2, which had a maximum particle area of less than 60 μm, was judged to have a bending strength of A but a thermal shock resistance of C. It was confirmed that the presence of an appropriate number of large particles is effective in ensuring thermal shock resistance. It is presumed that when large particles are present appropriately, the toughness of the insulator is improved.

[0053] For Sample No. 2, since the maximum particle area was less than 60 μm, the number of large particles was 0 particles / mm. 2 For Sample No. 2, since the maximum particle area was less than 60 μm, it is also considered that the thermal shock resistance was judged to be C. It is presumed that a maximum area of 60 μm or more is effective in improving the toughness of the insulator. 2 For Sample No. 2, since the maximum particle area was less than 60 μm, the number of large particles was 0 particles / mm. 2 For Sample No. 2, since the maximum particle area was less than 60 μm, it is also considered that the thermal shock resistance was judged to be C. A maximum area of 60 μm or more is presumed to be effective in improving the toughness of the insulator. 2 It is presumed that having a maximum area of 60 μm or more is effective in improving the toughness of the insulator.

[0054] Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Sample No. 1, which had an average area of less than 4.4 μm, the bending strength was judged to be A but the thermal shock resistance was judged to be C. It was confirmed that when the average area was less than 4.4 μm, the thermal shock resistance decreased. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Sample No. 1, which had an average area of less than 4.4 μm, the bending strength was judged to be A but the thermal shock resistance was judged to be C. It was confirmed that when the average area was less than 4.4 μm, the thermal shock resistance decreased. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Sample No. 1, which had an average area of less than 4.4 μm, the bending strength was judged to be A but the thermal shock resistance was judged to be C. It was confirmed that when the average area was less than 4.4 μm, the thermal shock resistance decreased. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Sample No. 1, which had an average area of less than 4.4 μm, the bending strength was judged to be A but the thermal shock resistance was judged to be C. It was confirmed that when the average area was less than 4.4 μm, the thermal shock resistance decreased. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Sample No. 1, which had an average area of less than 4.4 μm, the bending strength was judged to be A but the thermal shock resistance was judged to be C. It was confirmed that when the average area was less than 4.4 μm, the thermal shock resistance decreased.

[0055] Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the average particle area was 4.4 μm or more and 8.0 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had a maximum area exceeding 600 μm, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease.

[0056] Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had an average area of 8.0 μm or more, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had an average area of 8.0 μm or more, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2 Even when the maximum particle area was 600 μm or less and the number of large particles was 0.1 particles / mm or more, for Samples No. 9 and 10, which had an average area of 8.0 μm or more, the thermal shock resistance was judged to be A but the bending strength was judged to be C. It was confirmed that when the maximum area exceeded 600 μm, the bending strength tended to decrease. 2Sample Nos. 11 and 12, which exceeded [a certain value], were judged to have A in terms of thermal shock resistance, but C in terms of bending strength. It was confirmed that when the average area exceeded 8.0 μm 2 a tendency for the bending strength to decrease was observed.

[0057] Sample No. 12 had 7.8 large particles per mm 2 Since the number of large particles was large, the average area of the particles including the large particles was 10.1 μm 2 It was. The average area of the particles was 4.4 μm 2 or more and 8.0 μm 2 or less. In order to make it within the range, the number of large particles was preferably 6.2 particles / mm or less like Sample No. 8. 2

[0058] For Samples Nos. 4 - 8 where the average area of the particles was 4.4 μm 2 or more and 8.0 μm 2 or less, the maximum area was 600 μm 2 or less, the number of large particles was 0.1 particles / mm 2 or more, and furthermore the number of small particles was 613 particles / mm 2 or more and 2270 particles / mm 2 or less, both the judgment of bending strength and the judgment of thermal shock resistance were A. The presence of small particles being 613 particles / mm 2 or more and 2270 particles / mm 2 or less was found to be effective in improving the thermal shock resistance.

[0059] As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above embodiments at all, and it can be easily inferred that various improvements and modifications are possible without departing from the gist of the present invention. For example, the shape of the insulator 11 is an example and can be set as appropriate.

[0060] ​In the embodiment, the case where the insulator 11 is broken to create a fracture surface in accordance with the insulator bending strength test of the spark plug defined in JIS B8031:2006 has been described, but it is not necessarily limited to this. The insulator 11 before assembling the center electrode 13, the main body fitting 15, etc. to form the spark plug can naturally be broken to create a fracture surface in accordance with the three-point bending test or four-point bending test defined in JIS R1601:2008.

[0061] In the three-point bending test or four-point bending test defined in JIS R1601:2008, a force is applied between the fulcrums of the insulator 11 supported by two fulcrums to break the insulator 11. The side where the fulcrum contacts becomes the range 21 including the starting point of the fracture.

[0062] In the embodiment, the spark plug 10 in which the ground electrode 17 is exposed in the combustion chamber when the spark plug 10 is attached to an engine (not shown) has been described, but it is not necessarily limited to this. It is naturally possible to apply the configuration of the embodiment to a spark plug (a spark plug provided with a sub-chamber in the combustion chamber) in which the ground electrode 17 is covered with a cap provided with a through hole.

[0063] In the embodiment, the spark plug 10 in which a spark discharge occurs between the center electrode 13 and the ground electrode 17 has been described, but it is not necessarily limited to this. It is naturally possible to apply the configuration of the embodiment to a spark plug that utilizes non-equilibrium plasma generated around the center electrode 13.

Explanation of Reference Numerals

[0064] 10 Spark plug 11 Insulator (insulator for spark plug) 12 Axial hole 20 Plane 21 Range C Axis

Claims

1. An insulator for a spark plug made of an alumina-based sintered body provided with an axial hole extending along an axis, when a force for bending the insulator for the spark plug is applied to the insulator for the spark plug and it is broken, of the range obtained by dividing the fracture surface formed by the fracture into two by a plane perpendicular to the direction of the force and including the axis, the range including the origin of the fracture is, The average of the areas of the particles appearing in the planar image within the above range is 4.4 μm ２ or more and 8.0 μm ２ or less, and the maximum of the areas of the particles is 600 μm ２ or less, and The particles include large particles with an area of 60 μm ２ or more and 600 μm ２ or less, The large particles are insulators for spark plugs that are present at 0.1 pieces / mm or more per unit area of the planar image. ２ or more.

2. The large particles are 6.2 particles / mm per unit area of the planar image ２ The insulator for a spark plug according to claim 1, which is present as follows.

3. The particles include small particles having an area of 20 μm ２ or more and 59 μm ２ or less, The small particles are present at 613 particles / mm or more and 2270 particles / mm or less per unit area of the planar image. The insulator for a spark plug according to claim 1 or 2. ２ per unit area of the planar image. ２ The insulator for a spark plug according to claim 1 or 2.

4. A spark plug comprising the insulator for a spark plug according to Claim 1 or 2.

Images