spark plug

Abstract

Spark plug (10) comprising an insulator (11) formed from an aluminum oxide-based sintered body, wherein the insulator (11) contains 90 wt.% or more of an aluminum component in the form of oxide, and wherein the crystal grains of the insulator (11) have an average grain size of 1.5 µm or less and a standard deviation of the grain size of 1.2 µm or less.

Description

GENERAL STATE OF THE ART

[0001] The present invention relates to a spark plug, in particular of the type with an insulator, with improved mechanical strength.

[0002] A spark plug for an internal combustion engine, in which an insulator has the form of an aluminum oxide-based sintered body containing aluminum oxide as its main component, is known. Japanese Patent Publication No. 2000-313657 and Japanese Patent Publication No. 2011-70929 each disclose a technique for improving the dielectric strength performance of the insulator by regulating the ratio of aluminum oxide to sintering additives such as CaO or BaO in the aluminum oxide-based sintered body. BRIEF SUMMARY OF THE INVENTION

[0003] Recently, there has been a demand for a further improvement in the mechanical strength of the insulator to a higher level than that achieved by the above technique.

[0004] The present invention was developed to meet this requirement. A major advantage of the present invention is that it provides a spark plug which has an insulator with improved mechanical strength.

[0005] According to one aspect of the present invention, a spark plug is provided which has an insulator formed from an aluminum oxide-based sintered body, wherein the insulator contains 90 wt.% or more of an aluminum component in the form of oxide, and wherein crystal grains of the insulator have an average grain size of 1.5 µm or less and a standard deviation of the grain size of 1.2 µm or less.

[0006] Further tasks and features of the present invention will also be understood from the following description. List of characters Fig. Figure 1 is a side view of a spark plug according to an embodiment of the present invention. DETAILED DESCRIPTION

[0007] An exemplary embodiment of the present invention is described in detail below with reference to the drawing, without being limited thereto.

[0008] Fig. 1 is a half-cut side view of a spark plug 10 for an internal combustion engine according to an embodiment of the present invention. In Fig. 1 is one side of the spark plug 10 shown in a cross-section with respect to its axis O. In the present description, the upper and lower sides of Fig. 1 each as the front and back of the spark plug 10designated.

[0009] As in Fig. As shown in 1, the spark plug 10 an insulator 11 , a central electrode 15 and a metal case 17 on.

[0010] The insulator 11 It has the form of an aluminum oxide-based sintered body with good mechanical properties and good insulating properties under high-temperature conditions. The insulator 11 is essentially cylindrical with an axial opening formed along the axis O 12 , and features a forward-facing inclined surface on its outer circumference as the first engagement section 13 and a rearward-facing inclined surface as a second intervention section 14 on.

[0011] The center electrode 15 is formed from a metal material (such as a nickel-based alloy) in a rod shape and is cut into a front face of the axial opening. 12inserted and held in place. A metal connector 16 , which is formed from a conductive metallic material (such as low-carbon steel), has the shape of a rod to which a high-voltage cable (not shown) is attached, and is inserted into a rear side of the axial opening 12 inserted and held in it, and is located at its front end in the axial opening. 12 electrically with the center electrode 15 tied together.

[0012] The metal casing 17 It is formed from a conductive metal material (such as low-carbon steel) and is essentially cylindrical in shape to be fixed in a threaded mounting opening (not shown) of the internal combustion engine. The metal housing 17 is around the outer circumference of the insulator 11 arranged that it was the insulator 11by clamping the first and second engagement sections 13 and 14 of the insulator 11 a ground electrode, held from both sides in the direction of axis O. 18 , which is formed from a metal material (such as a nickel-based alloy) in a rod shape, is connected to the metal casing 17 connected and lies at the center electrode 15 so opposite each other that between the center electrode 15 and the ground electrode 18 a spark gap is defined.

[0013] In the present embodiment, the insulator contains 11 Aluminium oxide (Al2O3) as a main component and a sintering aid as an additional component.

[0014] More precisely, the aluminum oxide-based sintered body, which forms the insulator, contains 11The aluminum oxide-based sintered body contains 90 wt.% or more of an aluminum (Al) component in the form of Al₂O₃. The amount of Al component contained in the aluminum oxide-based sintered body is preferably 98 wt.% or less. When the amount of Al component contained in the aluminum oxide-based sintered body is 90 wt.% or more, the aluminum oxide-based sintered body ensures a high degree of sintering and achieves good tensile strength performance. The Al component forms a crystalline phase of Al₂O₃, BaAl₂. 12 O 19 , BaAl2Si2O8 or the like, and is present in crystal grains or grain boundaries of the aluminium oxide-based sintered body.

[0015] The sintering aid is present in the crystal grains or grain boundaries of the aluminum oxide-based sintered body as oxides, ions, or the like. Examples of sintering aids include inorganic compounds such as oxides, mixed oxides, hydroxides, carbonates, chlorides, sulfates, and nitrates of Si, Mg, Ba, Ca, rare earth elements, and the like; and natural minerals. These sintering aids can be used individually or in combination with other types. The sintering aid generally promotes the densification of the sintered body by melting during sintering and forming a liquid phase. Depending on its specific type, the sintering aid can also suppress abnormal aluminum oxide grain growth and ensure the mechanical strength of the insulator. 11 fulfill.

[0016] The aluminum oxide-based sintered body may additionally contain any other elements, such as unavoidable impurities, in an amount that does not impair the effects of the present invention. Examples of such additional elements are Na, S, N, B, Ti, Mn, Ni, K, Fe, and the like.

[0017] The insulator 11 and the spark plug 10 can be produced using the following procedures.

[0018] As raw materials for the insulator 11An AI compound and a sintering aid are provided in powder form. Mixing these raw material powders with a binder and a solvent creates a slurry. Other additives, such as a plasticizer, a defoaming agent, and a dispersant, can be added to the slurry as required. Preferably, the raw material powders are mixed for eight hours or longer to ensure a uniform mixture and allow for high compaction of the sintered body.

[0019] There are no particular restrictions on the type of aluminum compound used, as long as the aluminum compound is converted to aluminum oxide in the subsequent firing step. Generally, aluminum oxide powder is used for the raw material powder. Since the aluminum compound powder inevitably contains a sodium (Na) compound as an unavoidable impurity, a high-purity aluminum compound powder is preferably used. For example, the purity of the aluminum compound powder is preferably 99.5% or higher.

[0020] To ensure good mechanical strength of the sintered body, the Al compound is preferably used in such an amount that the sintered body contains 90 to 98 wt.% aluminum in the form of oxide, based on the total weight of the sintered body (in the form of oxide) after the firing step of 100 wt.%.

[0021] Furthermore, it is preferred that the Al compound powder has a particle size D50(also referred to as "mean diameter") of 0.4 to 1.5 µm and a particle size D90 The particle size range is 1.0 to 3.5 µm to achieve a dense sintered body. The term "particle size D50" refers, when measuring a cumulative particle size distribution, to a particle size value at 50% of the cumulative distribution. Similarly, the term "particle size D90" refers to a particle size value at 90% (undersize) of the cumulative distribution. In this description, each of the "particle size D50" and "particle size D90" values ​​is measured using a laser diffraction / light scattering particle size distribution analyzer (available as the LA950 from HORIBA Ltd.).

[0022] The preferred purity range and particle size distribution of the sintering aid powder are essentially the same as those of the Al compound powder.

[0023] There are no particular restrictions on the type of binder used, as long as it is capable of imparting good formability to the raw material powders. A hydrophilic binder can be used as such. Examples of hydrophilic binders include polyvinyl alcohol, water-soluble acrylic resin, gum arabic, dextrin, and the like. These binders can be used individually or in combination with two or more types.

[0024] The amount of binder is preferably 0.1 to 7 parts by mass and even better 1 to 5 parts by mass in relation to 100 parts by mass of the raw material.

[0025] There are no particular restrictions on the type of solvent used, as long as the raw materials are dispersed in the solvent. Examples of solvents include water, alcohol, and the like. These solvents can be used individually or in combination with other types.

[0026] The solvent is preferably used in an amount of 40 to 120 parts by mass, and even better 50 to 100 parts by mass, in relation to 100 parts by mass of the raw material.

[0027] The slurry formed above is granulated into a granular powder of spherical particles by spray drying, etc. Preferably, the granular powder has an average particle size of 30 to 200 µm, and even better, 50 to 150 µm. In this description, the "average particle size" is a value measured by a laser diffraction / light scattering particle size distribution analyzer (available as the LA950 from HORIBA Ltd.).

[0028] The granular powder is press-molded using a rubber press, a molding press, or similar equipment. Of course, the process for shaping the granular powder is not limited to press molding. The granular powder can alternatively be shaped using any other method, such as injection molding. The resulting shape is then formed by grinding with a resin-bonded grinding wheel into the desired form.

[0029] The shaped body is heated in an atmospheric environment to a maximum temperature of 1450 °C or higher within four hours, fired at this maximum temperature for one to one and a half hours, and then cooled. This results in the aluminum oxide-based sintered body. The mechanical strength of the aluminum-based sintered body (as the insulator) 11 ) is ensured by suppressing abnormal grain growth of the aluminum oxide.

[0030] On the other hand, the center electrode 13 and the ground electrode 16 Each electrode is formed by machining the electrode material, such as a nickel-based alloy, into a predetermined shape and dimensions. Furthermore, the metal housing is... 15 formed by plastic manipulation into a predetermined shape and predetermined dimensions. The mass electrode 16 is joined to the metal housing by resistance welding, etc.15 connected. The center electrode 13 and the metal connection 16 are placed in the insulator using a known method. 11 set. The insulator 11 , into which the central electrode 13 and the metal connection 16 were set, is in the metal housing 15 , with which the ground electrode 16 was connected and mounted. Then a distal end section of the ground electrode is 16 so to the center electrode 13 bent so that a distal end of the ground electrode 16 a front end of the center electrode 13 opposite. This causes the spark plug 10 completed.

[0031] In the present embodiment, the crystal grains of the insulator have 11The average grain size is 1.5 µm or smaller, and the grain size standard deviation is 1.2 µm or smaller. The grain size of the crystal grains is determined by the intercept method, as will be described in detail later.

[0032] The average grain size and grain size standard deviation of the insulator crystals 11 The particle size distributions of the aluminum compound powder and the sintering aid powder, the amount of sintering aid powder used, and similar factors can be controlled. Grain growth, resulting in a larger grain size, will likely occur in the aluminum oxide-based sintered body if the particle size of the aluminum compound powder or the sintering aid powder is small, or if the amount of sintering aid powder used is large. If the crystal grains of the insulator 11exhibiting an average grain size of 1.5 µm or smaller and a grain size standard deviation of 1.2 µm or smaller, the insulator will be subject to stress. 11 The generated stress is effectively dispersed, thus preventing the formation of a crack due to stress concentration on any one of the crystal grains. Accordingly, the mechanical strength of the insulator is increased. 11 improved.

[0033] To prevent cracks even more effectively through stress dispersion and thereby increase the mechanical strength of the insulator. 11 To further improve the grain size standard deviation of the crystal grains of the insulator, it is preferred that the grain size standard deviation of the insulator crystal grains be increased. 11 1.0 µm or less. It is also preferred that the average grain size of the insulator crystal grains is 1.0 µm or less. 111.0 µm or less to prevent cracks even more effectively through stress dispersion and thereby increase the mechanical strength of the insulator. 11 to improve further. If the lower limit of the grain size standard deviation of the crystal grains of the insulator 11 With a particle size of 0.2 µm, it is possible to simplify process control for managing the particle size standard deviation to a low level, and at the same time to reduce the toughness of the insulator. 11 to ensure this by allowing the crystal grains, which are of a relatively large size, to fulfill a function in preventing the development of cracks.

[0034] Here is the grain size of the crystal grains of the insulator. 11 a value obtained by polishing or grinding a cross-section of part of the insulator 11 , which is positioned in the forward direction in front of the first intervention section 13located, and considering the polished or ground cross-section of the insulator 11 is measured. In a state where the spark plug 10 That part of the insulator is attached to the internal combustion engine. 11 , which is before the first intervention section 13 is located, exposed to the combustion gas. The part of the insulator 11 , which is before the first intervention section 13 The area in the insulator is subject to bending stress from the pressure of the combustion gas and therefore poses a risk of breakage due to this bending stress. Improving the mechanical strength of this front part of the insulator can help mitigate this risk. 11 The occurrence of damage, such as a break in the insulator, will be 11 effectively prevented. EXAMPLES

[0035] The following invention is described in more detail below by means of the following examples. It should be understood that the following examples are for illustrative purposes only and that the present invention is not limited to them. Production of aluminum oxide-based sintered bodies

[0036] Aluminum oxide powder and sintering aid powders with varying particle size distributions were provided as raw material powders. The sintering aid powders used were SiO₂ powders and powders of Ba, Ca, and Mg carbonates. These raw material powders were mixed in various ratios. Each of the resulting powdered raw materials was mixed with polyvinyl alcohol as a binder and water as a solvent, forming a slurry. The slurry was granulated by spray drying into a granular powder consisting of spherical particles with an average particle size of approximately 100 µm. The granular powder was then press-molded into a columnar body with a rectangular cross-section. The columnar body was fired in an air atmosphere for one to eight hours at a temperature range of 1450 °C to 1650 °C.The resulting sintered body was machined with a surface grinding machine, etc., to form a test piece as defined in JIS R1601:2008 with a thickness of 3 mm and a width of 4 mm.

[0037] In this way, by appropriately defining the particle size distributions (D50 and D90) of the raw material powder, aluminum oxide-based sintered bodies with different average grain sizes and grain size distributions were produced as test pieces No. 1 to No. 23. The respective aluminum oxide-based sintered bodies exhibited a relative density of 94 to 99%.

[0038] The sintered bodies produced above were evaluated using the following methods. Composition of the sintered body

[0039] Each sintered body was analyzed by X-ray fluorescence analysis or chemical analysis. Using the analysis results, the Al₂O₃ and sintering aid content in each sintered body was determined based on the total weight of the respective detected components (in oxide form) as 100%. Particle size distributions of the raw material powders

[0040] The particle sizes D50 and D90 of the respective raw material powders were measured using a laser diffraction / light scattering particle size distribution analyzer (available as LA950 from HORIBA Ltd.). Average grain size and standard deviation of the crystal grains

[0041] Each of the sintered bodies was sectioned. The resulting cross-section was treated by high-gloss polishing and then by thermal etching. Chemical etching can be used instead of thermal etching. The treated cross-section of the specimen was examined using a scanning electron microscope (SEM). For SEM examination, the accelerating voltage of the SEM was set to 15 kV, and the working distance of the SEM was set to 10 to 12 mm. SEM images of the cross-section were acquired at ten random locations, each with a rectangular field of view of 20 µm × 15 µm.

[0042] The acquired SEM images were each binarized using image analysis software ("Analysis Five," available from Soft Imaging System GmbH). The binarization threshold was set as follows. The secondary electron image and the reflected electron image were examined from the SEM images. Lines were drawn along dark-colored boundaries (corresponding to grain boundaries) on the reflected electron image to determine the positions of the grain boundaries. The reflected electron image was smoothed while preserving the grain boundary edges. A curve was then obtained from the reflected electron image, with brightness on the horizontal axis and frequency on the vertical axis. Since the resulting curve had two peaks, the brightness at the midpoint between these two peaks was set as the binarization threshold.

[0043] Using binarized SEM images, the grain size of the crystal grains was determined by the following intercept method. First, those crystal grains were selected that intersected at least one of the two diagonal lines of the rectangular area. The largest diameter of each of the selected crystal grains was measured as the longer diameter D1. The largest diameter was defined here as the maximum value among the outer diameters of the crystal grain measured in all directions. Furthermore, the outer diameter of each of the selected crystal grains along a straight line passing through the midpoint of the longer diameter D1 and extending perpendicular to it was measured as the shorter diameter D2. The average of the longer and shorter diameters D1 and D2 was calculated as the apparent grain size of the crystal grain.The average of the apparent particle size values ​​of the number n crystal grains that intersected at least one of the two diagonal lines of each image was determined to be the average grain size of the crystal grains in the image's field of view. The standard deviation of the distribution of the apparent particle size values ​​of the number n crystal grains that intersected at least one of the two diagonal lines of each image was determined to be the grain size standard deviation of the crystal grains in the image's field of view. Given that the average grain size and grain size standard deviation vary somewhat from one image's field of view to another, the average values ​​of the average grain sizes and grain size standard deviations in ten different fields of view were each taken as the average grain size and grain size standard deviation of the crystal grains in the sintered body. Flexural strength of the sintered body

[0044] The sintered specimens (ten test pieces for each type of sintered body) were tested for their three-point bending strength at room temperature (5 to 35 °C) according to JIS R1601:2008. The average of the three-point bending test results was used to determine the bending strength of the sintered body.

[0045] The compositions and evaluation results of the respective sintered bodies are shown in TABLE 1. TABLE 1 Nr. Composition (wt%) Particle diameter distribution (µm) Crystal grain size (µm) Flexural strength (MPa) Al2O3 Sintering aids D50 D90 average grain size Standard deviation 1 92,61 7,39 0,58 1,23 0,79 0,44 654 2 93,04 6,96 0,58 1,20 0,87 0,42 729 3 93,26 6,74 0,58 1,21 0,89 0,48 708 4 93,28 6,72 0,56 1,20 0,86 0,48 674 5 93,13 6,87 0,60 1,23 0,86 0,47 709 6 93,19 6,81 0,64 1,27 0,88 0,42 673 7 95,46 4,54 0,49 1,01 0,66 0,30 737 8 95,26 4,74 0,97 1,98 0,95 0,78 601 9 93,44 6,56 0,89 1,69 0,92 0,58 631 10 94,55 5,45 0,89 1,70 0,93 0,59 610 11 94,11 5,89 0,80 1,69 0,92 0,61 621 12 93,69 6,31 0,90 1,70 0,92 0,57 630 13 93,45 6,55 0,88 1,67 0,93 0,56 622 14 94,12 5,88 0,89 1,67 0,91 0,56 620 15 92,88 7,12 0,97 2,00 1,03 0,89 598 16 93,45 6,55 1,08 2,10 1,21 0,80 581 17 93,14 6,86 1,10 2,22 1,10 0,98 582 18 93,29 6,71 0,59 1,23 1,15 1,08 568 19 94,21 5,79 1,37 3,47 1,47 1,18 552 20 93,40 6,60 1,68 3,80 1,69 0,95 525 21 93,38 6,62 1,55 3,76 1,54 1,06 545 22 93,34 6,66 0,67 2,07 1,03 1,23 532 23 93,31 6,69 0,88 5,59 1,62 1,38 520

[0046] As shown in TABLE 1, specimens 1 to 19, in which the crystal grains had an average grain size of 1.5 µm or less and a grain size standard deviation of 1.2 µm or less, exhibited a flexural strength of 550 MPa or more. Specimens 1 to 19 had a higher flexural strength than specimens 20 to 23, in which the crystal grains had an average grain size greater than 1.5 µm or a grain size standard deviation greater than 1.2 µm.

[0047] Test specimens No. 1 to No. 17, in which the crystal grains had an average grain size of 1.5 µm or less and a grain size standard deviation of 1.0 µm or less, showed a flexural strength of 580 MPa or more, which was higher than that of test specimens No. 18 and No. 19 with an average grain size of the crystal grains of 1.5 µm or less and a grain size standard deviation greater than 1.0 µm and less than or equal to 1.2 µm.

[0048] Test specimens No. 1 to No. 14, in which the crystal grains had an average grain size of 1.0 µm or less and a grain size standard deviation of 1.0 µm or less, showed a flexural strength of 600 MPa or more, which was higher than that of test specimens No. 15 to No. 17 with an average grain size of the crystal grains greater than 1.0 µm and less than or equal to 1.5 µm and a grain size standard deviation of 1.0 µm or less.

[0049] When each of the test specimens No. 1 to No. 19 was subjected to stress under the influence of a load, it was able to disperse the stress and suppress the occurrence of a crack due to stress concentration on any one of the crystal grains. Therefore, the mechanical strength of test specimens No. 1 to No. 19 is assumed to have been improved compared with test specimens No. 20 to No. 23. It is assumed that a greater effect in suppressing stress concentration and preventing cracking was obtained in test specimens No. 1 to No. 17 than in test specimens No. 18 and No. 19, since the grain size standard deviation of the crystal grains was smaller in test specimens No. 1 to No. 17 than in test specimens No. 18 and No. 19. Furthermore, it is assumed that in test specimens No. 1 to No. 19, the grain size standard deviation of the crystal grains was lower than in test specimens No. 18 and No. 19.14 a greater effect in suppressing stress concentration and preventing crack formation was obtained than in test specimens No. 15 to No. 17, because the average grain size of the crystal grains in test specimens No. 1 to No. 14 was smaller than in test specimens No. 15 to No. 17.

[0050] Although the present invention has been described above with reference to a specific embodiment and examples, these embodiments and examples are intended to facilitate understanding of the present invention but are not intended to limit it. Various modifications and variations can be made to the above embodiment and examples without altering the scope of the present invention.

[0051] In the above embodiment, the present invention was applied to the spark plug 10applied, in which the center electrode 15 and the one with the metal casing 18 connected ground electrode 18 a spark discharge is generated. However, the present invention is not limited to this type of spark plug, but is applicable to any type of spark plug in which the insulator has the form of an aluminum oxide-based sintered body. For example, the present invention can alternatively be applied to a spark plug in which a barrier discharge is generated around the insulator surrounding the center electrode, or to a spark plug in which a corona discharge is generated at a front end of the center electrode passing through the insulator.

[0052] The entire contents of Japanese patent application no. 2018-188625 (filed on October 3, 2018) are incorporated herein by reference. The scope of the present invention is defined by reference to the following claims. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2018188625

[0052]

Claims

[1] Spark plug (10) comprising an insulator (11) formed from an aluminum oxide-based sintered body, wherein the insulator (11) contains 90 wt.% or more of an aluminum component in the form of oxide, and wherein crystal grains of the insulator (11) have an average grain size of 1.5 µm or smaller and a standard deviation of grain size of 1.2 µm or smaller. [2] Spark plug (10) according to claim 1, wherein the grain size standard deviation of the crystal grains of the insulator (11) is 1.0 µm or less. [3] Spark plug (10) according to claim 1 or 2, wherein the average grain size of the crystal grains of the insulator (11) is 1.0 µm or less.

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