spark plug

Abstract

Spark plug (10), comprising: a first electrode (18) comprising a base material (19) and a discharge element (20) comprising, wherein at least a part of the discharge element (20) is connected to the base material (19) with an intermediate diffusion layer (25); and a second electrode (13) which faces the discharge element (25) with an intermediate spark gap (22), wherein the base material (19) contains at least 50 wt% of Ni, at least 8 wt% and at most 40 wt% of Cr, at least 0.05 wt% and at most 2 wt% of Si, at least 0.01 wt% and at most 2 wt% of Al, at least 0.01 wt% and at most 2 wt% of Mn, at least 0.01 wt% and at most 0.1 wt% of C and at least 0.001 wt% and at most 0.04 wt% of Fe.

Description

Technical area

[0001] The present invention relates to a spark plug and in particular to a spark plug in which at least a part of a discharge element is connected to a base material with an intermediate diffusion layer. background

[0002] As a result of increased engine performance, improved combustion efficiency, and the like, there is a trend toward high spark plug electrode temperatures under operating conditions. In a spark plug where a first electrode, comprising a discharge element bonded to a base material, faces a second electrode with an intervening spark gap, an increase in the temperature of the first electrode leads to an increase in the thermal stress of a connection section of the discharge element, and therefore concerns arise regarding discharge element detachment. In the technology disclosed in PTL 1, a base material contains at least 0.05 wt% and at most 5 wt% Fe, thereby preventing discharge element detachment while improving high-temperature strength and high-temperature corrosion resistance.In an example in PTL 2, a base material contains 2 wt% of Fe to ensure the high temperature strength of the base material, thereby preventing the detachment of a discharge element. List of patent literature PTL 1: Unexamined Japanese patent application, publication number JP 2003 - 105 467 A PTL 2: Unexamined Japanese patent application, publication number JP 2007 - 173 116 A Technical problem

[0003] However, with the aforementioned technology, it was discovered that the resistance to delamination of the discharge element may not be sufficiently ensured with further temperature increases of the electrodes. In other words, a thermal stress is generated in the junction between the base material and the discharge element due to a difference in their coefficients of thermal expansion, and a crack readily forms. When oxygen enters the crack, it combines with iron from the base material, forming iron oxide in the junction. Under the operating conditions of a motor, the volume of the iron oxide changes due to alterations in its crystal structure through oxidation-reduction. Consequently, the stress in the junction between the base material and the discharge element continues to increase.Accordingly, there is a possibility that the discharge element could easily detach from the base material.

[0004] Particularly in the case of an electrode where at least part of a discharge element is connected to a base material with an intervening diffusion layer, the voltage buffering effect of the diffusion layer is weak compared to an electrode where a discharge element is connected to a base material by an intervening laser-welded fused section. Therefore, there is a possibility that the discharge element may detach more easily.

[0005] The present invention was made to solve this problem, and its aim is to provide a spark plug which can prevent the simple detachment of a discharge element connected to a base material. Solution to the problem

[0006] To achieve this objective, a spark plug according to the present invention comprises: a first electrode comprising a base material and a discharge element, wherein at least a portion of the discharge element is connected to the base material with an intervening diffusion layer; and a second electrode facing the discharge element with an intervening spark gap. The base material contains at least 50 wt% of Ni, at least 8 wt% and at most 40 wt% of Cr, at least 0.05 wt% and at most 2 wt% of Si, at least 0.01 wt% and at most 2 wt% of Al, at least 0.01 wt% and at most 2 wt% of Mn, at least 0.01 wt% and at most 0.1 wt% of C, and at least 0.001 wt% and at most 0.04 wt% of Fe. Advantageous effects of the invention

[0007] According to the spark plug described in claim 1, the base material contains at least 0.001 wt% and at most 0.04 wt% of Fe and at least 0.05 wt% and at most 2 wt% of Si, which has a higher affinity for oxygen than Fe. It is therefore possible to prevent the formation of iron oxide along the interface between the diffusion layer and the discharge element, along the interface between the diffusion layer and the base material, and within the diffusion layer itself, thereby preventing the base material from becoming brittle. Accordingly, it is possible to ensure the strength of the base material and to further reduce the stress in the diffusion layer caused by the volume change of the iron oxide. It is therefore possible to prevent the discharge element bonded to the base material from easily detaching.

[0008] According to the spark plug described in claim 2, the base material contains at least 22 wt% and at most 28 wt% of Cr, at least 0.7 wt% and at most 1.3 wt% of Si, at least 0.6 wt% and at most 1.2 wt% of Al, at least 0.1 wt% and at most 1.1 wt% of Mn, and at least 0.01 wt% and at most 0.07 wt% of C. Accordingly, it is possible to further prevent simple detachment of the discharge element.

[0009] According to the spark plug described in claims 3 and 4, the condition 2.5 ≤ X / Y is met if X (mass %) represents a proportion of silicon in the base material and Y (mass %) represents a proportion of iron in the base material. The silicon contained in the base material can improve the effectiveness of preventing iron oxidation and volume changes in the iron oxide. It is therefore possible to further prevent the discharge element from simply detaching.

[0010] According to the spark plug described in claim 5, the base material contains a solid solution containing nickel, wherein the solid solution contains a segregation, and wherein, in a cross-section of the base material, the area of ​​the segregation occupying a surface of the base material is at least 0.01% and at most 4%. It is possible to further prevent simple detachment of the discharge element by ensuring the high-temperature strength of the base material. Brief description of the drawings [ Fig. 1] Fig. Figure 1 is a half-section view of a spark plug according to one embodiment. [ Fig. 2] Fig. Figure 2 is a sectional view of a ground electrode. [ Fig. 3] Fig. Figure 3 shows an element distribution in the area of ​​a diffusion layer. [ Fig. 4] Fig. Figure 4 is a sectional view of a base material. [ Fig. 5] Fig. Figure 5 shows an element distribution in the area of ​​a merged section. Description of embodiments

[0011] A preferred embodiment of the present invention is described below with reference to the accompanying drawings. Fig. Figure 1 is a half-section view of a spark plug 10 according to an embodiment with an axis O as the boundary. The lower side in Fig. 1 is referred to as the front of spark plug 10, and the top side in Fig. 1 is referred to as the back of spark plug 10 (the same applies to Fig. 2).

[0012] As in Fig. As shown in Figure 1, the spark plug 10 comprises an insulator 11, a central electrode 13 (second electrode), a metal sleeve 17, and a ground electrode 18 (first electrode). The insulator 11 is an essentially cylindrical element with excellent mechanical and insulating properties at high temperatures and is made of aluminum oxide or the like. The insulator 11 has an axial hole 12 extending along the axis O through it.

[0013] The central electrode 13 is a rod-shaped electrode inserted into the axial hole 12 and held by the insulator 11. The central electrode 13 comprises a base material 14 and a discharge element 15, which is connected to the front end of the base material 14. A core material with excellent thermal conductivity is embedded in the base material 14. The base material 14 is made of an alloy consisting mainly of nickel or of a metallic material consisting mainly of nickel. The core material is made of copper or an alloy containing copper as the main component. It is, of course, possible to omit the core material.The discharge element 15 is, for example, made of a precious metal, such as Pt, Ir, Ru, Rh and the like, or W, whose ignition wear resistance is greater than that of the base material 14, or of an alloy which is mainly made of such a precious metal or W.

[0014] A metal terminal 16 is a rod-shaped element to which a high-voltage cable (not shown) is connected, and the front of the metal terminal 16 is located in the insulator 11. The metal terminal 16 is electrically connected to the central electrode 13 in the axial hole 12.

[0015] The metal sleeve 17 is a substantially cylindrical metal element that is connected to a screw hole (not shown) of an internal combustion engine. The metal sleeve 17 is made of a conductive metallic material (for example, low-carbon steel or the like). The metal sleeve 17 is connected to the outer circumference of the insulator 11. The ground electrode 18 is connected to the front end of the metal sleeve 17.

[0016] The ground electrode 18 comprises a base material 19, which is connected to the metal sleeve 17, and a discharge element 20, which is also connected to the base material 19. A core material with excellent thermal conductivity is embedded in the base material 19. The base material 19 is a metallic material formed from an alloy consisting primarily of nickel. The core material is copper or an alloy containing copper as its main component. It is, of course, possible to omit the core material and form the entire base material 19 from an alloy consisting primarily of nickel. The base material 19 contains nickel, chromium, silicon, aluminum, manganese, carbon, and iron. It should be noted that it may also contain other elements besides these.

[0017] The discharge element 20 is, for example, made of a precious metal, such as Pt, Ir, Ru, Rh, and the like, or of tungsten whose ignition wear resistance is higher than that of the base material 19, or of an alloy consisting mainly of such a precious metal or tungsten. A discharge surface 21 of the discharge element 20 faces the central electrode 13 with an intervening spark gap 22. In the present embodiment, the discharge element 20 is an alloy consisting mainly of Pt and containing Ni, and has a disc shape with a circular discharge surface 21.

[0018] The spark plug 10 is manufactured, for example, by the following procedure. First, the central electrode 13 is inserted into the axial hole 12 of the insulator 11. After the metal terminal 16 is inserted into the axial hole 12 and conductivity between the metal terminal 16 and the central electrode 13 is ensured, the metal sleeve 17, to which the base material 19 was previously connected, is attached to the outer circumference of the insulator 11. After the discharge element 20 has been connected to the base material 19 by resistance welding, the base material 19 is bent so that the discharge element 20 faces the central electrode 13 in the axial direction, thus forming the spark plug 10. It is possible to subject the base material 19, to which the discharge element 20 has been connected, to a heat treatment after resistance welding.

[0019] Fig. Figure 2 is a sectional view of the ground electrode 18, which includes the straight line 24, parallel to axis O, from the straight lines passing through a center 23 of the discharge surface 21 of the discharge element 20. In the present embodiment, axis O of the spark plug 10 and the straight line 24 coincide. At least a portion of the discharge element 20 is connected to the base material 29 with an intervening diffusion layer 25. The diffusion layer 25 connects the base material 19 and the discharge element 20 by atomic diffusion (interatomic bonding) that occurs between the base material 19 and the discharge element 20. A fused section, where the discharge element 20 and the base material 19 have been fused and solidified, may be formed at a section of the interface between the discharge element 20 and the base material 19.However, the fused section is not included in diffusion layer 25.

[0020] Fig. Figure 3 shows an element distribution in the area of ​​the diffusion layer 25. Fig. Figure 3 shows the proportions of Pt and Ni. The proportions were measured on a polished surface of the ground electrode 18, which encompasses the straight line 24 and is perpendicular to the diffusion layer 25. The measurement was performed from the discharge element 20 to the base material 19 at predetermined intervals (e.g., 1 µm). The horizontal axis of the Fig. Figure 3 represents the percentage by mass of the elements, with the percentages decreasing towards the left. The vertical axis represents the distance (i.e., the position on spark plug 10 in the direction of axis O), and the bottom edge indicates the front of spark plug 10.

[0021] The elemental concentrations contained in the base material 19 and the discharge element 20 are determined by WDS analysis using FE-EPMA (JXA 8500F manufactured by JEOL Ltd.) equipped with a hot cathode field emission electron gun. Following a qualitative WDS analysis, the mass composition is measured by quantitative analysis, determining the concentrations (mass %) relative to the total mass compositions of the elements detected.

[0022] In the present embodiment, the base material 19, which is formed from an alloy consisting mainly of nickel, contains no platinum. In contrast, the discharge element 20 is composed mainly of platinum and contains nickel. The proportion of nickel in the discharge element 20 is lower than the proportion of nickel in the base material 19. If the distribution of platinum and nickel is known, it is therefore possible to specify the position of the diffusion layer 25 between the base material 19 and the discharge element 20, into which atoms diffuse.

[0023] In the diffusion layer 25, the diffusion of atoms is generated by hot-pressure bonding between the discharge element 20 and the base material 19. In the diffusion layer 25, the concentration of a specific element (Pt in the present embodiment) contained in the discharge element 20 decreases continuously from the discharge element 20 towards the base material 19. Similarly, in the diffusion layer 25, the concentration of a specific element (Ni in the present embodiment) contained in the base material 19 decreases continuously from the base material 19 towards the discharge element 20.

[0024] A fused section 26 is described, which is formed by laser welding. Fig. Figure 5 shows an element distribution in the region of the fused section 26 in a sample in which the fused section 26 formed by laser welding is located between the base material 19 and the discharge element 20. Fig. Figure 5 shows the proportions of Pt and Ni. The proportions were measured from the discharge element 20 towards the base material 19 across the fused section 26 at predetermined intervals (e.g., 1 µm). The horizontal axis in Fig. 5 represents the content fractions (mass %), and these fractions decrease towards the left. The vertical axis represents the distance (i.e., the position on the spark plug in the direction of axis O), and the lower side indicates the front of the spark plug. The base material 19 and the discharge element 20, which have been liquefied, flow into the fused section 26 and solidify; and unlike the diffusion layer 25, the elements (Pt and Ni) are thereby mixed with each other without relation to the distance from the discharge element 20 or from the base material 19.

[0025] Referring again to Fig. Section 2 describes a method for measuring the thickness T of the diffusion layer 25. Fig. 2 intersects the straight line 24, which runs through the center 23 of the discharge surface 21 of the discharge element 20, the diffusion layer 25 under perpendicular incidence, and therefore the content fractions of Pt and Ni at measuring points on the straight line 24 from the discharge element 20 in the direction of the base material 19 are measured by a WDS analysis using a FE-EPMA.

[0026] First, a measuring point A, located 10 µm away from the discharge surface 21 of the discharge element 20 in the direction of the base material 19, was defined as the initial measuring point (base point) of the discharge element 20. A quantitative analysis was then performed at five measuring points arranged at 10 µm intervals in the direction of the base material 19. The mean value of the Pt content at the five measuring points is considered to be the Pt content W1 in the discharge element 20.

[0027] Next, a quantitative analysis is performed on measurement points arranged at constant intervals (e.g., 1 µm) along a straight line 24 in the direction of the base material 19, starting from the measurement point of the five measurement points of the discharge element 20 that is closest to the base material 19. From these measurement points, all those are identified where the Pt content W2 is at most W1 and where the Pt content at measurement points closer to the base material 19 than the measurement point itself is at most W2. From all these measurement points, a measurement point B is determined that is closest to the discharge element 20. The position of measurement point B is considered to be the position of the boundary between the discharge element 20 and the diffusion layer 25 for which Pt was measured.

[0028] Next, a measurement point C on the straight line 24, located 100 µm away from measurement point B in the direction away from the discharge element 20, is determined as an initial measurement point (base point) of the base material 19, and a quantitative analysis is performed at five measurement points arranged at 10 µm intervals on the straight line 24 in the direction away from the discharge element 20. The mean value of the Pt content at the five measurement points is considered to be the Pt content W3 in the base material 19.

[0029] Next, a quantitative analysis is performed on measurement points. These five measurement points of the base material 19 are arranged at constant intervals (e.g., 1 µm) along a straight line 24 in the direction of the discharge element 20, starting from measurement point C, which is the closest to the discharge element 20. From these measurement points, all those are identified where the Pt content W4 is at least W3 and where the Pt content at measurement points closer to the discharge element 20 than the measurement point itself is at least W4. From all these measurement points, a measurement point D is determined that is closest to the base material 19. The position of measurement point D is considered to be the position of the boundary between the base material 19 and the diffusion layer 25 for which Pt was measured.A distance in the axial direction between measuring point B and measuring point D is considered to be a thickness T1 of the diffusion layer 25 for which Pt was measured.

[0030] Similarly, measuring point A, located 10 µm away from the discharge surface 21 of the discharge element 20 in the direction of the base material 19, is defined as the initial measuring point (base point) of the discharge element 20, and a quantitative analysis is performed at five measuring points arranged at 10 µm intervals along the straight line 24 in the direction of the base material 19. The mean value of the Ni content at the five measuring points is considered to be the Ni content W5 in the discharge element 20.

[0031] Next, a quantitative analysis is performed on measurement points arranged at constant intervals (e.g., 1 µm) along a straight line 24 in the direction of the base material 19, starting from the measurement point of the five measurement points of the discharge element 20 that is closest to the base material 19. From these measurement points, all those are identified where the Ni content W6 is at least W5 and where the Ni content at measurement points closer to the base material 19 than the measurement point itself is at least W6. From all these measurement points, a measurement point E is determined that is closest to the discharge element 20. The position of measurement point E is considered to be the position of the boundary between the discharge element 20 and the diffusion layer 25 for which Ni was measured.

[0032] Next, a measurement point F on the straight line 24, located 100 µm away from measurement point E in a direction away from the discharge element 20, is determined as an initial measurement point (base point) of the base material 19, and a quantitative analysis is performed at five measurement points arranged at 10 µm intervals on the straight line 24 in the direction away from the discharge element 20. The mean value of the Ni content at the five measurement points is considered to be the Ni content W7 in the base material 19.

[0033] Next, a quantitative analysis is performed on measurement points. These five measurement points of the base material 19 are arranged at constant intervals (e.g., 1 µm) along a straight line 24 in the direction of the discharge element 20, starting from the measurement point F closest to the discharge element 20. From these measurement points, all those are identified where the Ni content W8 is at most W7 and where the Ni content at measurement points closer to the discharge element 20 than the measurement point itself is at most W8. From all these measurement points, a measurement point G is determined that is closest to the base material 19. The position of measurement point G is considered to be the position of the boundary between the base material 19 and the diffusion layer 25 for which Ni was measured.A distance in the axial direction between measuring point E and measuring point G is considered to be a thickness T2 of the diffusion layer 25 for which Ni was measured.

[0034] From the thickness T2 and the thickness T1 of the diffusion layer 25, for which Pt was measured, the greater thickness is considered to be the thickness T of the diffusion layer 25 (see Fig. 3) Taking into account the resistance to detachment of the discharge element 20, the thickness T of the diffusion layer 25 is preferably at least 5 µm, but is usually less than 70 µm.

[0035] A WDS analysis using FE-EPMA to determine the mass compositions of the base material 19 and the discharge element 20 at each set of five measurement points, with measurement points A, C, and F as their respective base points, is performed under conditions of an accelerating voltage of 20 kV and a spot diameter of 10 µm. A WDS analysis to define measurement points B, D, E, and G for determining the thickness of the diffusion layer 25 is performed under conditions of an accelerating voltage of 20 kV and a spot diameter of 1 µm.

[0036] The elements to be analyzed are not limited to Pt and Ni. Two types of elements can be selected as needed from those contained in the base material 19 or the discharge element 20. The thickness of the diffusion layer 25 is considered easy to measure if Ni, the most abundant element in the base material 19, is selected, and an element predominantly contained in the discharge element 20 is selected.

[0037] Depending on the surface shape of the discharge surface 21 of the discharge element 20 or the thickness of the diffusion layer 25, there is a possibility that a concentration gradient exists below the measuring points A, C, and F, or that the measuring points A, C, and F lie within the diffusion layer 25. In such a case, the measured values ​​at the measuring points A, C, and F do not represent the compositions of the discharge element 20 and the base material 19. Therefore, the measurement is carried out with suitably modified positions of the measuring points A, C, and F.In short: Measuring point A can be determined at any section as long as measured values ​​can be obtained that represent the composition of the discharge element 20 before connection, and measuring points C and F can be determined at any section as long as measured values ​​can be obtained that represent the composition of the base material 19 before connection.

[0038] Fig. Figure 4 is a sectional view of the base material 19. If, for example, a segregation 27 of the discharge element 20 or of the base material 19 is present on the straight line 24, if a fused section (not shown) is present next to the diffusion layer 25, or if a cavity (not shown) is present in the base material 19 or in the discharge element 20 on the straight line 24, i.e., if measured values ​​are considered to be influenced by the segregation 27, a cavity, or the like, instead of the measurement points of the measurement, two measurement points that are closest to the measurement points and are not influenced by the segregation 27, the cavity, or the like are selected, and an average of the values ​​measured at the two measurement points is used.

[0039] The base material 19 is a solid solution containing nickel. The segregation 27 has a crystal structure that differs from that of the solid solution of the base material 19. The segregation 27 is, for example, an element that forms the base material 19, or impurities such as carbides, nitrides, oxides, and intermetallic compounds. A suitable amount of the segregation 27 helps to ensure the strength of the base material 19.

[0040] Furthermore, in a spark plug where at least part of a discharge element is connected to a base material with an intervening diffusion layer, and the base material contains iron, a problem arises: the iron can potentially have a significant impact on the discharge element's resistance to delamination. In other words, as the temperature of a ground electrode increases under operating conditions, oxygen atoms diffuse along the interface between the diffusion layer and the discharge element, and along the interface between the diffusion layer and the base material, into the inner section of the diffusion layer. Subsequently, the iron from the base material combines with oxygen to form iron oxide. The volume of the iron oxide changes due to alterations in its crystal structure through oxidation-reduction, thus increasing the stress on the diffusion layer.As a result, the discharge element, which is connected to the base material with the intermediate diffusion layer, detaches easily.

[0041] In contrast, in the spark plug, the discharge element is fused to the base material by the intermediate laser-welded section 26 (see Fig. 5) is connected, a thermal stress generated by a difference in the linear thermal expansion coefficient between the base material and the discharge element is buffered by the fused section 26. The Fe contained in the base material therefore has no significant influence on the detachment of the discharge element.

[0042] According to the present embodiment with the spark plug 10, in which at least a part of the discharge element 20 is connected to the base material 19 with the intermediate diffusion layer 25, the base material 19 contains at least 50 wt% of Ni, at least 8 wt% and at most 40 wt% of Cr, at least 0.05 wt% and at most 2 wt% of Si, at least 0.01 wt% and at most 2 wt% of Al, at least 0.01 wt% and at most 2 wt% of Mn, at least 0.01 wt% and at most 0.1 wt% of C and at least 0.001 wt% and at most 0.04 wt% of Fe.

[0043] The content fraction (mass %) of each element of the base material 19 is calculated based on analysis results of the mass composition by WDS analysis using an FE-EPMA at the five measuring points that correspond to measuring point C (see Fig. 2) as the base point. The content fraction (mass %) of each element of the base material 19 can be calculated from the five measuring points, which replace measuring point C with measuring point F (see Fig. 2) as the base point. In short: A measurement can be carried out on any section as long as measured values ​​can be obtained that represent the composition of the base material 19 before joining.

[0044] By containing at least 50 wt% Ni, the base material 19 can ensure heat resistance properties. By containing at least 8 wt% and at most 40 wt% Cr, it is possible to ensure oxidation resistance of the base material 19 by means of a Cr oxide film formed on the surface of the base material 19 and to prevent the formation of segregation 27, for example as Cr nitrides and Cr carbides. By containing at least 0.05 wt% and at most 2 wt% Si, it is possible to ensure oxidation resistance of the base material 19 and to prevent the formation of segregation 27, which is formed from a Si compound. By containing at least 0.01 wt% and at most 2 wt% Al, it is possible to ensure high-temperature strength and high-temperature corrosion resistance.

[0045] By containing at least 0.01 wt% and at most 2 wt% of Mn, the base material 19 can prevent itself from becoming brittle due to desulfurization and can inhibit the formation of segregation 27, for example as Mn sulfides. By containing at least 0.01 wt% and at most 0.1 wt% of C, it is possible to ensure high-temperature strength and to inhibit the formation of segregation 27, for example as Cr carbides. By containing at least 0.001 wt% and at most 0.04 wt% of Fe, it is possible to inhibit the formation of iron oxide. The proportions of elements of the base material 19 other than Ni, Cr, Si, Al, Mn, C and Fe, and the proportions of elements of unavoidable impurities preferably total at most 1 wt% and further preferably total at most 0.4 wt%.

[0046] The base material 19 contains at least 0.001 wt% and at most 0.04 wt% of Fe and at least 0.05 wt% and at most 2 wt% of Si, which has a higher affinity for oxygen than Fe. Since there is more Si, which has a higher affinity for oxygen than Fe and readily diffuses into a portion exposed in a combustion chamber (not shown) of the engine, than Fe, oxygen preferentially combines with the Si originating from the base material 19, and a Si oxide film is formed along the interface between the diffusion layer 25 and the discharge element 20, along the interface between the diffusion layer 25 and the base material 19, and on the diffusion layer 25 itself. The presence of this Si oxide film prevents the formation of iron oxide, in which oxygen combines with the Fe originating from the base material 19.

[0047] Since the silicon content is at most 2% by mass, it is also possible to prevent the formation of iron oxide and thus prevent the base material 19 from becoming brittle. Therefore, it is possible to ensure the strength of the base material 19 and to further reduce the stress in the diffusion layer 25 caused by the volume change of the iron oxide. As a result, it is possible to prevent the discharge element 20 from easily detaching.

[0048] If the Si content in the base material 19 is represented by X (wt%) and the Fe content in the base material 19 is represented by Y (wt%), then for a ratio X / Y, X / Y ≥ 2.5 is preferred. This serves to improve the effect of preventing oxidation of the Fe and volume change of the iron oxide by the Si contained in the base material 19 and thereby further prevent easy detachment of the discharge element 20.

[0049] The area of ​​the segregation 27, which occupies the surface of the base material 19, preferably comprises at least 0.01% and at most 4% of the cross-section of the base material 19. This serves to prevent the base material 19 from becoming brittle and to ensure its strength. If the area of ​​the segregation 27 is at least 0.01%, the high-temperature strength of the base material 19 is further increased, and the base material 19 is therefore less prone to deformation. Accordingly, this prevents the oxide film, which forms on the exposed part in the combustion chamber of the engine, from easily detaching.It is therefore possible to further prevent the formation of iron oxide due to the diffusion of oxygen atoms along the interface between the diffusion layer 25 and the discharge element 20, along the interface between the diffusion layer 25 and the base material 19, and into the inner section of the diffusion layer 25. If the area of ​​the segregation 27 is at most 4%, brittleness of the base material 19 is prevented. If the area of ​​the segregation 27 is at least 0.01% and at most 4%, it is therefore possible to further prevent easy detachment of the discharge element 20 by ensuring the high-temperature strength of the base material 19.

[0050] The segregation 27 can be detected by imaging or analyzing composition images using an EPMA equipped with a wavelength-dispersive X-ray spectrometer detector (WDX or WDS), a SEM equipped with an energy-dispersive X-ray spectrometer detector (EDX or EDS), or similar instruments. After photographing a cross-section of the base material 19 with a rectangular field of view measuring 400 µm × 600 µm, the area (%) of the segregation 27 occupying the area of ​​the base material 19 is obtained by image processing. Examples

[0051] The present invention is described in detail with an example. However, the present invention is not limited to this example. (Preparing samples 1 to 45)

[0052] An inspector prepared various types of the base material 19 with the compositions specified in Table 1 and the disc-shaped discharge element 20, which consisted of Pt: 80 wt%, Rh: 20 wt%, and unavoidable impurities up to a maximum detection limit. The inspector joined the discharge element 20 to the base materials 19 by resistance welding and obtained the spark plugs 10 according to samples 1 to 45. For each sample, a large number of specimens prepared under the same conditions were prepared for cross-sectional observation of the base material 19 and the like, in addition to assessing the peel resistance, which was carried out on each specimen. The thickness T of the diffusion layer 25 formed between the base material 19 and the discharge element 20 was less than 70 µm for all specimens.

[0053] Table 1 shows the X / Y ratio, where X (wt%) represents the Si content in the base material and Y (wt%) represents the Fe content. After photographing a cross-section of the base material 19 in a rectangular field of view measuring 400 µm × 600 µm, the area (%) of the segregation 27 covering the area of ​​the base material 19 was also determined by image processing. In the segregation column of Table 1, samples with a segregation value of at least 0.01% and at most 4%, and samples with a segregation value of less than 0.01% or greater than 4%, are marked as "good" and "poor," respectively. (Delamination resistance test)

[0054] The tester conducted a 100-hour test in which each sample was fitted with each cylinder of a four-cylinder, two-liter engine and each sample was repeatedly subjected to a load of 4000 rpm for one minute, followed by a load of idle speed for one minute. The temperature of the discharge element 20 at 4000 rpm was 950 °C. Using a spark plug with a hole extending into the area of ​​the discharge element 20, the temperature of the discharge element 20 was measured before the start of the peel resistance test, with the temperature-measuring portion of a thermocouple positioned on a front section of the base material 19 near the discharge element 20. The amount of energy delivered by an ignition coil to each sample during an ignition discharge was 100 mJ.

[0055] Following the tests, each sample was subjected to an examination of a cross-section of the ground electrode 18 using a scanning electron microscope (SEM). This cross-section included the straight lines 24 passing through the center 23 of the discharge surface 21 of the discharge element 20, the straight line 24 parallel to axis O, and the lengths L1 and L2 of cracks extending from the two ends of the diffusion layer 25 towards its center. A value M, obtained by dividing the total length of the cracks (L1 + L2) by the length L of the discharge surface 21 (i.e., (L1 + L2) / L), was calculated. Based on this value M, the samples were classified into five grades from A to E. The criterion was as follows: A: M < 20%, B: 20% ≤ M < 30%, C: 30% ≤ M < 40%, D: 40% ≤ M < 50%, and E: M ≥ 50% or the discharge element 20 was discharged.The results of the peel resistance tests are given in the peel property column in Table 1.

[0056] As shown in Table 1, samples 7, 16, 23, 29, 34, and 39 to 45 were rated E in the peel resistance test. In sample 7, the Cr content was greater than 40 wt%. In sample 16, the Si content was greater than 2 wt%. In sample 23, the Al content was greater than 2 wt%. In sample 29, the Mn content was greater than 2 wt%. In sample 34, the C content was greater than 0.1 wt%. In sample 39, the Cr content was less than 8 wt%.

[0057] In sample 40, the Cr content was greater than 40 wt%, the Si, Al, and Mn contents were each greater than 2 wt%, and the C content was greater than 0.1 wt%. In samples 41 to 43, the Fe content was greater than 0.04 wt%. In sample 44, the Si content was greater than 2 wt%. In sample 45, the Al content was greater than 2 wt%.

[0058] Samples 1 to 7 differ from each other mainly in their chromium content. Samples 1 to 6 were rated A or B in the peel resistance test. Samples 2, 3, and 5 were rated A, and samples 1, 4, and 6 were rated B. The chromium content in sample 1 was at least 8 wt% and less than 22 wt%, and in sample 6 it was greater than 28 wt% and at most 40 wt%. In sample 4, the area of ​​segregation was not at least 0.01% and at most 4%.

[0059] Samples 8 to 16 differ from each other mainly in their silicon content. Samples 8 to 15 were rated A, B, or C in the peel resistance test. Samples 10 to 12 were rated A, samples 9 and 13 to 15 were rated B, and sample 8 was rated C. The silicon content in samples 8 and 9 was at least 0.05 wt% and less than 0.7 wt%, and in samples 13 to 15 it was greater than 1.3 wt% and at most 2 wt%. Furthermore, sample 8 met the X / Y criterion of < 2.5.

[0060] Samples 17 to 23 differ from each other mainly in their aluminum content. Samples 17 to 22 were rated A or B in the peel resistance test. Samples 19 and 20 were rated A, and samples 17, 18, 21, and 22 were rated B. The aluminum content in samples 17 and 18 was at least 0.01 wt% and less than 0.6 wt%, and in samples 21 and 22 it was greater than 1.2 wt% and at most 2 wt%.

[0061] Samples 24 to 29 differ from each other mainly in their Mn content. Samples 24 to 28 were rated A or B in the peel resistance test. Samples 25 and 26 were rated A, and samples 24, 27, and 28 were rated B. The Mn content in sample 24 was at least 0.01 wt% and less than 0.1 wt%, and in samples 27 and 28 it was greater than 1.1 wt% and at most 2 wt%.

[0062] Samples 30 to 34 differ from each other mainly in their carbon content. Samples 30 to 33 were rated A, B, or C in the peel resistance test. Samples 30 and 31 were rated A, sample 32 was rated B, and sample 33 was rated C. In sample 32, the area of ​​segregation was not at least 0.01% and at most 4%. In sample 33, the carbon content was greater than 0.07% by mass and at most 0.1% by mass, and the area of ​​segregation was not at least 0.01% and at most 4%.

[0063] Samples 35 to 38 were rated B, C, or D in the peel resistance test. Samples 35 and 36 were rated B, sample 37 was rated C, and sample 38 was rated D. In sample 35, the aluminum content was at least 0.01 wt% and less than 0.6 wt%, and the manganese content was greater than 1.1 wt% and at most 2 wt%. In sample 36, the chromium content was greater than 28 wt% and at most 40 wt%. In sample 37, the aluminum content was at least 0.01 wt% and less than 0.6 wt%, and the segregation area was not at least 0.01% and at most 4%. In sample 38, the content of Al was at least 0.01 mass-% and less than 0.6 mass-%, the content of Mn was at least 0.01 mass-% and less than 0.1 mass-%, and X / Y < 2.5 was fulfilled.

[0064] Samples 2, 3, 5, 10 to 12, 19, 20, 25, 26, 30 and 31, which were rated as A, each contained at least 50 wt% of Ni, at least 22 wt% and at most 28 wt% of Cr, at least 0.7 wt% and at most 1.3 wt% of Si, at least 0.6 wt% and at most 1.2 wt% of Al, at least 0.1 wt% and at most 1.1 wt% of Mn, at least 0.01 wt% and at most 0.07 wt% of C and at least 0.001 wt% and at most 0.04 wt% of Fe and met X / Y ≥ 2.5. For each of these samples, the area of ​​segregation was at least 0.01% and at most 4%.

[0065] The example demonstrated that ratings of A to D in the peel resistance test can be obtained by ensuring that the base material contains at least 50 wt% of Ni, at least 8 wt% and at most 40 wt% of Cr, at least 0.05 wt% and at most 2 wt% of Si, at least 0.01 wt% and at most 2 wt% of Al, at least 0.01 wt% and at most 2 wt% of Mn, at least 0.01 wt% and at most 0.1 wt% of C and at least 0.001 wt% and at most 0.04 wt% of Fe.

[0066] Furthermore, it was discovered that ratings of A or B in the peel resistance test can be obtained if the base material contains at least 50 wt% of Ni, at least 22 wt% and at most 28 wt% of Cr, at least 0.7 wt% and at most 1.3 wt% of Si, at least 0.6 wt% and at most 1.2 wt% of Al, at least 0.1 wt% and at most 1.1 wt% of Mn, at least 0.01 wt% and at most 0.07 wt% of C, and at least 0.001 wt% and at most 0.04 wt% of Fe. It was also discovered that a rating of A in the peel resistance test can be obtained if X / Y ≥ 2.5 is satisfied and the area of ​​segregation is at least 0.01% and at most 4%.

[0067] The present invention has been described above based on the embodiment described above. However, the present invention is not limited to the aforementioned embodiment and it can readily be assumed that it can be improved or modified in various ways within the spirit of the present invention.

[0068] The embodiment described is one case in which the discharge element 20 has a disc shape; however, the embodiment is not necessarily limited to this, and it is of course possible to use a different shape. Other shapes of the discharge element 20 include, for example, a truncated shape, an elliptical cylindrical shape, and prism shapes, such as a triangular prism and a quadrilateral prism.

[0069] The embodiment described includes a case in which the discharge element 20 is connected to one end section of the base material 19 and in which the other end section of the base material 19 is connected to the metal sleeve 17; however, the embodiment is not necessarily limited to this. It is, of course, possible to insert an intermediate material between one end section of the base material 19 and the discharge element 20. In this case, the intermediate material is a section of the base material 19, and the discharge element 20 is connected to the intermediate material (base material 19) via the intervening diffusion layer 25.

[0070] Using the ground electrode 18 as an example of the first electrode, the diffusion layer 25 between the base material 19 of the ground electrode 18 and the discharge element 20 was described in the first embodiment; however, the embodiment is not necessarily limited to this. It is, of course, possible to use the central electrode 13 as the first electrode and the ground electrode 18 as the second electrode. In this case, the base material 14 of the central electrode 13 and the discharge element 15 are connected to each other with the diffusion layer 25 in between. As in the previously mentioned embodiment, it is possible to prevent the discharge element 15 from detaching from the base material 14 by making the composition of the base material 14 of the central electrode 13 similar to the composition of the base material 19 of the ground electrode 18.

[0071] In the embodiment described, a case was formed in which the diffusion layer 25 between the base material 19 and the discharge element 20 is formed by resistance welding; however, the embodiment is not necessarily limited to this. It is of course possible to form the diffusion layer 25 by diffusion of atoms with the base material 19 and the discharge element 20 in close contact with each other to a degree that minimizes plastic deformation, provided that the temperature is at most the melting points of the base material 19 and the discharge element 20, in order to join the base material 19 and the discharge element 20 together (conventionally known as diffusion joining).

[0072] The embodiment described above features a case in which the base material 19 connected to the metal sleeve 17 is bent. However, the present embodiment is not necessarily limited to this. It is, of course, possible to use a straight base material 19 instead of the bent one. In this case, the straight base material is connected to the metal sleeve 17, with the front face of the metal sleeve 17 being extended along the direction of axis O such that the base material faces the central electrode 13.

[0073] In this embodiment, a case is described in which the axis O of the central electrode 13 coincides with the center 23 of the discharge electrode 21 of the discharge element 20, and in which the ground electrode 18 is arranged such that the discharge element 20 faces the central electrode 13 in the axial direction. However, the embodiment is not necessarily limited to this, and the positional relationship between the ground electrode 18 and the central electrode 13 can be adjusted as required. For example, another positional relationship between the ground electrode 18 and the central electrode 13 is an arrangement in which the ground electrode 18 is positioned such that a lateral surface of the central electrode 13 and the discharge element 20 of the ground electrode 18 face each other. Reference symbol list 10 Spark plug 13 central electrode (second electrode) 18 Ground electrode (first electrode) 19 basic materials 20 discharge element 22 spark gap 25 diffusion layer 27 Segregation

Claims

[1] Spark plug (10), comprising: a first electrode (18) comprising a base material (19) and a discharge element (20) comprising, wherein at least a part of the discharge element (20) is connected to the base material (19) with an intermediate diffusion layer (25); and a second electrode (13) which faces the discharge element (25) with an intermediate spark gap (22), wherein the base material (19) contains at least 50 wt% of Ni, at least 8 wt% and at most 40 wt% of Cr, at least 0.05 wt% and at most 2 wt% of Si, at least 0.01 wt% and at most 2 wt% of Al, at least 0.01 wt% and at most 2 wt% of Mn, at least 0.01 wt% and at most 0.1 wt% of C and at least 0.001 wt% and at most 0.04 wt% of Fe. [2] Spark plug (10) according to claim 1, wherein the base material (19) contains at least 22 wt% and at most 28 wt% of Cr, at least 0.7 wt% and at most 1.3 wt% of Si, at least 0.6 wt% and at most 1.2 wt% of Al, at least 0.1 wt% and at most 1.1 wt% of Mn and at least 0.01 wt% and at most 0.07 wt% of C. [3] Spark plug (10) according to claim 1 or claim 2, wherein if X (mass-%) represents a content fraction of Si in the base material (19) and Y (mass-%) represents a content fraction of Fe in the base material (19), 2.5 ≤ X / Y is satisfied. [4] Spark plug (10) according to claim 3, wherein, if X (mass-%) represents the content of Si in the base material (19) and Y (mass-%) represents the content of Fe in the base material (19), 2.5 ≤ X / Y ≤ 400 is satisfied. [5] Spark plug (10) according to one of claims 1 to 4, wherein the base material (19) contains a solid solution containing Ni, wherein the solid solution contains a segregation (27) present therein, and wherein in a cross-section of the base material (19) an area of ​​the segregation (27) which occupies an area of ​​the base material (19) is at least 0.01% and at most 4%.

Images