Patent Publication Number: US-10312669-B2

Title: Spark plug

Description:
RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP16/04601 filed Oct. 17, 2016, which claims the benefit of Japanese Patent Application No. 2016-012657, filed Jan. 26, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The disclosure of the present specification relates to a spark plug used in an internal combustion engine or the like. 
     BACKGROUND OF THE INVENTION 
     A noble metal including platinum (Pt) is known to be used as an electrode of a spark plug used in an internal combustion engine. For example, in the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. H06-60959, a discharge member formed from platinum or a platinum-iridium alloy is bonded to an electrode base material via an intermediate member formed from a platinum-nickel alloy. A diffusion layer is formed between the discharge member and the intermediate member. This suppresses peeling or falling off of the discharge member due to thermal stress between the members. 
     However, in recent years, in order to further improve fuel economy, the temperature within a combustion chamber of an internal combustion engine needs to be further increased, and a spark plug needs to operate under a higher-temperature environment. Under such a high-temperature environment, since wear of a discharge member due to spark, oxidation, or the like, and peeling of a discharge member due to thermal stress or the like, are more likely to occur, wear resistance and peeling resistance need to be further improved. 
     For example, in the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. H06-60959, platinum or a platinum-iridium alloy is used as a discharge member, and a platinum-nickel alloy is used as an intermediate member. However, under the above-described high-temperature environment, there is a possibility of embrittlement and decrease in thermal conductivity due to increase of elements in the diffusion layer and Kirkendall voids that occur due to progressing of interdiffusion between the discharge member and the intermediate member. In addition, for example, when platinum is used as a discharge member, crystal grains are likely to grow in platinum and intercrystalline cracking is likely to occur. Since high-temperature combustion atmosphere is likely to reach the vicinity of an interface with the diffusion layer due to the intercrystalline cracking, diffusion may progress and intercrystalline cracking may be thus increased. Therefore, peeling resistance and wear resistance are likely to decrease. When a platinum-iridium alloy is used as a discharge member, oxidation wear of iridium is likely to occur under a high-temperature environment, and the diffusion layer is likely to be embrittled due to iridium and nickel being mixed in the diffusion layer. Since iridium is reduced due to oxidation, crystal grains on the surface of the discharge member gradually grow as seen in platinum, and the crystal grains fall off as in the case of platinum. As a result, under the high-temperature environment, temperature is likely to increase in the vicinity of an interface between the discharge member and the diffusion layer, diffusion may progress, and the wear resistance and peeling resistance of a spark plug may be decreased. 
     The present specification discloses a spark plug that can achieve both wear resistance and peeling resistance of a spark plug under a high-temperature environment. 
     SUMMARY OF THE INVENTION 
     Application Example 1 
     In accordance with a first aspect of the present invention, there is provided a spark plug comprising: 
     a center electrode extending in an axial direction; and 
     a ground electrode forming a gap between the ground electrode and the center electrode, 
     wherein at least one of the center electrode and the ground electrode includes: 
     an electrode base material; 
     a discharge member having a discharge surface that forms the gap; 
     an intermediate member disposed between the discharge member and the electrode base material; and 
     a diffusion layer formed between the discharge member and the intermediate member, 
     the electrode base material contains not less than 50 wt. % of nickel (Ni), 
     the discharge member contains not less than 45 wt. % of platinum (Pt), and at least one of nickel and rhodium (Rh), 
     the intermediate member contains platinum and nickel, 
     in the discharge member, a content of platinum is highest, and a total content of platinum, rhodium, and nickel is not less than 92 wt. %, 
     in the intermediate member, a content of one of platinum and nickel is not less than 50 wt. %, a content of nickel is higher than a content of nickel in the discharge member, and a total content of platinum, rhodium, and nickel is not less than 85 wt. %, and 
     a thickness of the diffusion layer is not less than 0.002 mm and not more than 0.065 mm. 
     According to the above-described structure, it is possible to achieve improvement of oxidation resistance of the discharge member, suppression of progressing of interdiffusion between the discharge member and the intermediate member, decrease in thermal stress, suppression of embrittlement of the diffusion layer, and suppression of decrease in thermal conductivity of the diffusion layer. As a result, it is possible to achieve both wear resistance and peeling resistance in the spark plug. 
     Application Example 2 
     In accordance to a second aspect of the present invention, there is provided a spark plug, as described in the application example 1, wherein, 
     the thickness of the diffusion layer is not less than 0.005 mm and not more than 0.065 mm. 
     According to the above-described structure, since it is thus possible to more effectively suppress peeling between the discharge member and the intermediate member due to thermal stress, it is possible to further improve peeling resistance and wear resistance in a spark plug  100 . 
     Application Example 3 
     In accordance with a third aspect of the present invention, there is provided a spark plug, as described in the application example 1 or 2, wherein 
     in the case where D 1  represents a distance between the diffusion layer and the discharge surface of the discharge member, and G represents a length of the gap, 
     D 1 ≥0.1 mm and (D 1 /G)≥0.1 are satisfied. 
     Thus, it is possible to further suppress progressing of interdiffusion between the discharge member and the intermediate member. 
     Application Example 4 
     In accordance with a fourth aspect of the present invention, there is provided a spark plug, as described in any of the application examples 1 to 3, wherein 
     in the discharge member, a total content of platinum, rhodium, and nickel is not less than 96 wt. %. 
     Thus, by further decreasing the components other than platinum, rhodium, and nickel in the discharge member, it is possible to further suppress embrittlement of the diffusion layer and decrease in thermal conductivity of the diffusion layer. 
     Application Example 5 
     In accordance with a fifth aspect of the present invention, there is provided a spark plug, as described in any of the application examples 1 to 4, wherein 
     in the intermediate member, a total content of platinum, rhodium, and nickel is not less than 96 wt. %. 
     Thus, by further decreasing the components other than platinum, rhodium, and nickel in the intermediate member, it is possible to further suppress the embrittlement of the diffusion layer and decrease in thermal conductivity of the diffusion layer as described above. 
     Application Example 6 
     In accordance with a sixth aspect of the present invention, there is provided a spark plug, as described in any of the application examples 1 to 5, wherein 
     a content of nickel in the intermediate member is higher than a content of nickel in the discharge member by not less than 2.5 wt. %. 
     Thus, since it is possible to more effectively decrease thermal stress generated between the intermediate member and the electrode base material, it is possible to further improve peeling resistance of a discharge member  351 . 
     Application Example 7 
     In accordance with a seventh aspect of the present invention, there is provided a spark plug, as described in the application example 4, wherein, 
     in the discharge member, a total content of platinum and rhodium is not less than 88 wt. %. 
     Thus, by decreasing, in the discharge member, the component other than platinum superior in wear resistance and rhodium which allows suppression of grain growth of platinum, it is possible to further improve wear resistance of the spark plug. 
     It is noted that the technique disclosed in the present specification can be implemented in various forms. For example, the technique may be implemented as a spark plug, an ignition device using the spark plug, an internal combustion engine to which the spark plug is mounted, and an electrode for the spark plug, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a spark plug  100  of an embodiment of the present invention. 
         FIGS. 2(A) and 2(B)  are diagrams illustrating a portion in the vicinity of the front end of the spark plug  100 . 
         FIGS. 3(A) and 3(B)  are explanatory views of a diffusion layer  352 . 
         FIG. 4  is an explanatory view of a comparative sample. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Embodiment 
     A-1. Structure of Spark Plug: 
     Hereinafter, a mode of the present invention will be described on the basis of an embodiment.  FIG. 1  is a cross-sectional view of a spark plug  100  of the present embodiment. In  FIG. 1 , an alternate long and short dashed line indicates an axis CL of the spark plug  100 . A direction (an upward-downward direction in  FIG. 1 ) in parallel with the axis CL is also referred to as an axial direction. The radial direction of a circle about the axis CL on a plane perpendicular to the axis CL is also referred to merely as a “radial direction”, and the circumferential direction of the circle is also referred to merely as a “circumferential direction”. In  FIG. 1 , the downward direction is referred to as a front end direction FD, and the upward direction is also referred to as a rear end direction BD. The lower side in  FIG. 1  is referred to as a front side of the spark plug  100 , and the upper side in  FIG. 1  is referred to as a rear side of the spark plug  100 . 
     The spark plug  100  is mounted to an internal combustion engine and is used to ignite fuel gas within a combustion chamber of the internal combustion engine. The spark plug  100  is assumed to operate under a relatively high-temperature environment. Specifically, temperature in the vicinity of a discharge member (electrode tip) within the combustion chamber is assumed to be not less than 600° C. The spark plug  100  includes a ceramic insulator  10  as an insulator, a center electrode  20 , a ground electrode  30 , a metal terminal  40 , and a metal shell  50 . 
     The ceramic insulator  10  is formed by baking alumina or the like. The ceramic insulator  10  is a substantially cylindrical member having a through hole  12  (axial hole) that extends along the axial direction to penetrate through the ceramic insulator  10 . The ceramic insulator  10  includes a flange portion  19 , a rear trunk portion  18 , a front trunk portion  17 , a step portion  15 , and a leg portion  13 . The rear trunk portion  18  is located rearward of the flange portion  19  and has an outer diameter smaller than the flange portion  19 . The front trunk portion  17  is located forward of the flange portion  19  and has an outer diameter smaller than the flange portion  19 . The leg portion  13  is located forward of the front trunk portion  17  and has an outer diameter smaller than the front trunk portion  17 . The leg portion  13  is exposed to a combustion chamber of an internal combustion engine (not shown) when the spark plug  100  is mounted to the internal combustion engine. The step portion  15  is formed between the leg portion  13  and the front trunk portion  17 . 
     The metal shell  50  is formed from a conductive metal material (e.g., a low-carbon steel material) and is a cylindrical metal member for fixing the spark plug  100  to an engine head (not shown) of the internal combustion engine. The metal shell  50  has an insertion hole  59  penetrating along the axis CL. The metal shell  50  is disposed around the outer circumference of the ceramic insulator  10 . That is, the ceramic insulator  10  is inserted and held in the insertion hole  59  of the metal shell  50 . The front end of the ceramic insulator  10  projects forward of the front end of the metal shell  50 . The rear end of the ceramic insulator  10  projects rearward of the rear end of the metal shell  50 . 
     The metal shell  50  includes a hexagonal-prism-shaped tool engagement portion  51  with which a spark plug wrench engages, a mounting screw portion  52  for mounting the spark plug  100  to the internal combustion engine, a flange-like seat portion  54  formed between the tool engagement portion  51  and the mounting screw portion  52 . The nominal diameter of the mounting screw portion  52  is, for example, any of M8 (8 mm), M10, M12, M14, and M18. 
     An annular gasket  5  which is formed by bending a metal plate is inserted between the mounting screw portion  52  and the seat portion  54  of the metal shell  50 . The gasket  5  seals a gap between the spark plug  100  and the internal combustion engine (engine head) when the spark plug  100  is mounted to the internal combustion engine. 
     The metal shell  50  further includes: a thin crimp portion  53  provided at the rear side of the tool engagement portion  51 ; and a thin compressive deformation portion  58  provided between the seat portion  54  and the tool engagement portion  51 . Annular ring members  6  and  7  are disposed in annular regions each formed between: the inner peripheral surface of a portion of the metal shell  50  from the tool engagement portion  51  to the crimp portion  53 ; and the outer peripheral surface of the rear trunk portion  18  of the ceramic insulator  10 . Powder of a talc  9  is filled between the two ring members  6  and  7  in the regions. The rear end of the crimp portion  53  is bent radially inward and fixed to the outer peripheral surface of the ceramic insulator  10 . The compressive deformation portion  58  of the metal shell  50  is compressively deformed by the crimp portion  53 , which is fixed to the outer peripheral surface of the ceramic insulator  10 , being pressed toward the front side during manufacturing. The ceramic insulator  10  is pressed within the metal shell  50  toward the front side via the ring members  6  and  7  and the talc  9  due to the compressive deformation of the compressive deformation portion  58 . The step portion  15  (ceramic-insulator-side step portion) of the ceramic insulator  10  is pressed by a step portion  56  (metal-shell-side step portion), which is formed on the inner periphery of the mounting screw portion  52  of the metal shell  50 , via an annular plate packing  8  formed from metal. As a result, the plate packing  8  prevents gas within the combustion chamber of the internal combustion engine from leaking to the outside through a gap between the metal shell  50  and the ceramic insulator  10 . 
     The center electrode  20  includes: a rod-shaped center electrode body  21  extending in the axial direction; and a columnar center electrode tip  29  bonded to the front end of the center electrode body  21 . The center electrode body  21  is disposed within the through hole  12  and at a front end side portion of the ceramic insulator  10 . The center electrode body  21  is structured to include an electrode base material  21 A, and a core portion  21 B embedded in the electrode base material  21 A. The electrode base material  21 A is formed from, for example, nickel or an alloy containing nickel as a principal component. In the present embodiment, the electrode base material  21 A is formed from INCONEL 601 (“INCONEL” is a registered trademark). The core portion  21 B is formed from copper or an alloy containing copper as a principal component, the copper and the alloy having more excellent thermal conductivity than the alloy forming the electrode base material  21 A. In the present embodiment, the core portion  21 B is formed from copper. 
     In addition, the center electrode body  21  includes: a flange portion  24  provided at a predetermined position in the axial direction; a head portion  23  (electrode head portion) disposed rearward of the flange portion  24 ; and a leg portion  25  (electrode leg portion) disposed forward of the flange portion  24 . The flange portion  24  is supported by a step portion  16  of the ceramic insulator  10 . A front end portion of the leg portion  25 , that is, the front end of the center electrode body  21  projects forward of the front end of the ceramic insulator  10 . The center electrode tip  29  will be described later. 
     The ground electrode  30  includes: a ground electrode base material  31  bonded to the front end of the metal shell  50 ; and a clad electrode  35 . The ground electrode  30  will be described later. 
     The metal terminal  40  is a rod-shaped member extending in the axial direction. The metal terminal  40  is formed from a conductive metal material (e.g., low-carbon steel), and a metal layer (e.g., Ni layer) for anticorrosion is formed on the surface of the metal terminal  40  by means of plating or the like. The metal terminal  40  includes a flange portion  42  (terminal flange portion) formed at a predetermined position in the axial direction, a cap mounting portion  41  located rearward of the flange portion  42 , and a leg portion  43  (terminal leg portion) located forward of the flange portion  42 . The cap mounting portion  41  of the metal terminal  40  is exposed on the side rearward of the ceramic insulator  10 . The leg portion  43  of the metal terminal  40  is inserted into the through hole  12  of the ceramic insulator  10 . A plug cap to which a high-voltage cable (not shown) is connected is mounted on the cap mounting portion  41 , and a high voltage is applied to cause spark discharge. 
     In the through hole  12  of the ceramic insulator  10 , a resistor  70  for reducing electric wave noise generated when spark occurs is disposed between the front end of the metal terminal  40  (the front end of the leg portion  43 ) and the rear end of the center electrode  20  (the rear end of the head portion  23 ). The resistor  70  is formed from, for example, a composition containing glass particles as a principal component, ceramic particles other than glass, and a conductive material. In the through hole  12 , a gap between the resistor  70  and the center electrode  20  is filled with a conductive seal  60 , and a gap between the resistor  70  and the metal terminal  40  is filled with a conductive seal  80 . Each of the conductive seals  60  and  80  is formed from, for example, a composition containing particles of B 2 O 3 —SiO 2  glass or the like, and metal particles (Cu, Fe, etc.). 
     A-2. Structure of Front End Portion of Spark Plug  100 : 
     The structure of the vicinity of the front end of the above-described spark plug  100  will be described further in detail.  FIGS. 2(A) and 2(B)  are diagrams illustrating a portion in the vicinity of the front end of the spark plug  100 .  FIG. 2(A)  illustrates a specific cross section obtained by cutting a portion in the vicinity of the front end of the spark plug  100  at a specific plane including the axis CL.  FIG. 2(B)  is an enlarged view of the vicinity of the clad electrode  35  on the specific cross section in  FIG. 2(A) . 
     The center electrode tip  29  has a cylindrical shape, and is bonded to the front end of the center electrode body  21  (the front end of the leg portion  25 ) via, for example, a melt portion  27  formed by laser welding ( FIG. 2(A) ). The melt portion  27  is a portion that includes the component of the center electrode tip  29  and the component of the center electrode body  21 . The center electrode tip  29  is formed of a material containing, as a principal component, a noble metal having a high melting point. As the material of the center electrode tip  29 , for example, iridium (Ir), an alloy containing iridium as a principal component, platinum (Pt), or an alloy containing platinum as a principal component is used. 
     The ground electrode base material  31  is a bent bar-shaped body having a quadrangular cross section. A rear end portion  31 B of the ground electrode base material  31  is bonded to a front end surface  50 A of the metal shell  50 . Accordingly, the metal shell  50  and the ground electrode base material  31  are electrically connected to each other. A front end portion  31 A of the ground electrode base material  31  is a free end. 
     The ground electrode base material  31  is, for example, formed by using a nickel alloy, which will be described later in detail. In the ground electrode base material  31 , a core material formed by using a metal having a thermal conductivity higher than a nickel alloy, for example, copper or an alloy containing copper may be embedded. 
     The clad electrode  35  includes a discharge member  351 , an intermediate member  353 , and a diffusion layer  352  formed between the discharge member  351  and the intermediate member  353 . 
     The discharge member  351  has a cylindrical shape extending in the axial direction, and is formed by using an alloy containing platinum as a principal component, which will be described later in detail. The rear end surface of the discharge member  351  is a discharge surface  351 B that forms a spark gap between the discharge surface  351 B and a discharge surface  29 A on the front side of the center electrode tip  29 . 
     The intermediate member  353  has a cylindrical shape extending in the axial direction, and is formed by using an alloy containing platinum and nickel, which will be described later in detail. The intermediate member  353  is disposed between the discharge member  351  and the ground electrode base material  31 . Specifically, the intermediate member  353  and the discharge member  351  are bonded to each other by diffusion bonding. That is, a rear end surface  353 B of the intermediate member  353  is bonded, via the diffusion layer  352 , to a front end surface  351 A of the discharge member  351 . A front end surface  353 A of the intermediate member  353  is bonded to the rear side of the front end portion  31 A of the ground electrode base material  31  by using resistance welding. A front side portion that includes the front end surface  353 A of the intermediate member  353  is embedded in the front end portion  31 A of the ground electrode base material  31 . 
     The diffusion layer  352  is formed between the discharge member  351  and the intermediate member  353 .  FIGS. 3(A) and 3(B)  are explanatory diagrams of the diffusion layer  352 .  FIG. 3(A)  indicates the content (unit is wt. %) of platinum at the position on the axis CL of the ground electrode  30 . As indicated in  FIG. 3(A) , the content of platinum in the discharge member  351  is represented as W 1 (Pt), and the content of platinum in the intermediate member  353  is represented as W 2 (Pt). The content of platinum in the diffusion layer  352  continuously varies, from W 1 (Pt) to W 2 (Pt), going from the discharge member  351  side toward the intermediate member  353 .  FIG. 3(B)  indicates the content (unit is wt. %) of nickel at the position on the axis CL of the ground electrode  30 . The content of nickel in the discharge member  351  is represented as W 1 (Ni), and the content of nickel in the intermediate member  353  is represented as W 2 (Ni). The content of nickel in the diffusion layer  352  continuously varies, from W 1 (Ni) to W 2 (Ni), going from the discharge member  351  side toward the intermediate member  353 . The same can be applied to another component (e.g., rhodium). That is, it can be said that the diffusion layer  352  is a layer in which the content of a specific component continuously varies, from the content of a specific component in the discharge member  351  to the content of a specific component in the intermediate member  353 , going from the discharge member  351  toward the intermediate member  353 . When each of the discharge member  351  and the intermediate member  353  includes an element other than platinum, rhodium, and nickel, an intermetallic compound may be formed in the diffusion layer  352 . A combination of materials of the discharge member  351  and the intermediate member  353  is more preferably a combination of materials which does not form such an intermetallic compound. 
     As illustrated in  FIG. 2(A) , the length of a gap between the ground electrode  30  and the center electrode  20 , that is, the shortest distance between the discharge surface  29 A of the center electrode tip  29  and the discharge surface  351 B of the discharge member  351  is represented as G. In addition, as illustrated in  FIG. 2(B) , the outer diameter of the discharge member  351  is represented as R 1 , and the outer diameter of the intermediate member  353  is represented as R 2 . In an example of  FIG. 2(B) , the outer diameter R 1  of the discharge member  351  is equal to the outer diameter R 2  of the intermediate member  353 . In modification, the outer diameter R 1  of the discharge member  351  may be smaller than the outer diameter R 2  of the intermediate member  353 . In addition, as illustrated in  FIG. 2(B) , a distance, along the axis CL direction, between the diffusion layer  352  and the discharge surface  351 B of the discharge member  351  is represented as D 1 . In addition, the thickness of the diffusion layer  352 , that is, the length of the diffusion layer  352  in the axial direction is represented as D 2 , and the thickness of the intermediate member  353  is represented as D 3 . In addition, the length (also called projecting length) from the discharge surface  351 B of the discharge member  351  to the surface of the ground electrode base material  31  is represented as D 4 . 
     In the present embodiment, the thickness D 2  of the diffusion layer  352  is not less than 0.002 mm and not more than 0.065 mm. As a result, it is possible to improve peeling resistance and wear resistance of the spark plug  100 . 
     Details will be described. When the thickness D 2  of the diffusion layer  352  is less than 0.002 mm, thermal stress between the discharge member  351  and the intermediate member  353  cannot be reduced by the diffusion layer  352 . Therefore, the discharge member  351  and the intermediate member  353  are likely to be peeled off each other, and peeling resistance deteriorates. In addition, peeling between the discharge member  351  and the intermediate member  353  causes decrease in thermal conductivity between the discharge member  351  and the intermediate member  353 . As a result, heat conduction decreases and the temperature of the discharge member  351  increases, and wear of the discharge member  351  increases, and wear resistance deteriorates. 
     When the thickness D 2  of the diffusion layer  352  exceeds 0.065 mm, the thermal conductivity between the discharge member  351  and the intermediate member  353  is decreased since the diffusion layer  352  has thermal conductivity lower than the discharge member  351  and the intermediate member  353 . Consequently, since the temperature of the discharge member  351  increases, the use of the spark plug  100  leads to progressing of interdiffusion between the discharge member  351  and the intermediate member  353 , and therefore the thickness D 2  of the diffusion layer  352  is further increased. As a result, the thermal conductivity between the discharge member  351  and the intermediate member  353  further decreases. As a result, heat conduction further decreases, the temperature of the discharge member  351  increases, and wear of the discharge member  351  increases. As a result, wear resistance deteriorates. 
     As described above, when the thickness D 2  of the diffusion layer  352  is not less than 0.002 mm and not more than 0.065 mm, it is possible to suppress the above-described peeling due to thermal stress and increase in the thickness of the diffusion layer  352  due to the progressing of diffusion. Therefore, as described above, it is possible to improve peeling resistance and wear resistance of the spark plug  100 . 
     A method for measuring a composition (e.g., contents of platinum and nickel) of the discharge member  351  and the intermediate member  353  and the thickness D 2  of the diffusion layer  352  will be described with reference to  FIGS. 3(A) and 3(B) . These values can be obtained as follows, by performing point analysis by using FE-EPMA (Field Emission-Electron Probe Micro Analysis), specifically, by using WDS (Wavelength Dispersive X-ray Spectrometer) of JXA-8500F manufactured by JEOL Ltd. 
     An exemplary case where the discharge member  351  is Pt and the intermediate member  353  is Pt-10Ni will be described. First, the ground electrode  30  (the discharge member  351 , the intermediate member  353 , and the ground electrode base material  31 ) of the spark plug  100  is cut on a plane including the axis CL, and the ground electrode  30  on which the cross-sectional surface is polished is prepared as a sample for analysis. In the polished surface of the sample, point analysis is performed at five points starting from an originating point A ( FIG. 3(A) ) located forward (on the intermediate member  353  side) of a surface S ( FIG. 3(A) ) of the discharge member  351  on the axis CL, by 10 μm, along the axial direction, and the five points are located at intervals of 10 μm toward the front end side. As a result, the average of Pt contents v 1  to v 5  ( FIG. 3(A) ) measured at five points is determined as a platinum content W 1  (Pt) of the discharge member  351 . 
     Next, point analysis is performed at intervals of 0.5 μm from the point A along the axial direction toward the front side (toward the intermediate member  353 ), and a platinum content at each point is plotted. In the plotted graph, among points at which a platinum content Vb is less than W 1 (Pt) such that, at all the points forward of the point at which Vb=W 1 (Pt) is satisfied, the platinum content is not more than Vb, the rearmost point B ( FIG. 3(A) ) is specified. A position of the point B in the axial direction is regarded as the position of a boundary between the discharge member  351  and the diffusion layer  352  in the axial direction. 
     Next, a point C ( FIG. 3(A) ) forward of the point B by 150 μm along the axial direction is specified. From the originating point C, point analysis is performed at five points toward the front side at intervals of 10 μm. As a result, the average of Pt contents v 6  to v 10  ( FIG. 3(A) ) measured at the five points is determined as a platinum content W 2  (Pt) of the intermediate member  353 . 
     Next, point analysis is performed at intervals of 0.5 μm from the point C along the axial direction toward the rear side (toward the discharge member  351 ), and a platinum content at each point is plotted. In the plotted graph, among points at which a platinum content Vd is more than W 2 (Pt) such that, at all the points rearward of the point at which Vd=W 2 (Pt) is satisfied, the platinum content is not less than Vd, the foremost point D ( FIG. 3(A) ) is specified. A position of the point D in the axial direction is regarded as the position of a boundary between the intermediate member  353  and the diffusion layer  352  in the axial direction. 
     As described above, a distance between the specified points B and D in the axial direction is regarded as a thickness D(Pt)( FIG. 3(A) ). 
     Such an analysis is performed for another component contained in any of the discharge member  351  and the intermediate member  353 . In this example, the same analysis is performed for nickel. That is, an average of nickel contents u 1  to u 5  ( FIG. 3(B) ) measured at intervals of 10 μm at the five points starting from an originating point A ( FIG. 3(B) ) is determined as a nickel content W 1 (Ni) of the discharge member  351 . Then, point analysis is performed at intervals of 0.5 μm from the point A along the axial direction toward the front side, and a nickel content at each point is plotted. In the plotted graph, among points at which the nickel content ue is more than W 1 (Ni) such that, at all the points forward of the point at which ue=W 1 (Ni) is satisfied, the nickel content is not less than ue, the rearmost point E ( FIG. 3(B) ) is specified. A position of the point E in the axial direction is regarded as the position of a boundary between the discharge member  351  and the diffusion layer  352  in the axial direction. 
     Next, a point F ( FIG. 3(B) ) forward of the point E by 150 μm along the axial direction is specified. From the originating point F, point analysis is performed at five points toward the front side at intervals of 10 μm. As a result, an average of nickel contents u 6  to u 10  ( FIG. 3(B) ) measured at the five points is determined as a nickel content W 2  (Ni) of the intermediate member  353 . 
     Next, point analysis is performed at intervals of 0.5 μm from the point F along the axial direction toward the rear side, and a nickel content at each point is plotted. In the plotted graph, among points at which the nickel content ug is less than W 2 (Ni) such that, at all the points rearward of the point at which ug=W 2 (Ni) is satisfied, nickel content is not more than ug, the foremost point G ( FIG. 3(B) ) is specified. A position of the point G in the axial direction is regarded as the position of a boundary between the intermediate member  353  and the diffusion layer  352  in the axial direction. 
     As described above, a distance between the specified points E and G in the axial direction is regarded as a thickness D(Ni) ( FIG. 3(B) ). 
     The maximum value of the thicknesses measured for respective components as described above is determined as the thickness D 2  of the diffusion layer  352 . In this example, the larger of values of the thickness D(Pt) measured for platinum and the thickness D(Ni) measured for nickel is determined as the thickness D 2  of the diffusion layer  352 . 
     Here, the point analysis for each of v 1  to v 10  and u 1  to u 10  as described above (point analysis at intervals of 10 μm) was performed at an accelerating voltage of 20 kV with a spot diameter of 10 μm, and point analysis for specifying the points B, D, E, and G (point analysis at intervals of 0.5 μm) was performed at an accelerating voltage of 20 kV with a spot diameter of 1 μm. 
     It is noted that, when the measured values vary depending on positions, the same measurement as described above is performed five times by displacing the measurement position (e.g., the position of the point A) in the radial direction, and the average of values obtained from the five times of measurement is determined as the final thickness D 2  of the diffusion layer  352 . 
     It is noted that the contents W 1 (Pt), W 1 (Ni), W 2 (Pt), and W 2 (Ni) measured at the points A, C, and F are values indicating the compositions of the discharge member  351  and the intermediate member  353 . Depending on the surface state of the discharge member  351 , and the thicknesses of the respective members  351 ,  352 , and  353 , in the points A, C, and F, there may be density gradient or these points may be located within the diffusion layer  352 . Therefore, when the measured contents W 1 (Pt), W 1 (Ni), W 2 (Pt), and W 2 (Ni) do not appear to represent the compositions of the discharge member  351  and the intermediate member  353 , the positions of the points A, C, and F are changed as appropriate and then measurement is performed. 
     In addition, in the above-described measurement, when precipitates or voids are included in the member  351 ,  352 , and  353 , if the measured value and an observation result of the composition indicate that there is a point at which precipitates or voids seem to affect the measured value, the average of values at two points that do not seem to be affected by precipitates or voids, are located forward and rearward of the point, and are closest to the point is used, instead of the value measured at the point. 
     It is noted that the thickness D 2  of the diffusion layer  352  is preferably not less than 0.005 mm and not more than 0.065 mm. Thus, it is possible to more effectively suppress peeling between the discharge member  351  and the intermediate member  353  due to thermal stress. Therefore, it is possible to further improve peeling resistance and wear resistance of the spark plug  100 . 
     Here, when a distance D 1  between the diffusion layer  352  and the discharge surface  351 B of the discharge member  351  is excessively short, rising of temperature of the discharge surface  351 B due to discharge may cause the temperature in the vicinity of the diffusion layer  352  to be also high. As a result, the above-described interdiffusion is likely to progress when the spark plug  100  is used. Since a discharge voltage increases when a gap length G is excessively large, wear of the discharge member  351  increases. Since the above-described distance D 1  becomes short early when the wear of the discharge member  351  increases, the use of the spark plug  100  causes the progressing of the above described interdiffusion. Therefore, it is preferable that as the gap length G is larger, the distance D 1  is larger. Specifically, the gap length G and the distance D 1  between the diffusion layer  352  and the discharge surface  351 B of the discharge member  351  preferably satisfy (D 1 /G)≥0.1. That is, the distance D 1  is preferably not less than 10% of the gap length G. Thus, since it is possible to suppress the progressing of the above described interdiffusion, it is possible to further improve peeling resistance and wear resistance of the spark plug  100 . However, when the distance D 1  is excessively small, even if (D 1 /G) is controlled so as to satisfy (D 1 /G)≥0.1, it is difficult to obtain an effect of suppressing the progressing of interdiffusion. Therefore, it is preferable to satisfy D 1 ≥0.1. 
     It is noted that, since the discharge member  351  contains relatively expensive platinum as a principal component, it is not preferable that the distance D 1  is unnecessarily large. For example, the distance D 1  is preferably less than 0.4 mm. 
     The ground electrode  30  is, for example, manufactured as follows. The discharge member  351  and the intermediate member  353  are bonded to each other by diffusion bonding. Specifically, for example, a manufacturer performs preliminary bonding of the discharge member  351  and the intermediate member  353  by resistance welding. The manufacturer performs heat treatment, under a predetermined condition, for the discharge member  351  and the intermediate member  353  that have been preliminarily bonded to each other, to perform diffusion bonding of the discharge member  351  and the intermediate member  353 . As a result, the diffusion layer  352  is formed between the discharge member  351  and the diffusion layer  352 . The heat treatment is a treatment of holding, for example, in a furnace in vacuum or inert gas atmosphere, the discharge member  351  and the intermediate member  353  that have been preliminarily bonded to each other at a temperature of 700° C. to 1300° C. for 0 to 100 hours. The reason for including 0 hour is not that no heat treatment is performed, but that, when the temperature has risen to a target temperature, the temperature may be decreased without maintaining the temperature. If time and temperature are properly controlled, heat treatment may be performed in the air. By adjusting the conditions of heat treatment, it is possible to control the thickness D 2  of the diffusion layer  352 . For example, the higher the temperature to be maintained is, the larger the thickness D 2  of the diffusion layer  352  can be. The longer the holding time is, the larger the thickness D 2  of the diffusion layer  352  can be. In addition, the discharge member  351  and the intermediate member  353  bonded to each other by diffusion bonding may be formed by: processing melted materials obtained by blending and melting the components of the discharge member  351  and the intermediate member  353  into the respective plate materials by rolling or the like; stacking the two plate materials and then further performing rolling of the two plate materials at room temperature or hot rolling thereof; and punching the two plate materials after the rolling into predetermined shapes. Also in this case, in order to form the desired diffusion layer  352  by progressing of solid phase diffusion, heat treatment may be performed for the two plate materials after the rolling or punching as appropriate. 
     The manufacturer bonds the clad electrode  35 , that is, the discharge member  351  and the intermediate member  353  that have been bonded to each other by diffusion bonding, to the ground electrode base material  31  by resistance welding. It is possible to control the distance of the intermediate member  353  embedded in the ground electrode base material  31 , by controlling applied pressure, current, and energizing time during resistance welding. 
     A-2. Material of Ground Electrode  30   
     Next, a material that forms the ground electrode  30  will be described. The material of the ground electrode base material  31  of the ground electrode  30  is a metallic material that contains not less than 50 wt. % of nickel (Ni). Specifically, INCONEL 601 (Ni content of about 60 wt. %), INCONEL 600 (Ni content of about 75 wt. %), a Ni alloy (Ni content of about not less than 90 wt. %) having higher Ni content, or the like is used as the material of the ground electrode base material  31 . Here, the ground electrode base material  31  represents a member that includes at least a part that includes a surface to which the clad electrode  35  is bonded and that is formed from the same material as that of the part that includes the surface to which the clad electrode  35  is bonded. For example, here, when the ground electrode base material  31  has a multilayer structure including a core material such as copper, the core material is not included in the ground electrode base material  31 . 
     The material of the discharge member  351  of the ground electrode  30  is a material that satisfies the following (1) to (3). 
     (1) Not less than 45 wt. % of platinum (Pt) and at least one of nickel and rhodium (Rh) are contained. 
     (2) The content of platinum is highest. 
     (3) The total content of platinum, rhodium, and nickel is not less than 92 wt. %. 
     When the above-described (1) to (3) are satisfied, it is possible to improve peeling resistance and wear resistance of the spark plug  100 . 
     Details will be described. When the content of platinum is highest (the above-described (2)), for example, it is possible to improve wear resistance at high temperature as compared to a case where iridium that is poor in oxidation resistance because of forming a volatile oxide at a high temperature is a principal component. Furthermore, by adding nickel or rhodium (the above-described (1)), it is possible to suppress grain growth of platinum and generation of intercrystalline cracking, whereby it is possible to improve peeling resistance. Since decrease in heat conduction occurs when the discharge member  351  is peeled, this results in deterioration of wear resistance. Although grain growth can be suppressed also when iridium is added, iridium is easily oxidized and volatilized. Therefore, wear occurs in iridium during use of the spark plug  100 , and the effect of suppressing grain growth is lost as time elapses. Furthermore, since nickel and rhodium are superior to iridium also in oxidation resistance, the added amount is less likely to decrease during use of the spark plug  100 . Accordingly, since it is possible to suppress grain growth and intercrystalline cracking for a long time, it is possible to improve peeling resistance and wear resistance. 
     Furthermore, in order to improve peeling resistance and wear resistance, by use of the spark plug  100 , embrittlement of the diffusion layer  352  that occurs due to progressing of interdiffusion between the discharge member  351  and the intermediate member  353  and decrease in thermal conductivity of the diffusion layer  352  need to be reduced. This is because the embrittlement of the diffusion layer  352  causes peeling of the discharge member  351 . In addition, decrease in thermal conductivity of the diffusion layer  352  causes decrease in heat conduction, resulting in increase in wear of the discharge member  351 . 
     The embrittlement of the diffusion layer  352  and the decrease in thermal conductivity of the diffusion layer  352  are caused by increased kinds of elements having different characteristics being mixed in the diffusion layer  352 . Therefore, in order to suppress the embrittlement of the diffusion layer  352  and the decrease in thermal conductivity of the diffusion layer  352 , elements to be added to the discharge member  351  are preferably similar to nickel and platinum each of which is a principal component of the intermediate member  353  described later, or elements whose properties are similar to those of platinum and nickel. Nickel and rhodium that are elements added to the discharge member  351  are similar to platinum in properties such as crystal structure. Therefore, even if interdiffusion between the discharge member  351  and the intermediate member  353  progresses, it is possible to suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352 . 
     Furthermore, when the total content of platinum, rhodium, and nickel is not less than 92 wt. % (the above-described (3)), the proportions of elements superior in oxidation resistance is increased and it is possible to improve wear resistance. In addition, by suppressing the proportions of other elements, it is possible to suppress the mixture of other elements in the diffusion layer  352  due to interdiffusion as described above. Therefore, it is possible to suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. 
     Furthermore, the total content of platinum, rhodium, and nickel in the discharge member  351  is more preferably not less than 96 wt. %. Thus, by further decreasing the components other than platinum, rhodium, and nickel in the discharge member  351 , it is possible to further suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. 
     Furthermore, it is particularly preferable that, in the discharge member  351 , the total content of platinum, rhodium, and nickel is not less than 96 wt. % and the total content of platinum and rhodium is not less than 88 wt. %. Thus, by further decreasing the components other than platinum, rhodium, and nickel in the discharge member  351 , it is possible to further suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above, and by decreasing the components other than platinum superior in wear resistance and rhodium which allows suppression of the grain growth of the platinum, it is possible to further improve wear resistance. 
     The material of the intermediate member  353  of the ground electrode  30  is a material that satisfies the following (4) to (6). 
     (4) Platinum and nickel are contained, and the content of one of platinum and nickel is not less than 50 wt. %. 
     (5) The nickel content is higher than the nickel content in the discharge member  351 . 
     (6) The total content of platinum, rhodium, and nickel is not less than 85 wt. %. 
     Details will be described. Platinum and nickel are contained and the content of one of platinum and nickel is not less than 50 wt. % (the above-described (4)), and the total content of platinum, rhodium, and nickel is not less than 85 wt. % (the above-described (6)). Therefore, the principal component of the intermediate member  353  can be platinum, nickel, or rhodium of the same component contained in the discharge member  351 , or can be a component having properties similar thereto. As a result, it is possible to suppress the mixture of another component in the diffusion layer  352  due to the above-described interdiffusion. Therefore, it is possible to suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. 
     Furthermore, the intermediate member  353  contains platinum and nickel (the above-described (4)) and the content of nickel in the intermediate member  353  is higher than the content of nickel in the discharge member  351  (the above-described (5)). Therefore, a thermal expansion coefficient of the intermediate member  353  can be set so as to be between a thermal expansion coefficient of the discharge member  351  and a thermal expansion coefficient of the ground electrode base material  31 . As a result, since it is possible to decrease thermal stress generated between the intermediate member  353  and the discharge member  351 , and between the intermediate member  353  and the ground electrode base material  31 , it is possible to improve peeling resistance of the discharge member  351 . 
     As described above, by satisfying the above-described (4) to (6), it is possible to improve peeling resistance and wear resistance of the spark plug  100 . 
     Furthermore, in the intermediate member  353 , the total content of platinum, rhodium, and nickel is more preferably not less than 96 wt. %. Thus, by further decreasing the components other than platinum, rhodium, and nickel in the intermediate member  353 , it is possible to further suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. 
     Furthermore, in the intermediate member  353 , it is more preferable that the nickel content is higher than the nickel content in the discharge member  351  by not less than 2.5 wt. %. Thus, the thermal expansion coefficient of the intermediate member  353  can be a more appropriate value between the thermal expansion coefficient of the discharge member and the thermal expansion coefficient of the electrode base material. As a result, especially, it is possible to more effectively decrease thermal stress generated in a portion, between the ground electrode base material  31  and the intermediate member  353 , at which the diffusion layer  352  is not formed. Therefore, it is possible to further improve peeling resistance of the discharge member  351 . 
     B. Evaluation Test 
     By using samples of spark plugs, evaluation test was performed to evaluate wear resistance and peeling resistance. In the evaluation test, as shown in Tables 1 to 4, 66 types of samples 1 to 66 were produced. In each sample, components other than the ground electrode  30  are the same as those of the spark plug  100  described above and are common thereamong. 
     In Table 1, regarding samples 1 to 50, for each sample, a material of the discharge member  351 , the distance D 1  from the diffusion layer  352  to the discharge surface  351 B of the discharge member  351 , the gap length G, a value of (D 1 /G), the thickness D 2  of the diffusion layer  352  are indicated. It is noted that, in Table 1, a calculated value of (D 1 /G), the total content (Pt+Rh+Ni) of platinum, rhodium, and nickel in the discharge member  351 , and the total content (Pt+Rh) of platinum and rhodium in the discharge member  351  are also indicated. In Table 2, regarding samples 1 to 50, a material of the intermediate member  353 , and evaluation results of wear resistance and peeling resistance are indicated. In addition, in Table 2, the total content (Pt+Rh+Ni) of platinum, rhodium, and nickel in the intermediate member  353 , and a difference ΔW(Ni) obtained by subtracting the nickel content in the discharge member  351  from the nickel content in the intermediate member  353  are also indicated. In Tables 3 and 4, regarding samples 51 to 66, the same items as those in Tables 1 and 2 are indicated. 
     The clad electrode  35  of each of the 66 types of samples was manufactured by diffusion bonding of the cylindrical intermediate member  353  having an outer diameter of 1.6 mm and a thickness of 0.4 mm to the cylindrical discharge member  351  having an outer diameter of 1.6 mm. The clad electrode  35  of each of the manufactured samples was welded to the ground electrode base material  31  by resistance welding, such that the projecting length D 4  ( FIGS. 2(A) and 2(B) ) were less than 0.4 mm. It is noted that for all samples, INCONEL 601 was used as the material of the ground electrode base material  31 . 
     Then, as indicated in Tables 1 to 4, the material of each of the discharge member  351  and the intermediate member  353  was changed for each sample. Furthermore, the 66 types of samples 1 to 66 as indicated in Tables 1 to 4 were manufactured by changing a length of the discharge member  351  in the axial direction, a condition of heat treatment in diffusion bonding, and the gap length G. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Discharge member 
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Pt + Rh + 
                   
                   
                   
                   
                   
               
               
                 No. 
                 Pt 
                 Rh 
                 Ni 
                 Ir 
                 Re 
                 Pd 
                 Au 
                 Ni 
                 Pt + Rh 
                 D1 
                 G 
                 D1/G 
                 D2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 100 
                   
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 2 
                 80 
                   
                   
                 20 
                   
                   
                   
                 80 
                 80 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 3 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.27 
                 1 
                 0.27 
                 0.027 
               
               
                 4 
                 80 
                   
                   
                 20 
                   
                   
                   
                 80 
                 80 
                 0.1 
                 1 
                 0.10 
                 0.012 
               
               
                 5 
                 80 
                   
                   
                 20 
                   
                   
                   
                 80 
                 80 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 6 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 7 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.25 
                 1.3 
                 0.19 
                 0.001 
               
               
                 8 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.25 
                 1.3 
                 0.19 
                 0.075 
               
               
                 9 
                 85 
                 5 
                   
                   
                 10 
                   
                   
                 90 
                 90 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 10 
                 85 
                 5 
                   
                   
                   
                 10 
                   
                 90 
                 90 
                 0.1 
                 1 
                 0.10 
                 0.001 
               
               
                 11 
                 88.5 
                   
                 1.5 
                   
                 10 
                   
                   
                 90 
                 88.5 
                 0.13 
                 1.3 
                 0.10 
                 0.007 
               
               
                 12 
                 88.5 
                   
                 1.5 
                   
                   
                   
                 10 
                 90 
                 88.5 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 13 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.12 
                 1.3 
                 0.09 
                 0.047 
               
               
                 14 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.13 
                 1.4 
                 0.09 
                 0.007 
               
               
                 15 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.09 
                 0.4 
                 0.23 
                 0.065 
               
               
                 16 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.1 
                 1 
                 0.10 
                 0.002 
               
               
                 17 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 18 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.3 
                 1 
                 0.30 
                 0.002 
               
               
                 19 
                 87 
                 5 
                   
                   
                   
                 8 
                   
                 92 
                 92 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 20 
                 91 
                 5 
                   
                   
                   
                 4 
                   
                 96 
                 96 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 21 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.13 
                 1.3 
                 0.10 
                 0.002 
               
               
                 22 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 23 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 24 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 0.4 
                 0.50 
                 0.065 
               
               
                 25 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 0.8 
                 0.25 
                 0.065 
               
               
                 26 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 1 
                 0.20 
                 0.008 
               
               
                 27 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.002 
               
               
                 28 
                 95 
                 5 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.065 
               
               
                 29 
                 90 
                 10 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.002 
               
               
                 30 
                 90 
                 10 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.005 
               
               
                 31 
                 90 
                 10 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.005 
               
               
                 32 
                 90 
                 10 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.13 
                 1.3 
                 0.10 
                 0.007 
               
               
                 33 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.005 
               
               
                 34 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.005 
               
               
                 35 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.007 
               
               
                 36 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.2 
                 0.4 
                 0.50 
                 0.012 
               
               
                 37 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.25 
                 1.3 
                 0.19 
                 0.002 
               
               
                 38 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.25 
                 1.3 
                 0.19 
                 0.005 
               
               
                 39 
                 80 
                 20 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.25 
                 1.3 
                 0.19 
                 0.065 
               
               
                 40 
                 60 
                 40 
                   
                   
                   
                   
                   
                 100 
                 100 
                 0.15 
                 1.1 
                 0.14 
                 0.002 
               
               
                 41 
                 48 
                 40 
                 12 
                   
                   
                   
                   
                 100 
                 88 
                 0.15 
                 1.1 
                 0.14 
                 0.009 
               
               
                 42 
                 45 
                 40 
                 15 
                   
                   
                   
                   
                 100 
                 85 
                 0.15 
                 1.1 
                 0.14 
                 0.005 
               
               
                 43 
                 50 
                 45 
                 5 
                   
                   
                   
                   
                 100 
                 95 
                 0.15 
                 1.1 
                 0.14 
                 0.012 
               
               
                 44 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.12 
                 1.3 
                 0.09 
                 0.002 
               
               
                 45 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.12 
                 1.3 
                 0.09 
                 0.002 
               
               
                 46 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.12 
                 1.3 
                 0.09 
                 0.002 
               
               
                 47 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.15 
                 1 
                 0.15 
                 0.002 
               
               
                 48 
                 94.5 
                   
                 1.5 
                   
                   
                   
                 4 
                 96 
                 94.5 
                 0.11 
                 1.3 
                 0.08 
                 0.007 
               
               
                 49 
                 90.5 
                   
                 1.5 
                   
                   
                   
                 8 
                 92 
                 90.5 
                 0.1 
                 1.2 
                 0.08 
                 0.004 
               
               
                 50 
                 90.5 
                   
                 1.5 
                   
                   
                   
                 8 
                 92 
                 90.5 
                 0.1 
                 1.2 
                 0.08 
                 0.007 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Intermediate member 
                 Evaluation results 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Pt + 
                   
                 Peeling 
                 Peeling 
               
               
                   
                   
                   
                   
                 ΔW 
                   
                   
                   
                   
                   
                 Rh + 
                 Wear 
                 resistance 
                 resistance 
               
               
                 No. 
                 Pt 
                 Rh 
                 Ni 
                 (Ni)  
                 Ir 
                 Pd 
                 Au 
                 Cr 
                 Co 
                 Ni 
                 resistance  
                 1 
                 2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 2 
                 80 
                   
                   
                 0 
                   
                   
                 20 
                   
                   
                 80 
                 C 
                 C 
                 C 
               
               
                 3 
                 0 
                   
                 99 
                 99 
                 1 
                   
                   
                   
                   
                 99 
                 C 
                 C 
                 A 
               
               
                 4 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 5 
                 40 
                   
                   
                 0 
                   
                 60 
                   
                   
                   
                 40 
                 C 
                 C 
                 C 
               
               
                 6 
                 98.5 
                   
                   
                 0 
                   
                   
                   
                   
                 1.5 
                 98.5 
                 A 
                 C 
                 C 
               
               
                 7 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 8 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 9 
                 97.5 
                   
                 2.5 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 10 
                 98 
                   
                 2 
                 2 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 11 
                 96 
                   
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 C 
                 C 
                 A 
               
               
                 12 
                 92 
                   
                 4 
                 2.5 
                 4 
                   
                   
                   
                   
                 96 
                 C 
                 C 
                 A 
               
               
                 13 
                 66 
                 5 
                 12 
                 2 
                   
                   
                   
                   
                 17 
                 83 
                 C 
                 C 
                 B 
               
               
                 14  
                 97.5 
                   
                 2.5 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 15  
                 97.5 
                   
                 2.5 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 16  
                 98.5 
                   
                 1.5 
                 1.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 B 
               
               
                 17  
                 98.5 
                   
                 1.5 
                 1.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 B 
               
               
                 18  
                 98.5 
                   
                 1.5 
                 1.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 B 
               
               
                 19  
                 98.5 
                   
                 1.5 
                 1.5 
                   
                   
                   
                   
                   
                 100 
                 B 
                 S 
                 B 
               
               
                 20  
                 98.5 
                   
                 1.5 
                 1.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 B 
               
               
                 21  
                 97.5 
                   
                 2.5 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 22  
                 97.5 
                   
                 2.5 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 23 
                 95 
                   
                 5 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 24 
                 95 
                   
                 5 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 25 
                 95 
                   
                 5 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 26 
                 91 
                   
                 5 
                 5 
                   
                 4 
                   
                   
                   
                 96 
                 A 
                 SSS 
                 A 
               
               
                 27 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 28 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 29 
                 95 
                   
                 5 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 30 
                 91 
                   
                 5 
                 5 
                 2 
                 2 
                   
                   
                   
                 96 
                 A 
                 SSS 
                 A 
               
               
                 31 
                 90 
                   
                 10 
                 10 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 32 
                 80 
                   
                 10 
                 10 
                 5 
                 5 
                   
                   
                   
                 90 
                 A 
                 SS 
                 A 
               
               
                 33 
                 95 
                   
                 5 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 34 
                 90 
                   
                 10 
                 10 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 35 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 36 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 37 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 38 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 39 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 40 
                 80 
                   
                 20 
                 20 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 41 
                 80 
                   
                 20 
                 8 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 42 
                 80 
                   
                 20 
                 5 
                   
                   
                   
                   
                   
                 100 
                 B 
                 SSS 
                 A 
               
               
                 43 
                 80 
                   
                 20 
                 15 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 44 
                 92 
                   
                 3 
                 1.5 
                   
                   
                   
                 5 
                   
                 95 
                 A 
                 A 
                 B 
               
               
                 45 
                 91 
                   
                 4 
                 2.5 
                   
                   
                   
                 5 
                   
                 95 
                 A 
                 A 
                 A 
               
               
                 46 
                 96 
                   
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 S 
                 A 
               
               
                 47  
                 93.5 
                 3  
                 3.5 
                 2 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 B 
               
               
                 48 
                 91 
                   
                 4 
                 2.5 
                 5 
                   
                   
                   
                   
                 95 
                 A 
                 S 
                 A 
               
               
                 49 
                 91 
                   
                 4 
                 2.5 
                 5 
                   
                   
                   
                   
                 95 
                 B 
                 B 
                 A 
               
               
                 50 
                 91 
                   
                 4 
                 2.5 
                 5 
                   
                   
                   
                   
                 95 
                 B 
                 A 
                 A 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Discharge member 
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Pt + Rh + 
                   
                   
                   
                   
                   
               
               
                 No. 
                 Pt 
                 Rh 
                 Ni 
                 Ir 
                 Re 
                 Pd 
                 Au 
                 Ni 
                 Pt +Rh 
                 D1 
                 G 
                 D1/G 
                 D2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 51 
                 90.5 
                   
                 1.5 
                   
                   
                   
                 8 
                 92 
                 90.5 
                 0.1 
                 1 
                 0.10 
                 0.007 
               
               
                 52 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 53 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.38 
                 1 
                 0.38 
                 0.002 
               
               
                 54 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.2 
                 1 
                 0.20 
                 0.002 
               
               
                 55 
                 98.5 
                   
                 1.5 
                   
                   
                   
                   
                 100 
                 98.5 
                 0.2 
                 1.1 
                 0.18 
                 0.005 
               
               
                 56 
                 90.5 
                   
                 1.5 
                   
                 8 
                   
                   
                 92 
                 90.5 
                 0.2 
                 1.1 
                 0.18 
                 0.012 
               
               
                 57 
                 90.5 
                   
                 1.5 
                   
                   
                   
                 8 
                 92 
                 90.5 
                 0.13 
                 1.3 
                 0.10 
                 0.007 
               
               
                 58 
                 95 
                   
                 5 
                   
                   
                   
                   
                 100 
                 95 
                 0.2 
                 1.1 
                 0.18 
                 0.007 
               
               
                 59 
                 95 
                   
                 5 
                   
                   
                   
                   
                 100 
                 95 
                 0.2 
                 1.1 
                 0.18 
                 0.007 
               
               
                 60 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.12 
                 1.3 
                 0.09 
                 0.047 
               
               
                 61 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.2 
                 1.1 
                 0.18 
                 0.022 
               
               
                 62 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.2 
                 1.1 
                 0.18 
                 0.022 
               
               
                 63 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.2 
                 1.1 
                 0.18 
                 0.017 
               
               
                 64 
                 90 
                   
                 10 
                   
                   
                   
                   
                 100 
                 90 
                 0.2 
                 1.1 
                 0.18 
                 0.022 
               
               
                 65 
                 80 
                   
                 20 
                   
                   
                   
                   
                 100 
                 80 
                 0.2 
                 1.1 
                 0.18 
                 0.017 
               
               
                 66 
                 75 
                   
                 25 
                   
                   
                   
                   
                 100 
                 75 
                 0.2 
                 1.1 
                 0.18 
                 0.012 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Intermediate member 
                 Evaluation results 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Pt +  
                   
                 Peeling 
                 Peeling 
               
               
                   
                   
                   
                   
                   
                 ΔW 
                   
                   
                   
                   
                 Rh + 
                 Wear 
                 resistance 
                 resistance 
               
               
                 No. 
                 Pt 
                 Rh 
                 Ni 
                 Ir 
                 (Ni) 
                 Pd  
                 Au 
                 Cr 
                 Co 
                 Ni 
                 resistance 
                 1 
                 2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 51 
                 91 
                   
                 4 
                 2.5 
                 5 
                   
                   
                   
                   
                 95 
                 B 
                 S 
                 A 
               
               
                 52 
                 96 
                   
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 53 
                 96 
                   
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 54 
                 96 
                   
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SS 
                 A 
               
               
                 55 
                 93 
                 3 
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 56 
                 93 
                 3 
                 4 
                 2.5 
                   
                   
                   
                   
                   
                 100 
                 B 
                 SS 
                 A 
               
               
                 57 
                 92 
                   
                 4 
                 2.5 
                 4 
                   
                   
                   
                   
                 96 
                 B 
                 SS 
                 A 
               
               
                 58 
                 89 
                   
                 7 
                 2 
                   
                   
                   
                   
                 4 
                 96 
                 A 
                 SSS 
                 B 
               
               
                 59 
                 90 
                   
                 10 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 60 
                 68 
                 5 
                 12 
                 2 
                   
                   
                   
                   
                 15 
                 85 
                 A 
                 S 
                 B 
               
               
                 61 
                 85 
                   
                 15 
                 5 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 62 
                 60 
                   
                 40 
                 30 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 63 
                 30 
                   
                 70 
                 60 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 64 
                 15 
                   
                 85 
                 75 
                   
                   
                   
                   
                   
                 100 
                 A 
                 SSS 
                 A 
               
               
                 65 
                 50 
                   
                 50 
                 30 
                   
                   
                   
                   
                   
                 100 
                 B 
                 SSS 
                 A 
               
               
                 66 
                 20 
                   
                 80 
                 55 
                   
                   
                   
                   
                   
                 100 
                 B 
                 SSS 
                 A 
               
               
                   
               
            
           
         
       
     
     In all of samples 1 to 66, the discharge member  351  contains platinum, and, as the platinum content for samples 1 to 66, 45 wt. %, 48 wt. %, 50 wt. %, 60 wt. %, 75 wt. %, 80 wt. %, 85 wt. %, 87 wt. %, 88.5 wt. %, 90 wt. %, 90.5 wt. %, 91 wt. %, 94.5 wt. %, 95 wt. %, 98.5 wt. %, and 100 wt. % are indicated. 
     The discharge member  351  of sample 1 is 100 wt. % of platinum (pure platinum). In samples 2 to 64 excluding sample 1, the discharge member  351  contains at least one of rhodium (Rh), nickel (Ni), iridium (Ir), rhenium (Re), palladium (Pd), and gold (Au). 
     In samples 3, 6 to 10, 14 to 43 each of which contains rhodium, as a rhodium content, 5 wt. %, 10 wt. %, 20 wt. %, 40 wt. %, and 45 wt. % are indicated. In samples 11 to 13, 41 to 66 each of which contains nickel, as a nickel content, 1.5 wt. %, 5 wt. %, 10 wt. %, 12 wt. %, 15 wt. 5, 20 wt. %, and 25 wt. % are indicated. In samples 2, 4, 5 each of which contains iridium, an iridium content is 20 wt. %. In samples 9, 11, and 56 each of which contains rhenium, as a rhenium content, 8 wt. % and 10 wt. % are indicated. In samples 10, 19, 20 each of which contains palladium, as a palladium content, 4 wt. %, 8 wt. %, and 10 wt. % are indicated. In samples 12, 48 to 51, and 57 each of which contains gold, as a gold content, 4 wt. %, 8 wt. %, and 10 wt. % are indicated. 
     In samples 1, 2, 4 to 66 excluding sample 3, the intermediate member  353  contains platinum, and, as a platinum content, 15 wt. %, 20 wt. %, 30 wt. %, 40 wt. %, 50 wt. 5, 60 wt. %, 66 wt. 5, 68 wt. %, 80 wt. %, 85 wt. %, 89 wt. %, 90 wt. %, 91 wt. %, 92 wt. %, 93 wt. %, 93.5 wt. %, 95 wt. %, 96 wt. %, 97.5 wt. %, 98 wt. %, and 98.5 wt. % are indicated. 
     In samples 1, 3, 4, 7 to 66 excluding samples 2, 5, 6, the intermediate member  353  contains nickel, and, as a nickel content, 2 wt. %, 2.5 wt. %, 3 wt. %, 3.5 wt. %, 4 wt. %, 5 wt. %, 7 wt. %, 10 wt. %, 12 wt. %, 15 wt. %, 20 wt. %, 40 wt. %, 50 wt. %, 70 wt. %, 80 wt. %, 85 wt. %, 90 wt. %, and 99 wt. % are indicated. 
     The intermediate member  353  may further contain one or more of rhodium, iridium, palladium, gold, chromium (Cr), and cobalt (Co). In samples 13, 47, 55, 56, and 60 each of which contains rhodium, as a rhodium content, 3 wt. % and 5 wt. % are indicated. In samples 3, 12, 30, 32, 48 to 51, 57 each of which contains iridium, as an iridium content, 1 wt. %, 2 wt. %, 4 wt. %, and 5 wt. % are indicated. In samples 5, 26, 30, and 32 each of which contains palladium, as a palladium content, 2 wt. %, 4 wt. %, 5 wt. %, and 60 wt. % are indicated. In sample 2 that contains gold, a gold content is 20 wt. %. In samples 44, 45 each of which contains chromium, a chromium content is 5 wt. %. In samples 6, 13, 58, and 60 each of which contains cobalt, as a cobalt content, 1.5 wt. %, 4 wt. %, 15 wt. %, and 17 wt. % are indicated. 
     In samples 1 to 66, as the distance D 1  from the diffusion layer  352  to the discharge surface  351 B of the discharge member  351 , 0.09 mm, 0.1 mm, 0.11 mm, 0.12 mm, 0.13 mm, 0.15 mm, 0.2 mm, 0.25 mm, 0.27 mm, 0.3 mm, and 0.38 mm are indicated. In addition, as the gap length G, 0.4 mm, 0.8 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, and 1.4 mm are indicated. In addition, as the thickness D 2  of the diffusion layer  352 , 0.001 mm, 0.002 mm, 0.004 mm, 0.005 mm, 0.007 mm, 0.008 mm, 0.009 mm, 0.012 mm, 0.017 mm, 0.022 mm, 0.027 mm, 0.047 mm, 0.065 mm, and 0.075 mm are indicated. 
     It is noted that, for samples 1 to 66, comparative samples that correspond to the respective samples were prepared.  FIG. 4  is an explanatory diagram of a comparative example.  FIG. 4  is a cross-sectional view obtained by cutting a portion in the vicinity of the front end of the comparative example at a cross section including the axis CL. As shown in  FIG. 4 , in each comparative sample, an electrode tip  355  was prepared, and the electrode tip  355  had the same composition as the discharge member  351  of the corresponding sample and had the same shape and size as the entirety of the clad electrode  35  of the corresponding sample. Then, the comparative sample was manufactured by performing resistance welding of the electrode tip  355  in place of the clad electrode  35  of the corresponding sample, onto the ground electrode base material  31 . The structure excluding the electrode tip  355  of the comparative sample, such as the gap length G and the projecting length D 4 , is the same as that of a corresponding one of samples 1 to 66. 
     In the evaluation test, durability test was performed in which each of the samples and the comparative samples was mounted to a 3-cylinder gasoline engine having a displacement of 0.66 L. In the durability test, driving at a rotation speed of 6000 rpm for one minute and driving in an idling state for one minute were repeatedly performed at full throttle over 150 hours. Then, driving at a rotation speed of 6000 rpm for 100 hours at full throttle was further performed. It is noted that conditions, such as a quantity of fuel injection, of this gasoline engine was adjusted, such that, when a spark plug having the same shape as that used in the test except that a thermocouple was mounted in the ground electrode was used, the temperature at a position which was distant, by 1 mm, toward the joint surface at which a metal shell was joined, from the front end of the spark plug (the front end of the ground electrode  30 ) was 1000° C. at full throttle. 
     After the durability test, regarding the discharge member  351  of each sample and the electrode tip  355  of each comparative sample, a decreased amount (hereinafter called consumed volume), after the test, by which the volume before the test has decreased, was measured by using a CT scanner (TOSCANER-32250 μhd manufactured by Toshiba IT &amp; Control Systems Corporation). Then, the consumed volume of the discharge member  351  in the case of the consumed volume of the electrode tip  355  of the corresponding comparative sample being 1 was calculated as a consumed volume of each sample. Then, wear resistance of a sample for which the consumed volume was not more than 0.95 was evaluated as “C”. Wear resistance of a sample for which the consumed volume was not less than 0.9 and less than 0.95 was evaluated as “B”. Wear resistance of a sample for which the consumed volume was less than 0.9 was evaluated as “A”. Wear resistance is superior in the order of A, B, and C. 
     Furthermore, the ground electrode  30  of each sample and comparative sample after the durability test was cut on the cross section including the axis CL, and the oxide scale generation proportion at the cross section was measured. 
     First, the comparative samples will be described. A case where oxide scale OS indicated by a bold solid line has been generated in a cross section of the comparative sample in  FIG. 4 , will be described. In the cross section in  FIG. 4 , oxide scale OS was generated at an interface having the entire length of R 1  between the ground electrode base material  31  and a front end surface  355 A of the electrode tip  355 . On the interface, a length L 0  of a part in which oxide scale OS was generated was measured. The length L 0 , in the example shown in  FIG. 4 , is a total of L 01  and L 02  (L 0 =L 01 +L 02 ), and a ratio (L 0 /R) of the length L 0  of a part in which oxide scale was generated relative to the entire length R 1  of the interface was calculated as an oxide scale generation proportion SR 0  of each comparative sample. 
     Next, the samples will be described. For each of the samples, the oxide scale generation proportion SR 1  between the discharge member  351  and the intermediate member  353 , and the oxide scale generation proportion SR 2  between the intermediate member  353  and the ground electrode base material  31  were measured. Specifically, a length L 1  (not shown) of a part in which oxide scale was generated between the front end surface  351 A of the discharge member  351  and a front end surface  352 A of the diffusion layer  352  was measured. Then, the length L 1  with respect to the entire length R 1  of the interface was calculated as the oxide scale generation proportion SR 1  (SR 1 =(L 1 /R)) for each sample. Furthermore, a length L 2  (not shown) of a part in which oxide scale was generated at the interface between the intermediate member  353  and the ground electrode base material  31  was measured. Then, the length L 2  with respect to the entire length R 1  of the interface was calculated as the oxide scale generation proportion SR 2  (SR 2 =(L 2 /R 1 )) for each sample. 
     Then, the oxide scale generation proportion SR 1  in the case of the oxide scale generation proportion SR 0  of the corresponding comparative sample being 1 was calculated as an evaluation value E 1  of peeling resistance  1  of each sample. Peeling resistance  1  means resistance to peeling between the discharge member  351  and the intermediate member  353 . In addition, the oxide scale generation proportion SR 2  in the case of the oxide scale generation proportion SR 0  of the corresponding comparative sample being 1 was calculated as an evaluation value E 2  of peeling resistance  2  of each sample. Peeling resistance  2  means resistance to peeling between the intermediate member  353  and the ground electrode base material  31 . 
     Peeling resistance  1  of a sample in which the evaluation value E 1  was not less than 0.95 or the oxide scale generation proportion SR 1  was not less than 0.5 was evaluated as “C”. Peeling resistance  1  of a sample in which the evaluation value E 1  was not less than 0.9 and less than 0.95 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “B”. Peeling resistance  1  of a sample in which the evaluation value E 1  was not less than 0.85 and less than 0.9 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “A”. Peeling resistance  1  of a sample in which the evaluation value E 1  was not less than 0.5 and less than 0.85 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “S”. Peeling resistance  1  of a sample in which the evaluation value E 1  was not less than 0.3 and less than 0.5 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “SS”. Peeling resistance  1  of a sample in which the evaluation value E 1  was less than 0.3 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “SSS”. Resistance to peeling between the discharge member  351  and the intermediate member  353  is superior in the order of SSS, SS, S, A, B, and C. 
     Furthermore, peeling resistance  2  of a sample in which the evaluation value E 2  was not less than 0.95, or the oxide scale generation proportion SR 2  was not less than 0.5 was evaluated as “C”. Peeling resistance  2  of a sample in which the evaluation value E 2  was not less than 0.8 and less than 0.95 and the oxide scale generation proportion SR 2  was less than 0.5 was evaluated as “B”. Peeling resistance  2  of a sample in which the evaluation value E 2  was less than 0.8 and the oxide scale generation proportion SR 1  was less than 0.5 was evaluated as “A”. Resistance to peeling between the intermediate member  353  and the ground electrode base material  31  is superior in the order of A, B, and C. 
     Results of the evaluation test are as indicated in Tables 1 to 4. In samples 7 and 8 in which the thickness D 2  of the diffusion layer  352  was not within a range of not less than 0.002 mm to not more than 0.065 mm, although the material of the discharge member  351  satisfied the above-described (1) to (3) and the material of the intermediate member  353  satisfied the above-described (4) to (6), wear resistance and peeling resistance  1  were evaluated as “C”. This may be because, as described above, it is not possible to suppress peeling due to thermal stress, or increase in thickness of the diffusion layer  352  due to progressing of diffusion. 
     In samples 1, 2, 4, 5, 9 to 12 in which the discharge member  351  did not satisfy any of the above-described (1) to (3), wear resistance and peeling resistance  1  were evaluated as “C”. Especially, in samples 1, 4, 9, 11, and 12, although the thickness D 2  of the diffusion layer  352  was within a range of not less than 0.002 mm to not more than 0.065 mm and the intermediate member  353  satisfied the above-described (4) to (6), wear resistance and peeling resistance  1  were evaluated as “C”. This may be because, for example, in sample 1 in which the discharge member  351  was pure platinum, it is not possible to suppress the increase of the diffusion layer  352  due to the above-described intercrystalline cracking of platinum. In addition, in samples 2, 4, 5, 9 to 12 in which the total content of platinum, rhodium, and nickel in the discharge member  351  was less than 92 wt. %, it may not be possible to suppress increase in the proportion of an element (e.g., iridium) inferior in oxidation resistance and embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352 . 
     In samples 2, 3, 4, 5, 6, and 13 in which the intermediate member  353  did not satisfy any of the above-described (4) to (6), at least one of wear resistance, peeling resistance  1 , and peeling resistance  2  was evaluated as “C”. For example, in all of samples 2, 5, and 6 in which the intermediate member  353  did not contain nickel, peeling resistance  2  was evaluated as “C”. This may be because, since the intermediate member  353  did not contain nickel, it is not possible to sufficiently decrease thermal stress between the intermediate member  353  and the ground electrode base material  31  containing nickel as a principal component. In addition, in sample 3 in which the intermediate member  353  did not contain platinum, peeling resistance  1  was evaluated as “C”. This may be because, since the intermediate member  353  did not contain platinum, it is not possible to sufficiently decrease thermal stress between the intermediate member  353  and the discharge member  351 . Furthermore, in all of samples 2, 5, and 13 in which the total content of platinum, rhodium, and nickel was less than 85 wt. %, wear resistance and peeling resistance  1  were evaluated as “C”. Especially, in sample 13, although the discharge member  351  satisfied the above-described (1) to (3), the intermediate member  353  satisfied the above-described (4) and (5), and the thickness D 2  of the diffusion layer  352  was within a range of not less than 0.002 mm to not more than 0.065 mm, wear resistance and peeling resistance  1  were evaluated as “C”. This may be because, since the total content of platinum, rhodium, and nickel in the intermediate member  353  was less than 85 wt. %, it is not possible to suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. In addition, in sample 5 in which both the platinum content and the nickel content in the intermediate member  353  were less than 50 wt. %, wear resistance, peeling resistance  1 , and peeling resistance  2  were all evaluated as “C”. This may be because it is not possible to suppress embrittlement of the diffusion layer  352  and decrease in thermal conductivity of the diffusion layer  352  as described above. 
     Meanwhile, in samples 14 to 66 in which the discharge member  351  satisfied the above-described (1) to (3), the intermediate member  353  satisfied the above-described (4) to (6), and the thickness D 2  of the diffusion layer  352  was within a range of not less than 0.002 mm to not more than 0.065 mm, wear resistance, peeling resistance  1 , and peeling resistance  2  were all evaluated as “B” or higher. 
     As is understood from the above description, it has been confirmed that in a case where the discharge member  351  satisfies the above-described (1) to (3), the intermediate member  353  satisfies the above-described (4) to (6), and the thickness D 2  of the diffusion layer  352  is not less than 0.002 mm and not more than 0.065 mm, it is possible to achieve both wear resistance and peeling resistance in the spark plug  100 . 
     In samples, among samples 14 to 66, which further satisfied one or more of the following (7) to (10), it has been found that resistance to peeling between the discharge member  351  and the intermediate member  353  was further improved. 
     (7) The thickness of the diffusion layer  352  is not less than 0.005 mm and not more than 0.065 mm. 
     (8) The distance D 1  between the diffusion layer  352  and the discharge surface  351 B of the discharge member  351 , and the gap length G satisfy D 1 ≥0.1 mm and (D 1 /G)≥0.1. 
     (9) In the discharge member  351 , the total content of platinum, rhodium, and nickel is not less than 96 wt. %. 
     (10) In the intermediate member  353 , the total content of platinum, rhodium, and nickel is not less than 96 wt. %. 
     It is noted that among samples 14 to 66, samples that satisfy the above-described (7) are samples 14, 15, 24 to 26, 28, 30 to 36, 38, 39, 41 to 43, 48, 50, 51, and 55 to 66. Among samples 14 to 66, samples that satisfy the above-described (8) are samples 16 to 43, 47, 51 to 59, and 61 to 66. Among samples 14 to 66, samples that satisfy the above-described (9) are samples 14 to 18, 20 to 48, 52 to 55, and 58 to 66. Among samples 14 to 66, samples that satisfy the above-described (10) are samples 14 to 31, 33 to 43, 46, 47, 52 to 59, and 61 to 66. 
     For example, among samples 14 to 66, peeling resistance  1  of sample 49 that does not satisfy any of the above-described (7) to (10) was evaluated as “B”. Meanwhile, among samples 14 to 66, peeling resistance  1  of each of samples 44, 45, and 50 that satisfies only one of the above-described (7) to (10) was evaluated as “A”. In addition, among samples 14 to 66, peeling resistance  1  of each of samples 19, 46, 48, 51, and 60 that satisfies two of the above-described (7) to (10) was evaluated as “S”. In addition, among samples 14 to 66, peeling resistance  1  of each of samples 14 to 18, 20 to 23, 27, 29, 32, 37, 40, 47, 52 to 54, 56, and 57 that satisfies three of the above-described (7) to (10) was evaluated as “SS”. Among samples 14 to 66, peeling resistance  1  of each of samples 24 to 26, 28, 30, 31, 33 to 36, 38, 39, 41 to 43, 55, 58, 59, and 61 to 66 that satisfies all of the above-described (7) to (10) was evaluated as “SSS”. 
     Thus, it has been confirmed that it is more preferable that at least one of the above-descried (7) to (10) is satisfied. Thus, it is possible to further improve resistance to peeling between the discharge member  351  and the intermediate member  353 . 
     Furthermore, among samples 14 to 66, peeling resistance  2  of each of samples 16 to 20, 44, 47, 58, and 60 in each of which the above-described ΔW(Ni) was less than 2.5 was evaluated as “B”. Peeling resistance  2  of each of samples 14, 15, 21 to 43, 45, 46, 48 to 57, 59, and 61 to 66 in which ΔW(Ni) was not less than 2.5 was evaluated as “A”. 
     Thus, it has been confirmed that it is more preferable that ΔW(Ni) is not less than 2.5, that is, that the nickel content in the intermediate member  353  is higher than the nickel content in the discharge member  351  by not less than 2.5 wt. %. Thus, it is possible to further improve resistance to peeling between the intermediate member  353  and the ground electrode base material  31 . 
     Furthermore, among samples 14 to 66, wear resistance of each of samples 19, 42, 49 to 51, 56, 57, 65, 66 in which, in the discharge member  351 , the total content of platinum and rhodium was less than 88 wt. %, or the total content of platinum, rhodium, and nickel was less than 96 wt. % was evaluated as “B”. Wear resistance of each of samples 14 to 18, 20 to 41, 43 to 48, 52 to 55, and 58 to 64 in which, in the discharge member  351 , the total content of platinum and rhodium was not less than 88 wt. %, and the total content of platinum, rhodium, and nickel was not less than 96 wt. % was evaluated as “A”. 
     Thus, it has been confirmed that it is more preferable that, in the discharge member  351 , the total content of platinum and rhodium is not less than 88 wt. % and the total content of platinum, rhodium, and nickel is not less than 96 wt. %. Thus, by decreasing, in the discharge member, the components other than platinum superior in wear resistance and rhodium which allows suppression of the grain growth of the platinum, it is possible to further improve wear resistance of a spark plug. 
     C. Modification 
     (a) In the above-described embodiment, the ground electrode  30  and the center electrode  20  oppose each other in the axis CL direction of the spark plug  100 , and form a gap (vacant space) to cause spark discharge. Instead thereof, the ground electrode  30  and the center electrode  20  may oppose each other in a direction perpendicular to the axis CL, and form a gap to cause spark discharge. 
     (b) In the above-described embodiment, the clad electrode  35  is used for the ground electrode  30 . However, the clad electrode  35  may be used for the center electrode  20 . That is, the clad electrode  35  may be welded to the front end surface of the leg portion  25  of the center electrode  20  by resistance welding. 
     (c) The materials of the general components of the spark plug  100  of the above-described embodiment, for example, the materials of the metal shell  50 , the center electrode  20 , and the ceramic insulator  10  can be variously changed. In addition, specific dimensions of the metal shell  50 , the center electrode  20 , and the ceramic insulator  10  can be variously changed. For example, the material of the metal shell  50  may be low-carbon steel that is plated with zinc or nickel, and may be unplated low-carbon steel. In addition, the material of the ceramic insulator  10  may be various insulating ceramics other than alumina. 
     Although the present invention has been described above based on the embodiments and the modified embodiments, the above-described embodiments of the invention are intended to facilitate understanding of the present invention, but not as limiting the present invention. The present invention can be changed and modified without departing from the gist thereof and the scope of the claims and equivalents thereof are encompassed in the present invention. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           5 : gasket 
           6 : ring member 
           8 : plate packing 
           9 : talc 
           10 : ceramic insulator 
           12 : through hole 
           13 : leg portion 
           15 : step portion 
           16 : step portion 
           17 : front trunk portion 
           18 : rear trunk portion 
           19 : flange portion 
           20 : center electrode 
           21 : center electrode body 
           21 A: electrode base material 
           21 B: core portion 
           23 : head portion 
           24 : flange portion 
           25 : leg portion 
           27 : melt portion 
           29 : center electrode tip 
           29 A: discharge surface 
           30 : ground electrode 
           31 : ground electrode base material 
           35 : clad electrode 
           40 : metal terminal 
           41 : cap mounting portion 
           42 : flange portion 
           43 : leg portion 
           50 : metal shell 
           51 : tool engagement portion 
           52 : mounting screw portion 
           53 : crimp portion 
           54 : seat portion 
           56 : step portion 
           58 : compressive deformation portion 
           59 : insertion hole 
           60 : conductive seal 
           70 : resistor 
           80 : conductive seal 
           100 : spark plug 
           351 : discharge member 
           352 : diffusion layer 
           353 : intermediate member