JP2012114055A - Spark plug and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the progress of oxidized scale at a joint boundary and to improve the peeling resistance of a noble metal chip.SOLUTION: A spark plug 1 includes a center electrode 5, a ground electrode 27, and a noble metal chip 32 connected to the ground electrode 27. The noble metal chip 32 includes a main body 32A and a covering section 32B. A gap 33 is formed between the main body 32A and the center electrode 5. The covering section 32B extends from the main body 32A to a back surface side 27B of the ground electrode 27 and covers at least a part of a joint boundary BO between the ground electrode 27 in a tip end surface 27F and the noble metal chip 32. When the length of the joint boundary BO in the tip end surface 27F is D(mm) and the length of a portion covered by the covering section 32B in the joint boundary BO is C(mm), C/D≥0.5 is satisfied, and the minimum value LAof the length of the covering section 32B from the joint boundary BO toward the back surface side 27B is equal to or larger than 0.05 mm.

Description

  The present invention relates to a spark plug used for an internal combustion engine or the like, and a manufacturing method thereof.

  A spark plug used in a combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in the axial direction, an insulator provided on the outer periphery of the center electrode, a cylindrical metal shell provided outside the insulator, and a base And a ground electrode having an end joined to a tip of the metal shell. The ground electrode is arranged with its middle part bent back so that the tip of the ground electrode faces the tip of the center electrode, thereby forming a gap between the tip of the center electrode and the tip of the ground electrode. Is done.

  In addition, a technique is known in which a noble metal tip is provided at a portion where the gap is formed in the tip portion of the ground electrode to improve wear resistance and ignition performance. In general, a noble metal tip is provided by joining its end face to a ground electrode by, for example, resistance welding (see, for example, Patent Document 1).

JP 2006-269436 A

  By the way, since the ground electrode is disposed so as to protrude toward the center of the combustion chamber, the temperature tends to be high, and the noble metal tip joined to the ground electrode is likely to become even higher. Therefore, the oxide scale rapidly develops at the junction boundary between the ground electrode and the noble metal tip, and a thermal stress difference generated between the ground electrode and the noble metal tip is added to the junction boundary, so that the noble metal tip is removed from the ground electrode. There is a risk of peeling (dropping). In particular, in high-power, high-compression engines in recent years, the combustion chamber becomes even hotter, so the oxide scale tends to develop more rapidly, and the thermal stress difference that occurs between the ground electrode and the noble metal tip Becomes larger. For this reason, there is a greater concern about peeling of the noble metal tip.

  The present invention has been made in view of the above circumstances, and its purpose is to effectively suppress the development of oxide scale at the joining boundary, and to improve the peel resistance of the noble metal tip, and It is in providing the manufacturing method.

  Hereinafter, each configuration suitable for solving the above-described object will be described in terms of items. In addition, the effect specific to the corresponding structure is added as needed.

Configuration 1. The spark plug of this configuration includes a center electrode extending in the axial direction,
A cylindrical insulator provided on the outer periphery of the center electrode;
A cylindrical metal shell provided on the outer periphery of the insulator;
A rod-shaped ground electrode fixed to the tip of the metal shell;
A noble metal tip which is bonded to the surface of the ground electrode on the side of the center electrode and forms a gap with the tip of the center electrode; and
A spark plug in which a longitudinal tip surface of the ground electrode and a side surface of the noble metal tip are provided on substantially the same plane,
The noble metal tip is disposed on a surface of the ground electrode on the side of the center electrode, and a main body portion that forms the gap with the center electrode;
It is formed integrally with the main body, and extends from the main body toward the back side of the ground electrode, and at least a part of the bonding boundary between the ground electrode and the noble metal tip on the front end surface of the ground electrode. A covering portion covering,
When the length of the joint boundary on the tip surface of the ground electrode is D (mm) and the length of the part of the joint boundary covered with the covering portion is C (mm), C / D ≧ 0.5
The minimum value LA _Min of the length of the covering portion from the joint boundary toward the back side of the ground electrode is 0.05 mm or more.

  Note that “substantially on the same plane” means not only when the front end surface of the ground electrode and the side surface of the noble metal tip are arranged exactly on the same plane, but also along the longitudinal direction of the ground electrode. This includes the case where there is a slight deviation (for example, less than 0.3 mm) between the front end surface of the metal and the side surface of the noble metal tip. Further, “the back surface of the ground electrode” refers to a surface on the back of the surface of the ground electrode located on the center electrode side. Furthermore, in the case where a melted portion is formed between the ground electrode and the noble metal tip by laser welding or the like, the "bonding boundary between the ground electrode and the noble metal tip" is the gap between the melted portion and the ground electrode. The boundary located on the back side of the ground electrode among the boundaries.

According to the above-described configuration 1, the covering portion that covers at least a part of the bonding boundary between the ground electrode and the noble metal tip (main body portion) is provided on the tip surface of the ground electrode. While satisfying ≧ 0.5, the minimum value LA _{Min of the} length is 0.05 mm or more. That is, more than half of the bonding boundary is covered with the covering portion on the front end surface of the ground electrode, and the portion between the end edge of the covering portion on the back side of the ground electrode and the front end surface of the ground electrode (that is, the bonding portion) A sufficiently large distance of 0.05 mm or more is ensured from the junction) to the junction boundary). Therefore, the invasion of oxygen to the junction boundary can be effectively suppressed, and as a result, the progress of oxide scale at the junction boundary can be more reliably prevented. As a result, the peel resistance of the noble metal tip can be improved.

Configuration 2. In the spark plug of this configuration, in the configuration 1, the maximum value of the length of the covering portion from the main body portion toward the back side of the ground electrode is LB _Max (mm), and the length of the gap is G ( mm), and when the thickness of the main body part from the connecting part of the covering part and the main body part to the surface of the noble metal tip forming the gap is E (mm),
LB _Max <G + E
It is characterized by satisfying.

  Note that the connection portion means a boundary portion between the main body portion and the covering portion.

  According to the above configuration 2, even if the covering portion is peeled off from the ground electrode and falls to the gap side with the connecting portion as a base point, the covering portion does not contact the center electrode. That is, according to the configuration 2, even when the covering portion is peeled off, the center electrode and the ground electrode are electrically connected (so-called bridge is generated) by the peeled covering portion. Can be prevented from occurring.

  Configuration 3. The spark plug of this configuration is characterized in that, in the above configuration 1 or 2, C / D ≧ 0.8 is satisfied.

  According to the configuration 3, since 80% or more of the bonding boundary is covered with the covering portion on the tip surface of the ground electrode, it is possible to further suppress the intrusion of oxygen into the bonding boundary. As a result, the peel resistance can be further improved.

  Configuration 4. The spark plug of this configuration satisfies C / D = 1.0 in any one of the above configurations 1 to 3.

  According to the above-described configuration 4, the entire bonding boundary is covered with the covering portion on the tip surface of the ground electrode. Therefore, the invasion of oxygen to the bonding boundary can be extremely effectively suppressed, and the peel resistance can be greatly improved.

  Configuration 5. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 4, when the maximum thickness of the covering portion is T (mm), 0 <T ≦ 0.15 is satisfied.

  According to the configuration 5, since the maximum thickness T of the covering portion is as small as 0.15 mm, peeling of the covering portion can be more reliably prevented. As a result, it is possible to maintain the peeling resistance improving effect exhibited by the above-described configurations over a long period of time.

Configuration 6. The spark plug manufacturing method of this configuration is the spark plug manufacturing method according to any one of the above configurations 1 to 5,
A joining step of joining a tip member formed of the same material as the noble metal tip to a side surface on one end side of a rod-like member formed of the same material as the ground electrode;
After the joining step, one end portion of the rod-shaped member and the tip member are cut by a predetermined cutting blade, and the noble metal tip provided with the main body portion and the covering portion, and the cutting step for obtaining the ground electrode. Including
In the cutting step, the rod-shaped member and the tip member are cut by the cutting blade from the surface of the rod-shaped member to which the tip member is joined toward the back side of the surface. To do.

  According to the configuration 6, the bar-shaped member and the chip member are cut by the cutting blade from the surface of the bar-shaped member to which the chip member is joined toward the back side of the surface. Therefore, a part of the tip member extends along the one end surface of the rod-shaped member after cutting (that is, the tip surface of the ground electrode) in a form that is dragged by the movement of the cutting blade. A covering portion that covers the boundary between the noble metal tip and the noble metal tip can be formed. That is, the spark plug having the above-described configuration 1 can be easily obtained only through a relatively simple process of cutting the chip member and the like.

Configuration 7. The spark plug manufacturing method of this configuration is the above configuration 6, wherein, in the cutting step, the rod-shaped member is placed on the pedestal in a state where one end of the rod-shaped member protrudes from one end surface of a predetermined pedestal. And
The rod-shaped member and the tip member are cut by the cutting blade in a state where the distance between the cutting blade along the longitudinal direction of the rod-shaped member and one end surface of the pedestal is 1.5 mm or less. It is characterized by.

The size such as the length C (mm) and the minimum value LA _Min in the covering portion can be changed by adjusting the clearance between the base on which the rod-shaped member is placed and the cutting blade. However, if the clearance between the pedestal and the cutting blade is excessively widened, the rod-shaped member is bent, and the load applied to the cutting blade during cutting increases. As a result, the cutting blade may be damaged, leading to a decrease in productivity.

  In this regard, according to the configuration 7, the distance (clearance) between the cutting blade and the one end surface of the pedestal is 1.5 mm or less at the time of cutting the chip member or the like. It can be made sufficiently small. As a result, damage to the cutting blade can be prevented more reliably, and productivity can be improved.

  Note that the thickness of the covering portion increases as the clearance increases, and decreases as the clearance decreases. Therefore, setting the thickness of the covering portion to be small leads to a decrease in clearance, and thus contributes to prevention of damage to the cutting blade. That is, setting the maximum thickness T of the covering portion to be 0.15 mm or less as in the above configuration 5 is not only in terms of suppressing peeling of the covering portion, but also preventing damage to the cutting blade during manufacturing. This is also effective.

It is a partially broken front view which shows the structure of a spark plug. It is a partially broken expanded front view which shows the structure of the front-end | tip part of a spark plug. (A) is a partial expanded sectional view which shows structures, such as a noble metal chip | tip, (b) is a partial expanded side view which shows structures, such as a noble metal chip | tip. It is an expanded sectional schematic diagram which shows typically the case where a coating | coated part peels. It is a schematic diagram which shows the process in which a chip | tip member is joined to a rod-shaped member by resistance welding. It is a schematic diagram which shows a rod-shaped member and a chip member after a joining process. It is a schematic diagram which shows the relative positional relationship etc. of a cutting blade and a base in a cutting process. It is a schematic diagram which shows the ground electrode and noble metal chip | tip obtained through the cutting process. It is a cross-sectional schematic diagram for demonstrating the measurement position of the length of a joining boundary and the length of an oxide scale in a desktop burner test. It is a graph which shows the result of the desktop burner test of the sample which made C / D 0.3 or 0.5, and changed length LA _Min variously. It is a graph which shows the result of the desktop burner test which made the heating temperature 1000 _degreeC in the sample which changed C / D and length LA _Min variously. It is a graph which shows the result of the desktop burner test which made the heating temperature 1000 degreeC in the sample which changed C / D variously after setting LA _Min to 0.05 mm or 0.6 mm. It is a partial expanded sectional view which shows the joining aspect of the noble metal tip with respect to the ground electrode in another embodiment. It is a partial enlarged plan view which shows the structure of the ground electrode in another embodiment. (A), (b) is a partial expanded side view which shows the structure of the coating | coated part in another embodiment.

  Hereinafter, an embodiment will be described with reference to the drawings. FIG. 1 is a partially cutaway front view showing a spark plug 1. In FIG. 1, the direction of the axis CL <b> 1 of the spark plug 1 is the vertical direction in the drawing, the lower side is the front end side of the spark plug 1, and the upper side is the rear end side.

  The spark plug 1 includes an insulator 2 as a cylindrical insulator, a cylindrical metal shell 3 that holds the insulator 2, and the like.

  As is well known, the insulator 2 is formed by firing alumina or the like, and in its outer portion, a rear end side body portion 10 formed on the rear end side, and a front end than the rear end side body portion 10. A large-diameter portion 11 that protrudes radially outward on the side, a middle body portion 12 that is smaller in diameter than the large-diameter portion 11, and a tip portion that is more distal than the middle body portion 12. On the side, a leg length part 13 formed with a smaller diameter than this is provided. In addition, of the insulator 2, the large diameter portion 11, the middle trunk portion 12, and most of the leg long portions 13 are accommodated inside the metal shell 3. A tapered step portion 14 is formed at the connecting portion between the middle body portion 12 and the long leg portion 13, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  Further, the insulator 2 is formed with a shaft hole 4 penetrating along the axis CL1, and a center electrode 5 is inserted and fixed to the tip end side of the shaft hole 4. The center electrode 5 includes an inner layer 5A made of copper or a copper alloy having excellent thermal conductivity, and an outer layer 5B made of a Ni alloy containing nickel (Ni) as a main component. Furthermore, the center electrode 5 has a rod shape (cylindrical shape) as a whole, and its tip end surface is formed flat and protrudes from the tip of the insulator 2. Further, a noble metal portion 31 made of a predetermined noble metal alloy (for example, a platinum alloy or an iridium alloy) is provided at the tip of the center electrode 5.

  A terminal electrode 6 is inserted and fixed on the rear end side of the shaft hole 4 in a state of protruding from the rear end of the insulator 2.

  Further, a cylindrical resistor 7 is disposed between the center electrode 5 and the terminal electrode 6 of the shaft hole 4. Both ends of the resistor 7 are electrically connected to the center electrode 5 and the terminal electrode 6 through conductive glass seal layers 8 and 9, respectively.

  In addition, the metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a spark plug 1 is attached to the outer peripheral surface of the metal shell 3 such as an internal combustion engine or a fuel cell reformer. A threaded portion (male threaded portion) 15 for attachment to the hole is formed. In addition, a seat portion 16 is formed on the outer peripheral surface on the rear end side of the screw portion 15, and a ring-shaped gasket 18 is fitted on the screw neck 17 on the rear end of the screw portion 15. Further, on the rear end side of the metal shell 3, a tool engaging portion 19 having a hexagonal cross section for engaging a tool such as a wrench when the metal shell 3 is attached to the combustion device is provided. 1 is provided with a caulking portion 20 for holding the insulator 2.

  A tapered step portion 21 for locking the insulator 2 is provided on the inner peripheral surface of the metal shell 3. The insulator 2 is inserted from the rear end side to the front end side of the metal shell 3, and the rear end of the metal shell 3 is engaged with the step portion 14 of the metal shell 3. It is fixed to the metal shell 3 by caulking the opening on the side inward in the radial direction, that is, by forming the caulking portion 20. An annular plate packing 22 is interposed between the step portions 14 and 21 of both the insulator 2 and the metal shell 3. Thereby, the airtightness in the combustion chamber is maintained, and the fuel gas entering the gap between the leg long portion 13 of the insulator 2 exposed to the combustion chamber and the inner peripheral surface of the metal shell 3 is prevented from leaking outside.

  Further, in order to make the sealing by caulking more complete, annular ring members 23 and 24 are interposed between the metal shell 3 and the insulator 2 on the rear end side of the metal shell 3, and the ring member 23 , 24 is filled with powder of talc (talc) 25. That is, the metal shell 3 holds the insulator 2 via the plate packing 22, the ring members 23 and 24, and the talc 25.

  Further, as shown in FIG. 2, the front end portion 26 of the metal shell 3 is bent back at a substantially middle portion, and the front end side surface 27 </ b> S faces the front end portion (the noble metal portion 31) of the center electrode 5. The electrode 27 is joined. The ground electrode 27 is made of an alloy containing Ni as a main component, and has substantially the same thickness along its longitudinal direction. Further, a noble metal tip 32 made of a noble metal alloy (for example, platinum alloy such as Pt-10Ni, Pt-20Ni, etc.) is formed on the surface facing the noble metal portion 31 of the tip side surface 27S of the ground electrode 27 by resistance welding. It is joined. A gap 33 is formed between the noble metal portion 31 and the noble metal tip 32, and spark discharge is performed in the gap 33 in a direction substantially along the axis CL1.

  In addition, in the present embodiment, as shown in FIGS. 3A and 3B, the front end surface 27 </ b> F along the longitudinal direction of the ground electrode 27 and the front end of the ground electrode 27 among the side surfaces of the noble metal tip 32. The side surface 32S located on the side is provided on substantially the same plane.

  Furthermore, the noble metal tip 32 includes a main body portion 32A and a covering portion 32B.

  The main body portion 32A has a rectangular parallelepiped shape, and is joined to the front end side surface 27S of the ground electrode 27 in a state where one end portion of the main body portion 32A is embedded. Further, the other end surface of the main body portion 32A has a flat shape, and the gap 33 is formed between the other end surface and the center electrode 5 (the noble metal portion 31).

  The covering portion 32B is formed integrally with the main body portion 32A, and has a thin plate shape that extends from one end portion of the main body portion 32A toward the back surface 27B of the ground electrode 27 (the surface on the back side of the tip side surface 27S). Yes. At the front end surface 27F of the ground electrode 27, at least a part of the joining boundary BO between the ground electrode 27 and the noble metal tip 32 (main body portion 32A) is covered with the covering portion 32B.

  Further, in the present embodiment, the length of the junction boundary BO on the tip surface 27F of the ground electrode 27 is D (mm), and the length of the portion of the junction boundary BO covered with the covering portion 32B is C (mm). In this case, C / D ≧ 0.5 (more preferably, C / D ≧ 0.8, and even more preferably, C / D = 1.0) is satisfied. In other words, the wide area of the bonding boundary BO is covered with the covering portion 32B on the front end surface 27F of the ground electrode 27.

Further, the minimum value LA _Min of the length of the covering portion 32B from the bonding boundary BO toward the back surface 27B side of the ground electrode 27 is set to 0.05 mm or more. In other words, a sufficiently large distance from the edge on the back surface 27B side of the covering portion 32B and the front end surface 27F of the ground electrode 27 (a portion serving as an oxygen entry port) to the bonding boundary BO is secured.

  Furthermore, when the maximum thickness of the covering portion 32B along the longitudinal direction of the ground electrode 27 is T (mm), 0 <T ≦ 0.15 is satisfied.

In addition, the maximum length of the covering portion 32B from the main body portion 32A toward the back surface 27B side of the ground electrode 27 is LB _Max (mm), and the length of the gap 33 is G (mm) (see FIG. 2). When the thickness of the main body portion 32A along the central axis CL2 of the noble metal tip 32 from the connecting portion 32X of the main body portion 32A and the covering portion 32B to the surface of the noble metal tip 32 forming the gap 33 is defined as E (mm). , LB _Max <G + E. That is, as shown in FIG. 4, even if the covering portion 32B is peeled off from the distal end surface 27F of the ground electrode 27 and falls to the gap 33 side from the connecting portion 32X, the end portion of the covering portion 32B remains at the center electrode 5 ( It is configured not to contact the noble metal part 31).

  Next, the manufacturing method of the spark plug 1 comprised as mentioned above is demonstrated.

  First, the metal shell 3 is processed in advance. That is, a through-hole is formed by subjecting a cylindrical metal material (for example, an iron-based material such as S17C or S25C or a stainless steel material) to a cold forging process, and a rough shape is manufactured. Thereafter, the outer shape is adjusted by cutting to obtain a metal shell intermediate.

  Subsequently, a straight bar-shaped member made of a Ni alloy (which becomes the ground electrode 27) is resistance-welded to the front end surface of the metal shell intermediate. When the welding is performed, so-called “sag” is generated. After the “sag” is removed, the threaded portion 15 is formed by rolling at a predetermined portion of the metal shell intermediate body. Thereby, the metal shell 3 to which the ground electrode 27 is welded is obtained. It should be noted that the length of the rod-shaped member at this time is slightly longer than the length of the ground electrode 27.

  The metal shell 3 to which the ground electrode 27 is welded is galvanized or nickel plated. In order to improve the corrosion resistance, the surface may be further subjected to chromate treatment.

  On the other hand, the insulator 2 is formed separately from the metal shell 3. For example, a raw material powder containing alumina as a main component and containing a binder or the like is used to prepare a green granulated material for molding, and a rubber molded product is used to obtain a cylindrical molded body. Then, the insulator 2 is obtained by subjecting the obtained molded body to grinding and shaping the outer shape, followed by firing.

  Separately from the metal shell 3 and the insulator 2, the center electrode 5 is manufactured. That is, the center electrode 5 is produced by forging a Ni alloy in which a copper alloy or the like for improving heat dissipation is arranged at the center. Next, a noble metal portion 31 made of a noble metal alloy is joined to the tip portion of the center electrode 5 by laser welding or the like.

  Then, the insulator 2 and the center electrode 5, the resistor 7, and the terminal electrode 6 obtained as described above are sealed and fixed by the glass seal layers 8 and 9. The glass seal layers 8 and 9 are generally prepared by mixing borosilicate glass and metal powder, and the prepared material is injected into the shaft hole 4 of the insulator 2 with the resistor 7 interposed therebetween. Then, the terminal electrode 6 is pressed from behind, and then baked in a baking furnace. At this time, the glaze layer may be fired simultaneously on the surface of the rear end body portion 10 of the insulator 2 or the glaze layer may be formed in advance.

  Thereafter, the insulator 2 provided with the center electrode 5 and the terminal electrode 6 and the metal shell 3 provided with the ground electrode 27 are fixed. More specifically, after the insulator 2 is inserted into the metal shell 3, the opening on the rear end side of the metal shell 3 formed relatively thin is caulked radially inward, that is, the caulking portion 20 is By forming, the insulator 2 and the metal shell 3 are fixed.

  Next, in the joining step, a rectangular parallelepiped tip member (becomes the noble metal tip 32) made of a predetermined noble metal alloy is joined to the tip side of the rod-like member. That is, as shown in FIG. 5, after the tip member CC is placed on one end side of the surface corresponding to the tip side surface 27S of the ground electrode 27 in the rod-shaped member SC, the predetermined welding electrode rod WS is attached to the tip member. Contact CC. Then, the welding electrode rod WS is moved to the rod-shaped member SC side, and the tip member CC is pressed by the welding electrode rod WS with a predetermined pressure, while the welding electrode rod WS is moved from the welding electrode rod WS to the tip member CC side with a predetermined current value. Energize. Thereby, as shown in FIG. 6, the chip member CC is joined to the rod-shaped member SC in a state where a part thereof is buried in the rod-shaped member SC.

Next, in the cutting step, as shown in FIG. 7, one end of the rod-shaped member SC and a part of the chip member CC are cut using a cutting blade CB made of predetermined hard steel. The cutting process will be described in detail. First, the rod-shaped member SC is placed on a pedestal PE formed of a predetermined metal material. At this time, one end of the rod-shaped member SC is projected from the one end surface of the pedestal PE, and the distance L between the cutting blade CB and the one end surface of the pedestal PE along the longitudinal direction of the rod-shaped member SC is 1. .5 mm or less. After the rod-shaped member SC or the like is arranged at a predetermined position, cut along the direction orthogonal to the longitudinal direction of the rod-shaped member SC from the surface of the rod-shaped member SC where the chip member CC is joined toward the back side of the surface. The blade CB is moved to cut one end of the rod-shaped member SC and a part of the chip member CC. As a result, a part of the tip member CC extends along the cut surface of the rod-shaped member CC (that is, the front end surface 27F of the ground electrode 27) while being dragged to the side surface of the cutting blade CB. As a result, as shown in FIG. As described above, the noble metal tip 32 including the main body portion 32A and the covering portion 32B that covers the bonding boundary BO is formed. It should be noted that the length C of the covering portion 32B covering the joint boundary BO, the length of the covering portion 32A (the minimum value LA _Min and the maximum value LB _Max ), and the maximum thickness T (mm) of the covering portion 32B. Can be changed by adjusting the distance L between the cutting blade CB and the base PE.

  After the cutting step, the above-described spark plug 1 is obtained by bending the ground electrode 27 toward the center electrode 5 and adjusting the size of the spark discharge gap 33 between the noble metal portion 31 and the noble metal tip 32.

  As described above in detail, according to the present embodiment, at the front end surface 27F of the ground electrode 27, more than half of the joint boundary BO is covered with the covering portion 32B, and the back surface 27B side of the ground electrode 27 in the covering portion 32B. The distance from the portion located between the end edge of the ground electrode 27 and the front end surface 27F of the ground electrode 27 (the portion serving as an oxygen intrusion port for the bonding boundary BO) to the bonding boundary BO is sufficiently large as 0.05 mm or more. ing. Therefore, it is possible to effectively suppress the intrusion of oxygen into the junction boundary BO, and thus more reliably prevent the progress of oxide scale at the junction boundary BO. As a result, the peel resistance of the noble metal tip 32 can be improved.

  Furthermore, since the maximum thickness T of the covering portion 32B is very small as 0.15 mm, peeling of the covering portion 32B can be more reliably prevented. As a result, the effect of improving peel resistance can be maintained over a long period of time.

Moreover, since it is configured to satisfy LB _Max <G + E, even if the covering portion 32B is peeled off, the peeled covering portion 32B causes the center electrode 5 (the noble metal portion 31) and the ground electrode 27 to be separated. The situation of being electrically connected is prevented from occurring.

  In addition, in this embodiment, when obtaining the noble metal tip 32 having the above-described configuration, a relatively simple method of cutting the tip member CC or the like is used. Therefore, the above-described noble metal tip 32 and, by extension, the spark plug 1 can be easily manufactured.

  Further, when cutting the tip member CC, the distance L between the cutting blade CB and the base PE is set to 1.5 mm or less, so that the load applied to the cutting blade CB at the time of cutting can be sufficiently reduced. . As a result, damage to the cutting blade CB can be more reliably prevented, and productivity can be improved.

Next, in order to confirm the operational effects achieved by the above embodiment, the length C of the portion of the joint boundary covered by the covering portion with respect to the length D (mm) of the joint boundary on the tip surface of the ground electrode. Samples of spark plugs in which the ratio (C / D) of (mm) was set to 0.3 or 0.5 and the minimum length LA _Min of the covering portion was changed in various _ways were prepared. A desktop burner test was performed. The outline of the desktop burner test is as follows. That is, the sample was subjected to 1000 cycles, with one cycle consisting of heating with a burner for 2 minutes and then gradually cooling for 1 minute so that the temperature of the noble metal tip was 900 ° C. Then, after the end of 1000 cycles, as shown in FIG. 9, the sample is observed in a cross section including the central axis of the ground electrode, and the junction boundary BO with respect to the length X of the junction boundary BO between the noble metal tip and the ground electrode is observed. The ratio of the length Y (Y / X; hereinafter referred to as “oxidized scale ratio”) of the oxide scale OS (the part indicated by the bold line in FIG. 9) was measured. FIG. 10 shows the test results of the test. In FIG. 10, the test results of the sample with C / D of 0.3 are indicated by circles, and the test results of the sample with C / D of 0.5 are indicated by squares. In each sample, the noble metal tip was formed of a platinum alloy, and the maximum thickness T of the covering portion was set to 0.1 mm. In addition, the minimum value LA _Min was changed by cutting the coated portion.

As shown in FIG. 10, the sample with C / D of 0.3 or length LA _Min of less than 0.05 mm has a relatively large oxide scale ratio, which makes the noble metal tip peel resistant. It became clear that it was inferior. This is because most of the bonding boundary is exposed to the outside on the front end surface of the ground electrode, or the portion between the edge located on the back side of the ground electrode and the front end surface of the ground electrode ( This is thought to be because the oxygen entry point becomes close to the bonding boundary, and oxygen easily enters the bonding boundary.

On the other hand, a sample with a sufficiently large length LA _Min of 0.05 mm or more while C / D is set to 0.5 (that is, half of the joining boundary is covered with the covering portion on the tip surface of the ground electrode) It was found that almost no oxide scale was formed and had excellent peeling resistance.

Next, with respect to a spark plug in which C / D is set to 0.5, 0.8, or 1.0 and the length LA _Min is variously changed, the heating temperature of the noble metal tip is set to 1000 ° C. (that is, the bonding boundary) The above-mentioned desk-top burner test was conducted as a condition in which oxygen is very easy to enter. FIG. 11 shows the test results of the test. In FIG. 11, the test result of the sample with C / D of 0.5 is indicated by a circle, the test result of a sample with C / D of 0.8 is indicated by a triangle, and C / D is 1. The test result of the sample set to 0 is shown by a square.

In addition, the length LA _Min was set to 0.05 mm or 0.6 mm, and the length D (mm) was kept constant, and the length C (mm) was changed to variously change the C / D. Samples of spark plugs were prepared, and the above-described desktop burner test was performed on each sample with a precious metal tip heating temperature of 1000 ° C. FIG. 12 shows the test results of the test. In FIG. 12, the test results of the sample with LA _Min of 0.05 mm are indicated by circles, and the test results of the sample with LA _Min of 0.6 mm are indicated by squares. In both tests, as in the above test, in each sample, the noble metal tip was formed of a platinum alloy and the thickness T was set to 0.1 mm.

As shown in FIG. 11 and FIG. 12, the sample in which LA _Min is 0.05 mm or more and C / D is 0.8 or more is in spite of the condition that oxygen easily enters the bonding boundary. The oxide scale ratio became 20% or less, and it became clear that very excellent peel resistance could be realized. In particular, a sample in which LA _Min is 0.05 mm or more and C / D is 1.0 has an oxide scale ratio of 10% or less, and it has been confirmed that the peel resistance can be remarkably improved.

From the above test results, in order to improve the peel resistance of the noble metal tip, a covering portion covering at least a part of the junction boundary between the ground electrode and the noble metal tip is provided on the tip surface of the ground electrode, and the length D of the junction boundary is provided. (Mm) and the length C (mm) of the portion of the joint boundary covered by the covering portion that satisfies C / D ≧ 0.5 and is covered from the joint boundary toward the back side of the ground electrode It can be said that the minimum value LA _Min of the part length is preferably 0.05 mm or more.

  Further, from the viewpoint of further improving the peel resistance, it is more preferable to configure so as to satisfy C / D ≧ 0.8, and it is even more preferable to configure so as to satisfy C / D = 1.0. It can be said that it is preferable.

  In addition, it is not limited to the description content of the said embodiment, For example, you may implement as follows. Of course, other application examples and modification examples not illustrated below are also possible.

  (A) In the above-described embodiment, the noble metal tip 32 is joined to the ground electrode 27 by resistance welding. For example, the noble metal tip 32 is formed of an iridium alloy or the like, and the noble metal tip 32 is somewhat joined by resistance welding. If difficult, the noble metal tip may be joined to the ground electrode by laser welding or electron beam welding. In this case, as shown in FIG. 13, a molten portion 45 is formed by melting the metal constituting the ground electrode 37 and the metal constituting the noble metal tip 42 by irradiation with a laser beam or the like. The “bonding boundary between the ground electrode and the noble metal tip” means the bonding boundary BO2 between the ground electrode 37 and the melting portion 45 [that is, the “bonding boundary” refers to the noble metal in the ground electrode. A point where the tip is joined directly or indirectly by a melted part or the like (a part that is particularly important when the noble metal tip is firmly joined to the ground electrode) and a noble metal tip or a melted part is pointed out].

  (B) In the above embodiment, the ground electrode 27 is configured to have substantially the same thickness along its longitudinal direction. However, as shown in FIG. The tip of the ground electrode 47 may be tapered by making the surface tapered. In this case, the flame kernel growth inhibition by the ground electrode 47 is suppressed, and the ignitability can be improved. On the other hand, since the cross-sectional area of the ground electrode 47 is reduced and the noble metal tip 52 is likely to have a higher temperature, it causes rapid progress of oxide scale and an increase in the thermal stress difference between the ground electrode 47 and the noble metal tip 52. However, it may be difficult to ensure sufficient peel resistance. However, by employing the present invention, sufficient peel resistance can be maintained. That is, according to the present invention, it is possible to effectively eliminate the demerit (decrease in peel resistance) caused by the tip of the ground electrode 47 having a tapered shape, while the advantage (improvement of ignitability) by using the tapered shape. Can be fully exhibited.

(C) The shape of the covering portion when viewed from the front end side of the ground electrode 27 is not particularly limited. For example, when viewed from the front end side of the ground electrode 27 as shown in FIG. It is good also as comprising so that the coating | coated part 62B may make a rectangular shape. Also, as shown in FIG. 15B, the minimum length LA _Min is configured to be less than 0.05 mm at a portion of the covering portion 72B located on the end side in the width direction of the ground electrode 27. It is good to do. That is, on the tip surface 27F of the ground electrode 27, the covering portion 72B having the minimum length LA _Min of 0.05 mm or more may be formed over half or more of the bonding boundary BO (therefore, accordingly). For example, on the front end surface of the ground electrode, the entire joining boundary may be covered with the covering portion, while the minimum value LA _{Min of the} length may be less than 0.05 mm in a portion of the covering portion).

  (D) Although the center electrode 5 includes the noble metal portion 31 in the above embodiment, the noble metal portion 31 may not be provided.

  (E) In the above embodiment, the ground electrode 27 is made of a single Ni alloy, but the inner layer formed of a metal having excellent thermal conductivity (for example, copper, copper alloy, or pure Ni) It is good also as a multilayer structure which consists of an outer layer which covers an inner layer.

  (F) In the above embodiment, the case where the ground electrode 27 is joined to the distal end portion 26 of the metal shell 3 is embodied. However, a part of the metal shell (or the metal tip that is pre-welded to the metal shell) The present invention can also be applied to the case where the ground electrode is formed so as to cut out a part of (see Japanese Patent Laid-Open No. 2006-236906, etc.).

  (G) In the above embodiment, the tool engagement portion 19 has a hexagonal cross section, but the shape of the tool engagement portion 19 is not limited to such a shape. For example, it may be a Bi-HEX (deformed 12-angle) shape [ISO 22777: 2005 (E)].

1 ... Spark plug 2 ... Insulator (insulator)
3 ... metal shell 5 ... center electrode 27 ... ground electrode 27B ... back surface (of ground electrode) 27F ... tip surface (of ground electrode) 32 ... precious metal tip 32A ... main body portion 32B ... covering portion 32S ... side surface (of precious metal tip) 32X ... Connection part 33 ... Gap BO ... Junction boundary CL1 ... Axis

Claims (7)

A central electrode extending in the axial direction;
A cylindrical insulator provided on the outer periphery of the center electrode;
A cylindrical metal shell provided on the outer periphery of the insulator;
A rod-shaped ground electrode fixed to the tip of the metal shell;
A noble metal tip which is bonded to the surface of the ground electrode on the side of the center electrode and forms a gap with the tip of the center electrode; and
A spark plug in which a longitudinal tip surface of the ground electrode and a side surface of the noble metal tip are provided on substantially the same plane,
The noble metal tip is disposed on a surface of the ground electrode on the side of the center electrode, and a main body portion that forms the gap with the center electrode;
It is formed integrally with the main body, and extends from the main body toward the back side of the ground electrode, and at least a part of the bonding boundary between the ground electrode and the noble metal tip on the front end surface of the ground electrode. A covering portion covering,
When the length of the joint boundary on the tip surface of the ground electrode is D (mm) and the length of the part of the joint boundary covered with the covering portion is C (mm), C / D ≧ 0.5
The spark plug according to claim 1, wherein a minimum value LA _Min of the length of the covering portion from the joining boundary toward the back side of the ground electrode is 0.05 mm or more. The maximum value of the length of the covering part from the main body part toward the back side of the ground electrode is LB _Max (mm), the length of the gap is G (mm), and the covering part and the main body part When the thickness of the main body from the connecting portion to the surface of the noble metal tip forming the gap is E (mm),
LB _Max <G + E
The spark plug according to claim 1, wherein:   The spark plug according to claim 1, wherein C / D ≧ 0.8 is satisfied.   The spark plug according to claim 1, wherein C / D = 1.0 is satisfied.   5. The spark plug according to claim 1, wherein 0 <T ≦ 0.15 is satisfied, where T (mm) is the maximum thickness of the covering portion. A spark plug manufacturing method according to any one of claims 1 to 5,
A joining step of joining a tip member formed of the same material as the noble metal tip to a side surface on one end side of a rod-like member formed of the same material as the ground electrode;
After the joining step, one end portion of the rod-shaped member and the tip member are cut by a predetermined cutting blade, and the noble metal tip provided with the main body portion and the covering portion, and the cutting step for obtaining the ground electrode. Including
In the cutting step, the rod-shaped member and the tip member are cut by the cutting blade from the surface of the rod-shaped member to which the tip member is joined toward the back side of the surface. Manufacturing method of spark plug. In the cutting step, the rod-shaped member is placed on the pedestal in a state where one end of the rod-shaped member protrudes from one end surface of a predetermined pedestal,
The rod-shaped member and the tip member are cut by the cutting blade in a state where the distance between the cutting blade along the longitudinal direction of the rod-shaped member and one end surface of the pedestal is 1.5 mm or less. The method for manufacturing a spark plug according to claim 6.

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