Abstract:
A spark plug comprises a shell having a substantially cylindrical threaded portion for threadable engagement in a cylinder head of an internal combustion engine, an insulator disposed coaxially in the shell, a center electrode disposed coaxially in the insulator, a side ground electrode having a first end coupled to the shell and a second end facing an end of the center electrode to define a spark discharge gap therebetween, and an electrode tip portion secured to either the side ground electrode or the center electrode proximate the spark discharge gap. The tip portion is formed from an alloy comprising from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, from 0 to about 10 percent by weight nickel, from about 3500 to about 4500 parts per million tantalum, and from about 100 to about 200 parts per million zirconium.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/272,208 filed Nov. 17, 2008, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/988,262 filed Nov. 15, 2007, the contents each of which are incorporated herein by reference thereto. 
     
    
     BACKGROUND 
       [0002]    Exemplary embodiments of the present invention relate to spark plugs for use in internal combustion engines, and, more particularly, to spark plugs having an electrode which includes a tip portion that is capable of being resistance welded to nickel-based electrodes and resistant to wear in oxidizing conditions at high temperatures. 
         [0003]    Spark plugs are widely used to ignite fuel in internal combustion engines. The electrodes of a spark plug are subject to intense heat and to an extremely corrosive environment generated by the formation of sparks and combustion of the air/fuel mixture. An electrode suffers whenever there is discharge during sparking, which can result in very high local temperatures and wear because of sputtering, melting, ablation, splashing, and particle erosion. In addition, the primary cause of wear for spark plugs is the failure of the electrodes caused by exposure to oxidizing conditions at elevated temperatures with high-sparking voltages. Oxidation proceeds from the surface of a healthy electrode, and spark discharge can remove weakened oxidized scales from the electrode surface causing wear and significantly reducing the life of the spark plug. 
         [0004]    Thus, as an electrode becomes hotter, the speed of both sparking wear and oxidation wear accelerates. With combustion engines moving toward more lean burn to reduce emissions, and with cylinder pressures, compression ratios, and ignition voltages increasing, the reliability and lifetime performance of spark plugs are critical to accommodate further advances in engine development. Therefore, to provide durability and erosion resistance, the material used for spark plug electrodes should have a high melting point and be highly resistant to oxidation to withstand the high temperatures and corrosive environment that result from the chemical reactions between air, fuel, and fuel additives within the combustion chamber. 
         [0005]    The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs has been accomplished in various ways. For instance, U.S. Pat. No. 4,705,486 (“the &#39;486 patent”), entitled “Method for Manufacturing a Center Electrode for a Spark Plug” the contents of which are incorporated herein in their entirety by reference thereto, discusses a method for manufacturing a center electrode that provides a degree of longevity for the spark plug. The center electrode is made from a good heat conducting material such as copper surrounded by a jacket of a corrosion resistant material such as nickel. Nickel, however, is susceptible to selective oxidation at high temperatures, which limits the life of the spark plug. Moreover, after a period of operating at higher temperatures in recirculation gases, corrosion/erosion can occur in the nickel-based electrode. Once corrosion has taken place, the electrical flow path will deteriorate and result in lower fuel efficiency. 
         [0006]    To resist erosion caused during service in oxidizing conditions at elevated temperatures with high sparking voltages, heat- and wear-resistant tips consisting of high-cost precious metal alloys can be added to the discharge end of the spark plug electrode. While these spark plug electrode tips are tougher and more erosion resistant than the balance of the electrodes, they are situated at the points at which the spark crosses over between the sparking electrodes and therefore are among the most critical working parts of a spark plug. 
         [0007]    The &#39;486 patent, for example, provides a method of manufacturing an electrode for a spark plug in which a platinum (Pt) tip is attached to a body composed of a nickel alloy such as Inconel that is disposed about a copper core. Other illustrative examples of publications relating to various wear-resistant spark plug electrode tips, and to spark plugs including such electrode tips, may be found in U.S. Pat. Nos. 6,597,089, 6,166,479, 6,094,000, 6,071,163, 5,998,913, 5,980,345, 5,793,793, 5,973,443, and 5,456,624; PCT Pub. No. WO 01/18925; and U.S. Pat. Pub. Nos. 2004/0027042 and 2002/0171346, the contents of each of which are incorporated herein in their entirety by reference thereto 
         [0008]    Some wear-resistant spark plug electrode tips incorporate precious metals such as platinum because they provide reasonably good resistance to oxidation and erosion under exposure to a combustion chamber environment. Platinum, however, is susceptible to intergranular cracking and attack by oxidation and lead found in certain fuels being used with internal combustion engines. Progressive oxidation and crack growth can result in a substantial increase in electric resistance and thus breakdown voltage for ignition to continue. The erosion and deterioration of the platinum tip portion causes the sparking gap to widen, thus weakening the spark that the spark plug produces. Furthermore, platinum is a very expensive raw material, as are the other noble metals, and it is therefore advantageous to strictly control the amount of noble metal which is incorporated into each spark plug. 
         [0009]    Iridium (Ir) has shown excellent resistance to attack by a wide range of molten metals. For instance, iridium is superior to platinum in withstanding attack by lead. Furthermore, iridium can provide superior wear resistance with a narrower center diameter, which allows for improved ignition. The coefficient of thermal expansion of iridium, however, differs significantly from nickel. Under high thermal stress, this difference can cause weakening or fracture to occur at the area where an iridium-based electrode tip portion and a nickel-based electrode are joined as the tip portion and electrode heat up during use of the spark plug, and may even lead to physical separation of the noble metal and base metal. Thus, iridium and its alloys have heretofore been extremely difficult to resistance weld or otherwise secure to an electrode comprised of nickel-based substrate alloys without experiencing gradually cracking and/or breaking at the joint between these components, particularly in side electrodes where the thermal stresses are most severe. 
         [0010]    Although various designs for spark plugs having wear-resistant electrode tips are known, the inventors herein have recognized a need for a spark plug having an electrode construction that allows for a long life of operation before the spark plug requires replacing, is highly wear-resistant and resistant to oxidation at high operating temperatures, and can provide a reliable, oxidation-resistant weld between the tip portion and the substrate portion. 
       SUMMARY 
       [0011]    Exemplary embodiments of the present invention relate to a spark plug that comprises a shell having a substantially cylindrical threaded portion for threadable engagement in a cylinder head of an internal combustion engine, an insulator disposed coaxially in the shell, a center electrode disposed coaxially in the insulator, a side ground electrode having a first end coupled to the shell and a second end facing an end of the center electrode to define a spark discharge gap therebetween, and an electrode tip portion secured to either the side ground electrode or the center electrode proximate the spark discharge gap. The tip portion is formed from an alloy comprising from about 60 to about 70 percent by weight iridium (Ir), from about 30 to about 35 percent by weight rhodium (Rh), from 0 to about 10 percent by weight nickel (Ni), from about 3500 to about 4500 parts per million tantalum, and from about 100 to about 200 parts per million zirconium. 
         [0012]    Exemplary embodiments of the present invention relate to a spark plug that comprises a shell having a substantially cylindrical threaded portion for threadable engagement in a cylinder head of an internal combustion engine, an insulator disposed coaxially in the shell, a center electrode disposed coaxially in the insulator, a side ground electrode having a first end coupled to the shell and a second end facing an end of the center electrode to define a spark discharge gap therebetween, and an electrode tip portion secured to either the side ground electrode or the center electrode proximate the spark discharge gap. The tip portion is formed from an alloy comprising from about 60 to about 70 percent by weight iridium (Ir), from about 30 to about 35 percent by weight rhodium (Rh), from 0 to about 10 percent by weight nickel (Ni), and from about 50 to about 100 parts per million cerium. 
         [0013]    Exemplary embodiments of the present invention also relate to a spark plug that comprises a center electrode disposed coaxially in an insulator, a side ground electrode facing the center electrode to define a spark discharge gap therebetween, and an electrode tip portion secured to either the side ground electrode or the center electrode proximate the spark discharge gap. The tip portion is formed from an alloy comprising from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, from 0 to about 10 percent by weight nickel, from about 3500 to about 4500 parts per million tantalum, and from about 100 to about 200 parts per million zirconium. 
         [0014]    Exemplary embodiments of the present invention also relate to a spark plug that comprises a center electrode disposed coaxially in an insulator, a side ground electrode facing the center electrode to define a spark discharge gap therebetween, and an electrode tip portion secured to either the side ground electrode or the center electrode proximate the spark discharge gap. The tip portion is formed from an alloy comprising from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, and from about 50 to about 100 parts per million cerium. 
         [0015]    Exemplary embodiments of the present invention also relate to a wear-resistant electrode tip portion for securing to a spark plug electrode. The tip portion comprises from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, from 0 to about 10 percent by weight nickel, from about 3500 to about 4500 parts per million tantalum, and from about 100 to about 200 parts per million zirconium. 
         [0016]    Exemplary embodiments of the present invention also relate to a wear-resistant electrode tip portion for securing to a spark plug electrode. The tip portion comprises from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, and from about 50 to about 100 parts per million cerium. 
         [0017]    Exemplary embodiments of the present invention also relate to a method for constructing an electrode for a spark plug. The method comprises obtaining an electrode tip portion formed from an alloy comprising from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, from 0 to about 10 percent by weight nickel, from about 3500 to about 4500 parts per million tantalum, and from about 100 to about 200 parts per million zirconium. The method further comprises placing the tip portion in a welding fixture. The method further comprises aligning the tip portion with the electrode. The method further comprises welding the tip portion to the electrode. 
         [0018]    Exemplary embodiments of the present invention also relate to a method for constructing an electrode for a spark plug. The method comprises obtaining an electrode tip portion formed from an alloy comprising from about 60 to about 70 percent by weight iridium, from about 30 to about 35 percent by weight rhodium, from 0 to about 10 percent by weight nickel, and from about 50 to about 100 parts per million cerium. The method further comprises placing the tip portion in a welding fixture. The method further comprises aligning the tip portion with the electrode. The method further comprises welding the tip portion to the electrode. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a cross-sectional view of a spark plug in accordance with an exemplary embodiment of the present invention; 
           [0020]      FIG. 2  is a side elevational detail view, partially broken away and partially shown in cross-section, of an end portion of the exemplary spark plug of  FIG. 1 ; 
           [0021]      FIG. 3  is a cross-sectional detail view of a center electrode which is one component of the exemplary spark plug of  FIG. 1 ; 
           [0022]      FIG. 4  is a vertical cross-sectional detail view of an exemplary embodiment of a rivet-shaped electrode tip; 
           [0023]      FIG. 5  is an elevational view of an exemplary embodiment of a spherical electrode tip; and 
           [0024]      FIG. 6  is a schematic view of an exemplary embodiment of a resistance welding machine. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0025]    Referring to  FIGS. 1-4 , a spark plug in accordance with an exemplary embodiment of the present invention is shown generally at  10 . Spark plug  10  includes an annular metal casing or shell  12  having a cylindrical base  14  with external threads  16  formed thereon for threadable engagement in a cylinder head (not shown). Cylindrical base  14  of spark plug shell  12  has a generally flattened lower surface  18 . A ground or side electrode  20 , formed from nickel (Ni) or a nickel-based alloy, is welded on to lower surface  18  of threaded base  14 . Throughout the present description of exemplary embodiments, the terms “ground electrode” and “side electrode” refer to the same component, and these terms are used interchangeably. 
         [0026]    Spark plug  10  further includes a hollow ceramic insulator  24  disposed concentrically within shell  12  and a center electrode  26  disposed concentrically within insulator  24 . In the present exemplary embodiment, center electrode  26  includes a central core  28  that is made of a thermally and electrically conductive material, such as copper (Cu) or a copper-based alloy, and an outer cladding  30  that is formed from a nickel-based alloy. In exemplary embodiments, cladding  30  can be formed from commercially available nickel-based alloys such as Inconel 600 or 601, or Hoskins 831 or 592. 
         [0027]    In the present exemplary embodiment, ground electrode  16  has a wear-resistant electrode tip portion  22  affixed thereon (by, for example, brazing, resistance welding, or laser welding) adjacent the end thereof. Center electrode  26  also has a wear-resistant electrode tip portion  32  affixed to a lower end  34  thereof. Electrode tips  22  and  32 , shown in the shape of a rivet in  FIGS. 1-4 , are comprised of materials that can provide a reliable, oxidation-resistant weld with a nickel-based alloy, as will be further described herein. 
         [0028]    An electrically conductive insert or rod  36  fits into the upper end  38  of insulator  24  opposite center electrode  26 , and a refractory glass-carbon composite material is disposed within insulator  24  between the lower end of insert  36  and center electrode  26  to provide an internal resistor  40  with spark plug  10 . 
         [0029]    As illustrated in  FIG. 1 , spark plug shell  12  is a substantially cylindrical sleeve having a hollow bore  42  formed therethrough. As noted above, spark plug shell  12  includes cylindrical base portion  14  which generally has threads  16  formed on the exterior surface thereof. Spark plug shell  12  includes a sealing surface  44  for contacting a cylinder head (not shown) and, on the shell above the sealing surface, a generally hexagonal boss  46  for allowing spark plug  10  to be grasped and turned by a conventional spark plug socket wrench for installation or removal thereof. 
         [0030]    During operation, it is desirable to maintain the spacing, or gap G, between center electrode  26  and ground or side electrode  20  throughout the life of spark plug  10 . 
         [0031]    In exemplary embodiments, wear-resistant tip  32  of center electrode  26  can be formed in the shape of a post, rivet, or sphere. Spark plugs generally using fine wire rivet firing tips and methods of attaching such rivet tips to electrodes are described generally in U.S. Pat. Nos. 5,456,624 and 6,071,163, the contents of which are incorporated herein in their entirety by reference thereto. In the present exemplary embodiment, as shown in  FIG. 4 , wear-resistant electrode tip  32  is provided in the form of a rivet  48  that includes a head  50  having a continuous, semi-spherical outer surface  52  and a flat portion  54  opposite the outer surface of the head. A generally cylindrical shank  56  extends from the flat portion  54  and terminates in a generally flattened base  60 . In alternative exemplary embodiments in which wear-resistant tip  32  takes the form of a post, it can resemble shank  56  of rivet  48 , with head  50  removed therefrom. In the alternative exemplary embodiment illustrated in  FIG. 5 , electrode tip  32   a  is formed in the shape of a sphere  50   a . In non-limiting exemplary embodiments, the diameter of the sphere may vary significantly, but can be in the range from about 0.38 to about 1.14 mm, and, in non-limiting exemplary embodiments, about 0.80 mm. 
         [0032]    In exemplary embodiments, each wear-resistant spark plug electrode tip can be formed from a wire made of an alloy comprising iridium (Ir) and rhodium (Rh). Such an electrode tip can exhibit improved resistance to both sparking discharge and oxidation, as well as enhanced durability at high temperatures. Specifically, iridium, which has a high melting point, and rhodium both provide excellent sparking wear resistance, and the addition of rhodium as an alloying metal element in an iridium-based alloy is effective in improving the oxidation resistance of iridium and inhibiting volatility. In exemplary embodiments, the electrode tips can also comprise small or micro amounts of tantalum (Ta), zirconium (Zr), and/or cerium (Ce). These alloying elements additions can help to further ensure each electrode tip portion against welding cracks that might occur due to differing coefficients of thermal expansion between the nickel-based electrode substrate and the iridium-rhodium tip portions. 
         [0033]    In exemplary embodiments, the alloy for an electrode tip can comprise iridium in a range from about 60 to about 70 percent by weight, rhodium in a range from about 30 to about 35 percent by weight, and nickel in a range from 0 to about 10 percent by weight, as well micro additions of tantalum in a range of about 3500 to about 4500 ppm, zirconium in a range of about 100 to about 200 ppm, and/or cerium in a range of about 50 to about 100 ppm. A non-limiting exemplary embodiment of a mixture that is usable for the electrode tip alloy is 65 percent by weight iridium, 35 percent by weight rhodium, 4000+/−500 ppm tantalum, and 150+/−50 ppm zirconium. A second non-limiting exemplary embodiment of a mixture that is usable for the electrode tip alloy is 70 percent by weight iridium, 30 percent by weight rhodium, and 75+/−25 ppm cerium. Another non-limiting exemplary embodiment of a mixture which is usable for the electrode tip alloy is 60 percent by weight iridium, 30 percent by weight rhodium, 10 percent by weight nickel, 4000+/−500 ppm tantalum, and 150+/−50 ppm zirconium. Yet another non-limiting exemplary embodiment of a mixture that is usable for the electrode tip alloy is 60 percent by weight iridium, 30 percent by weight rhodium, 10 percent by weight nickel, and 75+/−25 ppm cerium. 
         [0034]    Exemplary processes of forming a spherical shaped electrode tip portion and welding it to an electrode substrate are described in U.S. Pat. No. 5,980,345, the contents of which are incorporated herein in their entirety by reference thereto. An exemplary process of forming a rivet-shaped tip portion and welding it to an electrode substrate in accordance with the present invention will now be described. A length of wire made from one of the aforementioned exemplary iridium-rhodium alloys is cut to a predetermined length. A shank end of the rivet is then finished and formed, and a head of the rivet is formed in a conventional high speed ball former. The electrode substrate may be formed as described in U.S. Pat. No. 4,705,486. 
         [0035]    As indicated schematically in  FIG. 6 , which shows an exemplary resistance welding machine having a firing tip of a rivet-shaped electrode tip  148  and an electrode substrate  126  in the positions they assume just before being brought into welding contact, the electrode substrate and the rivet are clamped respectively in a lower welding head  162  and an upper welding head  164 . In such a conventional electric resistance welding machine, upper welding head  164  is movable relative to lower welding head  162 . Upper welding head  164  has a recess formed in an upper surface thereof for holding and maintaining rivet  148  stationary during the welding process. Electrode substrate  126  may be either a portion of a center electrode or a side electrode. 
         [0036]    Upper welding head  164  is then moved toward lower welding head  162  until an outer surface of rivet  148  makes an initial point contact with a lower end  134  of electrode substrate  126 . An electrical current is then applied through parts  148 ,  126  that varies from 500 to 1,000 amps, and upper welding head  164  forces electrode substrate  126  against rivet  148  with a force that varies from about 10 to about 30 pounds. This welding operation generates an alloying of the iridium-rhodium alloy of rivet  148  and a nickel-based alloy cladding of electrode substrate  126  at the weld interface. The outer surface of rivet  148  penetrates into lower end  134  to a depth that is controlled by varying the current and the applied force between the two parts during application of the current, and embeds the outer surface into the cladding about 0.006 inches to about 0.012 inches deep, thereby forcing the nickel-based material of the cladding, which is displaced by a head of rivet  148 , to flow around the outer surface to capture the rivet. In this manner, rivet  148  becomes securely fixed to electrode substrate  126 . If a flat electrode tip is desired, the tip may, optionally, be flattened in place on the finished electrode part. The finished part is then removed from the welding machine and may be assembled into a finished spark plug following standard procedures and using standard components for the balance of the parts. 
         [0037]    In exemplary embodiments, the resistance welding process can be used to achieve better bonding strength between the iridium-rhodium alloy tip and the nickel alloy electrode as a result of the inter-diffusion of nickel with micro additions of tantalum, zirconium, and/or cerium. Intermediate phases can be formed between the nickel alloy electrode and tantalum, zirconium, and/or cerium that will minimize the mismatch in the thermal expansion coefficient between iridium and the nickel alloy. Thus, these small additions serve to better match the coefficient of thermal expansion of the electrode substrate to the tip portion to ensure against cracks and stress fractures as the electrode goes from room temperature to an operating temperature during use. 
         [0038]    Thus, while the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular exemplary embodiments disclosed herein, but that the invention will include all embodiments falling within the scope of the appended claims and their legal equivalence.