Abstract:
An ignition device such as a spark plug having ground and center electrodes, at least one of which includes a firing tip formed from an alloy containing iridium, rhodium, tungsten, and zirconium. With the inclusion of tungsten and zirconium in the alloy, the percentage of rhodium can be kept relatively low without sacrificing the erosion resistance or reduced sparking voltage of the firing tip. In one embodiment, the firing tip contains 2% rhodium, 0.3% tungsten, 0.02% zirconium, and the balance iridium.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 10/390,075, filed Mar. 17, 2003, the complete disclosure of which is hereby incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates generally to spark plugs and other ignition devices used in internal combustion engines and, more particularly, to such ignition devices having noble metal firing tips. As used herein, the term “ignition device” means spark plugs, igniters, and other such devices that are used to initiate the combustion of a gas or fuel.  
         BACKGROUND OF THE INVENTION  
         [0003]    A variety of iridium-based alloys have been proposed for use in spark plug electrodes to increase the erosion resistance of the firing surfaces of the electrodes. Iridium-based alloys typically have a relatively high melting point and are more resistant to spark erosion than many of the metals widely used today. The iridium-based alloy is typically used in the form of a pad or rivet that is laser welded or otherwise metallurgically bonded to the center and ground electrodes on either side of the spark gap. There are, however, known disadvantages to the use of certain iridium-based alloys, including difficulty in bonding that material to the electrodes and oxidative volatilization of the alloy at higher temperatures. The present invention addresses the latter of these two problems.  
           [0004]    A known approach for reducing the oxidative loss of iridium is to utilize it in the form of an alloy combined with rhodium. U.S. Pat. No. 6,094,000 and published UK patent application GB 2,302,367 to Osamura et al. discloses such an alloy in which rhodium can be included in an amount ranging from 1-60 wt %. Group  3 A and  4 A elements such as yttria or zirconium oxide can also be added to help reduce consumption resistance. Notwithstanding Osamura et al.&#39;s teaching of use of rhodium in amounts as low as  1 %, it has been found that minimization of oxidative loss of the iridium at higher temperatures requires much higher amounts of rhodium. This is borne out in the test data presented by Osamura et al. and their patent notes that the amount of rhodium is preferably at least 3%.  
           [0005]    U.S. Pat. No. 5,793,793 to Matsutani et al. reports a similar finding, wherein the amount of rhodium is kept within the range of 3-50 wt % and, most preferably, is at least 18%. In U.S. Pat. No. 5,998,913, Matsutani identifies some disadvantages of the inclusion of high percentages of rhodium and, in an effort to reduce the amount of rhodium in the alloy, proposes the addition of rhenium or ruthenium. According to this patent, by adding rhenium and/or ruthenium in amounts up to 17 wt %, the amount of rhodium needed to maintain good resistance to oxidative consumption can be lowered to as little as 0.1 wt %.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to an ignition device having a pair of electrodes defining a spark gap therebetween, with at least one of the electrodes including a firing tip formed from an alloy of iridium, rhodium, tungsten, and zirconium. The combination of these constituent elements permits the known benefits of good erosion resistance and lowered sparking voltages to be obtained at much lower percentages of rhodium than has been found desirable in alloys containing only iridium and rhodium. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    A preferred exemplary embodiment of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and:  
         [0008]    [0008]FIG. 1 is a fragmentary view and a partially cross-sectional view of a spark plug constructed in accordance with a preferred embodiment of the invention;  
         [0009]    [0009]FIG. 2 is a side view of a rivet that can be used in place of the firing tip pads used on the spark plug of FIG. 1; and  
         [0010]    [0010]FIG. 3 depicts a wire that can be used in place of the firing tip pads shown in FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]    Referring to FIG. 1, there is shown the working end of a spark plug  10  that includes a metal casing or housing  12 , an insulator  14  secured within the housing, a center electrode  16 , a ground electrode  18 , and a pair of firing tips  20 ,  22  located opposite each other on the center and ground electrodes  16 ,  18 , respectively. Housing  12  can be constructed in a conventional manner and can include standard threads  24  along with an annular lower end  26  to which the ground electrode  18  is welded or otherwise attached. Similarly, all other components of the spark plug  10  (including those not shown) can be constructed using known techniques and materials, excepting of course the ground and/or center electrodes  16 ,  18  which are constructed with firing tip  20  and/or  22 , as will be described below.  
         [0012]    As is known, the annular end  26  of housing  12  defines an opening  28  through which insulator  14  protrudes. Center electrode  16  is permanently mounted within insulator  14  by a glass seal or using any other suitable technique. It extends out of insulator  14  through an exposed, axial end  30 . Ground electrode  18  is in the form of a conventional ninety-degree elbow that is mechanically and electrically attached to housing  12  at one end  32  and that terminates opposite center electrode  16  at its other end  34 . This free end  34  comprises a firing end of the ground electrode  18  that, along with the corresponding firing end of center electrode  16 , defines a spark gap  36  therebetween.  
         [0013]    The firing tips  20 ,  22  are each located at the firing ends of their respective electrodes  16 ,  18  so that they provide sparking surfaces for the emission and reception of electrons across the spark gap  36 . These firing ends are shown in cross-section for purposes of illustrating the firing tips which, in this embodiment, comprise pads welded into place on the firing ends. As shown, the firing tips  20 ,  22  can be welded into partial recesses on each electrode. Optionally, one or both of the pads can be fully recessed on its associated electrode or can be welded onto an outer surface of the electrode without being recessed at all.  
         [0014]    In accordance with the invention, each firing tip is formed from an alloy containing iridium, rhodium, tungsten, and zirconium. Preferably, the alloy is formed from a combination of iridium with 1-3 wt % rhodium, 0.1-0.5 wt % tungsten, and 0-0.5 wt % zirconium with no more than minor amounts of anything else. “Minor amounts,” means a combined maximum of 2000 ppm of unspecified base metal and PGM (platinum group metals) impurities. In a highly preferred embodiment, the alloy is formed from about 2.0 wt % rhodium, about 0.3 wt % tungsten, about 0.02 wt % zirconium, and the balance iridium with no more than trace amounts of anything else. The alloy can be formed by known processes such as by melting the desired amounts of iridium, rhodium, tungsten, and zirconium together. After melting, the alloy can be converted into a powdered form by an atomization process, as is known to those skilled in the art. The powdered alloy can then be isostatically pressed into solid form, with secondary shaping operations being used if necessary to achieve the desired final form. Techniques and procedures for accomplishing these steps are known to those skilled in the art.  
         [0015]    Although the electrodes can be made directly from the alloy, preferably they are separately formed from a more conventional electrically-conductive material, with the alloy being formed into firing tips for subsequent attachment to the electrodes. Once both the firing tips and electrodes are formed, the firing tips are then permanently attached, both mechanically and electrically, to their associated electrodes by metallurgical bonding, such as laser welding, laser joining, or other suitable means. This results in the electrodes each having an integral firing tip that provides an exposed sparking surface for the electrode. Laser welding can be done according to any of a number of techniques well known to those skilled in the art. Laser joining involves forming a mechanical interlock of the electrode to the firing tip by using laser light to melt the electrode material so that it can flow into a recess or other surface feature of the firing tip, with the electrode thereafter being allowed to solidify and lock the firing tip in place. This laser joining technique is more fully described in European Patent Office publication no. EP 1 286 442 A1, the complete disclosure of which is hereby incorporated by reference.  
         [0016]    As will be appreciated, the firing tips  20 ,  22  need not be pads, but can take the form of a rivet  40  (shown in FIG. 2), a wire  42  (shown in FIG. 3), a ball (not shown), or any other suitable shape. Although a round-end rivet is shown in FIG. 2, a rivet having a conical or frusto-conical head could also be used. As indicated in FIG. 3, the firing tip can, but need not, include one or more surface features such as grooves  44  to permit it to be interlocked to the electrode using the laser joining technique discussed above. The construction and mounting of these various types of firing tips is known to those skilled in the art. Also, although the firing ends of both the center and ground electrodes are shown having a firing tip formed from the iridium/rhodium/tungsten/zirconium alloy, it will be appreciated that the alloy could be used on only one of the electrodes. The other electrode can be utilized without any firing tip or can include a firing tip formed from another precious metal or precious metal alloy. For example, in one embodiment, the center electrode firing tip  20  can be formed from the iridium/rhodium/tungsten/zirconium alloy and the ground electrode firing tip  20  can be formed from platinum or a platinum alloy.  
         [0017]    The combination of iridium, rhodium, tungsten, and zirconium has been found to yield an alloy that exhibits good resistance to both spark and oxidative consumption, and the present invention permits these benefits to be maintained using relatively small amounts of rhodium.  
         [0018]    It will thus be apparent that there has been provided in accordance with the present invention an ignition device and manufacturing method therefor which achieves the aims and advantages specified herein. It will, of course, be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific embodiments shown. Various changes and modifications will become apparent to those skilled in the art. For example, although an ignition device in the form of a spark plug has been illustrated, it will be appreciated that the invention can be incorporated into an igniter of the type in which sparking occurs across the surface of a semiconducting material disposed between the center electrode and an annular ground electrode. All such changes and modifications are intended to be within the scope of the present invention.