Abstract:
A method of coating a metal spark plug shell with a titanium-containing compound involves placing the shell into a vacuum chamber, lowering the pressure in the chamber to a level below atmospheric pressure, and applying a protective coating, containing a titanium compound, to a portion of the exterior of the spark plug shell by physical vapor deposition. A preferred titanium compound is titanium nitride. The coating on the spark plug shell improves corrosion resistance thereof, provides a pleasing appearance, and resists seizing of the spark plug in place in a cylinder head portion of an engine. A spark plug incorporating the coated shell is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to spark plugs for internal combustion engines. More particularly, the present invention relates to a spark plug having a protective coating on a metal shell portion thereof, and to a method of making such a spark plug. 
     2. Description of the Background Art 
     Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plugs of many types are known and are commercially available. Spark plug electrodes are subject to intense heat and to a highly corrosive environment generated by the exploding air/fuel mixture. To improve durability and erosion resistance, spark plug electrode tips must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, and fuel additives within a combustion chamber. 
     Spark plugs generally include a hollow ferrous metal shell, and a ceramic insulator partially enclosed within the shell. This spark plug shell usually has male threads formed on the outside thereof The metal shell is the portion of the spark plug which engages with a threaded hole in an engine cylinder head when the spark plug is rotatably installed therein. 
     A problem of ‘seizing’ sometimes occurs, in which the metal shell portion of a spark plug, normally made of iron or steel, may become locked in place in an aluminum cylinder head, over time, if left undisturbed therein. Since cylinder heads are commonly made out of aluminum-based alloys in most modern internal combustion engines, this potential for seizing is of concern. 
     Seizing of this type is particularly a risk where spark plugs are left in place for extended time intervals. Automotive manufacturers are now selling vehicles with engines that can go many thousands of miles between tune ups, and as a result, spark plugs are being left in useful service for extended time periods. Durability and corrosion resistance are also important concerns in such extended life spark plugs. 
     Accordingly, a need exists for an improved spark plug, which is resistant to seizing in place in internal combustion engines, particularly in aluminum alloy cylinder heads. Preferably, such a spark plug would be seize-resistant even where left in place for extended service intervals. Most preferably, such a seize-resistant spark plug would also have superior durability and corrosion resistance. 
     Titanium compounds have been suggested for use as one component of an internal spark plug resistor by Yamada et al. in U.S. Pat. No. 4,173,582, and have been used as coatings to harden some tools such as cutting tools, but are not presently used as coatings for spark plugs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of making a spark plug having a metal shell with a thin protective coating thereon, in which the protective coating is resistant to the spark plug&#39;s seizing in place. The preferred coating includes a titanium compound. The present invention also provides a spark plug which is a product of the described method. 
     Optionally, the protective coating hereof may be a two-part coating comprising an inner coating layer, which comprises a corrosion-resistant material, and an outer coating layer which comprises a titanium compound. Where the two-part coating is used, a preferred material for the inner coating layer is selected from the group consisting of zinc, zinc chromate, nickel, and nickel alloys. 
     The method of coating a metal spark plug shell with a titanium-containing compound involves placing the spark plug shell into a vacuum chamber, lowering the pressure in the chamber to a level below atmospheric pressure, and applying a protective coating, containing a titanium compound, to a portion of the exterior of the spark plug shell by physical vapor deposition. 
     The coating on the spark plug shell improves corrosion resistance thereof, provides a pleasing appearance, and resists seizing of the spark plug in place in a cylinder head portion of an engine. 
     Accordingly, it is an object of the present invention to provide an extended life spark plug having a seize-resistant coating on the threaded base thereof, 
     It is another object of the invention to provide an extended life spark plug of the type described which also exhibits improved durability and corrosion resistance as compared to known spark plugs. 
     It is a further object of the present invention to provide a method of making a seize-resistant spark plug. 
     For a more complete understanding of the present invention, including further objects, features, and advantages, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a spark plug in accordance with present invention; 
     FIG. 2A is a cross sectional view of a spark plug shell which is usable as one possible component of the spark plug of FIG. 1, swing a single layer wear-resistant coating applied to the exterior thereof; 
     FIG. 2B is a cross sectional view of the spark plug shell of FIG. 2A, further modified to include an external layer of wax covering the first protective coating. 
     FIG. 3 is a cross sectional view of a spark plug shell which is usable as an alternative component of the spark plug of FIG. 1, instead of the spark plug shell of FIG. 2, and showing a two-layer wear-resistant coating applied to the exterior thereof, and 
     FIG. 4 is a flow chart showing steps in one method according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout this description, relative terms like “upper”, “lower”, “above”, “below”, and the like are used in reference to the components of the spark plug  10  as oriented in the illustration of FIG.  1 . It should be understood that these terms are used for purposes of illustration, and are not intended to limit the invention. The spark plug  10  could be inverted or turned on its side in a particular application thereof, and if it were so inverted, or otherwise placed in an orientation different from that shown in FIG. 1, then such relative positional terms would no longer be accurate. 
     Referring now to the drawings, and particularly to FIG. 1, a spark plug in accordance with the present invention is shown generally at  10 . The spark plug  10  includes a metal casing or shell  12  having a cylindrical base  14 , which may have external threads  16  formed thereon for threadable engagement in a cylinder head (not shown). The threaded portion of the base  14  acts as an engaging portion. The cylindrical base  14  of the spark plug shell  12  has a generally flattened lower surface  18 . A ground electrode  20  is welded on to the lower surface  18  of the cylindrical base  14 , and after attachment to the base, the ground electrode is bent in an approximately 90 degree angle. 
     Optionally, the ground electrode  20  may have a wear-resistant electrode tip  22  welded thereon adjacent the end thereof. 
     The spark plug  10  further includes a hollow ceramic insulator  24  disposed concentrically within the shell  12 . The lower portion of the insulator  24  is housed within the shell  14 , and the upper portion thereof extends upwardly away from the shell. 
     The spark plug further includes a center electrode  26  disposed concentrically within the insulator  24  at the bottom of the shell  12 . The lower end of the center electrode  26  extends outwardly and downwardly from the insulator  24  adjacent the ground electrode  20 . 
     The center electrode  26  is preferred to include a central core  28  made of a thermally and electrically conductive material, such as copper or a copper alloy, with an outer cladding  30  which is preferably formed from a nickel alloy. The center electrode  26  may also have a wear-resistant electrode tip  32  affixed to a lower end  34  thereof, if desired. 
     An electrically conductive metal insert or stud  36  fits into the upper end  38  of the insulator  24 , opposite the center electrode  26 . The lower end of the stud  36  is installed inside of the ceramic insulator  24 , while the upper end of the stud is outside and above the insulator for receiving an ignition wire connector (not shown) thereon. 
     Also, a refractory glass-carbon composite material is disposed within the insulator  24 , between the lower end of the insert  36  and the center electrode  26 , to provide an internal resistor  40  within the spark plug  10 . 
     THE SPARK PLUG SHELL 
     Referring in particular to FIG. 2, it may be seen that the spark plug shell  12  is a substantially cylindrical sleeve having a hollow bore  42  formed therethrough. 
     While it is noted that the spark plug shell illustrated in FIG. 2 is not identical to the spark plug shell used in the spark plug of FIG. 1, the differences between the shells of FIGS. 1 and 2 are for specific application geometries, and the two depicted shells are otherwise functionally equivalent. 
     As previously noted, the spark plug shell  12  includes a cylindrical base portion  14  which generally has male threads  16  formed on the exterior surface thereof The spark plug shell  12  includes a sealing surface  44  for cooperatively contacting a complimentary sealing surface of a cylinder head (not shown). The spark plug shell  12  also includes a generally hexagonal boss  46  thereon above the sealing surface, for allowing the spark plug to be grasped and turned by a conventional spark plug socket wrench for installation or removal thereof 
     SINGLE LAYER COATING 
     Referring now to FIG. 2A, it has been discovered, in accordance with the present invention, that if a thin layer of a titanium-containing compound is applied to the exterior surface of the spark plug shell  12 , to form a protective coating  50 , that a number of significant benefits are obtained over an uncoated shell. The relative thickness of the protective coating  50  is exaggerated in the drawings for purposes of illustration. 
     Preferably, the compound used to form the coating  50  on the spark plug shell is a titanium compound, selected from the group consisting of titanium nitride, titanium carbonitride, titanium zirconium nitride, and mixtures thereof. 
     In particular, in a first embodiment of the present invention, it is preferred to apply a coating  50  of the type described to the threads  16  on the exterior of the shell  12 , to minimize the likelihood of the spark plug threads galling or seizing in a cylinder head. 
     Conventional methods of applying the coating  50  to the spark plug shell, using a physical vapor deposition process may be used. Physical vapor deposition is a relatively well established coating process in the relevant art. One acceptable method of applying this type of coating is outlined in U.S. Pat. No. 4,929,322 to Sue et al., the disclosure of which is hereby incorporated by reference. 
     A first benefit of applying such a coating  50  to the exterior of the spark plug shell  12  is that the corrosion resistance of the shell is improved. Also, it has been found that the durability and external hardness of the spark plug shell  12  is increased. Further, it has been discovered that the removal of the spark plug from engagement with a substrate is made easier than it would be in the absence of such a coating; that is, the likelihood of a spark plug seizing in place is reduced. This is particularly beneficial where a ferrous spark plug shell is installed in a cylinder head made of aluminum or an aluminum alloy. In addition, the coating  50  gives the spark plug shell  12  a pleasing appearance. 
     The coating  50  may be applied selectively, if desired, so as to be present on the threads  16  of the shell base  14 , while omitted from other parts of the spark plug shell such as, e.g., the inner surface of the ground electrode  20 . 
     TWO-LAYER COATINGS 
     It has been found that when applied very thinly such as in a thickness in a range of about 2-6 micrometers, titanium-based compounds may provide coatings which are somewhat porous and which may allow some corrosion of the underlying metal in extreme conditions. Accordingly, supplemental materials may be used to augment the corrosion resistance of the coating  50 , if extra corrosion resistance is desired. 
     For example, with reference to FIG. 2B, an outer covering layer  52  of a wax material, such as carnauba wax, a synthetic polymeric wax, or another suitable wax known in the art, may, optionally, be applied to the spark plug shell  12  on top of the base coating  50 , to further increase corrosion resistance as may be appropriate for a particular application. 
     Referring now to FIG. 3, a spark plug shell  112  is shown having a two-layer coating  145  thereon in accordance with a second embodiment of the present invention. 
     A first or base coat  150 , in accordance with this embodiment, is provided to enhance corrosion resistance, and may be a metallic coating, or a coating comprising a metallic salt. The base coat  150  is applied in a thickness between 2 and 6 micrometers. Preferred materials for use in forming the base coat  150  in this embodiment may be selected from the group consisting of zinc compounds and nickel compounds. Specific preferred materials include zinc, zinc chromate, and nickel. 
     This base coat  150  is preferably applied to the spark plug shell  112  by immersing the shell in a plating bath, which may be an electroplating bath or an electroless plating bath, as is most appropriate. Electroplating methods are well established and known to those in the art. Electroless plating is a technique in which the object to be coated is soaked in a chemical solution containing metallic salts, in the presence of a chemical reducing agent, and without using any electrical current. Further detail on the electroless plating process may be found in the disclosure of U.S. patent application Ser. No. 09/114,448, the disclosure of which is incorporated herein by reference. 
     The step of applying the base coat using an immersive plating process is shown at  160  in the flow chart of FIG.  4 . 
     Subsequent to the application of the base coat  150 , the spark plug shell  112  is then placed into a vacuum chamber. Pressure in the chamber is then reduced to a level below atmospheric pressure, and a top coat  152  is applied to a portion of the exterior of the spark plug shell by physical vapor deposition. The top coat  152  is a titanium-containing compound selected from the group consisting of titanium nitride, titanium carbonitride, titanium zirconium nitride, and mixtures thereof 
     The step of applying the seize-resistant top coat of a titanium compound using physical vapor deposition is shown at  162  in the flow chart of FIG.  4 . 
     After the top coat  152  has been applied to the spark plug shell  112 , the shell is used as one component, along with other known components, to assemble a spark plug in the normal way. The step of assembling the spark plug shell and other components into a complete spark plug is shown at  164  in the flow chart of FIG.  4 . 
     Although the present invention has been described herein with respect to a preferred embodiment thereof, the foregoing description is intended to be illustrative, and not restrictive. Those skilled in the art will realize that many modifications of the preferred embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.