Spark plug for internal combustion engines having an alloy layer between the electrodes and tip ends

Abstract

A spark plug for an internal combustion engine including a center electrode, an earth electrode, a metal tip joined by welding to an ignition section of at least one of the center electrode and the earth electrode, and an alloy layer located at a welding portion between the metal tip and the at least one electrode. The metal tip is formed of material highly resistant to heat and wear which is distinct in the coefficient of thermal expansion from the material of the electrodes. The alloy layer is formed to have a thickness of at least about 10μ, so as to avoid the rupture of the metal tip along the surface thereof adjacent to the welding portion.

What is claimed is: 
     
       1. A spark plug for an internal combustion engine comprising: a center electrode;   an earth electrode;   a metal tip joined by welding to an ignition section of one of said electrodes, said metal tip being highly resistant to heat and wear and formed of material differing in the co-efficient of thermal expansion from the metal forming said one electrode; and   an alloy layer of the metals of said one electrode and said metal tip, said alloy layer being provided for reducing thermal stresses applied to said metal tip and located at a welding portion between said metal tip and said one electrode and having a thickness of at least about 10 μm.   
     
     
       2. A spark plug as claimed in claim 1, wherein said metal tip is formed of material containing about 10 to 30% by weight of iridium, and the balance of platinum and incidental impurities. 
     
     
       3. A spark plug as claimed in claim 1, wherein said metal tip is formed of material containing about 10 to 30% by weight of iridium, about 0.5 to 10% by weight of nickel, and the balance of platinum and incidental impurities. 
     
     
       4. A spark plug as defined in claim 1 including: a metal tip joined by welding to an ignition section of the other of said electrodes, said metal tip being highly resistant to heat and wear and formed of material differing in the co-efficient of thermal expansion from the metal forming said other electrode; and   an alloy layer of the metals of said other electrode and said metal tip, said alloy layer being provided for reducing thermal stresses applied to said metal tip and located at a welding portion between said metal tip and other electrode and having a thickness of at least 10 μm. .Iadd.   
     
     
       5.  A spark plug as claimed in claim 1, wherein said alloy layer comprises an alloy of platinum and nickel. .Iaddend. .Iadd.6. A spark plug as claimed in claim 1, wherein said alloy layer is formed by holding said welding portion between said metal tip and said one electrode in an atmosphere of predetermined temperature for a predetermined period. .Iaddend. .Iadd.7. A spark plug as claimed in claim 1, wherein said thickness is at least about 15 μm. .Iaddend. .Iadd.8. A spark plug as claimed in claim 1, wherein said metal tip has a surface welded to said ignition section of said one electrode and another surface facing the other of said electrodes through a gap. .Iaddend. .Iadd.9. A spark plug as claimed in claim 1, wherein said one electrode is a center electrode having a reduced diameter, tapered end portion with a flat end and said metal tip is welded on said flat end. .Iaddend. .Iadd.10. A spark plug as claimed in claim 1, wherein said metal tip has a disc shape. .Iaddend. 
     
     
        .Iadd.11.  A spark plug as claimed in claim 1, wherein said metal tip is made of precious metal and has a diameter larger than a thickness of said 
     
     
        metal tip. .Iaddend. .Iadd.12.  A spark plug as claimed in claim 4, wherein said one electrode is said center electrode, said other electrode is said earth electrode, and said metal tips face each other so that a spark gap is formed therebetween. .Iaddend. .Iadd.13. A spark plug as claimed in claim 12, wherein said center electrode has a reduced diameter, tapered end portion with a flat end and said metal tip is welded on said flat end. .Iaddend. .Iadd.14. A spark plug as claimed in claim 13, wherein said metal tip welded on said center electrode has a surface welded on said flat end and another surface facing said metal tip welded on said earth electrode so that said spark gap is formed. .Iaddend. .Iadd.15. A spark plug as claimed in any one of claims 1-14, wherein said thickness is an initial thickness. .Iaddend.