| 摘要 |
PROBLEM TO BE SOLVED: To provide a long-life spark plug for an internal combustion engine having a noble metal chip of high strength and toughness. SOLUTION: A spark plug 10 for an internal combustion engine comprises a center electrode 2 held in a through hole 41 in an insulator 4, a housing 5 for holding the insulator 4, and an earth electrode 3 provided on the housing 5. A spark gap G is provided between the center electrode 2 and the earth electrode 3. At least, one of the electrodes 2, 3 is provided with a noble metal chip 6 joined thereto and having a groove part on a surface thereof confronting the other electrode. The size of crystal grains in crystal grain structure of the noble metal chip 6 satisfies relations of N>K and (N+K)/2<=50μm, where the mean grain size in the axial direction of the noble metal chip 6 is Nμm and the mean grain size in the radial direction thereof is Kμm.
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