In general, spark plugs used for ignition of an internal combustion engine such as a gasoline engine include a center electrode, an insulator provided to the outside of the center electrode, a metal shell provided to the outside of the insulator, and a ground electrode installed to the metal shell and forming a spark discharge gap between itself and the center electrode. The insulator of such spark plug is made of an aluminum sintered compact containing a rare earth element constituent, a silicon constituent, and a group 2 element (see Japanese Patent No. 4607253, “Patent Document 1”).
Conventionally, improvement in the dielectric strength property has been desired for the insulator used for the spark plug. In order to improve the dielectric strength property of the insulator, the composition of the materials of the insulator has been optimized or the material has been micronized.
However, the change in the composition of the material of the insulator is likely to cause the problem of the degradation of a part of the performance and, further, the micronized material may cause the excessive sintering resulting in the abnormal particle growth and the decreased density, which is likely to cause the reduction in the strength and/or the dielectric strength performance of the insulator. Improvement in strength and dielectric strength performance of an insulator used in a spark plug is desired.