Spark plug having a metal layer in a terminal metal piece

A spark plug comprising:

• a metallic shell having a ground electrode;
• an insulator disposed within the metallic shell and having an axially extending through-hole;
• a center electrode disposed within the through-hole of the insulator;
• a terminal metal piece disposed within the through-hole of the insulator;
• and
• a conductive coupling layer disposed within the through-hole and located between the center electrode and the terminal metal piece, the conductive coupling layer having a conductive glass seal layer on at least an end facing the terminal metal piece, wherein
• the terminal metal piece has a surface layer region in contact with the conductive glass seal layer, and the surface layer region is formed of a conductive or semi-conductive oxide layer having a thickness of 0.1 μm or greater.

A spark plug comprising:

• a metallic shell having a ground electrode;
• an insulator disposed within the metallic shell and having an axially extending through-hole;
• a center electrode disposed within the through-hole of the insulator;
• a terminal metal piece disposed within the through-hole of the insulator;
• and
• a conductive coupling layer disposed within the through-hole and located between the center electrode and the terminal metal piece, the conductive coupling layer having a conductive glass seal layer on at least an end facing the terminal metal piece, the conductive glass seal layer being formed of a mixture of at least one metal and glass, said mixture comprising from 35 to 70 wt. % metal selected from the group consisting of Cu, Fe and mixtures thereof and containing, as an auxiliary metal component, at least one metal selected from Zn, Sb, Sn, Ag, and Ni in an amount of 0.1 to 10 wt. %.