Sintered ceramic body for a spark plug insulator and method of sintering the same

A sintered ceramic spark plug insulator body consisting essentially of silicon nitride (Si.sub.3 N.sub.4), aluminum nitride (AlN) and alumina (Al.sub.2 O.sub.3), the ratio of the sum of the weight of aluminum nitride (AlN) and alumina (Al.sub.2 O.sub.3) to silicon nitride (Si.sub.3 N.sub.4) being in the range 0.093-1.439 and at least one sintering additive selected from the group consisting of yttrium oxide (Y.sub.2 O.sub.3), calcium oxide (CaO), lanthanum oxide (La.sub.2 O.sub.3), magnesia (MgO), cerium oxide (CeO.sub.2), scandium oxide (Sc.sub.2 O.sub.3), strontium oxide (SrO), barium oxide (BaO) and neodymium (Nd.sub.2 O.sub.3), the weight percentage of the sintering additive being in the range from 0.3 to 15.0 of the total weight of the silicon nitride (Si.sub.3 N.sub.4), aluminum nitride (AlN) and alumina (Al.sub.2 O.sub.3), said sintered ceramic body having an electrical resistance of 50 M/.OMEGA. or more at 700.degree. C., the composition of said spark plug insulator body being within the area bounded by the straight lines connected points A, C, E, J, T, R and A shown in the three-component or triaxial diagram of FIG. 1, said three component diagram being made up of the compounds (Si.sub.3 N.sub.4), aluminum nitride (AlN) and alumina (Al.sub.2 O.sub.3).