Abstract: A silicon nitride based composition for manufacturing sintered ceramic articles, in particular cutting tool inserts, having improved density, hardness and fracture toughness characteristics is described. The amounts of yttrium oxide, aluminum nitride and titanium nitride contained in the silicon nitride based mixture are interrelated by a formula to attain substantially improved abrasion resistance.

Abstract: A composition is described for hard sintered ceramic articles having densities which are 97% or higher than the theoretical density. The composition contains up to 98.5 vol. % titanium diboride, 0-59 vol % aluminum nitride, 0-59 vol. % hexagonal boron nitride, 0-59 vol. % zirconium nitride, and in addition, zirconia, hafnia or ceria having particle size 0.7 .mu.m or less, in 1.5 vol. % and optionally, a sintering aid in less 2 vol. %. The mixture of the above components is further mixed and ground by ball-milling and vibro-milling in the presence of an aliphatic alcohol, cast into required shapes and sintered in an inert gas at temperatures below 1950.degree. C. The sintered ceramic articles obtained of this composition include ceramic cutting tool inserts, impact-resistant ceramic structures, nozzles, extrusion dies and ceramic evaporating boats.

Abstract: An improved ceramic heater to be incorporated in a glow plug is described. The heater is made up of a ceramic core enclosed by two layers sintered together to form a unitary ceramic heater. All three component elements of the heater are composed of silicon nitride containing different concentrations of an electrically conductive ceramic substance, such as titanium nitride, titanium carbonitride or molybdenum disilicide. Each component additionally contains low amounts of sintering additives. The core of the heater has the highest concentration (46-75 vol. %) of the electrically conductive substance. The core enclosed in a ceramic layer which is an electrical insulator and is composed of silicon nitride, less than 28 vol. % of the electrically conductive ceramic substance and sintering additives. An outer layer over the core contains the electrically conductive ceramic substance in a concentration (33-50 vol. %) which is in between that in the core and that in the insulator layer.