Hot Pressed Si.sub.3N.sub.4 Ceramics Using MgO-Al.sub.2O.sub.3 as Sintering Additive for Vehicle Engine Parts

In an attempt to optimize a qualified ceramic material with enhanced and outstanding characteristics, the current work aims to study the processing of dense Si.sub.3N.sub.4 based ceramic composites by the hot pressing (HP) method. The proposed material consists of an equimolecular mixture of [alpha]- and [beta]-Si.sub.3N.sub.4 with the addition of various amounts of MgO-Al.sub.2O.sub.3 spinel as sintering additive. Investigation of the different characteristics of the obtained ceramics with respect to densification parameters, hardness, fracture toughness, and bending strength and cold crushing strength was performed and correlated with their microstructure behavior. Results indicated that a dense, well distributed structure of silicon nitride ceramics was attained by the addition of various amounts of magnesium aluminate spinel. Moreover, insertion of MgO-Al.sub.2O.sub.3 mixtures into Si.sub.3N.sub.4 ceramics was established to enhance their grain growth demeanor and increase the elongated [beta]-Si.sub.3N.sub.4 grains and their aspect ratio. Furthermore, it was revealed that increasing the MgO ratio relative to that of Al.sub.2O.sub.3 in the spinel structure was responsible for enhancing the different properties of the produced nitride ceramics. Results suggested that the obtained hot pressed Si.sub.3N.sub.4 ceramics seeded with [beta]- Si.sub.3N.sub.4 particles can be a successful candidate in the automotive industry as alternative parts for vehicle engines.