Densification, microstructure and properties of electroconductive Si3N4–TaN composites. Part I: Densification and microstructure

Dense Si3N4–TaN composites with the TaN phase in the range of 5–50 vol.% were produced by a hot pressing technique in reducing (CO) and neutral (N2) atmospheres. Two types of ceramic items in the form of three dimensional components and functionally graded material were evaluated. The influence of densification parameters, amount and grain size of TaN particles and geometry of the functional zone on the microstructure, mechanical properties and electrical resistivity was investigated. It was found that the use of a coarse, non stoichiometric TaN powder as a second component of the composite, combined with a reducing atmosphere in the sintering process, resulted in the formation of a chain type network of the electroconductive phase. The formation of new phases such as SiC, Ta5Si3, TaC0.5Si in the composite body were also recorded. A quite different microstructure was observed when the composite material was sintered in a nitrogen atmosphere or a fine TaN powder was used. The latter composites were characterised by a ring-type network of the conductive phase in the insulating matrix. The conducting particles formed the monolithic and isometric structure of the conductive clusters. Apart from the Si3N4 and TaN such new phases as Ta5Si3N7 and TaSi2 were identified in the sintered composite material.