Spark plasma sintering of Si₃N₄-B₄C composites

Abstract

In this study the production of Si₃N₄-B₄C composites using spark plasma sintering technique was studied. This technique was preferred in order to minimize the reactions between starting constituents. Fully dense Si₃N₄ ceramic was obtained by SPS. The bulk densities of all the produced composites was found to be significantly less than that of Si₃N₄, and this was attributed to the formation of the relatively low-density reaction products. In the B₄C-containing composites there was a trend for the bulk density to increase slightly with decreasing particle size of B₄C powder. Composites containing both B₄C and TiO₂ had somewhat higher bulk densities when compared with those just containing B₄C. This was related to the formation of relatively high-density reaction products in significant amounts. The fine B₄C added to the Si₃N₄ base composition containing Al₂O₃ and Y₂O₃ as sintering additives, even when incorporated in significant amounts, was consumed readily during the fast sintering process. Using coarse B₄C particles reduced the reaction kinetics to some extent. As a result of reactions between Si₃N₄ and B₄C particles SiC, h-BN and metallic Si were formed. When both B₄C and TiO₂ were added together additional phases of Ti (C, N) and TiB₂ were formed. Possible reactions that explain the formation of the in-situ phases were proposed through thermodynamic considerations.