Spark plasma sintering of Si_3N_4-B_4C composites(<Feature>SiAlons and Non-oxides)

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In this study the production of Si_3N_4-B_4C composites using spark plasma sintering technique was studied. This technique was preferred in order to minimize the reactions between starting constituents. Fully dense Si_3N_4 ceramic was obtained by SPS. The bulk densities of all the produced composites was found to be significantly less than that of Si3_3N_4, and this was attributed to the formation of the relatively low-density reaction products. In the B_4C-containing composites there was a trend for the bulk density to increase slightly with decreasing particle size of B4C powder. Composites containing both B_4C and TiO_2 had somewhat higher bulk densities when compared with those just containing B_4C. This was related to the formation of relatively high-density reaction products in significant amounts. The fine B_4C added to the Si_3N_4 base composition containing Al_2O_3 and Y_2O_3 as sintering additives, even when incorporated in significant amounts, was consumed readily during the fast sintering process. Using coarse B_4C particles reduced the reaction kinetics to some extent. As a result of reactions between Si_3N_4 and B_4C particles SiC, h-BN and metallic Si were formed. When both B_4C and TiO_2 were added together additional phases of Ti (C, N) and TiB_2 were formed. Possible reactions that explain the formation of the in-situ phases were proposed through thermodynamic considerations.
2008-06-01