Fabrication Of Si₃N₄ Composites And Porous Ceramics With Reaction-Sintering Method

Si3N4-BN, Si3N4-MoSi2-BN composites and Si3N4-based porous ceramics were fabricated by reaction-sintering with another phase adding by using Si powder as the main materials in this paper. The effects of raw materials ratio, sintering schedule, sintering aid and pore-forming agent on the phase compositions, microstructures and mechanical properties of Si3N4-based composites and porous ceramics were investigated. The synthesis of BN and transition behavior was investigated in order to improve the mechanical properties of Si3N4-BN composites firstly. And nearly nano-size flake BN was also synthesized by molten salt method with controlling reaction condition and the formation mechanism was discussed. Si3N4-BN composites were fabricated by reaction-sintering method and the effect mechanisms of sintering aids were discussed. Si3N4-MoSi2-BN composites were prepared by in-situ synthesis with adding Mo powder to the raw materials and the formation mechanism of Mo5Si3 was investigated. Porous Si3N4 ceramics were successfully fabricated by adding. pore-forming agent. And Si3N4-SiC composites were prepared with SiC adding to improve the strength of porous ceramics. In addition, Si3N4-BN composites were also fabricated with BN adding. The following conclusions were obtained.(1) Turbostratic boron nitride powder was prepared with boric acid and urea used as raw materials in nitrogen (N2) gas flow at 900℃. The transition behavior of t-BN to h-BN at different temperature with different additives was investigated. The results showed that the transition of t-BN to h-BN gets better increasingly with temperature rising and more additive contents, but the specific surface area decrease. When the content of sodium tetraborate was 9%, ball-liked and uniform size of h-BN was prepared and the crystallization of h-BN obtained at 1300℃was even better than which obtained at 1600℃without additions.(2) Nearly nano-size and flake BN was synthesized by molten salt method using boric acid and urea as raw materials and sodium tetraborate as salt. The effects of different sodium tetraborate content, reaction temperature and holding time on BN crystallization were investigated and the synthesis mechanism of BN was discussed. The results showed the crystallization of BN gets better increasingly with more sodium tetraborate contents, higher reaction temperature and longer holding time. And nearly nano-size and flake BN can be synthesized when the mass ratio of raw materials and sodium tetraborate is 1:1 at 1200℃with holding 2 hours.(3) Si3N4-BN composites withβ-Si3N4 as the main phase and a-Si3N4 the second were fabricated by reaction-sintering method using Si and BN powder as raw materials. The effects of different sintering schedules and BN contents on composites were examined. The different properties between composites obtained with synthesis BN and commercial BN was compared. And the effect mechanism of sintering aids was discussed. The results showed that all the three sintering aids such as CaF2, Fe2O3 and Al2O3+Y2O3 can accelerate the nitriding of Si and the transition of a-Si3N4 toβ-Si3N4. And also CaF2 can promote the formed of a-Si3N4 fibre. The porosity of composite was low and bulk density was high with Fe2O3 addition. But the bending strength of composite was high with Al2O3+Y2O3 addition. In addition, high strength and fine microstructure of Si3N4-BN composites can obtain by adding synthesized BN.(4) Si3N4-MoSi2-BN composites were obtained by the reaction-sintering method using Si, Mo and BN as raw materials. The effect of different Mo content on composites was investigated and the formation mechanism of Mo5Si3 was discussed. The results showed that apparent porosity of the obtained composites increased, the bulk density and bending strength decreased with increasing the Mo content. The in-situ synthesized MoSi2 were dispersed uniformly and their sizes were 2-3μm. In addition, the content of Mo5Si3 in the obtained composites increased with increasing the Mo content.(5) Porous Si3N4 ceramics were fabricated by reaction-sintering with adding pore-forming agent. The effects of presintering, different pore-forming agent and content on composites were investigated. The results showed that apparent porosity of the obtained composites increased and bending strength decreased with increasing the pore-forming agent content. The properties and microstructures of composites with adding KCl were better than which with urea. In addition, presintering can promote the strength of composites, but decreasing the porosity.(6) Porous Si3N4-SiC and Si3N4-BN composites were fabricated by adding SiC and BN in the raw materials respectively in order to improve the performance of porous Si3N4 ceramics. The effects of presintering, SiC and BN on composites were investigated. The results showed that bending strength of composites increased with SiC content increasing, but decreased with BN content increasing. And presintering can promote the strength of composites, but decreasing the porosity. In addition, small amounts of Si2N2O will be formed after presintering in the composites.