Preparation of Reaction Bonded Silicon Nitride (RBSN) through commercial silicon gel-casting and investigation the effect of premix pH and sintering aid compounds on silicon nitride formation and porosity body

The aim of the present study was to manufacture porous Si3N4 ceramic through gel-casting of Si powder and RBSN method. The effects of gel solution pH and amount of sintering aids were investigated. Acrylamide  and Methylenebisacrylamide were used as monomers solution (ratio of 15 AM: 1 MBAM). Al2O3, Y2O3, and SiO2 powders (as sintering aids) and Si (as main powder) were added to obtain a slurry with solid loading of 60wt%. Heat treatment and nitridation process were carried out up to 1350ºC and in N2 atmosphere for 22 hr. Results of XRD analysis showed that the final phase in all samples was metallic Si and β-Si3N4 along with some percentages of cristobalite. Analysis of the effects of pH on the final phase of  the samples made of Si powder (pure and without sintering aids) revealed that preparation of the gel solution in acid environment (pH:2~3) led to an increased intensity of β-Si3N4 phase peaks and reduced metallic Si phase in comparison with the alkaline environment (pH:11~12), and the slurry prepared in the acidic environment reacted more effectively with N2 atmosphere. Regarding the effect of sintering aids on the formation of the final phase, the findings suggested that increasing the amount of SiO2 and reducing Al2O3 and Y2O3 did result in the formation of more β- Si3N4 (from 66% to 80%) and decrease in the Si metal (from 18% to 7%). Moreover, increased amount of sintering aids and formation of more liquid phase facilitated the proces of β-Si3N4.