Fabrication and flexural strength of porous Si3N4 ceramics with Li2CO3 and Y2O3 as sintering additives

By employing sintering additives of Li2CO3 and Y2O3,porous Si3N4 ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550°C,respectively.The experimental results demonstrate the completed phase transformation fromαtoβ-Si3N4 in Si3N4 ceramic samples with a amount of 1.60 wt%Li2CO3(0.65 wt%Li2O)and 0.33 wt%Y2O3 additives.The as-synthesized porous Si3N4 ceramics exhibit high flexural strength((126.7±2.7)MPa)and high open porosity of 50.4%at elevated temperature(1200°C).These results are attributed to the significant role of added Li2CO3 as sintering additive,where the volatilization of intergranular glassy phase occurs during sintering process.Therefore,porous Si3N4 ceramics with desired mechanical property prepared by altering the addition of sintering additives demonstrate their great potential as a promising candidate for high temperature applications.

Microstructure and brazing mechanism of porous Si_3N_4/Invar joint brazed with Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler

Porous Si3N4 was brazed to Invar alloy in this study, and Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was designed to inhibit the formation of Fe2Ti and Ni3Ti intermetallic compounds. The effects of the brazing temperature and the thickness of Cu interlayer on the microstructure and mechanical properties of brazed joints were investigated. The typical microstructure of the joint brazed with multi-layered filler was porous Si3N4/TiN ＋ Ti5Si3/Ag-Cu eutectic[Cu[Ag-Cu eutectic/Cu-rich layer ＋ diffusion layer/Invar. When the brazing temperature increased, the reaction layer at the ceramic/filler interface grew thicker and the Cu interlayer turned thinner. As the thickness of Cu interlayer increased from 50 to 150 μm, the joint strength first increased and then decreased. In this research, the maximum shear strength （73 MPa） was obtained when being brazed at 1173 K with a 100 μm Cu interlayer applied in the filler, which was 55% higher than that brazed with single Ag-Cu-Ti brazing alloy and had reached 86% of the ceramic. The release of residual stress and the barrier effect of Cu interlayer to inhibit the formation of Fe2Ti and Ni3Ti intermetallics played the major role in the improvement of joint strength.

Effect of agarose content on microstructures and mechanical properties of porous silicon nitride ceramics produced by gelcasting

Porous Si3N4 self-reinforce ceramics were prepared by gelcasting using agarose solutions. By changing the agarose content in the slurries, the porous silicon nitride ceramics with different porosities, α→β-Si3N4 phase transformation, and mechanical properties were obtained. When the agarose content changed from 0.2% to 0.8% (w/w, based on powder), the porosities increased from 10.3% to 21.4%, while the fracture strength decreased from 455 to 316 MPa and the fracture toughness decreased from 6.6 to 5.5 MPa·m1/2. Many fibrous β-Si3N4 grains grown from the internal wall of the round pores is the typical microstructure of the gelcasting porous silicon nitride ceramic. Both elongated β-Si3N4 grains and suitable interfacial bonding strength contributes to high fracture toughness by favoring crack deflection and bridging. The growth mechanisms of fibrous grains resulted from the synergy of solution-diffusion-reprecipitation and vapor-liquid-solid (VLS).

Fabrication and Properties of Porous Si_3N_4 Ceramic with High Porosity

A novel process combining oxidation-bonding and sintering was developed to fabricate porous Si3N4 ceramic with high porosity. After sintering at 1800℃, the SiO2 in porous Si3N4 ceramic transforms into Si2N2O because of the reaction of SiO2 and Si3N4 . Due to the reaction of SiO2 and carbon, the porosity of porous Si3N4 ceramic increases obviously from 40.2% to 76.8% with the weight decreasing by 35.6%. As the porosity increases, the dielectric constant and loss of the porous Si3N4 ceramic decrease obviously from 3.08 to 1.61 and from 3.70×10（-3） to 0.74×10（-3） , but due to the production of Si2N2O whose mechanical properties are much higher than SiO2 and the increase of the bonding strength among Si3N4 particles, the flexural strength and the fracture toughness of the porous Si3N4 ceramic decrease from 55 to 39 MPa and from 0.7 to 0.5 MPa·m（1/2） , respectively, but its Vickers hardness increases from 1.2 to 1.3 GPa.