Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications,
because the ceramics can be made highly mechanical strength, fracture toughness, electrical
resistivity and high thermal conductivity. Generally, relatively large amount of additives are
required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including
surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy
phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering.
Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced
sintering process. Dense materials were sintered by this process, but their thermal conductivities
were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed
to reduce the excess amount of glassy phase.
An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as
the sintering aid. These ceramics could be sintered to almost full density at relatively low
temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials
have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added.
Based on the creation of low temperature pressureless sintering without packing powder, a novel
two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at
1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and
kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be
fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure,
mechanical properties, oxygen content and chemical composition were discussed.
Effects of processing parameters and heat treatment on thermal conductivity of additively manufactured AlSi10Mg by selective laser melting
