Sinter+HIP which is the most common HIP method is composed of sintering and HIPing processes. Both are performed separately and continuatively in two furnaces. On the other hand, in Sinter/HIP, sintering and HIPing is performed in one furnace. Therefore, Sinter/HIP eliminates extra heating-up, cooling, and handling in Sinter+HIP.
In the present work, at full dense Si3N4 sintered body which was added 6 wt% Y2O3 and 2 wt% Al2O3 as additives was tried to fabricate by Sinter/HIP, whose cycle was composed of the following stages;
1) hot evacuation; at 1 400°C, for 2 h, under 70 Pa in vacuum
2) gas pressure sintering; at 1 800°C, for 2 h, under 1 MPa of N2
3) HIPing; at 1 750°C, for 3.5 h, under 190 MPa of 0.5 vol% N2+Ar.
At the same time, gas pressure sintering was attempted in which its heating pattern was just the same as that of Sinter/HIP, but its gas pressure which was kept constant was 1 MPa of N2.
Three point bending strengths of Sinter/HIPed bodies measured at room temperature and 1 200°C were 1.09 GPa and 605 MPa, respectively. On the contrary, those of gas pressure sintered body were 814 MPa at room temperature and 420 MPa at 1 200°C. Porosity of Sinter/HIPed body was 0.15 % which was much smaller than 0.98% of porosity of gas pressure sintered body. Large pores of 310 μm were observed in gas pressure sintered body, whereas they were eliminated in Sinter/HIPed body. It was concluded that the increase in strength by Sinter/HIP was due to the decrease in porosity and elimination of large pores.