Mechanical properties and thermal conductivity of Si3N4 ceramics with YF3 and MgO as sintering additives

Si3N4 ceramic was densified by hot pressing sintering at 1750 °C for 1 h under the uniaxial pressure of 20 MPa in N2 atmosphere with YF3 and MgO as sintering additives. The thermal conductivities of SN-YF specimen were both higher than that of Si3N4 ceramic sintered with Y2O3 and MgO before and after annealing treatment. The grain size and aspect ratio in SN-YF specimen were both bigger than those in the SN-YO specimen, which was beneficial for the creation of high thermal conductive path. On the other hand, the improvement of thermal conductivity by the addition of YF3 might be attributed to the reduction of the grain boundary phase due to the evaporation of SiF4, and the resultant reduction of the lattice oxygen due to the reduction of SiO2 in the grain boundary phase. The {0001} direction of grains had the probability of growing along the hot pressing direction in the SN-YF specimen, which was beneficial for the improvement of thermal conductivity while the {0001} direction grew along the X0-Y0 plane in the SN-YO specimen. The mechanical properties of SN-YF specimen were comparable to those of SN-YO specimen.