Thermal conductivity and mechanical properties of Si3N4 ceramics with binary fluoride sintering additives

Silicon nitride (Si3N4) ceramics were fabricated by gas pressure sintering (GPS) using four sintering additives: Y2O3-MgO, Y2O3-MgE2, YF3-MgO, and YF3-MgF2. The phase composition, grain growth kinetics, mechanical properties, and thermal conductivities of the Si3N4 ceramics were compared. The results indicated that the reduction of YF3 on SiO2, induced a high Y2O3/SiO2 secondary phase ratio, which improved the thermal conductivity of the Si3N4 ceramics. The depolymerization of F atom reduces the diffusion energy barrier of solute atom and weakens the viscous resistance of anion group, which was beneficial to grain boundary migration. Besides exhibiting a lower grain growth exponent(n = 2.5)and growth activation energy (Q = 587.94 +/- 15.35 kJ/mol), samples doped with binary fluorides showed excellent properties, including appreciable thermal conductivity (69 W m(-1) K-1), hardness (14.63 +/- 0.12 GPa), and fracture toughness (8.75 +/- 0.18 MPa m 1/2), as well as desirable bending strength (751 +/- 14 MPa).