Comparative study of fluoride and non-fluoride additives in high thermal conductive silicon nitride ceramics fabricated by spark plasma sintering and post-sintering heat treatment

Abstract High thermal conductivity silicon nitride (Si 3 N 4 ) ceramics fabricated using fluoride and non-fluoride additives were comparatively investigated in this study for the first time. Three series of sintering aids, namely, MgF 2 -Y 2 O 3 , MgSiN 2 -Y 2 O 3 and MgO-Y 2 O 3 were used to fabricate Si 3 N 4 ceramics by spark plasma sintering (SPS) at 1650 ℃ for 5 min and post-sintering heat treatment at 1900 ℃ for 4 h. Microstructure, thermal conductivity, and mechanical properties of Si 3 N 4 ceramics were comparative investigated. Results indicate that MgF 2 as sintering additive for Si 3 N 4 ceramics is able to effectively reduce the temperature of liquid phase in SPS process. The proportion of large grains is about 34% in fluoride-doped sample, which is more than those samples using MgSiN 2 and MgO as sintering aids (24 and 25%, respectively). Thermal conductivity of ceramics is 76, 64, and 60 Wm −1 K −1 for samples with MgF 2 , MgSiN 2 and MgO, respectively. Fluoride-doped sample displayed good mechanical properties including bending strength (857.6 MPa), hardness (14.9 GPa), and fracture toughness (7.7 MPa m 1/2 ).