Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si(3)N(4) Ceramics

In this study, coarse Beta silicon nitride (β-Si(3)N(4)) powder was used as the raw material to fabricate dense Si(3)N(4) ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si(3)N(4) ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si(3)N(4) grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si(3)N(4) powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m(1/2)). The addition of partial fine α-Si(3)N(4) powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si(3)N(4) ceramics SPS sintered with fine α-Si(3)N(4) powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m(1/2), and thermal conductivity of 71 W·m(−1)·K(−1).