Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

Abstract

In this study, the mechanism of the effect of ZrB2 on phase transformation of Si3N4 at a low temperature and the influence of its content on Si3N4-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B2O3 and ZrO2 on ZrB2 particles, alone cannot contribute to phase transformation of Si3N4 at a low temperature. But, the introduction of 0.5 vol% ZrB2 into Si3N4 ceramics can promote the α-β phase transformation of Si3N4, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB2 content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si3N4-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB2 increases to 5 vol%, the Si3N4-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si3N4-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB2.