Microstructure, mechanical, wave-transparent and heat insulation properties of Si₃N₄ foam ceramic by organic foam impregnation combined with CVI

An ultralight and high-strength Silicon nitride (Si₃N₄) foam ceramic with highly efficient wave-transparent and heat insulation properties was successfully prepared using the organic foam impregnation technique combined with chemical vapor infiltration (CVI) process. The microstructure of Si₃N₄ foam ceramic inherits the pore structure characteristics of the template, and the ceramic skeleton is formed by coupling the Si₃N₄ whiskers with the CVI Si₃N₄ matrix. The use of CVI process avoids the volume shrinkage problem during the ceramic formation process by traditional sintering method at high temperature, and ensures the near-net-shape preparation of ceramic. Meanwhile, CVI Si₃N₄ matrix has pure components without impurities like low melting point metal oxides by sintering method or free carbon by polymer-derived ceramic method, which is beneficial to the improvement of wave-transparent ability and thermal stability. The addition of whisker reinforcement greatly increases the compressive strength of ceramic through whisker pulling out, crack deflection and other strengthening and toughening mechanisms. Moreover, we discuss in detail the relationship between the structural parameters of Si₃N₄ foam ceramic, such as whiskers-matrix solid content ratio, open-closed porosity ratio, pore size and mechanical, wave-transparent as well as heat insulation properties. In this study, the compressive strength of 40PPI Si₃N₄ foam ceramic with total porosity of 91.97% is 1.06 MPa, the thermal conductivity is only 0.624 W/(m·K) at 25 °C and the dielectric constant and loss tangent are 1.40 (10 GHz) and 6.35 × 10⁻³ (10 GHz), respectively.