Design,thermal shock resistance of dense BaO-Al2O3-SiO2 glass/Si3N4-barium feldspar coating for porous Si3N4 ceramic

Silicon nitride-monoclinic barium feldspar (Si₃N₄-m-BAS) composite possesses great dielectric properties, low density, and low thermal expansion coefficient (CTE). Preparing dense Si₃N₄-m-BAS coating on porous Si₃N₄ ceramic is an effective strategy to improve its water resistance and ensure its dielectric performances. However, this promising coating has not been reported yet, because the synthesis of m-BAS is difficult, and the densification of Si₃N₄-BAS composite requires very high temperature. Here, the BaO-Al₂O₃-SiO₂ glass/Si₃N₄-BAS coating was first fabricated by a manual spray method and pressureless sintering at 1450°C. Combining the influence of Si⁴⁺ on the crystal phase composition of BAS and the volume expansion effect of silicon in N₂, an effective coating structure design scheme was proposed. By changing the content of silicon powder, the CTE and horizontal shrinkage of the coating during sintering were controlled. Besides, the prepared coatings exhibited low water absorption and high bonding strength. During the thermal shock tests, SiO₂ produced by the oxidation of Si₃N₄ healed the cracks in the coating, thus delaying the degradation of the properties. The coating prepared in this work is expected to be applied to radome in extreme service environments.