An investigation into a multilayered BN/Si3N4/BN interfacial coating

Abstract

In order to prevent environmental degradation of the interface, a triplex coating was employed as the interface in ceramic matrix composites (CMC). This interface consists of an initial BN layer followed by a Si3N4 layer and lastly another BN layer. Single strand unidirectional mini-composites using BN/Si3N4/BN coated ceramic grade Nicalon™ fibers as the reinforcement and chemical vapor infiltrated (CVI) SiC as the matrix were fabricated to understand the initial properties of the interfacial coating. Field emission scanning electron microscopy (FE-SEM) confirmed the thickness of the triplex coating before and after mini-composite fabrication. FE-SEM micrographs after mechanical and environmental testing of the single strand unidirectional mini-composites showed the consequences of using the triplex interfacial coating. Finally, eight ply continuous fiber reinforced (CFR) CMCs with the BN/Si3N4/BN triplex interface and the traditional BN/Si3N4 duplex interface were fabricated using the polymer impregnation and pyrolysis (PIP) process. The PIP process has gained popularity in recent years and this allows for the fabrication of larger CMC panels as compared with the CVI process. Mechanical testing for the PIP-fabricated CFR-CMC panels showed that the composites using the triplex interface had better mechanical properties than those fabricated with a BN/Si3N4 duplex interface after environmental testing.