Influence of a multilayered matrix on the lifetime of SiC/BN/SiC minicomposites

The mechanical behaviour at room temperature and the lifetime in air at 700°C under static loading of SiC/BN/SiC minicomposites have been investigated. The minicomposites consisted of a single tow of Hi-Nicalon fibres coated with a BN interphase and a CVI-SiC matrix. For few of them, a BN layer was introduced within the matrix. All the minicomposites were heat treated at high temperature to improve the BN crystallinity. In some cases, the BN interphase was submitted to a specific treatment before the infiltration of the SiC matrix, to further improve its crystalline state. The differences in interfacial zone, as assessed by TEM, were correlated with those in mechanical properties. A significant improvement of the mechanical behaviour at room temperature and the lifetime of the minicomposites with a multilayered matrix was observed. The multilayered matrix is efficient when a silica layer on both sides of the BN layer within the matrix is present.