Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation‐Resistant Ceramic Composites

Reaction‐couple experiments have been pursued in order to evaluate the potential of a phyllosiiicate to act as a chemically protective, fracture‐deflecting, oxidation‐resistant interphase for oxide fiber–oxide matrix composites. The synthetic mica fluorophiogopite (KMg 3 [AlSi 3 ]O 10 F 2 ) was reacted with single‐phase substrates of alumina (Al 2 O 3 ), mullite (3Al 2 O 3 ·2SiO 2 ), forsterite (Mg 2 SiO 4 ), or enstatite (MgSiO 3 ). X‐ray spectroscopy, X‐ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reaction couples. Fluorophlogopite reacts strongly with alumina, mullite, and enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica–alumina and mica–mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl 2 O 4 ) layer separating the substrate from the breakdown products of the mica. This unvarying result suggests, therefore, that a spinel diffusion barrier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is established in the layer sequence alumina–spinel–fluorophlogopite; i.e., planar interfaces are established amongst these phases that are stable under conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on an understanding of its intrinsic fracture energy, the fluoromica interphase is expected to be effective in mechanically protecting adjacent oxides from propagating cracks, a behavior qualitatively demonstrated by indentation experiments on the kinetically persistent alumina–spinel–fluorophlogopite–spinel–alumina laminates.