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 phyllosilicate to act as a chemically protective, frac ture-deflecting, oxidation-resistant interphase for oxide fiber-oxide matrix composites. The synthetic mica fluorophlogopite (KMg3[AlSi3]O10 F2) was reacted with single-phase substrates of alumina (Al2O3), mulli te (3Al2O3.2SiO2), forsterite (Mg2SiO4), or enstatite (MgSiO3). X-ray spectroscopy, X-ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reacti on couples. Fluorophlogopite reacts strongly with alumina, mullite, an d enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica-alum ina and mica-mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl2O4) lay er separating the substrate from the breakdown products of the mica. T his unvarying result suggests, therefore, that a spinel diffusion barr ier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is establi shed in the layer sequence alumina-spinel-fluorophlogopite; i.e., plan ar interfaces are established amongst these phases that are stable und er conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on a n understanding of its intrinsic fracture energy, the fluoromica inter phase 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-spine l-fluorophlogopite-spinel-alumina laminates.