Thermochemical reactions and equilibria between fluoromicas and silicate matrices: A petromimetic perspective on structural ceramic composites

A petrominetic (geological-analog) approach is applied to the design of alu mina-fiber-reinforced glass-ceramic-matrix composites that use a refractory , trioctahedral fluoromica fiber-matrix interphase and feldspar matrixes. S tudies of the solid-state reaction couples between these silicate phases ar e pursued to address the chemical tailorability of the interphase/matrix in terface from an engineering perspective, The minimization of alumina and si lica activities within polyphase, feldspar-based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. A n anorthite-2-vol%-alumina (CaAl2Si2O8 + alpha-Al2O3) substrate, chemically buffered with MgO, is shown to exhibit thermodynamic stability against flu orokinoshitalite (BaMg3[Al2Si2]O10F2), up to temperatures potentially as hi gh as 1460 degrees C. The keg to the approach is the reduction of alumina a ctivity via the formation of MgAl2O4 spinel. Similarly, the formation of fo rsterite (Mg2SiO4) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between sol id-solution feldspars and fluoromicas also is characterized. Coupled interd iffusion of K+ and Si4+ in exchange for Ba2+ and APC was observed between K -Ba solid-solution celsian and the barium-rich solid-solution end-member fl uorokinoshitalite at 1300 degrees C. A similar cationic exchange also is ob served against the potassium-rich end-member fluorophlogopite (KMg3[AlSi3]O 10F2), although in a reverse direction, at temperatures of <1280 degrees C, The interfacial compositions identified via electron microprobe analysis s pecify one set of local equilibrium conditions from which global ceramic co mposite equilibrium can be achieved.