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Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices: A Petromimetic Perspective on Structural Ceramic Composites
Author(s) -
King Todd T.,
Grayeski Walter,
Cooper Reid F.
Publication year - 2000
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2000.tb01549.x
Subject(s) - materials science , forsterite , anorthite , solid solution , silicate , spinel , composite material , ceramic , cationic polymerization , chemical engineering , gehlenite , ceramic matrix composite , mineralogy , phase (matter) , chemistry , metallurgy , organic chemistry , polymer chemistry , engineering
A petromimetic (geological–analog) approach is applied to the design of alumina‐fiber‐reinforced glass‐ceramic‐matrix composites that use a refractory, trioctahedral fluoromica fiber–matrix interphase and feldspar matrixes. Studies of the solid‐state reaction couples between these silicate phases are pursued to address the chemical tailorability of the interphase/matrix interface from an engineering perspective. The minimization of alumina and silica activities within polyphase, feldspar‐based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. An anorthite–2‐vol%‐alumina (CaAl 2 Si 2 O 8 +α‐Al 2 O 3 ) substrate, chemically buffered with MgO, is shown to exhibit thermodynamic stability against fluorokinoshitalite (BaMg 3 [Al 2 Si 2 ]O 10 F 2 ), up to temperatures potentially as high as 1460°C. The key to the approach is the reduction of alumina activity via the formation of MgAl 2 O 4 spinel. Similarly, the formation of forsterite (Mg 2 SiO 4 ) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between solid‐solution feldspars and fluoromicas also is characterized. Coupled interdiffusion of K + and Si 4+ in exchange for Ba 2+ and Al 3+ was observed between K‐Ba solid‐solution celsian and the barium‐rich solid‐solution end‐member fluorokinoshitalite at 1300°C. A similar cationic exchange also is observed against the potassium‐rich end‐member fluorophlogopite (KMg 3 [AlSi 3 ]O 10 F 2 ), although in a reverse direction, at temperatures of <1280°C. The interfacial compositions identified via electron microprobe analysis specify one set of local equilibrium conditions from which global ceramic composite equilibrium can be achieved.