Microstructural Evolution in Triaxial Porcelain

Abstract

Microstructural evolution in a model triaxial porcelain was studied by X‐ray diffractometry and electron microscopy of quenched samples after firing for 3 h at 600°–1500°C. The clay component dehydroxylated to metakaolin at ∼550°C. Metastable sanidine formed from decomposition of the feldspar at >600°C and dissolved at >900°C. Liquid formation at ∼1000°C was associated with melting of feldspar and silica discarded from metakaolin formation via the K2O–Al2O3–SiO2 eutectic. Liquid content increased at 1000°–1200°C with further feldspar melting and additionally at >1200°C because of quartz dissolution. Small (≤7 nm) mullite and γ‐alumina crystals precipitated in pure clay relicts and larger (≤30 nm) mullite crystals in mixed clay‐feldspar relicts at 1000°C. In the evolving microstructures, three regions were observed. These regions were derived from pure clay relicts containing primary (type‐I) mullite; feldspar‐penetrated clay relicts, also containing secondary (granular type‐II) mullite; and the matrix of fine clay, feldspar, and quartz, containing secondary (granular type‐II and elongated type‐III) mullite. In addition to shape, the mullite size changed, increasing from regions containing type‐I to type‐III mullite, because the increasingly fluid liquid enhanced crystal growth. Below 1300°C, primary mullite was richer in Al2O3 than the secondary mullite, and the glass composition was inhomogeneous, with the K2O and Al2O3 contents varying throughout the microstructure. Above 1400°C, mullite began to dissolve.