Impact of spark plasma sintering (SPS) on mullite formation in porcelains

The effect of the spark plasma sintering (SPS) process on mullite formation in porcelains was studied using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. SPS affected the kinetics and morphology of formed mullite. After sintering at 1100°C, unlike conventional sintering, SPS promoted the formation of mullite due to the combination of vacuum and applied pressure. Mullite crystal growth was altered by the atmosphere (vacuum), dwell time (0‐15 minutes), and temperature (1000‐1200°C). The applied pressure caused the mullite needles to orient perpendicular to the direction of the applied load. Depending on SPS dwell time, the mullite formed after sintering at 1100°C also had different crystal structure (tetragonal for short dwell time of 0‐5 minutes and orthorhombic for a long dwell time of 10‐15 minutes). Dissolution of mullite was observed at 1100°C by extending the dwell time by up to 15 minutes and the dissolved mullite reprecipitated on the small needles (~40 nm) and coarsened via Oswald ripening resulting in larger mullite needles (~60 nm).