Mullite monoliths, coatings and composites from a preceramic polymer containing alumina nano-sized particles
Pressed silicone resin powders containing alumina nano-particles enabled the production of dense and crack-free mullite monoliths upon heat treatment at 1250°C in air. The chemical interaction between the preceramic polymer and the nano-filler determined a low energy barrier of nucleation (<700 kJ/mol) for the nucleation of mullite, with a high yield (about 80 vol%, after 100 s at 1350°C). The composite powders were obtained by dissolving the silicone resin in a solvent and dispersing the nano-sized alumina filler particles, followed by drying. The drying of the suspensions directly on a substrate, such as Si-SiC monoliths or SiC open-celled foams, allowed the preparation of thin and strongly bonded anti-oxidation coatings (tested in air at 1300°C). With the introduction of secondary micro- and nano-powders (mullite, yttria-stabilized or non-stabilized zirconia) in the suspensions, tetragonal zirconia-reinforced mullite matrix composites and thick mullite coatings on Si -SiC were obtained, after treatment at 1350°C for 1-3 h.