Development of mullite and mullite/Al2O3 precursor sols for electrophoretic deposition of oxidation protection coatings

Mullite and mullite/Al2O3 precursor sols have been developed for the deposition of oxidation barrier coatings on carbon fibre reinforced composites using a combination of sol–gel synthesis and electrophoresis. ls were synthesised by controlled hydrolysis and condensation of TEOS (tetraethoxysilane) and Al(OBus)3 (aluminium tri-sec-butylate). The main objective was the definition of synthesis conditions which yield sols suitable for the electrophoretic deposition (EPD). Measurements of the Electrokinetic Sonic Amplitude were used to investigate the electrokinetic properties of the sols in the as-prepared state and depending on the later addition of H2O. 29Si CP/MAS NMR spectra of dried precursor samples were recorded to study the homogeneity of Al/Si mixture. The progress of crystallisation with increasing temperature of heat treatment was examined by XRD. Oxidation protection coatings on C/C–SiC composites were prepared by EPD. s a low H2O to TEOS ratio during the sol synthesis was advantageous for a low mullite formation temperature, a high H2O to TEOS ratio was necessary to enable the EPD. The synthesis of a sol with a low H2O to TEOS ratio in the first step and the later modification of this sol by the addition of water was a successful method to combine the required electrokinetic properties and mullitisation temperatures below 1200 °C.