Formulation of oxide suspensions for liquid processing of ceramic matrix composites

International audienceThis study deals with the formulation of oxide suspensions for liquid processing of ceramic matrix composites. Those processes are more advantageous than conventional ones regarding: cycle time, quality, flexibility and cost. To properly impregnate the fiber reinforcement, the suspension must be stable, well dispersed and have a low viscosity (< 1 Pa.s). Regarding those requirements, the dispersion of an alpha-alumina ultrafine powder (AKP50, d50 = 0.3 µm, Sumitomo Chemical) in aqueous suspension, using an ammonium polymethacrylate (Darvan® C-N, Vanderbilt Minerals) as dispersant, was investigated by zeta potential, sedimentation and rheological measurements. The dispersant concentration minimizing the viscosity was found to be 0.26 wt.%. Moreover, this concentration permits to shift the isoelectric point (IEP) from pH = 9.5 (without dispersant) to pH = 5.1. The influence of powder concentration on suspension viscosity is well described by aQuemada model. The maximum volume fraction was found to be equal to 40.16 vol.%, thanks to the viscosity. No significant sedimentation was observed, regarding the operating time of the process. This work comprises also the study of a commercial suspension of colloidal silica (Ludox AS-40, 40 wt.% suspension in water, W.R. Grace & Co.-Conn.). The zeta potential is negative from pH = 2 to pH = 11. The impact of dilution on viscosity was studied. For all powder loadings (from 1 to 40 wt.%), the viscosity is lower than 1 Pa.s (Newtonian) and the variation is also well described by a Quemada model. No sedimentation was observed. Furthermore, the mixture of both suspensions in a stoichiometric ratio of 3Al2O3-2SiO2, corresponding to mullite, was investigated. The natural pH and the IEP were measured at pH = 9.5 and pH = 3.0, respectively. The viscosity, as a function of solid concentration, is well described by a Quemada model. No significant sedimentation was observed