Environmental standards require the use of new materials to increase the performance of propulsion systems. Given their high mechanical strength and stiffness and low density, ceramic materials could allow the required increase in operating temperatures. To overcome their brittleness, ceramic matrix composites (CMC) are developed. Especially, oxide/oxide CMCs composed of an oxide ceramic matrix reinforced with oxide fibres, are considered for thermomechanical applications at 600 - 800 °C. One major difference with other CMCs is the stability of the oxide fibres in oxidizing atmosphere. High porosity in the matrix, so called “weak matrix”, enables damage tolerance into the composite by crack deflexion without the need of fibre coating or interphase. However, the main obstacle to the industrialisation of these materials remains their production cost, which mainly comes from the fibres (up to 70%).
In this work, a new processing route for oxide/oxide CMC manufacturing is investigated: tow pre-impregnation (tow-preg). It consists of impregnating NextelTM 610 alumina fibre tows with an aqueous alumina slurry, on an automated impregnation line. This allows both automation of the impregnation step and reduction in fibre cost. In this way, the slurry must meet several criteria, such as being stable, having a viscosity compatible with the process and the desired amount of matrix. Moreover, the impregnated tows must have sufficient flexibility and powder adhesion to make easier the handling of the tow-preg and the shaping of the composite.
As the initial slurry developed for pre-preg oxide/oxide composite does not fulfil the mentioned requirements of this new process, a formulation and rheological study has been performed to optimise the slurry. Thus, the addition of different organic additives was assessed regarding the processability on the impregnation line. Different natures of polymers (polyethylene glycols PEG or Sorbitol), and in various ratios were investigated (2-40 wt.%). Interactions between the different components of the slurries were analysed. The rheology of the slurries was studied, so as to achieve viscosities within the operating ranges. The aspect of impregnated tows has been characterized, especially in terms of flexibility and powder adhesion. Then, CMCs have been developed from these tow-pregs, by autoclaving and sintering. The microstructures of the tow-pregs and the CMCs were studied by SEM (Scanning Electron Microscope) observations.
The addition of an organic additive increases the viscosity of the slurry, which needs to be adjusted. On the one hand, a slurry with a little amount of PEG does not give a proper handling and powder adhesion to the tows, but provides a CMC with the desired microstructure (25vol.% porosity, 47vol.% fibres). One the other hand, a high amount of a hygroscopic additive (Sorbitol) can reactivate the flexibility of the tows by humidity conditioning. This greatly simplifies the elaboration of CMCs from tow-pregs, but causes problems of compaction and increases the porosity in the resulting CMCs. Hence, thermal analyzes were carried out to adapt the process and obtain the desired fiber content and porosity, in order to achieve mechanical properties similar to pre-preg materials.