Abstract
The use of manufacturing methods commonly used for polymer matrix composites (PMCs) in the production of ceramic matrix composites (CMCs), as opposed to more traditional ceramic manufacturing methods, has the potential to reduce the cost of CMC components. To date, the work in oxide/oxide ceramic matrix composite has been focused on flat laminates, while to enhance the adoption of these material in, for instance, the aerospace sector, complex shapes are required.
In this study, the viability of processing a commercially available Nextel 610®/alumina-silica matrix pre-impregnated material using three typical PMC manufacturing methods (vacuum bag, autoclave process and warm press) has been investigated. From these methods, autoclave processing was chosen for further optimisation, as it resulted in the best properties combination, including thickness uniformity, macro-porosity shape and distribution, and 4-pt flexural strength.
The key process parameters associated with autoclave processing were assessed, e.g., ply layup sequence, debulking steps (application of vacuum during layup process to obtain near net shapes), autoclave parameters and sintering cycle. These parameters were then related to physical properties, porosity, microstructural features and flexural strength. The main outcome was, that the sintering cycle had a major effect on the nano-porosity content of the laminates, which also affected the failure mode.
There are further considerations if complex shapes are to be produced. The locking angle of the prepreg, which limits the maximum distortion of fibres that can lead to wrinkles, was measured using picture frame testing and simple shapes were manufactured to understand the limit of the fibre conformability. Additional modifications were required to produce these features, e.g., consumables modifications to avoid material wrinkling as well as allowing through thickness bleeding or adjusting the autoclave cycle to avoid matrix leak in the internal radii. It was found that even with those modifications, 5 mm radius was the fibre limit.
To verify the appropriateness of the manufacturing process developed, a small-scale complex shape demonstrator was designed and manufactured. The focus was primarily modifications to the ply designs (shapes), as four trials showed that this parameter was critical to producing a component. Alongside the demonstrators, 3 mm thick laminates were manufactured to understand the effect of the modifications on the laminate uniformity, microstructure and mechanical properties. Compared to a baseline panel, the changes to the manufacturing resulted in a drop in mechanical properties as well as a change in the failure mode. Nevertheless, the potential for use of PMC techniques to manufacture CMCs has been demonstrated.