Manufacture of long-fibre reinforced ceramics by reaction bonding
Licentiate thesis, 2009
For several decades ceramic matrix composites has been of interest to be used as hot parts in certain high-performance gas turbine aero engines. In general, ceramic materials resist higher temperatures than superalloys. If the parts in the combustion chamber are replaced by ceramic matrix composites, the combustion temperature can be increased, which improves the thermal efficiency and reduces the need of cooling of the surfaces with incoming bypassed air. The mechanical, physical and chemical requirements of ceramic matrix composites for such applications are extensive. This put demands on the manufacturing techniques of such composites. By utilizing continuous ceramic fibres as reinforcement, large “uniting” long-fibre reinforced composites can be manufactured. Normally, the fibres consist of fibre yarns containing up to a couple of thousands of filaments and these yarns often occur in the shape of woven weaves. One of the difficulties during manufacture is how to incorporate the ceramic matrix material in the fibre structure and how to fill all the pores between the yarns and the filaments. This can for instance be carried out by various optimized slurry infiltration routes.
The most difficult part is to obtain sufficient mechanical strength of the slurry infiltrated composites without deteriorating the properties of the fibres during heat treatment. Additionally, the fibres obstruct densification of the matrix since the fibres have already reached their final size. Since the target is to maximize the density of the matrix to attain as good mechanical properties as possible, alternative ways to fill out the pore network between the ceramic matrix powder and increase the strength must be developed. One such method is reaction bonding. By reaction bonding, a metal or metal alloy powder is oxidized or nitrided at elevated temperatures. The metal powder will react to its corresponding ceramic compound while the open porosity in the compact will be reduced significantly. Due to strong inter-granular bonding between particles, the obtained reaction bonded ceramic matrix will exhibit relatively good mechanical strength.
Two reaction bonding systems have been evaluated in this study, reaction bonded silicon nitride and reaction bonded mullite. The first is a nonoxide system and the second is an all-oxide system, both with continuous fibres suited for each system. It has been shown that both systems can be used to produce fairly good long-fibre reinforced ceramic composites.
ceramic matrix composite