Applications of CMCs Made via the Liquid Silicon Infiltration (LSI) Technique

Abstract

Ceramic matrix composites (CMC) are very promising materials for use in structural applica-tions when one considers their low mass, excellent thermal shock stability and strength at high temperatures. There are different processes for manufacturing complex shaped CMC components based on a fibre architecture of continuous carbon or silicon carbide fibres. DLR in Stuttgart has developed the liquid silicon infiltration (LSI) process which allows the fabrication of thin walled, extremely lightweight C/C-SiC structures. It is a three step pro-cess starting with carbon fibre reinforced polymer (CFRP) manufacture followed by carboni-zation and conluding with liquid silicon infiltration. The advantage of the LSI process is that it is a near net shape process without reinfiltration steps. Moreover, the LSI process enables the joining of substructures in-situ without the need for additional metallic bolts or ceramic. An homogeneous, and therefore strong, joint can be produced by implementing molten silicon as a joining material which reacts with carbon, ei-ther from the joining specimen or introduced as a paste to the joining surfaces, to stoichiomet-rically form silicon carbide so the joint is as strong and thermally stable as the basis C/C-SiC material. The typical areas of application for CMC lie where metals, or superalloys, can no longer be taken into consideration due to high thermomechanical loading, and contains all areas of lightweight structures. However, if the application is in an oxidising atmosphere carbon burn-out of both the fibres and the matrix will start at temperatures as low as 450 °C. As a conse-quence, carbon based CMCs can only be utilised for limited lifetime applications. Even oxi-dation protective coatings will only reduce material degradation rather than to prevent oxida-tion completely. The LSI process and examples of CMC applications are discussed in this paper. Successful applications for the high temperature regime are space reentry structures. Commercially at-tractive applications within the medium temperature regime take advantage of the excellent tribological properties (e.g. brake disks) of the material. For temperatures below 450 °C (car-bon burn-out), CMCs are suitable for expansion sensitive applications due to their near to zero coefficient of expansion.