Melt-infiltration processes

Kurzfassung

CMC materials offer a unique combination of properties at high as well as at medium and low temperatures, which can not be achieved by any other materials like metals or monolithic ceramics. However, CMC materials are also very expensive materials due to high raw material and process cost. These drawback could be cut down partially by adding melt infiltration as an economically manufacturing process to well established CVI and PIP / LPI. MI processes are characterised by the use of low cost matrix precursors and carbon fibres, which, in some cases, even do not have to be coated, as well as by short process times. The SiC-matrix is build up almost simultaneously in the whole volume of the part in one single process step and no multiple infiltration cycles are needed to densify C- as well as SiC fibre reinforced CMC. Therefore, even very large, complex shaped and thin walled lightweight structures as well as very thick walled components can be realized in near net shape technique. Compared to CMC materials derived from CVI and LPI, MI based C/SiC, C/C-SiC and SiC/SiC materials offer significantly lower open porosities, leading to higher shear strength and thermal conductivity. However, lower tensile strength of C/SiC and C/C-SiC as well as limited lifetime of SiC/SiC materials caused by free Si in the matrix, attacking the fibres in long term use at high temperatures are still a challenge. After more than 20 years of development and of gaining operational experiences in first aerospace applications, MI-CMC materials have just found their way out of the laboratories into high performance products for industrial market. Typical application areas are TPS structures for spacecraft, hot structures for rocket propulsion, gas turbines and nuclear reactors, thermally stable structures and friction materials. The introduction of C/SiC automotive brake disks in serial production is a breakthrough and an important milestone in CMC technology, offering a high potential for further applications of C/SiC and C/C-SiC materials in other industrial areas. The further development will not only be focused on low cost processes and the improvement of material properties and environmental barrier coatings, but will also concentrate on providing design and simulation tools in combination with convincing, non destructive evaluation methods. With the later, scientists and engineers should be able to reliable calculate lifetime as well as damage and failure behaviour, which is a basic prerequisite for future applications of CMC components in very promising, but challenging, safety critical applications fields.