J Rebelo Kornmeier et al. Materials Characterization 58(2007)922-92 1. 2. Description of the C/SiC nozzle fabrication great importance to determine the residual stress state of the C/SiC composite nozzles Several processing methods have been developed for fabricating continuous fibre ceramic matrix composites. 1.4. Connection between C/SiC nozzle and metallic The C/SiC composites are usually manufactured by an component infiltration processes. In the following the Liquid Polymer Infiltration(LPD) process, which was used for In order to connect the C/SiC composite nozzle to a the samples analysed in this study, will be briefly metallic ring a special brazing method was developed 3]. Prior to sold Here, the C/SiC composite is made via the so called composite is perforated with a solid state small impulse polymer route. A carbon fibre bundle coated with a laser NdYAG, see Fig. 1(a). The perforation method was polymer is impregnated with a powder-filled polymer, optimised with respect to the optimal shape, dimensions the precursor, and laminated to form prepregs. Subse- and distributions of the perforations and the surface area quently, the wound fibre cloth structure is laminated, to be perforated. The cavities were then filled with compacted in an autoclave and cross linked. Afterward solder in a high vacuum, Fig. 1(b)3 this green composite is pyrolysed without pressure and without moulding tools at temperatures around 1300- 2. Experimental 1900 K in an inert gas atmosphere. Such processes are relatively flexible since the composition of the precursor 2.1. Tomography an be tailored. A shrinking of the matrix occurs during the pyrolysis step owing to the generation of gaseous Tomography in general is a method which provides ecies. As a consequence, several pyrolysis sequences cross sectional images of an object from transmission and re-impregnations have to be applied in order to data, obtained by irradiating it with specific radiation achieve a low enough residual porosity [1] from many different directions. From these projections a tomographic image is then mathematically recon- 1.3. Residual stress of the fibre ceramic matrix structed. Here the projection at a given angle represents composite material (a) Generally, temperature gradients occurring within the omponent during the fabrication processes can lead to residual macros-stresses on the scale of the component. Moreover, when the material has more than one phase phase-specific residual stresses arise during cooling as a consequence mainly of the difference of the thermal expansion coefficient of the phases, i.e. between fibre and the matrix in the case of composite materials. The phase-specific residual stresses of one phase is the sum of residual macro stresses plus load stresses plus micro residual stresses of this phase [2]. The material resistance and consequently the lifetime of the compos- (b) ite component under service conditions depends on the residual stress state. The fracture of ceramic matrix composites is usually governed by matrix cracking followed by interactions of the newly-formed cracks with fibres and interfaces such as fibre pull-out and debonding. These fracture mechanisms are activated by tensile stresses acting parallel to the fibre axis. Furthermore, it is obvious that the stresses acting norma to the fibre axis have an important influence on the load transfer from the fibre to the matrix as well as on the debonding process. Therefore, in order to optimise the aterial behaviour for certain external loads. it is of Fig. 1.(a) Laser perforation; (b) cavities filled with solder.1.2. Description of the C/SiC nozzle fabrication Several processing methods have been developed for fabricating continuous fibre ceramic matrix composites. The C/SiC composites are usually manufactured by an infiltration processes. In the following the Liquid Polymer Infiltration (LPI) process, which was used for the samples analysed in this study, will be briefly described [1]. Here, the C/SiC composite is made via the so called polymer route. A carbon fibre bundle coated with a polymer is impregnated with a powder-filled polymer, the precursor, and laminated to form prepregs. Subse￾quently, the wound fibre cloth structure is laminated, compacted in an autoclave and cross linked. Afterwards this green composite is pyrolysed without pressure and without moulding tools at temperatures around 1300– 1900 K in an inert gas atmosphere. Such processes are relatively flexible since the composition of the precursor can be tailored. A shrinking of the matrix occurs during the pyrolysis step owing to the generation of gaseous species. As a consequence, several pyrolysis sequences and re-impregnations have to be applied in order to achieve a low enough residual porosity [1]. 1.3. Residual stress of the fibre ceramic matrix composite material Generally, temperature gradients occurring within the component during the fabrication processes can lead to residual macros-stresses on the scale of the component. Moreover, when the material has more than one phase, phase-specific residual stresses arise during cooling as a consequence mainly of the difference of the thermal expansion coefficient of the phases, i.e. between fibre and the matrix in the case of composite materials. The phase-specific residual stresses of one phase is the sum of residual macro stresses plus load stresses plus micro residual stresses of this phase [2]. The material resistance and consequently the lifetime of the compos￾ite component under service conditions depends on the residual stress state. The fracture of ceramic matrix composites is usually governed by matrix cracking followed by interactions of the newly-formed cracks with fibres and interfaces such as fibre pull-out and debonding. These fracture mechanisms are activated by tensile stresses acting parallel to the fibre axis. Furthermore, it is obvious that the stresses acting normal to the fibre axis have an important influence on the load transfer from the fibre to the matrix as well as on the debonding process. Therefore, in order to optimise the material behaviour for certain external loads, it is of great importance to determine the residual stress state of the C/SiC composite nozzles. 1.4. Connection between C/SiC nozzle and metallic component In order to connect the C/SiC composite nozzle to a metallic ring a special brazing method was developed [3]. Prior to soldering, the surface of the C/SiC composite is perforated with a solid state small impulse laser NdYAG, see Fig. 1(a). The perforation method was optimised with respect to the optimal shape, dimensions and distributions of the perforations and the surface area to be perforated. The cavities were then filled with solder in a high vacuum, Fig. 1(b) [3]. 2. Experimental 2.1. Tomography Tomography in general is a method which provides cross sectional images of an object from transmission data, obtained by irradiating it with specific radiation from many different directions. From these projections a tomographic image is then mathematically recon￾structed. Here the projection at a given angle represents Fig. 1. (a) Laser perforation; (b) cavities filled with solder. J. Rebelo Kornmeier et al. / Materials Characterization 58 (2007) 922–927 923