Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Kurzfassung

Damage-tolerant ceramic matrix composites (CMC) are prone to high temperature applications under severe environmental conditions and usually utilize carbon or ceramic fibres (e.g. SiC) as reinforcements of ceramic matrices with inherent low elongation to break compared to common metals. However, CMC reveal an elongation to break and stiffness similar to the ceramic matrices, and thus need a fibre coating in order to improve the elongation to break length and thus to achieve damage tolerance of the composite. In addition, such fibers often expose a low ductility during failure. As a consequence, design criteria for components of such CMC materials are limited by the low strain of failure. In order to overcome this problem, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibres to reach pseudo ductile behaviour during failure. Tungsten (W) and molybdenum (Mo) fibers were chosen as reinforcement in SiCN CMC manufactured by polymer infiltration and pyrolysis process. These fibres are commercially available since they are widespread used in light bulbs, etc. , and possess an intrinsic higher elongation to break, compared to ceramic fibres, as well as high stiffness even at high temperatures. W/SiCN and Mo/SiCN composites were manufactured via filament winding and resin transfer moulding of commercially available polysilazanes, pyrolysed and re-densified by multiple reinfiltration and pyrolysis steps. These composites were investigated with respect to microstructure, flexural and tensile strength. Single fibre strengths for W and Mo were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. W fibre became brittle across the entire cross section, while the Mo fibre showed a superficial, brittle reaction zone but kept ductile on the inside.