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978-3-8439-2846-5, Reihe Werkstoffwissenschaften
Reaktive Randschichtmodifizierung polymerabgeleiteter Keramikmaterialien
187 Seiten, Dissertation Universität Erlangen-Nürnberg (2016), Softcover, A5
In this work a novel ceramic system based on preceramic polymers was investigated. The following pioneering results were achieved:
A defined amount of low-melting Cr- and Fe-silicides in combination with stabilizing SiC-particles facilitates production of a reaction bonded ceramic microcomposite with low porosity < 1 %. Special attention was drawn to adjusting the sintering processs to the reaction pyrolysis. The active filler reaction with atmospheric carbon and N2 starting at 1100 °C leads to a strengthening of the matrix material. Before material expansion results in anisotropic volume expansion, the diffusion of reactive gases into the material is stopped by open pore channel closure due to FeSiCr-sintering. The influence of particle size on reaction and sintering behavior was investigated and resulting material properties were measured.
The filler controlled surface reaction layer formation in air or N2 may flatten surface roughness and fill surface defects. The key reactive filler CrSi2 is triggering nitridation as well as oxidation which leads to an increase in fracture strength, fracture toughness and hardness in the surface edge zone. In comparison to common ceramic materials this surface layer gives rise for a near-netshape production without cost intensive grinding.
Reaction layer formation induces crack healing of damaged surfaces. In N2 a complete crack healing reaction was verified after 90 min at 1300 °C. The temperatures of crack healing in air range from 600 – 1300 °C. Due to high reactivity of the CrSi2 crack healing times of 2 min were achieved up to a crack depth of 50 µm. In an oxidizing atmosphere the material is building a passive reaction layer up to 1000 °C consisting of Cr2O3 and SiO2. Increasing Cr-diffusion at temperatures > 1100 °C from the core to the surface leads to material degradation for long time temperature operations. Additionally, the degradation of the SiOC-matrix at 1300 °C increases material degradation but also allows fast crack healing and material strengthening for short time heat treatment.
The production process of dense polymer derived ceramics was further developed by starting with the shaping of single platelets followed by milling or grinding shaping steps to a continuous near-netshape extrusion process. Furthermore complex three-dimensional parts like springs or volume-increased parts like bearing rings were produced without distortion due to high homogeneity of microstructure.