SURFACE CRACK HEALING OF POLYMER-FILLER DERIVED
CERAMICS
L. Schlier1, N. Travitzky1, P. Greil1
1
University of Erlangen-Nuernberg, Department of Materials Science (Glass and Ceramics), Martensstr. 5, D-91058 Erlangen, Germany - e-mail: Lorenz.Schlier@ww.uni-erlangen.de; Nahum.Travitzky@ww.uni-erlangen.de; Peter.Greil@ww.uni-erlangen.de
Keywords: Polymer derived ceramics, surface nitridation, crack healing
ABSTRACT
Healing of surface cracks and pores on the surface of polymer derived Si-O-C ceramics by annealing in nitrogen atmosphere was investigated. Thermodynamic calculations as well as experimental results show Si3N4 and Si2N2O to be the major
condensed reaction products. High mobility of vapour phase reactants SiO(g) and N2(g) facilitates long range transport and nitride and oxinitride reaction products may
fill open pores and cracks. While the filler free Si-O-C system requires annealing temperatures exceeding 1200 °C significantly lower reaction temperatures can be achieved by adding transition metal filler (CrSi2) which catalyzes nitridation reaction.
Maximum crack length and crack opening to be healed were correlated to the penetration depth of nitrogen perpendicular to the surface and specific volume expansion of the nitridation reaction phases. Penetration of nitrogen causes effective reduction of porosity at least near the surface which gave rise for a pronounced improvement of fracture strength of (heal/0) 1.5. Removing the surface nitridation
reaction layer by grinding and reannealing in nitrogen atmosphere resulted in a complete recovery of the initial modulus of rupture. Since post-fabrication treatment in a reactive atmosphere is independent on the component shape and size formation of a surface reaction zone with reduced porosity and flaws may offer a versatile route for improving properties of bulk polymer-filler derived ceramic components.
