 |
Carbon fibres-reinforced carbon composites (CC composites) are candidate materials for advanced structures, which could work under dynamic load at elevated temperature. Their mechanical properties are retained even until 2000 oC, and due to low values of the coefficients of thermal expansion (CTE) and high heat of sublimation they have good ablation resistance. Additionally, CC composites exhibit thermal shock resistance and chemical resistance in non-oxidizing atmosphere.
However, application of CC composites in high temperature structures is limited due to the potential oxidation damage and erosion in air above 400 oC [1]. Much effort is done in order to protect CC composites against air – oxidation. Several methods have been developed to improve oxidation resistance: chemical vapor deposition (CVD) coatings, multilayer coatings, impregnation of CCs composite with organosilicon compounds, i.e. polysiloxanes and heat treatment [1]. A cross-linked polysiloxane resin during pyrolysis up to 1000 oC can be transformed into a silicon oxycarbide (structure containing Si-C-O bonds, blackglass), and during pyrolysis up to 1700 oC silicon carbide crystallizes [2]. Our previous work indicate that depending on the structure of polysiloxane resins it is possible to obtain ceramic samples with high ceramic yield, i.e. from 82 to 86 wt % at 1000 oC and from 61 to 70 wt % at 1700 oC [2].
The aim of this work is to compare mechanical behavior of phenol-formaldehyde resin-derived CC (carbon) composite with CC/ceramic (carbon/ceramic) composites obtained by the impregnation of CC composite with commercially available polysiloxane-based solutions of preceram and their subsequent heat treatment at 1000 oC, 1500 oC and 1700 oC. CC/ceramic composites heat treated at 1000 oC and 1500 oC contain silicon oxycarbide [2] and CC/ceramic composite heat treated at 1700 oC contains silicon carbide [2]. As a reinforcement HTS 5131 carbon fibers (Tenax) in a form of roving were used. Phenol-formaldehyde resin (Organika-Sarzyna, Poland) and Lukosil 901 polysiloxane substrate (Lucebni zavody, Czech Republic) used in this experiments were inexpensive (cost about 10$/kg).
Dynamic mechanical analysis (DMA) in three-point clamping mode was carried out with TA Q800 DMA analyzer equipped with a TA Universal Analysis 2000 software. All tests were carried out in air at 450 oC at an oscillatory frequency of 20 Hz. A multifrequency-strain mode was set up under a strain of 80 mm. CC composite was used as a reference.
In comparison to the reference CC composite, CC/ceramic composites exhibited at least three times longer lifetime.
Acknowledgements
This work was supported by the National Science Centre, grant No. 2011/01/B/ST8/07451, decision No. DEC-2011/01/B/ST8/07451.
References
[1] Savage G., Carbon – carbon composites. Chapman and Hall, London (1992).
[2] Gumula T., Paluszkiewicz C., Blazewicz S.: Study on thermal decomposition processes of polysiloxane polymers – from polymer to nanosized silicon carbide. Journal of Analytical and Applied Pyrolysis, 86, 375-380 (2009).
