The majority of work in graphene nanocomposites has focused on polymer matrices. Here we report for the first time the use of graphene to enhance the toughness of bulk silicon nitride ceramics. Ceramics are ideally suited for high-temperature applications but suffer from poor toughness. Our approach uses graphene platelets (GPL) that are homogeneously dispersed with silicon nitride particles and densified, at ∼1650 °C, using spark plasma sintering. The sintering parameters are selected to enable the GPL to survive the harsh processing environment, as confirmed by Raman spectroscopy. We find that the ceramic's fracture toughness increases by up to ∼235% (from ∼2.8 to ∼6.6 MPa·m(1/2)) at ∼1.5% GPL volume fraction. Most interestingly, novel toughening mechanisms were observed that show GPL wrapping and anchoring themselves around individual ceramic grains to resist sheet pullout. The resulting cage-like graphene structures that encapsulate the individual grains were observed to deflect propagating cracks in not just two but three dimensions.
Abstract:
Multilayer ceramin coatings appear to offer the best oxidation protection for carbon-carbon composites that make up the structure of future hypersonic spcae vehicles.
Abstract:
Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon-carbon (C-C) composites at high temperature (1500°C). Our approach combines using a precursor to ZrB2 and processing them with B4C particles as filler material within the C-C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B4C, and ZrB2-B4C filled C-C composites from 800 to 1500°C. Ablation rates decreased by 30% when C-C composites were filled with a combination of ZrB2-B4C particles over carbon black and B4C filled C-C composites. We also investigated using a sol-gel precursor method as an alternative processing route to incorporate ZrB2 particles within C-C composites. We successfully converted ZrB2 particles within C-C composites at relatively low temperatures (1200°C). Our ablation results suggest that a combination of ZrB2-B4C particles is effective in inhibiting the oxidation of C-C composites at temperatures greater than 1500°C. © 2010 Elsevier Ltd.
Using an organic solvent-based formulation, flexible and homogeneous ZrB2 tapes were cast for potential use as advanced aerospace exploration vehicles. Dispersant concentrations were optimized for attrition-milled ZrB2 powder using gravitational sedimentation and viscosity measurements. Tape cast slurry formulations with varying amounts of binder (6-8 vol%), plasticizer (7-11 vol%), and solids loading (20-23 vol%) were used to optimize the casting slurry. An optimal slurry formulation was determined based on cast tape uniform particle distribution and flexibility without cracking. Thermal gravimetric analysis of the optimized tape was used to create a binder burnout schedule that did not alter the tape structure or particle distribution. Our organic solvent-based tape casting approach results in minimum oxygen contamination after colloidal processing and robust cast tapes with a thickness of 280 mu m and green density of similar to 41% theoretic density after binder burnout.
