Spark plasma joining is used to join ZrB2-SiC composites with seamless microstructures at the joint that results in retention of high-temperature mechanical and oxidation properties after joining. Our approach uses a spark plasma sintering furnace and Zr-B powder filler layers to join the parts together. The joining processing parameters used to optimize the joint microstructure were filler composition, target temperature, hold time, and volume of filler. A filler of 1 mm(3) and spark plasma joining conditions at 1800 degrees C for 300 s resulted in the formation of a joint region that was indistinguishable from the bulk substrates. Room and high-temperature (1350 degrees C) shear strengths of joined substrates measured equal to baseline substrates and oxidation behavior for joined and baseline substrates were equivalent after static air oxidation at 1700 degrees C. X-Ray diffraction measurements show the joint is composed of ZrB2 and ZrC. We found the joining mechanism to be solid-state bonding of ZrB2 that formed from the Zr-B filler and reaction bonding by the formation of ZrC. Spark plasma joining rapidly joins ZrB2-SiC and probably other conductive ultra high-temperature ceramic composites, and has the potential to impact the rapid assembly and joining of complex thermal protection material systems.
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.
Multilayer ceramin coatings appear to offer the best oxidation protection for carbon-carbon composites that make up the structure of future hypersonic spcae vehicles.
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.