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.
Carbon–carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500°C and need thermal protection systems to survive aerothermal heating. We investigated using multilayers of high‐temperature ceramics such as ZrB2 and SiC to protect C–C against oxidation. Our approach combines pretreatment and processing steps to create continuous and adherent high‐temperature ceramic coatings from infiltrated preceramic polymers. We tested our protective coatings at temperatures above 2600°C at the National Solar Thermal Testing Facility using controlled cold‐wall heat flux profiles reaching a maximum of 680 W/cm2.
Colloidal processing was used to make highly dispersed aqueous composite suspensions containing single‐wall carbon nanotubes (SWNTs) and Si3N4 particles. The SWNTs and Si3N4 particles were stabilized into composite suspensions using a cationic surfactant at low pH values. Bulk nanocomposites containing 1.0, 2.0, and 6.0 vol% SWNTs were successfully fabricated using rapid prototyping. The survival of SWNTs was detected, using Raman spectroscopy, after high‐temperature sintering, up to 1800°C. The nanocomposites have densities up to 97% of the composite theoretical density. The engineered nanostructures reveal an increase in grindability and damage tolerance behavior over the monolithic ceramic. We also observed toughening mechanisms such as SWNT crack bridging and pull‐out, indicating that SWNTs have the potential to serve as toughening agents in ceramics. Increased fracture toughness values over the monolithic Si3N4 were observed for the 2.0‐vol% SWNT–Si3N4 nanocomposite when a given sintered microstructure was present. We report here the effects of colloidal processing on mechanical behavior of SWNT reinforced nonoxide ceramic nanocomposites.
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 mm3 and spark plasma joining conditions at 1800°C for 300 s resulted in the formation of a joint region that was indistinguishable from the bulk substrates. Room and high‐temperature (1350°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°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.
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