Erica L. Corral

Toughening in Graphene Ceramic Composites

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 ceramics 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.

Ultra High Temperature Ceramics for Hypersonic Vehicle Applications

HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems.

Microstructural development during aging of 2014 aluminum alloy composite

The 2014 aluminum alloy reinforced with 0.1 and 0.15 volume fraction of alumina particles (VFAP) have been solutionized for a range of time from 1.5 to 20 h at 813 K. The effect of solutionizing time (ST) on the age hardening response of the composites has been studied and compared with the characteristics exhibited by the monolith. The results indicate that increasing the ST decreases the time required to get the peak hardness (TPH) values in the monolith but the composites do not show a systematic monotonic behavior. The TPH values first decrease and then increase with an increase in ST at an aging temperature of 473 K for the composite. It has been speculated that he ST influences the concentration of quenched-in vacancies and continued heating may affect the bonding between particles and matrix which can generate additional dislocations throughout the solutionizing process due to curvature effects.

Tape Casting of Magnesium Oxide

A tape casting procedure for fabricating ceramic magnesium oxide tapes has been developed as a method to produce flat sheets of sintered MgO that are thin and porous. Thickness of single layer tapes is in the range of 200-400 {micro}m with corresponding surface roughness values in the range of 10-20 {micro}m as measured by laser profilometry. Development of the tape casting technique required optimization of pretreatment for the starting magnesium oxide (MgO) powder as well as a detailed study of the casting slurry preparation and subsequent heat treatments for sintering and final tape flattening. Milling time of the ceramic powder, plasticizer, and binder mixture was identified as a primary factor affecting surface morphology of the tapes. In general, longer milling times resulted in green tapes with a noticeably smoother surface. This work demonstrates that meticulous control of the entire tape casting operation is necessary to obtain high-quality MgO tapes.

Influence of processing on microstructures of Ti-44Al-11Nb alloy

Abstract The cyclic and static oxidation behavior in the processing of Ti-44A1-11Nb alloy has been studied at 900, 950, and 1000°C in air for a period of up to 168 hours (or a week). A recreation of diffusion paths for the oxide to penetrate into the base metal during every cycle results in the lower oxidation rates during cyclic mode compared to static mode. An extended period of heating time allows for the coarsening of the structure for both forms of alloys: polycrystalline and directionally solidified crystals. Coarsening and phase transformation in the heating period involves large amounts of dislocation activities in the lamellar structure as well as at the interface of lamellar structure and islands of γ.