Abstract:
High-temperature ceramics, such as silicon nitride, are considered the best-suited materials for use in extreme environments because they posess high melting temperatures, high strength and toughness, and good thermal shock resistance. The goal of this research is to create bulk multifunctional high-temperature ceramic nanocomposites using single-wall carbon nanotubes in order to tailor electrical and thermal conductivity properties, while also enhancing the mechanical properties of the monolith. Colloidal processing methods were used to develop aqueous single-walled carbon nanotube (SWNT)-Si3N4 suspensions that were directly fabricated into bulk parts using a rapid prototyping method. High-density sintered nanocomposites were produced using spark plasma sintering, at temperatures greater than 1600 °C, and evidence of SWNTs in the final sintered microstructure was observed using scanning electron microscopy and Raman spectroscopy. The multifunctional nanocomposites show exceptional fracture toughness (8.48 MPa-m1/2) properties and was directly measured using conventional fracture toughness testing methods (ASMT C41 ). Our results suggest that the use of SWNTs in optimized sintered ceramic microstructures can enhance the toughness of the ceramic by at least 30% over the monolith. In addition, the observation of hallmark toughening mechanisms and enhanced damage tolerance behavior over the monolith was directly observed. The nanocomposites also measured for reductions in electrical resistivity values over the monolith, making them high-temperature electrical conductors. These novel nanocomposites systems have enhanced electrical conductivity, and enhanced toughness over the monolith which make them unique high-temperature multifunctional nanocomposites.
Abstract:
Colloidal processing was used to make highly dispersed aqueous composite suspensions containing single-wall carbon nanotubes (SWNTs) and Si 3N4 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 Si 3N4 were observed for the 2.0-vol% SWNT-Si 3N4 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. © 2008 The American Ceramic Society.
Silicon carbide nanofiber dispersion within a silicon oxycarbide glassy ceramic was achieved through a combination of a fast solgel procedure for in situ ceramic matrix synthesis and nanofiber conversion from sacrificial multiwalled carbon nanotube templates. Nanotubes were dispersed using both surfactant adsorption and a covalent sidewall modification scheme with gel-grafting capabilities. The combination of high temperature processing and silicon monoxide precursor concentrations allowed substantial carbothermal reduction of the nanotube templates, yielding silicon carbide nanofibers. The resulting nanocomposites were examined for density, Vickers microhardness, Young's modulus, and fracture toughness. The surfactant-assisted route inhibited ceramic densification, offering virtually no mechanical property enhancement. In contrast, the covalently functionalized nanotube templates at 0.8 wt% loading enhanced tensile modulus of 77% while simultaneously maintaining both Vickers microhardness and fracture toughness. These results indicate strong interfacial adhesion between the nanofiber surface and host matrix despite the abrupt chemical changes experienced during the high temperature processing.
Abstract:
A solutionizing heat treatment of 2014 aluminum alloy reinforced with 0.15 volume fraction of alumina particles (VFAP) results in deformation-induced precipitation during rolling and tensile deformation, with 0.10 VFAP, at room temperature. The extent of precipitation increases with increase in time and/or temperature of solutionizing. An attempt has been made to identify the various types of precipitates in the samples deformed to a given strain and in fractured conditions. The work-hardening curves and tensile properties of the composites have been shown to be dependent on the time and temperature combination of the solutionizing process.
Abstract:
A study has been conducted to determine the cyclic and static thermal stability of Ti-44Al-11Nb alloy in air in a range of temperature from 900 to 1000 °C. Weight gain method shows that the static oxidation rates are higher than cyclic oxidation rates under identical experimental conditions at a given temperature. The oxide layer at the surface penetrates the base metal in a direction parallel to the lamellae of the two phases, α2 and γ, confirmed by scanning electron micrographs. The thermal stability of up to 168 hours indicates that there are phase transformations taking placed affecting the microstructures in such a way that large amount of dislocation activities are involved.
