Bernard H. Kear

High pressure/low temperature sintering of nanocrystalline alumina

Abstract Bulknanocrystalline α-Al2O3samples with a relative density >98% and a grain size

Processing and properties of nanostructured WC-Co

Abstract A novel chemical processing method is described for making nanostructured WC-Co powders. Critical to the success of the process is the control of thermodynamics and kinetics of gas-solid reactions in a fluid bed reactor. Of particular importance is the precise control of carbon and oxygen activities in the fluidizing gas stream. The as-synthesized powders have a high surface area and each powder particle is composite in nature. In contrast to conventional WC-Co powders, the extremely good mixing of the ceramic and metal phases in the powder particles, and the interconnected nanoprosity enables these powders to be consolidated by solid state sintering at relatively low temperatures. A low sintering temperature and a short sintering time ensure retention of a nanostructure in the consolidated material. Preliminary evidence indicates that as the scale of the nanostructure is reduced, the mechanical properties, such as hardness, are enhanced, which is in keeping with the well-established trend in properties of conventional WC-Co material.

Chemical processing and properties of nanostructured WC-Co materials

Abstract A new thermochemical processing method for preparing high surface area powders, starting from homogeneous precursor compounds, has been developed. The method has been applied successfully to the synthesis of nanostructured WC-Co powders. These powders have been consolidated by low pressure plasma spraying and by cold compaction and liquid phase sintering. The use of VC as a grain growth inhibitor is essential to mitigate the WC grain growth during liquid phase sintering. Theoretically dense VC-doped WC-Co materials display superior hardness, wear resistance and cracking resistance.

On the processing of nanocrystalline and nanocomposite ceramics

Abstract Two methods are being developed for the production of nanostructured bulk ceramics. The first method utilizes hot pressing to consolidate a flame-synthesized metastable nanopowder; taking advantage of a pressure-induced phase transformation to suppress grain growth during consolidation. This process; called Transformation Assisted Consolidation (TAC), is being used to prepare test samples of single phase nanoceramics- nanocrystalline ceramics. The second method utilizes plasma spraying of an aggregated powder feed to generate a splat-quenched metastable phase with extended solid solubility. In this case; TAC processing promotes co-nucleation of two or more nanophases from the rapidly quenched material; which provides an additional mechanism to limit grain growth during sintering. This variant on TAC processing is being used to prepare test samples of multiphase nanoceramics- nanocomposite ceramics.

Synthesis of oxide nanoparticles in low pressure flames

A method is described for the production of oxide nanopowders by controlled thermochemical decomposition of metalorganic precursor/carrier gas mixtures in low pressure flames. A specially designed burner is used to achieve a flat flame, extending a few millimeters out of the burner, which ensures that the temperature distribution and gas phase residence time are identical over the entire burner surface. Metalorganic precursors introduced along with the fuel/air mixture, therefore, experience completely uniform decomposition, thereby yielding a uniform nanopowder product. The substantial heat release in the flame allows the burner to support a high precursor flow rate. In this paper, we describe the progress made in flame synthesis of nanophase SiO2 (from Hexamethyldisilazane), TiO2 (Titanium ethoxide) and Al2O3 (Aluminum tri-sec butoxide) powders.

Chemical processing and applications for nanostructured materials

Abstract This paper presents an overview of recent research performed at Rutgers University and the University of Connecticut on the synthesis and processing of nanostructured materials. Highlights of this collaborative research program include: (1) synthesis of carbide strengthened steel and hard cermet powders from aqueous solution precursors, (2) synthesis of ceramic powders and ceramic matrix composites from metalorganic precursors, (3) densification of powder compacts by liquid phase sintering, (4) formation of high quality coatings by thermal spraying, and (6) demonstration of superior hardness and wear resistance in bulk materials and coatings.

Factors controlling decarburization in HVOF sprayed nano-WC/Co hardcoatings

Abstract The mechanism of decarburization in HVOF sprayed nano-WC/Co coatings has been investigated. The controlling factor is heterogeneous melting and localized superheating of the high surface area feed powder during spraying. This has the effect of causing extensive dissolution of the WC nanoparticles in the liquid Co, accompanied by rapid reaction of the dissolved C with oxygen in the flame environment. Upon cooling down from the peak temperature in the flame, the Co-rich liquid, now deficient in C, forms W 2 C and/or W phases, depending on the loss of carbon by gasification. Thus, it is concluded that HVOF sprayed high surface area nano-WC/Co powder is more susceptible to decarburization than conventional WC/Co powder, which is in agreement with observation.

Chemical vapor processing and applications for nanostructured ceramic powders and whiskers

Abstract A new chemical synthetic process for producing non-agglomerated nanostructured ceramic (n-ceramic) powders from metalorganic precursors is described. The process combines rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor with rapid condensation of the product particle species on a cold substrate under a reduced inert gas pressure of 1–50 mbar. A wide variety of metalorganic precursors is available commercially, all of which can be utilized in this process to produce n-ceramic powders, including single phase, multiphase and multicomponent systems. The process has been used to synthesize nonagglomerated n-SiC x N y and n-ZrO x C y powders, with controlled particle size in the range 2–20 nm. Heat treatment of the as-synthesized n-powders in various high purity gas streams causes compositional and structural modifications, including particle purification and crystallization, as well as transformation to a whisker-like morphology. Non-agglomerated n-ceramic powders form uniformly dense powder compacts by cold pressing, which can be sintered to theoretical density at temperatures as low as 0.5 T m .

High density, ultrafine precipitates in YBa2Cu3O7−x thin films prepared by plasma‐enhanced metalorganic chemical vapor deposition

Thin films of yttrium‐rich YBa2Cu3O7−x with c‐axis orientation prepared by plasma‐enhanced metalorganic chemical vapor deposition have been examined by high‐resolution transmission electron microscopy. Yttria precipitates smaller than 50 (A) in size have been identified in the matrix. They are uniformly distributed, have a high density as large as 1024 per cubic meter and are oriented with respect to the matrix. The magnetic field dependence of the critical current density of the thin films indicates that the yttria precipitates are effective flux pinning centers.

Plasma-sprayed nanostructured Al2O3/TiO2 powders and coatings

Air plasma spray has been used to produce metastable oxide-ceramic powders and coatings, starting with commercially available Al2O3/13TiO2 powder feed. The feed material undergoes rapid melting and homogenization in the high-temperature zone of the plasma jet. A metastablex-Al2O3·TiO2 phase is formed when the molten droplets are quenched on a chilled substrate. The metastable phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting ofα-Al2O3 andβ-Al2O3·TiO2. Both types of ceramic materials have potential as hard, wear-resistant coatings.