While monazite (LaPO4) does not flash sinter even at high fields of 1130 V/cm and temperatures of 1450°C, composite systems of 8YSZ–LaPO4and Al2O3–LaPO4have been found to more readily flash sinter. 8YSZ added to LaPO4greatly lowered the furnace temperature for flash to 1100°C using a field of only 250 V/cm. In these experiments,
The addition of small quantities of aluminum oxide (Al2O3) to 8 mol% yttria‐stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al2O3are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD‐coated powders were flash sintered using voltage‐to‐current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al2O3films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al2O3can enhance grain growth and sintering of 8YSZ. This suggests that Al2O3dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.
more » « less- NSF-PAR ID:
- 10452575
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 104
- Issue:
- 6
- ISSN:
- 0002-7820
- Page Range / eLocation ID:
- p. 2472-2482
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The mechanical properties of Al2O3–LaPO4composites with varying microstructures produced by flash sintering and conventional sintering are evaluated. Specifically, Vickers and Knoop hardness values were measured and calculated for different resultant microstructures, including eutectic microstructures with varying layer thickness, polycrystalline (noneutectic) microstructures, and single‐phase samples of Al2O3, LaPO4, and 8YSZ. The findings indicate that eutectic microstructures exhibited higher hardness values than polycrystalline counterparts on the flash‐sintered sample. However, the hardness values of eutectic microstructures with varying layer thicknesses show no significant or systematic variation. The grain size, indentation size, eutectic colony size, indentation shape (elastic recovery in Knoop indentations), and crack propagation pathways in the indented samples are also discussed. Overall, the results suggest that Al2O3–LaPO4eutectic composites have higher hardness than their polycrystalline counterparts and have great potential as abradable coatings with high machinability and durability.
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Abstract Aluminum nitride (AlN) is a promising material for electronic substrates and heat sinks. However, AlN powders react with water that adversely affects final part properties and necessitates processing in organic solvents, increasing the cost of AlN parts. Small quantities of yttrium oxide (Y2O3) are commonly added to AlN particles to enable liquid phase sintering. To mitigate the reaction of AlN particles with water, particle atomic layer deposition (ALD) was used to coat AlN powders with conformal films of Y2O3prior to densification and powder processing. When AlN particles were coated with 6 nm thick films of amorphous Y2O3, the hydrolysis reaction was significantly suppressed over 48 h, demonstrating that Y2O3nanofilms on AlN powders act as a barrier coating in an aqueous solution. AlN powders with Y2O3addition by particle ALD sintered to high relative densities (≥90% theoretical) after sintering at 1800°C for 50 min.
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Abstract The properties of technical ceramics are highly dependent on their microstructure, which evolves during sintering. Sintering is the process by which ceramic parts are subjected to high temperatures to activate chemical diffusion and the consumption of porosity. During the initial stage of sintering, interparticle necks between neighboring particles form and subsequently increase in size, consuming porosity as the particle centers move closer together. To experimentally determine how this process depends on particle surface composition, particle atomic layer deposition (ALD) was used to deposit a thin film of amorphous aluminum oxide (Al2O3) onto yttria‐stabilized tetragonal zirconia (3YSZ) particles, producing core‐shell structured powders. The uniformity of the Al2O3film was confirmed with transmission electron microscopy and energy dispersive spectroscopy. Scanning electron microscopy was used to observe microstructural evolution during sintering, and the dihedral angles of Al2O3and 3YSZ grains were measured to determine the ratio of interfacial energies between the 3YSZ|3YSZ, 3YSZ|Al2O3, and Al2O3|Al2O3interfaces. Analysis of the densification kinetics revealed that the initial stage of densification is dependent on the material at the surface of the particles (ie, the Al2O3film) and is controlled by the diffusion of Al3+cations through Al2O3. Once the Al2O3film has coalesced, the sintering behavior is controlled by the densification of the core material (3YSZ). Thus, core‐shell powders fabricated by particle ALD sinter by a two‐step process where the kinetics are dependent on the material present at interparticle contacts.
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