Publication Date

12-2011

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Darryl P. Butt, Ph.D.

Abstract

Tungsten casting is impractical, and tungsten compacts are most often produced by press-and-sinter or by hot pressing. The temperatures typically required to sinter tungsten above 90% dense are above 1800 ºC, and sintering times are typically hours. Alternatively, spark plasma sintering (SPS) can be used to consolidate materials to high densities at lower temperatures and shorter times than traditional sintering techniques. In this study, pure tungsten and tungsten with 1, 4, 10, 15, and 20 weight percent ceria were spark plasma sintered at varying pressures, temperatures, and times to investigate the microstructures and the kinetics of sintering.

Densification of tungsten and tungsten with 10 weight percent ceria begins between 800 and 900 ºC and densities greater than 90% can be achieved at temperatures as low as 1500 ºC. Grain growth is limited in the tungsten with 1, 10, and 20 weight percent ceria samples relative to the pure tungsten. The limited grain growth may be due to boundary pinning effects in the tungsten with 1 weight percent ceria, and it may be due to an increased diffusion distance in the tungsten with 10 and 20 weight percent ceria samples. The hardness of the tungsten and tungsten with 1 weight percent ceria is dependent on the density of the samples; however, the hardness of the tungsten with 10 and 20 weight percent ceria may be dependent on grain size and/or flaws in the microstructure. The ceria phase in these samples contained microscopic cracks, and these fractures may be due to a mismatch in thermal expansion between the tungsten and ceria phases or they may be due to thermal shock from rapid heating or rapid cooling during SPS.

Ceria loss was observed in tungsten samples containing 10, 15, and 20 weight percent ceria that were spark plasma sintered above 1600 ºC. Using X-ray photoelectron spectroscopy, it was found that both the Ce3+ and Ce4+ ions were present in the samples, indicating that the ceria phase was reduced from CeO2 to Ce2O3. This reaction likely releases oxygen gas, forming pores in the tungsten-ceria microstructure.

The densification kinetics of SPS tungsten has been determined using traditional hot pressing models. The models for plastic flow, lattice diffusion, and power-law creep were inadequate to describe the densification kinetics. Between 1100 and 1500 ºC, the rate-limiting mechanism for densification appears to be boundary diffusion. The apparent activation energy for boundary diffusion was found to be 360±20 kJ/mol, and the resulting diffusion constant was found to be 4.3±0.1 m2/s. The densification kinetics data from this study are limited, and future experiments on spark plasma sintering of tungsten are necessary to confirm the results in this thesis. An attempt was also made to determine the grain growth kinetics of SPS tungsten. Grain size data from tungsten spark plasma sintered at 1200, 1500, and 1800 ºC were used to determine the apparent activation energy for grain growth. Realistic values for the grain growth exponents could not be determined, and future work is necessary.

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