Preparation, microstructure and properties of silicon carbide-dysprosia composites
Kim, Shin
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Permalink
https://hdl.handle.net/2142/20033
Description
Title
Preparation, microstructure and properties of silicon carbide-dysprosia composites
Author(s)
Kim, Shin
Issue Date
1991
Doctoral Committee Chair(s)
Kriven, Waltraud M.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
Composites of silicon carbide with up to 30 vol% dysprosia (Dy$\sb2$O$\sb3$) were fabricated by hot pressing and hot isostatic pressing. Dy$\sb2$O$\sb3$, one of the lanthanide sesquioxides (or rare-earth oxides), undergoes a displacive (monoclinic to cubic) phase transformation with volume expansion during cooling from high temperatures ($\approx$1850$\sp\circ$C). The effects of Dy$\sb2$O$\sb3$ additions on the microstructure and on selected mechanical properties of the composites were investigated. The phase analysis and microstructure of the composites were investigated using X-ray diffraction, SEM and TEM. Processing parameters influencing composite density and the retention of monoclinic dysprosia in a silicon carbide matrix were identified. Some of the mechanical properties evaluated for the composites were: modulus of elasticity, hardness, flexure strength and fracture toughness. The hardness of the composites was determined using the Vickers indentation method. Elastic properties were determined by the ultrasonic pulse echo technique. Flexure strengths were determined using four point bend testing. Fracture toughness was determined by both the three point, single-edge notched beam test and by the microindentation methods.
As hot pressed or as-hipped composites always contained cubic Dy$\sb2$O$\sb3$ in a SiC matrix. In order to retain the monoclinic Dy$\sb2$O$\sb3$ in the composite, rapid cooling from above the transformation temperature was necessary. Oxygen, which is present as impurities coating SiC powders or from the atmosphere during densification, was shown to be detrimental in retaining the monoclinic Dy$\sb2$O$\sb3$ phase in the composite. Pressure-assisted densification was necessary in this composite system due to the decrease of sinterability with the addition of Dy$\sb2$O$\sb3$.
With the dispersion of 10% to 15 vol% Dy$\sb2$O$\sb3$ in SiC matrix, the hardness and Young's modulus of the composites decreased but the fracture toughness increased by $\approx$40%, while the flexural strength was comparable to that of unreinforced SiC. This increase in fracture toughness was not related to a phase transformation of Dy$\sb2$O$\sb3$ in the matrix. The polymorphs of Dy$\sb2$O$\sb3$ in SiC matrix did not have a significant effect on the fracture toughness of the composite. Examination of crack propagation profiles and fracture surfaces suggested that the increased fracture toughness was probably due to crack deflection in conjunction with crack-interface grain bridging.
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