Date of Award
Doctor of Philosophy (PhD)
Materials Science and Engineering
Alumina composites with 10, 20 and 30 volume % SiC whiskers were fabricated using colloidal processing methods followed by uniaxial hot pressing. The tensile creep properties of these materials have been studied between 1200°C and 1400°C. The composite slurries showed the best stability at pH = 2, which led to uniform distribution of whiskers in the final products. However, at pH ≥ 6 flocculation occurred between whiskers, resulting in whisker agglomerates in the matrix. Distribution of whiskers was characterised using neutron diffraction methods, which indicated that the whisker orientation could not be altered significantly by adjusting pH. All the composites showed much superior tensile creep resistance compared to pure alumina and the effect of increasing whisker volume fraction was significant up to 30%. Relatively high stress exponents were found, which is most probably associated with much enhanced cavitational creep in tension. The activation energy varied with whisker volume fraction, temperature and applied stress in a complex manner. This combined with the temperature-dependent stress exponents makes the identification of creep mechanisms difficult. Nevertheless, it appears that at moderate stress level grain boundary diffusion and grain boundary sliding (GBS) become more significant as whisker volume fraction increases. The composites containing 20 and 30% whiskers showed significant anelastic strain recovery (∼0.001) following tensile creep, which is consistent with earlier reports that involved bending creep tests. The whisker bending effect was studied by measuring the peak width of (111) SiC planes (perpendicular to the whisker axis) at various conditions. The difference in the peak width at room temperature was found to be insignificant before and after creep. Moreover, during in-situ neutron diffraction measurement at 1400°C, no measurable variation in the peak width was recorded from the crept samples that were cooled under load. It may be that the neutron diffraction technique used in this study is not sufficiently sensitive to measure the small bending strains developed. However, These results along with other evidence in the literature suggest that Herztian contact deformation of networked whiskers, rather than bending deformation of whiskers, may be the dominant mechanism to explain the observed anelastic strain recovery. This mechanism predicts similar strain recovery in any composite that contains constrained 'hard' inclusions with sufficient contact numbers. Models based on this mechanism have been developed, which seem to predict the magnitude of the recoverable strain reasonably well.
Quan, Guang-Chun, "Tensile creep of silicon carbide whisker-reinforced alumina composites" (2004). Open Access Dissertations and Theses. Paper 2745.