Infoscience

Thesis

High temperature mechanical spectroscopy of fine-grained zirconia and alumina containing nano-sized reinforcements

Polycrystalline zirconia (3Y-TZP grade) unreinforced and reinforced with multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw) or silicon carbide particles (SiCp) have been processed and studied by mechanical spectroscopy with complementary observations of electron microscopy and creep tests. Moreover, for comparison with zirconia, polycrystalline alumina specimens, unreinforced and reinforced with silicon carbide nanoparticles, have also been studied. The high-temperature mechanical loss spectrum of pure zirconia presents an exponential background (exponential increase with temperature) accompanied by a decrease of the dynamic shear modulus above 1200 K. A dissipation peak, which transforms into an exponential increase, appears in the spectra obtained as a function of frequency (isothermal spectrum). The peak does not depend on the amplitude of the applied stress, whereas the exponential increase is stress dependent above a certain threshold. This result has been linked to the interface reaction, which plays an important role in deformation accommodation during creep. The high-temperature mechanical loss is grain size dependent. Smaller the grain size, higher the mechanical loss. The presence of the impurities in the starting powder of zirconia leads to a shift of the entire mechanical- loss spectrum towards lower temperatures. Doping zirconia with different amounts of CNTs results in a decrease of the isothermal-mechanical loss level with respect to pure zirconia. A dissipation peak that transforms into an exponential increase is present for a higher amount of CNTs in the mechanical-loss spectrum as a function of temperature. Zirconia doped with CNTs exhibits a lower creep strain than pure zirconia when submitted to a compressive stress at high temperature. Also, addition of different amounts of SiCw to zirconia leads to a decrease of the mechanical-loss level. A high-temperature dissipation peak is presents in the isochronal spectrum. The peak is better resolved for a higher amount of SiCw. The creep-strain curve is lower in SiCw-doped zirconia than in pure zirconia at high temperature. Addition of SiCp to zirconia may shift or not the mechanical-loss spectrum towards lower temperatures. Doping zirconia with different amounts of SiCp results in the appearance of a peak or peaks in the high temperature isochronal and isothermal spectrum. The height of the peaks depends on the amount of SiCp to zirconia. Similar to zirconia, the high-temperature mechanical loss spectrum of alumina consists of an exponential increase of the mechanical loss and a decrease in the dynamic shear modulus above 1200 K. Additions of SiCp lower the high-temperature mechanical loss with respect to the one in pure alumina. High-temperature plastic deformation of pure zirconia and alumina is interpreted in terms of a theoretical model of GB sliding. When GB sliding is not limited by obstacles like multiple junctions, GB asperities or other defects, creep occur and an exponential background is observed in the mechanical-loss spectrum. Addition of nano-sized reinforcements on the GBs results in a decrease of the exponential background. This means that the GB sliding is more difficult and as a consequence a better creep resistance is observed. The activation parameters obtained from the Arrhenius plots were apparent. Two methods are used to correct the apparent values. However, even if the corrected values seem more reasonable than the values obtained from the Arrhenius plots, they do not bring useful information. It is proposed that apparent too high values of the activation enthalpy account for the evolution of the GB microstructure with temperature. Thermal evolution of the GB microstructure is believed to offer short circuit paths for GB diffusion, which allows the pinning nano-sized reinforcements to be overcome. Then pinning effect is less efficient.

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