High temperature mechanical spectroscopy of fine-grained ceramics

Mechanical loss measurements were performed at high temperature in fine-grained ceramics, such as zirconia and alumina. At high temperature, the mechanical loss increases exponentially with temperature, which accounts for material creep. The results are analyzed with a model of grain-boundary sliding lubricated by an intergranular glassy phase. When the amount of the intergranular glassy phase is higher, the mechanical loss level is globally higher and so is the creep rate. On the other hand, doping finegrained ceramics with nano-sized reinforcements such as multiwall carbon nanotubes results in a decrease in the high-temperature mechanical loss. In this case grain-boundary sliding is more difficult and as a consequence better creep resistance is observed.

High Temperature Behavior of Nano-Structured Ceramic Composites Studied by Mechanical Spectroscopy

Mehdi Mazaheri

Silicon nitride based ceramics (SiAlONs); tetragonal polycrystalline zirconia (3Y‐TZP); alumina and their composites reinforced with different amount of multi‐ walled carbon‐nanotubes (CNTs) have been processed by Spark Plasma Sintering (SPS). High temperature mechanical spectroscopy measurements were performed in each material between room temperature and 1600 K. In each of these materials, anelastic and viscoplastic relaxation phenomena were investigated and responsible mechanisms were explained. In particular, grain boundary (GB) sliding, which is responsible for high temperature plasticity in fine grained ceramics, gives rise to a peak or an exponential increase in the high temperature mechanical loss. Peak and exponential background depend on two forces, which control the GB sliding: a friction force due to the GB viscosity and a restoring force due to the elasticity of the surrounding grains. In the present thesis, ceramics and composites have been processed, which allow one to study the role played by these forces on the thermo‐ mechanical behavior of these refractory materials. SiAlON ceramics were chosen for studying the viscous force due to the inter‐granular glassy phase. In zirconia and alumina, it is shown how the CNT reinforcements may improve the restoring force and consequently the creep resistance. Different grades of SiAlON ceramics, processed with different sintering aids (Ca2+, Y3+, Yb3+), with oxygen rich (CaO, Y2O3 and Yb2O3) and nitrogen rich (CaH2, YN and YbN) compounds, have been studied. Mechanical spectroscopy has been used to analyze the behavior of the residual glassy phase, present after sintering, either as grain‐boundary glassy (GB) films or glass pockets located in GB triple junctions. The mechanical loss spectra show a relaxation peak, which is due to a relaxation phenomenon (the so called “α‐relaxation”) associated with the glass transition in the amorphous phase. The peak position depends on the glass viscosity and the peak height is mainly affected by the glassy phase amount and the SiAlON bimodal microstructure, namely equiaxed versus elongated grains. Moreover the peak height depends on the restoring force provided by the neighboring grains, which limit the GB sliding process. As a matter of fact, a good correlation between the mechanical loss peak and plastic deformation in a compression test has been observed. It is concluded that the elongated grains in the bimodal microstructure provide a higher restoring force, which limits the GB sliding of smaller equiaxed grains. In the case of equiaxed fine grained oxide ceramics, such as zirconia and alumina, the restoring force has been increased by CNT additions, which can reinforce the GBs. 3Y‐TZPcomposites with a homogenous distributions of CNTs, ranging within 0.5 – 5 wt%, were processed by SPS. A significant improvement in room temperature fracture toughness and shear modulus as well as creep performance at high temperature were obtained, and interpreted as due to the GB reinforcing role of the CNTs. To support this interpretation, high‐resolution electron microscopy and Raman spectroscopy have been carried out. Moreover, a remarkable enhancement of the electrical conductivity up to ten orders of magnitude due to CNT additions has been obtained with respect to the pure ceramics. The isothermal spectrum of the 3Y‐TZP composites (measured at 1600 K) is composed of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at lower frequency. This is interpreted as primarily due to GB sliding. The absence of a well‐marked peak in monolithic 3Y‐ TZP is justified by considering that the restoring force decreases at low frequencies or high temperatures. Therefore, GB sliding is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. With CNT additions the mechanical loss decreases and the relaxation peak is better resolved with respect to the background. This is interpreted by the pinning effect of CNTs on GBs, providing addition source of restoring force, which can hinder GB sliding at high temperature, resulting in a creep resistance improvement. Similarly to 3Y‐TZP based composites, alumina specimens (3 different types of alumina powders) reinforced with different amount of CNTs were sintered by SPS. It is shown how the initial particle size of the powders may affect the dispersion of CNTs. Measurement of different properties, such as hardness, fracture toughness and mechanical loss at high temperature, have evidenced the crucial role of the CNT dispersion in the obtained mechanical properties.

EPFL2013

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

Claudia Ionascu

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.

EPFL2008

High temperature mechanical loss spectrum of 3Y-TZP zirconia reinforced with carbon nanotubes or silicon carbide whiskers

Claudia Ionascu, Robert Schaller

High temperature plasticity of fine-grained ceramics (ZrO2, Al2O3, etc) is usually associated with a grain boundary sliding process. The aim of the present research is then to improve the high-temperature mechanical strength of polycrystalline zirconia (3Y-TZP) through the insertion of multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw), which are susceptible to pin the grain boundaries. The effect of these nano-sized particles on grain boundary sliding has been studied by mechanical spectroscopy.