M. Castillo-Rodríguez

Superplasticity in nanocrystalline ceramics: pure grain boundary phenomena or not?

Abstract Superplasticity in ceramics has been the subject of intense research activity for the last two decades. Quite recently, the fabrication of fully dense nanocrystalline oxides with grain size below 100 nm enabled examination of their superplastic behaviour. This work presents a critical analysis of the plasticity of two important nanostructured oxide systems: MgO and yttria tetragonal zirconia polycrystals. A thorough comparison of their plastic deformation reveals that nano-structuring may be a necessary, but not a sufficient condition for superplasticity in ceramics as commonly assumed. Instead, the changes in the chemical composition and the transport properties, through the bulk and at grain boundaries, versus temperature and grain size can induce a rich variety of mechanical responses.

Correlation between plasticity and dislocation dissociation in ceramics

The influence of dislocation core extension on plastic properties is discussed. Dislocation dissociation has been studied in ceramics. Such investigation has been carried out for the past decades, but in spite of new results, there are still pending questions. Here, we review cases of glide and climb dissociation. The former has been found in 4H-SiC with implications not only on the plasticity but also on the crystal structure changes. The latter has been found in several oxides (sapphire, spinel, strontium titanate, etc.). It is, sometimes, associated with a maximum in the critical resolved shear stress (CRSS) dependence on temperature. We discuss the reasons for such effects, emphasising factors that can reduce the mobility of dislocations to justify why dissociation can influence plasticity and, sometimes, has negligible effects.

High Temperature Internal Friction in Fine Grain and Nano-Crystalline Zirconia

Engineering ceramics are being developed to improve their high-temperature mechanical properties and in particular creep resistance. Recently the production of fine grain ceramics undergoes another step-forward with the development of new technologies to produce nanocrystalline materials. The question is whether the properties depending on the grain size can be extrapolated at nanoscale or, on the contrary, new microscopic mechanisms could appear to be dominant at this nanometer grain size. In the present work we study, by mechanical spectroscopy, the high temperature behavior up to 1350°C of a fine grain Zirconia and a nanocrystalline Zirconia sintered in a conventional way. A new forced torsion pendulum, recently built, has been used for the mechanical spectroscopy measurements. The high temperature background (HTB) of internal friction has been measured as a function of temperature for different frequencies in both materials. The analysis of the HTB shows that the fine grain Zirconia exhibits a single process of defects mobility, with an apparent activation enthalpy similar to the one measured by creep. On the contrary, the HTB of the nanocrystalline sample becomes more complex, showing a much higher energy loss, which will be discussed at the light of the internal friction spectra analysis.