|M.Sc Student||David Ehre|
|Subject||Processing of Nanocrystalline Ceramics and the Effects of|
Grain Size on Mechanical Properties
|Department||Department of Materials Science and Engineering||Supervisors||Professor Chaim Rachman|
|Full Professor Gutmanas Elazar|
Recent years have brought increased interest in nanocrystalline materials in general and particularly in nanocrystalline ceramics. One of the main reasons for this interest is their unique mechanical properties. Among the more documented and controversial properties is the abnormal "Hall-Petch Effect”. This effect describes the reduction in the material’s resistance to plastic deformation with decreasing grain size in the nanocrystalline regime. This is in contrast to “normal” behavior where hardness increases with decreasing grain size.
Many studies deal with densification of ceramics with attempts to preserve the nanocrystalline form of the material. This task is not simple, as in most cases substantial grain growth occurs when the material reaches around 92% of its theoretical density during the sintering.
This study focuses on nanocrystalline Magnesium Oxide (MgO), a stochiometric ceramic with a cubic form (Rock-Salt), very high melting temprature (2852ºC) and relatively low yield point (160MPa).
The study included investigation of two densification methods: pressureless sintering and hot-pressing through which nanocrystalline MgO was densified. It was found that the densification mechanism during sintering at 1100oC to 1200oC is controlled by volume diffusion (activation energy of 280±13 kJ/mole). In hot-pressing the densification mechanism at lower temperatures (below 400oC) was plastic flow and at higher temperatures (650oC-900oC) diffusional creep (activation energy of 192±25 kJ/mole). Hot-pressing resulted in samples of nanocrystalline magnesia with grain size of 70nm and 99% theoretical density.
Hardness results revealed abnormal Hall-Petch behavior in the nanocrystalline range. A simple model, based on composite material was used to describe the experimental results for a wide range of grain sizes.