Within the framework of the development of protections for mobile structures (space and aeronautical systems,...) or solutions for the protection of personnel against mechanical impact, manufacturers are seeking to improve the mechanical strength/mass ratio of onboard protections by seeking the lowest possible densities of the constituent materials of these structures while maintaining sufficient mechanical performance. Polycrystalline ceramics are of particular interest because they combine lightness and high mechanical performance in compression. The introduction of porosities in these materials allows considerable weight savings, while preserving mechanical properties adapted to severe stresses. The objective of this work is to evaluate the effects of the microstructure on the mechanical behavior of a quasi-brittle and heterogeneous aluminas manufactured by gel-casting under quasi-static compression loading.

Three types of porous alumina have been manufactured. Two of them have a relative density of 80% containing microporosities induced by subsintering or mesoporosities induced by the addition of polymeric spheres. The last one has a relative density of 40% containing mesoporosities. A dense one (98.5% of relative density) was also produced to serve as a reference.

These aluminas were finely characterized by different observation means in order to detect critical defects. The heterogeneous structure induced by the presence of porosities at different scales was observed by Scanning Electron Microscopy (SEM) in order to qualitatively analyze the morphology of porosities. These observations were coupled with X-ray microtomography in the case of mesoporosities to quantify their distributions (size, sphericity, ...). Quasi-static uniaxial compression tests were performed on a standard tension/compression experimental set-up coupled with a high speed camera to observe fracture processes during testing.

Subsintering leads to a tortuous and interconnected microporosity observable at the grain scale (lower than 3 µm). The addition of polymer spheres introduces quasi-spherical mesopores with the majority of the pores diameter between 10 and 50 µm. These pores are qualified as "mesoporosity". Some mesoporosities can reach a diameter greater than 100 µm. For alumina with a relative density of 80%, the pores are mainly isolated but some of them show interconnection which leads to non-spherical pores. For alumina with a relative density of 40%, the pores are strongly interconnected by the presence of holes in the walls between porosities. Mechanical results in quasi-static regime show that stress/strain curve is linear for all relative density. A non-linearity just before fracture is observed for the sample with the lowest relative density (40%) which indicates a quasi-brittle behavior rather than a brittle behavior for the more porous samples. In addition, results show the increase of porosity rate leads to a decrease of elastic modulusand compression strength according to a decreasing polynomial law. For low density alumina (relative density of 40%), one main crack is observed thanks to the high speed camera. On the contrary, dense and porous aluminas with a relative density of 80% (micro and mesoporous samples) exhibit multicracking mechanisms.