Plastic deformation of nanostructured Al2O3 at room temperature

A glass that won't break Oxide glasses are important for applications ranging from smartphone screens to window panes. One familiar feature of glass is that it fractures and shatters when rapidly deformed, limiting the number of potential uses. However, Frankberg et al. found that they could deform thin films of glassy alumina (Al 2 O 3 ) with high strain rates at room temperature (see the Perspective by Wondraczek). This surprising observation is supported by simulations of the material that show that dense and flawless glassy alumina samples can deform this way. The discovery provides important insight into designing new glasses that might be more fracture resistant. Science , this issue p. 864 ; see also p. 804

Introduction Recent TEM in situ mechanical experiments on single alumina nanoparticles have shown unexpected plasticity in room temperature alumina [1, 2]. These results push the theoretical boundaries of ceramics mechanical ductility towards comparable levels with metals. The important questions for materials science now are: (i) whether the plastic behaviour can be transferred into polycrystalline systems; (ii) what is the microstructure of such plastic polycrystalline system and (iii) what is the mechanism behind the hypothetical plasticity of the polycrystalline system. Relatively cheap and abundantly available engineering ceramic, such as alumina, with room temperature plasticity would be a breakthrough in the engineering ceramics field. We report the findings of our study of polycrystalline alumina thin films, produced by pulsed laser deposition, with crystal size of < 5 nm using TEM and in situ TEM. Pulsed laser deposition is an extreme fabrication method where the deposition material is transformed into plasma by a short laser pulse. As the plasma quickly expands into vacuum or background gas the nucleation and growth of nanoparticles is rapidly quenched. Alumina produced this way has an exotic, nanocrystalline microstructure, and is a strong candidate for having the capability for room temperature plasticity. The more conventional TEM studies are focused on determining the as-received state of the material, grain size, morphology, crystal structure, grain boundary structure and whether any structural defects pre-exist since they have major impact on the mechanical response of the material. In situ TEM studies are focused on analysis of the material’s mechanical response (strain, dislocation activity, fracture etc.) to compression and indentation forces and look for evidence of the mechanism behind the mechanical response. Experimental Pulsed laser deposition (PLD) of Al2O3 thin films was done on various substrates including silicon, sapphire and sodium chloride using PLD coating equipment (Nano2Energy Laboratory, Italian Institute of Technology and Coldabtm PLD coating system, Picodeon Ltd Finland). Two techniques were used to prepare TEM characterization samples from the PLD alumina coatings. First, TEM samples were prepared using a focused ion beam (FIB) lift-out technique and second, NaCl crystals coated with PLD thin film alumina were dissolved in water and the free-standing alumina film was deposited on a TEM grid. Figure 1 shows a TEM image of the microstructure of the PLD alumina film prepared using FIB lift-out method. Figure 2 shows a selected area electron diffraction pattern taken from the Figure 1 site indicating the presence of polycrystalline gamma-Al2O3. For in situ TEM mechanical testing, R-plane sapphire substrates were used. Sapphire substrate was prepared using broad ion milling (Ilion II, Gatan Inc.) to produce an electron transparent, roughly 20° edge on the sapphire substrate. Furthermore a part of the edge was modified with FIB to produce electron transparent anvils with flattened tip in order to quantify the area of compression. The produced edge and anvils were either directly PLD coated or a PLD film separated from the NaCl substrate was transported on the sapphire edge or anvils. The in situ tests were conducted using Nanofactorytm and Hysitron® PI 95 in situ TEM sample holders with JEOL 2010F and FEI Titan microscopes. In the test the PLD alumina film was compressed between the sapphire substrate and a diamond tip and the deformation process was filmed in situ together with synchronized strain and force measurement.

Latest qualitative and quantitative results + preliminary MD simulation results on room temperature plastic deformation of nanostructured Al2O3 thin films.