Researchers at IMDEA Materials Institute and members of the ESTRUMAT, programme have implemented new high temperature nanomechanical testing techniques. The objective is two-fold. On the one hand, these techniques are necessary to study size effects in the mechanical properties of materials, relevant for applications in which miniaturization is leading to smaller and smaller devices. On the other hand, they open the door to screen the mechanical properties when the amount of material available is so limited that conventional testing is precluded.
Micropillar compression tests, in which a microscopic pillar of the material is compressed on a dedicated nanoindentation platform, is a good example of this strategy (Fig. 1). Micropillar compression at room temperature has been proven an effective method to study size effects in the deformation and fracture mechanisms of very small volumes of metallic materials, of the order of 1 m3, but the extension of such tests to high temperature is still an uncharted world. This is due to the difficulty of achieving the required thermo-mechanical stability to measure loads and displacements with nanoNewton and nanometer resolution, respectively.
Figure 1. SEM image of compressed micropillars of nanoscale Al/SiC multilayers at: (a) room temperature and (b) 100 ºC.
A current example of application of these tests is found in the case of nanoscale multilayers. These materials, consisting of alternating layers of two materials, are attractive composites due to their unique electrical, magnetic, optical, and mechanical properties. In particular, metal-ceramic systems, like Al/SiC, display an attractive combination of strength, hardness and toughnes. However, little is known about their high temperature mechanical behavior as they are manufactured in the form of very thin (≈ µm) coatings on Si substrates by magnetron sputtering and there are not standard methods to test their high temperature mechanical properties.
Researchers at IMDEA Materials Institute have overcome this limitation by using Focused Ion Beam machining to fabricate micropillars of 500 nanometers in diameter, that were deformed in compression using a high temperature nanoindentation system. The images in Fig. 1 show two micropillars compressed at ambient temperature and 100 ºC, highlighting the large difference induced by temperature in the behavior of the confined plastic deformation of the metallic layers1. At room temperature, the compressive strength of the multilayers was very large (≈3 GPa) as a result of the constrained plastic deformation of the nanometer thick metallic layers confined between the stiff ceramic layers. However, the enhanced plastic flow of the nanostructured Al layers at 100 ºC (showed by the large extrusions on the edges of the micropillars, Fig. 1b) led to a large reduction in strength. The study reveals the mechanisms at the microscale responsible for the attractive combination of strength and toughness, but also highlights the large sensitivity to the temperature, that results in a 50% reduction in strength at 100 ºC (Fig. 2), due to the nanostructure of the Al layers.
Figure 2. Compressive stress-strain curves of Al/SiC nanoscale multilayers at room temperature and 100 ºC.
The research activities above mentioned have been developed within the framework of the projects SIZEMATERS (MICINN) and ESTRUMAT (Madrid Regional Government). Besides, these promising results are the basis of the R&D cooperative transnational project recently approved by the Materials World Network (NSF) which will be carried out in collaboration with Arizona State University and Los Alamos National Laboratory (EE.UU).
1In collaboration with Prof. Chawla, Arizona State University