High temperature mechanical behavior of AI/SiC nanoscale multilayers
Nanoscale Al/SiC composite laminates are metal ceramic multilayers with unique mechanical properties at ambient temperature, such as high strength, high toughness, and damage tolerance, due to the nm scale thickness of their layers. Nevertheless, nothing is known about their high temperature mechanical properties and this is a key issue both from the fundamental viewpoint as well as from the in service behavior. This lack of information is mainly due to the difficulties associated with the characterization of the mechanical behavior of thin films at high temperature, a rather unexplored area. High temperature instrumented nanoindentation and micropillar compression were carried out in this thesis to study the mechanical properties of Al/SiC nanolaminates as a function of layer thickness from room temperature up to 300oC. Mechanical tests were complemented with detailed transmission electron microscopy (TEM) analysis of the deformed structures to elucidate the effect of temperature on the deformation mechanisms at the nm scale. In addition, finite element simulations of the multilayer deformation were used to clarify the influence of the Al flow stress and of the interface properties (strength, friction coefficient) on the overall stress strain response of Al/SiC multilayers. The combination of nanoindentation, micropillar compression tests, TEM observations and numerical simulations provided a better understanding of the key parameters influencing the high temperature mechanical behavior of Al/SiC nanoscale multilayers. It was found that the mechanical behavior at ambient temperature was controlled by the high strength of the Al nanograins and the constraint induced by the stiff SiC nanolayers on the Al plastic flow. Changes in the Al SiC interface behavior, in the form of interface sliding, limited the constraint on plastic flow at 100oC. This phenomenon, together with the softening of the Al nanolayers, resulted in a marked reduction in the flow stress and in the strain hardening capacity of the nanoscale multilayers. The role of the Al SiC interfaces in plastic flow was also apparent in the creep activation energies, which showed a marked decrease with the reduction in Al layer thicknesses, reaching values close to the activation energy for grain boundary diffusion in Al for layer thicknesses of 10 nm. Finally, above 200oC, chemical reactions between Al and SiC promoted a large degradation in the mechanical properties of the nanoscale multilayers.