Deformation, strengthening and fracture mechanisms of nanoscale Al/SiC multilayers

Resum

Nanoscale Al/SiC composite laminates are metal-ceramic multilayers with unique mechanical properties. In particular, extremely high strength has been reported at ambient temperature when the individual layer thicknesses are below 100 nm, and this behavior has been attributed to the large density of interfaces and to the nanoscale layer dimensions. Nevertheless, there are a number of fundamental questions about the deformation of metal-ceramic nanolaminates that remain unanswered. First, it is not clear how plasticity in the Al layers takes place when the length scale is reduced to the nanometer scale and what is the role of the ceramic layers on deformation. Second, although their behavior is potentially anisotropic, the fact that nanolaminates are often produced in the form of micrometer thick coatings has prevented the mechanical properties to be measured in different loading directions. And, finally, it has been proposed that the large density of interfaces can make these materials extremely damage tolerant but the fracture toughness has been rarely measured and the fracture mechanisms are unclear. The work carried out in this thesis contributes to clarify these open issues by employing a set of novel nano-mechanical testing techniques like nanoindentation, micropillar compression, microtensile testing and micropillar splitting tests on Al/SiC nanolaminates with layer thicknesses in the range 10-100 nm. The mechanical tests were complemented with detailed transmission electron microscopy analysis of the deformed structures and finite element simulations to clarify the influence of the individual layer properties and other microstructural features (the layer waviness or columnar grains) on the overall stress-strain response of Al/SiC multilayers in different directions. The strength of the Al layers as a function of layer thickness and temperature was obtained from the hardness of the nanolaminates by an inverse methodology based on the numerical simulation of the nanoindentation tests by means of the finite element method. The room temperature yield stress of the Al layers showed a large “the thinner, the stronger” effect, which depended not only on the layer thickness but also on grain size. This was confirmed by in-situ mechanical tests within the transmission electron microscope that showed that dislocations were nucleated at the Al/SiC interfaces, swept the nanocrystalline Al grains and were absorbed at the opposite interface. There was a dramatic reduction in the Al yield stress with temperature, which increased as the Al layer thickness decreased, and led to an inverse size effect at 100ºC. This behavior was compatible with plastic deformation mechanisms controlled by grain boundary and interface diffusion at 100ºC, which limit the strength of the ultra-thin Al layers. Then, the hardness measured by nanoindentation was compared with the strength determined by micropillar compression. It was found that the strain hardening rate and the compressive strength of the micropillars increased much more rapidly with layer thickness reduction than the hardness. Finite element simulations revealed that the constraint imposed by the SiC layers on the plastic deformation of the Al layers was responsible for the large strain hardening rate observed. Moreover, it was demonstrated that the degree of constraint depended on the ratio between layer thickness and micropillar radius due to the contribution of the free surface of the micropillars to deformation. This work shows that taking these effects into account is essential to understand the mechanical performance of metal-ceramic nanoscale multilayers measured under different testing conditions. The anisotropic response of Al/SiC nanolaminates was also explored by performing micropillar compression tests in directions perpendicular (90º), parallel (0º) and oblique (45º) to the layer orientation. The 0º orientation showed the highest strength because the reinforcing SiC layers were aligned with the loading direction, while the 45º orientation showed the lowest strength because it promoted the deformation of the Al layers by shear. The strength for perpendicular loading was limited by the cracking of the SiC layers perpendicular to the layers as a result of the tensile stresses that developed in the SiC layers due to the constraint. The strength for parallel and oblique loading was, however, limited by buckling of the layers and the development of shear bands along the columnar boundaries. While the strength for perpendicular loading increased abruptly with layer thickness reduction, the strength for parallel loading was independent of layer thickness and controlled by the layer waviness, in agreement with buckling-induced failure. Finally, the fracture toughness of the Al/SiC nanolaminates was determined, for the first time, by using the micropillar splitting method. The fracture toughness was relatively low, in the range 0.7-1.2 MPa√m. It increased as the layer thickness was reduced from 100 nm to 25 nm to decrease again for the smallest layer thickness of 10 nm. Analysis of the fracture surfaces revealed a tortuous crack path, with clear signs of ductile fracture of the Al layers and crack deflection at the Al-SiC interfaces for layer thicknesses between 25 and 100 nm but not for 10 nm. The results highlight the role of the Al-SiC interfaces and the columnar boundaries on the fracture of Al/SiC nanolaminates, as demonstrated by microtensile testing in directions perpendicular and parallel to the layers. Under tensile loading, the nanolaminates showed a very brittle behavior, independent of layer thickness. The tensile strength was found to be limited by the Al-SiC interface strength in the direction perpendicular to the layers and by the strength of the pre-existing columnar boundaries in the parallel direction. The results point out that, even though a reduction in the size of the ceramic layers has obvious benefits on delaying cracking of the SiC layers, the reduction in the thickness of the Al layers hinders energy dissipation by plasticity of the Al layers. As a result, Al/SiC nanolaminates are very strong, but cannot be deemed as damage tolerant materials.