Length scale effects on brittle-ductile interface failure

The route to gain exceptional structural or functional properties in advanced materials is to combine materials with different, often contradictory, physical properties. Heterogeneous structures comprising of distinct materials are therefore widely encountered in technological and natural materials as composites at different length scales. The common boundaries of the different material layers, denoted as interfaces, are known to be particularly weak and therefore dictate the overall deformation behavior and reliability of composite structures. One prevailing materials combination occurring in various technological and natural materials is the combination of brittle (strong) materials with ductile (soft) materials such as protective coatings, thin film components in microelectronic packages and biomaterials. In this prospective project, it is the aim to study the role of the interface on the deformation behavior of a model brittle-ductile material system at different length scales ranging from the nanoscale to the macroscopic regime. As a chemically inert model material system ductile silver (Ag) films are deposited onto brittle single crystalline magnesium oxide substrates (MgO) by means of physical vapor deposition techniques. It is envisioned to build a fundamental framework describing the deformation behavior and predominant crack propagation mechanisms of the MgO-Ag interface. Advanced experimental techniques encompassing monotonic and cyclic loading setups will be applied in order to study the interfacial behavior of bi-materials interface structures. It is further planned to conduct selected in-situ experiments under the scanning and transmission electron microscope to directly observe the involved crack initiation and propagation along the bi-material interface and examine dislocation formation and motion which accompany interface failure. The main focus will be on understanding the role of the interface on the deformation processes as a function of film thickness, microstructure, and loading conditions and how these phenomena influence measured interfacial fracture toughness. Therefore, a systematic study of the failure mechanisms occurring at different length-scales encompassing roughly eight orders of magnitude will be conducted. It is the aim to thoroughly understand the involved mechanisms especially at the nano-scale, which is becoming more and more important due to ongoing miniaturization of materials structures and composites in novel applications.

Achieving exceptional structural and functional properties in advanced materials often involves combining materials with diverse and sometimes contradictory physical properties. Heterogeneous structures, represented by distinct materials in composites, are prevalent in both technological and natural materials at various length scales. The interfaces between these materials play a crucial role in determining the overall deformation behavior and reliability of composite structures, as for example encountered in applications such as microelectronics. This project focused on the examination of thin film properties and their impact on their delamination behavior at different length scales, with a particular emphasis on brittle-ductile interfaces- which are interfaces separating ductile thin films from brittle substrates. The conventional Hutchinson & Suo model, rooted in elastic beam theory, is commonly used to quantify interface adhesion of such material systems but does not consider plastic deformation during film deformation. Our novel experimental approach, conducted at a microscopic scale, demonstrates that a thin film's ability to undergo plastic deformation enhances its resistance to delamination, thereby improving adhesion. This trend could also be confirmed using finite element simulations. Advanced experimental techniques, including monotonic and cyclic loading setups, were developed to study the interfacial behavior of bi-material interface structures. In situ transmission electron microscopy, coupled with a picoindentation sample holder, was employed to trigger delamination while observing thin film deformation mechanisms. This comprehensive approach aims to unravel the involved mechanisms, particularly at the nano-scale, as the ongoing miniaturization of materials structures and composites becomes increasingly important in novel applications. The insights gained from this study have the potential to advance our understanding of interface reliability and contribute to the design of more reliable composite materials for diverse technological applications.