# Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review
## 5 Conclusions and Future Developments  
In this overview, we have emphasized the broad trends observed in the fracture properties of nanoscale metallic multilayer films, such as fracture toughness, wear resistance, adhesion energy, and plastic instability, which roughly summarize the development of NMMF in recent 10 years. A transition from bulk-like feature behavior to one controlled by individual dislocation and interface-mediated fracture has been reviewed. Interface structure and length scale are believed to be two important factors deciding the fracture response. In sharp contrast to tough monolithic crystalline metals, the presence of interfaces enables the very high strength associated with the multilayered structure. Accordingly, through properly tailoring the internal features (length scale and interface structure), an optimized combination of fracture toughness and strength can obtained, which runs counter to the popular belief that increasing the strength leads to a decrease in the fracture toughness in materials. This finding is potentially able to explain effectively'structure-properties' relationships that are used to advantage in multilayered composites, where the microstructure can be tailored with the goal of achieving a high density of fcc/fcc coherent interfaces [15]. Hopefully, the theoretical analyses and available experimental results can provide further impetus for understanding the nanoscale damage of multilayers, with which it is possible to artificially control the constituent phases or dimension configurations to achieve target performance in technological applications. While there are still many key issues that remain puzzling. Their resolution would undoubtedly lead to improved fracture properties and superior understanding of the deformation map for NMMFs. In the following, we list a few questions with which we are concerned, which may shed light on further development. 1. The length scale-dependent toughening map [15] is mainly build on an integrated interface which acts as effective barriers for crack propagation, and there is largely ignored interdiffusion at the interfaces, especially at small \(h\) and in multilayers with negative heat of mixing [148; 149; 150]. The formation of alloys and amorphous is a result of interface diffusion [95; 151; 152], which should be considered in building the deformation map for NMMFs. For \(h<1\) nm, even a small amount of interdiffusion may have a pronounced effect on interface stress and strength [148; 153]. Additionally, residual stress in the layers, e.g., growth stress in NMMFs could also be important in influencing the toughening model [154]. 2. Plastic deformation of NMMFs has been described using various mechanisms with varying levels of success. While the majority of the mechanisms focus on the NMMFs with equal individual  
Figure 27: Deformation map for crystalline/crystalline nanolayered micropillars: shear localization in RI, shear + extrusion deformation in RII, extrusion and barreling deformation in RIII [61]. layer thickness (modulation ratio \(\eta\) = 1), few works concern NMMFs with varying modulation ratios. Under certain conditions, such as modulation-induced structure transition (Section 2.2.1) and length-scale-induced toughening (Section 4.1), it is not only interesting, but also of practical requirement to establish more comprehensive models for exploring the size effect. 3. Recent studies show that interface structure also plays an important role in shear instability in NMMFs. Dislocation transmission favors the formation of crystallographic band, whereas dislocations do not transmit which cause interface tilting and are associated with shear band formation [155]. It is also argued that the interface with low shear strength and strong barriers for slip transmission indicates a new mechanism for shear banding and fracture [13].