# Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review
## 3 Basic Mechanical Properties
### Experiment Methods
#### 3.1.3 Flat Punch Micropillar Compression  
For NMMFs, when \(h\) is below 20 nm, tensile tests reveal that insignificant deformability occurs in the test specimens as a result of small flaws [56]. In this condition, stress-strain curves and the ductility are not available using traditional methods. However, micropillar compression testing which enables measurement of stress-strain response at small \(h\) has been successfully used to investigate the mechanical performance of NMMFs [5; 13; 57]. Focused ion beam machining has created opportunities for fabricating micropillar NMMFs, where a sample is shaped with a high precision using a Ga+ ion beam [58; 59]. Figure 9 shows such a pillar with the layer direction perpendicular to the cylinder axis.  
The microcompression test was performed with a flat punch diamond indenter. During testing, force-displacement data were continuously recorded, and a constant volume is used to calculate the true stress-strain curves, the homogeneous deformation assumption model is applied to characterize  
Figure 8: Respective SEM images showing the buckles in Cu/Nb NMMFs after uniaxial tensile testing [52].  
Figure 9: SEM micrograph of the micropillar of Cu/Nb multilayer with \(h\) = 5 nm. The layer interfaces are perpendicular to the cylinder axis [5].  
the deformation behavior [57; 60]. Considering the substrate effect and taper influences, the true strain \(\varepsilon_{T}\) and true stress \(\sigma_{T}\) are expressed as [60]:  
\[\varepsilon_{T}=\frac{1+(L_{0}/r_{0})\tan\psi}{E_{measured}}\frac{PL_{p}}{A_{0} L_{0}}+\ln(\frac{L_{0}}{L_{p}}) \tag{13}\]  
and:  
\[\sigma_{T}=\frac{P}{A_{0}L_{0}}\bigg{\{}L_{0}-[\mu_{tot}-\frac{PL_{Su}}{E_{Su} A_{Su}}-\frac{\sqrt{\pi}P(1-v^{2})}{2E_{Su}\sqrt{A_{Su}}}]\bigg{\}} \tag{14}\]  
where \(A_{0}\) is the cross-sectional area at half initial height (\(L_{0}\)) of the pillar; \(r_{0}\) is the radius at the top of the pillar; \(L_{p}\) is the final height of the pillar; \(P\) is the load and \(E_{measured}\) is the measured modulus of the tapered NMMFs determined from the unloading segments at strains between 1% and 5%; \(\mu_{tot}\) is the total displacement; \(A_{Su}\), \(L_{Su}\) and \(E_{Su}\) are the average cross-sectional area, the total length and the modulus of the substrate pillar, respectively. The yield strength of micropillars is no longer expressed by flow stress at strain of 0.2%. Since large stress strain scatter is generally observed in the initial stage of plastic flow, the flow stress at a relatively large amount of strain (2%) is chosen [60; 61].  
In general, the macroscopic deformability of these NMMFs micropillars, as well as the deformation behavior (homogeneous vs. in-homogeneous), has been a hot topics in the current literatures. Additionally, most of the micropillar work pays close attention to the effects of sample size i.e., pillar diameter on the yield strength [62; 63; 64]. In contrast, in small-scaled materials such as NMMFs, the extrinsic size and intrinsic size overlap in general. Unlike traditional nanoindentation test leading to a non-uniform stress state in the plastic zone within the sample [65; 66], the microcompression methodology opened a new avenue for studying the origin of deformation instability in NMMFs with a nominally homogeneous stress state [61].