Optimizing film thickness to delay strut fracture in high-entropy alloy composite microlattices

Incorporating high-entropy alloys (HEAs) in composite microlattice structures yields superior mechanical performance and desirable functional properties compared to conventional metallic lattices. However, the modulus mismatch and relatively poor adhesion between the soft polymer core and stiff metallic film coating often results in film delamination and brittle strut fracture at relatively low strain levels (typically below 10%). In this work, we demonstrate that optimizing the HEA film thickness of a CoCrNiFe-coated microlattice completely suppresses delamination, significantly delays the onset of strut fracture (∼100% increase in compressive strain), and increases the specific strength by up to 50%. This work presents an efficient strategy to improve the properties of metal-composite mechanical metamaterials for structural applications.

Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. been implemented in composite micro-and nanolattices to exhibit superior specific strength and tunable properties compared to conventional metals/alloys [22][23][24]. Despite their high strength [22], these HEA microlattices still experience brittle strut fracture at low compressive strains (∼7%). The majority of composite microlattices reported thus far exhibit this phenomenon, which is primarily caused by weak cores or film delamination [12,[18][19][20]. Hence, in this work, we employed projection micro-stereolithography (PµSL) and magnetron sputtering to fabricate CoCrNiFe-coated composite microlattice. Due to its liquid crystal on silicon spatial light modulator and multi-projection stitching process, PµSL offers a large, high-resolution build area, enabling the fabrication of centimeter-scale samples with micro-scale resolution [10,25,26]. We show that optimizing the film thickness completely suppresses delamination and drastically delays the onset of strut fracture. Ultimately, we managed to fabricate composite microlattices that exhibit higher specific strength and compressive strain compared to previous works.

Material and methods
An octet lattice (figure 1(a)) was designed and optimized using a CAD software (SolidWorks™). Polymer microlattices were fabricated via PµSL (BMF™ Technologies) using 1,6hexanediol diacrylate (HDDA) resin ( figure 1(b)). The octet geometry demonstrates stretching-dominated behavior, allowing it to withstand larger loads than bending-dominated lattices at similar relative densities [27,28]. The octet lattice had a unit cell size of 2 mm and a strut diameter of 0.2 mm. Subsequently, a thin layer of CoCrNiFe HEA film was deposited onto the polymer scaffold via magnetron sputtering at room temperature (figure 1(c)). High purity (⩾99.99%) CoCrNiFe alloy targets were used, and the chamber was evacuated below 4.0 × 10 −4 Pa. The total argon flow rate was fixed at 12 sccm, and the rotation speed was 10 r min −1 to homogenize the alloy composition and film thickness. The sputtering power was 200 W. The substrates were neither cooled nor heated during deposition. The thickness of HEA films was adjusted by the sputtering time, while all other parameters were kept constant. Gatan™ Microtest 200 was used to perform the uniaxial compression experiments at a strain rate of 10 −3 s −1 .
For characterizations, FEI™ Quanta 450 FEG scanning electron microscope (SEM), JEOL™ JEM-2100 F transmission electron microscope (TEM), and Rigaku™ SmartLab x-ray diffraction (XRD) with a scanning range from 30 • to 60 • were used to capture the post-compression microlattice morphologies and microstructures of CoCrNiFe HEA thin films, respectively. The HEA film was deposited on a Si wafer during the deposition of the microlattice [24]. The coated Si wafer was used in the characterizations described above. The TEM sample was prepared via manual milling followed by ion-milling at liquid nitrogen temperature to minimize potential ion damages. A cross-sectional view of HEA film with a single-tilt holder was observed using TEM at an acceleration voltage of 200 kV. shows (111), (200) and (220) peaks, further verifying the FCC structure. Figure 3 shows the in situ uniaxial compression experiments performed on the polymer and composite microlattices. Figures 3(a) and (b) show the deformation process of the microlattices during compression and the corresponding stress-strain curves, respectively. All of the microlattices in figure 3(a) show elastic strut buckling at low compressive strains. However, the microlattice with a thicker coating (i.e. 400 nm) exhibited localized strut fracture before the pristine polymer (i.e. 0 nm) and composite microlattice, which had a thinner coating (i.e. 100 nm). The thinly coated microlattice displayed strut fracture at a significantly higher compressive strain (∼17%) compared to that of the composite microlattice with a thick coating (∼8.5%). The thinly coated microlattice performed is similarly to the pure polymer microlattice (∼18%). This observation agrees with the corresponding stress-strain curves shown in figure 3(b). Although the microlattice with a thinner coating possess a slightly lower modulus, its peak stress was about 50% higher than the microlattice with a thick coating. The increased peak stress is due to the enhanced compressive strain at which strut fracture initiates. As shown in figure 4(a), the thinly coated microlattice possessed the highest specific modulus and strength compared to the polymer and composite microlattice with the thicker coating. The obtained values were higher than previously reported microlattices, such as NiP [3] and SiNcoated [4] microlattices. For the calculation of specific modulus and strength, the density of the composite microlattices were approximated using the following equation:

Results and discussion
where ρ MEA and ρ poly are the estimated densities of the CoCrNiFe HEA film and HDDA photoresist, respectively. The density of the film was determined through the rule of mixtures and the respective densities of each element. V MEA and V poly are the volumes of the CoCrNiTi 0.1 film and polymer core, respectively. To elucidate the delayed strut fracture in the thin-coated (i.e. 100 nm) composite microlattice, the dominant deformation behavior was determined using a simple buckling model where the composite configuration was approximated by a cylindrical shell (i.e. HEA film) with an elastic core (i.e. polymer) [29]. The loading conditions for the compression of core-shell pillars was used to mimic the stretching-dominated behavior of the octet lattice geometry [27,30]. The critical buckling stress, σ cr , for the composite pillars under uniaxial compression was calculated by modifying the results for the buckling of a hollow cylinder (equation (1)) [31] to account for an elastic, complaint core as a two-dimensional (2D) foundation supporting a longitudinal strip, mn ( figure 4(b)), of the shell.
In equation (1), σ o represents the critical buckling stress of a hollow cylinder with radius, r, and film or wall thickness, t. Equation (2) gives the sinusoidal radial displacement, w, during composite strip buckling: In this case, l/m represents the half-buckled wavelength, l ′ , and w m is the maximum radial displacement. The compliant polymer core is treated as a half elastic space with a spring constant of k poly , as expressed in equation (3) [32,33]: where the buckling wavelength parameter λ = l/mπ = l ′ /π . Consequently, the axial load (N axial ) in the buckled composite strip can be represented as equation (4): and α = E poly /E HEA . E poly and E HEA are the elastic modulus of the core (∼3 GPa) and HEA film (∼216 GPa). v HEA and v poly are the Poisson's ratio of the HEA film and core, which were taken as 0.3 and 0.4, respectively [34]. The minimum or critical buckling load can then be calculated by setting the derivative of N axial with respect to λ equal to zero. In our case, (r/t) ( E poly /E HEA ) > 10. Thus the critical wavelength parameter, λ cr , can be calculated by the result for the wrinkling of a flat sheet (i.e. HEA film) on an elastic core (i.e. polymer), shown in the following (equation (5)) [33]: For the 2D composite strip, σ crit = N axial /t, and as , thus, the critical buckling stress in the shell can be expressed by the following (equation (6)): where f is given by: . (7) Alternately, by combining equations (1) and (6), the critical buckling stress, σ cr , could be written as: The calculated buckling wavelength parameter to thickness ratios, λ cr /t, are plotted in figure 4(c) (adapted from Karam and Gibson) [29]. Our composite microlattices lie well outside of the solid black line, indicating that the microlattices possess a core that is strong enough to support the HEA film and result in a synergistic enhancement in load bearing capability. This explains the buckling and gradual localized strut fracture observed in the deformation of our composite microlattice as opposed to the catastrophic brittle failure, represented by an extreme drop in stress levels, that results from insufficient core strength [2,8].
The improved strength and ductility of the thin-coated composite microlattice is mainly attributed to the brittle-toductile transition of the HEA film caused by the size reduction. Figure 5 shows the SEM images of deformed composite microlattices after compression. The microlattice with 100 nm coating retained its smooth surface morphology without any apparent cracks even after deformation. No delamination was observed after closer inspection of a fractured strut, which indicates a good adhesion of the film to the polymer core. Furthermore, the rough and serrated cross-sectional morphology of the fractured polymer core implies that the composite microlattice has an elasto-plastic deformation behavior and experiences significant plastic deformation prior to fracture. Conversely, the microlattice with 400 nm coating had multiple cracks on its strut surfaces after compression. The cross-sectional morphology of the fractured strut indicates some delamination and brittle fracture surface, implying that it undergoes little to no plastic deformation. In a coreshell configuration, the metallic film is the dominant loadbearing component. Thus, strut fracture initiates from the film as opposed to the polymer core (i.e. cracks start from outer surface). Once the film cracks, the polymer core cannot withstand the load by itself, causing the strut to fracture. Therefore, as the fracture expands from the HEA film to the polymer core, it travels in the direction of the slip planes of the HEA film as opposed to the build direction of the polymer. Overall, the post-mortem morphologies of the fractured lattices explain the largely delayed onset of fracture in the composite microlattices with thinner film (100 nm), which results in higher peak stress at lower densities.

Conclusion
In this work, we designed and fabricated CoCrNiFe HEAcomposite microlattices via PµSL 3D printing and magnetron sputtering coating. After investigating the mechanical properties of octet microlattices coated with different film thicknesses (100 nm and 400 nm) via in situ compression experiments, we found that reducing the HEA film coating thickness significantly delayed strut fracture onset. This increases the compressive strain at which a stress drop occurs and the strength of a microlattice despite its lower density. We attribute these results to the size-induced brittle-to-ductile transition of the HEA films, as shown by absence of cracks, delamination and obvious plastic deformation in the fractured strut of the thin-coated composite microlattices. Our strategy may apply to other advanced metallic/alloy composite microlattices designed for robust mechanical and functional applications.