A combined compression and indentation study of mechanical metamaterials based on inverse opal coatings

A relatively new class of tailored photonic metamaterials based on the inverse opal (IO) structure shows multifunctionality with exceptional mechanical response due to its periodic porous arch-like structure. Exploiting the smaller is stronger paradigm through varying its pore size and the addition of atomic layer deposition (ALD) films, allow tailorable strength and elastic modulus. Quantification of such properties are achieved through flat punch nanoindentation testing. Results are validated by comparison to microcompression tests, a widely used technique to circumvent the complex stress state normally imposed by indentation, but in the case of high porosity is greatly simplified, approximating uniaxial stress; results from both mechanical loading approaches show strong similarities. All results showed a dependence of strength and elastic modulus on the ratio of the deformation size, i.e., micropillar or flat punch diameter, to the pore size, a trend which is well described by the influence of the boundary conditions of the test method rather than an intrinsic size effect. At larger ratios the values approach a constant value. Furthermore, the mechanical response can be tailored through the deposition of a thin film on the shell structure of the silica IO structures; 34 nm of TiO2 was shown to produce a 10-fold increase in strength and 5-fold increase in elastic modulus.