High-strength cellular ceramic composites with 3D microarchitecture

Abstract

To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m(3); only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina-polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m(3).

Fig. 1.

Computer-aided design models (Upper) and SEM images (Lower) of examined cellular microarchitectures (scale bars: 10 µm). Design C is an orthotropic construction with nonrigid cubic unit cells (C); therefore it generally is considered to behave in a rather bending-dominated manner. We also introduced global diagonal bracings to obtain a more stretching-dominated and collapse-resistant behavior (B). When applied to all faces of every unit cell (A), the structural stability is enhanced further. Stretching domination is maximized because the structure is fully triangular. The design may be regarded as behaving fairly isotropically. Depending on the stiffness of junctions, collapse mechanisms are less urgent and topologies may be designed so that a maximum of structural elements are arranged in loading direction, without risking early global buckling. We realized that approach with a hexagonal truss structure (D), whose unit cell geometry is not rigid, just like that of design C, and with a shape-optimized honeycomb design (E). Both constructions behave anisotropically.

Fig. 2.

All cellular material designs were tested mechanically under uniaxial compressive loading (compare Movies S1–S4). (A) An in situ test of a polymeric truss structure (side view). (B) Stress–strain curves of fully triangular trusses (Fig. 1A). Alumina layers of the indicated thicknesses have been deposited onto a polymeric core structure. With increasing layer thickness, the compressive strength and stiffness (Young’s modulus ) increase strongly compared with bare polymeric structures. (C) Specific compressive strength as a function of the coating thickness . Labels refer to the nomenclature of Fig. 1. The estimated stress at failure inside the alumina layer, (Eq. 5), increases with decreasing layer thickness. All data points correspond to at least three measurements. Error bars give the corresponding maximal upper and lower deviation.

Fig. 3.

Failed samples of different coating thickness (see headings) and architecture (Upper, design A; Lower, designs C–E). With growing layer thickness of alumina, the failure mechanism changes from buckling to brittle fracture. (A) Polymeric truss designs A and B buckle with large plastic deformations. (B–E) Once coated with Al₂O₃, plastic deformation is reduced significantly (compare Movies S1 and S2)_. (B) Face buckling, fine networks of surface cracks, and squamous flaking occur. (C) In the range of 50-nm thick coatings, brittle fracture of the vertical compressive bars, normal to the loading direction as well as close to the junctions, was observed. (D) Reaching 100 nm, fracture was detected to appear exclusively at the junctions. (E) For 200 nm, we found the structures and especially their vertical compressive bars to burst into small pieces. (F) Polymeric honeycomb structures buckle with large plastic deformations. (G) For 10-nm thick coatings, buckling causes fine networks of surface cracks, leading to vertical crack propagation (compare Movie S4). (J) Thicker-coated honeycomb designs burst in a brittle manner. (H) Designs C and D buckle globally and fracture without notable plastic deformation for both bare polymeric structures (compare Movie S3) and coated ones. (I) Reaching 100 nm, we observed cracking of the vertical compressive bars normal to the loading direction and fracture close to the junctions.

Fig. 4.

Compressive strength–density Ashby chart showing the cellular ceramic composite materials described in this report compared with other materials (compare CES EduPack, Granta Design Ltd.). The truss structures A, B, and D outperform all technical foam materials. The optimized honeycomb designs achieve strength-to-weight ratios comparable to those of technical ceramics and high-strength steels. The nomenclature refers to Fig. 1. Labels indicate the thicknesses of the deposited alumina layers.