Tough metal-ceramic composites with multifunctional nacre-like architecture

Abstract

The brick-and-mortar architecture of biological nacre has inspired the development of synthetic composites with enhanced fracture toughness and multiple functionalities. While the use of metals as the "mortar" phase is an attractive option to maximize fracture toughness of bulk composites, non-mechanical functionalities potentially enabled by the presence of a metal in the structure remain relatively limited and unexplored. Using iron as the mortar phase, we develop and investigate nacre-like composites with high fracture toughness and stiffness combined with unique magnetic, electrical and thermal functionalities. Such metal-ceramic composites are prepared through the sol-gel deposition of iron-based coatings on alumina platelets and the magnetically-driven assembly of the pre-coated platelets into nacre-like architectures, followed by pressure-assisted densification at 1450 °C. With the help of state-of-the-art characterization techniques, we show that this processing route leads to lightweight inorganic structures that display outstanding fracture resistance, show noticeable magnetization and are amenable to fast induction heating. Materials with this set of properties might find use in transport, aerospace and robotic applications that require weight minimization combined with magnetic, electrical or thermal functionalities.

Figure 1

Processing route used for the fabrication of metal–ceramic nacre-like composites through the assembly of coated platelets followed by densification at high temperature. BnOH: benzyl alcohol. M: metal. MO_x: metal oxide. H_rot: rotating magnetic field. MASC: magnetically-assisted slip casting. SPS: Spark Plasma Sintering.

Figure 2

Alumina platelets coated with different concentration of hematite. Scanning electron micrographs of (a) bare platelets and of platelets coated with (b) 10, (c) 23 and (d) 43 vol% of hematite. The imaged platelets were obtained after the sol–gel coating procedure followed by a burn-out step. Platelets shown in (b) and (c) were prepared using precursor/platelet mass ratios of 1 and 2, respectively. The high fraction of hematite formed on the platelet displayed in (d) results from two successive coating reactions performed with the same powder. (e) Volume fraction of hematite formed on the platelet surface as a function of the initial precursor/platelet mass ratio. The dotted line indicates the predicted volume fraction if all the iron added as precursor for the sol–gel reaction is converted into oxide particles on the surface of the platelets. Optical images of the powders synthesized with increasing precursor/platelet ratios. Scale bars: 500 μm.

Figure 3

Crystallography and morphology of the platelet coating after heat treatment at distinct temperatures under reducing atmosphere in a small-batch process. (a) Volume fraction of each crystalline phase present in coated platelets subjected to different reduction temperatures for 8 h. The initial coated platelets consist of 89% Al₂O₃ and 11% Fe₂O₃. The lines between experimental data points are guides to the eye. (b) SEM images of the platelet coating at selected temperatures, highlighting the morphological changes associated with the phase transformations of the initial hematite particles. Magnification: 85 k ×.

Figure 4

Mechanical properties of metal-ceramic composites with nacre-like architecture. (a) Stress–strain curves obtained from three-point bending tests on un-notched specimens containing different iron fractions. (b) Effect of the metal fraction on the flexural strength and the elastic modulus of nacre-like composites. (c) Ashby plot depicting the specific elastic modulus and specific strength of our metal-ceramic composites in comparison to literature data for conventional materials. (d) Force–displacement data obtained for single-edge notched beam samples with varying iron content. (f) Effect of the metal fraction on the critical stress intensity factor for crack initiation (K_IC) and the maximum intensity factor after crack propagation (K_Jmax). (e) Stress intensity factor (K_J) of the composite as a function of the propagated crack length (R-curve). (g,h) Scanning electron microscopy (SEM) images of the fracture surface of a specimen containing 12.4 vol% Fe highlight (g) the plastic deformation of the metallic phase and (h) the homogeneous distribution of metallic iron within the structure. Magnification: 88 k ×. In image (h) the EDX intensity spectra for Fe and O elements across a selected line is displayed on top of a false coloured micrograph to illustrate the distribution of metallic and ceramic phases in the structure. Magnification: 10 k ×. (i) Light microscopy image of the fracture generated in a notched specimen after testing. Magnification: 100 ×.

Figure 5

Magnetic properties of the metal-ceramic composites with nacre-like architecture. (a) Picture displaying the magnetic attraction between a nacre-like metal-ceramic specimen and a stack of commercial neodymium magnets. (b) Magnetization as a function of the applied magnetic field strength of metal-ceramic composites made with increasing volume fractions of iron and hercynite. (c) Closer view of the magnetization curves in (b) around the origin. (d) Saturation magnetization as a function of the volume fraction of metallic iron in the composite. (e,f) Volume magnetic susceptibility χ, the remanent magnetization M_r and the coercive field H_c as function of the content of metallic iron in the specimens.

Figure 6

Inductive heating behavior of the metal-ceramic composites upon exposure to oscillating magnetic fields. (a) Pictures showing the coil used to generate the magnetic fields and the position of the specimen in the beginning of an experiment (left) and when it reached the peak temperature (right). (b) Temperature recorded over time during the inductive heating experiment performed with specimens of different iron fractions. In this experiment, the magnetic field was turned on at time zero and was switched off at the time points indicated with a star symbol inside the plot. (c) Average heating rate of composites with varying volume fraction of iron (b).