Anisotropy effect of bioinspired ceramic/ceramic composites: Can the platelet orientation enhance the mechanical properties at micro- and submicrometric length scale?
Introduction
In general, improving fracture toughness of a material decreases its hardness. In ceramic materials, improved fracture toughness can be achieved by the addition of a soft ductile phase, which thus reduces the inherent brittleness of such materials. To improve the hardness and stiffness required for its final structural application, a large amount of inorganic and/or organic elements can be incorporated, even surpassing the 95 % vol. [1,2].
Many natural materials are both strong and tough. The literature on bio-inspired materials is extensive in number of published papers dealing with multiple biological systems, including: woodpecker [1], nacre [2,3], desert scorpion [4], arterial walls [5], elk antlers [[6], [7], [8], [9]], porcupine quills [10], horse hooves [11,12], sea snail eggs [13,14], buriti palm [15], pomelo fruit [16], plant tissues [17], flax [18], kelp [19], among others. It is also broad in terms of the methodology employed to characterize these complex materials (i.e. mechanical characterization, numerical modeling, environmental assessment, composite fabrication and prototyping, among others) as well as the length scale at which the properties were evaluated. Bioinspired designs are thus promising to enhance the fracture toughness of engineering and structural materials.
A commonly reported strategy to enhance the fracture toughness is to alternate layers of weak and ductile materials between brittle inorganic layers which delay crack propagation [5,6]. Nacre is a well-known example of such microstructure, with hard microscale mineral layers bonded together by soft organic layers. Accordingly, whenever a crack nucleates, it quickly encounters an organic layer, which provides avenues for crack deflection and energy dissipation. Cracks in such microstructures can thus be controlled and stopped before spreading through the whole shell, causing serious damage. However, ductile layers in such synthetic designs are either polymeric or metallic; and hence, the composite materials cannot thus be used at high temperature.
Bioinspired brick-and-mortar ceramic/ceramic designs have recently proven able to combine high strength and high toughness up to reasonably high temperatures (600 °C) [[20], [21], [22], [23]]. They are made of anisotropic alumina platelets, bonded by a glassy interphase with various compositions.
Despite the importance of the mechanical behavior of such structures at the micro-scale, there are very scarce studies of their micromechanical properties, which have mainly focused on macro-scale properties [24,25]. The role of the lamella orientation, for instance, is still unclear. A deeper knowledge of the deformation behavior of such materials as a function of lamellar orientation and temperature at the micro-scale is crucial in order to improve the performance of these materials and to enhance their lifetime under severe working conditions.
Here, we conducted a systematic micro- and nanomechanical study of strong, tough and stiff bio-inspired ceramic samples made from brittle constituents. In order to correlate the mechanical properties at different length scales with the microstructure, we investigated three different platelet orientations. Special attention was paid to analyze the main damage and fracture mechanisms as a function of the platelet orientation by FESEM/FIB.
Section snippets
Material
The ceramic/ceramic composite specimens were processed by Field-Assisted Sintering Technology (FAST). A suspension containing the Al2O3 platelets and the glass-phase precursors is initially frozen with liquid nitrogen, then freeze-dried in order to avoid platelet agglomeration. The sintering, on the other side, allows the user to align the platelets and to keep a fine microstructure of the material due to the short heating time. More information about the processing route is available in Ref. [
Microstructural characterization
After the sintering process, the ceramic/ceramic composites presented a heterogeneous microstructure composed of alumina (Al2O3) platelets and a minor secondary phase mainly based on silicon oxide (SiO2) and calcium carbonate (CaCO3), located between the platelets. Fig. 3 shows the SEM micrographs of the ceramic/ceramic composites with 0 and 90° oriented platelets. The 45° oriented sample (not shown here) has a microstructure similar to the 90° sample. The spatial distribution of the main
Conclusions
In this study, the microstructure and the micro- and submicrometric properties, in terms of hardness and elastic modulus, from room temperature up to 550 °C as well as the induced damage produced during the indentation process were investigated in detail as a function of the Al2O3-platelet orientation. The following conclusions can be drawn:
- (i)
Hardness and elastic modulus values were assessed in the 21–24 GPa and 400–426 GPa ranges respectively for the Al2O3-platelets/glassy phase composite system
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
J. J. Roa acknowledges the Serra Hunter program of the Generalitat de Catalunya. S. Deville and H. Saad acknowledge the financial support from Agency National de la Recherche, project BICUIT (ANR-16-CE08-0006).
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