Toughening mechanisms in nacre-like alumina revealed by in-situ imaging of stress
Introduction
Strength and toughness are mutually exclusive properties of materials in general and ceramics in particular. Designs found in biological structural materials such as nacre may provide useful guidelines to alleviate this trade-off [1]. Nacre-like alumina, made of brittle constituents only, combines the hardness and strength of standard alumina materials, and comparatively high toughness. This toughness (damage resistance) is achieved mostly by the deviation of cracks along the platelets interface in their brick-and-mortar microstructure [2]. While the deviation of cracks in brick-and-mortar microstructures is well documented, several other toughening mechanisms can exist in biological or bio-inspired materials [3], [4]. Discrete elements modeling (DEM) has been implemented to understand the toughening mechanisms of nacre-like alumina and provide guidelines of microstructural features to implement to improve the damage resistance of these composites [5]. Establishing quantitative links between nano- to microscopic structures and bulk mechanical properties determined in standardized fracture tests requires additional mechanical modeling at various scales and methods that enable imaging the effects of microstructures on the mechanical behavior. For instance, structural defects (such as local misalignments, variable platelet size) of real microstructures spanning few tens of micrometers can be modeled by DEM, which suggests that they may have a significant effect on crack propagation [6]. To validate these results, we developed in situ spectroscopic and scanning electron microscopy (SEM) imaging of stress field and microstructure during a bending test on nacre-like alumina.
Section snippets
Materials and methods
Samples were prepared as standard single-edge notched beams (SENB), and mirror polished before testing [7]. 3 × 6 × 36 mm samples were used for fluorescence spectroscopy and SEM characterization. The fluorescence of trivalent chromium (Cr3+), a ubiquitous impurity in alumina-based polycrystalline ceramics, was used to map stresses in the composite [8], [9]. Even in very low quantities (< 0.5 wt%), Cr3+ causes an intense fluorescence characterized by two sharp and intense lines, noted R1 and R2,
Results
Starting from an initial state with negligible stress, the evolution of the stress map calculated from large fluorescence maps (Fig. 1) shows, the tensile stress field that develops around the crack tip as applied force increases (Fig. 1). Tensile stresses are detected well before the crack initiation. The stress field is asymmetric with the side at higher stress (left of the notch in Fig. 1) eventually fracturing with a crack initiating at about 70° of the notch plane for an applied force of
Discussion
When SENB four-point experiments are performed on non-textured ceramics, the crack initiates and propagates along the notch plane. Strong crack deviation from the notch plane here is due to the anisotropic microstructure of the material. This crack deviation is linked to the contrasted mechanical properties between alumina platelets (brick) and the aluminosilicate glass phase (mortar) [2], [4], [13], [14]. Optimization of brick-and-mortar materials has thus far relied on mechanical tests on
Conclusion
In situ fluorescence mapping of stress and SEM imaging during mechanical tests prove useful to identify local mechanisms and microstructural features that contribute to toughening in these bioinspired composites. Future improvements will focus on measurements that can be combined on the same sample and use the orientation information provided by EBSD to locally analyze the stress from the scale of the individual components (here the alumina platelets) to the scale of the aggregate (platelets
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.
Acknowledgments
We acknowledge the technical support of Saint-Gobain Research Provence. This research was funded through project BICUIT by Agence Nationale pour la Recherche (ANR-16-CE08–0006, project BICUIT). INSU-CNRS and LABEX Lyon Institute of Origins (ANR-10-LABX-0066) support the Raman facility at ENS de Lyon. An anonymous reviewer is thanked for thoughtful remarks that helped clarifying the manuscript.
References (23)
- et al.
Compos. Part B: Eng.
(2020) - et al.
Scr. Mater.
(2021) - et al.
Materialia
(2020) - et al.
Wear
(2012) - et al.
Acta Mater.
(2005) - et al.
Materialia
(2021) - et al.
J. Eur. Ceram. Soc.
(2020) - et al.
J. Mech. Phys. Solids
(2020) - et al.
Scr. Mater.
(2017) - et al.
Theor. Appl. Fract. Mech.
(2019)