Nacre-like alumina composites based on heteroaggregation

High strength and high toughness are usually mutually exclusive in materials. Among all material classes, ceramics exhibit a high stiﬀness and strength, but they present a limited plastic deformation, which results in a moderate toughness. However, tough ceramics have been obtained using anisotropic particles organized in a ’brick and mortar’ microstructure, inspired by the structure of the natural nacre. Here, we propose to build nacre-like ceramic composites from colloidal suspensions using heteroaggregation of particles. Two diﬀerent shaping processes are used: direct settling of suspensions or freeze-granulation. After sintering, in both cases, the platelets alignment is very good, close to that of platelets in natural nacre, with a slightly better one noted for direct settling. Despite a better platelet alignment, the toughness is lower than in previous studies showing that further improvement of the in-terfacial phases present in the material must now be considered to reinforce its mechanical behavior.


Introduction
1 Nacre, found in several species of seashells, is a natural material composed of 95vol.% of 2 aragonite (CaCO 3 ) and 5vol.% of organic materials (proteins), that exhibits a toughness at 3 least three orders of magnitude higher than those of calcium carbonate [1,2,3]. The high 4 toughness of nacre is conferred by its 'brick and mortar' hierarchical architecture composed 5 of layers of inorganic and organic materials [2]. The alignment of inorganic bricks in nacre 6 is remarkable, and has been shown to induce several toughening mechanisms [4]. In the last 7 two decades, natural materials like nacre have been replicated to obtain brick and mortar 8 composites with mechanical properties greater than those of their elementary constituents 9 [1,5,6,7]. 10 To build brick and mortar materials, the control of platelet-shape particles alignment is 11 critical [8]. To control this alignment, different techniques have been proposed [9] such as 12 magnetic fields [10, 11], gravitational settling [12] or ice-templating [7,13]. Most of these 13 processes use colloidal suspensions which have to be well-dispersed. For that purpose, the 14 use of organic binders or dispersants is common, which often imposes a debinding phase 15 before sintering. An alternative, that avoids the introduction of organic compounds in the 16 microstructure, is to take advantage of surface charges present at the surface of particles in 17 aqueous suspensions. Previous studies have shown that the heteroaggregation of particles 18 under the action of the electrostatic forces represents a promising method to control the 19 arrangement of particles [14,15,16]. 20 Here, a nacre-like ceramic composite is elaborated without adding dispersant or carbonated  Here, we show that such nacre-like ceramic composites can be obtained without addition of 31 organic additives using simple shaping methods. Brownian dynamics simulations and exper-32 iments have indeed shown that using the heteroaggregation between the alumina platelets 33 and the silica nanoparticles in aqueous suspensions yields at the same time a compact ar-34 rangement of the platelets and a good spatial distribution of the silica in between them [16]. 35 settling of suspensions and the second in freeze-granulation and lyophilisation. A thermal 37 treatment is then applied to consolidate green samples obtained from both shaping methods. The zeta potential of particles as a function of pH is measured using Acoustosizer IIs from 49 Colloidal Dynamics. For the measurements, aqueous suspensions are prepared with a solid 50 loading of 1wt.% in osmosed water and pH is adjusted using HCl and NaOH.

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The mixed suspensions, used for shaping the nacre-like ceramic composites, are prepared by 52 adding the various constituents in osmosed water and are deagglomerated by an ultrasonic 53 treatment (300 W, 30 s, pulse on 3 s, pulse off 1 s). The volume ratio between the various 54 components Al 2 O 3 /αAl 2 O 3 /SiO 2 is fixed at 94.6%/3.6%/1.8%.

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Sedimentation tests are performed for suspensions prepared with a solid loading of 3vol.%.

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For that, suspensions are prepared and introduced in closed tubes, and allowed to settle.  In this study, two methods are applied to shape the composites:  The alignment of platelets in the sintered samples is characterized by X-ray diffraction (XRD), 82 using a Bruker D8 "Discover" diffractometer equipped with a copper target (Cu K α1 radia-  113 a n = a n−1 + W − a n−1 2 with a and C the crack length and the compliance respectively, calculated at the n and  The first step in this study is to characterize the particles surface charges in aqueous 134 suspensions. For each type of particle, zeta potential measurements as a function of pH are 135 performed (Fig. 1a).

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The natural pH of both alumina platelets and nanoparticles suspensions is around 6.5.

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An isoelectric point around 9 is found for both suspensions: for pH less than 9, both Al 2 O 3 138 platelets and nanoparticles are mainly positively charged and for pH greater than 9, they are 139 negatively charged. For silica particles, the natural pH is around 9, which can be explained The three-component suspensions are then used to shape nacre-like ceramic composites.

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First, we use sedimentation to shape pellets. After drying, some pellets are broken in half 165 and the rupture surface is observed by SEM. Figure 2a shows that in the foreground some  observed by SEM (Fig. 3a and 3b). Alumina platelets are easily recognized on the pictures  The strength of the samples is measured by three points bending. Table 1