Soutenance de thèse : Hassan SAAD

Elaboration, microstructural and mechanical characterization of nacre-like alumina

Doctorant : Hassan SAAD

Laboratoire INSA : MATEIS
Ecole doctorale : ED34 Matériaux de Lyon

The development of strong and damage-resistant materials that can withstand harsh environments is a major issue in materials science and engineering. To improve the toughness of ceramic materials while keeping their intrinsic strength and stiffness, lessons have been taken from materials found in nature. Bioinspired brick-and-mortar designs have recently been obtained using different techniques such as ice-templating or a combination of magnetic field and slip casting, opening the way to the development of highly textured ceramic materials showing both high strength and toughness.
In 2014, Bouville and al. developed a new bio-inspired ceramic material with unique damage resistant behavior, using the ice-templating technique. The objective was to use the growth of crystals as a driving force for the bulk self- assembly of elementary building blocks (alumina platelets) leading to a hierarchical structure similar to that of nacre. The resulting material had a unique combination of high strength (470MPa), high toughness (22MPa.m1/2) and high stiffness (290 GPa), and presented a R-curve behavior that bears out the presence of extrinsic reinforcement mechanisms leading to a damage-resistant behavior. However, the small specimens arising from this method are difficult to study, in particular through mechanical tests. An important point resulting from this work is to understand the effect of the sample size which could potentially overestimates these mechanical properties. Moreover, reproducibility and scalability issues related to this process have impeded further developments and industrialization of such materials.
Here we present a simpler approach for the elaboration of such ceramic/ceramic composite with a brick and mortar structure based on field-assisted sintering (FAST) [1]. Raw alumina platelets are aligned thanks to the uniaxial pressure of FAST process and coated by an aluminosilicate glass-phase formed at high temperature. The final bulk material, with thickness up to 1cm, presents a nacre-like microstructure, retains the mechanical strength of alumina (430 ± 30 MPa) and exhibits excellent damage resistance characterized by an R-curve effect with a crack initiation toughness comprised between 5 and 6 MPa.m1/2.
We are also interested, during this PhD, in the understanding of the mechanisms at the origin of this nacre-like microstructure and of the good damage-resistance properties. A complete characterization is thus carried out. On the one hand, a parametric study of the sintering process was performed, combined with a microstructural characterization to understand how a simple pressure-assisted sintering process can lead to such complex brick and mortar structures. On the other hand, an exhaustive mechanical characterization revealed that crack deflection is the main toughening mechanism of such composites and highlighted the critical effect of the interfaces in these systems. The mechanical characterization was performed both at the macroscopic and the microscopic scale in order to evaluate the effect of the interface strength and composition in the global mechanical response of the material.