Anisotropy effect of bioinspired ceramic/ceramic composites: Can the platelet orientation enhance the mechanical properties at micro- and submicrometric length scale?

Abstract

In advanced ceramics, improving toughness usually relies on the introduction of a soft metallic or polymeric ductile phase, which decreases the mechanical properties. Some natural materials are strong, stiff and tough due to a combination of mechanisms operating at different length scales. However, such structures have been extremely difficult to replicate into synthetic materials. Here we investigate the microstructure and the micromechanical properties of a bioinspired ceramic-ceramic composite. The micromechanical properties at room temperature show slight differences as a function of the platelet orientation. The hardness strongly decreases with increasing temperatures (up to 550 °C) for all the investigated orientations. The elastic strain to failure, defined as the H/E ratio, was used to estimate the wear resistance of materials, which is higher at room temperature because the dislocation mobility is lower than that at high temperature.

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.