XIAO Lingbin, WANG Hongjie, SU Lei, PENG Kang, LU De, NIU Min
(State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University , Xi'an 710049, Shaanxi, China)

Extended Abstract: [Significance] The inherent brittleness of ceramic materials originates from the strong and directional covalent or ionic bonds, coupled with their complex crystal structures. The formation and movement of dislocations are difficult, making plastic deformation challenging and leading to failures that are more sudden and unpredictable. Therefore, overcoming the brittleness of ceramic materials and enhancing their fracture toughness are crucial for ensuring their safe use. It is well known that the essence of toughening ceramic materials lies in increasing their resistance to crack initiation and propagation under external forces. Natural nacre is a composite structure commonly found on the inner surface of mollusk shells and is renowned for its excellent mechanical properties of simultaneous high strength and toughness. Due to its unique multiscale effects, when the nacre structure is introduced into ceramic materials, high fracture toughness can be achieved. Inspired by this, extensive research on nacre-mimetic ceramic materials has been conducted, providing an important approach for the design of high-performance ceramic materials.[Progress] The properties and toughening mechanisms of natural nacre are discussed, starting from its "brick-and-mortar" structural characteristics, while the differences between laminated ceramics and nacre-mimetic ceramics are then introduced. Natural nacre typically consists of 95 vol% aragonite and 5 vol% organic matrix. This structure has alternating hard and soft layers, which can cause crack deflection, increase the plastic zone of crack influence, alleviate stress concentration and reduce the crack propagation rate. The adhesive action of the organic matrix can consume large amount of fracture energy, while the fracture of mineral bridges between aragonite crystals can also hinder crack propagation and consume fracture energy. Various methods, such as layer-by-layer deposition, freeze-drying and 3D printing, have been used to prepare nacre-mimetic ceramic materials with ceramic/organic, ceramic/metal and ceramic/ceramic structures. When the soft phase is an organic material, the flexural strength of the composite material is significantly lower than that of the composite materials with soft phases of metal and ceramic, due to the low inherent strength of organic materials. It is not the case that the higher the deformation ability of the soft phase, the higher the fracture toughness of the material will be. The overall level of fracture toughness of the composite materials with organic soft phase is the lowest among the three systems. The nacre-mimetic ceramic materials with the ceramic/metal and ceramic/ceramic compositions are both strong and tough. Laminated ceramics are a general term for a series of ceramics that have a layered structure at the macro or micro level. Both laminated ceramics and nacre-mimetic ceramics include layered structures into the design and manufacturing of ceramic materials to enhance fracture toughness. Although both have layered structures, there are differences in microstructures, toughening mechanisms and composites systems. Firstly, the layered structure in laminated ceramics is mainly obtained by using simple physical lamination methods, such as tape casting, etc., with two phases stacked in long-range order. The layered structure in nacre-mimetic ceramics is currently mainly obtained by means of directional freeze-drying methods. Secondly, laminated ceramics can be divided into strong interface bonding laminated ceramics and weak interface bonding laminated ceramics, depending on the interlayer bonding strength. Strong interface bonding laminated ceramics rely on the residual stress between layers to hinder crack propagation and thus enhance toughness, while weak interface bonding laminated ceramics rely on interface separation leading to crack deflection to enhance toughness. Nacre-mimetic ceramics have a multiscale synergistic toughening mechanism, including but not limited to crack deflection caused by the layered structure, plastic deformation and adhesion of the soft phase, formation of interlayer mineral bridges and friction of nanoscale protrusions on the lamellar surface. Thirdly, laminated ceramics are mainly composed of ceramic/ceramic layers stacked in sequence, while nacre-mimetic ceramics have multiple systems such as ceramic/organic, ceramic/metal and ceramic/ceramic. In the ceramic/ceramic system, to simulate the nacre structure, ceramic nanoparticles, ceramic fibers, etc., are often compounded with the ceramic matrix.[Conclusions and Prospects] Natural nacre, thanks to the multi-level "brick-and-mortar" structure composed of aragonite and organic materials, has excellent mechanical properties, being both strong and tough. Introducing nacre structure into ceramic materials can greatly increase their fracture toughness. The nacre-mimetic ceramic materials of the ceramic/ceramic system not only have high fracture toughness but can also basically maintain their original properties at high temperatures, making them a good high-temperature resistant structural material with comprehensive performance. In the future, the development of nacre-mimetic ceramic materials can follow three directions: expanding the selection range of soft phases in nacre-mimetic ceramic materials, such as elastic ceramic aerogels, exploring the functional characteristics of nacre-mimetic ceramic materials to expand their application scope and analyzing the multiscale effects in nacre-mimetic ceramic materials and clarifying the contributions of various toughening mechanisms through experiments and simulations.
Key words: nacre-like ceramics; toughening mechanisms; preparation methods; mechanical properties