Making Ultra‐Tough Nanoceramics by Columnar Submicrocrystals with Three‐Level Micro‐Nano Structures

Abstract

leading to the intergranular fracture. The fracture surface area of equiaxed nanocrystalline ceramics is independent of grain size (Figure S1a,b and Equation S3, Supporting Information). Therefore, the total fracture energy (Equation S4, Supporting Information) of brittle ceramics does not change under the invariant interface energy, according to Griffith theory. This is the fundamental reason why the toughness of nanocrystalline ceramics cannot be greatly improved.The toughness of nanoceramics with high-strength can be greatly improved by changing the grain geometry or the way of grain fracture. In the past, high-toughness ceramics were mainly prepared by adding whiskers, [11] fibers, [12] and nanoparticles. [13] Due to the limitation of equilibrium phase transition, these ceramics can only form one-or two-level micro-nano structures (Figure 1b,c), which not only have low-content second-level structure but also cannot achieve three-level structure. In order to prepare high-density ceramics, the whisker content should not exceed 20-30 vol%, [14] and whiskers are damaged during the mixing process, thus limiting the improvement of ceramic toughness. In situ self-generated whiskers or rod-like crystals can not only increase their content up to 30-50 vol%, but also improve the bonding strength between them and the matrix, which greatly exerts their toughening effect. The widely used in situ self-generated β-Si 3 N 4 rod-like microcrystal [15,16] ceramics and Al 2 O 3 ceramics with a small amount of α-Al 2 O 3 columnar microcrystals [17][18][19] can improve the ceramic toughness because of the rod-like/columnar structures. However, most of these ceramics need to add inducer or seeds, and the Si 3 N 4 usually undergo α→β phase transformation, which depends on the equilibrium phase diagram, so the method of rod-like/columnar crystals toughened ceramics is restricted. Through the nonequilibrium phase transformation, the three-level or multi-level structure of high-content nanoparticles, whiskers, rod-like/ columnar crystals, or plate-like crystals can be realized by in situ growth, breaking the limit of the phase diagram. The multi-level structure of ceramics with intergranular fracture will increase the fracture surface area, thereby improving the toughness. The multi-level micro-nano structure is similar to the fractal structure (Figure 1a), so the higher the number of levels it has, the larger the fracture surface area of ceramics. We designed threelevel structure of crystals based on the fractal theory of romanesco broccoli [20] (Figure 1a). The intergranular fracture of these two three-level micro-nano grains (Figure 1b,c) can increase the The low fracture toughness of equiaxed nanocrystalline ceramics is the main bottleneck of its wide range of commercial applications. Here, the authors report a method to overcome this limitation for preparing ultra-tough nanoceramics from using amorphous and supersaturated Al 2 O 3 /ZrO 2 solid solution micro-powders, which is fabricated by Al-O 2 ultrahigh-temperature combustion ...

Strong and Tough Alumina Ceramic from the Particle‐Stabilized Foam

“…They have grain boundary energy and grain face energy, respectively. [ 51 ] The diffusion rate of grain boundary atoms/ions is much faster than that of grain faces and intragranular atoms. The primary factor is that the atoms deviate from the equilibrium position and have higher kinetic energy at the grain boundary.…”

“…Grain boundary directions with higher surface energies coarsen at a much greater rate than grain faces directions, resulting in the formation of abnormally grown grains with unique plate‐like morphologies. [ 51 ] Plate grains have a similar toughening mechanism to fibers. [ 56 ] In addition, scanning electron microscopy (SEM) micrographs indicated sintering pressure have a marked effect on the quantity and morphology of plate‐like grains.…”

“…[49,50] In addition, the almost no-linear shrinkage of the equiaxed grains constituting the fragments during sintering will further reduce porosity at the grain boundaries. [51,52] Using equiaxed grains as seeds to replace powder particles for hot-pressing sintering not only improves the relative density and flexural strength of bulk ceramics, but also increases the fracture toughness by coarsening to form plate-like grains induced by additives. This strategy solves the conflicts between strength and toughness and realizes synergistic enhancement.…”

Perspectives Toward Damage‐Tolerant Nanostructure Ceramics

“…Reproduced with permission. [ 59 ] Copyright 2022, John Wiley and Sons. f) SiC‐reinforced porous Si 3 N 4 ceramics and its fracture toughness compared with literature.…”

“…The sample has extremely high hardness (HV20 GPa) and excellent toughness (16 MPa m 1/2 ), and the bending strength reaches 1300 MPa (Figure 10e ). [ 59 ] Wang et al. prepared high‐porosity and high‐strength porous Si 3 N 4 ceramics.…”