贝壳中单个文石板片的力学性能和变形机理研究

The toughening origin of nacre is long attributted to its elaborate "brick-mortar" microarchitecture. It is widely accepted in the previous literature that the individual aragonite (CaCO3) platelets in nacre are composed of pure monocrystal, thus exhibit mechanical brittleness. However, recent experimental progress has shown that individual aragonite platelets are built up with the highly-oriented nanoparticles. The nanoparticle-architecture can tune crack to propagate in an intergranular manner inside aragontie platelet, in sharp contrast with the transgranular crack trajectory in monocrystal aragonite mineral. These experimental results reveal that the aragonite platelets in nacre are mechanically optimized by the nanoparticle architecture design. In this research, we will use FIB technique to fabricate the micropillars and microbeams from the individual aragonite platelets of nacre, which are then tested by the nanocompression and nanoindentation to determine their mechanical properties. We will focus on exploiting the elastic modulus, poisson ratio, strength, ducility, toughness, small-scale deformation/fracture mechanism of aragonite platelet in nacre,and compare the mechanical manifestation of the micropillars/microbeams test from geological aragonite mineral (pure monocrystal). We emphasize the mechanistic understanding using TEM observation after failure from these two different aragonite minerals. The design principle of flaw-tolerance of aragonite platelet is highly expected to be uncovered through comparing with the fracture modes from geo-aragonite and bio-aragonite minerals. The research topic will provide in-depth understanding of toughening origin of nacre and can inspire us to design stronger-and-tougher engineered nanoceramic.

贝壳的韧化起源来自“砖-砌”层状结构的巧妙设计。长期以来的观点认为：贝壳中的文石板片（Aragonite platelet）由纯单晶构成，在力学性能上表现为脆性。但是，近期的研究结果表明，单个文石板片是由高度有序纳米颗粒排列成的单晶，在抗裂纹扩展行为上与纯单晶文石表现迥异。本项目拟开展贝壳中单个文石板片的力学性能研究，重点探索从单个文石板片“裁剪”成的微柱、悬臂梁的压缩、弯曲力学性质，通过与单晶文石（地质文石）相同的小试样力学性能对比，研究两种相同成分、不同微结构的文石的变形、失效机理，明确由纳米颗粒构成的文石板片力学性能优化的临界尺度设计准则，揭示贝壳在纳米尺度上对缺陷容忍度的设计原理，为设计高强-高韧的工程纳米陶瓷提供理论依据和实用途径。

贝壳的强韧化机制起源于其巧妙设计的“砖-砌”纳米层状结构。近几十年来的研究认为：贝壳中裂纹扩展沿着高分子层，碰到文石板片(Aragonite platelet)发生转向提高裂纹扩展的抗力来实现增韧的效果。长期的研究观点认为：贝壳中的文石板片由成分和结构单一的矿物单晶构成，在力学性能上表现为脆性。然而，近期我们的研究结果发现，单个文石板片是由高度有序纳米颗粒排列成的“伪单晶”，在抗裂纹扩展行为上与地质文石矿物单晶表现迥异。本项目开展了贝壳中单个文石板片的力学性能研究，通过与地质文石矿物单晶的纳米力学性能对比，研究两种相同成分、不同微结构的文石晶体的变形、失效机理，揭示了贝壳在纳米尺度上的结构设计对缺陷不敏感的微观机理。取得的重要研究进展有：.1、采用纳米压入实验对贝壳文石板片和地质文石单晶进行纳米力学性能测试，发现地质文石比贝壳文石的硬度和强度高；地质文石单晶的纳米压入载荷-位移曲线上可以看见明显的应变爆发（strain burst）现象，而贝壳文石板片的载荷-位移曲线没有观察到应变爆发现象，直接证明了由纳米颗粒组成的贝壳文石板片可以有效释放局部内应力。.2、采用不同加载速率的纳米压入实验对比发现：地质文石的载荷-位移曲线对应变速率不敏感，而在贝壳文石板片中展现出应变速率越高，强度越高的现象，即存在应变速率敏感性。.3、采用第一性原理对贝壳文石碳酸钙的层错能、孪晶形成能、不稳层错能、不稳孪晶形成能进行了计算，发现贝壳文石的不稳层错能与层错能的比值为81.7，我们对其 它很多体系材料的比值数据进行了采集，得出了贝壳中不稳层错能与层错能的比值是最高的，为我们解释在贝壳文石中容易形成变形孪晶提供了机理解释。.项目实施以来一直尝试在单个贝壳文石板片内部微加工Micropillar和Microbeam试样，但效果一直不佳，目前还在进一步尝试相关研究工作。