摘要
Nanotwinned diamond（nt-diamond）,which demonstrates unprecedented hardness and stability,is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature（HPHT）.Its hardness and stability increase with decreasing twin thickness at the nanoscale.However,the formation mechanism of nanotwinning substructures within diamond nanograins is not well established.Here,we characterize the nanotwins in nt-diamonds synthesized under different HPHT conditions.Our observation shows that the nanotwin thickness reaches a minimum at ~20 GPa,below which phase-transformation twins and deformation twins coexist.Then,we use the density-functional-based tight-binding method and kinetic dislocation theory to investigate the subsequent plastic deformation mechanism in these pre-existing phase-transformation diamond twins.Our results suggest that pressure-dependent conversion of the plastic deformation mechanism occurs at a critical synthetic pressure for nt-diamond,which explains the existence of the minimum twin thickness.Our findings provide guidance on optimizing the synthetic conditions for fabricating nt-diamond with higher hardness and stability.
在高温高压条件下以洋葱碳为原料合成的纳米孪晶金刚石具有前所未有的硬度和稳定性,且二者随纳米孪晶厚度的减小而提高.目前为止,在金刚石纳米晶中纳米孪晶的形成机制尚不明确本研究通过分析在不同条件下合成的纳米孪晶金刚石块材中的孪晶厚度,发现在合成压力约为20 GPa时孪晶厚度达到一个极小值(~5 nm).TEM结果表明在合成压力低于20 GPa时,纳米孪晶金刚石中同时存在因马氏体相变而形成的相变孪晶和塑性形变所导致的形变孪晶.针对马氏体相变后形成的相变孪晶内部塑性变形,基于密度泛函理论的紧束缚方法和位错运动学理论的分析表明:纳米孪晶金刚石的塑性变形存在一个依赖于压力的机制转变,而机制转变的临界压力能够解释孪晶厚度随压力变化时出现的极小值.
基金
supported by the National Natural Science Foundation of China(51421091,51332005,51572225,51272227, 51172197,51525205 and 51672239)
the US National Science Foundation(EAR-1361276)
