Veprek S
Department of Chemistry, Technical University Munich, Lichtenbergstr. 4, D-85747 Garching, Germany.
J Nanosci Nanotechnol. 2011 Jan;11(1):14-35. doi: 10.1166/jnn.2011.3815.
A brief overview of recent attempts to design new super- and ultrahard materials, which are based on the assumption that materials with high elastic moduli should be super- or ultrahard, is presented in order to show that meeting this condition is not sufficient. Instead, electronic and structural stability upon a finite, relatively large shear strain at atomic level is necessary to avoid structural transformations to softer phases or even a collapse of the structure. We discuss several examples where very high hardness of > 70 GPa has been obtained due to the nano-sized and nano-structured effects. Superhard nano-sized and/or nano-structured ("nanocomposites") materials can be prepared either by limited diffusion or by spinodal phase segregation during their synthesis. The advantages of the latter mentioned approach is the formation of a stable nanostructure with strong interfaces, that avoids grain boundary shear and concomitant softening when the crystallite size decreases below about 10-20 nm. In such a way, hardness enhancement by a factor of 4 to 5 has been achieved. We shall show that nc-TiN/a-Si3N4 nanocomposites can achieve hardness in excess of 100 GPa when properly designed, and prepared with low density of flaws and impurities. The paper finishes with a short overview of industrial applications of the nanocomposites as wear protection coatings on tools for machining.
本文简要概述了近期设计新型超硬和极硬材料的尝试，这些尝试基于高弹性模量的材料应为超硬或极硬的假设，目的是表明满足这一条件并不充分。相反，在原子水平上承受有限且相对较大的剪切应变时的电子和结构稳定性对于避免结构转变为较软相甚至结构坍塌是必要的。我们讨论了几个由于纳米尺寸和纳米结构效应而获得超过70 GPa极高硬度的例子。超硬纳米尺寸和/或纳米结构（“纳米复合材料”）材料可以在其合成过程中通过有限扩散或旋节线相分离来制备。后一种方法的优点是形成具有强界面的稳定纳米结构，当微晶尺寸减小到约10 - 20 nm以下时，可避免晶界剪切和随之而来的软化。通过这种方式，硬度提高了4至5倍。我们将表明，当经过适当设计并以低密度缺陷和杂质制备时，nc - TiN/a - Si₃N₄纳米复合材料可以实现超过100 GPa的硬度。本文最后简要概述了纳米复合材料作为加工工具耐磨保护涂层的工业应用。
