基于分子动力学单晶硅的纳米压痕过程研究（英文）

基于分子动力学的基本理论,在微纳米尺度下建立了单晶硅的纳米压痕分子动力学模型。研究了在纳米压痕过程中单晶硅基体的变形机理、势能变化和温度变化。研究发现:在纳米压痕过程中基体上出现了位错、空位及滑移带,基体两侧有凸起现象。当压头撤离时,基体与压头间存在颈缩现象。在系统达到平衡时系统的势能出现不同,这是因为原子位错运动使得系统增加的势能小于压头原子所做的功。温度的变化与位错变形的程度相关,位错变形越剧烈系统温度升高的越快。     

纳米压痕有缺陷单晶硅的分子动力学分析

为了研究单晶硅中纳米裂纹对加工过程的影响,建立了单晶硅纳米压痕的分子动力学模型。在该研究中,分析了不同压痕速度对单晶硅中纳米裂纹演化的影响。对纳米压痕工艺、温度变化、势能变化、加载力、裂纹扩展、配位数等进行了深入的研究。研究结果表明,工件中的纳米裂纹在加载过程中有愈合的趋势;压头加载速度越大,工件纳米压痕区温度越高,势能越大,但加载速度对载荷变化趋势的影响不大;另外,纳米裂纹会使工件中Bct5-Si与Si-II的量显著下降。该研究为包含缺陷的单晶硅半导体的实际加工提供了理论指导。     

纳米压痕形变过程的分子动力学模拟

根据EAM多体势，利用分子动力学方法模拟了Ni压头压入Al基体的纳米压痕全过程．包括压头接近和离开基体时的原子组态；压入和上升时的载荷一位移曲线以及位错的发射和形变带的产生和变化；同时模拟了纳米尺度的应力弛豫行为．结果表明，当压头尚未接触基体时就能吸引基体原子，通过缩颈而互相连接．当压入应力Ts为1．9MPa时，基体Al开始发射位错；当分切应力Td=6．4MPa时，出现形变带．压头上升过程出现反向的拉应力，使基体反向屈服，在卸载过程中基体残留位错的组态不断改变．当压头上升离开基体后能拉着基体通过缩颈而相连，当压头和基体分离后仍粘有基体原子．在纳米尺度也存在应力弛豫现象，其原因是热激活引起的位错发射和运动．     

基于分子动力学的金属钛纳米切削过程研究（英文）

采用基于分子动力学的仿真方法建立了金属钛纳米切削分子动力学模型,选择了有代表性的切削条件,通过仿真得到瞬间原子位置图像并对切削过程中材料去除现象、加工表面形成过程、系统势能和工件温度等的变化进行了分析。发现在金属钛的纳米切削过程中切屑和加工表面是由于晶格能的释放和位错的不断延伸扩展形成的。已加工表面原子的弹性恢复和晶格重构能够减缓总势能和温度不断增加的趋势,并使其伴随有微小波动。     

纳米级铱单晶不同温度拉伸的分子动力学解析

针对面心立方金属铱单晶独特的韧脆变形特征,采用分子动力学方法研究了纳观尺度下的铱单晶在不同温度下的拉伸变形行为。通过分析不同温度拉伸过程中的应力应变关系,势能变化和原子构型图,认为随着温度的上升,纳米级铱单晶沿[100]晶向的弹性模量逐渐下降,抗拉强度也逐渐降低。温度为300 K时拉伸变形过程中在晶体内仅有少量空位和位错产生,600和800 K拉伸变形过程中在晶体内有滑移,位错和空位产生。     

基于分子动力学的金属钛纳米切削过程研究

本文采用基于分子动力学的仿真方法建立了金属钛纳米切削分子动力学模型,选择了有代表性的切削条件,通过仿真得到瞬间原子位置图像并对切削过程中材料去除现象、加工表面形成过程、系统势能和工件温度等的变化进行了分析。发现在金属钛的纳米切削过程中切屑和加工表面是由于晶格能的释放和位错的不断延伸扩展形成的。已加工表面原子的弹性恢复和晶格重构能够减缓总势能和温度不断增加的趋势,并使其伴随有微小波动。     

晶粒尺寸对γ-TiAl力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-TiAl合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-TiAl模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-TiAl的硬度。结果表明：当晶粒尺寸小于9.9nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-TiAl在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

晶粒尺寸对γ-TiAl合金力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-Ti Al合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-Ti Al模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-Ti Al的硬度。结果表明:当晶粒尺寸小于9.9 nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9 nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-Ti Al在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

片层厚度对双相TiAl合金力学性能影响的纳米压痕分子动力学研究

为研究纳米压痕过程中片层厚度和γ/α2相界对双相TiAl合金变形行为及力学性能的影响,本文针对5种不同厚度的双相TiAl合金模型,采用分子动力学的方法模拟计算了金刚石压头以垂直于γ/α2相界方向分别压入γ和α2相的纳米压痕过程。结果表明：材料的硬度随片层厚度的减小而增大,当片层厚度减小至7nm时,材料的硬度达到最大值,进一步减小片层厚度时,材料的硬度反而减小。材料的弹性模量也会随片层厚度的变化而改变,与硬度呈现正比关系。此外,在纳米压痕过程中,压头压入γ相时,变形行为以{111}面的层错为主,γ/α2相界会阻碍位错的运动;压头压入α2相时,变形行为以(0001)基面的堆垛层错为主,基面上Shockley不全位错的运动会导致材料表面产生相变,且棱柱面滑移被激活。     

Au_(85)原子纳米团簇热力学性质分子动力学模拟研究

采用了Johnson的(EAM)模型并结合分子动力学方法,模拟了Au_(85)原子纳米团簇熔化和凝固过程。研究了Au_(85)原子纳米团簇熔点、凝固点以及该团簇的结构,在分析势能和热容量随温度变化关系的过程中,发现Au_(85)原子纳米团簇的熔化和凝固过程中的熔点和凝固点不是线性变化,均出现了负热容现象,并且团簇的凝固点低于熔点。为了探究出现这种负热容现象的原因,对熔化过程和凝固过程中该团簇的内部结构进行了对比。结果表明,该纳米团簇在加热熔化(降温凝固)的过程中表面原子的比例显著变化,引起团簇内部势能的急剧增加(快速减少),使得动能和势能之间相互快速转化。为了维持整个系统的能量平衡,势能-温度图像发生跳变,在物理上表现为负热容现象。     

Prediction of the threshold load of dislocation emission in silicon during nanoscratching

This paper establishes an analytical model to predict the normal threshold load that causes the emission of partial dislocations in silicon during nanoscratching. In the modeling, the deformation mechanisms and the sequence of microstructural changes already explored by experiment and molecular dynamics will be exactly followed; that is, with increasing the normal load of nanoscratching, phase transformation first takes place, followed by partial dislocation emission from the interface between the phase transformed zone and the original crystalline silicon when the scratching load reaches its threshold. The model postulates that the emission process represents the generation of a dipole of Shockley partial dislocations. One partial dislocation of the dipole, located at the interface, is considered immobile, while the other partial dislocation moves into the bulk of the crystalline silicon. The mobile partial dislocation slips along a crystallographic plane, and a stacking fault is formed in its wake. Based on the above, the threshold normal scratching load for the emission of a partial dislocation is determined by means of the energy criterion. The influence of the indenter geometry and the location of dislocation nucleation on the threshold normal scratching load is then investigated. Compared with the deformation of silicon under nanoindentation, the present study concludes that the threshold load under scratching is always smaller, and that a sharp indenter leads to a relatively smaller threshold load. The model prediction is well verified by scratching experiments.     

Multiscale modeling of ductile crystals at the nanoscale subjected to cyclic indentation

A multiscale method is applied to study the response of an aluminum single-crystal substrate to cyclic indentation at finite temperature. The evolution of contact-induced deformation on the nanoscale is controlled based on defect nucleation beneath the indenter. The method allows for visualization of atomistic deformation during loading and unloading. Although there are inherent limitations to our two-dimensional model, we have found qualitative similarities to the mechanisms of homogeneous defect nucleation and deformation in three-dimensional face-centered cubic crystals. It is shown that the atomistic surface roughening process mostly arises from homogeneous dislocation nucleation during successive loading/unloading processes. These sub-surface defects cause major permanent deformation of the substrate during indentation. The slip steps forming on the surface of the indented substrate contribute their own dislocation activity, sending dislocations directly from the surface into the crystal, but those activities mostly remain localized near the indented surface. Force–displacement curves and the hysteresis which occurs due to inelastic deformation and heating of the substrate are studied for each cycle, and correlated with sub-surface and surface nucleation of defects.     

分子动力学模拟不同层错能单晶Ni及其合金拉伸变形行为

采用分子动力学模拟了不同尺寸模型的单晶Ni及Ni₅₇Cr₁₉Co₁₉Al₅合金[100]晶向拉伸变形过程,确定了具有稳定塑性流变应力的模型尺寸,进一步研究了在具有稳定塑性流变应力的相同模型下单晶Ni及其合金拉伸变形行为。结果表明,层错能较低的单晶Ni₅₇Cr₁₉Co₁₉Al₅合金在小尺寸模型拉伸变形时,容易形成多层孪晶结构或变形孪晶;模型的横截面边长大于30倍的晶格常数时,塑性流变阶段流变应力、相结构及位错密度随应变起伏趋于平稳。具有稳定流变应力的相同尺寸单晶Ni及其合金拉伸时,层错能越低,塑性变形时层错面的面积越大。Shockley不全位错在单晶Ni及其合金塑性变形过程中起主导作用,多层孪晶的形成伴随着位错耗尽,变形孪晶的形成与湮灭则主要由位错饥饿机制主导。     

Deformation mechanisms at pop-out in monocrystalline silicon under nanoindentation

This paper clarifies a common misunderstanding of the phase transformation in monocrystalline silicon under nanoindentation, namely that a pop-out represents the onset of a phase transition. Through a detailed investigation into the indentation-induced deformation of monocrystalline silicon using a Berkovich indenter, it was found that a pop-out does not correspond to the onset of the transformation. The critical contact pressure for initiating phase transformation during unloading is independent of the maximum indentation load or of the unloading rate. The size of a pop-out depends on the time it takes place (earlier and later), and its location alters the proportion of the transferred phases (amorphous and crystalline phases) after complete unloading. A lower unloading rate or a higher maximum indentation load promotes the occurrence of a pop-out.     

半导体硅器件接触时粘着产生起因的原子尺度分析

目的 实现对半导体硅器件接触变形与相变转化的微观认识和理解其粘着产生起因.方法 基于分子动力学法的Morse和Tersoff混合势函数,对单晶硅受加载和卸载时的接触特性与粘着起因展开分析,并用剪切应变和配位数分别描述硅器件接触变形与相变行为.结果 加载期时,硅基与探针紧密接触区应变程度由内到外逐渐衰减;卸载期时,应变由外到内逐渐增强,且卸载时接触边缘两侧硅原子会形成桥搭,表明硅基与探针接触时存有粘着,该粘着是诱导硅基原子粘附于探针表面的主因.粘着产生是由于硅基受载时,硅发生相变转化的键能被破坏引起.加载期积累的部分应变能在卸载时得以释放,以致硅基与探针紧密接触区的部分破坏原子粘附于探针外围轮廓,而产生明显粘着增强.另外,加载和卸载时的硅基相变主要以Bct5-Si为主,且单晶硅粘着接触变形与相变行为受温度依赖性显著.温度越高,硅基表面容易有随机粗糙波纹出现,卸载时更容易受温度影响而产生粘着增强效应,这是诱导半导体微/纳器件失效的根本原因.结论 半导体硅器件的动态接触变形与相变转化受温度依赖性显著,温度升高引起的材料软化变形是造成粘附增强的主要原因.此次研究对高温重载工况的半导体器件接触行为和粘着起因的理解有更深层次认识.     

铜离子催化作用下单晶硅表面微纳结构的制备

目的结合金属辅助化学湿法刻蚀原理,在单晶硅表面制备高效减反的微纳米结构。方法以单晶硅为基体,提出用Cu2+作为催化剂,在单晶硅表面两步化学刻蚀出多种微纳米减反结构,运用SEM/AFM表面分析方法,对形成的表面形貌和制备工艺进行分析,详细介绍了铜离子催化作用下制备微纳米结构的机理、反应现象及主要影响因素。结果铜离子催化化学刻蚀单晶硅可以得到均匀分布的微纳米减反结构,所得结构在250~800 nm范围内的反射率达5%以下。结论与传统碱性刻蚀技术相比,该技术所得微结构具有更高的光吸收率,并且稳定性好,容易控制。     

Mesoscale modelling of the yield point properties of silicon crystals

The yield point properties that characterize silicon crystals in the temperature domain where Peierls forces govern the dislocation mobility have been investigated in easy glide conditions with the help of a three-dimensional mesoscopic simulation of dislocation dynamics. The influence of temperature, applied strain rate and initial dislocation microstructure, the latter consisting of a random initial distribution of dislocation sources, were examined in detail and globally found in fair agreement with the available experimental results. A critical examination of a well-known model by Alexander and Haasen has been performed, leading to a discussion and reformulation of the law accounting for the multiplication rate of dislocations. The present lack of knowledge on the mechanisms by which new dislocation sources are formed during the plastic deformation of silicon crystals is emphasized.     

Effects of grain size and indentation sensitivity on deformation mechanism of nanocrystalline tantalum

Nanoindentation process is performed by molecular dynamics simulations to investigate the plastic deformation phenomena and mechanical properties of the polycrystalline tantalum (Ta) substrate. The comparison of mechanical behaviors between the single crystal and polycrystalline Ta is analyzed. The effects of grain size, indenter radius, indentation velocity, and temperature are deeply investigated. Further, the important factors of mechanical property such as the plastic deformation, dislocation, amorphization process, indentation force, yield pressure, and von Mises stress are evaluated. The results show that the phase transformations and dislocations almost absent in the grains. Instead, the dislocations densely occur in the grain boundaries, the amorphization regions are formed around the indentation areas and develop along the grain boundaries. The high von Mises stresses are mainly distributed around the indentation areas and in the grain boundaries. The single crystal tantalum shows better mechanical characteristics than the polycrystalline tantalum such as harder to deform, higher yield pressure, and lower dislocation.