Direct observation of plasticity and quantitative hardness measurements in single crystal cyclotrimethylene trinitramine by nanoindentation

Investigation of deformation beginning with elasticity and continuing through the elastic–plastic transition to incipient cracking has been conducted for (210), (021) and (001) oriented single crystals of the explosive cyclotrimethylene trinitramine, commonly known as “RDX”. Nanoindentation was performed with a conical tip over a range of loads. The resulting load–depth data exhibited distinct, reproducible, orientation-dependent load excursions demonstrating elastic–plastic transitions. Indent impressions were imaged by atomic force microscopy revealing deformation features consistent with slip on six planes. Impressions on the (210) and (001) planes showed deformation pile-up features associated with the zone axes of slip planes. Slip traces were evident on the (210) plane indicating slip on four planes and suggesting cross-slip. Height data, for impressions formed by progressively increasing loads, indicated one additional slip system consistent with (010) slip. All of the orientations exhibited cracking thresholds at very low loads. The reduced elastic moduli were anisotropic and the hardness values were isotropic indicating limited plasticity. Maximum shear stresses estimated from a Hertzian model, at load excursions, were within 1/15 to 1/10 of published shear moduli, indicating deformation initiated near the theoretical yield strength, presumably by homogeneous nucleation of dislocations. The material strength parameters and deformation pathways inferred from this work are compared to previous microhardness investigations in which the ambiguity of results can be attributed to the effects of cracking and simultaneous slip on multiple systems. A mechanistic explanation for the hindered plasticity, and cracking, observed for RDX is offered in terms of compatibility conditions.     

Analytical model for nanoscale viscoelastic properties characterization using dynamic nanoindentation

In the last few decades, nanoindentation has gained widespread acceptance as a technique for materials properties characterization at micron and submicron length scales. Accurate and precise characterization of material properties with a nanoindenter is critically dependent on the ability to correctly model the response of the test equipment in contact with the material. In dynamic nanoindention analysis, a simple Kelvin–Voigt model is commonly used to capture the viscoelastic response. However, this model oversimplifies the response of real viscoelastic materials such as polymers. A model is developed that captures the dynamic nanoindentation response of a viscoelastic material. Indenter tip-sample contact forces are modelled using a generalized Maxwell model. The results on a silicon elastomer were analysed using conventional two element Kelvin–Voigt model and contrasted to analysis done using the Maxwell model. The results show that conventional Kelvin–Voigt model overestimates the storage modulus of the silicone elastomer by ~30%. Maxwell model represents a significant improvement in capturing the viscoelastic material behaviour over the Voigt model.     

Nanoscale mechanical characterization of polymeric fibers

The mechanical and viscoelastic behaviors of polymeric fibers varying in molecular weight have been determined at the nanometer scale with a surface force apparatus. The existence of a skin–core structure resulting from the elaboration process is pointed out. The role of fiber anisotropy is also discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 264–275, 2005     

Designing Elastic Organic Crystals: Highly Flexible Polyhalogenated N‐Benzylideneanilines

The intermolecular interactions and structural features in crystals of seven halogenated N‐benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison d’être for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1) A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2) corrugated packing patterns that would get interlocked and, in the process, prevent long‐range sliding of molecular planes.     

石墨烯/铝基复合材料在纳米压痕过程中位错与石墨烯相互作用机制的模拟研究

石墨烯因其优异的力学性能已成为增强金属基复合材料的理想增强体.然而,目前对石墨烯/金属基复合材料在纳米压痕过程中嵌入石墨烯与位错之间的相互作用仍不清晰.本文采用分子动力学模拟方法,对90°,45°和0°位向的石墨烯/铝基复合材料进行了纳米压痕模拟,研究了压痕加载和卸载过程中石墨烯/铝基复合材料的位错形核及演化,以获取不同位向的石墨烯与位错的相互作用机制,并分析其对塑性区的影响.研究发现,石墨烯可以有效阻碍位错运动,并且石墨烯会沿着位错滑移方向发生弹性变形.在纳米压痕过程中,位错与不同位向石墨烯之间的相互作用差异导致塑性区的变化趋势不同.研究结果表明,在石墨烯/铝基复合材料中,位向不同的石墨烯对位错阻碍强度和方式不同,且石墨烯位向为45°的复合材料的硬度高于其他模型.此外,石墨烯/铝基复合材料的位错线总长度的演化规律与石墨烯位向紧密相关.本文研究可为设计和制备高性能石墨烯/金属基复合材料提供一定的理论指导.     

初始压入位置对Ni基单晶合金纳米压痕影响研究

针对Ni基单晶合金建立初始压入γ 相的γ /γ' 模型和初始压入γ'相的γ'/γ 模型, 采用分子动力学方法模拟金刚石压头压入两种模型的纳米压痕过程, 计算两种模型[001]晶向硬度. 采用中心对称参数分析两种模型(001)相界面错配位错对纳米压痕过程的影响. 结果显示: 弛豫后, 两种模型(001)相界面错配位错形式不同, 其中γ'/γ 模型(001)相界面错配位错以面角位错形式存在; 压入深度在0.930 nm 之前, 两种模型(001)相界面错配位错变化不大, 压入载荷-压入深度及硬度-压入深度曲线较符合; 压入深度在0.930 nm之后, γ'/γ 模型(001)相界面错配位错长大很多, 导致相同压入深度时γ'/γ 模型比γ /γ'模型压入载荷和硬度计算结果小; 压入深度在2.055 nm之后, γ /γ'模型(001)相界面错配位错对γ 相中位错进入γ'相有阻碍作用, 但仍有部分位错越过(001) 相界面进入γ' 相中, γ'/γ 模型(001)相界面处面角位错对γ' 相中位错进入γ 相有更明显的阻碍作用, 几乎无位错越过(001) 相界面进入γ 相中, 面角位错的强化作用更明显, 所以γ'/γ 模型比γ /γ'模型压入载荷上升速度快.     

高温仪器化压入测试中热接触对位移测量漂移的影响

基于对室温压头和热试样接触后传热过程的分析, 重点研究热接触引起的压头基托热膨胀对高温仪器化压入测试中位移测量漂移的影响. 首先, 通过热传导理论分析, 获得热接触后基托内温度场分布的解析解, 进而研究基托热膨胀引起的位移测量漂移量; 然后, 建立有限元分析模型, 数值模拟高温仪器化压入过程, 验证理论模型的准确性. 研究发现, 压头与热试样接触面间的热传导性质显著影响基托内的温度场分布; 对于不同材料的试样, 接触面间传热性能不同, 基托的热膨胀量差异可以达到几个数量级. 研究结果有助于优化高温压入测试程序, 提高测试的可靠性.      

Mechanics and surface ultrastructure changes of poly(3-hydroxybutyrate) films during enzymatic degradation in pancreatic lipase solution

A study of in vitro biodegradation of poly(3-hydroxybutyrate) (PHB) in model conditions was performed. The porcine pancreatic lipase solutions at different concentrations in the two buffer systems (phosphate-buffered saline (PBS) and simulating body fluid (SBF)) were chosen as model biodegradation media. At first, optimal concentration of pancreatic lipase (0.25 mg/ml in PBS) was determined: in these media the decomposition of PHB films realized faster according to the data of gravimetric analysis. Differential scanning calorimetry showed an increase in the crystallinity of the samples (from 49% to 59%) after enzymatic degradation. These data are confirmed by method of nanoindentation, where the increase of the Young's modulus during the degradation (from 1.37 GPa to 4.4 GPa) was shown. This is due to the crystallization of the amorphous polymer component, and its decomposition and dissolving. During biodegradation three types of polymer ultrastructure changes were observed on the surfaces of the films: appearance of new lamellae, disappearance of lamellae and disintegration of lamellae into shorter fragments.     

含缺陷双层石墨烯的纳米压痕模拟研究

石墨烯具有独特的力学、电学性能,被誉为是具有战略意义的新材料,具有广泛的应用前景. 目前生产的石墨烯含有各种缺陷,相较于完美石墨烯,其仍有较大应用价值. 因此有必要研究和掌握缺陷对石墨烯性能的影响,以便在目前的生产技术下,推动其工业化应用. 采用Tersoffff 势来模拟C—C 共价键的相互作用,Lernnard-Jones 势来模拟非成键碳原子之间相互作用力,基于分子动力学模拟了金刚石压头压入含缺陷双层石墨烯的纳米压痕过程,讨论了Lernnard-Jones 势函数的截断半径最佳值以及得到了典型的载荷-位移曲线. 重点探讨了Stone-Thrower-Wales、空位(包括单空位和双空位缺陷) 以及圆孔缺陷当位置不同和数目不同时对石墨烯力学性能的影响. 得出结论:薄膜中心存在缺陷时,破坏强度下降幅度特别明显. 空位缺陷在压头半径范围内存在时,临界载荷与缺陷与薄膜中心的距离成线性关系;缺陷数目越多,其杨氏模量、破坏强度等就越低. 圆孔缺陷数目在压头范围外达到一定浓度后会使石墨烯的力学性质显著降低. 本文结论也说明石墨烯结构稳定,对小缺陷不敏感,缺陷石墨烯仍具有较好的性能和使用价值.      

不锈钢表面等离子合金化渗铜层的纳米力学性能研究

为了研究不锈钢渗铜层的纳米力学性能,本文采用等离子表面合金化技术在304不锈钢表面制备了渗铜层。利用纳米压入硬度仪,采用连续刚度测试法,对渗铜层以及不锈钢基体的表面和横截面的纳米力学性能进行了测试,得到了纳米压入过程的载荷-压入深度曲线,发现渗铜层抵抗外载荷的能力低于不锈钢基体。并得到了渗铜层和不锈钢基体的表面、横截面方向的硬度以及杨氏模量,经对比得到渗铜层的杨氏模量和硬度都要比不锈钢基体的低,并且渗铜层的力学性能表现出各向异性。对纳米压入的数据进行分析,发现渗铜层在小尺度压入时硬度和杨氏模量表现出明显的"尺度效应"。