润滑介质对单晶硅相变与脆塑行为的影响

研究了分别在十五烷、无水乙醇和蒸馏水润滑下单晶硅的摩擦磨损行为及其相变和脆塑行为.结果表明:在十五烷润滑下单晶硅的摩擦系数和磨损体积损失最低,而在蒸馏水润滑下的摩擦系数和磨损体积损失最大;单晶硅在非极性溶剂十五烷润滑下发生明显的Si-I→a-Si相变,磨损表面光滑并呈现明显的金属塑性特征;单晶硅在无水乙醇润滑下发生轻度Si-I→a-Si相变,磨损表面特征为微弱的塑性变形和微断裂共存;在蒸馏水润滑下,单晶硅发生轻度的Si-I→Si-III相变,磨损表面变得粗糙并伴有大量微断裂;润滑介质的极性是影响单晶硅磨损表面相变和脆塑行为的主要因素之一.     

纳米压痕试验中压头尺寸效应的准连续介质法分析

采用准连续介质法模拟了单晶铝纳米压痕试验过程,分析了不同宽度的刚性矩形压头所引起的初始塑性变形特点,获得了载荷-压深、应变能-位移和硬度-压深曲线.从位错理论的角度分析了压头尺寸对纳米压痕测试结果的影响.研究发现:随着压头宽度的不断增大,压头下方位错形核所需要的载荷和压深程度增大,需要的应变能增加,应变能的变化速率递增,纳米硬度值减小,呈现出明显的尺寸效应.同时表明在一定的压入深度下,硬度与压头尺寸之间存在着一定的比例关系,不同尺寸压头获得的硬度值可以相互换算,但当矩形刚性压头宽度大于或等于120时这种尺寸效应消失.研究结果为纳米压痕实验过程中压头尺寸的选择提供了参考依据.     

纳米晶钽在单向拉伸载荷下的分子动力学模拟

利用分子动力学方法模拟了纳米晶钽在单轴拉伸载荷作用下的微观结构演化情况. 结果表明纳米晶钽在塑性变形过程中可以发生从BCC到FCC, HCP结构的应力诱导相变. FCC 结构原子百分比的最大值和试样的抗拉强度成线性关系，据此可推出一个相变发生的临界应 力值. 应变率越大，相变滞后于应力越严重. 当应变达到一定值时，试样会发生晶间断裂现 象，定量分析发现纳米晶钽晶间裂纹初始形成应变不受平均晶粒尺寸的影响，而与应变率和 模拟温度有着密切的关系.     

凸结构的形成-低载下单晶硅表面的划痕损伤研究

利用纳米划痕仪及曲率半径为3μm的球形金刚石针尖,在单晶硅(100)表面进行了不同载荷下的划痕试验.结果表明:随载荷的增加,单晶硅表面的划痕损伤先后经历了从凸起形成、凸起与凹槽并存到材料去除的变化过程.当载荷为0.5～3.0mN时,单晶硅上的划痕损伤表现为凸结构的形成,且凸起的高度和体积随载荷的增加而增大;当载荷为3～50mN时,凸起和凹槽同时出现,但损伤区域体积未见减少,损伤仍以凸结构形成为主导;当载荷大于50mN时,凹槽深度和磨损体积明显增大,划痕损伤表现为典型的材料去除.进一步的分析显示,单晶硅的划痕损伤特征与其接触区的应力状态密切相关,低载下的摩擦剪切是凸结构产生的主要原因.     

纳米压痕中位错运动的原子模拟

利用分子动力学方法模拟研究了金刚石压头压入Ni薄膜(111)晶面的纳米压痕过程中薄膜进入初始塑性后的纳观机制,采用中心对称参数(CSP)研究不同压入深度时薄膜内部的位错的萌生和生长情况.结果表明:压痕力-压痕深度曲线的每一次的剧烈的振荡,都是一次能量释放的过程,在薄膜内部的位错生长也最剧烈.加载过程中,压入深度为0.66nm时出现位错(层错),压入深度为0.93nm时出现明显的位错形核,随着压入深度的增加,多个位错形核相互作用形成梯杆位错.压入深度为1.4nm时,梯杆位错旁出现了棱柱形不全位错环,随着压入深度的增加,棱柱形不全位错环沿着{111}滑移面运动.在最大压入深度处,薄膜塑性形变达到最大.     

单晶硅滑动磨损性能及其相变研究

考察了单晶硅在室温和低接触应力条件下的摩擦磨损行为随速度的变化情况．结果表明，单晶硅的摩擦系数和磨损率随滑动速度的提高而呈现降低趋势；单品硅在低速、短滑动时间下的磨损表面形貌特征以微断裂为主，并伴有一定程度的塑性变形;随着滑动时间的延长，塑性变形特征逐渐减弱，塑性变形同具有金属延性特征的p—Sn(简称si—II)相密切相关，Si—II在滑动过程中可转变为体心立方结构(简称Si-III)、斜方六面体结构(简称Si—XII)和非晶硅相;在高速条件下，单晶硅磨损表面呈现微断裂和较弱的塑性变形特征；尽管通过Raman分析证实磨损表面存在si—III相，但其对磨损机制的影响有待于进一步研究．     

纳米金刚石薄膜结构相变及摩擦性能的研究

论文用第一性原理分子动力学模拟研究了纳米金刚石薄膜的结构相变和表面重构,以及金刚石薄膜(110)表面不同方向上表面形态和抛光残余的本质.研究发现,纳米金刚石的表面碳团簇通过断开(111)面的σ键,形成具有碳六元环结构的石墨碎片;内部原子sp3杂化向sp2杂化转化的发生是从(111)面上成对C原子向石墨相转化时形成π键的过程中获得了能量,驱动石墨的转变由表层向心部逐渐进行.转变过程中存在一种洋葱状富勒烯和金刚石结构共存的过渡相—Bucky-diamond,表面悬空键的消除和表层的富勒烯外壳最大限度地降低了表面能和系统总能量,Bucky-Diamond结构稳定存在.sp2杂化碳的缓冲作用对薄膜中残余应力有较大的影响.     

影响纳米压入测试结果的因素

纳米硬度计 ,又称深度测量压入仪。该仪器在刚性压针上施加特定载荷 ,同时记录压入试样深度。此技术广泛应用于微纳米尺度力学性能的研究。以MTSNanoIndenter○RXP为测试手段 ,参考试样熔融硅为研究对象 ,进行了不同压入深度的测试。结果显示 ,硬度和模量有随压入深度减小而增大的趋势。分析了接触零点的确定、压针尖端缺陷、试样表面的吸湿和粗糙度、弹塑性转变等因素对测试结果的影响。在压入深度为微米和亚微米量级时 ,上述因素对测试结果无显著影响 ;而在纳米量级时 ,有显著影响。所以 ,当压入深度为几十纳米时 ,纳米压入测试结果的可靠性值得注意     

基于微纳米压入法提取无铅焊料合金弹塑性力学参数的研究

本文通过微纳米压入法结合数值模拟研究了无铅焊料合金SnAg3.5 的弹塑性力学性能，分别采用圆柱形压头及两种不同锥角压头对无铅焊料合金进行压入测试：基于圆柱形压头测试过程中接触面积恒定的特点得到了无铅焊料的弹性模量，进一步采用塑性应变梯度理论对两种锥角压头的测试结果予以修正并通过数值模拟反分析得到相应的特征应力值，同时基于压入特征塑性应变与压头锥角的关系式得到两种不同锥角压头下的特征应变值，在此基础上经求解方程组得到焊料合金的初始屈服应力与应变强化因子，进而得到了焊料合金的幂强化弹塑性本构关系．该方法剔除了压入尺度效应的影响并保证了所得本构关系的唯一性，给出了一种通过原位压入测试表征金属材料弹塑性力学性能的有效方法．     

单晶硅表面等离子体基离子注入碳纳米薄膜的摩擦学特性

用等离子体基离子注入(PBII)技术在单晶硅表面制备了碳纳米薄膜,考察了薄膜在不同载荷及速度下同Si3N4球对摩时的摩擦学性能,并采用扫描电子显微镜观察分析了磨痕表面形貌.结果表明,所制备的碳纳米薄膜光滑致密,为高硬度富弹性的类金刚石碳(DLC)膜,薄膜通过C-Si键合作用而同硅片表面形成牢固结合,且成分及结构呈现某种梯度变化特征,单晶硅经改性后摩擦学性能大幅度改善:在低载荷(0.5 N)下其耐磨寿命达3 h以上,摩擦系数处于0.10～0.30之间,磨痕不明显;在高载荷(4 N)下其耐磨寿命及摩擦系数(0.03～0.20之间)均明显降低.这是由于较高载荷或滑动速度导致DLC薄膜石墨化加剧所致.     

高载下单晶铜和单晶硅的径向纳动与损伤行为研究

采用纳米压痕仪研究了单晶铜和单晶硅径向纳动的运行特点和损伤过程.结果表明:径向纳动的残余压痕深度随循环次数增加急剧减小,而纳动循环中载荷-位移曲线在闭合前表现为1个迟滞环;试样在首次径向纳动循环中耗散的能量最大,其后逐渐减小并趋于稳定;材料的接触刚度和弹性模量在最初几次纳动循环中增加较快,随后变化趋于平缓;尽管2种材料的压痕投影面积均随纳动循环次数增加而增大,但由于损伤机制不同,使其径向纳动损伤显示出各自不同特点,其中单晶铜主要表现为压痕边缘的皱褶堆积,而单晶硅表现为塑性区边界裂纹的萌生与扩展.     

High Temperature Nanoindentation of PMR-15 Polyimide

This paper presents the high temperature nanoindentation experiments performed on an aerospace polymer resin–PMR-15 polyimide. The sharp-tipped Berkovich nanoindenter equipped with a hot-stage heating system was used. The indentation experiments were performed using the “hold-at-the-peak” method at various indenter holding times and unloading rates. The creep effect was seen to decrease with increasing holding time and/or unloading rate. Procedures used to minimize the creep effect are investigated at both ambient and elevated temperatures so that the correct contact depth (together with modulus and hardness) can be determined from nanoindentation load-depth curve. The temperature dependent mechanical properties of PMR-15 are measured through the current nanoindenter and results are consistent with those obtained from macroscopic tests.     

A thermomechanical constitutive model for phase transformations in silicon under pressure and contact loading conditions

Most of the technologically relevant abrasive machining techniques for silicon (Si) such as lapping, sawing and grinding are based on the interaction of the silicon surface with a hard particle or asperity. It has been long established that the governing deformation mechanism for Si under such contact loading conditions is stress induced phase transformation. The present work introduces a novel phenomenological constitutive model for phase transformations of silicon set up in a thermomechanical framework of broad applicability. Taking into account experimental observations as well as first principle and molecular dynamics calculations, it captures both the cd-Si → β-Si transition upon compression and the β-Si → a-Si transition upon rapid decompression, which are most relevant for indenter loading. The model was numerically implemented in analogy to incremental plasticity and successfully applied for finite-element (FE) simulations of nanoindentation.     

Mechanics of indentation of plastically graded materials—I: Analysis

The introduction of controlled gradients in plastic properties is known to influence the resistance to damage and cracking at contact surfaces in many tribological applications. In order to assess potentially beneficial effects of plastic property gradients in tribological applications, it is essential first to develop a comprehensive and quantitative understanding of the effects of yield strength and strain hardening exponent on contact deformation under the most fundamental contact condition: normal indentation. To date, however, systematic and quantitative studies of plasticity gradient effects on indentation response have not been completed. A comprehensive parametric study of the mechanics of normal indentation of plastically graded materials was therefore undertaken in this work by recourse to finite element method (FEM) computations. On the basis of a large number of computational simulations, a general methodology for assessing instrumented indentation response of plastically graded materials is formulated so that quantitative interpretations of depth-sensing indentation experiments could be performed. The specific case of linear variation in yield strength with depth below the indented surface is explored in detail. Universal dimensionless functions are extracted from FEM simulations so as to predict the indentation load versus depth of penetration curves for a wide variety of plastically graded engineering metals and alloys for interpretation of, and comparisons with, experimental results. Furthermore, the effect of plasticity gradient on the residual indentation pile-up profile is systematically studied. The computations reveal that pile-up of the graded alloy around the indenter, for indentation with increasing yield strength beneath the surface, is noticeably higher than that for the two homogeneous reference materials that constitute the bounding conditions for the graded material. Pile-up is also found to be an increasing function of yield strength gradient and a decreasing function of frictional coefficient. The stress and plastic strain distributions under the indenter tip with and without plasticity gradient are also examined to rationalize the predicted trends. In Part II of this paper, we compare the predictions of depth-sensing indentation and pile-up response with experiments on a specially made, graded model Ni-W alloy with controlled gradients in nanocrystalline grain size.     

Two new expanding cavity models for indentation deformations of elastic strain-hardening materials

Two expanding cavity models (ECMs) are developed for describing indentation deformations of elastic power-law hardening and elastic linear-hardening materials. The derivations are based on two elastic–plastic solutions for internally pressurized thick-walled spherical shells of strain-hardening materials. Closed-form formulas are provided for both conical and spherical indentations, which explicitly show that for a given indenter geometry indentation hardness depends on Young’s modulus, yield stress and strain-hardening index of the indented material. The two new models reduce to Johnson’s ECM for elastic-perfectly plastic materials when the strain-hardening effect is not considered. The sample numerical results obtained using the two newly developed models reveal that the indentation hardness increases with the Young’s modulus and strain-hardening level of the indented material. For conical indentations the values of the indentation hardness are found to depend on the sharpness of the indenter: the sharper the indenter, the larger the hardness. For spherical indentations it is shown that the hardness is significantly affected by the strain-hardening level when the indented material is stiff (i.e., with a large ratio of Young’s modulus to yield stress) and/or the indentation depth is large. When the indentation depth is small such that little or no plastic deformation is induced by the spherical indenter, the hardness appears to be independent of the strain-hardening level. These predicted trends for spherical indentations are in fairly good agreement with the recent finite element results of Park and Pharr.     

Finite-element analysis of indentation experiments in surface-coated materials

There are only a few methods suitable for a quantitative characterization of the mechanical properties of surface-coated materials; indentation testing is one of those methods. Within the last decade, a great deal of effort has been made to improve the indentation test and to gather information on the complete deformation process from experimental and numerical investigations. Following this line, this contribution concentrates on the numerical calculation of the elastic-plastic field of deformation in the specimen during the indentation process and the corresponding load-depth of indentation curves in dependence on the dominating parameters. The basic idea is to determine the influence of geometrical imperfections of the indenter on the—experimentally obtained—mechanical properties such as hardness and to provide methods which enable one to distinguish between properties of the system used for testing and the material investigated. Results obtained for uncoated and coated materials are compared. Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

金属材料的强度与应力-应变关系的球压入测试方法

压入法获取材料单轴应力-应变关系和抗拉强度对服役结构完整性评价有重要的基础意义.假定材料均匀连续、各向同性、应力应变关系符合Hollomon律,基于能量等效假定,即代表性体积单元(representativevolume element, RVE)的vonMises等效和有效变形域内能量中值等效假定,本文提出了关联材料载荷、深度、球压头直径和Hollomon律的四参数半解析球压入(semi-analyticalspherical indentation,SSI)模型.通过球压入载荷-深度试验关系获得材料的应力-应变关系和抗拉强度.考虑压入过程中的损伤效应,针对金属材料提出了用于球压入测试的材料弹性模量修正模型.对11种延性金属材料完成了球压入试验,采用本文提出的球压入试验方法测到的弹性模量、应力-应变关系和抗拉强度与单轴拉伸试验结果吻合良好.      

Numerical Modeling of Elastic Spherical Contact for Mohr-Coulomb Type Failures in Micro-Geomaterials

The contact behavior for geological materials, such as reservoir shale rock, is simulated using the finite element method by considering a nano-indenter tip indenting into a geomaterial obeying the Mohr-Coulomb failure criterion. The deformation and slip at the micro-scale along the shear direction in grain-to-grain contact follows the Coulomb frictional/sliding failure criterion, while the linear elastic force-displacement law is enforced in the direction normal to the contact surface. A series of simulations are performed to study the effect of cohesion, friction angle, and tensile strength on the contact response. For a material with very high cohesion and frictionless contact, the indented geomaterial behaves almost purely as an elastic medium. In this case, the indentation process converges to the classic Hertz grain-to-grain spherical contact model. For a material with extremely low cohesion, the geomaterial behaves like cohesionless granular material at the micro-scale. For materials with finite cohesion values, such as shales, the force-displacement responses are analyzed and reported. This simulation is compared to micro-indentation tests using a spherical indenter tip conducted on preserved samples of Woodford shale.