平头压头下基体对压痕规律的影响研究

本文通过对软薄膜/硬基体两相材料体系的平头压痕弹塑性模拟.重点研究了平压头压入过程中,不同屈服强度比(软薄膜屈服强度与硬基体屈服强度之比)以及不同压头尺寸下硬基体对压痕规律的影响.研究发现硬基体对压痕规律的影响与屈服强度比近似满足线性关系,且这种线性关系不随压头尺寸的改变而改变,相同压头半径下,屈服强度比越大,影响就越明显;相同屈服强度比下,压头半径越大,影响就越小.研究还发现压头压入过程中,材料的堆积对压入深度没有影响.     

反向法确定钨合金基体细观性能及其宏观验证

为了确定钨合金基体相的细观性能,采用纳米压痕实验测试了钨合金材料中基体相的细观硬度与弹性模量.利用反向确定法给出了钨合金材料中基体相的细观弹、塑性性质,根据纳米压痕实验中得到的载荷-位移关系,通过三维有限元计算给出了基体相的原位细观屈服强度和硬化模量.为探讨反向法确定钨合金基体细观性能的有效性,采用宏观实验给出了其宏观验证.宏观验证结果表明,用反向法确定钨合金基体材料的细观弹、塑性性质是可靠的.     

硅橡胶压磁复合材料力学性能纳米压痕实验研究

通过纳米压痕测试技术对非晶Fe73.5Cu1Nb3Si13.5B9/硅橡胶压磁复合材料薄膜的力学性能进行了研究,讨论了加载速率、保载时间、峰值载荷等试验参数对模量和硬度测试结果的影响.进一步分析了纳米压痕实验表征非晶Fe73.5Cu1Nb3Si13.5B9/硅橡胶压磁复合材料薄膜蠕变行为的可行性,通过合理确定压痕蠕变实验参数,获得该材料的蠕变应力指数.结果表明:相对峰值载荷,加载速率对测试结果影响更为显著,而保载时间对硬度和模量测试结果几乎没有影响.     

一种钠钙硅酸盐玻璃的纳米压痕测试分析

采用纳米压痕测试技术对一种钠钙硅酸盐玻璃进行微观力学性能的测试分析.测得加载-卸载过程载荷与压入深度曲线,发现被测玻璃的最大压深、残余深度和弹性回复量随最大加载力的增加而增大,但其相对弹性回复率系数基本稳定,平均值为58.2%.通过电子显微镜观察了不同最大载荷下的压痕形貌,发现压痕区域出现了边界沉陷现象.当最大加载力为1 000 mN左右时,三棱锥工具头测试的压痕区域出现了较明显的微裂纹;采用四棱锥工具头时出现微裂纹的最大加载力要小于该值,且裂纹取向均与金刚石工具头的棱角取向一致.利用非线性有限元软件MSC.Marc对纳米压痕过程进行了仿真分析,得到载荷与压入深度的仿真曲线,该曲线与试验结果基本相符;分析了载荷作用下材料内部的应力分布.利用Oliver-Pharr模型得到不同压入深度下被测玻璃的接触刚度值,该值随压入深度的增加而增大.     

仪器化压痕法在在役设备检测中的应用

仪器化压痕技术(IIT)是一种新型的检测在役设备弹性模量、屈服强度、抗拉强度和硬度等力学性能的技术。IIT是通过压头在材料表面下压得到载荷-深度曲线,然后通过分析载荷-深度曲线得到材料的各项力学性能。通过该技术可以实现对在役设备的非破坏力学性能检测。     

硅橡胶压磁复合材料力学性能纳米压痕实验研究

通过纳米压痕测试技术对非晶Fe_73.5Cu₁Nb₃Si_13.5B₉/硅橡胶压磁复合材料薄膜的力学性能进行了研究, 讨论了加载速率、 保载时间、 峰值载荷等试验参数对模量和硬度测试结果的影响。进一步分析了纳米压痕实验表征非晶Fe_73.5Cu₁Nb₃Si_13.5B₉/硅橡胶压磁复合材料薄膜蠕变行为的可行性, 通过合理确定压痕蠕变实验参数, 获得该材料的蠕变应力指数。结果表明: 相对峰值载荷, 加载速率对测试结果影响更为显著, 而保载时间对硬度和模量测试结果几乎没有影响。     

基于EBSD方法测定单晶Cu纳米压痕的各向异性实验

为研究单晶Cu材料的各向异性力学特性,针对单晶连铸技术制备的单晶Cu,采用电子背散射衍射(EBSD)法对其3个不同晶粒的晶面进行定向,利用原位纳米压痕仪在不同晶面进行不同压入载荷的纳米压痕实验.通过EBSD分析,发现用单晶连铸技术制备的单晶Cu在拉拔方向上具有较强的择优取向,单个晶粒较大,且晶粒内部没有(亚)晶界存在.纳米压痕实验结果表明单晶Cu样件在各种压痕载荷下的约化模量为50 GPa～120GPa,材料的晶体取向对纳米压痕载荷-位移曲线和约化模量有很大影响,面(032)比面(119)和面(041)有更大的约化模量.不同载荷下,硬度值在0.8 GPa左右变动,晶体取向对硬度的影响较小.实验所得单晶Cu各晶面约化模量与采用金属弹性力学理论计算所得数值吻合较好.     

残余应力对固体氧化物燃料电池弹塑性性能的影响

半电池结构NiO-YSZ/YSZ由于弹性模量不同和热膨胀系数不匹配,导致烧结过程中产生残余应力. 残余应力对于燃料电池的性能和使用带来一定的影响, 本文把残余应力引入到计算薄膜性能的逆向分析模型中, 建立了考虑残余应力影响的薄膜的纳米压痕分析模型. 利用纳米压痕方法测试了离双层材料界面不同距离处的载荷-位移曲线和相应的材料性能, 用本文建立的模型计算了不同点的性能的变化, 发现离界面越远, 电解质YSZ薄膜的硬度越大. 将热力分析得到的残余应力场作为压痕模拟的初始应力场, 计算三棱锥压头下压痕载荷-位移曲线, 结果显示考虑残余应力时的载荷-位移曲线更接近实验曲线, 并给出了残余应力下压痕形貌图, 发现有残余应力时的压痕形貌更深更大.     

仪器化压痕法单次加载数据绘制应力-应变曲线计算拉伸试验结果的方法初探

采用压痕仪自动采集的位移数据减去压痕仪的弹性变形量得到压痕深度,进而计算出压痕直径,再根据TABOR提出的关于压痕方法测试拉伸性能的系列关系式,计算出真应力、工程应力和应变,绘制出压痕法的连续应力-应变曲线和lg W-lg d曲线,根据两种曲线计算出抗拉强度、屈服强度和应变硬化指数(n值)。实测数据显示压痕法的连续应力-应变曲线与拉伸法的应力-应变曲线存在显著差异。对压痕法测得的抗拉强度、屈服强度和n值的测量不确定度进行了详细分析,结果表明:抗拉强度的测量不确定度小于抗拉强度值的10%,而n值的相对测量不确定度显著大于抗拉强度的相对测量不确定度,目前尚未找到可靠评估屈服强度测量不确定度的方法。     

基于压入试验的材料硬化指数及弹性模量计算方法

为深入研究压入实验在测量材料局部力学性能上的应用,采用ABAQUS有限元模拟结合压痕理论,以RO关系为材料本构模型获得通过压痕响应获得材料硬化指数以及弹性模量的经验公式。通过结合有限元数值模拟,避免物理实验在数据测试阶段的误差,通过研究压入实验卸载段得到能够表征弹性模量的独立参量,另外新引入载荷-位移曲线在对数坐标系的表达,在压痕响应中提取出以硬化指数为单一变量的特征量,将弹性模量的计算与硬化指数独立,得到更加精确的计算结果。经过对理想数学模型以及实验数据进行验证,均能够在有效误差范围内得到合理的硬化指数及弹性模量数值。     

Finite element analysis of the indentation stress characteristics of the thin film/substrate systems by flat cylindrical indenters

The indentation stress characteristics of thin film/substrate systems by the flat cylindrical indenters have been simulated by means of the finite element method (FEM). The emphasis was put on the stress distribution ahead of the indenters. The influences of the friction coefficient between the indenter and the thin film, the thickness and hardening modulus of the thin film have been considered. It is found that the stress distribution was not affected by the friction coefficient. But the influence of the thickness and hardening modulus of the thin film on the stress distribution was obvious. At small indentation depth, the plastic deformation occurs at the edge of the indenter only, and the zone will propagation both vertically and laterally with the indentation depth increasing. When the indentation depth reaches a certain value, the thin film at the interface will occur the deformation plastic zone for the case studied in this paper. At lager depths, the two plastic zones will connect, and then the plastic zone propagates along the lateral direction. Beside, it is also found that the maximum of the Mises stress and the shearing stress on the interface occur at 0.8r and r(r is the radius of the indenter), respectively.     

Numerical Study of Creep Damage Behavior of Thin Film/Substrate Systems under Indentation

A numerical study has been performed on the creep damage development of the thin film/substrate systems by the Kachanov‐Rabothov damage law. The emphasis was to study the influence of the modulus ratio of the substrate to the thin film, the size of the indenter and the indentation stress. Results show that two obvious damage zones are found ahead of the indenter. One is at the edge of the indenter, the other is at the interface ahead of the indenter edge. The influence of the modulus ratio of the substrate to the thin film on the indentation damage is not obvious before a certain creep time, and later, the greater modulus ratio of the substrate to the thin film has the smaller damage rate. And the indentation depth rate and the damage rate are also affected by the size of the indenter and indentation stress.     

Evaluation of the elastic modulus of thin film considering the substrate effect and geometry effect of indenter tip

A method using finite element method (FEM) is proposed to evaluate the geometry effect of indenter tip on indentation behavior of film/substrate system. For the nanoindentation of film/substrate system, the power function relationship is proposed to describe the loading curve of the thin film indentation process due to substrate effect. The exponent of the power function and the maximum indentation load can reflect the geometry effect of indenter and substrate effect. In the forward analysis, FEM is used to simulate the indentation behavior of thin film with different apex angles of numerical conical indenter tip, and maximum indentation load and loading curve exponent are obtained from the numerical loading curves. Meanwhile, the dimensionless equations between the loading curve exponent, the maximum load, elastic properties of film/substrate system and apex angle of indenter are established considering substrate effect. In the reverse analysis, a nanoindentation test was performed on thin film to obtain the maximum indentation load and the loading curve exponent, and then the experimental data is substituted into the dimensionless equations. The elastic modulus of thin film and the real apex angle of indenter can be obtained by solving the dimensionless equations. The results can be helpful to the measurement of the mechanical properties of thin films by means of nanoindentation.     

Indentation of an incompressible elastic film

The problem of impressing a rigid flat-ended cylindrical indenter onto an incompressible elastic film is considered. An integral transform solution is developed to reduce the solution of the problem to a Fredholm integral equation of the second kind with a symmetrical kernel depending on the boundary conditions (frictionless/bonded) in the areas of contact. The relationship between the applied load and the indentation depth is derived, which provides a guideline for measuring the elastic constants of thin films and determining the degree of adhesion between a thin film and a stiffer substrate.     

A Reverse Numerical Approach to Determine Elastic-plastic Properties of Multi-layer Material Systems with Flat Cylindrical Indenters

In the present study, the indentation testing with a flat cylindrical indenter on typical multi-layer material systems was simulated successfully by finite element method. The emphasis was put on the methods of extracting the yield stresses and strain-hardening modulus of upper and middle-layers of three-layer material systems from the indentation testing. The slope of the indentation depth to the applied indentation stress curve was found to have a turning point, which can be used to determine the yield stress of the upper-layer. Then, a different method was also presented to determine the yield stress of the middle-layer. This method was based on a set of assumed applied indentation stresses which were to be intersected by the experimental results in order to meet the requirement of having the experimental indentation depth. At last, a reverse numerical algorithm was explored to determine the yield stresses of upper and middle-layers simultaneously by using the indentation testing with two different size indenters. This method assumed two ranges of yield stresses to simulate the indentation behavior. The experimental depth behavior was used to intersect the simulated indentation behavior. And the intersection corresponded to the values of yield stresses of upper and middle-layers. This method was also used further to determine the strain-hardening modulus of upper and middle-layers simultaneously.     

A numerical fracture analysis of indentation into thin hard films on soft substrates

In this paper, finite element simulations of spherical indentation of a thin hard film deposited on a soft substrate are carried out. The primary objective of this work is to understand the mechanics of fracture of the film due to formation of cylindrical or circumferential cracks extending inwards from the film surface. Also, the role of plastic yielding in the substrate on the above mechanics is studied. To this end, the plastic zone development in the substrate and its influence on the load versus indentation depth characteristics and the stress distribution in the film are first examined. Next, the energy release rate J associated with cylindrical cracks is computed. The variation of J with indentation depth and crack length is investigated. The results show that for cracks located near the indenter axis and at small indentation depth, J decreases over a range of crack lengths, which implies stability of crack growth. This regime vanishes as the location of the crack from the axis increases, particularly for a substrate with low yield strength. Finally, a method for combining experimental load versus indentation depth data with simulation results in order to obtain the fracture energy of the film is proposed.     

A numerical analysis of flexure induced cylindrical cracks during indentation of thin hard films on soft substrates

In this paper, the fracture behavior of a thin hard film, perfectly bonded to a soft substrate, containing circumferential (cylindrical) cracks subjected to spherical indentation is studied using the finite element method. These cracks emanate upwards from the film-substrate interface and are driven by the flexure of the film over the soft substrate under indentation. The film is taken to be linear elastic while the substrate obeys an elastic-plastic constitutive model with linear strain hardening. Three values for the substrate yield strength are considered in the analysis. The variation of energy release rate and mode mixity are examined as functions of crack length and load, for cracks located near and away from the indentation axis. The results show that, when the crack length is small, predominantly mode I conditions prevail due to tensile radial stresses near the interface. As the crack length increases, the mode mixity gradually changes from mode I to II. For cracks located near the axis, the crack growth process is stable over a range of crack lengths up to about a third of the film thickness and thereafter becomes unstable. The role of the substrate yield strength on the above issues is investigated.     

Influence of contact geometry on hardness behavior in nano-indentation

In this paper the influence of contact geometry, including the round tip of the indenter and the roughness of the specimen, on hardness behavior for elastic-plastic materials is studied by means of finite element simulation. We idealize the actual indenter by an equivalent rigid conic indenter fitted smoothly with a spherical tip and examine the interaction of this indenter with both a flat surface and a rough surface. In the latter case the rough surface is represented by either a single spherical asperity or a dent (cavity). Indented solids include elastic perfectly plastic materials and strain hardening elastic–plastic materials, and the effects of the yield stress and strain hardening index are explored. Our results show that due to the finite curvature of the indenter tip the hardness versus indentation depth curve rises or drops (depending on the material properties of the indented solids) as the indentation depth decreases, in qualitative agreement with experimental results. Surface asperities and dents of curvature comparable to that of the indenter tip can appreciably modify the hardness value at small indentation depth. Their effects would appear as random variation in hardness.     

Effect of surface roughness on nanoindentation test of thin films

By using the two-dimensional quasicontinuum method, the nanoindentation process on a single crystal copper thin film with surface roughness is simulated to study the effect of surface morphology on the measurements of mechanical parameters. The nanohardness and elastic modulus are calculated according to Oliver-Pharr’s method. The obtained results show a good agreement with relevant theoretical and experimental results. It is found that surface roughness has a significant influence on both the nanohardness and elastic modulus of thin films determined from nanoindentation tests. The effect of such factors as the indenter size, indentation depth and surface morphology are also examined. To rule out the influence of surface morphology, the indentation depth should be much greater than the characteristic size of surface roughness and a reasonable indenter size should be chosen. This study is helpful for identifying the mechanical parameters of rough thin films by nanoindentation test and designing nanoindentation experiments.