Dependence of microcrack number density on microstructural parameters during plastic deformation of WC–Co composite

Previous studies have shown that WC–Co material can plastically deform during indentation test, and microcracking is one of the dominant mechanisms for the plastic deformation. Microcracks follow several paths of microstructure: transgranular cracks within WC grains, intergranular cracks along grain boundary and cracks at WC/Co interface. In this study, six commercial grades of WC–Co material were tested using the Hertzian indentation technique at the same indentation load with the same size of an indenter. The microcrack number density of each grade was measured and its correlation with microstructural parameters was analyzed. Materials with high cobalt content, large grain size, large mean free path and low contiguity form more microcracks in the course of plastic deformation. The microcrack number density is also correlated with mechanical properties. High microcrack number density corresponds to low hardness and high toughness. The mechanism of how the microcracking affects the fracture toughness is also discussed.     

Influence of indenter angle on cracking in Si and Ge during nanoindentation

The influence of indenter angle on the nanoindentation cracking behavior of single crystal Si and Ge was systematically explored through nanoindentation experiments with a series of triangular pyramidal indenters with different centerline-to-face angles in the range 35.3–85.0°. The relationships between indentation load, crack length and indentation size and their dependence on indenter angle were carefully examined and compared with previous indentation cracking studies. The results are discussed in terms of ways to estimate fracture toughness and indentation cracking threshold loads more precisely through nanoindentation.     

纳米压痕技术及其在薄膜/涂层体系中的应用

综述了纳米压痕技术的发展历程及其在薄膜领域的应用。介绍了当前实验室条件下主要采用的电磁驱动式纳米压痕仪的构造和工作过程。为了保证测试结果的准确性,要在合适的温度、湿度下进行压入实验,借助保载来消除一些可以避免的误差。阐述了压头的分类和选择原则,玻氏压头相比于维氏压头具有更小的中心线与棱面夹角,避免了尖端横刃对于压入结果准确性的影响,因此最常用的压头为玻氏压头;表征断裂韧性最合适的压头为立方角压头;表征微机电系统的弯曲采用楔形压头。总结了通过最大载荷和压入面积得到涂层力学参量的分析流程。归纳了将纳米压痕法应用于表征薄膜涂层的硬度和弹性模量、室温下蠕变性能、断裂韧性、残余应力、塑性性能等力学量的研究,如表征硬度和弹性模量的Oliver-Pharr法的应用,识别蠕变柔量的Lee-Radok模型的应用,分析断裂韧性的Lawn-Evans-Marshall模型的应用。在涂层制备过程中,制备参数的改变可以使得涂层具有不同的力学性能,涂层厚度远小于表面尺寸,硬度和弹性模量仍然存在各向异性,非晶态结构涂层具有更高的硬度和弹性模量。采用碳纳米管强化可以提高涂层的断裂韧性,涂层内存在适量的残余应力数值和合...     

用裂纹密度参数评价PVD硬质涂层断裂韧性的研究

本文根据裂纹体自洽模型,推导出裂纹密度参数关系式,并采用压痕法,通过统计压痕域的裂纹几何,计算裂纹密度参数,以此来评价涂层的断裂韧性.涂层的裂纹密度参数与涂层的膜/基结合力有良好的对应关系,裂纹密度参数的设置对研究涂层的断裂韧性,揭示涂层的磨损与剥落过程,在理论上和工程上都颇有意义.     

Nanoindentation Stress–Strain for Fracture Analysis and Computational Modeling for Hardness and Modulus

Stress–Strain plots based on nanoindentation load–depth curves were obtained to study phenomena like internal fracture and ductile to brittle transitions. Fracture phenomena during the indentation process were analyzed based on the stress–strain plots. A transition from ductile to brittle fracture was observed on increasing the depth or load of indenter penetration. A new approach with shape factors in the fracture studies based on radial crack branching and micro-cracking was done. Hardness and modulus plots were fitted with polynomials. The fitting parameters were varied to obtain different hardness and modulus responses.     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

Friction stir welding (FSW) process

A method for evaluating the microstructural stress–strain relationship of materials, using a triangular pyramidal indenter, is proposed in order to investigate the mechanical properties of steels and weld zones. An existing evaluation method, using a ball indenter, is correspondingly applied to the evaluation method using a triangular pyramidal indenter because the strain distribution under the indenter or the indentation curve on the unloading process between the ball and pyramidal indentation has a similarity. A corresponding ball indenter whose projection area is equal to that of the triangular pyramidal indenter is used to replace the triangular pyramidal indentation with the ball indentation, and the representative stress and strain that express the complicated deformation under the indenter are determined. The stress–strain relationships of single-phase steels in microstructural size are estimated by the proposed method, and on average correspond with those measured by macro-tensile tests. The difference in the stress–strain relationships due to the difference of the crystal orientation of each grain is possibly negligible with this method. It is expected to clearly estimate the difference in the stress–strain relationship of each phase in, for example, dual-phase steels by the proposed method.     

Interfacial indentation test of FeB/Fe2B coatings

The present study uses interfacial indentation testing to estimate the adhesion of the FeB/Fe₂B coating formed on the surface of borided AISI 316 steel. This technique creates and propagates a crack along the FeB/Fe₂B interface and defines the apparent fracture toughness, which can then be related to the adhesion and mechanical support of the aforementioned interface. The boriding process was performed on the surface of AISI 316 steel by means of the powder-pack method at temperatures of 1123, 1173, 1223 and 1273 K with 2, 4, 6, 8 and 10 h for each temperature. The Young's modulus for each surface layer was obtained by Knoop microindentation at a constant indentation load. Vickers microindentation fracture technique was used to generate microcracks at the FeB/Fe₂B interface with varying indentation loads. The applied load, Young's modulus, hardness, and lateral crack lengths generated from the corners of the indentations, along with the depth of the FeB layer, were used to determine the apparent fracture toughness and adhesion of the FeB/Fe₂B interface. The apparent fracture toughness of the FeB/Fe₂B interface varied between 3.56 and 4.45 MPa . Finally, the intensity of residual stress at the FeB/Fe₂B interface was estimated as a function of the FeB layer thickness.     

PZT-5铁电陶瓷恒载荷压痕裂纹在空气和水中的扩展过程

用Vickers硬度计作为加载装置,研究了极化锆钛酸铅(PZT-5)铁电陶瓷压痕裂纹恒载荷下在室温空气和水中的扩展规律.结果表明,恒载荷下,压痕裂纹在空气和水中不断扩展, 120 h后趋于稳定,从而就可获得裂纹扩展速率和裂纹止裂的门槛应力强度因子KISCC,它们均显示各向异性.研究表明,裂纹扩展速率和KISCC的各向异性与铁电陶瓷断裂韧性的各向异性有关,即裂纹扩展速率随KIC的增加而降低,而KISCC随KIC的增加而升高.平行极化方向裂纹的断裂韧性比垂直极化方向高,即KCICdn/dt;与在空气中相比,在水中应力腐蚀更敏感,即裂纹扩展速率更高、门槛值更低.     

A numerical fracture analysis of a stationary semi-circular interface crack during interfacial indentation test

The aim of this work is to investigate the fracture behavior of a stationary crack lying along the interface in a coated system during interfacial indentation test. One traditional 3D numerical model and one cohesive element numerical model have been made to study the Vickers indentation test process. A crack-block approach is applied to generate 3D meshes containing the crack front along the interface. The stress intensity factors and the energy release rates (G) on the crack front are computed. The fracture mode mixity is also presented. It is found that the near surface propagation of the crack can be prevented during loading due to compressive stresses. It may occur during unloading due to residual stresses resulting from plastic strain.     

Investigation of Adhesion and Fracture Toughness of Thermally Grown Oxide Scales by Interface Indentation Test

HIGH TEMPERATURE ALLOYS rely on theformation of a dense layer against oxidation.Howeverthe crack,spallation and detachment of oxide layeunder growth stress and thermal stress fromtemperature difference cause the failure of theprotective layer.Therefore,high temperature materialsare required forming a good oxide scale with highstrength and high interface bonding strength.Anappropriate measuring method and evaluation fooxide/metal interfacial adhesion is of great importanceto understand …     

Depth-sensing indentation of low-density brittle nanoporous solids

Many applications of low-density nanoporous dielectrics are limited by their poor mechanical properties. Although nanoindentation is often used to evaluate the mechanical behavior of such materials, the physical meaning of the inelastic parameters measured by various indentation methods is not clear. Here, we study low-density nanoporous silica monoliths (aerogels) by nanoindentation using the most common indenter geometries (spherical, pyramidal and flat punches) and discuss the parameters measured. Results suggest that the deformation of the nanoporous silica monoliths studied in this work is controlled by elastic bending and fracture of nanoligaments with no signs of plasticity. The contact pressure (Meyer hardness) increases with increasing strain and does not represent the foam “crushing pressure”. The critical load for Hertzian fracture obeys the Auerbach law, and the formation of radial cracks and brittleness are strongly suppressed by the presence of nanopores. We also discuss the choice of indenter geometry and provide recommendations that can be used to overcome some typical challenges of indentation studies of low-density nanoporous solids, including very low contact stiffness for indenters with small contact areas, large surface roughness inherent to this class of materials, spatial non-uniformity (skin layers on monoliths), viscoelasticity and elastic nonlinearity.     

晶粒尺寸对纳晶双峰材料断裂韧性的影响

为了描述由纳晶基体和粗晶颗粒组成的纳晶双峰材料的断裂韧性,通过建立一个粘聚力模型来研究纳晶双峰材料的临界应力强度因子K_(IC)(表征材料断裂韧性)。考虑到纳晶双峰材料的一个典型情况:裂纹位于2个纳晶颗粒的交界面处,裂纹尖端与粗晶粒的晶界相交,假设粘聚区的尺寸等于纳晶颗粒的尺寸d。裂纹的钝化和扩展过程受位错和粘聚力的共同影响,刃型位错是从粘聚力裂纹的尖端发射,该过程对裂纹产生屏蔽效应。模型计算结果显示:当粗晶颗粒尺寸D确定时,K_(IC)随着纳晶材料晶粒尺寸d的增大而增大;当纳晶材料晶粒尺寸d确定时,K_(IC)随着粗晶材料晶粒尺寸D的增大而增大;相对于纳晶颗粒的尺寸,断裂韧性对粗晶晶粒的尺寸更加敏感。     

Microstructure and mechanical properties of a SiC containing advanced structural ceramics

The fracture toughness of SiC-AlN structural ceramics can be improved by obtaining fine grain size distribution of both SiC and AlN phases and a weak SiC-AlN interfacial solid solution layer which helps in crack deflection. In order to achieve this, a SiC containing ceramics i.e. 55wt.%SiC+45wt.%AlN is prepared by spark plasma sintering (1850 °C/50 MPa/5 min.). High density (96.6%) with a very fine grain microstructure comprising of intimate mixture of both the SiC and AlN phases are obtained in the product. Vol% & wt% of SiC & AlN phases estimated through the EDS mapping and Rietveld analysis are found to be 54 vol% & 46 vol%, and 54.2 wt% & 45.8 wt% respectively. Formation of solid solution layer between hexagonal AlN (a = 3.1114 Å, c = 4.9792 Å) and hexagonal SiC (a = 3.081 Å, c = 5.031 Å) phase is confirmed indirectly by change in AlN lattice parameter and directly by EDS mapping. Electron probe micro analysis (EPMA) study reveals that, the solid solution between AlN and SiC phase is formed by mainly diffusion of SiC in AlN phase. Young modulus and hardness of the SPS product are similar to those reported in the literature. The product shows higher indentation fracture toughness (6.7 ± 0.9 MPa.m^0.5 and 4.8 ± 0.6 MPa.m^0.5 obtained using Median and Palmqvist crack model respectively) than the fracture toughness of a well known ultra-high temperature structural material i.e. ZrB2–20SiC (3.9 ± 1.1 MPa.m^1/2). The indentation cracks are found to initiate at the AlN-SiC interface (Hv1263–1641) and are found to propagate along either the AlN phase (Hv1434–1720) or AlN-SiC interface. The cracks are found to stop when these encounter either SiC phase (Hv1896–2476) or porosity. The higher value of fracture toughness may be attributed to the mismatch in coefficient of thermal expansion (CTE) of SiC and AlN phase, fine grain size distribution, secondary crack generation and a weak AlN-SiC interfacial solid solution layer which helps in crack deflection. This product may find potential application as high temperature structural material since SiC containing materials exhibits outstanding high temperature oxidation and corrosion resistance.     

评价强界面涂层界面结合能力的横截面压入法

为了解强界面脆性涂层/硬性金属基体涂层结构横截面压入时的涂层剥落特点,以电镀铬层/硬性金属基体为研究模型,在原位观察系统下进行横截面压入试验,其结果表明:铬层断裂和剥落与载荷-位移曲线上出现的3个载荷下降点相对应,首先是在铬层横截面上形成一条与压头轴线平行的中间裂纹;随后又出现了以第1条裂纹为对称轴的2条斜裂纹;随着3条裂纹的稳态扩展和汇合,形成了与周围铬层相隔离、仅靠基体支撑的2块1/4圆形局部"孤岛"铬层;最后"孤岛"铬层沿界面剥落.以此"孤岛" 剥落为力学计算模型,提出定量评价强界面脆性涂层/硬性金属基体的界面结合能力的横截面压入法,该方法利用载荷-位移曲线计算"孤岛"剥落所需要的总能量,测量界面剥落面积,并计算两者的比值,给出临界界面能量释放率,以此数据作为评价界面结合能力的指标,并给出了应用举例.     

X70管线钢不同温度下断裂韧性实验研究

以三维弹塑性断裂理论为基础，对有限厚度板裂纹端部应力场、三维应力约束进行了分析，通过对不同厚度、不同初始裂纹长度在不同温度下三点弯曲试件的断裂韧性测试断口观测和理论分析获得如下结果：离面约束对裂尖应力场及断裂韧性有强烈的影响；断口均产生分层裂纹，其位置、大小和数量与试样厚度、温度和裂纹初始长度有关；温度较低时，分层裂纹距主裂纹根部一定距离，分层裂纹宽度较小，对厚度效应影响较小；温度较高时，分层裂纹首先出现在主裂纹根部，分层裂纹宽度较大且充分张开，降低了试样的有效厚度，对X70管线钢进行性能评价时必须考虑管道壁厚、层裂和环境温度的耦合作用。     

Transverse fracture in thin-film coatings under spherical indentation

The competition between transverse surface and sub-surface cracks in a thin, hard coating bonded to polycarbonate substrate due to spherical indentation is investigated in real-time as a function of coating thickness and indenter radius. Fine grain (Y-TZP) and medium grain (alumina) ceramics and pre-abraded amorphous glass are used for the coating. As the coating thickness is reduced, the familiar star-shape sub-surface damage is suppressed, resulting in the top-surface ring crack as the dominant fracture mode. In the intermediate thickness range, the sub-surface damage occurs as a set of off-axis cracks. LEFM in conjunction of a large-strain FEM contact code is used to predict the onset of transverse fracture in the coating. Guided by the test results, the damage on both coating surfaces is assumed as a cylindrical surface crack. In consistency with their polycrystalline nature, the coatings are assumed to contain a distribution of cracks, with the least fracture load among all possible crack lengths taken as the critical load. The numerical predictions compare well with the tests results, and they help identify the applicability range of the simpler point loading case as well as a fracture analysis that is based on a critical stress criterion in terms of the system parameters.     

Local and global fracture toughness of a flame sprayed molybdenum coating

This study deals with the failure of a flame sprayed molybdenum coating and its fracture mechanical characterization. The microstructure of the coating was examined by scanning electron microscopy. Local hardness and Young’s modulus values were measured by nanoindentation methods. X-ray measurements were used to estimate the grain size within the molybdenum splats. Special fracture mechanical tests were made to study the fracture mechanical behavior of cracks parallel and perpendicular to the coating-substrate interface. Indentation fracture toughness tests were made to examine the local fracture behavior of the material. Using these local toughness values and taking into account the microstructure, crack path, and mismatch effects it was possible to explain the global fracture toughness values.     

Nano-impact test on a TiAlN PVD coating and correlation between experimental and FEM results

Nano-impact test on PVD coatings is an efficient method for investigating film failure mechanisms. During this test, the coating is subjected to repetitive impacts by a diamond indenter, inducing high local deformations and stresses into the film material, which may lead to coating failure.In the paper, coated specimens with a TiAlN PVD film were investigated by nano-impact tests. The nano-impacts were conducted at several loads and for various test durations. For explaining the attained results, the nano-impact test was simulated by a developed three dimensional finite elements method (FEM) model, considering a piecewise linear plasticity material law. The employed software was the LS-DYNA package; its feature of constrained tied nodes failure was used for simulating crack formation and propagation, as the plastic strain develops and exceeds the coating failure strain. The film elasto-plastic properties, used in the FEM-calculations, were determined by nanoindentations and analytical evaluation of the related results. During the nano-impact indenter penetration, it was assumed that the coating material at the FEM model node regions can withstand the applied load up to a maximum value, which corresponds to the coating rupture stress. Over this load limit, the related nodes are disconnected from the neighboring finite elements. If all nodes of an element are disconnected, the element is released for simulating a crack formation and it becomes an inactive separate entity. In this way, the stress fields developed in the film material and its coating fracture progress in terms of imprint depth versus the repetitive indenter penetrations are analytically described. The attained results converge sufficiently with the experimental ones. The developed nano-impact FEM-simulation predicts the film failure initiation and evolution, which depend on the impact load.