Effect of nitrogen ion irradiation on the nano-tribological and surface mechanical properties of ultra-high molecular weight polyethylene

Generation of wear debris is the principal obstacle limiting the durability of ultra-high molecular weight polyethylene (UHMWPE) in biomedical applications. Aiming to enhance UHMWPE wear resistance, surface modification with swift heavy ion irradiation (SHI) appears as a potential and attractive methodology. Contrary to ion implantation techniques, the swift heavy ions range can reach tens to hundreds microns and its extremely high linear energy is able to induce effective chemical modifications using low fluence values. Nano-wear performance and surface mechanical properties of samples of pristine and SHI irradiated (using N₂⁺ ions at 33 MeV and a fluence of 1 × 10¹² ions/cm²) were characterized by depth sensing indentation (DSI) and scanning probe microscopy (SPM). It turned out that modifications induced by irradiation at the surface layers were successful to reduce nano-wear volume and creep deformation. These improvements were related to beneficial changes in hardness, elastic modulus, hardness to elastic modulus ratio and friction coefficient.     

Effects of the substrate on the determination of thin film mechanical properties by nanoindentation

We examine the effects of the substrate on the determination of mechanical properties of thin films by nanoindentation. The properties of aluminum and tungsten films on the following substrates have been studied: aluminum, glass, silicon and sapphire. By studying both soft films on hard substrates and hard films on soft substrates we are able to assess the effects of elastic and plastic inhomogeneity, as well as material pile-up, on the nanoindentation response. The data set includes Al/glass and W/sapphire, with the film and substrate having nearly the same elastic properties. These systems permit the true contact area and true hardness of the film to be determined from the measured contact stiffness, irrespective of the effects of pile-up or sink-in. Knowledge of the true hardness of the film permits a study of the effects of the elastic modulus mismatch on the nanoindentation properties, using the measured contact stiffness as a function of depth of indentation.     

On the relationship between plastic zone radius and maximum depth during nanoindentation

The relationship between plastic zone size and plastic depth during indentation has been studied by a number of workers and an expression relating the plastic zone radius to the residual indentation depth (assumed to be the plastic depth) was developed by Lawn et al. [B.R. Lawn, A.G. Evans and D.B. Marshall, J. Am. Ceram. Soc. 63 (1980)198.] based on microindentation testing. In this study, the relationship between the plastic zone radius and residual indentation depth was examined using finite element analysis for conical indentation in elastic-perfectly-plastic bulk materials. The simulations show that the Lawn method overestimates the plastic zone size for different materials with a wide range of Young's modulus over hardness ratio and for indenters with different geometries and it does not consider tip rounding effects. Therefore, an analytical expression is outlined which agrees well with finite element data. For practical application, the relationship between the radius of the plastic deformation zone and the maximum penetration depth is developed here which has been used to modify the energy-based model developed at Newcastle University to predict the hardness and Young's modulus of coated systems. It is found that the new relationship is easy to apply and predictions of the hardness and Young's modulus of coated glass show good agreement with experimental results.     

Study on the indentation size effect in CaF2: Dislocation structure and hardness

In this study nanoindentations have been performed on a cleaved surface of a CaF₂ single crystal and the dislocation structure has been investigated by the etch pit technique using atomic force microscopy. The deformation during indentation is first purely elastic until dislocations are created observable in a pop-in in the load displacement data, as well as in a dislocation rosette around the indentation. After pop-in a relatively high hardness is observed, which gradually decreases, until at 3 μm a nearly constant hardness is found. By using sequential polishing, etching and imaging, the dislocation structure underneath indentations with indentation depths of 300 nm and 110 nm (load: 5 mN, 1 mN) is quantified. The dislocation density and radial distribution of dislocation density depend on the indentation depth, where a smaller indentation depth leads to a higher dislocation density, which is in qualitative agreement with the observed increase in hardness.     

单晶SnO2纳米带裂纹形核的临界应力和断裂韧性

使用纳米压痕法测量了单晶SnO2纳米带的硬度、断裂韧性以及裂纹形核的临界应力。结果表明，当载荷大于临界值后微裂纹就从压痕顶端形核、扩展；与此相应，在载荷一位移曲线上出现位移突变平台，根据平台载荷计算出压痕裂纹形核的临界应力σc=3．4GPa；利用裂纹的长度计算出SnO2纳米带的断裂韧性为0．028—0．066MPa-m^1／2，其平均值为KIc=0．044MPa-rn^1／2，它比其它块体脆性材料的断裂韧性小一个数量级,实验测出SnO2纳米带的硬度H=6．25GPa和弹性模量E=86．7GPa。     

Indentation properties of plasma sprayed Al2O3–13% TiO2 nanocoatings

Young’s modulus and hardness were determined by depth sensing indentation in plasma sprayed Al₂O₃–13% TiO₂ nanocoatings. Results were compared to conventional coatings and the relevance of the nanostructure was analyzed. An indentation size effect was observed. Data provided by indentation tests at different maximum loads were used to estimate size-independent hardness and elastic modulus. Enhanced properties were observed in the nanostructured coating compared to the conventional one. Partially melted zones in the nanocoating, which act as reinforcements in the ceramic matrix composite, are likely responsible for the enhancement.     

An application of universal hardness test to metal powder particles

Powder metallurgy is a “net shape” components producing technology from metal powders by compaction with following sintering processes. For today actual trends of powder metallurgy are associated with modern powder grades, alloyed by elements with high affinity to oxygen (Cr, Mn, Si, etc.). Contamination of powder particles by oxides and/or other secondary phases have a negative effect on their compressibility and sinterability. The geometry properties of powders give integral information about powder quality. Evaluation of yield strength and/or rather the strain hardening exponent, characterizing the mechanical properties on the level of individual particles, really is not possible. One of available approaches could be measurement of the microhardness of particles. The contribution deals with the evaluation of the microhardness of powder particles and specification of the factors affecting measured values. Using standard Vickers microhardness HV0.01 measurements for two different powders the results obtained showed large scattering from the average. This gave no possibility to identify the influence of alloying and particle matrix purity on microhardness. Problem was solved utilizing instrumented indentation test using NanoIndenter XP. This is usable technique for estimation of microhardness of powder particle matrix and gives possibility to recognize differences between different size fractions of particles. Based on the obtained results it was concluded, that absolute results of indentation hardness and indentation modulus are strongly affected by mounting resin type. Utilizing DSI method and mounting resin of proper hardness enabled to evaluate the microhardness of powders with different alloying element content. Influence of particles purity/size on powder microhardness was established as well. Indentation hardness and indentation modulus for sintered materials are in good agreement with the data for corresponding bulk materials. Obtained results confirm that universal hardness test is valuable instrument for evaluating of sintered materials properties.     

Influence of the residual stress on the nanoindentation-evaluated hardness for zirconiumnitride films

The influence of the residual stress on the evaluated hardness and modulus for zirconium nitride films has been investigated using nanoindentation experiments in this work, and a variety of indentation load–displacement curves have been examined by analyzing the contribution of the residual stress to the indentation load. Atomic force microscopy (AFM) is performed to reveal the behavior of deformation (e.g. pile-up) around the indent on the surface of the film. The pile-up occurs for the film under a compressive stress, and is enlarged with increasing the compressive stress, which leads to that the actual contact area by indenter significantly deviates to the one calculated by Oliver–Pharr method. After correcting the contact area contributed by pile-up via AFM experiments, the residual stress does not affect the nanoindentation-measured hardness and modulus.     

对几种薄膜硬度测试方法的评定

用传统的显微硬度计直接测量薄膜硬度难以避免基体的影响且小载荷压入时产生的较小的压痕不易被准确测量。纳米压入仪是一种可以精确控制及测量压入深度的高精度仪器。这种通过测压入深度测硬度值的技术可以减少基体硬度对薄膜测量硬度的影响同时产生的误差较直接测压痕对角线的误差小。但是，纳米压入仪对试样表面光洁度要求高，仪器价格昂贵。这些都限制其在实际工业中的应用。较好的一种硬度测试方法是通过传统显微硬度计在不同的大载荷下测得膜基复合硬度，同时建立适当的模型计算出薄膜的本征硬度值。此方法可能更适合于常规应用。     

Nanoindentation of porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3−δ

In this paper we show how reliable measurements on porous ceramic films can be made by appropriate nanoindentation experiments and analysis. Room-temperature mechanical properties of the mixed-conducting perovskite material La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF6428) were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 °C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45% to 5% after sintering at 900–1200 °C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the “dense” sample was found to account for the discrepancy between the elastic moduli measured by indentation and by impulse excitation. Crack-free LSCF6428 films of acceptable surface roughness for indentation were also prepared by sintering at 900–1200 °C. Reliable measurements of the true properties of the films were obtained by data extrapolation provided that the ratio of indentation depth to film thickness was in the range 0.1–0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3-D microstructures of films before and after indentation were characterized using focused ion beam/scanning electron microscopy tomography. Finite-element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.     

Indentation properties of copper leadframe with hard coatings

Hardness and elastic modulus of a typical leadframe material which consists of a Cu alloy substrate with Ni and Pd bi-layer coatings were characterised using Vicker’s micro-hardness and nano-indentation tests. The two different testing methods produced similar results in that ‘composite’ hardness for all coated specimens decreased (except at very low loads in the nano-indentation test) with increasing indentation depth and tended to converge at sufficiently high loads due to the strong influence of the coating/substrate interaction. The hardness values of substrate and Ni and Pd coatings were successfully determined from the ‘composite’ hardness values based on the empirical linear relationship with the inverse of indentation diagonal and the rule of mixtures principle.     

The Instrumented Indentation Study of HVOF-Sprayed Hardmetal Coatings

Elastic-plastic properties, namely, hardness and Young’s modulus, of four HVOF-sprayed hardmetal coatings were measured by instrumented indentation using Oliver-Pharr method Nanoindenter XP MTS with a continuous stiffness measurement (CSM) module. The results show that with sufficient number of CSM measurements, one can distinguish between indents made in the hard particles and indents made in the binder material. This can be accomplished by analyzing the plots of hardness and Young’s modulus versus load (or versus indentation depth). Further development of the dependence curves enables the load (or indentation depth) to be set to correspond to the point of transition from a single structure component to the composite material and to determine the properties of both. Comparison of results of CSM measurement with the results of single indentation measurement at a defined load reveals a new perspective on the origin of the indentation size effect in hardmetal coatings. The measurements show that the increase in both the hardness and Young’s modulus with decreasing load is caused mainly by the predominant influence of hard particles in the coatings.     

Morphological, structural, and mechanical characterizations of InGaN thin films deposited by MOCVD

Presented in this study are surface roughness, crystalline structure, and nanomechanical properties of InGaN thin films deposited under various growth temperatures, obtained by means of X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation techniques. The InGaN thin films with different In contents were deposited on sapphire substrates through a metal-organic chemical-vapor deposition (MOCVD) system. Changes in mechanical properties for InGaN thin films are discussed in conjunction with deposition temperature, surface morphology and crystalline structure. The XRD measurements showed that there was no phase separation of In as the In composition went from 25 at.% to 34 at.%. Moreover, both XRD and AFM showed larger grain and surface roughness in In_0.25Ga_0.75N thin films. Nanoindentation results indicate that hardness and Young's modulus both decreased as the indentation depth increased. The contact stress–strain relationships were also analyzed.     

聚苯乙烯薄膜表面粘弹性行为的纳米压痕研究

目的通过研究聚苯乙烯薄膜不同深度下的加载力曲线,分析得到其随着深度的不同而导致测量硬度和模量的差异,研究聚苯乙烯薄膜表面粘弹性行为,从而为高分子薄膜受限效应的理论和应用探索提供参考和借鉴。方法采用纳米压痕方法对聚苯乙烯薄膜样品(厚度约500 nm)进行了系列纳米压痕测量与分析。结果加载速率较低时,其加载曲线与P-h~2关系会发生不同程度的偏离。加载速率为0.01、0.02、0.03、0.06、0.1 mN/s时,P-h~2关系中的指数从1.609逐渐增大到1.628。通过在最大载荷时保载60 s,得到了压入深度随时间变化的关系,进而计算得到蠕变柔量,并根据Zener模型拟合得到压入深度为300~400 nm时,薄膜样品存在具有不同粘弹性的两种结构。而根据不同压入深度的卸载曲线,得到样品的硬度和折合弹性模量都在压入深度约为200~400 nm的区域存在粘弹性的非均匀现象。结论在不同压入深度处存在两种不同的粘弹性结构,根据当前人们普遍接受的高分子薄膜层模型,这两种结构分别对应聚苯乙烯薄膜样品的自由表面层和本体层。     

硅化镍对镍硅合金表面气体渗硼层硬度、弹性模量和断裂韧性的影响

在N2?H2?BCl3气氛下对镍硅合金进行两级气体渗硼(910℃、2 h)制备双区硼化层.显微组织由两种具有不同相成分的区域组成.外层区域仅含有硼化镍的混合物(Ni2B,Ni3B),内层区域除了硼化镍还含有硅化镍(Ni2Si,Ni3Si).研究硅化镍的存在对镍基合金表面硼化层力学性能的影响.使用带有Berkovich金...     

Nanoindentation characteristics of Zr69.5Al7.5 − xGaxCu12Ni11 glasses and their nanocomposites

This work deals with the indentation behavior of Zr_69.5Al_7.5 − xGa_xCu₁₂Ni₁₁ (x = 0, 1.5, 7.5 at.%) alloys. A comparison between their nanohardness and reduced elastic modulus values of the as-synthesized glassy phase with their nanocomposites has been made. The indentation characteristics of a novel Ga substituted glass composition corresponding to x = 7.5 have shown significant improvement in regard to hardness and elastic modulus. The evidence of pile up has been observed in case of as-synthesized glassy ribbons. The load (P) versus depth (h) curves for as-synthesized melt-spun ribbons displayed the presence of displacement burst, which are known as pop-ins. The amount of energy per unit volume required for the shear band formation in glassy state has been estimated based on the pop-ins observed in P-h curve. This seems to decrease with Ga addition. Based on transmission electron microscopic observations of indented glassy specimen, the possibility of nanocrystallization has been ruled out.     

Nano-indentation fracture test of Pb(Zr0.52Ti0.48)O3 ferroelectric thin films

In this paper, we propose an elastic groundsill beam model with piezoelectric effect considered to assess the interfacial adhesion of ferroelectrics thin films, complemented and validated by nano-indentation fracture test on Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films. It was observed that the hardness and elastic modulus of thin films depend on the indentation depth. It was also observed from the load-indentation depth curves and atomic force microscopy (AFM) images that the fracture failure of PZT thin films induced by nano-indentations can be divided into three typical stages: no damage, bulging and spallation. The delamination of thin film systems was modeled as an interfacial crack propagation problem, with the energy release rate determined from the elastic groundsill beam model. Good agreement was observed between the indentation load and the radius of the largest imprint. For PZT thin films deposited on single Si substrate with thickness of 350 nm and 450 nm, the energy release rates per unit new crack area are in the range of 3.4~52.4 J/m² and the phase angles are constant of 13.4°. The corresponding mode I and mode II stress intensity factors are in the range of K_I=0.4–1.6MPa·m^1/2 and K_II=0.6–2.2MPa·m^1/2.     

多层膜晶界和膜界间竞比变形及其对硬度测量的影响

使用磁控溅射法制备了不同调制波长的Ni/Al多层膜,利用X射线衍射(XRD)和高分辨电子显微术(HRTEM)对薄膜进行了微结构表征,采用连续刚度法(CSM)研究了不同压入深度下多层膜的硬度.结果表明,随调制波长减小,薄膜呈纳米晶结构特征且存在超硬效应.调制波长L大于30nm时,纳米压入硬度随压入深度的增加而升高;L小于30nm时,最大压入深度的硬度测量值反而最小.同时发现压入深度较小时硬度相对大小对调制波长不敏感.结合微结构表征,从晶界和膜界的竞比变形角度进行了分析讨论.     

Deformation in a Zr57Ti5Cu20Ni8Al10 bulk metallic glass during nanoindentation

Nanoindentation experiments of a Zr₅₇Ti₅Cu₂₀Ni₈Al₁₀ bulk metallic glass were performed with indentation loads ranging from 200 to 2000 μN. Both the indentation hardness and the reduced contact modulus decreased with the increase in the indentation load due to the propagation of shear bands underneath the indenter – the occurrence of a softening effect. The ratio of the indentation hardness to the reduced contact stiffness was a function of the reciprocal of the indentation depth. Based on the concept of diffusion-induced stresses, a one-dimensional constitutive relation between the change of the excessive free volume and the flow stress was proposed. The indentation-size effect as observed in the indentation tests was explained through the consideration of the contribution of the strain gradient in the constitutive relation.     

Effect of Strain Rate on Deformation Behavior of AlCoCrFeNi High-Entropy Alloy by Nanoindentation

In this study, nanoindentation tests with continuous stiffness measurement technique were measured to investigate the deformation behavior of a high-entropy alloy AlCoCrFeNi under different indentation strain rates at room temperature. Results suggest that the creep behavior exhibits remarkable strain rate dependence. In-situ scanning images showed a conspicuous pileup around the indents, indicating that an extremely localized plastic deformation occurred during the nanoindentation. Under different strain rates, elastic modulus basically remains unchanged, while the hardness decreases with increasing indentation depth due to the indentation size effect. Furthermore, the modulus and hardness of AlCoCrFeNi HEAs are greater than that of the Al _x CoCrFeNi (x = 0.3,0.5) at the strain rate of 0.2 s^?1 due to its higher negative enthalpy of mixing related to the atomic binding force, and the solid solution strengthening induced by the lattice distortion, respectively.