Depth-sensing Indentation Method Research Articles

Sb effect on micro structural and mechanical properties of rapidly solidified Al–12Si alloy

The effects of antimony (Sb) addition on the microstructure and mechanical properties of rapidly solidified eutectic Al–Si alloy were investigated. Al–12 wt.%Si– X wt.%Sb ( X = 0, 0.5 and 1.0) alloys were fabricated using a melt-spinner at rotating speeds of 20 m/s. The as-melt-spun ribbons were characterized by a combination of scanning electron microscopy (SEM), X-ray diffraction (XRD) and depth-sensing indentation (DSI) methods. SEM observations showed that 0.5%Sb addition refined the microstructure whereas 1.0%Sb addition led to the formation of coarse Si particles. The wheel side of the ribbons exhibited a finer microstructure than those on the free or air sides exposed to the atmosphere. According to the XRD results, the diffraction patterns of melt-spun ribbons consisted solely of α-Al and Si phases, and a possible volume fraction of AlSb or an intermetallic phase in the alloy was not observed. DSI tests were performed on the ribbons under different loads ranging from 200 to 1200 mN at room temperature. The resulting indentation load-displacement ( P– h) curves were analyzed using the work of indentation approach (WIA) to evaluate the ribbons’ microhardness ( H) and reduced modulus ( Er). We found that the H and Er values of the ribbon decrease with increasing Sb content. This can be attributed to the over modification and changes in the microstructure of Al–12%Si alloy when a minimum of 1.0%Sb is present as a modifier.

Depth-sensing indentation modeling for determination of Elastic modulus of thin films

There are various methods to address the problem of determining the hardness of thin films when the substrate is involved in the deformation process produced during conventional indentation tests. For the determination of the elastic modulus using depth-sensing indentation methods, the problem is more complex due to the deformation of the equipment that comes in addition to the effect of the substrate. In the paper we discuss the use of Oliver and Pharr’s method to take into account the deformation of the equipment for the measurement of the elastic modulus of TiCN thin films. For micro-indentation tests we proposed a new model to precise the effect of the substrate. The elastic moduli that are calculated are in very good agreement to those found in literature. For nano-indentation tests it is necessary to correct the data to take into account the shape of the indenter tip. We show that this correction, proposed at the origin for massive materials, is not able to explain the discrepancies between the calculated values and those coming from the literature for the elastic modulus of thin films.

Energy dissipation in depth-sensing indentation as a characteristic of the nanoscratch behavior of coatings

Wear behavior of coatings has usually been described in terms of mechanical properties such as hardness ( H) and effective elastic modulus ( E*). Alternatively, an energy approach appears as a promising analysis taking into account the influence of those properties. In a nanoindentation test, the dissipated energy depends not only on the hardness and elastic modulus, but also on the elastic recovery ( W e ). This work aims to establish a relation between plastic deformation energy ( E p ) during depth-sensing indentation method and the grooving resistance of coatings in nanoscratch tests. An energy dissipation coefficient ( K d ) was defined, calculated as the ratio of the plastic to the total deformation energy ( E p / E t ), which represents the energy dissipation of materials. Reactive depositions using titanium as the target and nitrogen and methane as reactive gases were obtained by triode magnetron sputtering, in order to assess wear and nanoindentation data. A topographical, chemical and microstructural characterization has been conducted using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), wave dispersion spectroscopy (WDS), scanning electron (SEM) and atomic force microscopy (AFM) techniques. Nanoscratch results showed that the groove depth was well correlated to the energy dissipation coefficient of the coatings. On the other hand, a reduction in the coefficient was found when the elastic recovery was increased.

Cyclic Depth-Sensing Indentation of Gold Wire

Conventional depth-sending indentation and novel cyclic indentation were performed to study the mechanical properties and strengthening mechanism of pure gold and Ca-doped gold of up to 90 ppm. The system was chosen because it is popular materials for microelectronics interconnection, yet the strengthening mechanism is not well-understood. Conventional depthsensing indentation showed an increase in hardness and modulus of gold with increasing Ca content of up to 90 ppm, which remained significantly higher after annealing. Cyclic depth sensing indentation, where the specimen is loaded to a specific value, unloaded and immediately reloaded, produced a cyclic loop between unloading and reloading curves. The path of the unloading and reloading curves tell the story of the different response of the dislocations generated under the indentation, in different materials, pure gold and Ca-doped gold at different level. Thus their dislocation movement and deformation behavior could be studied by cyclic depth-sensing indentation method. This paper reports cyclic indentation study performed in calcium doped gold widely used in wire bonding and shows that a few ppm level of calcium could result in dislocationssolute interactions, leading to significant strengthening of the materials. TEM micrographs of the material shows that it consists of elongated grains parallel to the drawing direction about 200 nanometers in diameter and a few micrometers in length. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.

Effective elastic modulus of film-on-substrate systems under normal and tangential contact

Load and depth sensing indentation methods have been widely used to characterize the mechanical properties of the thin film–substrate systems. The measurement accuracy critically depends on our knowledge of the effective elastic modulus of this heterogeneous system. In this work, based on the exact solution of the Green's function in Fourier space, we have derived an analytical relationship between the surface tractions and displacements, which depends on the ratio of the film thickness to contact size and the generalized Dundurs parameters that describe the modulus mismatch between the film and substrate materials. The use of the cumulative superposition method shows that the contact stiffness of any axisymmetric contact is the same as that of a flat-ended punch contact. Therefore, assuming a surface traction of the form of [1−( r/ a) 2] −1/2 with radial coordinate r and contact size a, we can obtain an approximate representation of the effective elastic moduli, which agree extremely well with the finite element simulations for both normal and tangential contacts. Motivated by a recently developed multidimensional nanocontact system, we also explore the dependence of the ratio of tangential to normal contact stiffness on the ratio of film thickness to contact radius and the Dundurs parameters. The analytical representations of the correction factors in the relationship between the contact stiffness and effective modulus are derived at infinite friction conditions.

An improved relation for the effective elastic compliance of a film/substrate system during indentation by a flat cylindrical punch

Measurement of the mechanical properties of thin films on substrates by load and depth sensing indentation methods such as nanoindentation often requires accurate descriptions for the effective elastic compliance of the film/substrate system. Here, a simple modification of the commonly used solution derived by Gao et al. [H. Gao, C.H. Chiu, J. Lee, Int. J. Solids Struct. (1992) 2471] is presented, that significantly improves its accuracy and range of applicability, as demonstrated by comparison with finite element simulations.

Indentation creep behavior of nano-scale Ag/Co multilayers

The creep behavior of nano-scale Ag/Co multilayers prepared by magnetron sputtering with a periodicity of 4–60 nm was studied using the depth-sensing indentation method at room temperature. Results show that the total creep depth increased with decreasing hardness, and the stress exponent values obtained by dimensional analysis were in the range from 2.5 to 5.0, which indicates that dislocation climb is a possible mechanism for indentation creep deformation. Transient effects during the hold-loading segment were more notable at higher strain rates.

Investigation of the relationship between elastic modulus and hardness based on depth-sensing indentation measurements

An analytical relationship between the reduced modulus E r and hardness H for solid materials is established based on the conventional depth-sensing indentation method of Oliver and Pharr. It is found that the two properties are related through a material parameter that is defined as the recovery resistance R s. This parameter is shown to represent the energy dissipation during indentation. Based on indentation measurements with the use of a Berkovich indenter, the relationship is given as E r = 0.6647 HR s . Also presented is a simple set of procedures to determine the area of indent. The procedures require three measured quantities, i.e., the peak load and corresponding displacements as well as the depth of residual indentation, but do not require complicated curve fitting process and regression analysis which themselves involve the specimen material. Nano-indentation tests were conducted using a Berkovich indenter on five materials spanning a wide range of hardness and plasticity. Experimental results revealed two important features: (a) the reduced modulus predicted by the new E r– H relationship is the same as that obtained by the conventional method; (b) the elastic modulus and hardness values determined by the simple set of procedures are comparable to those obtained by using the conventional method.

Development of pile-up during spherical indentation of elastic–plastic solids

Finite element modeling has been used to study the development of pile-up during indentation with a rigid sphere as it relates to the measurement of mechanical properties by load and depth sensing indentation. A wide range of materials with different elastic moduli, yield stresses, strain-hardening exponents, and friction coefficients were examined. Results show that during a significant portion of the spherical indentation process, the amount of pile-up cannot be related solely to the strain-hardening exponent, as is often assumed. Rather, after initially sinking-in at small depths of penetration, the pile-up for many materials evolves and increases gradually as the indenter is driven into the material. Even when deformation enters the fully developed plastic stage, the pile-up geometry continues to change in manner that can significantly affect the contact area. It is also shown that contact friction affects the pile-up geometry, and that the pile-up heights before and after the indenter is unloaded can be quite different. Implications for the measurement of mechanical properties by load and depth sensing indentation methods are discussed.

Depth-sensing indentation method for evaluation of efficiency of secondary cementitious materials

An application of a depth-sensing indentation (DSI) system to characterize properties of hardened cement paste is described. The test method developed, based on the microhardness testing concept, was implemented on a universal testing frame. Preliminary experiments were dedicated to evaluate efficiency of mineral additives in cement paste from Vickers hardness data. A suitable method of specimen preparation for microhardness tests was elaborated and successfully applied. The investigation revealed a clearly linear correlation between Vickers hardness and water-to-cement (w/c) ratio. An evaluation of the efficiency of mineral additives in cement paste was possible assuming a criterion of equal Vickers hardness. The DSI test method seems to be a valuable method for studying the bulk properties of cementitious composites.

Direct measurement of residual contact area and volume during the nanoindentation of coated materials as an alternative method of calculating hardness

Abstract The mechanical properties of thin films can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus and thus to gain better insight into the response of a material to controlled deformation at such small scales. However, previous work by Tsui and Pharr has shown that the effects of pile-up, particularly in soft films deposited on hard substrates, can produce significant overestimation of the hardness and modulus due to an underestimation of the true contact area by common nanoindentation analysis procedures. By measuring the topography of the residual indent using scanning force microscopy (SFM) and combining this information with the indentation data, it is possible to gain a fuller understanding of the indentation method and its effects on the material being tested. In addition, the true contact area can be directly measured from the SFM images and subsequently used to recalculate the hardness of the material more accurately. Moreover, the SFM allows the plastic volume of indentation to be measured, from which hardness can also be calculated in terms of plastic work. Experimental results are presented for two types of thin film deposited on hard substrates where SFM analysis of indentations at various depths gives significant additional information concerning the true response of the system to instrumented indentation at a nanometric scale. Pile-up effects can be precisely monitored as a function of depth and correlated to hardness variations encountered across the coating-substrate interface.

Direct measurement of residual contact area and volume during the nanoindentation of coated materials as an alternative method of calculating hardness

Abstract The mechanical properties of thin films can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus and thus to gain better insight into the response of a material to controlled deformation at such small scales. However, previous work by Tsui and Pharr has shown that the effects of pile-up, particularly in soft films deposited on hard substrates, can produce significant overestimation of the hardness and modulus due to an underestimation of the true contact area by common nanoindentation analysis procedures. By measuring the topography of the residual indent using scanning force microscopy (SFM) and combining this information with the indentation data, it is possible to gain a fuller understanding of the indentation method and its effects on the material being tested. In addition, the true contact area...

Die Kraft-Eindring-Methode

For determination of mechanical properties and hardness of coatings and in small materials volumina the depth sensing indentation method is used in microscale down to nanoscale range. The decisive factor for the reliability of the test results is the calibration of load and displacement measurement as well as the knowledge about the compliance of the instrument and the true geometry of the indenter. This paper discusses the features of this test method and reports about a European research project carried out under the coordination of the National Physical Laboratory (NPL) in UK. The works included the development and test of calibration methods, preparation of reference samples and round-robins in different laboratories with determination of hardness and Young's modulus.

Measurement of Residual Stresses by Load and Depth Sensing Spherical Indentation

AbstractThe finite element method was used to determine whether load and depth sensing indentation with spherical indenters may be useful in the measurement of residual stresses in materials. The spherical indentation process for a wide range of elastic/ideal-plastic materials to which compressive and tensile biaxial stresses were applied was simulated using standard finite element techniques. The elastic moduli and yield stresses of the materials were varied systematically to model the behavior of a wide variety of metals and ceramics. Elastic-ideal-plastic materials were considered in the study with residual stress levels varied from zero up to the yield stress. All three indentation regimes - elastic, elastic-plastic, and plastic - were examined, with emphasis given to the elastic and the early part of the elastic-plastic regimes, where differences in the load-displacement characteristics caused by residual stress were found to have a significant effect. Systematic examination of the relationships among residual stress, contact pressure, and elastic recovery revealed a simple, measurable indentation parameter which correlates well with the residual stress. Using this parameter, an experimental technique is proposed by which residual stresses can be estimated from measurements of the load of an indentation in the elastic-plastic regime, the yield stress, and the elastic modulus of the material, all of which can be determined by load and depth sensing indentation methods. Based on a critical examination of the technique by finite element simulation, the technique appears promising.

Combining scanning force microscopy with nanoindentation for more complete characterisation of bulk and coated materials

The mechanical properties of thin films as well as bulk materials can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus, and thus gain better insight into the response of a material to controlled deformation at such small scales. However, the calculation of such values from conventional load-displacement curves can prove inaccurate owing to the effects of pile-up and sink-in which invariably occur during the indentation of many types of material and which seriously affect the calculation of residual contact area. This paper addresses the problem and suggests the use of scanning force microscopy (SFM) as a complimentary tool which allows the true residual contact area to be measured. In addition, some experimental results are presented for a variety of bulk and coated materials, these confirming the use of the SFM for obtaining significant additional information concerning the true response of a material to instrumented indentation at a nanometric scale, e.g., elastic/plastic deformation, cracking, phase transformation, pile-up/sink-in effects, etc.

Characterization of integrated circuit aluminium bonding pads by nanoindentation and scanning force microscopy

The mechanical properties of thin films and surface modified layers can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. It is particularly suited to the characterization of bonding pads used to connect individual microcircuits and chips. Such pads are commonly made of aluminium and have a thickness of approximately 1 μm. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus, and thus gain better insight into the response of a bonding pad material to controlled deformation at such small scales. The nano hardness tester (NHT) is a recently developed instrument using an already established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a preset maximum value, the normal load is reduced until partial or complete relaxation occurs. At each stage of the experiment the position of the indenter relative to the sample surface is precisely monitored with a differential capacitive sensor, giving a load/displacement curve characteristic of the sample material. By consequently measuring the topography of the residual indent using a surface imaging technique (e.g., scanning force microscopy) and combining this information with the indentation data, it is possible to gain a fuller understanding of the material response, as well as additional information on surface roughness and frictional properties, the latter being of particular importance in bonding applications. Experimental results are presented for a variety of integrated circuit bonding pads, these confirming the ability of the NHT to becoming a common test in quality assurance and process development of thin films and coatings for the microelectronics industry.

Evolution of Contact Area and Pile-Up During The Nanoindentation of Soft Coatings on Hard Substrates

The mechanical properties of thin films can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus, and thus gain better insight into the response of a material to controlled deformation at such small scales. However, previous work [1] has shown that the effects of pile-up, particularly in soft films deposited on hard substrates, can produce significant overestimation of the hardness and modulus due to an underestimation of the true contact area by common nanoindentation analysis procedures. By measuring the topography of the residual indent using Scanning Force Microscopy (SFM) and combining this information with the indentation data, it is possible to gain a fuller understanding of the indentation method and its effects on the material being tested. In addition, the true contact area can be directly measured from the SFM images and subsequently used to calculate the hardness of the material more accurately. Experimental results are presented for a selection of soft films on hard substrates where SFM analysis of indentations at varying depths gives significant additional information concerning the true response of the system to instrumented indentation at a nanometric scale. Pile-up effects can be precisely monitored as a function of depth and correlated to hardness variations encountered across the coating/substrate interface.

Post-deposition reduction of internal stress in thin films: The case of HfN coatings bombarded with Au ions

Abstract The post-deposition bombardment of reactively sputtered HfN thin films with energetic (1, 2.5 and 5 MeV) gold ions to a fluence of 10 14 cm −2 resulted in a considerable reduction of high internal stresses which were estimated from the measurements of the surface curvature. The higher the energy of the gold species the lower degree of relaxation was registered. The observed phenomena are attributed here to the transport of the interstitial defects within the thermal spikes induced by bombarding HfN with Au ions. An alternative explanation of stress-reduction caused by an increase in vacancy concentration is also presented. Significant softening of HfN films after ion-beam treatment was registered by the high accuracy depth-sensing indentation method and attributed to the formation of an inter-layer of amorphous silicon, directly under the film. A structural analysis has been carried out using thin film X-ray diffraction. Nitride film resistivity was found to be slightly enhanced by bombardment with Au ions.

Evaluation of Hard TiN Coatings by Depth Sensing Indentation and Scratch Testing Methods

The IAR Nanomechanical Probe (NMP) was used in the depth sensing indentation mode to evaluate the Vickers hardness (HV) and elastic modulus E of five commercial TiN coatings applied to a 17-4 PH stainless steel substrate. The HV values of the TiN layers at depth to thickness (DTT) ratios of less than 0.1 varied between about 28 and 39 GPa, depending on the deposition process. The elastic modulus was within the range of 300 to 400 GPa, corresponding to published values, at DTT ratios of less than or equal to 0.05. At higher DTT values, the elastic modulus decreased with increasing DTT ratio due to larger and larger interference from the less rigid stainless steel substrate. Depth sensing scratch tests were also performed on the samples to determine the critical load crvalues needed to cause spallation of the coatings. For each coating, the interfacial fracture toughness Kic was calculated from crand used to describe the adhesive strength. Distinct differences between the Kic values of the different samples were observed, reflecting differences in adhesive strength. The significance of these results is discussed in terms of the application of titanium nitride coatings to gas turbine engines.

Localised micro-hardness measurements with a combined scanning force microscope/nanoindentation system

Thin-film coatings, modified surface layers and multiple phases are increasingly being used to improve the properties and functionality of engineering materials. Nanoindentation effectively meets the industrial need to measure hardness on a micro- and even nanometre scale and has proved its worth in caracterising the mechanical properties of surfaces, subsurface regions, interfaces, phases and grain boundaries in a quasi non-destructive way. The depth-sensing indentation method provides a means of evaluating the elastic and plastic deformations of the tested material, from which the hardness and the elastic modulus can be calculated. The combination of such an instrument with high resolution scanning force microscopy (SFM) and a conventional optical microscope provides a system capable of a more complete analysis of important surface parameters such as surface micro-roughness, topography and grain structure. In addition to the precise measurement of the indentation depth, the displaced material volume can also be measured together with other important information concerning the processes taking place on a microscale whilst performing such an indentation, e.g. plastic flow, micro-fracture, material compression and other interfacial effects. First results obtained on a multiphase high speed steel are presented, showing the net advantages of such a system in carrying out highly localised indentation studies together with precise measurement of imprints via SFM.