Showing papers on "Indentation published in 1996"

Influences of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in an aluminum alloy

Abstract: The influence of applied stress on the measurement of hardness and elastic modulus using nanoindentation methods has been experimentally investigated using special specimens of aluminum alloy 8009 to which controlled stresses could be applied by bending. When analyzed according to standard methods, the nanoindentation data reveal changes in hardness with stress similar to those observed in conventional hardness tests. However, the same analysis shows that the elastic modulus changes with stress by as much as 10%, thus suggesting that the analysis procedure is somehow deficient. Comparison of the real indentation contact areas measured optically to those determined from the nanoindentation data shows that the apparent stress dependence of the modulus results from an underestimation of the contact area by the nanoindentation analysis procedures.

Influences of stress on the measurement of mechanical properties using nanoindentation: Part II. Finite element simulations

Abstract: The finite element method has been used to study the behavior of aluminum alloy 8009 during elastic-plastic indentation to establish how the indentation process is influenced by applied or residual stress. The study was motivated by the experiments of the preceding paper which show that nanoindentation data analysis procedures underestimate indentation contact areas and therefore overestimate hardness and elastic modulus in stressed specimens. The NIKE2D finite element code was used to simulate indentation contact by a rigid, conical indenter in a cylindrical specimen to which biaxial stresses were applied as boundary conditions. Indentation load-displacement curves were generated and analyzed according to standard methods for determining hardness and elastic modulus. The simulations show that the properties measured in this way are inaccurate because pileup is not accounted for in the contact area determination. When the proper contact area is used, the hardness and elastic modulus are not significantly affected by the applied stress.

Analysis of nanoindentation load-displacement loading curves

Abstract: Nanoindentation load-displacement curves provide a “mechanical fingerprint” of a materials response to contact deformation. Over the last few years, much attention has been focused on understanding the factors controlling the detailed shape of unloading curves so that parameters such as true contact area, Young's modulus, and an indentation hardness number can be derived. When the unloading curve is well behaved (by which we mean approximating to linear behavior, or alternatively, fitting a power-law relationship), then this approach can be very successful. However, when the test volume displays considerable elastic recovery as the load is removed [e.g., for many stiff hard materials and many inhomogeneous systems (e.g., those employing thin hard coatings)], then the unloading curve fits no existing model particularly well. This results in considerable difficulty in obtaining valid mechanical property data for these types of materials. An alternative approach, described here, is to attempt to understand the shapes of nanoindentation loading curve and thus quantitatively model the relationship between Young's modulus, indentation hardness, indenter geometry, and the resultant maximum displacement for a given load. This paper describes the development and refinement of a previous approach by Loubet et al1 originally suggested for a Vickers indenter, but applied here to understand the factors that control the shape of the loading curve during nanoindentation experiments with a pointed, trigonal (Berkovich) indenter. For a range of materials, the relationship P = Kmδ2 was found to describe the indenter displacement, δ, in terms of the applied load P. For each material, Km can be predicted from the Young's modulus (E) and the hardness (H). The result is that if either E or H is known, then the other may be calculated from the experimental loading curve. This approach provides an attractive alternative to finite element modeling and is a tractable approach for those cases where analysis of unloading curves is infeasible.

Indentation hardness: Fifty years on a personal view

Abstract: This introductory paper is a brief revised account of basic studies carried out between 1940 and 1946 on the meaning of indentation hardness. Using very simple concepts, it shows that the indentation hardness of metals is directly related to the uniaxial yield stress as augmented by the deformation and work hardening produced by the indentation process itself. The work originally carried out on metals is applicable to many other solids, particularly if they have a crystalline structure. The paper concludes with a short review of current studies in the field of microhardness and nanohardness.

An ultrasound indentation system for biomechanical properties assessment of soft tissues in-vivo

Abstract: An ultrasound indentation system for biomechanical assessment of soft tissues in vivo was developed. The pen-size, hand-held probe was composed of an ultrasound transducer and a load cell. The ultrasound transducer was at the tip of the probe serving also as the indentor. The thickness and deformation of the soft tissue layer were determined from the ultrasound echo. A compressive load cell was connected in series with the ultrasound transducer to record the force response. A validation experiment was performed on porcine tissues. Force and deformation acquired with the present system was in good comparison with those obtained from a Housfield material testing machine. Material constants were obtained via a curve-fitting procedure by predicting the force transient response from the deformation-time data using a quasilinear viscoelastic model. In addition, deformation in the fat and in the muscle could be differentiated. The potential applications of this type of indentation probes are many. The specific application of this current development is for stump tissue assessment in the design of prosthetics.

Nanoindentation and picoindentation measurements using a capacitive transducer system in atomic force microscopy

Abstract: An indentation system is developed to directly apply loads ranging from 1μN to 10 mN and to make load–displacement measurements with subnanometre indentation depth capability. This system is used here in conjunction with a commercial atomic force microscope to provide in-situ imaging. A three-sided pyramidal (Berkovich) diamond tip has been used to obtain a load–displacement curve with residual depths of the order of 1 nm. The load–displacement data have been used to obtain indentation hardness and Young's modulus of elasticity for singlecrystal silicon and GaAs. Hardness on the nanoscales is found to be higher than that on the microscale. Ceramics exhibit significant plasticity and creep on the nanoscale.

Improvements in the indentation method with a surface force apparatus

Abstract: On the nanometre scale, the actual indenter-material contact area must be carefully determined to obtain reliable values of mechanical properties from an indentation test. On this scale, the contact area is greatly affected by the geometrical tip defect and by the possible formation of plastic pile-up (or sink-in) around the indent. Parameters such as local surface roughness and heterogeneity in surface and in thickness make it di5dt to conduct and to interpret nanoindentation tests. A new method, which couples nanoindentation experiments and imaging procedures, has been developed. Nanoindentation tests and topographic images are performed with a surface force apparatus developed in our laboratory. The important point of our method is the ability of our three-axis device to generate topographic images without having to move the sample. This allows us to determine precisely the actual tip–sample contact area, while performing accurate continuous quantitative quasistatic load measurement and simult...

Measurement of the adhesion of a brittle film on a ductile substrate by indentation

Abstract: The indentation of a brittle film on a ductile substrate is analysed for obtaining the interface toughness This measurement method of adhesion provides a simple technique for sampling a small area of the interface on practical geometries with common laboratory equipment Mechanics solutions are presented to access the interface toughness, I 9c, from measurements of the applied load, delamination radius, film thickness and film-substrate material properties The test is found to be particularly effective for films under large residual compression Measurements of the interface toughness ( I 9c ~ 45 J m -2 ) are made for a diamond-coated titanium alloy

The effect of indentation-induced cracking on the apparent microhardness

Abstract: The phenomenon of apparent microhardness increase with increasing applied indentation test load, the reverse indentation size effect (RISE), was addressed from the viewpoint of indentation-induced cracking. The apparent microhardness when the cracking occurs was found to be related to the applied indentation test load as P5/3. Previously published results on single crystals of silicon, GaAs, GaP and InP, which differ by a factor of four, all fall on the same line when analysed through this concept. It is concluded that the RISE is a result of the specimen cracking during the indentation.

Hardness and fracture toughness of bulk single crystal gallium nitride

Abstract: Basic mechanical properties of single crystal gallium nitride are measured A Vickers (diamond) indentation method was used to determine the hardness and fracture toughness under an applied load of 2N The average hardness was measured as 12±2 GPa and the average fracture toughness was measured as 079±010 MPa√m These values are consistent with the properties of brittle ceramic materials and about twice the values for GaAs A methodology for examining fracture problems in GaN is discussed

Indentation size effect: reality or artefact?

Abstract: The purpose of this investigation was to study the load dependence of the microhardness, typically in the range 5–500 gf. This well known phenomena is called the indentation size effect (ISE) and was investigated for two sets of specimens: titanium and aluminium alloys. Variation of the hardness with applied load was first compared with various existing models and the surface profile, near the indent, was measured by confocal microscopy. The formation of pile-ups near the indentation print led to the correction of the indent diagonal which is found to fit well with our experimental data as well as with other results in the literature. For the materials investigated, the ISE effect is an artefact, i.e. the variation of hardness with the applied load is only a consequence of the variation of the contact surface between the specimen and the indenter.

The Elastoplastic Response of Poly(Methyl Methacrylate) to Indentation

Abstract: This paper describes an experimental and analytical study of the contact compliance and hardness of a glassy polymer, poly (methyl methacrylate), which has an elastoplastic response to indentation using rigid conical and spherical indenters. The experimental method and the associated intrinsic errors are described. The nature and importance of these errors are reviewed and a variety of corrections are applied. The potential errors addressed include the zero error, the indenter tip defect correction and the problems associated with various extrapolations and interpolations of the experimental data necessary in order to abstract the material characteristics of the polymer. The analytical description of the data has been facilitated by an adaptation of the Box-Cox transformation (Box, G. E. P. Sz Cox, D. R. 1964 J. R. Statist. Soc. 26, 211), which is shown to be a most effective way of abstracting the required material characteristics data and minimizing the influence of the intrinsic errors inherent in the compliance method. The deduced material response characteristics of the polymer, in particular the Young’s modulus and the yield stress, are shown to be consistent with data obtained by conventional mechanical testing methods.

Damage-resistant alumina-based layer composites

Abstract: A new philosophy for tailoring layer composites for damage resistance is developed, specifically for alumina-based ceramics. The underlying key to the approach is microstructural control in the adjacent layers, alternating a traditional homogeneous fine-grain alumina (layer A) for hardness and wear resistance with a heterogeneous alumina : calcium-hexaluminate composite (layer C) for toughness and crack dispersion, with strong bonding between the interlayers. Two trilayer sequences, ACA and CAC, are investigated. Hertzian indentation tests are used to demonstrate the capacity of the trilayers to absorb damage. In the constituent materials, the indentation responses are fundamentally different: ideally brittle in material A, with classical cone cracking outside the contact; quasi-plastic in material C, with distributed microdamage beneath the contact. In the ACA laminates, shallow cone cracks form in the outer A layer, together with a partial microdamage zone in the inner C layer. A feature of the cone cracking is that it is substantially shallower than in the bulk A specimens and does not penetrate to the underlayer, even when the applied load is increased. This indicates that the subsurface microdamage absorbs significant energy from the applied loads, and thereby “shields” the surface cone crack. Comparative tests on CAC laminates show a constrained microdamage zone in the outer C layer, with no cone crack, again indicating some kind of shielding. Importantly, interlayer delamination plays no role in either layer configuration; the mechanism of damage control is by crack suppression rather than by deflection. Implications for the design of synergistic microstructures for damage-resistant laminates are considered.

Time dependent deformation during indentation testing

Abstract: Constant loading rate/load indentation tests (1/P dP/dt) and constant rate of loading followed by constant load (CRL/Hold) indentation creep tests have been conducted on high purity electropolished indium. It is shown that for a material with a constant hardness as a function of depth, a constant (1/P dP/dt) load-time history results in a constant indentation strain rate (1/h dh/dt). The results of the two types of tests are discussed and compared to data in the literature for constant stress tensile tests. The results from the constant (1/P dP/dt) experiments appear to give the best correlation to steady-state uniaxial data.

Engineered Interfaces for Adherent Diamond Coatings on Large Thermal-Expansion Coefficient Mismatched Substrates

Abstract: Adhesion of thin or thick films on substrates is a critical issue in systems where the thermal-expansion coefficients of the coating and bulk material are significantly different from each other. The large mismatch of the expansion coefficients results in the generation of very high stresses in the coating that may lead to delamination, cracking, or other deleterious effects. A method to increase the adherence of diamond coatings on tungsten-carbide and stainless steel substrates is reported based on a substrate-modification process that creates a three-dimensional thermally and compositionally graded interface. Scratch and indentation tests on diamond-coated steel and tungsten-carbide samples did not exhibit film fracture at the interface and concomitant catastrophic propagation of interfacial cracks.

An elastic-plastic indentation model and its solutions

Abstract: An analytical model of hardness has been developed. Four major indentation tests, namely indentation by cones, wedges, spheres, and flat-ended, axisymmetric cylinders have been analyzed based on the model. Analytical relationships among hardness, yield stress, elastic modulus, Poisson's ratio, and indenter geometries have been found. These results enable hardness to be calculated in terms of uniaxial material properties and indenter geometries for a wide variety of elastic and plastic materials. These relationships can also be used for evaluating other mechanical properties through hardness measurements and for converting hardness from one type of hardness test into those of a different test. Comparison with experimental data and numerical calculations is excellent.

Indentation of composite sandwich beams

Abstract: Indentation of a sandwich beam is analysed as linear elastic bending of the top skin on a rigid-perfectly plastic foundation (the core). The theoretical predictions of fracture load from this simple theory are shown to be in good agreement with experimental results from indentation tests on strips of sandwich panel, with glass-fibre-reinforced plastic skins and foam core, supported on a rigid base.

Indentation hardness and fracture toughness in single crystal TiC0.96

Abstract: Using the indentation technique, both the hardness and fracture toughness anisotropy of a titanium carbide single crystal have been investigated on the (001), (110) and (111) planes. The hardness values varied between 20 and 32 GPa, and the anisotropies were consistent with {{110}} 〈 1 1 ¯ 0 〉 slip. The indentation critical stress intensity factors KIa were in the range 1.5–3.6 MPam1/2. The anisotropy of KIa on (001) and (110) could be explained by a bond-breaking model but not that on (111). The experimental values were not consistent with the predicted ones.

Mechanical characterization of plasma sprayed ceramic coatings on metal substrates by contact testing

Abstract: Hertzian indentation testing is used to generate contact damage in plasma sprayed ceramic coatings on metal substrates. Two basic ceramic/metal coating/substrate systems are examined: alumina on steel and zirconia on superalloy. Macroscopic mechanical responses are measured via indentation stress-strain curves, which quantify the relative role of the coating and substrate in the net deformation and facilitate evaluations of elastic moduli and yield stresses. Micromechanical damage processes within the coating and substrate subsurface layers are studied using a “bonded-interface” specimen. Degradation occurs primarily by delamination and other cracking at the coating/substrate interface or in the coating, but plastic deformation of the metal substrate contributes importantly to the crack driving force.