Characterization of Indentation Size Effect of Hardness Using a Loading Curve from Crystalline Materials
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Abstract:
Nanoindentation technique has been widely used for measuring mechanical properties from a very small volume of material. The hardness measured using the depth sensing nanoindentation technique often decreases with increasing indentation size, the so called indentation size effect (ISE)[1, 2]. It has been generally acknowledged that the ISE in crystalline materials originates from the density change of geometrically necessary dislocations (GND) needed to accommodate a permanent indentation imprint. Conventionally, to characterize an ISE often requires a series measurement of hardness values at different indentation size. Based on the celebrated Oliver-Pharr scheme[3]. We propose a method to derive the ISE from the loading curve of one single indentation test. The application and limitation of the proposed method will be discussed.Keywords:
Indentation
Characterization
Knoop hardness test
The Knoop microhardness anisotropy profile was determined for the basal plane of a Czochralski grown alumina single crystal for indentation test loads from 100 through 1000 g. Microhardness maxima occur at low indentation test loads for the long axis of the Knoop indenter parallel to the 〈2[Onemacr][Onemacr]0〉. Minima exist for the long axis parallel to the 〈10[Onemacr]0〉. This low indentation test load profile is attributed to slip on the primary slip system, the (0001)〈[Onemacr][Onemacr]20〉, as previously noted by Brookes and co‐workers. The degree of the microhardness anisotropy decreases for higher indentation test loads. This results from the activation of multiple slip systems to accommodate the greater amounts of plastic flow required by the larger indentation sizes. The microhardness profile becomes more uniform with increasing indentation test load until the Knoop microhardness approaches a test‐load‐independent, orientation‐independent microhardness of 1167 ± 34 kg/mm 2 . The indentation size effect (ISE) was further investigated through lubricated indentation hardness measurements. Lubrication of the test specimen surface significantly reduces the ISE. Results indicate that friction between the test specimen surface and the indenter facets is a major portion of the ISE.
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A three‐dimensional fracture analysis is applied to the Knoop and Vickers indentation fracture of ceramics. A brief discussion of the accuracy of the analysis applied to model the step load on the crack face caused by the residual stresses is given. A study is made of the effect of the elongated plastic zone in Knoop indentation on the unloaded radial fracture. It is shown that for small indentation loads the published experimental data can be verified by varying the depth reached by the semielliptical plastic zone with given surface length. An analysis and interpretation of the interaction between the two halfpenny‐shaped radial cracks induced by Vickers indentation is also given.
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An inelastic characteristic of film of an amorphous alloy is studied by using nanoindentation test. First, the hardness is evaluated as a function of applied load to obtain an optimal indentation load and the interaction between different indenters is investigated to obtain a optimal indentation spacing. Then, we performed the nanoindentation test around the damaged region introduced by a Knoop indentation test. As a result, the hardness is almost the same as the virgin material and its distribution is spatially uniform. This result implies that the characteristic length scale of the deformation region is extremely smaller than the resolution of the nanoindentation test.
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In this article molecular dynamics (MD) simulations have been conducted to investigate the effect of the indenter shape on the nanoindentation behaviours of iron with body centred cubic structure. Two types of indenters (hemispherical indenter and pyramidal indenter) with dimensions of several nanometres have been modelled. The simulation results have shown that the indenter shape significantly influences the nanoindentation behaviours at such small scale. The indentation force increases with indentation depth during loading for the hemispheri-cal indenter, while the indentation force is low in values for the pyramidal indenter. To validate the MD model nanoindentation experiments have been carried out. The calculated indentation hardness of the hemispherical indenter is in reasonable agreement with the experimental value.
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Indentation
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The instrumented indentation technique (IIT) has recently attracted significant research interest because it is nondestructive and easy to perform, and can characterize materials on local scales. Residual stress can be determined by analyzing the indentation load-depth curve from IIT. However, this technique using a symmetric indenter is limited to an equibiaxial residual stress state. In this study, we determine the directionality of the non-equibiaxial residual stress by using the Knoop indentation technique. Different indentation load-depth curves are obtained at nonequibiaxial residual stresses depending on the Knoop indentation direction. A model for Knoop indentation was developed through experiments and theoretical analysis.
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Knoop hardness test
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Nanoindentation technique has been widely used for measuring mechanical properties from a very small volume of material. The hardness measured using the depth sensing nanoindentation technique often decreases with increasing indentation size, the so called indentation size effect (ISE)[1, 2]. It has been generally acknowledged that the ISE in crystalline materials originates from the density change of geometrically necessary dislocations (GND) needed to accommodate a permanent indentation imprint. Conventionally, to characterize an ISE often requires a series measurement of hardness values at different indentation size. Based on the celebrated Oliver-Pharr scheme[3]. We propose a method to derive the ISE from the loading curve of one single indentation test. The application and limitation of the proposed method will be discussed.
Indentation
Characterization
Knoop hardness test
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Abstract Indentation hardness tests such as Brinell, Rockwell, Vickers, Knoop, and instrumented indentation are frequently used methods for determining hardness. The basic concept utilized in all of these tests is that a set force is applied to an indenter in order to determine the resistance of the material to penetration. If the material is hard, a relatively small or shallow indentation will result, whereas if the material is soft, a fairly large or deep indentation will result. These tests are often classified in one of the following two ways: by the extent of the test force applied or the measurement method used. A “macro” test refers to a test where a load >1 kg is applied; similarly “micro” refers to a test where a load of ≤1 kg of force is applied. Additionally, some instruments are capable of conducting tests with loads as light as 0.01 g and are commonly referred to as ultralight or nanoindentation testers. Rockwell and Brinell testers fall into the macro category, whereas Knoop testers are used for microindentation tests. Vickers and instrumented indentation testers can be employed for both macro‐ and microindentation tests. The measurement methods available include a visual observation of the indentation or a depth measurement of the indentation. Rockwell and instrumented indentation testers are capable of determining the depth of the indentation, whereas Brinell, Knoop, and Vickers testers require an indentation diameter measurement. These visual measurements can be automated, as will be discussed later in this article. Hardness is not a fundamental property of a material, yet hardness testing is considered a useful quality‐control tool. Many properties are predicted from hardness values when combined with additional information such as alloy composition. The following is a list of such properties: resistance to abrasives or wear, resistance to plastic deformation, modulus of elasticity, yield strength, ductility, and fracture toughness. Some of these properties, such as yield strength, have numerical relationships with hardness values, whereas others such as fracture toughness are based on observations of cracks surrounding the indentations. Data analysis and conversions will be discussed in greater detail later in this article.
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Nanoindentation size effect was investigated under very low loads on type 316 stainless steel. Nanoindentation measurements were carried out on the samples surfaces with a Berkovich pyramidal diamond indenter applying loads in the range of 25-1000μN. Simultaneously, AFM images of the sample surface were recorded before and after indentation process .For type 316 stainless steel, the indentation size effect was found. The results were discussed in the terms of the model of geometrically necessary dislocations proposed to interpret the indentation size effect.It can be seen that the square of the nanohardness, H 2, vs the inverse of indentation depth, 1/h, is linearly dependent on the indented depth in the range of 25-150nm,which is a good qualitative agreement with the predictions of the model. However, for shallow indents, the slope of the line severely changes.Some possible mechanisms for this change were proposed.
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