Indentation Size Effect Of Hardness Research Articles

Rate-dependent indentation size effect on hardness and creep behavior of a titanium metallization film on alumina substrate

Nanoindentation tests were performed at room temperature to investigate the indentation size effect (ISE) and creep behavior of a titanium metallization film on alumina substrate. The ISE of hardness was examined at different loading strain rate (LSR). The effects of LSR and penetrating depth on nanoindentation creep were evaluated at the steady creep state. Inverse ISE in titanium film was observed at indentation depth below 200 nm while normal ISE was observed at penetrating depth ranging from 200 nm to 1000 nm. Elastic contact theory and strain gradient plasticity theory were applied to interpret the variation trends of hardness respectively. In addition, the intrinsic hardness and characteristic length were found to be strongly dependent on LSR. Except for creep strain rate, creep displacement and creep stress exponent showed relatively conspicuous correlations to LSR and/or penetrating depths.

Indentation Strain Rate Sensitivity of CoCrFeNiAl0.3 High‐Entropy Alloy

In this paper, the mechanical properties of CoCrFeNiAl0.3 high‐entropy alloy (HEA) under different strain rates are investigated using nanoindentation at room temperature. Experimental results show that the contact stiffness increases almost linearly with indentation depth and the elastic modulus is invariable under different indentation strain rates, while the indentation hardness under higher strain rate is relatively larger, and the hardness under large indentation depth is relatively smaller for each indentation strain rate. The indentation size effect of hardness and strain rate sensitivity exponent m under different indentation strain rates are analyzed, then the hardness and m free from the effect of indentation depth are obtained. Finally, this investigation reveals the size effects of the hardness of HEA in light of the strain rate sensitivity and activation volume of the dislocation nucleation.

Indentation size and loading strain rate dependent creep deformation of nanocrystalline Mo

The creep behavior of nanocrystalline body-centered cubic Mo thin films was evaluated using nanoindentation testing. The creep rate and the corresponding strain rate sensitivity (m) were found to be strongly dependent upon indentation size and loading strain rate (LSR). Other than the mechanism of diffusion along indenter tip-sample interface previously proposed for face-centered cubic metals and metallic glasses, alternative mechanisms involving surface effect enhanced screw dislocation activities, inverse indentation size effect of hardness and LSR dependent microstructural and stress states, were proposed to interpret the observed dependence of penetration depth and LSR of m.

Depth dependent strain rate sensitivity and inverse indentation size effect of hardness in body-centered cubic nanocrystalline metals

Size effects on hardness (H) and strain rate sensitivity (m) of nanocrystalline (NC) body-centered cubic Mo thin film were examined under nanoindentation testing. Contrast to existing reports that there was no indentation size effect on hardness in NC metals, inverse indentation size effect (ISE) in NC Mo was observed for the first time at penetration depths ranging from 15 to 200nm, at all the loading strain rates applied. In addition, the strain rate sensitivity of NC Mo exhibited strong dependence on penetration depth, increasing dramatically with decreasing penetration depth. Surface effects related to two deformation mechanisms were proposed to be responsible for the observed inverse ISE on H and depth dependent m. Specifically, the mobility of screw dislocation/component and the diffusion length of interfacial diffusion were altered as the deformed region underneath the indenter was approaching the free surface, resulting in the unusual size effects in NC Mo.

Evaluation of Mechanical Properties Variations for Kr Ion‐Irradiated 6H‐SiC by Nanoindentation Methods

Nanoindentation experiments were performed to investigate the irradiation effects on the mechanical properties of 6H‐SiC irradiated by 4 MeV Kr ions at high fluences from room temperature (RT) to 550°C. The irradiation temperature is the primary factor that affects modifications of the comprehensive mechanical properties, while the effect of the fluence is less significant. Elastic modulus and hardness decrease drastically for RT‐irradiated samples, but they almost recover for elevated temperature samples, with hardness slightly higher than its original value. The hardness increases first and then decreases with increasing temperature, and the elastic modulus decreases linearly as the swelling increases. Meyer's index is related to the indentation size effect of hardness and the magnitude of the lattice damage. The ratio of irreversible work is associated with the degree of elastic recovery and the ratio of hardness to elastic modulus of SiC. Compared with the unirradiated value, fracture toughness changes slightly for RT irradiation, while increasing significantly for elevated temperature irradiation and has the same variation tendency of hardness. Results indicate that mechanical properties change with the variations of interatomic bond strength, dislocation mobility, and the behavior of crack propagation, which is strongly affected by the defects induced by heavy‐ion irradiation.

TEM analysis and determination of dislocation densities in nanostructured copper tube produced via parallel tubular channel angular pressing process

Parallel tubular channel angular pressing (PTCAP) is a recently developed novel intense plastic deformation method appropriate for fabrication ultrafine-grained and nanostructured cylindrical tubes. In the present work a commercially pure copper was processed via multi-pass PTCAP and the effects of number of passes on grain refinement and the dislocation density were studied. TEM analysis showed that after first pass elongated subgrains with interior tangled dislocations were formed. After pass number two the density of interior dislocations through the elongated grains was decreased. In the next stages of deformation, at pass number three, elongated grains almost disappeared and equiaxed grains with grain size about 150nm are formed as a result of dynamic recovery. The dislocation densities were measured by hardness indentation size effect using the Nix–Gao model. The results showed that increase in the number of PTCAP passes leads to decrease in the dislocation densities. The dislocation density is decreased to 2.48×109cm−2 after fourth passes from 18.1×109cm−2 after first pass. TEM results verified calculated values from the Nix–Gao model. Microhardness of the PTCAP processed tube through four passes increased to ∼142HV from initial value of about 62HV. Also, significant increase takes place after single pass and in the next passes the hardness value is saturated.

Indentation size effect of hardness of metallic glasses

It has been found that the hardness measured from different metallic glassy samples using the Oliver and Pharr scheme [Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564–83] [1] shows an apparent indentation size effect (ISE), i.e., the hardness decreases as indentation depth increases. A similar ISE of the pile-up ratio has also been found. A ribbon glassy sample fabricated at a larger cooling rate shows larger pile-up and lower hardness value than a bulk glassy sample under the same conditions. The ISE of hardness can be effectively eliminated by taking into account the different pile-up ratios at different indentation depths.

Characterization of Indentation Size Effect of Hardness Using a Loading Curve from Crystalline Materials

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 Size Effect on Hardness of Nanostructured Thin Films

Based on nanoindentation techniques, the evaluation of hardness of two nanostructured thin films, AlN and Ti-Al-N, is discussed. In the case of AlN films, the indentation size effect of hardness can be modeled using the concept of geometrically necessary dislocations, whereas in the case of Ti-Al-N films, the measured hardness increases exponentially as the indentation depth decreases. The results show that, as thin films approach superhard, dislocation-based plastic deformation is gradually replaced by grain-boundary mediated deformation.