Indentation Time Research Articles

Effect of indentation load and time on knoop and vickers microhardness tests for enamel and dentin

The aim of this study was to determine the effect of variations in indentation load and time on the Knoop and Vickers hardness numbers (KHN and VHN) for enamel and dentin. Twenty molar teeth were divided into twenty enamel and twenty dentin specimens. Each specimen was tested using a Knoop or Vickers microhardness tester at different loads and times. The difference in hardness between the groups was analyzed with two-way ANOVA followed by a Tukey test. The results revealed that a difference of indentation time did not influence the microhardness number of enamel and dentin. The KHN values of enamel and the VHN values of dentin were affected by variation of test loads. Therefore, the tooth hardness number for different loads may not be acceptable for comparison.

114 球状圧子ナノインデンテーションによる降伏応力同定の高精度化に関する検討(構造部材や加工プロセスの信頼性を支える計測・評価技術)

It is known that stress-strain relations of a material can be obtained using a nano-indentation technique with a spherical tip. In this work, the effects of the experimental parameters on the accuracy in estimating stress-strain curve and yield stress by multiple continuous loading method using a load-controlled nano-indentation. Experimental parameters such as initial indentation, indentation time relating to deformation rate, holding time at maximum loading, and unloading ratio are changed and the influence on the obtained result is investigated. It has been found that the indentation time and the holding time seems to affect the stress-strain curve and yield stress.

Thermo-mechanical probe storage at Mbps single-probe data rates andTbit in−2 densities

Future applications for parallel probes, including probe-based data storageand probe lithography, demand that probe technologies achieve patterningrates in the megahertz per probe range at feature sizes below 30 nm,i.e. ∼1 Tbit in−2. If thermo-mechanical indentation of polymers is employed, some uncertainty remains as towhether the physics governing the indentation kinetics at these short timescales permitsthe fast indentation processes required. We demonstrate the feasibility of using polymermedia for thermo-mechanical probe storage or lithography, at sliding speeds of15 mm s−1 and sub-microsecond indentation times, which fall into this previously untapped regime.

Determining the viscoelastic parameters of thin elastomer based materials using continuous microindentation

Depth-sensing indentation (DSI) was used to evaluate the viscoelastic properties of three proprietary silicone-based coatings and four butyl-based rubber sheets. The coatings are used as layers in silicone bilayer coatings. The elastic indentation modulus, E ∗, irrecoverable deformation energy and elastic energy were determined in an earlier study using the unloading data of DSI tests and applying a creep correction method proposed by Ngan et al. The model used in this study to characterize the coatings was the standard four parameter Maxwell–Voigt viscoelastic model combined with the elastic indentation load-depth model for a Berkovich indenter as proposed by Fischer-Cripps. This model fits the data quite well. The parameters E 1 ∗, E 2 ∗, η 1, and η 2 were found by curve fitting this model with the indentation depth versus time data while held at a constant maximum load. The parameter E 1 ∗ is equivalent to E ∗; values of E 1 ∗ obtained using the Fischer-Cripps technique were 6.8 ± 0.9 MPa, 109 ± 7 MPa, and 4.2 ± 0.5 MPa for bond coat A, bond coat B, and the top coat, respectively. One purpose of this paper is to demonstrate a technique to determine viscoelastic parameters of silicone-based elastomers. It is hoped that these parameters may then be used in the evaluation and development of more durable silicone-based foul release coatings. Another purpose is to apply this technique to other elastomer-based materials such as butyl-based rubbers.

High-temperature Deformation Kinetics of Gold at 473K to 773K

AbstractHigh-temperature constant-force indentation creep tests of 200 seconds duration were performed on an annealed gold specimen at 473K to 773K, to investigate the dependence of the micro-/nano-indentation deformation kinetics upon indentation stress, temperature and time. The indent stress displayed a clear indentation size effect at 473 K. An analysis of the measured indentation creep rate, and its dependence upon temperature and stress, indicate that the strength of the deformation rate limiting obstacles increases with temperature. This is consistent with the expected temperature dependent evolution of the dislocation cell structure whose boundaries become the primary obstacles to dislocation glide.

Vickers microhardness studies on cadmium mercury thiocyanate (CMTC) single crystals grown in silica gel

Vickers microhardness study was carried out on the (110) face of the gel-grown cadmium mercury thiocyanate (CMTC) single crystal, subjected to the loads 10, 25 and 50 g. The measurements were made at room temperature with the indentation time as 10 s. Vickers hardness number Hv was calculated and it is found to increase with the applied load. Mayer's index number ‘ n’ and elastic stiffness constant were calculated. The anisotropy of the crystal is also studied. The results are discussed in detail.

Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation. A Study on Pile-up and Viscoelasticity

The analysis of nanoindentation force curves collected on polymers through the common Oliver and Pharr procedure does not lead to a correct evaluation of Young's modulus. In particular, the estimated elastic modulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical characterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of their influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale, i.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations minimize the effect of viscoelasticity. However, changing the indentation time in a broad range of contact time (fractions of second up to hundreds of seconds) did not allow the correct estimation of Young's modulus for the polymers used in this work. The final result is that the Oliver and Pharr procedure as well as any other procedure analyzing the unloading curve with elastic contact mechanics models cannot be employed to measure Young's modulus of polymers because its application is incorrect from a theoretical point of view, unless the analysis is limited to the very first nanometers of penetration depth when the contact is perfectly elastic. Viscoelastic contact mechanics models should instead be employed to characterize these materials.

Exploiting Chemical Switching in a Diels–Alder Polymer for Nanoscale Probe Lithography and Data Storage

AbstractReversibly crosslinked polymer films have properties that are beneficial to scanned‐probe data storage and lithographic applications that use thermomechanical nanoindentation as a write or expose mechanism. The novel polymer under study contains linkages based on thermally reversible Diels–Alder crosslinking. Thermomechanical properties on the nanometer scale are analyzed by indentation experiments on polymer thin films using heated tips. The underlying indentation mechanism is studied at varying tip temperatures and indentation times, revealing Arrhenius kinetics. This is in contrast to the Williams–Landau–Ferry kinetics usually observed for polymer systems. The discrepancy is explained by the reversible crosslinking incorporated into the structure of the polymer that allows switching between two different states: a rigid, highly crosslinked, low‐temperature state, and a deformable, fragmented, high‐temperature state. An individual indentation volume of less than 10–20 L (10 000 molecule pairs) is estimated. These kinetics experiments demonstrate that a chemical reaction of only a few thousand molecules can be transduced into a mechanically measurable action. The ability to cycle between two sets of properties in these materials opens up new perspectives in lithography and data storage. Examples of data storage with densities up to 1 Tb in.–2 and maskless lithography with resolution below 20 nm are demonstrated at writing times of 10 μs per bit/pixel.

Nanoindentation behavior of ultrathin polymeric films

Measurement of the mechanical properties of nanoscale polymeric films is important for the fabrication and design of nanoscale layered materials. Nanoindentation was used to study the viscoelastic deformation of low modulus, ultrathin polymeric films with thicknesses of 47, 125 and 3000 nm on a high modulus substrate. The nominal reduced contact modulus increases with the indentation load and penetration depth due to the effect of substrate, which is quantitatively in agreement with an elastic contact model. The flow of the nanoscale films subjected to constant indentation loads is shear-thinning and can be described by a linear relation between the indentation depth and time with the stress exponent of 1/2.

Hertzian contact fatigue on alumina/alumina-zirconia laminated composites

The Hertzian contact fatigue of two Al 2O 3/Al 2O 3-ZrO 2 laminated composites with different layer thickness and, consequently, different residual stress at the surface, was studied both under cyclic loading and under static loading (stress corrosion cracking). The apparition of a superficial ring crack, considered as the first stage of damage, was recorded as a function of time or number of cycles. A monolithic alumina processed in a similar way than the composites and with comparable microstructure was also tested for comparison purposes. Results show that the laminated composites present better resistance to contact fatigue than monolithic alumina. Comparison between static and cyclic loading data reveals that the alumina suffers from fatigue, and not only from stress corrosion cracking under cyclic loading. This fatigue is attributed to grain bridging degradation of alumina. A simple relationship between indentation load and time for apparition of ring crack, based on the Paris law, is derived, yielding values of the exponents similar to values previously reported in the literature.

Study of the Effect of the Indentation Time and Load on Fracture Toughness and Crack Morphologies in WC-17Co Thermally Sprayed HVOF Coating

Study of the Effect of the Indentation Time and Load on Fracture Toughness and Crack Morphologies in WC-17Co Thermally Sprayed HVOF Coating

Nanoindentation creep of quartz, with implications for rate- and state-variable friction laws relevant to earthquake mechanics

The frictional behavior of rocks in the laboratory is reasonably well described by rate- and state-variable friction laws, which reproduce a rich variety of natural phenomena when used in models of earthquakes. Despite the widespread adoption of the rate and state formalism in earthquake mechanics, the physical mechanisms that occur at microscopic contacting asperities on the sliding surface, which give rise to the observed rate and state effects, are still poorly understood. In an attempt to identify these underlying mechanisms, a series of nanoindentation experiments on quartz, an abundant mineral in the earth’s crust, was conducted. These experiments demonstrate the utility of using continuous stiffness measurements as a means of obtaining reliable indentation creep data on hard materials like quartz at room temperature. The projected area of indentation in quartz increases linearly with the logarithm of the time of indentation, in agreement with the increase in real area of contact with log time inferred from slide-hold-slide friction experiments on quartz rocks. However, the increase in fractional area with time in the indentation tests was larger than that inferred from friction experiments by a factor of 1.7. Differences between the rates of fractional area increase in the two tests may indicate that the increase in contact area during the hold portion of slide-hold-slide tests was modulated by slip that occurs during reloading after the hold, as was observed for other materials. The nanoindentation results suggest that the increase in frictional strength (i.e., the increase of state in the rate- and state-variable friction laws) during slide-hold-slide friction experiments was caused by creep of the highly stressed asperity contacts.

The contact behavior between a foam core sandwich plate and a rigid indentor

The purpose of this paper is to determine the indentation produced by a rigid indentor falling on a sandwich plate with a height–density closed cellular foam core. Two different indentor shapes are considered: a rigid sphere and a cylindrical rigid punch. By assuming that the time of complete indentation of the indentors is much larger than the time required by the elastic waves to propagate from the point of first contact to the boundary of the plate, a static analysis is performed. A distribution of surface pressures reproducing the contact law of the rigid indentor on an elastic half-space is introduced and the sandwich plate theory proposed by Dundrová, Kovarı́k and Slapák (1970) is adopted; the explicit solutions with pre-assigned surface pressures are obtained and the contact force–indentation relations, before the sandwich plate is damaged, are found. The contact laws are obtained for both a simply supported and a clamped circular sandwich plate and the relevant influence of boundary conditions on the elastic response of the sandwich plate is shown.

Investigation of time-dependent characteristics of materials and micromechanisms of plastic deformation on a submicron scale by a new pulse indentation technique

A new pulse technique is proposed to investigate dynamics and micromechanisms of plastic deformations of materials underneath the indenter during microindentation. It is established that the process of indenter penetration under pulse loading conditions can be described by several distinct stages (as many as four in some cases) differing in kinetics and activation parameters. In each investigated material, the first stage was characterized by a high strain rate (≥103 s−1) and the high contact stresses (dynamic hardness), which exceeded static hardness by factor of 5–10. Typical values of activation volumes during the first stage were of the order of 10−30 m3, i.e. close to the volume occupied by an atom (ion) in a lattice. During the second and the subsequent stages, the activation volume in ionic crystals increased up to about 10−28 m3,( i.e. 10b 3, where b is Burger's vector of glide dislocations). Dynamics of initial stages of the indenter penetration was, therefore, determined by elastic and subsequently by plastic deformation, which is carried out by non-equilibrium point defects (most likely by interstitials or crowdions). A relative role of point defects in the process of mass transfer underneath the indenter and its contribution to microhardness is estimated. In soft materials (NaCl, LiF, Pb) during long indentation times (≥1 s) contribution of point defects can be estimated as more than 10%, and in the case of hard materials (Si, amorphous alloys) became closer to 100%. Dynamics of the final stages of the indenter penetration in soft crystals were governed by the dislocation creep. In all investigated materials for short indentation times (≪1 s) plastic deformation underneath the indenter occurred predominantly via generation and motion of interstitials and their clusters containing a few atoms.

Preparation and characterization of acrylic polymer titania hybrid. A study of organicinorganic interactions in hybrid materials by micro-hardness Vickers

AbstractMetal-oxide ceramer films have been developed using an acrylic polymer bearing a low amount of methacrylic acid units (ca. 4%mol) as the organic phase with titanium tetrabutoxide as the inorganic sol-gel precursor. The characterisation of free films was realized by various experimental methods. The formation of COOTi bonds prevents large scale phase separation between the organic component and the mineral network. Mechanical properties of the hybrid films have been investigated through dynamic mechanical analysis. The influence of the titania content on the damping peak amplitude suggests that titania is molecularly dispersed in the polymer matrix and that it significantly hinders the segmental motion of the polymer chains. However, the low content in potential carboxylic crosslinking sites explains why the glass transition temperature remains relatively unchanged when titania content increases. Vickers microhardness measurements used in this study allowed us to understand the contribution of the inorganic part (phase TiO2) to the mechanical properties of the polymer. The creep of hybrids has been studied carrying out hardness measurements under various indentation times. The mineral constituent leads to an important increase of the hardness and limits, in a significant way, the creep of polymer.

Nanoindentation of carbon materials

The compressive behaviour of carbon materials has been investigated by modelling the recently developed method of nanoindentation. Molecular dynamics simulations are employed to investigate the indentation of a diamond cube-corner apex into graphite {0 0 0 1}, diamond {0 0 1} and a C 60 substrate. Simulations performed on graphite show that the crystal undergoes an elastic deformation. Indentation of diamond shows some elastic behaviour but also tip damage. Indentation on a C 60 film reflects a weakly bonded material with the emission of surface C 60 molecules and a plastic deformation of the material. Penetration of the substrate surfaces shows compression of the tip apex and in the case of the diamond substrate, extraction causes blunting by the transfer of atoms from the tip to the substrate. Some qualitative comparisons are made to experiment but available computer power constrains feasible indentation depths to an order of magnitude smaller than experiment and over indentation times several orders of magnitude smaller.

Nanoindentation of diamond, graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C60 molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C60 films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.

Atomic force microscopy study of nanoindentation creep on the (100) face of MgO single crystals

The experimental results obtained on nanoindentation creep and evolution of indentation impressions made on the (100) face of MgO crystals from their in-situ observations by atomic force microscopy are described and discussed. The indentations were made by square pyramidal Si tips at a constant load of 2 μN and indentation times, t ind, between 0.001 and 10 s. It was found that: 1. the initial depth, d i, of indentations is practically constant for t ind<0.1 s but increases with increasing t ind for t ind>0.1 s; 2. the initial indentation diameter, a i, practically does not depend on t ind; 3. an indented surface always tends to heal with time, but the time, t, of healing increases with penetration depth of d (i.e. on indentation time); 4. the dependences of both d and a on recovery time, t, exhibit three recovery stages — an initial transient stage, a steady-state stage and a slow stage — and follow a power-law dependence in these stages; and 5. indentation creep occurs for t ind>0.1 s. Analysis of the kinetics suggests that the recovery of indentations and the indentation creep occur by local reorganization, and volume and surface diffusion processes, but the indentation creep is caused above a threshold value of the indentation pressure gradient.

Microhardness studies of chain-extended PE: II. Creep behaviour and temperature dependence

The variation of hardness with indentation time has been investigated for chain-extended polyethylene (PE), other PE samples crystallised under different conditions and paraffins. Hardness is shown to decrease with indentation time for all the samples investigated according to a power-law. Chain-extended PE, produced by high pressure crystallization or annealing, flows at the lowest rate under the indenter of all the PE samples considered. On the other hand, paraffins creep at the highest rate. Creep behaviour depends markedly on the crystal thickness of the material. The mechanical properties at long indentation times seem to be determined primarily by the deformation modes of the crystals. The temperature dependence of hardness and that of the creep behaviour has also been investigated. In chain-extended PE, the softening of the sample and the higher rate of creep with increasing temperature are discussed in terms of the thermal expansion of the unit cell.

Asymptotic methods in the axisymmetric dynamic non-stationary contact problem for an elastic half-space

Asymptotic methods for solving the axisymmetric dynamic non-stationary contact problem for short and long values of the time of indentation of a rigid punch into an elastic half-space are developed. Using Laplace integral transformations (with respect to time) and Hankel integral transformations (with respect to the coordinate) the contact problem is reduced to solving an integral equation in the unknown Laplace transformant of the contact stresses under the punch. The zeroth term of the asymptotic solution of the integral equation for large values of the Laplace parameter (short times) is constructed using a special approximation in the complex plane of the symbol of the integral-equation kernel. The asymptotic solution of the integral equation for small values of the Laplace parameter (long times) is constructed in powers of this parameter. The solution of the contact problem is obtained using an inverse Laplace transformation, applied to the solutions of the integral equation.