True Hardness Research Articles

Effect of target self-bias voltage on the mechanical properties of diamond-like carbon films deposited by RF magnetron sputtering

Diamond-like carbon (DLC) thin films were deposited by RF magnetron sputtering under different target self-bias voltages from −290 to −1090 V. Mechanical properties of these films in terms of hardness, Young's modulus, elastic recovery, and plastic resistance parameter ( H/ E) under different target self-bias voltages were analysed by the nanoindentation technique. Hardness and Young's modulus were found to increase in the range of 11–22 and 110–152 GPa, respectively, with increase of target self-bias voltage. The Korsunsky composite hardness model was used in order to determine the true hardness of DLC films from measurement on the film/substrate system. Residual stress in the films, measured using the bending beam method, was found to be compressive and increased with the increase of target self-bias voltage. The variation of intensity ratio, I d/ I g and the position of G-band of the DLC films with respect to target self-bias voltage were also investigated by Raman spectroscopy.

Superconducting and Vickers hardness properties of ZnO‐added YBCO polycrystalline superconductors

AbstractThe effects of ZnO addition on the microstructure and mechanical properties of the YBCO have been investigated through XRD, SEM, EDX, ac susceptibility measurements and the standard Vickers microhardness testing method. We have prepared the samples by a conventional solid‐state reaction method and used an additive method for the YBCO system (0, 0.5 and 1 wt% of ZnO). The orthorhombic strain Δ = (b – a)/(b + a) increases as the Zn content increases, where a and b are the relevant lattice parameters of the samples. The superconducting transition temperatures and hardness depend on the Zn content of the samples. The hardness is found to be load dependent. From the relation between applied load F and the square of the impression diagonal, d2, the true hardness, H0, and the dissipative portion of the indentation load F0, were derived from the slope and the intercept (at d2 = 0) of the straight line, respectively. The hardness was increased while the transition temperature decreased with the amount of ZnO addition to YBCO. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Instrumented pyramidal and spherical indentation of polycrystalline graphite

The elastoplastic surface deformation of a polycrystalline graphite was studied by examining the indenter’s geometry dependence of load P versus penetration depth h relation (P–h relation) in instrumented pyramidal/spherical indentation tests. The tetrahedral pyramid indenters included inclined face angles β of 10.0°, 22.0° (Vickers pyramid), and 40.0°. The tip radius of spherical indenters used were 32 μm, 200 μm, 794 μm, 1.59 mm, and 6.35 mm. The true hardness H as a measure for plasticity was singled out of the elastoplastic loading parameter k1 in the quadratic expression of P = k1h2 and then quantitatively related to the yield stress Y that was determined from the mean contact pressure for spherical indentation at the onset of plastic yielding. The size effect of Y, decreasing with the increase in the tip radius of spherical indenter, is discussed using the model of geometrically necessary dislocations in terms of the material length scales for a plastic field with strain gradient.

On the description of indentation size effect in hardness testing for ceramics: Analysis of the nanoindentation data

The nanoindentation hardnesses of a commercially available soda-lime glass, a tetragonal ZrO2 polycrystalline and a hot-pressed Si3N4 were measured in the peak load range from 7.5 to 500 mN. The experimental results revealed that, for each material, the measured hardness exhibits a peak-load- dependence, i.e., indentation size effect (ISE). Such a peak-load-dependence was then analyzed using the Meyer’ law, the Hay–Kendall approach, the proportional specimen resistance (PSR) model, the elastic recovery model and the modified PSR model. The analyses revealed that: (1) Meyer's law provides a satisfactory description for the experimental data for each material but cannot provide any knowledge of the origin of the observed ISE; (2) the Hays–Kendall approach, the elastic recovery model and the PSR model yield meaningless values of the parameters included in the corresponding equations, invalidating the applicability of these models in analyzing the ISE in the nanoindentation region; (3) the modified PSR model is sufficient for describing the observed ISE but the physical meaning of this model seems to be more complex than those proposed originally. For each material, the true hardness was also determined based on the PSR model, the elastic recovery model and the modified PSR model, respectively. It was found that the true hardness values deduced based on different models are similar with each other and this similarity was attributed to the similarity between the empirical equations adopted in these models.

Linear strain hardening in elastoplastic indentation contact

Finite-element analyses for elastoplastic cone indentations were conducted in which the effect of linear strain hardening on indentation behavior was intensively examined in relation to the influences of the frictional coefficient (μ) at the indenter/material contact interface and of the inclined face angle (β) of the cone indenter. A novel procedure of “graphical superposition” was proposed to determine the representative yield stress YR. It was confirmed that the concept of YR applied to elastic-perfectlyplastic solids is sufficient enough for describing the indentation behavior of strainhardening elastoplastic solids. The representative plastic strain of εR (plastic) ≈ 0.22 tan β, at which YR is prescribed, is applicable to the strain-hardening elastoplastic solids, affording a quantitative relationship of YR = Y + ε;R (plastic) × EP in terms of the strain-hardening modulus EP. The true hardness H as a measure for plasticity is estimated from the Meyer hardness HM and then successfully related to the yield stress Y as H = C(β,μ) × Y for elastic-perfectly-plastic solids and as H = C(β,μ) × YR for strain-hardening solids, by the use of a β- and μ-dependent constraint factor C(β,μ) ranging from 2.6 to 3.2.

Indentation fracture toughness and dynamic elastic moduli for commercial feldspathic dental porcelain materials

Objective. The purpose of this study was to evaluate and compare the indentation fracture toughness, true hardness and dynamic elastic moduli for 14 commercial dental porcelain materials. Methods. The specimens were fired according to manufacturer instructions. The density of the specimens ( n=3) was measured by means of the water displacement technique. Dynamic Young's shear and bulk moduli and Poisson's ratio ( n=3) were measured using a non-destructive ultrasonic technique using 10 MHz lithium niobate crystals. The true hardness ( n=3) was measured using a Knoop indenter and the fracture toughness ( n=3) was determined using a Vickers indenter and a Tukon hardness tester. Statistical analysis of the data was conducted using ANOVA and a Student–Newman–Keuls (SNK) rank order multiple comparative test. Results. The SNK rank test analysis for the mean dynamic Young's modulus and fracture toughness was able to separate 14 dental porcelain materials into seven and nine groups, respectively, at p=0.05. The elastic moduli, true hardness and indentation fracture toughness for opaque porcelains were significantly higher than incisal; and body materials at p=0.05. Significance. The indentation fracture toughness and the ultrasonic test methods exhibit lower coefficient of variation compared to conventional methods and have considerable advantage for ceramic dental materials in that only small specimens are required to produce an acceptable number of data for statistical analysis.

Mechanical properties of commercial high strength ceramic core materials

Objective. The objective of the present study is to evaluate and compare the flexural strength, dynamic elastic moduli and true hardness ( H o) values of commercial Vita In-Ceram ® alumina core and Vita In-Ceram ® matrix glass with the standard aluminous porcelain (Hi-Ceram ® and Vitadur ®), Vitadur N ® and Dicor ® glass and glass-ceramic. Methods. The flexural strength was evaluated ( n=5) using 3-point loading and a servo hydraulic Instron testing machine at a cross head speed of 0.5 mm/min. The density of the specimens ( n=3) was measured by means of the water displacement technique. Dynamic Young's shear and bulk moduli and Poisson's ratio ( n=3) were measured using a non-destructive ultrasonic technique using 10 MHz lithium niobate crystals. The true hardness ( n=3) was measured using a Knoop indenter and the fracture toughness ( n=3) was determined using a Vickers indenter and a Tukon hardness tester. Statistical analysis of the data was conducted using ANOVA and a Student–Newman–Keuls (SNK) rank order multiple comparative test. Results. The SNK rank order test analysis of the mean flexural strength was able to separate five commercial core materials into three significant groups at p=0.05. Vita In-Ceram ® alumina and IPS Empress ® 2 exhibited significantly higher flexural strength than aluminous porcelains and IPS Empress ® at p=0.05. The dynamic elastic moduli and true hardness of Vita In-Ceram ® alumina core were significantly higher than the rest of the commercial ceramic core materials at p=0.05. Significance. The ultrasonic test method is a valuable mechanical characterization tool and was able to statistically discriminate between the chemical and structural differences within dental ceramic materials. Significant correlation was obtained between the dynamic Young's modulus and true hardness, p=0.05.

Analysis of physical properties of III-nitride thin films by nanoindentation

The thin films of undoped GaN, GaN:Si, and Al0.12Ga0.88N on sapphire (0001) substrate using nanoindentation are investigated. The Young’s modulus, hardness, and plastic energy of the films were calculated from the loading-unloading curve. The true hardness, maximum shear stress, and degree of elastic recovery are then deduced from the preceding calculated data. In addition, the loading-unloading curve clearly shows the pop-in phenomena, which can be attributed to the dislocation nucleation. To better understand the factors affecting the quality of films produced, the stress-strain relationship, which is able to reflect the quality of the fabricated films, is also analyzed using nanoindentation.

Microhardness and radiation damage studies of proton irradiated Kapton films

Kapton films were irradiated with a 3 MeV proton beam at different fluences. The microhardness, and electrical properties were studied using microhardness tester and LCR bridge, respectively. It is observed that hardness of the film increases significantly as fluence increases. The true bulk hardness of the film was obtained at loads greater than 400 mN . There is an exponential increase in the conductivity with frequency but effect of irradiation is not significant. The dielectric loss and dielectric constant are observed to change significantly due to irradiation.

Development of Nondestructive Screening Methods for Single Kernel Characterization of Wheat

ABSTRACTThe development of nondestructive screening methods for single seed protein, vitreousness, density, and hardness index has been studied for single kernels of European wheat. A single kernel procedure was applied involving, image analysis, near‐infrared transmittance (NIT) spectroscopy, laboratory density determination, single kernel characterization system (SKCS), and finally Kjeldahl protein determination on the crushed single kernels. Single kernel NIT spectroscopy showed excellent ability to determine protein content, and some ability for determination of single kernel vitreousness. Nondestructive determination of single kernel density, either based on NIT spectroscopy or based on image analysis and kernel weight, needs to be further improved for practical use. The use of SKCS hardness index as a true single kernel hardness reference in a NIT prediction model resulted in a poor predictability. However, by applying an averaging approach, in which single seed replicate measurements are mathematically simulated, a very good NIT prediction model was achieved. This suggests that the single seed NIT spectra contain hardness information, but that a single seed hardness method with higher accuracy is needed to achieve a good NIT prediction model for single kernel hardness.

Indentation responses of plasma sprayed ceramic coatings

This investigation aims at finding the Vickers and Knoop indentation responses of a few plasma sprayed ceramic coatings, namely Indian alumina, imported alumina, zircon, plasma dissociated zircon (PDZ) and zircon—20 wt% CaO. Some of the sprayed coatings under Vickers indentation show typical Indentation Size Effect (ISE) while others exhibit Reverse Indentation Size Effect (RISE). Such coating behaviour may be attributed to the heterogeneous phase composition and microstructure of the coatings. “True Hardness” and “Elastic Modulus” of the ceramic coatings have been calculated. Also the ISE phenomenon observed for all the coatings undergoing Knoop indentation are explained using models such as Meyer's law, Normalised Meyer's law, Hays' Kendall Approach and Proportional Resistance models. These models have so far been used for explaining the ISE behaviour of only the sintered crystalline ceramics. In this paper, it has been shown that these models hold good to an appreciable extent for plasma sprayed ceramic coatings as well.

Elastoplastic load–depth hysteresis in pyramidal indentation

Extensive indentation tests were conducted for nineteen different engineering materials ranging from brittle to ductile materials, and including hard ceramics, ductile metals, and a soft organic polymer. Three tetrahedral pyramid indenters with specific face angles β [shallow pyramid (β = 10°), Vickers (β = 22°), and sharp pyramid (β = 40°) indenters] were used. All the materials tested were subjected to the quadratic load P and penetration depth h relationship P = k1h2 on loading, and most of the tested materials to the quadratic unloading relationship of P = k2(h − hr)2 with the residual depth hr after a complete unload. To determine the contact area at peak indentation load, a specially designed depth-sensing instrument was constructed, on which the contact behavior during loading/unloading was examined by through thickness observation of transparent specimens. All the characteristic indentation parameters were investigated on the basis of simple elastoplastic model, and correlated well with the nondimensional strain E′ tan β/H, in which the elastic modulus E′ was a measure for elasticity, true hardness H was a measure for plasticity, and the inclined face angle β characterized the indenter. The ratio of the conventional Meyer hardness HM to the true hardness H of the materials tested ranged from 0.2 to 0.9 as a function of E′ tan β/H. The cavity model suggested that true hardness H is expressed by the yield stress Y through a constraint factor C as H = C · Y with C ≈ 5.

Impact of puroindoline b alleles on the genetic variation for hardness in soft × hard wheat crosses

AbstractWheat grain hardness is controlled by one major genetic factor, the puroindoline hardness (ha) locus on the short arm of chromosome 5D, but there is also evidence for other minor genetic factors modifying the effect of puroindoline alleles. In this study, the progeny of nine soft × hard wheat crosses was evaluated for kernel texture, and the allelic state of puroindoline b (pinB) was assessed by polymerase chain reaction. The F2 populations of all nine crosses showed the expected 1:2:1 segregation ratio of homozygous soft, heterozygous medium and homozygous hard offspring. A model of variance components was constructed to separate the effects of pinB‐D1 allelic variation from other genetic factors affecting endosperm hardness. This model showed that pinB‐D1 allelic variation could explain 75‐93% of the genetic variation for hardness in the F2, but there were also significant contributions from other genetic factors in all the crosses. The feasibility of pinB‐D1 alleles as a molecular marker for hardness is demonstrated, and the results also indicate the possibility of breeding wheat varieties with true medium hardness.

Elastoplastic Indentation on Heat‐Treated Carbons

A Vickers indenter, as an efficient mechanical microprobe, was applied to carbon materials heat‐treated at temperatures in the range 880°–2600°C. The plasticity of the carbon materials, which was enhanced by increasing the heat‐treatment temperature (HTT), was assessed from the relation between the indentation load, P, and the penetration depth, h. Using the concept of the true hardness, H, as a measure of plasticity and the experimental estimate of the H‐value, the plasticity of the carbon materials was examined as a function of their crystallographic parameters. The residual impression of the carbons at HTT > 1800°C was hardly visible on the indented surface after unloading, because of the nearly complete elastic recovery of the indented surface, yielding a very unique indentation P–h hysteresis in the loading/unloading cycle. The microscopic processes associated with this unique elastic recovery during unloading are discussed here in relation to the reversible slip of the dislocation‐network structures on the graphitic basal planes.

Microstructural Tailoring and Characterization of a Calcium α‐SiAlON Composition

Three calcium α‐SiAlON microstructures—namely, fine‐grained, bimodal, and large elongated—were developed using powders of the same composition and then characterized. The evolution of grain size and morphology was determined to be a process of nucleation and growth that could be controlled with a two‐step sintering technique. The extent of texture was identified in the as‐hot‐pressed materials as a function of sintering conditions. Samples with different microstructures exhibited different hardness and fracture toughness. The true hardness was derived from the intrinsic relation between applied loads and indent sizes. The effect of microstructure on hardness and fracture toughness was analyzed.

EFFECT OF ION BEAM IRRADIATION ON HARDNESS OF POLYETHYLENE TEREPHTHALATE

ABSTRACT Ion beam irradiation as one of the effective methods of polymer modification has been used on the PET films. The ion beam used was Li+3(50 MeV) of various fluences. The Vickers' hardness of the films irradiated at different ion fluences was measured. The hardness tests on each film were carried out at different loads. The true bulk hardness of the film is obtained at loads greater than 400 mN. The irradiation has been found to significantly increase the hardness of the film. The increase follows a linear trend with the fluence and has been explained in terms of crosslinking effected by electronic stopping mode of the ions.

Effects of the substrate on the determination of thin film mechanical properties by nanoindentation

We examine the effects of the substrate on the determination of mechanical properties of thin films by nanoindentation. The properties of aluminum and tungsten films on the following substrates have been studied: aluminum, glass, silicon and sapphire. By studying both soft films on hard substrates and hard films on soft substrates we are able to assess the effects of elastic and plastic inhomogeneity, as well as material pile-up, on the nanoindentation response. The data set includes Al/glass and W/sapphire, with the film and substrate having nearly the same elastic properties. These systems permit the true contact area and true hardness of the film to be determined from the measured contact stiffness, irrespective of the effects of pile-up or sink-in. Knowledge of the true hardness of the film permits a study of the effects of the elastic modulus mismatch on the nanoindentation properties, using the measured contact stiffness as a function of depth of indentation.

Indentation of a soft metal film on a hard substrate: strain gradient hardening effects

A new type of nanoindentation experiment showing the effect of a strain gradient on the flow strength of a crystalline material is conducted and analyzed. We show that by indenting a soft metal film (Al) on a hard substrate (glass) with a sharp diamond indenter a strong gradient of plastic strain is created. The true hardness of the film is observed to increase with increasing depth of indentation when the indenter tip approaches the hard substrate, in sharp contrast to the falling hardness with increasing depth in bulk materials. We associate this rise in hardness with the strong gradient of plastic strain created between the indenter and the hard substrate. We use the mechanism-based strain gradient (MSG) plasticity theory to model the observed indentation behavior. The modeling shows that the MSG plasticity theory is capable of describing not only the decreasing hardness with increasing depth of indentation at shallow indentations, as observed in bulk materials, but also the rise in hardness that occurs when the indenter tip approaches the film/substrate interface.

Hardness and elastic properties of Bi2O3-based glasses

The hardness and elastic properties of 20PbO · xBi2O3 · (80 − x)B2O3 glasses with x = 20–60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (Tg = 295–421°C), Vickers hardness (Hv = 2.9–4.5 GPa), true hardness (H = 1.5–3.8 GPa) and Young's modulus (E = 24.4–72.6 GPa) decreased monotonously with increasing Bi2O3 content. This compositional trend demonstrates that the strength of Bi–O chemical bonds in these glasses is considerably weak compared with B–O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20–50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37–0.88 MPam1/2 from the values of Hv and E, and it was proposed that not only weak Bi–O bonds but also boron coordination polyhedra (BO3 or BO4) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi2O3 contents belong to the category to fragile glass-forming liquids.

The load dependence of hardness in alumina–silver composites

The room temperature Knoop micro-hardness properties of three alumina-silver composites vis-a-vis a monolithic alumina have been investigated at different indentation loads. The composites have been developed using conventional pressureless sintering at 1550°C and these consisted of 1.6, 3.38 and 5.01% by volume of silver particles. It has been shown that the measured Knoop hardness values of these materials sensitively depend on the applied load of indentation. But following an emerging analytical technique, the composites can be assigned some unique true hardness values, which are functions of their silver content.