Load Independent Hardness Values Research Articles

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl2Si3O10 crystalline phase on the crystallization behavior of a MgO–Al2O3–SiO2 (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl2Si3O10 crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl2Si3O10 near the surface of the prepared glass-ceramics was larger than that of MgAl2Si3O10 inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl2Si3O10 phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.

Seed supported solution growth and characterization of L-alanine single crystals for optoelectronics

The large growth of L-alanine single crystal (size = ~18 × 12 × 12 mm3) was achieved by seed supported solution growth technique for the first time. The structure and vibrational modes of the crystal were confirmed by X-ray diffraction (XRD) and FT-Raman analyses, respectively. UV–Vis-NIR absorbance and transmittance measurements were carried out and Tauc’s rule is used to calculate energy gap for the grown crystal and found to be lie within 5.26–5.36 eV. The grown crystal is of high transparency between 80 and 89%, and the energy gap makes it useful for optoelectronic devices. The mechanical strength and chemical etching studies were also carried out and discussed. The present load independent hardness values along different faces of L-alanine crystal are noted ~30, 48, 50 and 65 kg/mm2, respectively, which indicates that (202) face showing higher hardness value compare to all the faces. The etch pit density was observed to be ~4 × 102/cm2. Furthermore, dielectric and electrical studies were also done. The facile and low-cost growth, high optical transparency, energy gap, microhardness, dielectric and electrical of the grown crystal indicates that it is suitable for optoelectronic devices.

VGF bulk growth, crystalline perfection and mechanical studies of CdZnTe single crystal: A detector grade materials

Bulk size (∼80 mm long and 20 mm dia.) with high quality single crystal of Cadmium Zinc Telluride (CZT) has been successfully grown by Vertical Gradient Freeze (VGF) method using five zone furnace. The powder X-ray diffraction analysis confirms the phase of CZT crystal and growth direction was found to be (111). The grown crystal was cut and polished into different pieces and crystalline perfection was studied by high-resolution X-ray diffraction (HRXRD) by employing a multicrystal X-ray diffractometer. The HRXRD study shows that the crystalline perfection of the grown crystals is quite good. The mechanical properties were carried out on the (111) face of different portions (2–1, 2-2-, 6–2, 6–4) of CZT single crystal at room temperature in the load ranging from 0.1 to 5.0 N. The load independent hardness values for 2–1, 2-2, 6–2 and 6–4 are found to be 0.62, 0.65, 0.61 and 0.55 GPa, respectively. In addition, the observed crack profile of indentation mark has been recorded and various other parameters such as dependence of crack length, fracture toughness, brittle index number, yield strength were also calculated and discussed.

Mechanical, microstructural and magnetic properties of the bulk BSCCO superconductor prepared by two different methods

BSCCO bulk samples have been prepared by solid-state reaction and ammonium nitrate precipitation methods with Bi1.65Pb0.35Sr2Ca2Cu3O10±y stoichiometry. Structural, mechanical and magnetic characterizations of the samples were performed by the X-ray powder diffraction (XRD), the scanning electron microscopy (SEM), Microhardness measurements, AC susceptibility measurements. The XRD patterns showed that the diffraction peaks for our samples belong to the two main phase, namely 2223 and 2212 of the BSCCO. In this study we have focused on microhardness measurements to investigate the mechanical properties. Vickers microhardness, Young’s modulus and yield strength values were calculated separately for all samples. In addition to these, we calculated the load independent hardness values of samples. Experimental results of hardness measurements were analyzed using the Meyer’s law, proportional sample resistance (PSR) model, Elastic–Plastic deformation model (EPD) and indentation induced cracking (IIC) model. The critical transition temperature, phase purity, lattice parameter, surface morphology and crystallinity of the prepared bulk samples were compared with each other.

Indentation studies on 〈1 0 0〉, 〈0 0 1〉 and 〈1 0 1〉 face of ammonium dihydrogen phosphate (ADP) single crystals: A novel nonlinear optical material

In the present work, Vickers microhardness measurements were carried out on different faces i.e. (100), (001) and (101) of ammonium dihydrogen phosphate (ADP) single crystals grown by slow evaporation solution technique at room temperature in the load ranging from 0.2 to 2N. The obtained results show that the load independent hardness values are 0.8, 0.7 and 0.66GPa, respectively at different faces of ADP crystal. From the crack length measurements, the fracture toughness values (Kc) was estimated using Evans and Anstis model and the present studies suggest that Evans model is more relevant when compared to Anstis model. The load variation of some mechanical properties viz. brittle index number (Bi) and yield strength (σy) for ADP have also been calculated for the first time.

Nanoindentation hardness of hot-pressed boron suboxide

Abstract The existence of the indentation size effect implies the absence of a single hardness value for the material under investigation especially at low applied loads. In this paper we present an investigation of the indentation size dependence behaviour of nanoindentation hardness in boron suboxide ceramic compacts prepared by uniaxial hot-pressing. Berkovich nanohardness indentations were conducted and analyzed accordingly. In addition to the ordinary Oliver and Pharr method of nanoindentation data analysis, a quantitative approach for the loading curve analysis is proposed. Using the proposed approach, the description and characterization of the observed indentation size effect through the application of the Meyer’s law, and the classical and the modified proportional specimen resistance models as well as the multi-fractal scaling law was conducted and is reported. The load-independent hardness values deduced from our quantitative approach are comparable to the results calculated with conventional methods, especially with the multi-fractal scaling law.

Mechanical characteristics of solution grown potassium zinc chloride crystals doped with lithium ions

Abstract Results of indentation-induced hardness testing studies on potassium zinc chloride crystals doped with Li + ions, leading to an understanding of their mechanical behaviour, are presented. The Vickers hardness of these crystals for (1 0 0), (0 1 0) and (0 0 1) planes in the load range 20–160 g were studied. Load-independent values of hardness are estimated for the three crystallographic planes by applying Hays-Kendall’s and Li-Bradt models. The results showed that: (1) for the three crystallographic planes the load-independent hardness obtained by Li-Bradt model is higher than that predicted by Hays-Kendall’s, approach; (2) the load independent hardness of the (0 0 1) plane is higher than that of both (1 0 0) and (0 1 0) planes, (3) the values of load-independent hardness depend on Li + concentrations in the K 2 ZnCl 4 crystals, (4) the variations of crack length and crack morphology are described for studied crystal planes.

Microhardness, fracture mechanism and dielectric behaviour of flux-grown GdFeO 3 single crystals

Results of indentation-induced hardness testing studies on flux-grown GdFeO 3 crystals, leading to an understanding of their mechanical behaviour, are presented. The Vickers hardness of these crystals for (1 1 0) and (0 0 1) planes in the load range 0.098–0.98 N is 20.85–10.98 and 35.9–13.98 GN m −2, respectively. Load-independent values of hardness are estimated for both crystallographic planes by applying Hays and Kendall’s law. The values of fracture toughness, brittleness index and yield strength have also been calculated for Palmqvist and median cracks. The dielectric measurements on these crystals are carried out with a fully automated impedance analyser. The dielectric constants, dielectric loss and conductivity at different temperatures and frequencies of the applied field are measured and their behaviour analysed. The activation energy of electrical processes is also determined at various frequencies.

Hardness and microstructural characteristics of rapidly solidified Al–8–16 wt.%Si alloys

Al–Si alloys with nominal composition of Al–8 wt.%Si, Al–12 wt.%Si, and Al–16 wt.%Si were rapidly solidified by using melt-spinning technique to examine the influence of the cooling rate/conditions on microstructure and mechanical properties. The microstructures of the rapidly solidified ribbons and ingot samples were investigated by the optical microscopy, electron microscopy and X-ray diffraction (XRD) techniques. The results showed that the structures of all melt-spun ribbons were completely composed of finely dispersed α-Al and eutectic Si phase, and primary silicon was not observed. The XRD analysis indicated that the solubility of Si in the α-Al matrix was greatly increased with rapid solidification. Additionally, mechanical properties of both conventionally cast (ingot) and melt-spun ribbons were examined by using Vickers indenter for one applied load (0.098 N). The hardness values of the melt-spun ribbons were about three times higher than those of ingot counterparts. The high hardness of the rapidly solidified state can be attributed to the supersaturated solid solutions. Besides, hardness values with different applied loads were measured for melt-spun ribbons. The results indicated that Vickers hardness values ( H v) of the ribbons depended on the applied load. Applying the concept of Hays–Kendall, the load independent hardness values were calculated as 694.0, 982.8 and 1186.8 MN/m 2 for Al–8 wt.%Si, Al–12 wt.%Si and Al–16 wt.%Si, respectively.

Hertzian analysis of the self-consistency and reliability of the indentation hardness measurements on superhard nanocomposite coatings

The measurement of elastic properties of superhard nanocomposite coatings can be subject to a number of possible errors, such as indentation size effects (indenter tip blunting, non-representative small volume of the material to be tested upon nanoindentation and a too small stress under the indenter which does not reach the yield stress of that material if a too low load is used), the composite effect of the system of superhard coating on a softer substrate, high compressive or tensile stress in the coatings, drifts and/or stiffness of the indenter etc. We shall present a systematic study of these possible artefacts on superhard coatings using a large range of applied loads on a number of super- and ultrahard samples. The hardness values obtained from the indentation measurements are compared with the Vickers hardness calculated from the projected area of the plastic deformation. The data will be also compared with finite element method computer modeling in order to obtain a deeper insight into the complex problems. It will be shown that reliable results can be obtained if sufficiently thick coatings are used which allows one to obtain load independent values of hardness measured at sufficiently large indentation depths. Hertzian analysis of the non-linear elastic response upon unloading provides analytical solutions that can be used in order to check if the hardness values measured on the super- and ultrahard coatings are self-consistent. In particular, it is possible to estimate the maximum tensile stress that the coatings survive without failure. This stress occurs at the periphery of the contact between the coating and the indenter and, in the case of ultrahard coatings, it can reach values in the range of tens of GPa. The results show a very good agreement with the theoretical predictions based on the Universal Binding Energy Relation.

Possible Artefacts in Measurement of Hardness and Elastic Modulus on Superhard Coatings and the Verification of the Correctness of the Data

ABSTRACTMeasurements of the hardness and Young's modulus of superhard coatings (HV≥40 GPa) by means of automated load-depth-sensing indentation technique can be subject to a number of errors that are discussed and exemplified here. Only load-independent values of hardness for loads larger than 30–50 mN can be considered reliable when the technique of Doerner and Nix (linear extrapolation of the unloading curve) is used to determine the corrected indentation depth. The results are compared with values of Vickers hardness calculated from the contact area of the remaining plastic deformation which was measured by means of calibrated scanning electron microscope. The values of Young's modulus obtained from the indentation are close to the zero-pressure shear modulus of the coatings as measured by means of Vibrating Reed and surface Brillouin scattering techniques.

Investigations on indentation induced hardness and fracture mechanism in flux grown DyAlO 3 crystals

Results of Vickers' microhardness ( H v), fracture toughness ( K c), brittleness index ( B i) of (110) and (001) faces of flux grown dysprosium aluminate crystals, in the load range of 10–100 g are given. The variation of microhardness with load is found to be non-linear and is explained by using Hays and Kendall's law. Applicability of Hays and Kendall's law gives us the load independent values of hardness. Hardness anisotropies for both the crystal planes are reported. The cracks are classified into Palmqvist and Median types. There is a transition from Palmqvist to Median types of cracks at higher loads. The values of fracture toughness ( K c), brittleness index ( B i) and yield strength ( σ y) are calculated using median type of cracks.

Indentation-induced microhardness and dielectric studies of flux-grown gadolinium aluminate crystals

Results of microhardness measurements and dielectric studies carried out on flux-grown single crystals are presented. Load independent values of hardness are estimated for (110) and (001) planes by applying the law of Hays and Kendall. The values of fracture toughness and brittleness index are calculated for median cracks at higher loads. Hardness anisotropy for both the crystal planes considered is reported. The dielectric constant, dielectric loss and conductivity are found to be dependent on temperature as well as frequency of the applied a.c. field. The anomalous behaviour of dielectric constant against temperature suggests some transition taking place at .