True Hardness Research Articles

Estimation of True Hardness and Quantitative Evaluation of Auto-Tempering in As-Quenched Martensitic Steels

Estimation of True Hardness and Quantitative Evaluation of Auto-Tempering in As-Quenched Martensitic Steels

Retracted: Micromechanical properties characterization of 4H–SiC single crystal by indentation and scratch methods

Growth and micromachining of single crystal silicon carbide (SiC) have drawn extensive attention in wide bandgap semiconductors and remained challenging. The micromechanical properties (i.e., hardness, elastic modulus, fracture toughness), which are the key to machinability and machining quality (and therefore the yield of the grown SiC), of 4H–SiC were assessed by nanoindentation, microhardness, and microscratch tests. Consistent values of true indentation hardness of 4H–SiC can be obtained by different approaches without the need for the area function of the indenter. Accurate determination of elastic modulus (i.e., 583.5 GPa) and indentation hardness (i.e., 38 GPa) of 4H–SiC by the instrumented indentation technique requires low loads with slight indentation-induced damage. Consistent values of fracture toughness Kc of 4H–SiC were obtained by different methods such as the Vickers/Berkovich indenter-induced cracking method (2.1 MPa·m1/2), energy-based nanoindentation approaches (the critical void volume fraction of 0.062 should be used for 4H–SiC when the deterioration of indentation hardness is used), and linear elastic fracture mechanics (LEFM)-based scratch approach with a spherical indenter (2.6 MPa·m1/2). This work provides the guidance for the mechanical machining of 4H–SiC wafers and the paradigm of micromechanical characterization of brittle solids by indentation and scratch technologies.

Evolution of fundamental mechanical properties with aliovalent Co/Cu partial substitution and preparation method for Y-123 system

This study investigates the effect of aliovalent Co/Cu replacement and preparation method on fundamental mechanical performance features of YBa2Cu3−xCoxO7−δ (Y-123) ceramic system depending on the crack propagation mechanism by Vickers hardness measurements (Hv) and mechanical investigation models for the first time. All the findings are verified by the scanning electron microscopy (SEM) examinations. Besides, the electron-dispersive X-ray (EDX) technique verifies the successful substitution mechanism. Besides, the Vickers hardness parameters improve systematically with the increment in the Co/Cu partial substitution (serving as a barrier) level due to formation of operable slip systems, ionic bond formations, and decrement of stress-amplified strain fields. Moreover, the Y-123 ceramic produced by solid-state reaction method and molecular weight of 0.20% presents the densest and smoothest surface morphology with the largest particle distributions and well-linked cobblestone-like grains. On the other hand, the Y-123 ceramic compounds produced by the sol–gel method are more sensitive and responsive to the indentation test loads. All the findings are wholly supported by the mechanical performance properties, including the shear modulus, resilience, and degree of granularity. Furthermore, the mechanical models indicate that every compound prepared exhibits the untypical reverse indentation size effect (RISE). Additionally, the modeling studies display that the induced cracking (IIC) approach is found to be the most appropriate method to examine true Vickers hardness parameters in the plateau limit regions. All in all, this comprehensive study reports efficiently exploiting the process–structure–property relationships in Y-123 ceramic material design for physical science and mechanical application fields using the aliovalent partial substitution and preparation condition.

Micromechanical characterization of zirconia and silicon nitride ceramics using indentation and scratch methods

Micromechanical characterization of brittle ceramics remains challenging due to the difficulty in sample preparation required by conventional tests. The micromechanical properties (e.g., elastic modulus EIT, indentation hardness HIT, and fracture toughness KIC) of ZrO2 and Si3N4 were investigated by indentation and scratch tests with different indenters (e.g., Vickers, Knoop, and Berkovich indenters) under various loads. The ratio of the true hardness by the Vickers indenter over that by the Knoop indenter is approximately a constant of 1.13 and independent of material. Elastic recovery of residual imprint by the Knoop indenter was also used to assess elastic modulus of ZrO2 and Si3N4, and modified equations were proposed. The length of Vickers indenter-induced cracking was found to be proportional to the applied load under large loads, and a modified formula for calculating fracture toughness was proposed for ZrO2 and Si3N4, which have relatively large fracture toughness of about 9 MPa⋅m1/2. The critical void volume fraction f* in the nanoindentation energy-based methods was found to linearly decrease with the exponent mf of the power-law equation used for curve fitting of the degradation of HIT or EIT with hmax. The determination of fracture toughness by scratch methodology was found to be affected by mechanical properties of material, indenter geometry, and data range.

Estimation of True Hardness and Quantitative Evaluation of Auto-Tempering in As-Quenched Martensitic Steels

Estimation of True Hardness and Quantitative Evaluation of Auto-Tempering in As-Quenched Martensitic Steels

Simulating porcelain firing effect on the structure, corrosion and mechanical properties of Co–Cr–Mo dental alloy fabricated by soft milling

This study aims at evaluating the effect of simulating porcelain firing on the microstructure, corrosion behavior and mechanical properties of a Co–Cr–Mo alloy fabricated by Metal Soft Milling (MSM). Two groups of Co-28Cr-5Mo specimens (25 × 20 × 3 mm) were prepared by MSM: The as-sintered (AS) specimens and the post-fired (PF) specimens that were subjected to 5 simulating porcelain firing cycles without applying the ceramic mass onto their surface. Phase identification by X-ray Diffraction (XRD), microstructure examination by optical microscopy and Scanning Electron Microscopy combined with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), corrosion testing by cyclic polarization and chronoamperometry in simulated body fluid (SBF), the latter test accompanied by Cr3+ and Cr6+ detection in the electrolyte through the 1.5-diphenylcarbazide (DPC) method and UV/visible spectrophotometry, and mechanical testing by micro-/nano-indentation were conducted to evaluate the effect of the post-firing cycles on the properties of Co–Cr–Mo. The results were statistically analyzed by the t test (p < 0.05: statistically significant). All specimens had a mixed γ-fcc and ε-hcp cobalt-based microstructure with a dispersion of pores filled with SiO2 and a fine M23C6 intergranular presence. PF led to an increase in the ε-Co content and slight grain coarsening. Both AS and PF alloys showed high resistance to general and localized corrosion, whereas neither Cr6+ nor Cr3+ were detected during the passivity-breakdown stage. PF improved the mechanical properties of the AS-alloy, especially the indentation modulus and true hardness (statistically significant differences: p = 0.0009 and 0.006, respectively). MSM and MSM/simulating-porcelain firing have been proven trustworthy fabrication methods of Co–Cr–Mo substrates for metal-ceramic prostheses. Moreover, the post-firing cycles improve the mechanical behavior of Co–Cr–Mo, which is vital under the dynamically changing loads in the oral cavity, whereas they do not degrade the corrosion performance.

An Investigation of the Influence on Compacted Snow Hardness by Density, Temperature and Punch Head Velocity

The density, temperature, and punch head velocity are dominant factors to the variation of the compacted snow hardness measured by penetrometers. This effect is essential to the construction and operation of compacted snow roads. The Improved Motor-driven Snow Penetrometers (IMSP) are utilized in this research to control the penetration speed and measure the true cone hardness during snow penetration. This study employs a multi-method approach combining orthogonal experiments and the Support Vector Regression (SVR) technique to analyze the effects of these three factors on snow hardness. The results of this investigation indicate that, under identical conditions, density is positively correlated with the hardness of compacted snow, and its sensitivity and significance to the compacted snow hardness are the greatest. Temperature and penetration speed have an effect on hardness, which cannot be completely ignored. The hardness of snow close to its melting point, regardless of its density, decreases significantly at high penetration rates. This study investigates the factors that influence the hardness of compacted snow and provides substantial technical support for the design, construction, and maintenance of snow roads.

Microstructure and properties of MAX-based composites manufactured from Ti3SiC2-based and Ti/SiC powders by FAST/SPS in vacuum

The synthesis of MAX-based composites from Ti3SiC2-based and non-stoichiometric Ti/SiC powders was developed via field-assisted sintering technology/spark plasma sintering (FAST/SPS) in vacuum at temperatures ranging from 1350 to 1450 °C, with a heating rate of 200 °C/min, holding time from 1.5 to 15 min, and a compaction pressure of 60 MPa. The phase composition of the powders and composites was analyzed using X-ray diffraction (XRD), while the microstructure was examined by means of light microscopy (LM) and scanning electron microscopy (SEM) with energy dispersive X-Ray spectroscopy (EDS). The specific features of the MAX phase microstructure formation from the mechanically alloyed powder were defined. The hardness, nanohardness, Young's modulus, and strain gradient model parameters were determined. Calculations based on load-displacement curves allowed to indicate such materials' properties as Young's modulus, true hardness, and characteristic depth. The friction and wear behavior were investigated at temperatures ranging from 20 to 700 °C and test loads from 10 to 40 N, revealing self-lubricating effects during sliding. A near-zero wear effect at high temperatures was found for the composites sintered from non-stoichiometric Ti/SiC powder. The resulting composites exhibited hardness of 810–860 HV10 and fracture toughness of 5.5–6.5 MPa m1/2 for the samples sintered from Ti3SiC2-based powder and hardness of 1100–1200 HV10 and fracture toughness of 5.0–5.3 MPa m1/2 for the samples sintered from Ti/SiC powder. In both cases, Ti3SiC2 was the dominant phase, while TiSi2, SiC, and TiC were also present in the microstructure of the sintered samples.

Post-fatigue fracture load, stress concentration and mechanical properties of feldspathic, leucite- and lithium disilicate-reinforced glass ceramics

ObjectiveTo evaluate the mechanical properties of different CAD/CAM ceramic systems and the post-fatigue fracture and stress distribution when used as cemented crowns. Materials and methodsSixty (60) CAD/CAM monolithic crowns were milled using three different ceramic materials (FD – Feldspathic [Vita Mark II]), LE - Leucite-based ceramic [IPS Empress CAD] and LD - Lithium Disilicate [IPS e.max CAD]) and adhesively cemented on resin composite dyes. Specimens were stored in distillated water (37 °C) for 7 days. After, half of the crowns were submitted to immediate fracture load test while the other half was submitted to fatigue cycling. The average cement layer of approximately 80 μm was assessed using scanning electron microscopy (SEM). The average thickness was used in the three-dimensional (3D) Finite Element Analysis (FEA). For each ceramic material, the density, Poisson ratio, shear modulus, Young modulus, fracture toughness, and true hardness were assessed (n = 3). The data was used to assess the Maximum Principal Stress throughout 3D-FEA according to each material during load to fail and post-fatigue. Data were submitted to two-way ANOVA and Tukey test (α = 0.05). ResultsLD showed the highest compression load, density, shear modulus, Young modulus, fracture toughness and true hardness values. While LE presented the lowest mechanical properties values. There is no difference in the Poisson ratio between the evaluated ceramics. ConclusionLD was susceptible to aging process but presented stronger physicomechanical properties, showing the highest post-fatigue fracture load and highest stress magnitude.

Structure and Properties of 3D Printed Zirconia Applied in Dentistry

During the last years, the interest in the application of the additive manufacturing (AM), also known as 3D printing, becomes extremely popular in the various fields of the medicine including the dentistry. Currently, metal and ceramic materials are most often used for dental prosthetics manufactured by 3D printing. The yttria-stabilized zirconia (YSZ) ceramics has become the best alternative for metal-based dental restorations. In this regard, the main goal of this review deals with studying the effect of the 3D printing parameters on the macro- and microstructure and, accordingly, on the mechanical properties of the sintered YSZ, and on this basis, to give the practical recommendations to clinical dentistry and further prospects. As most researched in recent years, the 3D printing methods of such ceramics are the Laser-Stereolithography (Laser-SL) and the Stereolithography-Digital Light Processing (SL-DLP) based on the vat-photopolymerization technology and are discussed here. The physical foundations and the technological parameters of these AM technologies are considered. The main attention focuses on the effects of the thermal conditions during the 3D printing on the solidification microstructure (density, grain size, and crystalline phase composition), which is controlled by the manufacturing technologies. In addition, the true hardness and the biaxial flexural strength of the 3D printed YSZ samples are discussed. At last, the advantages of 3D printing ceramics in dentistry are mentioned.

Indentation size effect and hardness of materials

In recent decades, the Indentation Size Effect (ISE), which manifests itself as a change in fixed hardness depending on indentation conditions, has become one of the most urgent problems of modern materials science. Research efforts are aimed at identifying and eliminating the causes of ISE, because the discrepancy between actual and true hardness values greatly complicates the task of finding the optimal material for the manufacture of parts. The proposed paper gives a brief review of the theories explaining ISE and its relationship with hardness of materials. A more detailed analysis of the effect of ISE on hardness is presented using an energy model based on the determination of the specific work of plastic deformation during indenter indentation. In all cases of ISE manifestation, a proportional relationship was established between the fixed hardness and the specific work of plastic deformation. Thus, the main cause of ISE is the hardening of the material in the centre of deformation under the indenter. The degree of influence of ISE on the fixable hardness as a function of material pre-hardening was determined. At a high level of pre-hardening (at ε&gt;0.25) the influence of the ISE becomes insignificant, so it cannot be of practical importance. The proposed method of hardness determination will allow to assign material for manufacturing of parts to a greater extent corresponding to the operating conditions.

Micromechanical properties of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals by indentation and scratch tests

Three kinds of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals fabricated by the Czochralski method with 4 at.% Dy3+ ion doping were investigated by indentation and scratch techniques under Vickers, Knoop, Berkovich, and spherical indenters to understand the influence of Lu ion on micromechanical properties and fracture behavior of Y3Al5O12 (i.e. YAG for x = 0) single crystals. The largest (or smallest) values of hardness, elastic modulus, and fracture toughness were found for x = 1/3 (or 1/2). The indentation size effect was explained by four different models with the Hays-Kendall approach being the most suitable one to determine the true hardness. Fracture toughness values of YAG crystals obtained by the Vickers hardness method agreed with those obtained by scratching with a spherical indenter based on linear elastic fracture mechanics.

Moisture‐dependent engineering properties of underutilized browntop millet (unhulled and hulled) and characterization of milled flour

AbstractBrowntop millet is an underutilized indigenous cereal crop. It is termed as the nutricereal due to its superiority in terms of nutritional quality. The current study attempted to investigate the effect of moisture content (10%–27%) on properties such as physical, mechanical, aerodynamic, and textural properties of unhulled and hulled browntop millet to uncover the gaps in developing the efficient primary processing machinery. In addition, we investigated the impact of milling on the biochemical, functional, color, structural, and powder flow properties of browntop millet flour. The results revealed that as the moisture content increased, the principle dimensions, 1000 grain weight, terminal velocity, angle of repose, and static coefficient of friction were linearly increased. In the same moisture content range, characteristics such as true density, bulk density, porosity, and hardness displayed an inverse trend. Milling showed a substantial (p &lt; 0.05) influence on the nutritional, functional and color parameters. FTIR spectra and X‐ray diffraction patterns demonstrated a slight change in functional groups and a slight decrease in starch crystallinity respectively. All three millet flours were cohesive. These flours, however, exhibited a positive correlation with bulk density and a negative correlation with wall friction and effective internal friction angles as the principle consolidating stress increased.Practical ApplicationMillets are a globally significant food crop with major economic influence in developing nations. They are one of the most significant drought‐resistant crops, and a good source of health‐promoting complex carbohydrates, dietary fiber, phenolic compounds, and phytochemicals. Despite of their high nutritional superiority and environmental friendliness, production and consumption have declined over the years. The significant factor contributing to this decline is the lack of machinery or equipment available to process on a commercial and residential scale after harvest. The knowledge of the measured properties will encourage researchers to develop and modernize equipment used for destoning, grading, sorting, separation, dehulling, and conveying. The measured nutritional, mineral, functional, structural, and powder flow properties of milled browntop millet fractions will help develop a new product. So that millet can be used to its full potential.

Indentation size effect in steels with different carbon contents and microstructures

Indentation Size Effect (ISE) in steels having a wide spectrum of carbon (C) concentrations (wt-%) 0.002 (interstitial-free), 0.07 (microalloyed), 0.19 (low carbon), 0.32 (medium carbon), and 0.7 (high carbon), and microstructures were investigated using Vickers micro-hardness tester. A decrease in micro-hardness with increasing load, i.e. ISE, is observed in all the samples except microalloyed steel. The empirical relations, such as the Nix and Gao model, Minimum Resistance model, and Proportional Specimen Resistance (PSR) model, were used to determine the load-independent or true hardness values. Nix and Gao model was adopted to determine the plastically deformed zone (PDZ) size under the indenter and subsequently correlated with ISE in the materials. It is observed that ISE is absent when the PDZ size becomes comparable to or larger than the grain size of the material.

The Indentation Size Effect (ISE) of Metals

The literature regarding the Reverse Indentation Size Effect (RISE) is scarce, the occurrence of which is assumed for plastic materials, including metals. The content of this article is to study the relationship between applied load and measured values of the Vickers micro-hardness of 19 metals with different types of lattices, measured with a Hanemann tester. The values of the load ranged between 0.09807 N (10 g) and 0.9807 N (100 g). The size and character of the Indentation Size Effect (ISE) were evaluated by Meyer’s power law (index n), Proportional Specimen Resistance (PSR), and Hays—Kendall methods. Meyer’s index n ranged between 1.65 for Mo and 2.44 for Ni. A correlation was found between the micro-hardness and Meyer’s index for metals with FCC and HCP lattices. The measured value of Vickers micro-hardness is influenced by the size and nature of the ISE. If this is not taken into account, it may be misleading. For this reason, we recommend using the “true hardness”, determined by the presented method.

Evaluation of irradiation hardening in proton-irradiated [formula omitted]-zirconium hydride and Zr2.5Nb

A sample with a δ-zirconium hydride rim grown on a Zr2.5Nb pressure tube was proton irradiated to different damage levels and its mechanical properties were probed using nanoindentation. Irradiations were also carried out on non-hydrided Zr2.5Nb as a reference. A numerical model proposed by Dao et al. and developed by Wang et al. was used to calculate the yield strength and the work hardening exponent. The samples were subjected to 0.2, 0.4, and 0.8 displacements per atom (dpa), with the irradiation temperature kept below 100 ∘C. To account for the indentation size effect, the Nix-Gao model was used to calculate the true hardness (H0) of both materials. Both hydrided and non-hydrided samples show an increase in the true hardness and in the yield strength due to the irradiation hardening. The true hardness of the δ-zirconium hydride increased from 3 GPa at 0 dpa to 4.31 at 0.8 dpa. Similarly, the H0 of the Zr2.5Nb increased from 2.53 at 0 dpa to 3.03 at 0.8 dpa. Additionally based on the model analysis, the yield strength increased from 731 MPa to 1146 MPa for the δ-zirconium hydride, and 662 MPa to 850 MPa for the Zr2.5Nb, with the latter in good agreement with literature values for Zr2.5Nb. Lastly, evaluation of the work hardening exponent with the different dpa levels suggests the defects density evolves differently with irradiation in the two materials. These novel results demonstrate a clear change in mechanical properties of the δ-zirconium hydride with irradiation.

New method to determine the true hardness of structural steels

The hardness of the material hardened in the zone of penetration of the indenter is recorded by standard methods of measuring hardness. This disadvantage is the main reason for the appearance of the size effect (indentation size effect) and a decrease in the measurement accuracy. The aim of the work is to develop a mechanical method for determining the hardness of metals and alloys without taking into account hardening in the zone of penetration of the indenter. Based on the linear relationship between the hardness and the resistance to deformation of the material, a formula was obtained for determining the hardness without taking into account the effect of hardening in the indentation zone, which makes it possible to increase the accuracy of mechanical methods for determining the hardness, expands their functionality and scope. The essence of the proposed method is illustrated by the example of determining the hardness of specimens made of steel 20, subjected to compression at various degrees of deformation. Compression test method was used to construct a hardening curve, which was used to determine the resistance to deformation of the material in the zone of penetration of the indenter and then the hardness of the material without taking into account the hardening in the zone of penetration of the indenter. The developed method for measuring the hardness of metals and alloys makes it possible to increase the accuracy of mechanical methods, to avoid the influence of the indentation size effect on the final result, and can be used to optimize the technologies for manufacturing parts.

Determination of mechanical properties of Bi12TiO20 crystals by nanoindentation

Bi12TiO20 (BTO) single crystal was grown by Czochralski method and investigated mechanically by nanoindentation measurements. X-ray diffraction pattern of the crystal presented one intensive peak around 37.95° associated with (330) plane of cubic crystalline structure. Nanoindentation experiments were performed at various loads between 5 and 100 mN. Hardness and Young's modulus of the crystal were determined by Oliver-Pharr method. The hardness-load dependency exhibited behavior of indentation size effect. True hardness value of BTO crystal was revealed as 4.4 GPa. Young's modulus decreased with increase of load and load-independent Young's modulus was found around 93 GPa at high loads. The load-dependent elastic and plastic deformation components were calculated and it was observed that the dominant component in BTO single crystal is plastic deformation at the applied loads. The present paper reports for the first time the mechanical characteristics of the BTO single crystal by carrying out nanoindentation experiments.

Experimental and theoretical investigation of the mechanical characteristics of sillenite compound: Bi12GeO20

The present study reports the mechanical and elastic characteristics of Bi12GeO20 (BGO) compound by experimental nanoindentation measurements and density functional theory (DFT) calculations. X-ray diffraction pattern of BGO was plotted and revealed diffraction peaks were associated with Miller indices of cubic crystalline structure with lattice constant of a = 10.304 Å. Two- and three-dimensional representations of Young’s modulus, linear compressibility, shear modulus and Poisson’s ratio were presented according to DFT calculations. The calculated elastic constants pointed out the mechanically stable and anisotropic behavior of the BGO. The hardness and Young’s modulus ranges of the BGO calculated from DFT studies were found as 3.7–6.3 GPa and 61.7–98.9 GPa, respectively. Hardness and Young’s modulus of BGO single crystal were also obtained by analyzing force-dependent nanoindentation experimental data. It was observed that hardness and Young’s modulus decrease with increase of load in the low applied loads and then reaches saturation in the high applied loads. This behavior is known as indentation size effect. True hardness value was determined from proportional specimen resistance model as 4.1 GPa. The force independent region presented the Young’s modulus as 114 GPa.

Nanomechanical properties of the ZEP-520 electron beam resist film

The ZEP-520 electron beam resist has been widely used in electron beam lithography and mask fabrication owing to its high resolution and good sensitivity. The stiffness and stability of the ZEP-520 resist are related to its mechanical properties, i.e., its hardness and elastic modulus. A higher hardness and elastic modulus can endow the ZEP-520 resist with higher stiffness, thereby allowing it to overcome the issue related to the collapse of the electron beam resist pattern. However, to date, only few studies have reported the mechanical properties of the ZEP-520 resist, and the relationship between the thickness of the ZEP-520 resist and its hardness and elastic modulus has not yet been determined. To obtain this information, in the present work, silicon is used as a substrate to analyse the hardness and elastic modulus of the ZEP-520 resist with different film thicknesses via nanoindentation. Additionally, a local area function method is proposed, through which the hardness of thin films can be conveniently and accurately obtained without being affected by the substrate effect. It is found that the true hardness increases with the decrease of the thickness of the ZEP-520 resist film; however, the true elastic modulus has no clear relationship with the film thickness. For a film thickness of 412 nm, the true hardness and elastic modulus of the ZEP-520 resist film are found to reach the maximum values of 0.52 and 11 GPa, respectively.