Vickers microindentation hardness studies of β-Sn single crystals

The Vickers microindentation hardness anisotropy profile and load dependence of apparent hardness of white tin (β-Sn) single crystals having different growth directions were investigated. Indentation experiments were carried out on the (001) crystallographic plane at indentation test loads ranging from 10 to 50 mN. Examinations reveal that the degree of the hardness anisotropy decreases with increasing indentation test load. Also, the materials examined exhibit significant peak load dependence (i.e., indentation size effect (ISE)). The traditional Meyer's law, proportional specimen resistance (PSR) model and modified PSR (MPSR) model, were used to analyze the load dependence of the hardness. While Meyer's law can not provide any useful information about the observed ISE, the load-independent hardness (i.e., H_PSR and H_MPSR) values can be estimated for different crystallographic directions, using the PSR and MPSR models. Briefly, for microindentation hardness determinations of β-Sn single crystals, the MPSR model is found to be more effective than the PSR model.     

Study of indentation size effect and microhardness of SrLaAlO<Subscript>4</Subscript> and SrLaGaO<Subscript>4</Subscript> single crystals

The load dependence of the Vickers microhardness of SrLaAlO₄ and SrLaGaO₄ single crystals, using a PMT-3 hardness tester, has been investigated and analysed from the standpoint of various theoretical models. On the (100) and (001) planes of these crystals, reverse indentation size effect was observed. Analysis of the experimental data revealed: (1) the indentation size effect is best described by Meyer's law and the proportional specimen resistance model of Li and Bradt, (2) indentation-induced cracking model for reverse indentation size effect and Meyer's law cannot be used to determine the hardness of the crystals, (3) as shown by the negative values of the load-dependent quantities in Hays-Kendall's approach and Li-Bradt model, the origin of indentation size effect is associated with the processes of relaxation of indentation stresses, and (4) the load-dependent and load-independent quantities of different models are interrelated and are intimately connected with the orientation and chemical composition of the crystals. It was also found that the plots of the ratio of indentation load to indentation diagonal against indentation diagonal for a sample exhibit two different slopes with a transition in the slopes occurring at an indentation diagonal, whose value depends on the indenter orientation, indented plane and chemical composition of the crystals. The physical significance of the appearance of these transitions and the nature of load-independent indentation microhardness are discussed. Electronic Publication     

The resistance of silicon carbide to static and impact local loading

The physical nature of the resistance of SiC crystals to static and local impact loading has been examined. Investigation of the temperature dependence of hardness for SiC crystals allows the determination of the characteristic deformation temperature (T* ≈ 1600 K), the parameter that characterizes the degree of covalence in interatomic bonds ( ≈ 6) and the temperature range in which a phase transition under pressure during indentation is possible (T < 800 K). Indentation technique gives possibility to construct stress-strain curves for brittle materials and to determine Hugoniot Elastic Limit. During dynamic penetration of a kinetic projectile into a SiC target the phase transition takes place.     

Atmosphere-Induced Effect in Microhardness, Dislocation Mobility and Plasticity of C60 and Graphite Crystals

Formation of hard, brittle and toluene-insoluble near-surface layer (∼0.3 μm) of C₆₀ crystals under atmospheric exposure was observed. Similar atmosphere-induced effect was found for graphite crystals and might also be expected for other molecular solids. Data on ageing kinetics of C₆₀ and graphite crystals are presented. Variation of hardness with indentation depth can be described by the microhardness model for bilayer medium with different mechanical properties. Specific feature of C₆₀ and graphite crystals is that no size effect appears in the intrinsic microhardness and dislocation mobility characteristics in the indentation depth range of 0.6-4μm.     

Microhardness Indentation Size Effect in Flux-grown Crystals of Rare Earth Aluminates

The previously reported results of microhardness measurements on flux-grown crystals of rare earth aluminates were re-examined in this paper to explore the applicability of the proportional specimen resistance (PSR) model to describe the indentation size effect. It was found that the PSR model is insufficient for describing the experimental data and a modified form of this model was proposed based on the consideration of the effect of surface stress state on hardness testing.     

Strain rate dependence of the indentation size effect in Ti–10V–2Fe–3Al alloy

The influence of strain rates on the indentation size effect (ISE) was explored experimentally. A strong ISE phenomenon on the hardness of Ti–10V–2Fe–3Al (Ti-1023) alloy was found when the peak-load was less than 3500?mN, regardless of the variation of loading rates. However, as indenter strain rates increased, the degree of ISE reduced considerably, which was related to the decrease of the internal indentation length scale ranging from 23.08 to 6.80?µm. Furthermore, a positive strain rate sensitivity of the hardness in the whole peak-load range was found in Ti-1023 alloy, which showed a linear function of indenter load. The underlying mechanism was well explained by the variation of geometrically necessary dislocation and statistically stored dislocations in Ti-1023 alloy.     

Preparation of Shaped Gray Tin Crystals

A new approach is proposed for preparing shaped α-Sn crystals. The key feature of this approach is that the phase transformation of β-Sn to α-Sn occurs in ice used as a shaper. α-Sn nucleates at a contact between β-Sn and a seed as water freezes in a closed system. The pressure exerted by the expanding ice minimizes the amount of α-Sn nuclei and reduces the growth rate.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 671–675.Original Russian Text Copyright © 2005 by Styrkas.     

Determination of the elastic modulus of highly porous samples by nanoindentation: a case study on sea urchin spines

Nanoindenation studies were carried out on single crystal calcite and on sea urchin spines from Heterocentrotus mammillatus, Phyllacanthus imperialis, and Prinocidaris baculosa. Unlike dense calcite single crystals resin embedded porous sea urchin spine segments showed a strong dependence of the indentation modulus, but not the indentation hardness, on the local porosity. This implies that the sampled volume for the indentation modulus in nanoindentation with forces down to 15 mN is not nanoscopic but extends approximately 50 μm around the indentation spot. Only for indentation depths ≪100 nm more or less mount-unaffected values of the indentation modulus could be found. The Voigt model for composite materials (calcite/resin) was found to be applicable for the dependency of the indentation modulus on the porosity. This is attributed to the network type of porosity and opens strategies for the control of stiffness in porous networks.     

Vickers micromechanical indentation of BaMoO4 crystals

Vickers microhardness measurements have been carried out on flux-grown single crystals of barium molydate. The dependence of hardness on indentation load, and anisotropy in hardness in the (011) and (001) planes have been described. Attempts have also been made to understand the nature of the cracks developed around the microindentation.     

2024铝合金表面扫描式微弧氧化工艺研究

本研究利用小功率微弧氧化电源, 通过内充液式管状阴极的逐行扫描, 在2024铝合金样件表面生成微弧氧化陶瓷膜层, 对样件的局部受损部位进行了成功的修复, 从而突破了传统微弧氧化技术不能用于铝合金构件现场局部防护与修复的限制; 利用XRD、SEM、EDS等分析方法对陶瓷膜层的相组成与微观组织形貌进行了研究。利用纳米压痕仪测试了陶瓷膜层的纳米压痕硬度和弹性模量, 用动电位极化曲线测试陶瓷膜层的耐腐蚀性能。结果表明: 在恒电流模式下, 扫描式微弧氧化电压快速升高, 直接进入微弧放电阶段。其一次扫描成膜层厚度17 μm, 相对于传统微弧氧化具有很高的成膜效率。铝合金扫描式微弧氧化陶瓷膜层主要由α-Al₂O₃和γ-Al₂O₃组成, 膜层分为致密层和疏松层, 表面多微孔, 且有微裂纹; 纳米压痕测试结果表明, 陶瓷膜层纳米压痕硬度和弹性模量沿界面向外呈现先增加后减小的变化趋势。动电位极化曲线表明, 扫描式和传统微弧氧化陶瓷膜层都能够对基体起到有效的腐蚀防护作用, 传统微弧氧化陶瓷膜层的腐蚀防护作用高于扫描式。     

The microhardness indentation load/size effect in rutile and cassiterite single crystals

The microhardness indentation load/size effect (ISE) on the Knoop microhardness of single crystals of TiO₂ and SnO₂ has been investigated. Experimental results have been analysed using the classical power law approach and from an effective indentation test load viewpoint. The Hays/Kendall concept of a critical applied test load for the initiation of plastic deformation was considered, but rejected to explain the ISE. A proportional specimen resistance (PSR) model has been proposed that consists of the elastic resistance of the test specimen and frictional effects at the indentor facet/specimen interface during microindentation. The microhardness test load, P, and the resulting indentation size, d, have been found to follow the relationship $$P = a_1 d + a_2 d^2 = a_1 d + (P_c /d_0^2 ) d^2$$ The ISE is a consequence of the indentation-size proportional resistance of the test specimen as described by a ₁. a ₂ is found to be related to the load-independent indentation hardness. It consists of the critical indentation load, P _c, and the characteristic indentation size, d _o.     

Load dependence of the apparent Knoop hardness of SiC ceramics in a wide range of loads

Silicon carbide (SiC) ceramics is a material with increasing use, due to its excellent mechanical properties, especially high hardness. In order to integrate this material into design process, we need to know its hardness as precise as possible. The Knoop hardness number (HK) is calculated using the expression: HK = α·F/d², where F is the applied load, d is the long diagonal of the resulting 1₀indentation and a is the Knoop indenter geometrical constant. In this paper, the Knoop hardness of SiC ceramics was measured in the applied load range from 4.9 to 98.07 N. For some materials measured “apparent” hardness value decreases with increasing applied test load (normal indentation size effect – ISE), while for some materials measured “apparent” hardness increases with increasing applied test load (reverse indentation size effect – RISE). Obtained results show the measured hardness exhibits the ISE. In the literature several models are given for the phenomenon explanation. We used the following models: Meyer's law (F = K·dⁿ), proportional specimen resistance – PSR (F = a₁·d + a₂·d²) and modified proportional specimen resistance – MPSR model (F = a₀ + a₁·d + a₂·d²). Results of regression analysis for all applied models show they can all be used for ISE analysis. “True” hardness was determined based on the PSR and MPSR model (HK_T = α·a₂). The obtained results were similar. If the specimen surface is carefully prepared and the range of loads is wide, the a₀ coefficient from MPSR model reaches small values and can be excluded. Therefore, for the calculation of SiC ceramics Knoop hardness, the simpler model (PSR) can be used.     

Knoop microhardness of single crystal sulphur

The Knoop microhardness of single crystal sulphur was measured as a function of crystallographic orientation and applied test load on the (110) and (111) planes. Microhardnesses were determined to be in the range of 25–35 kg mm^–2. Anisotropy of the microhardness and a normal indentation size effect (ISE) were observed. The ISE was addressed by the application of the traditional power law and the proportional specimen resistance model (PSR) of Li and Bradt. The load-independent hardness was determined, from which it was concluded that the (111) plane is harder than the (110) plane and also that the (111) plane is more anisotropic in microhardness.     

Thin film hardness determination using indentation loading curve modelling

A methodology for determining the thin film hardness from a microindentation loading curve is proposed. The loading curve is modelled to compute the dynamic Martens hardness using the indentation depth reached during the test. Moreover, the indentation size effect is taken into account by applying the strain gradient plasticity theory. Then, the dynamic Martens hardness and the hardness length-scale factor are used to express the applied load as a function of the indentation depth. The proposed model involves three parameters: (i) the dynamic Martens macro-hardness, equivalent to the hardness obtained for an infinite applied load, (ii) the hardness length-scale factor, which represents the material resistance to plastic deformation under indentation and (iii) a corrective load, considering the rounded tip effect of the indenter and the zero shift. The model is validated on a 316L stainless steel which subsequently is used as a substrate material for two different Diamond Like-Carbon thin films. The coated systems involved both a hydrogen-free mostly amorphous carbon-chromium (a-C) film of ∼ 2.6 μm in thickness and a hydrogenated, amorphous carbon (a-C:H) solid lubricant of ∼ 2 μm.     

Microstructure of pulsed laser irradiated Sn and Ge-50 at.%Sn thin films on Ge single crystal substrates

The microstructures produced from pulsed laser irradiation of evaporated thin films of Sn and Ge-50 at.%Sn on single-crystal Ge substrates have been investigated as a function of laser energy density and film thickness. In general, the irradiated samples exhibit a trilayer morphology consisting of an overlayer of equiaxed β-Sn and/or -Ge in a β-Sn matrix, and epitaxial cellular and segregated-free -Ge(Sn) alloy layers. The overlayer thickness decreases and the cellular layer thickness and the cell size both increase with increasing laser energy density. The maximum Sn content in the segregation-free -Ge(Sn) alloy layers remains below 2 at.%Sn, but the maximum Sn content in the cellular -Ge(Sn) alloy layers can reach up to 55 at.%Sn, estimated from the selected area diffraction pattern.     

Temperature Dependence of the Microhardness and Dislocation Mobility in the Near-Surface Layer of C60 Crystals

Abstract

Temperature dependence of the Vickers microhardness and dislocation mobility of as grown and aged in air C60 single crystals in the range of 300-550K has been investigated. An anomalous change of the hardness in the near-surface layer of C₆₀crystals around 470K is observed which is explained by the phase transition from an oxygen-intercalated to an oxidised structure. the growth of the oxide film on heating the crystals in air at temperatures higher than 470K is detected, and hardness of 280 MPa of the oxide film is measured. the stress-promoted oxidation of fullerite in the local zones of indentation at 390-470K is discovered. It has been found that ageing of the crystals in air reduce the temperature of the stress-promoted oxidation.     

Creep activation energy of flow process in Bi2Te2·8Se0·2 single crystals

Temperature dependence of the Vickers microhardness of Bi₂Te_2·8Se_0·2 single crystals has been studied. Loading time dependence of microhardness at different temperatures has been used for creep study in the temperature range 303 K-373 K. The activation energy for indentation creep of the crystals has been evaluated.     

Some features of indentation creep of LiF single crystals

The dependence of the size of the indentation and dislocation rosettes on loading time was investigated on the (001) plane of LiF single crystals. The measurements were performed in temperature range from room temperature to 170° C. The indentation time was varied from 0.2 to 10³ sec. It was revealed that the change of the indentation size during creep was more significant than the change in dislocation ensemble tracks in the field of the concentrated load. It was shown that the dependence of the length of the dislocation rosette edge arms on loading time, when plotting in double logarithmic scale, was linear. This fact allowed the determination of parameterm, characterizing the dependence of the dislocation velocity on stress, using creep experiments. The values ofm proved to be in good agreement with the results obtained by different methods.     

Indentation creep surface morphology of nickel-based single crystal superalloys

The crystallographic orientation dependence of surface morphology of indentation creep on a nickel-based single crystal superalloy is investigated by using crystal plasticity slip theory with a three-dimensional (3D) finite element model. The numerical results show that the pile-up patterns developed around the indentation imprint exhibit four-, two-, and threefold symmetry on the surfaces of [0 0 1]-, [0 1 1]-, and [1 1 1]-oriented single crystals, respectively. The evolution of radial and hoop stresses around the crater provides important information for possible radial crack nucleation, whose critical locations depend on crystallographic orientations. These characteristics can be well explained in the viewpoint of crystallographic anisotropy. The findings may shed some light on understanding of the crystal structures and its time-dependent deformation mechanisms with the indentation method.     

Significant Change in Mechanical Properties of YBa2Cu3O7−x Bulk Superconductors Diffused with Sn Nanoparticles

Mechanical behavior of YBa₂Cu₃O_7?x (Y123) superconductors exposed to Sn nanoparticles diffusion is determined by the way of Vickers microhardness (H _v ) conducted at different applied loads (0.245N≤F≤2.940 N). Load dependent microhardness, load independent microhardness, elastic modulus, and yield strength values are estimated from the microhardness curves. Unpredictably, the findings of the H _v values reveal that the undiffused sample and Sn diffused sample prepared at 500 ^°C exhibit reverse indentation size effect (RISE) behavior while the other samples obey indentation size effect (ISE) nature. Further, we extract the load independent (true) microhardness using the Meyer’s law, proportional specimen resistance (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model, and compare the true hardness with the apparent hardness.