Mechanical properties of low porosity Dy123 bulks with different Dy211 content

Mechanical properties of low porosity Dy123 (DyBa₂Cu₃O_x) bulks with different Dy211 (Dy₂BaCuO₅) content were evaluated through bending tests and Vickers indentation tests of specimens cut from the bulks. The bending strength and the Young’s modulus of the low porosity bulks were higher than those of porous Dy123 bulks evaluated in the previous study. The bending strength of the low porosity bulks, which was decreased by the oxygen annealing, increased with increase of the Dy211 content. The fracture toughness of the area with uniform dispersion of the Dy211 particles was superior to that with low density of the Dy211 particles.     

Investigation on mechanical properties of some thiourea complex crystals: Important nonlinear optical materials

In this work the authors have grown good quality single crystals of zinc thiourea sulphate, bis thiourea cadmium acetate, bis zinc thiourea acetate and bis thiourea zinc chloride were grown from slow evaporation solution growth method at ambient temperature and studied their mechanical properties. The crystal system and lattice parameters were confirmed by powder X-ray diffraction analysis. Vickers microhardness of the grown crystals was investigated by using Leitz-Wetzlar (Miniload 2) hardness tester up to an applied load of 120 g. It was observed that, upto 40 g applied load the hardness of the grown crystals increases with increasing the load and thereafter it is practically independent of the indentation load. Meyer's law and Hays–Kendall's law fail to explain the observed load variations. But the variation could be satisfactorily explained by PSR model proposed by Li and Bradt. Classification of cracks and their transition from Palmqvist to median types is explained. The average value of fracture toughness, brittleness index, Young's modulus and yield strength were calculated using expressions for Palmqvist and median types of cracks. Anisotropic nature of the grown crystals was studied using Knoop indentation technique.     

Formation of indentation cracks and origin of indentation size effect in cadmium tartrate pentahydrate single crystals

The load dependence of the Vickers microhardness on the as-grown (0 1 0) and (0 0 1), and cleaved (0 0 1) faces of cadmium tartrate pentahydrate (CTPH) single crystals has been investigated. The experimental results showed that, with an increase in the applied load, the microhardness of the as-grown (0 1 0) and (0 0 1) faces decreases, while that of the unheated and heated (0 0 1) cleavage faces decreases first up to a load of 2.5 N and then increases. Analysis of the experimental results revealed that: (1) radial crack length, indentation size and applied indentation load are mutually related, and these dependences related with fracture mechanics are the basis of Meyer’s empirical law, (2) with increasing indentation load, changes in the mechanism of development of indentation cracks from radial cracks to lateral cracks and surface chipping of the material, followed by predominantly surface chipping of the material are responsible for indentation size effect in CTPH crystals, (3) proportional specimen resistance model and Meyer’s law not only explain the indentation size effect but also can be used to determine load-independent hardness H^∗, and (4) there is no direct relationship between microhardness and fracture toughness of different CTPH samples, while the values of load-independent hardness H^∗, and brittleness indices β and B of CTPH crystals increase linearly with the Meyer constant A. Procedures are given to determine load-independent hardness H^∗ from the transition values of load and corresponding indentation size.     

Indentation Behavior of Silica Optical Fibers Aged in Hot Water

Vickers indentations were performed on silica optical fibers subjected to water interaction by a hydrothermal treatment. The observation of the Vickers impression provided direct evidence of a piling-up of material of the fiber submitted to the hydrothermal treatment. This indentation behavior is consistent with a structural relaxation promoted by the water and the glass network interaction. It is suggested that the water could act as a modifier since the deformation combines the densification and the shear flow of the material. When the relaxed surface layer is chemically etched, the anomalous behavior of silica under indentation is restored as the deformation is controlled only by densification. The chemical etched samples exhibited an increase in hardness.     

Indentation Behavior of Silica Optical Fibers Aged in Hot Water

Vickers indentations were performed on silica optical fibers subjected to water interaction by a hydrothermal treatment. The observation of the Vickers impression provided direct evidence of a piling-up of material of the fiber submitted to the hydrothermal treatment. This indentation behavior is consistent with a structural relaxation promoted by the water and the glass network interaction. It is suggested that the water could act as a modifier since the deformation combines the densification and the shear flow of the material. When the relaxed surface layer is chemically etched, the anomalous behavior of silica under indentation is restored as the deformation is controlled only by densification. The chemical etched samples exhibited an increase in hardness.     

The pack-boronizing of pure vanadium under a controlled atmosphere

Pack boronizing of pure vanadium was performed at 1100 °C for 4, 8, 12 and 16 h under a controlled atmosphere. Characterization of the boride formed on the surface of pure vanadium was carried out by metallographic techniques, profilometry, SEM-EDS, XRD and microhardness measurements. The metallographic studies revealed that a single boride layer with dense, compact and relatively smooth morphology was formed on the surface of pure vanadium. The interface between boride layer and base metal was wavy in nature. The formation of only the VB₂ phase on pure vanadium was confirmed by surface and cross-sectional XRD analysis. The microhardness of the boride layer was approximately 3700 HV for all boriding times. Fracture toughness of the boride layer was evaluated using Vickers indentation, giving the value of 2.1-5.9 and 1.7-3.4 MPa m^1/2 for Palmqvist and median/radial approaches, respectively. Thickness of the boride layer increased almost parabolically from about 23 to 50 μm with boriding time. Surface roughness of the coating was relatively increased from approximately 0.58 to 2.25 μm by boriding duration.     

Hardness of microporous SiC ceramics

Samples of silicon carbide ceramics with variable microporosity and hardness are studied. By measuring the hardness of the ceramics as a function of the integral porosity, it is established that the hardness is proportional to the mean length of bridges connecting pores. Microporosity is found to influence the hardness via stress concentration at the bridges, as a result of which they break up. Indentation causes extensive breakup of the bridges. The mean stress coefficient as a function of the integral porosity of the ceramics is estimated. It is shown that the micromechanisms of quasi-static indentation and high-speed indentation (penetration of a striking bar) are in many ways similar to each other.     

Investigation of electrical and mechanical properties of 3Y-TZP/Cubic zirconia solid electrolytes with composite structure prepared by near net shape forming

Yttria doped zirconia solid electrolytes with non-equilibrium composite structure were prepared by impregnation of a porous 3Y-TZP matrix with a solution of yttrium nitrate. Microstructures were characterized by XRD and SEM. The electrical properties were studied by impedance spectroscopy as a function of temperature. Biaxial flexural strength and fracture toughness of composite samples were measured by ring on ring and Vickers microhardness indentation methods respectively. The yttria content of the electrolytes was increased by a multi impregnation process. It was concluded that electrical and mechanical properties of the electrolytes can be adjusted by controlling the amount of doped yttria especially if severe sintering treatments are avoided. Electrolytes with conductivity of 0.08 S/cm at 900 °C and fracture strength and fracture toughness of respectively 390 MPa and 2.9 MPa m^1/2 were obtained.     

Toughening through multilayering in TiN–AlTiN films

The damage response of columnar multilayers of TiN and AlTiN to Vickers indentation is studied through focused ion beam machining and elastic modelling. Multilayers display an enhanced resistance, which increases with layer refinement, to the multiple fracture modes that appear at high loads in these materials, including edge (nested) cracks and inclined shear cracks. Measurements of layer thickness reveal that multilayers display additional modes of plastic deformation that lead to permanent compression and bending of the film. An elastic model of contact deformation in a bilayer where plasticity is mimicked by greatly enhanced elastic compliance of the film is used to rationalize the trends in crack resistance. It is shown that the enhanced toughness is not due to any increase in the strain capacity (hardness/modulus) of the film material, brought about by multilayering.     

Computer simulation of the relation between mechanical behavior and structural evolution of oxide ceramics under dynamic loading

Computer simulation is used to investigate the deformation and damage processes taking place in brittle porous oxide ceramics under intense dynamic loading. The pore structure is shown to substantially affect the size of the fragments and the strength of the materials. In porous ceramics subjected to shock loading, deformation is localized in mesoscopic bands having characteristic orientations along, across, and at ∼45° to the direction of propagation of the shock wave front. The localized-deformation bands may be transformed into macroscopic cracks. A method is proposed for a theoretical estimation of the effective elastic moduli of ceramics with pore structure without resorting to well-known hypotheses for the relation between elastic moduli and porosity of the materials.     

Evaluation of fracture toughness of ZrO2 and Si3N4 engineering ceramics following CO2 and fibre laser surface treatment

The fracture toughness property (K_1C) of Si₃N₄ and ZrO₂ engineering ceramics was investigated by means of CO₂ and a fibre laser surface treatment. Near surface modifications in the hardness were investigated by employing the Vickers indentation method. Crack lengths and the crack geometry were then measured by using the optical microscopy. A co-ordinate measuring machine was used to investigate the diamond indentations and to measure the lengths of the cracks. Thereafter, computational and analytical methods were employed to determine the K_1C. An increase in the K_1C of both ceramics was found by the CO₂ and the fibre laser surface treatment in comparison to the as-received surfaces. The K_1C of the CO₂ laser radiated surface of the Si₃N₄ was over 3% higher in comparison to that of the fibre laser treated surface. This was by softening of the near surface layer of the Si₃N₄ which comprised of lowering of hardness, which in turn increased the crack resistance. The effects were not similar in ZrO₂ ceramic to that of the Si₃N₄ as the fibre laser radiation in this case had produced an increase of 34% compared to that of the CO₂ laser radiation. This occurred due to propagation of lower crack resulting from the Vickers indentation test during the fibre laser surface treatment which inherently affected the end K_1C through an induced compressive stress layer. The K_1C modification of the two ceramics treated by the CO₂ and the fibre laser was also believed to be influenced by the different laser wavelength and its absorption co-efficient, the beam delivery system as well as the differences in the brightness of the two lasers used.     

Mechanical properties of polycrystalline RuSr2GdCu2O8 superconductor

The main objective of this paper is to report the room temperature hardness and elastic modulus of the RuSr₂GdCu₂O₈ superconductor phase by instrumented indentation. Polycrystalline samples were produced by a solid state reaction technique. The samples were also characterized by scanning electron microscopy, X-ray diffraction and electrical resistivity measurements. The influence of porosity on the mechanical properties was avoided by considering only those indentations inside the grains. The hardness and elastic modulus were 8.6 GPa and 145 GPa, respectively. These values are comparable with those of Y-123. The indentation fracture toughness evaluated after conventional Vickers indentation was 1.9 MPa m^1/2.     

孔隙率及晶粒尺寸对多孔PZT陶瓷介电和压电性能的影响及机理研究

采用添加造孔剂的方法制备多孔锆钛酸铅(PZT)陶瓷，并研究了孔隙率和晶粒尺寸对多孔PZT陶瓷介电和压电性能的影响及机理.研究表明：孔隙率的增加降低了多孔PZT陶瓷的介电常数，提高了静水压优值，并证明在一定条件下孔隙率与介电常数关系可由Okazaki经验公式及Banno模型预测；晶粒尺寸增加，多孔PZT陶瓷的介电常数、压电系数和优值增加，并可用Okazaki空间电荷理论解释晶粒尺寸对试样介电和压电性能的影响.对于添加重量百分数为10%造孔剂的多孔PZT陶瓷，当烧结温度为1300℃时，孔隙率为34%，d关键词： 多孔PZT陶瓷 静水压优值 压电性能 介电性能     

Adhesive and cohesive properties by indentation method of plasma-sprayed hydroxyapatite coatings

Adhesive and cohesive properties of the plasma-sprayed hydroxyapatite (HA) coatings, deposited on Ti-6Al-4V substrates by varying the plasma power level and spray distance (SD), were evaluated by an indentation method. The crystallinity and the porosity decreased with increasing both of these two parameters. The microhardness value, Young's modulus (E) and coating fracture toughness (K_C) were found to increase with a combinational increase in spray power and SD. The Knoop and Vickers indentation methods were used to estimate E and K_C, respectively. The critical point at which no crack appears at the interface was determined by the interface indentation test. This was used to define the apparent interfacial toughness (K_Ca) which is representative of the crack initiation resistance of the interface. It was found that K_Ca reaches to a maximum at a medium increase in both spray power and SD, while other mechanical properties of the coatings reaches to the highest value with further increase in these two plasma parameters. The tensile adhesion strength of the coatings, measure by the standard adhesion test, ISO 13779-4, was shown to alter in the same manner with K_Ca results. It was deduced that a combinational increase in spray power and SD which leads to a higher mechanical properties in the coatings, does not necessarily tends to a better mechanical properties at the interface.     

光学元件亚表层裂纹成核临界条件研究

基于压痕实验和连续刚度测量法得到了熔石英材料硬度和弹性模量随压入深度的变化曲线, 系统分析了材料由延性到脆性转变的过程, 确定了熔石英晶体在静态/准静态印压和动态刻划时产生裂纹的临界载荷和临界深度。渐变载荷刻划实验结果表明, 划痕过程诱发的裂纹对法向载荷有很强的依赖性, 载荷较小时材料去除方式为延性域去除。随着法向载荷的增加, 首先产生垂直于试件表面的中位裂纹和平行于试件表面方向扩展的侧向裂纹, 而在试件表面上并没有产生明显的特征。载荷进一步增加后, 侧向裂纹扩展并形成了明亮区域, 最终诱发了沿垂直于或近似垂直于压头运动方向扩展的径向裂纹, 实现了材料的脆性去除。     

Fracture of ceramics and glasses

The rich variety of fracture behavior exhibited by glasses and ceramic materials is reviewed with particular emphasis on the understanding gained through the use of deliberately introduced, controlled cracks. After a brief summary of the mechanics of indentation cracks four major topics are discussed, the structure of crack tips, environment assisted crack growth, high temperature fracture and the toughening of ceramics. Resolution of the sharp vs blunt crack dilemma is presented together with recent microscopy observations of crack tips in brittle solids. In describing fracture in polycrystalline ceramics we explore some of the complexities beyond the simple Griffith behavior relating strength to flaw size, and show how the scale of the microstructure with respect to the crack length affects the observed toughness. It is shown that the interaction of a crack with the microstructure provides a unifying theme for interpreting much of the current work in the literature and leads to important concepts discussed here, such as the discrete-continuum transition, R-curve behavior, toughening due to crack deflection and crack bridging, transformation toughening and stress-induced microcrack toughening.     

Theoretical and experimental investigation of the photoacoustic effect in solids with residual stresses

Modern experiment and theory in the field of residual stress detection by the photoacoustic method are summarized and analyzed. A multimode approach based on the simultaneous application of several photothermal and photoacoustic methods is proposed for the study of thermal and thermoelastic effects in solids with residual stress. Some experimental results obtained within the framework of this approach for Vickers indentation zones in ceramics are presented. The effect of annealing on the photoacoustic, piezoelectric signal for ceramics and the influence of the given external loading on the behavior of the photoacoustic signal near the radial crack tips is investigated. It is experimentally shown that both compressive and shear stresses contribute to the photoacoustic signal near the radial crack tips. The model of the photoacoustic, thermoelastic effect in solids with residual stress is proposed. It is based on the modified Murnaghan model of non-linear elastic bodies, which takes into account a possible dependence of the thermoelastic constant on stress. This model is further developed to explain the photoacoustic signal behavior near the radial crack tips. It is demonstrated that this model of the photoacoustic effect agrees qualitatively with the available experimental data.     

烧结压力对碳化钽陶瓷维氏硬度的影响

为了探究烧结压力对不同晶粒尺寸碳化钽(TaC)力学性能的影响,通过高温高压技术对纳米、微米尺寸TaC粉末进行高温高压烧结,制备不同烧结条件下的块状TaC陶瓷。利用X射线衍射等表征方法对烧结样品的物相、元素分布、压痕形态进行表征,结果表明:TaC在烧结过程中物相稳定,且无杂质渗入。利用维氏硬度计对不同烧结压力(3.0、4.0和5.5 GPa)条件下的3种陶瓷样品进行维氏硬度测试,并进行微观结构分析,结果表明:随着烧结压力由3.0 GPa提升到5.5 GPa,微米尺寸TaC的维氏硬度(21.0 GPa)优于3.0、4.0 GPa下的纳米尺寸TaC维氏硬度(17.5、19.2 GPa)。此外,研究发现,测试维氏硬度时,3.0 kg应用载荷对测试TaC维氏硬度更加精确。研究结果对结构陶瓷烧结和超高温陶瓷硬度研究具有指导意义。     

Role of the rate factor in the hardness testing of β-irradiated silicon single crystals

The effect of the indenter penetration rate on the magnitude of changes in the hardness and length of radial cracks in silicon, induced by low-intensity β radiation has been studied using the dynamic microindentation method. The elementary volume taking part in plastic deformation of the initial and irradiated silicon during indentation have been calculated using load versus penetration depth curves.     

Indentation fracture and indentation delamination in ZnO film/Si substrate systems

ZnO films with thicknesses ranging from 0.202 to 1.535?µm were deposited by using the magnetron sputtering technique on Si (100) substrates 525?µm thick. Then, Vickers indentation tests were carried out on the ZnO/Si systems at room temperature, in which the applied load varied from 10?mN to 2.0?N. The experimental results show that only indentation-induced radial cracking occurred in the systems with film thicknesses equal to and thinner than 0.554?µm, from which the residual stress in the films was extracted to be 387?MPa in compression. For the systems with film thicknesses equal to and thicker than 0.832?µm, only indentation-induced delamination occurred when indentation loads were low. Under high indentation loads, radial cracking concurrently occurred with delamination. The radial cracks were invisible at the film surfaces because the crack length was smaller than the delamination size. The critical film thickness for indentation-induced delamination was found to be around 0.7?µm for the ZnO/Si systems. Combining the composite hardness models with the indentation-induced delamination model, we developed a method to determine the interfacial fracture energy between a film and its substrate. The novel method is particularly useful for indentation equipment without any displacement measurement devices. Using the new method, we extracted the interfacial fracture energy to be about 12.2?J?m^?2 and from 9.2 to 11.7?J?m^?2 for the cases without and with buckling respectively of delaminated films. Consequently, the pure mode I interfacial fracture energy was calculated to be 10.4?J?m^?2 for the ZnO/Si systems.