Local breaking stresses in loaded microporous SiC ceramics

The ballistic strength and hardness have been measured for a series of SiC ceramic samples with variable (from 1 to 20%) porosity. It is shown that interpore bridges are stress concentrators, and the dependence of the factor of stress concentration on porosity is determined. The conclusions based on the experimental results are confirmed by the finite-element calculations of stress fields in porous systems.     

Influence of microporosity on the strength properties of SiC ceramic materials

A series of silicon carbide ceramic samples with variable characteristics of the microporosity and strength, such as the ballistic strength σ _B and the static strength σ _S , are investigated. The dependences of the strength on the integral porosity for ceramic materials are determined. It is established that the strength (both σ _B and σ _S ) is directly proportional to the average length of the bridges between micropores. The mechanism of the influence of microporosity on the strength of the ceramic materials is elucidated. According to this mechanism, interpore bridges are concentrators of stresses and, hence, are broken when a load is applied to the ceramic material. Numerous breakings of bridges bring about the failure of the ceramic body. The average stress concentration coefficient is estimated as a function of the integral porosity of the ceramic material. It is demonstrated that the static strength of the ceramic material is determined by the presence of large micropores (50–100 μm).     

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。     

Scaling relationships in sharp conical indentation of shape memory alloys

Based on dimensional analysis and finite element calculations, several scaling relationships in the indentation of shape memory alloys with a sharp conical indenter were obtained. These scaling relationships illustrate the dependence of the indentation response on the material properties of shape memory alloys, such as phase transition and plastic deformation. It is shown that the yield stress and strain-hardening exponent of transformed martensite play important roles in the indentation response, in addition to the phase transition properties. Additionally, the general relationships between indentation hardness and phase transition stress, maximum transition strain, martensite yield stress and the strain-hardening exponent of shape memory alloys were obtained. The results show that the indentation hardness of shape memory alloys is not proportional to the phase transition stress or to the martensite yield stress, and cannot be used directly to measure the phase transition stress or the yield stress of shape memory alloys.     

The existence of a lateral size effect and the relationship between indentation and scratch hardness in copper

Indentation size effects (ISEs) are well known in static indentation of materials that deform by dislocation-based mechanisms. However, whilst instrumented indentation techniques have become rapidly established as a means of determining the near-surface mechanical properties of materials, scratch testing has been much less widely used. Hardness is used in wear models as a proxy for the yield stress, and the design of materials and hard coatings has often sought to exploit size-derived performance enhancements through length-scale engineering. Yet, it is not known directly whether (or not) length-scale effects also apply to scratch (and thus wear) performance at small scales, or what the functional form of this effect is. This work directly demonstrates that there is a lateral size effect (LSE) and shows that there are questions to be answered if the use of hardness as an indicator of wear performance is to remain valid. We report on constant load scratch experiments using a Berkovich indenter on single-crystal, annealed copper, using a range of applied normal forces and compare results from three scratch hardness calculation methods to indentation hardness (ISO 14577:2002) measured on the same sample at the same loads. Scratch tests were performed with the Berkovich indenter aligned either edge forward or face forward to the scratch direction. In all cases, we demonstrate that there is a very significant (approximate factor of two) effect of scratch size (an LSE) on scratch hardness. The results also show that the deformation mechanisms occurring in scratch tests are different to those occurring beneath a static indentation and that different mechanisms dominated for different stylus orientations (face-forward vs. edge-forward orientation). This is, to our knowledge, the first direct demonstration of an LSE akin to the ISE in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes.     

Special features of the grain structure of polycrystalline NaNbO<Subscript>3</Subscript> with 3-0 connection

Grain structure of porous sodium niobate ceramics (with 3-0 connection and variable porosity changing from 1.0 to 13.6%) fabricated under different conditions is investigated and the microstructure parameters are calculated for different degrees of porosity. It is demonstrated that with increasing degree of porosity, the character of the sodium niobate grain structure changes due to the formation and ordering of the pore -multilayered grain coating clusters, sharp increase in the total pore surface area, and its significant excess over the external sample surface area. These structural changes can influence significantly the integral electrophysical characteristics of the porous sodium niobate ceramics and determine a high degree of its piezoanisotropy.     

The interaction between Lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter

It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (X. D. Hou and N. M. Jennett, ‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). Recently, we identified a ‘lateral size effect (LSE)’ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (A. Kareer, X. D. Hou, N. M. Jennett and S. V. Hainsworth ‘The existence of a lateral size effect and the relationship between indentation and scratch hardness’ Philos. Mag. published online 24 March 2016). This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 and 44.4 μm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine in reciprocal length (as found previously for indentation) rather than as a superposition of individual stresses. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness.     

Mechanical properties of thin Ag films on a silicon substrate studied using the nanoindentation technique

The mechanical properties of thin Ag films of equal thickness containing grains of various sizes were studied. The film hardness was measured using the Oliver-Pharr techniques based on indentation work calculations or on direct measurements of the area of pyramid imprints in AFM images. In order to avoid the influence of a substrate on the measured hardness, a technique was developed to determine the true values of the film hardness. It was established that the hardness of Ag films decreases with an increase in mean grain size, whereas the elastic modulus remains almost unchanged. It was shown that the dependence of the yield stress of Ag films on grain size does not obey the classical Hall-Petch law.     

Derivation of tensile flow properties of thin films using nanoindentation technique

By regarding the tip blunting as a ball indentation at very low depth range (within about 80 nm in our experiments), the flow properties of Au thin films were derived from the indentation load–depth curve obtained by nanoindentation technique. The effects of pile-up or sink-in were considered in determining the real contact between the indenter and the specimen. The representative strain in indentation was defined in various ways and examined by comparing the flow properties derived from indentation load–depth curve with those measured by tensile test. The best definition was found to be the shear strain at contact edge multiplied by 0.1. When we considered the effects of pile-up or sink-in, the representative stress in indentation could also be determined, and was found to be one third of the mean contact pressure for fully plastic regime. As a more intrinsic property than hardness, the yield strengths of Au films with thickness of 0.56 and 0.99 μm were extrapolated from the derived true stress–true strain curve as 261±30 and 154±18 MPa, respectively.     

Mechanical examination of PZT microfibre defects structure

The major problem in the production process of efficient ultrasonic transducer is the preparation of defect-free PZT fibres. A considerable amount of empirical work is presently in progress to achieve this goal of special importance for high-sensitive transducers. However, there is a lack of basic research on the detection of residual stress and defects areas in these fibres due to difficulties in mechanical examination of such flexible elements. This work presents use of the nanoindenter for material characterisation of PZT fibres of 140 µm radius obtained by extrusion method. The sudden depth-excursions during indentation on the edge of fibres have been clarified using Piezoresponse Mode Atomic Force Microscopy method and XRD measurements. The nanoindentation method proves to be the efficient tool capable to detect contribution of defects along the radius, properly estimate hardness as well as corresponding Young's modulus and concluding on structural properties of the micrometre-range ceramics fibres.     

用纳米压痕法分析溶剂对聚醚醚酮(PEEK)力学性能的影响

Poly (ether ether ketone)(PEEK) is a high-performance semi-crystalline thermoplastic polymer.Exposure of the polymeric surface to solvents can have a strong effect like softening/swelling of polymeric network or dissolution.In this study,nano-indentation analysis was performed to study the effect of acetone on the surface mechanical properties of PEEK using different exposure time.The experiments were performed with a constant loading rate (10 nm/s) to a maximum indentation displacement (1000 nm).A 30-second hold segment was included at the maximum load to account for any creep effects followed by an unloading segment to 80% unloading.The indentation hardness and the elastic modulus were computed as a continuous function of the penetration displacement in the continuous stiffness mode (CSM) indentation.The experimental data showed that the peak load decreased from ~5.2 mN to ~1.7 mN as exposure time in solvent environment increased from 0 to 18 days.The elastic modulus and the hardness of PEEK samples also displayed a decreasing trend as a function of exposure time in the solvent environment.Two empirical models were used to fit the experimental data of hardness as a function of exposure time which showed a good agreement with the experimental values.     

Theoretical study on nanoindentation hardness measurement of a particle embedded in a matrix

The finite element method was used to simulate indentation tests on a particle embedded in a matrix, to investigate the influence of the properties of the particle and the matrix, and the indentation depth on the measured hardness. The particle’s work-hardening exponent and the mismatch in particle and matrix yield strength have a significant influence on the measured hardness. A particle-dominated indentation depth was identified, within which the measured nanoindentation hardness agrees very well with the true hardness of the particle material. Numerical results from the simulations of a wide range of material properties determined that the measured hardness is within 5% difference of the particle’s true hardness when the indentation depth is less than 13.5% of the particle’s radius. The results can be used in practice as a guideline to measure the hardness of a particle embedded in a matrix and provides the theoretical basis to develop a particle-embedded method to measure the hardness of individual particles.     

Indentation Load Effect on Young＇s Modulus and Hardness of Porous Sialon Ceramic by Depth Sensing Indentation Tests

Depth sensing indentation （DSI） tests at the range of 200-1800mN are performed on porous sialon ceramic to determine the indentation load on Young＇s modulus and hardness values. The Young modulus and hardness （Dynamic and Martens） values are deduced by analysing the unloading segments of the DSI test load-displacement curves using the Oliver-Pharr method. It is found that Young＇s modulus ET, the dynamic hardness HD and the Martens hardness HM exhibit significant indentation load dependences. The values of Young＇s modulus and hardness decrease with the increasing indentation load, as a result of indentation load effect. The experimental hf /hm ratios lower than the critical value 0.7, with hm being the maximum penetration depth during loading and hf the final unloading depth, indicate that our sample shows the work hardening behaviour.     

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.     

Influence of the composition of the tetragonal phase in the surface layers of zirconia-based ceramics on their strength

The strength of partially stabilized zirconia-based ceramics is analyzed as a function of the porosity, the grain size, and the degree of tetragonality of the tetragonal phase. It is found that the strength of the studied ceramics, unlike conventional materials, is virtually independent of the porosity and the average grain size and is determined primarily by the content of the easily transformed tetragonal phase.     

Finite-element analysis of the mechanical behavior of Au/Cu and Cu/Au multilayers on silicon substrate under nanoindentation

Finite-element analysis of the nanoindentation into Au/Cu and Cu/Au multilayers was performed to deduce their mechanical characteristics from nanoindentation response. Different bilayer thicknesses, numbers, and sequences were studied using the load–displacement curve, hardness, indentation, and the residual surface profile as well as the von Mises equivalent stress. The characteristics of the multilayers were found to be dispersed between the Au and Cu. Nevertheless, if the indentation depth is smaller than the uppermost individual layer thickness of the multilayers, the intrinsic properties can be obtained. Using the von Mises equivalent stress as a failure criterion, the results showed that thinner multilayers would induce a greater potential of shear banding deformation. PACS 61.43.Bn; 62.20.-x; 68.03.Hj; 68.05.Cf; 68.08.De     

Experimental and numerical approach on interfacial properties of W/Al bilayer films for electronic devices manufacturing

The aim of this article is to provide a systematic method for performing experimental tests and theoretical evaluations on interfacial adhesion properties of the W/Al bilayer thin films interface. Samples W/Al bilayer thin films assembly is deposited on the quartz glass by using radio frequency magnetron sputtering. Based on the analysis of the experimental indentation data, the elastic modulus and hardness of the sample are investigated. The test results show that both of the values are easily influenced by the indentation depth. At the meantime, a finite element model is built to simulate the interface mechanical properties. The analysis shows that stress is mainly centralized close to the indenter and the maximum stress occurs in the lower layer Al film, not in the upper W film. The comparison between the experiment and the simulation shows the validity of the test and the modeling of each other to a certain extent. The investigation builds a basis for future work such as the fabrication of W/Al bilayer thin films for micro/nano manufacturing.     

Analysis of the substrate effects of strain-hardening thin films on silicon under nanoindentation

Mechanical properties of thin films on substrates can be evaluated directly through nanoindentation. For a comprehensive study, thin films should be characterized via Young’s modulus, yield stress and strain-hardening exponent at constant temperature. In this paper, we evaluate these effects of thin films on silicon substrate through finite element analysis. Thin films, from soft to hard relative to the silicon substrate, are investigated in three categories: soft films on hard substrates, soft to hard films on no elastic mismatch substrates, and hard films on soft substrates. In addition to examining the load-displacement curve, the normalized hardness versus normalized indentation depth is checked as well to characterize its substrate effect. We found that the intrinsic film hardness can be acquired with indentation depths of less than 12% and 20% of their film thickness for soft films on hard substrates and for soft to hard films on no elastic mismatch substrates, respectively. Nevertheless, nanoindentation of hard films on soft substrates cannot determine the intrinsic film hardness due to the fact that a soft substrate cannot support a hard film. By examining the von Mises stresses, we discovered a significant bending phenomenon in the hard film on the soft substrate. PACS 61.43.Bn; 62.20.-x; 68.03.Hj; 68.05.Cf; 68.08.De     

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.