Size effect on hardness for micro-sized and nano-sized YAG transparent ceramics

Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding.     

Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Can ferrroelasticity be evaluated by nanoindentation?

The present study investigated the possibility of evaluating ferroelastic mechanical characteristics by spherical indentation. Finite element simulation of spherical indentation, with a relatively large sphere, of a ferroelastic-plastic material was performed using characteristic bulk data of a typical ferroelastic oxide (LaSrCoFeO). The simulation results showed that the ferroelastic mechanical behaviour cannot be observed in the indentation load vs depth curve, but is clearly observable in the indentation stress vs indentation strain curve, which can be obtained reliably in experiments by estimating the contact radius using load-partial unloading sequences. The method can be reliable when the indentation stress is under the upper ferroelastic critical stress. Therefore, in principle ferroelastic mechanical characteristics could be evaluated by spherical indentation by obtaining the indentation stress vs indentation strain curve using partial unloading to estimate the contact radius, although the requirements are very difficult to satisfy in actual experiments.     

Multi-parameter models of the viscoelastic/plastic mechanical properties of coatings via combined nanoindentation and non-linear finite element modeling

Coatings deteriorate from a variety of failure mechanisms. To improve coating durability and/or enable structure-performance correlations, it is necessary to develop more advanced methods of mechanical characterization. For complex multifunctional coatings, multi-parametric constitutive models that simultaneously account for elastic, viscoelastic, and plastic mechanical properties should be used, especially when mechanical properties in the (macro-scale) bulk state differ from properties that occur as a result of thin-film application, post-treatment processes, or aging effects. The nanoindentation creep experiment combined with non-linear finite element modeling of nanoindentation is an effective tool for characterizing the properties of such coatings. Three- and four-parameter viscoelastic/plastic finite element models, implemented using the ABAQUS™ commercial finite element software, have been developed to simulate the isotropic indentation response of coatings. Unified constitutive models where both plastic and viscoelastic deformation are considered simultaneously have not been published previously within the indentation modeling literature. The parameters are determined by an optimization program that automatically matches the load vs. indentation deformation plot from the nanoindentation experiment, with the load vs. indentation deformation plot obtained by the finite element simulation. The computed parameters become a unique “thumbprint” for a particular coating. These parameters may then be used as input data for more complex simulations, for example, capable of computing stress and strain fields, strain energy dissipation, residual stress, and residual strain during particulate scratching; or various other forms of mechanical loading.     

Friction and wear behaviour of CMCs based on Portland clinker against steel countermaterial

Abstract

The wear behaviour of ceramic materials against steel has been studied with respect to the viability of using clinker as an inex pensive component. Friction and wear behaviour of composites based on Portland clinker reinforced with 3, 6, or 9 wt-% of three different oxides (alumina, magnesia, silica) was evaluated against a steel countermaterial (910 HV) using a pin on disk test. The composites were prepared by dry mixing and compacting at 180 MPa using cold isostatic pressing; sintering was carried out at 1400°C in air. All samples were polished to 0.8 μm. Friction coefficients and wear rates were determined and the materials characterised by optical and scanning electron microscopy.     

Elastoplastic stress–strain relationship of Si2N2O–Si3N4 ultrafine-grained ceramics based on microhardness test and finite element simulation

A method for obtaining the stress–strain relationship of ceramic materials was proposed on the basis of the relationship between the maximum load and the indentation size obtained by microhardness test. The microhardness testing process of Si₂N₂O–Si₃N₄ ultrafine-grained ceramics was simulated using ABAQUS finite element software. The stress–strain relationship curve of the material was obtained by repeatedly modifying and comparing the experimental and simulation results. The hardness testing principle and elastic–plastic theory were comprehensively applied in this work in accordance with the geometrical characteristics of the Vickers diamond indenter. The theoretical formula for calculating the stress–strain relationship of hard and brittle materials using microhardness experimental data combined with finite element simulation was deduced. The elastic–plastic area division principle for calculating yield stress was proposed. The accuracy of the theoretical formula was verified by comparing the theoretical and simulation results.     

Analysis of nanoindentation creep of polydimethylsiloxane

Abstract

The scope of the present work is to present a modified model extracted by finite element method (FEM) analysis for the nanoindentation of polydimethylsiloxane (PDMS). The suggested model, based on Sneddon’s equation, takes into account the specimen finite dimensions and the indenter curvature. Load–displacement curves from FEM analysis are obtained and compared with the experimental and those based on Sneddon’s analysis. Sneddon’s solution and FEM analysis deviate from the experiment. Thus, a new analysis is applied considering the finite tip radius effect, which results in the prediction of the elastic modulus of PDMS with 2% accuracy. A fitting analysis of a typical nanoindentation creep curve is performed using the Maxwell two-element model in order to obtain information about the time dependent properties of untreated and ultraviolet treated PDMS. The aforementioned method describes well the creep curve, especially the part attributed to viscous flow.     

A mechanical and modelling study of magnetron sputtered cerium-titanium oxide film coatings on Si (100)

Ce/Ti mixed metal oxide thin films have well known optoelectrical properties amongst several other physio-chemical properties. Changes in the structural and mechanical properties of magnetron sputtered Ce/Ti oxide thin films on Si (100) wafers with different Ce:Ti ratios are investigated experimentally and by modelling. X-ray Photoemission Spectroscopy (XPS) and X-ray diffraction (XRD) confirm the primary phases as trigonal Ce₂O₃ and rutile form of TiO₂ with SiO₂ present in all prepared materials. FESEM imaging delivers information based on the variation of grain size, the mixed Ce/Ti oxides providing much smaller grain sizes in the thin film/substrate composite. Nanoindentation analysis concludes that the pure cerium oxide film has the highest hardness value (20.1?GPa), while the addition of excess titanium oxide decreases the hardness of the film coatings. High temperature in-situ XRD (up to 1000?°C) results indicate high thermal phase stability for all materials studied. The film with Ce:Ti?=?68%:32% has a new additional minor oxide phase above 800?°C. Contact angle experiments suggest that the chemical composition of the surface is insignificant affecting the water contact angle. Results show a narrow band of 87.7–95.7° contact angle. The finite element modelling (FEM) modelling of Ce/Ti thin film coatings based on Si(100); Si(110); silica and steel substrates shows a variation in stress concentration.     

MgO assisted densification of highly transparent YAG ceramics and their microstructural evolution

In current study, various amounts of MgO single dopant was adopted to fabricated high quality transparent YAG ceramics, by utilizing a simple one-step solid state reaction sintering method in vacuum. At a MgO doping amount of only 0.03 wt.%, YAG transparent ceramics with a transmittance of 84.5% at 1064 nm could be obtained, after sintering at 1820 °C for 8 h. The microstructure evolution and optical property of as-fabricated YAG ceramics as a function of MgO doping concentration were systematically investigated. MgO dopant could effectively promote densification of YAG ceramics when the sintering temperature was lower than 1660 °C, and dramatically accelerate its grain growth between 1540 °C and 1660 °C. Further increase the doping amount of MgO would not benefit to the optical quality of YAG ceramics, and the intragranular pores as well as the Mg-riched secondary phase were observed from the MgO heavily doped ceramics.     

Ameliorative energy-storage properties stemmed from the refined grains in PBLZS antiferroelectric ceramics via introducing liquid phase sintering

The low breakdown strength (BDS) of antiferroelectric ceramics, which become failure before undergoing electrical field induced antiferroelectric-ferroelectric phase transition, have seriously restricted the progress of pulsed power capacitors. The method of refining grain sizes via the incorporation of glass additive is supposed to be an outstanding strategy to boost the BDS. Herein, the (Pb_0.91Ba_0.015La_0.05)(Zr_0.6Sn_0.4)O₃ (PBLZS) antiferroelectric ceramics with the introduce of BaO-B₂O₃-Al₂O₃-SiO₂ (BBAS) glass are designed and synthesized by a traditional solid-state reaction. When the glass content is 0.4 wt%, the recoverable energy storage density (W_rec) increases by 215 % from 2.0 J/cm³ to 6.3 J/cm³, together with a greatly enhanced BDS up to 390 kV/cm versus 270 kV/cm of pure ceramics. Meanwhile, the corresponding sintering temperature is remarkably decreased from 1300℃ to 1100℃. The superior charge and discharge performance can be obtained under the electrical field of 310 kV/cm, including a giant current density (1184.7 A/cm²), a high power density (184.2 MW/cm³), and an ultra-fast discharge period (40 ns). The prominent energy storage properties and low sintering temperature make it become a good candidate for fabricating multilayer pulsed power ceramic capacitors.     

Physical Behavior of Nanoporous Anodic Alumina Using Nanoindentation and Microhardness Tests

In this paper, the mechanical response and deformation behavior of anodic aluminum oxide (AAO) were investigated using experimental nanoindentation and Vickers hardness tests. The results showed the contact angle for the nanoporous AAO specimen was 105° and the specimen exhibited hydrophobic behavior. The hardness and the fracture strength of AAO were discussed and a three-dimensional finite element model (FEM) was also conducted to understand the nanoindentation-induced mechanism.     

数值模拟方法在过程设备设计教学中的探索与实践

本文通过具体案例,介绍了数值模拟方法在过程设备设计课堂教学和实验教学的一些应用情况.实践表明,数值模拟方法的引入提高了学生的学习兴趣和知识技能,开阔了学生的学术视野,强化了学生解决实际工程问题的能力.     

Coagulated concentrated anatase slurry leads to improved strength of ceramic TiO2 bone scaffolds

Slurry behaviour has an important influence on the properties of ceramic scaffolds produced by the polymer sponge method. By adding chloride salts to the TiO₂ slurry, the viscosity was increased depending on the chloride concentration at low pH and high particle concentration. Slurries with higher viscosity led to closed and dense scaffold struts combined with high porosity, resulting in a compressive strength over 1.6 MPa. Furthermore, scaffold prepared with 0.1 M CaCl₂ and SrCl₂ showed the formation of Ca- and Sr-rich phases at the grain boundaries. These ions were also shown to reduce the activation energy for grain growth in the TiO₂ scaffold as indicated by the significantly larger grain size. Ca²⁺-doped scaffolds had the highest compressive strength, while the strength of Sr²⁺-doped scaffolds was reduced by the formation of a solid solution phase below the sintering temperature.     

Yb:YAG陶瓷纳米粉体的研究

本文采用化学均相共沉淀法,以Y(NO3)3.6H2O、Al(NO3)3.9H2O和Yb(NO3)3.5H2O为原料,尿素作为沉淀剂,聚乙二醇400为表面活性剂,制备了Yb:YAG陶瓷纳米粉体。利用扫描电镜、X射线衍射对纳米陶瓷粉体的形貌、物相结构进行了分析。结果表明:化学反应过程中添加一定量的表面活性剂,能获得尺寸均匀、分散性好、无明显团聚的纳米粉体。     

Superplasticity of a multiphase fine-grained Mo-Si-B alloy

Mo-9Si-8B-3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo₃Si and Mo₅SiB₂ was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3 × 10^− 4 s^− 1. The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding was the main mechanism of plastic deformation for this alloy.