Modeling of grain size and hardness for pulse current electroplating

A mathematical model was developed to describe the effect of the current shape waveform on the grain size, formation, growth rate, and deposits hardness for progressive nucleation (simultaneous nucleation and growth) when four pulse current waveforms are applied: rectangular, ramp up, ramp down, and triangular waveforms. In the model, it is considered that species diffusion across the limit layer is the rate determinant step. The Hall-Petch expression was used to relate the grain size in the metal to its hardness. The model results are compared with the experimental data for nickel electrodeposit hardness, which were presented by Wong et al. (K.P. Wong, K.C. Chan, T.M. Yue, J. Appl. Electrochem. 31 (2001) 25). The model predictions are consistent with the experimental results for the four current waveforms, with an average hardness deviation of about 10% for currents between 1 and 6 kA/m².     

Artificial neural network investigation of hardness and fracture toughness of hydroxylapatite

Hardness and fracture toughness of hydroxylapatite were investigated by artificial neural network (ANN). Hardness and fracture toughness of hydroxylapatite were predicted by using its sintering temperature, sintering time, relative density, and grain size with ANN. It was found that prediction results of its hardness and fracture toughness closely matched with the experimental results.     

Hot-Pressing of Nano-Size Alumina Powder and the Resulting Mechanical Properties

The optimum conditions for sintering nano-phase alumina powder to theoretical density with high hardness and strength were determined. As-received gamma alumina powder was transformed to alpha alumina by heating for 8 min at 1300°C, and 0.1 wt% MgO was added to the gamma powder. These conditions resulted in 100% alpha powder with minimum growth in particle size. The alpha powder was ball-milled for 24 hr to further reduce its particle size. The resulting powder was hot-pressed in vacuum at different temperatures from 1250°C to 1450°C. To achieve high hardness and strength, the minimum time to give theoretical density at each temperature was used. Under these conditions, a Vickers hardness of greater than 22 GPa and diametral strength of above 1 GPa were achieved, which are much greater than those properties of polycrystalline alumina with larger grain size. The activation energy of alpha alumina with a mean diameter of 46 nm to reach theoretical density was 366 ± 23 kJ/molK. From analysis of these results and those of other workers, we conclude that low porosity is a very important factor for increasing strength and hardness of alumina with small grains.     

Analysis of indentation size effect (ISE) in nanoindentation hardness in polycrystalline PMN-PT piezoceramics with different domain configurations

Piezoelectric materials contain microstructural features (e.g., domain walls, interdomain spacing, and grain size) that span across several length scales, i.e., few nm in the case of interdomain wall spacing to several μm in case grain sizes. Recent experimental findings indicated that the domain configurations have more influence on the hardness of these materials than the grain size. In this study, nanoindentation experiments are conducted on polycrystalline PMN-PT (a relaxor ferroelectric material) with a focus to investigate the influence of domain configurations on the indentation size effect (ISE) in hardness, H. Different domain configurations are achieved by selectively annealing the as poled samples above and below the Curie temperature. Nanoindentation hardness is obtained in the load range of 1–5 mN with the maximum penetration depth well below the grain size of the samples. The experimental results reveal that all the samples, albeit to a different order, exhibit strong Reverse Indentation Size Effect (RISE) and normal ISE in H. The observed ISE is then analyzed using classical Meyer's law, the proportional specimen resistance (PSR) model and modified PSR (mPSR) model. The critical analysis of nanoindentation data reveals that the PSR model provides a satisfactory understanding of the genesis of RISE and ISE considering the elastic resistance of test material and frictional resistance at indenter facet/test material.     

Effect of high-energy ball milling on the microstructure and properties of ultrafine gradient cemented carbides

Planetary low-temperature high-energy ball mill was used for preparing the mixed powders with different particle sizes by adjusting the milling time. The ultrafine grain gradient cemented carbides were prepared by sinter-HIP treatment. The effects of milling time on the gradient formation, grain growth, and mechanical properties of ultrafine grain gradient cemented carbides were investigated. The results show that the high-energy ball milling cannot effectively reduce the particle size of mixed powder with short milling time. In addition, the particle size of the mixed powder is significantly reduced, while the specific surface area is significantly increased when the ball milling time exceeds 25 hours. The gradient layer thickness and the grain size increase at the beginning and then decrease when the mixed powder particle size was decreased. Simultaneously, the density, hardness, and fracture toughness of the alloy increase gradually. On the contrary, the number of WC with abnormal grain growth is significantly increased. The thickness of the gradient layer reached 32 μm, and the mean WC grain size is 314 nm. Based on the analyzed results, an optimized gradient layer thickness, grain size, density, and hardness can be obtained when the ball milling time is 35 hours.     

A new approach to the grain-size dependent transition of stress exponents in yttria tetragonal zirconia polycrystals. The theoretical limit for superplasticity in ceramics

Superplastic behavior of fine-grained zirconia ceramics (YTZP) is still a major issue on the ceramic science. This work presents a theoretical approach to the deformation mechanism yielding superplastic response in YTZP. In this paper we prove that, in the absence of any relaxation mechanism of internal stresses, a linear dependence of strain rate with stress is obtained, but the invariance of the microstructure cannot be assured. On the other hand, if the grain dynamics is controlled by the relaxation mechanism, a parabolic dependence is expected. This model predicts a transition between parabolic to linear dependence as the grain size increases, in quantitative agreement with the experimental data reported in the literature for YTZP.     

Effect of grain size on the static and dynamic mechanical properties of magnesium aluminate spinel (MgAl2O4)

Samples of transparent polycrystalline spinel with average grain size varying from 0.14 to 170 μm were prepared by different sintering approaches. The effect of grain size on the flexural strength, hardness and Hugoniot elastic limit (impact loading) was investigated. It was found that values of hardness divided by three for samples with grain size in the 0.14–15 μm range were almost equal to the dynamic yield strength values, estimated based on the Hugoniot elastic limit. This led to the assumption that the onset of inelastic deformation at the Hugoniot elastic limit was brittle rather than ductile. The observed departure of the dynamic yield strength from the hardness divided by three value for ceramics with grain size >15 μm was associated with either impact-induced shear banding or twinning. The feasibility of such banding/twinning intervention in initiating inelastic deformation in the spinel is supported by the values of apparent Hall-Petch coefficients in the corresponding grain size domains.     

超声波对镍镀层硬度的影响

分别在超声波和无超声波条件下,制备了镍镀层。通过镍镀层硬度测试、金相组织观察和镀层内应力的测定,分析了超声波条件下镍镀层硬度提高的影响因素。实验结果表明:增加超声波功率,导致镀层硬度增大。在超声波作用下,镍镀层的晶粒细小,镀层呈现压应力状态。镍镀层晶粒细化、加工硬化和存在压应力是超声波电镀镍层硬度提高的主要因素。     

Influence of microstructure and phase composition on the nanoindentation characterization of bioceramic materials based on hydroxyapatite

The aim of the study was to investigate the influence of microstructure and phase composition on the mechanical behaviour of hydroxyapatite (HAp) and biphasic HAp/β-tricalcium phosphate (β-TCP) bioceramic materials using nanoindentation. The formation of β-TCP phase in the HAp ceramic had the predominant influence on the nanomechanical properties of compact ceramics. For investigated microstructures there appear to be a slight decrease in the elastic modulus with increasing load and a higher decrease in hardness, which are in agreement with upper bounds of the results reported in literature. Maximal value of reduced modulus and hardness is yielded with pure HAp, and is measured to be 133.76 GPa for average grain size of 3 μm and 12.18 GPa for average grain size of 140 nm, respectively. The average modulus and hardness results for HAp/β-TCP ceramics with higher (101.61 GPa, 6.76 GPa) and lower grain size (115.72 GPa, 8.76 GPa) show sufficient mechanical properties in order to serve as hard tissue replacement material.     

脉冲电流密度对电沉积纳米晶镍织构和硬度的影响

采用脉冲电沉积法制备了韧性较好的纳米晶镍镀层。考察了脉冲峰值电流密度对纳米晶镍镀层织构和硬度的影响。结果表明,随着脉冲峰值电流密度的增加,Ni(111)晶面择优取向程度逐渐增强,晶粒显著减小,镀层的硬度逐渐增加。纳米晶镍镀层硬度与晶粒尺寸的关系符合经典的Hall—Petch效应。     

Effect of sintering time on electronic properties of strontium hexaferrite

Polycrystalline M-type strontium hexaferrite was synthesized by a solid-state reaction, and the change in physical properties according to the change of sintering conditions was studied. Rietveld refinement analyses on the strontium hexaferrite sintered in different conditions indicate negligible change in the crystal structure, and the results from FE-EPMA indicate little change in stoichiometry. However, the results from FE-SEM measurements show grain growth in the ones sintered for longer time. In addition, this grain growth affects the decrease in the coercive field. In addition, results of impedance measurements show drastic difference in relaxation behavior. Double relaxation peaks appear in frequency domain in all samples. Double relaxation peaks appear due to grain and grain boundary effects, and two factors were confirmed through Z-view data fitting. As the grain size increases by longer sintering, the relaxation frequency shifts to the high frequency region, and overall normalized resistance decreases. From this result, only electrical properties were controlled without magnetic change through sintering condition control.     

Effect of open porosity on flexural strength and hardness of ZrB2-based composites

In this work, Taguchi L₃₂ experimental design was applied to optimize flexural strength and hardness of ZrB₂-based composites which were prepared by SPS. With this aim, batch ZrB₂-based composites tests were performed to achieve targeted experimental design with nine factors (SiC, C_f, MoSi₂, HfB₂ and ZrC content, milling time of C_f and SPS parameters such as temperature, time and pressure) at four different levels. Flexural strength of all composite was measured by three-point bending test. Hardness measurement was done by Vickers indenter. SEM was applied to evaluate microstructure. The results showed that SiC grain size plays important role on flexural strength and correlation between flexural strength and open porosity is low while hardness strongly depends to open porosity. Grain size variation in the range of ~1.5 µm to ~8 µm has little effect on hardness.     

Size-induced grain boundary energy increase may cause softening of nanocrystalline yttria-stabilized zirconia

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size-induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria-stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra-fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.     

Implications of particle size to transient stage of deep bed filtration

This study was aimed at investigating the effect of particle size, mostly in the submicron range, on break-through stage of filtration. Latex beads, with diameters ranging from 0.46- to 2.967-μm were filtered through filter grains of diameters 0.1-, 0.175- and 0.45-mm. Experimental conditions were chosen so as to obtain breakthrough curves. The experimental results showed that the initial efficiency follows the pattern reported by previous experimental and theoretical studies, i.e., lower efficiency for 0.825-μm particles which fall in the range of critical size. However, the particle removal during the transient stage increased with an increase in particle size for the range of sizes studied. This pattern is qualitatively confirmed by the theoretical predictions of Vigneswaran and Chang (1986) model. This study also provides experimental verification of the effect of the ratio of particle size and grain size at different stages of filtration.     

Modeling of Grain-Size-Dependent Microfracture-Controlled Sliding Wear in Polycrystalline Alumina

To quantify grain-size-dependent sliding wear of polycrystalline alumina induced by grain-boundary microfracture, an attempt is made to extend and combine Cho et al. 's fracture mechanics analysis and Fu and Evans' simple micro-cracking theory. An analytical equation is derived to relate wear with microstructural parameters. Wear by intergranular microfracture occurs, provided that the combined stresses are greater than a threshold value. The critical sliding time to the wear transition decreases with increasing grain size, and the wear rate after the threshold is proportional to the grain size. The theoretical predictions are correlated with the lubricated sliding wear data of aluminas with different grain sizes reported by Cho et al.     

Stress relaxation of foamed high-alumina cement paste

A series of stress relaxation tests on foamed high-alumina cement pastes with different relative densities under various temperatures and imposed fixed strains were conducted to study the effects of relative density and imposed strain on the stress relaxation rates of foamed high-alumina cement pastes. At the same time, the activation energy for stress relaxation of foamed high-alumina cement paste was determined from experimental results. Experimental results on the stress relaxation rates of foamed high-alumina cement pastes are also compared to a theoretical expression obtained from a cell-edge relaxation-bending model. Consequently, the microstructural coefficients included in the theoretical expression for describing the stress relaxation rates of foamed high-alumina cement pastes are found. Furthermore, the stress relaxation rates of foamed high-alumina cement pastes can be predicted from the theoretical expression once their relative density and the imposed strain are known.     

结构陶瓷的摩擦磨损

本文综述了目前国内外结构陶瓷摩擦磨损的研究现状,重点评述了外部因素（主要包括摩擦方式、环境、负载、滑动速度、温度及时间等）对陶瓷摩擦磨损的影响,内部因素（主要有硬度、强度、韧性、弹性模量、粒径、气孔率、晶界相等）与磨损量的关系及搽恋裂磨损过程的各种模型,并就目前结构陶瓷摩擦磨损研究中的一些问题和热点进行了简要的讨论。     

The Effect of Relative Density on the Mechanical Properties of Hot‐Pressed Cubic Li7La3Zr2O12

The effect of relative density on the hardness and fracture toughness of Al‐substituted cubic garnet Li_6.19Al_0.27La₃Zr₂O₁₂ (LLZO) was investigated. Polycrystalline LLZO was made using solid‐state synthesis and hot‐pressing. The relative density was controlled by varying the densification time at fixed temperature (1050°C) and pressure (62 MPa). After hot‐pressing, the average grain size varied from approximately 2.7–3.7 μm for the 85% and 98% relative density samples, respectively. Examination of fracture surfaces revealed a transition from inter‐ to intragranular fracture as the relative density increased. The Vickers hardness increased with relative density up to 96%, above which the hardness was constant. At 98% relative density, the Vickers hardness was equal to the hardness measured by nanoindentation 9.1 GPa, which is estimated as the single‐crystal hardness value. An inverse correlation between relative density and fracture toughness was observed. The fracture toughness increased linearly from 0.97 to 2.37 MPa√m for the 98% and 85% relative density samples, respectively. It is suggested that crack deflection along grain boundaries can explain the increase in fracture toughness with decreasing relative density. It was also observed that the total ionic conductivity increased from 0.0094 to 0.34 mS/cm for the 85%–98% relative density samples, respectively. The results of this study suggest that the microstructure of LLZO must be optimized to maximize mechanical integrity and ionic conductivity.     

Consolidation behavior and hardness of P/M molybdenum

In this study, the consolidation behavior and hardness of commercially available molybdenum powder were investigated. In order to analyze the compaction response of the powder theoretically, an elastoplastic constitutive equation based on the yield function presented by Shima and Oyane was applied to predict the compact density under uniaxial pressure from 100 MPa to 700 MPa. The compacts were sintered at 1400-1600 °C for 20-60 min. The sintered density and grain size of molybdenum were increased with increasing the compacting pressure and processing temperature and time. The effect of the porosity and grain size on the hardness of the specimens was explained based on the modified plasticity theory of porous material and the Hall-Petch type equation.     

Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT

The hardness characteristic of nano-polycrystalline cBN synthesized by direct conversion sintering was thoroughly investigated using Vickers and Knoop indenters. It was found that nano-polycrystals consisting of smaller cBN grains increase the elastic recovery of indentations during unloading of the indenters and the diagonal of Vickers indentations and the minor diagonal of Knoop indentations significantly decrease in length. Thus, if a Vickers indenter is used, the apparent hardness value increases, making it impossible to perform an accurate evaluation, e.g. incorrect Vickers hardness values in excess of 80 GPa were obtained from nano-polycrystalline cBN with a grain size of 50 nm or less. On the other hand, it was verified that a Knoop indenter ensures an accurate hardness evaluation even if the constituent grains are fine because its major diagonal length which is used for measurement is less susceptible to elastic recovery. In an accurate evaluation of the hardness of different types of nano-polycrystalline cBN using a Knoop indenter, the hardness of each type of cBN was around 45 GPa, and there was no clear Hall–Petch relationship between hardness and grain size without a slight bell-like correlation. These results suggest that reported hardness values higher than 80 GPa of similar nano-polycrystalline cBN evaluated using a Vickers indenter are incorrect values caused by elastic recovery occurring at the indentation.