Environment-dependent indentation recovery of select soda-lime silicate glasses

Indentations made on silicate glasses can easily be affected by the environment. In the present work, indentations were made on select commercial float glasses as well as on experimental soda-lime silicate glasses using a 1 mm diameter spherical tungsten carbide ball-mounted Brinell indenter. Recovery of indentations made on the glass samples was measured in different environments, namely, 100 °C, room temperature/room humidity and 100% relative humidity, as a function of time by using a Zygo laser non-contact profiliometer. Elastic (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) and indentation (Vickers hardness, fracture toughness, brittleness and fracture surface energy) properties of the glasses were also determined by a pulse-echo and Vickers indentation methods, respectively, to correlate with the recovery of indentations. The elastic properties and Vickers hardness are directly proportional to the packing ions present in the glass structure and the strength of an individual bond, whereas the brittleness and fracture toughness more likely depend on molar volume of the glasses. According to the applied environment, a recovery rate of indentations follows the order: room temperature/room humidity <100% relative humidity <100 °C, regardless of glass composition. The reason for higher recovery rate of indentations is attributed to the structural relaxation, which is promoted by a thermodynamic driving force at 100 °C, and stored strain energy in deformation zone, allowing the indentations to regain their original configurations at certain points.     

Effect of Load on the Hardness of Hot Isostatically Pressed Silicon Nitride

The hardness values of five hot isostatic pressed silicon nitride materials, with varying densities, were measured at loads between 1 and 200 N. For the fully dense materials, the calculated hardness decreased from about 30 to 15 GPa as the load increased to about 10 N, and the hardness remained constant at higher loads. For the samples that showed indentation size effect (ISE), cracks formed at the corners of the indentation, starting at the lowest load of 1 N. Materials with lower densities had lower hardness values, displayed a very small or no ISE, and formed corner cracks only at high loads. For the samples that displayed an ISE at low loads, the formation of cracks was analyzed using the Niihara et al . criterion for Palmqvist cracks.     

Experimental Measurement of Residual Stress Field around Sharp Indentation in Glass

A new technique is developed to measure the residual stress field around Vickers indentations in glass and ceramics. This technique uses a small indentation as a microprobe to measure the residual stress at a specific position near a large indentation. The approach is based on the observation that the crack lengths of the small indentation are changed under the influence of the residual stress field created by the large indentation. A simple fracture mechanics model is derived to calculate the residual stress from the measurement of the changes of the crack lengths of the small indentation. The results show that the residual stress around Vickers indentations is a nonequal biaxial field; both tensile and compressive stresses exist around a sharp indentation and decrease as the distance from the center of indentation increases. This technique can be easily extended to many other cases of residual stress in ceramics and composites.     

Fracture barrier estimation by the edge fracture test method

Known methods for determining the fracture resistance of brittle materials by indentation flaking of the rectangular specimen edge are compared. The edge fracture (EF) test method was chosen as an optimum one. The investigations demonstrated that silicon carbide ceramics, submicrometre alumina, and glasses possessed fracture resistance (F_R) values that can be located above the baseline in the EF base diagram. These results emphasize that the above materials display a higher resistance to the onset of fracture (fracture barrier). This barrier can be estimated by comparing EF test data resulting from Rockwell and Vickers indentations.     

Knoop Microhardness Anisotropy and the Indentation Size Effect on the Basal Plane of Single-Crystal Alumina (Sapphire)

The Knoop microhardness anisotropy profile was determined for the basal plane of a Czochralski grown alumina single crystal for indentation test loads from 100 through 1000 g. Microhardness maxima occur at low indentation test loads for the long axis of the Knoop indenter parallel to the 〈2[Onemacr][Onemacr]0〉. Minima exist for the long axis parallel to the 〈10[Onemacr]0〉. This low indentation test load profile is attributed to slip on the primary slip system, the (0001)〈[Onemacr][Onemacr]20〉, as previously noted by Brookes and co-workers. The degree of the microhardness anisotropy decreases for higher indentation test loads. This results from the activation of multiple slip systems to accommodate the greater amounts of plastic flow required by the larger indentation sizes. The microhardness profile becomes more uniform with increasing indentation test load until the Knoop microhardness approaches a test-load-independent, orientation-independent microhardness of 1167 ± 34 kg/mm². The indentation size effect (ISE) was further investigated through lubricated indentation hardness measurements. Lubrication of the test specimen surface significantly reduces the ISE. Results indicate that friction between the test specimen surface and the indenter facets is a major portion of the ISE.     

Modeling of Rigidity Percolation and Incipient Plasticity in Germanium–Selenium Glasses

We compute the bulk and surface structures of glasses in the germanium–selenium (Ge–Se) system using Monte Carlo simulations and our previously derived set of ab initio potentials. We investigate the elastic response of the Ge–Se glasses under a flat "micro"-indentation and incipient plasticity under a spherical nanoindentation. The glasses with a high average coordination number (〈 m 〉>2.4) display structural frustration owing to an excess of bond constraints, leading to permanent densification from both types of indentations. The glasses with a low average coordination number (〈 m 〉<2.4) exhibit a large number of floppy modes, enabling continuous shear flow. According to the Phillips theory of topological constraints, the ideal glass former is one in which the number of constraints exactly equals the number of degrees of freedom (GeSe₄, where 〈 m 〉=2.4). In both types of indentation simulations, we find that the GeSe₄ glass structure is most resistant to distortions of its basic structural unit.     

Field strength effect on elastoplastic behavior of aluminoborosilicate glass: I. Elastic moduli and indentation size effect

The modifier field strength (FS) is believed to play an important role in determining the elastic–plastic responses of aluminoborosilicate (ABS) glasses, but its effect is not well understood. Three novel alkali and three alkaline earth (AE) ABS compositions were created for this study which is the first part of two studies that explored the elastoplastic responses of these glasses. Six compositions were designed using different network modifiers (NWMs) to cover a range of cation FS. The glasses were also designed such that the concentrations of NWM and Al₂O₃ were similar, which maximized the three-coordinated boron fraction in the network. It is well known that modifier FS can affect the coordination number (CN) of Al and B in an ABS glass structure, for example, a higher FS modifier can promote B³ → B⁴ and higher [Al^5,6], but the degree of this depends on network former (NWF) ratios. Previous work used solid-state NMR spectroscopic analysis on the current glasses to find that there was variation between [B⁴] and [Al⁴] between the two glass series (alkali vs. AE) but that was attributed to synthesis factors and no trend with FS was associated with the varying NWF CN. Further, ²⁹Si ssNMR showed no evidence of NBOs which made sense based on composition. The conclusion, therefore, was that there was a far greater correlation with modifier FS for the increased mechanical and physical properties rather than the CN of Al and B. Part I of the current work focused on the elastic moduli, Poisson's ratio, the indentation size effect (ISE), and the bow-in parameter. This part laid out the foundation to investigate the intersection of these elastoplastic properties with hardness and crack resistance as a function of NWM FS. Results showed that: (i) the Young's, bulk, and shear moduli increased with modifier FS, whereas Poisson's ratio did not trend with FS; (ii) the alkali glasses had a significantly higher magnitudes of ISE compared to the AE glasses; and (iii) the bow-in parameter was load dependent and decreased with modifier FS at the highest indentation load.     

基于能量平衡方法的压痕尺寸效应分析

针对硬度测试中出现的压痕尺寸效应现象,采用纳米压痕技术与原子力显微技术相结合,得到压痕过程中的载荷与压深加载卸载曲线和压痕三维图,从中可以得到压痕过程中压头所做的塑性功、最大压深、塑性变形面积、塑性变形体积等,并以此为变量提出了一个基于能量平衡方法的改进模型,此模型能更好地解释压痕尺寸效应.单晶硅实验结果表明:在较大压深下,由于形成新表面所消耗的功是出现压痕尺寸效应的主要因素;随着压痕深度的减小,用于克服材料对压头的阻力而消耗的功所起的作用越来越大;而在更小的压深下,用于产生塑性变形的初始能量和测试系统误差所造成的影响会越来越大,不可忽视.     

Indentation Creep of Na2O · 3SiO2 Glasses with Various Water Contents

Time-dependent deformation behavior induced by an indentation, called indentation creep, was investigated for Na₂O · 3SiO₂ glasses with various water contents by measuring the indentation depth as a function of time using a conical indenter. It was found that the indentation creep behavior of the glasses could be best explained as steady-state inhomogeneous (non-Newtonian viscous) flow. Water in the glasses appeared to reduce the stress (or yield stress) needed to cause this flow. Crack initiation of the glasses was promoted by water through this inhomogeneous flow.     

Indentation Behavior of Soda-Lime Silica Glass, Fused Silica, and Single-Crystal Quartz at Liquid Nitrogen Temperature

In an attempt to elucidate the processes involved in the formation of indentation impressions, Vickers hardness measurements have been made on soda-lime silica glass, fused silica, and crystalline quartz indented at room temperature and 77 K. The hardness of all three materials increases by a factor of ∼2.5 on cooling to liquid nitrogen temperature. High-magnification SEM photographs revealed that the deformation and cracking patterns of the glasses changed strikingly: no shear lines were observed within the indentations, and ring cracking occurred instead of radial/median cracking. In addition, cracking occurs at much higher loads than at room temperature. The hardness results have been explained in terms of volume flow (densification) rather than shear flow (viscous or plastic) for the glasses at low temperature. The quartz crystal, on the other hand, deformed plastically at both room temperature and 77 K. Cracking differences result from changes in both flow and water activity     

黏弹材料动态纳米压入过程的尺度效应

提出柔度因子分析法表征压入尺寸效应（ISE)通讨聚甲基丙烯酸甲酯(PMMA)及热塑性聚氨酯(TPU)的动态纳米压痕试验,探讨黏弹材料动态压入过程中的压入尺度效应及其影响因素。结果表明,Berkovich针尖对黏弹材料进行纳米压痕动态测试时,主载荷是压入尺度效应产生与否的决定因素,频率对此影响较小,谐振载荷控制方式影响甚微;测试材料存在一临界值,当压入深度大于其临界值时,动态纳米压痕测试获得的黏弹材料参数与传统的动态力学分析(DMA)试验结果基本一致。     

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.     

Indentation fracture resistance test round robin on silicon nitride ceramics

Reproducibility of indentation fracture resistance of three commercial silicon nitrides including bearing balls was evaluated by an international round robin with six laboratories. The between-laboratory standard deviations for indentations at 196 N on the perfectly mirror-finished surfaces were in the range of 0.2–0.5 MPa m^1/2, demonstrating an excellent precession of the test results. The scatter in the fracture resistance increased as the indentation load decreased from 196 to 98 N. The errors in measuring crack lengths deduced from the deviation of each laboratory's readings from author's reading for the same indentations tended to increase with a decrease in the magnification of the lab's microscope, which suggested that finding exact crack tips with lower magnification was difficult especially for those samples with insufficiently mirror-finished surfaces indented at 98 N. Observation of indentations at the load of 196 N with powerful optics was advised to ensure the validity of the indentation technique which is used as the quality assessment of Si₃N₄ bearing balls.     

Investigation of the Indentation‐Size Effect (ISE) in a Commercial SiAlON: Multifractal Scaling Analysis and Underlying Mechanisms

The indentation‐size effect (ISE) in a commercial SiAlON was investigated by conducting Knoop indentation between 0.1 and 20 kg. The resulting ISE was analyzed utilizing Meyer's Law, Proportional Specimen Resistance (PSR) model, and a Multifractal Scaling Law (MFSL). Meyer's law and the PSR model fits to the hardness–load data were not excellent. Further analysis based on the PSR model and MFSL revealed three piecewise linear fits corresponding to load regimes 0.1–0.3, 0.5–2, and 5–20 kg. Physical inference of MFSL fit parameters suggested that these three load regimes correspond to where indentation behavior is governed by deformation mechanisms limited to single grains, grain boundaries, and multiple grains, respectively. Independent of the ISE analysis results, comprehensive examination of indents by scanning electron microscopy revealed that changes in deformation mechanisms could also be grouped into these three load regimes. Corresponding changes in deformation mechanisms were microcleavage cracking, grain‐boundary cracking, and macrocracking, respectively. These observations are consistent with the findings of both the PSR model and MFSL with respect to the physical aspects of the governing mechanisms. It is concluded that these mechanisms are responsible for the observed ISE in this commercial SiAlON.     

Residual Stress Fields at the Surface of Sharp Pyramid Indentations

This paper describes an experimental measurement of the residual stress field around Berkovich and Knoop indentations on the surface of soda-lime glass. The isostress contours of both the residual tensile and compressive stress are presented. The results are compared with similar measurements around Vickers indentations on the same glass. The residual stress field around all three different shapes of indentations is clearly nonequal biaxial, with characteristic differences in the form of the stress contours that relate to each specific shape. The results for Berkovich indentation are compared with finite element (FEM) numerical calculations. Some general conclusions related to the sharp indentations are deduced from this study.     

Thermal Shock Resistance of Sintered Alumina/Silicon Carbide Nanocomposites Evaluated by Indentation Techniques

The thermal shock resistance of sintered Al₂O₃/1, 2.5, and 5 vol% SiC nanocomposites was studied using two indentation techniques. In the first technique, "indentation thermal shock" measurements were made of the extension of median/radial cracks around Vickers indentations after quenching from various temperatures (up to 480°C) into a bath of boiling water. This technique allowed a critical thermal shock temperature, Δ T _C^Ind, to be quantitatively evaluated. In the second technique, "indentation fatigue" tests were conducted on the thermally shocked specimens; repeated indentations were made at the same site, and the number of load cycles needed to initiate lateral fracture was measured. The results showed that nanocomposites with an addition of SiC nanophase as low as 1 vol% had a thermal shock resistance superior to that of pure Al₂O₃.     

Towards understanding the scratchability in functional glasses

The chemical stability and transparency of functional glasses make them a suitable candidate for interactive screen displays, optical grade lenses, etc. Such applications demand glasses with increased scratch resistance to maintain their original optical and mechanical properties. For decades, indentation hardness has been used as a parameter to judge the scratch resistance of glasses. Recent technological advancements in research have shed light on the fundamental difference between the indentation hardness and scratch hardness values. Further, it is also essential to understand the fundamental behavior of glasses under various environments such as abrasive and corrosive conditions. To this extent, this review aims to provide a comprehensive overview of the experimental approaches to quantify and understand the scratch resistance of glasses. Specifically, we introduce the basic differences between indentation and scratch experiments, and various scratch quantification models available in the literature for glasses. We also discuss, in detail, the scratch experiments that have been performed on glasses and the role of various environmental conditions. Overall, through this review article, we outline the understanding on scratch behavior of glasses thus far, thereby, shedding light on the open questions which can enable the design of improved scratch-resistant glasses.     

Nanoscale elastic-plastic deformation and stress distributions of the C plane of sapphire single crystal during nanoindentation

The nanoscale elastic-plastic characteristics of the C plane of sapphire single crystal were studied by ultra-low nanoindentation loads with a Berkovich indenter within the indentation depth less than 60 nm. The smaller the loading rate is, the greater the corresponding critical pop-in loads and the width of pop-in extension become. It is shown that hardness obviously exhibits the indentation size effect (ISE), which is 46.7 ± 15 GPa at the ISE region and is equal to 27.5 ± 2 GPa at the non-ISE region. The indentation modulus of the C plane decreases with increasing the indentation depth and equals 420.6 ± 20 GPa at the steady-state when the indentation depth exceeds 60 nm. Based on the Schmidt law, Hertzian contact theory and crystallography, the possibilities of activation of primary slip systems indented on the C surface and the distributions of critical resolved shear stresses on the slip plane were analyzed.     

Elastic Response of Thermal Spray Deposits under Indentation Tests

The elastic response behavior of thermal spray deposits at Knoop indentations has been investigated using indentation techniques. The ratio of hardness to elastic modulus, which is an important prerequisite for the evaluation of indentation fracture toughness, is determined by measuring the elastic recovery of the in-surface dimensions of Knoop indentations. The elastic moduli of thermal spray deposits are in the range of 12%-78% of the comparable bulk materials and reveal the anisotropic behavior of thermal spray deposits. A variety of thermal spray deposits has been examined, including Al₂O₃, yttria-stabilized ZrO₂(YSZ), and NiAl. Statistical tools have been used to evaluate the error estimates of the data.     

Mechanical characterization at nanometric scale for heterogeneous graphite-salt phase change materials with a statistical approach

A statistical indentation analysis of a series of phase change graphite-salt composite materials for latent heat thermal energy storage applications was investigated using instrumented indentation technique with the aim of isolate the mechanical influence of each phase. This method employs the instrumented indentation technique to extract the in situ hardness and Young's modulus properties of individual components without the need to observe the residual imprints for heterogeneous materials. This approach relies on a large array of imprints (around 1000 indentations performed at 200 nm of indentation depth) and the statistical analysis of the resulting data. A statistical study by a Cumulative Distribution Function fit and Gaussian simulated distributions showed that the mechanical properties for each compound can be isolated when the indentation depth is much lower than the size of the secondary phases.