Berkovich Indenter Tip Research Articles

Stress distribution variations during nanoindentation failure of hard coatings on silicon substrates

Regarding quality inspection of technologically important nanocomposite hard coatings based on Ti, B, Si, C, and N and bioceramics such as hydroxyapatite that are used in small-scale high-precision devices and bio-implants, it is essential to study the failure mechanisms associated with nanoindentation, such as fracture, delamination, and chipping. The stress imposed by the indenter can affect the fracture morphology and the interfacial fracture energy, depending on indenter shape, substrate type, crystallographic properties, pre-existing flaws, internal micro-cracks, and pre-strain. Reported here are finite-element-based fracture studies that provide insights into the different cracking mechanisms related to the aforementioned failure process, showing that the fracture morphology is affected by the interaction of different cracking events. The interfacial fracture energy, toughness, and residual stress are calculated using existing models with minor adjustments, and it is found that increasing the indenter sharpness improves the shear stress distribution, making the coating more prone to separation. Depending on the prevailing type of stress, the stress distribution beneath the depression results in either crack formation or a dislocation pile-up leading to strain hardening. Different forms of resistances resulting from the indentation process are found to affect the tip–sample conduction, and because of its stronger induced plasticity than that of a Berkovich indenter tip, a sharper cube-corner tip produces more resistance.

Effects of crystallographic orientation and strain‐hardening on nanoindentation response of cyclotetramethylene tetranitramine (‐HMX) single crystals

AbstractMechanical behavior of cyclotetramethylene tetranitramine (HMX) is complex and insufficiently understood, but it is imperative as it affects the initiation of the decomposition reaction though hot spots formation. We present a single‐crystal plasticity model for ‐HMX which accounts for pressure dependence of both the elastic constants and the yield surface. We evaluate the relative importance of pressure sensitivity of the elastic constants and plasticity, and of strain hardening, to the quasi‐static mechanical behavior of HMX, in general, and to indentation, in particular. Further, we study the effect of the in‐plane crystallographic orientations of the substrate with respect to the Berkovich indenter tip on the indentation force‐displacement curve. We conclude that pressure sensitivity of plasticity makes the largest contribution to defining the response to indentation, followed by strain‐hardening, sensitivity of which depends on hardening modulus, while the pressure sensitivity of elastic constants play a smaller role. Both the indentation plane normal orientation and the in‐plane indenter tip rotation may influence properties determined from nanoindentation test (e. g. hardness).

Face Turning of Single Crystal (111)Ge: Cutting Mechanics and Surface/Subsurface Characteristics

Abstract Single crystal Ge is a semiconductor that has broad applications, especially in manipulation of infrared light. Diamond machining enables the efficient production of surfaces with tolerances required by the optical industry. During machining of anisotropic single crystals, the cutting direction with respect to the in-plane lattice orientation plays a fundamental role in the final quality of the surface and subsurface. In this study, on-axis face turning experiments were performed on an undoped (111)Ge wafer to investigate the effects of crystal anisotropy and feedrate on the surface and subsurface conditions. Atomic force microscopy and scanning white light interferometry were used to characterize the presence of brittle fracture on the machined surfaces and to evaluate the resultant surface roughness. Raman spectroscopy was performed to evaluate the residual stresses and lattice disorder induced by the tool during machining. Nanoindentation with Berkovich and cube corner indenter tips was performed to evaluate elastic modulus, hardness, and fracture toughness of the machined surfaces and to study their variations with feedrate and cutting direction. Post-indentation studies of selected indentations were also performed to characterize the corresponding quasi-plasticity mechanisms. It was found that an increase of feedrate produced a rotation of the resultant force imparted by the tool indicating a shift from indentation-dominant to cutting-dominant behavior. Fracture increased with the feedrate and showed a higher propensity when the cutting direction belonged to the <112¯> family.

Crmnv and Crmntiv Heas Elastic Properties for Use in Beam Exit Windows for MW Irradiation Facilities

The choice of a material and the engineering design that is intended to lead to a beam exit window capable of sustaining mechanical and thermal loads in a high-radiation environment, while minimizing the beam loss and maximizing the window lifetime is discussed. Thin titanium windows have been typically used in industrial accelerators in the tens of kW beam power [1].The focus of this research is to conduct fundamental studies related to the choice of a material, from a material science point of view, for MW-class beam exit windows and an engineering design, including parameters such as the window size, thickness, and cooling scheme, suitable for industrial accelerators. CrMnV and CrMnTiV HEAs were selected for this study. According to Yang et al., [2], 1.1 and 6.6% should be expected as the criteria for forming high entropy stabilized solid solution phases. The two parameters of the CrMnV and CrMnTiV alloys are and respectively. Therefore, the two alloys are expected to be solid solution.The mechanical properties of CrMnV and CrMnTiV solid solution HEAs exit window prior to E-beam irradiation were measured using nanoindentation. The nanoindentation testing was carried out using XP Nanoindenter tester equipped with a three-sided diamond Berkovich indenter tip with maximum displacement of 2000 nm. The hardness is measured as 5.2 and 6.1 GPa for the bulk CrMnV and CrMnTiV solid solution HEAs at a depth of indentation of 900 nm respectively, Fig. 1. We also observe that both solid solution CrMnV and CrMnTiV HEAs depict indentation size effect. References Chmielewski A.G., Zimek, Z., Panta P., Drabik W., Radiat. Phys. Chem. 45 1029 (1995). Yang, X. and Y. Zhang, Prediction of high-entropy stabilized solid-solution in multi-component alloys. Materials Chemistry and Physics, 2012. 132(2-3): p. 233-238. Figure 1

Spatial Resolution Limit for Nanoindentation Mapping on Metallic Glasses.

Spatial heterogeneity, as a crucial structural feature, has been intensively studied in metallic glasses (MGs) using various techniques, including two-dimensional nanoindentation mapping. However, the limiting spatial resolution of nanoindentation mapping on MGs remains unexplored. In this study, a comprehensive study on four representative MGs using nanoindentation mapping with a Berkovich indenter was carried out by considering the influence of a normalized indentation spacing d/h (indentation spacing/maximum indentation depth). It appeared to have no significant correlation with the measured hardness and elastic modulus when d/h > 10. The hardness and elastic modulus started to increase slightly (up to ~5%) when d/h < 10 and further started to decrease obviously when d/h < 5. The mechanism behind these phenomena was discussed based on a morphology analysis of residual indents using scanning electron microscopy and atomic force microscopy. It was found that the highest spatial resolution of ~200 nm could be achieved with d/h = 10 using a typical Berkovich indenter for nanoindentation mapping on MGs, which was roughly ten times the curvature radius of the Berkovich indenter tip (not an ideal triangular pyramid) used in this study. These results help to promote the heterogeneity studies of MGs using nanoindentation that are capable of covering a wide range of length scales with reliable and consistent results.

Deformation behaviour of ion-irradiated FeCr: A nanoindentation study

Understanding the mechanisms of plasticity in structural steels is essential for the operation of next-generation fusion reactors. This work on the deformation behaviour of FeCr, focusses on distinguishing the nucleation of dislocations to initiate plasticity, from their propagation through the material. Fe3Cr, Fe5Cr, and Fe10Cr were irradiated with 20 MeV Fe3+ ions at room temperature to doses of 0.008 dpa and 0.08 dpa. Nanoindentation was then carried out with Berkovich and spherical indenter tips. Our results show that the nucleation of dislocations is mainly from pre-existing sources, which are not significantly affected by the presence of irradiation defects or Cr%. Yield strength, an indicator of dislocation mobility, increases with irradiation damage and Cr content, while work hardening capacity decreases mainly due to irradiation defects. The synergistic effects of Cr and irradiation damage in FeCr appear to be more important for the propagation of dislocations than for their nucleation.Graphical abstract

Investigation of the mechanical properties of Mn15Si26 via EBSD-nanoindentation coupling and ab-initio calculation

The goal of this study is to investigate the mechanical and elastic characteristics of the Mn15Si26 compound via experimental nanoindentation measurements and ab-initio calculations. The mechanical properties such as Young’s modulus (E) and nanohardness are important inputs for improving the design and mechanical reliability of thermoelectric modules. The high-energy X-ray diffraction pattern of Mn15Si26 has been indexed with the Miller indices of a tetragonal crystalline structure whose cell parameters are the following: a = b = 5.535(3) Å and c = 65.552(4) Å. Nanoindentation measurements, with a Berkovich indenter tip have been performed on higher manganese silicide (HMS) compound mainly composed of Mn15Si26 grains. For the first time ever, it has been evidenced that both elastic modulus and nanohardness of the latter varied significantly depending on their crystallographic orientations provided by electron backscatter diffraction. Nanohardness and Young’s modulus along the< 001 > orientations are higher than the< 100 > ones. The nanohardness value of Mn15Si26 ranges from 16 GPa to 20 GPa and the Young’s modulus measured varies between 234 GPa and 300 GPa. The stiffness tensor (Sij= (Cij)−1) of Mn15Si26 has been deduced from these experimental measurements as well as calculated using Ab-initio calculations. The macroscopic elastic modulus (E, G, Β) and Poisson's coefficient have been examined and discussed and their 3D-representation has been plotted. The mechanical anisotropy hereby evidenced as the existence of anisotropy of the thermoelectric properties could be a significant factor for the mechanical reliability of thermoelectric modules which consisted of Mn15Si26 legs with a possible preferred crystallographic orientation induced during their fabrication.

Morphological characterization of semi-crystalline POM using nanoindentation

Nanoindentation (NI) is a contact method to investigate localized micromechanical properties of materials, whereby NI of semi-crystalline polymers is challenging. The influence of morphological structures such as spherulites or crystal-lamellae on localized NI depth-force behavior is discussed controversially in literature. Hence, the main objective of this study is to determine the influence of crystalline zones on NI results. Polyoxymethylene (POM) exhibits high crystallinity with the spherulitic structure on the micrometer scale and was therefore chosen to proof the influence of spherulite distribution on NI results concerning modulus. Furthermore, the correspondence between the mean elastic modulus from different NI experiments and macroscopic compression tests will be demonstrated. A POM tensile bar was investigated by NI with a large sphero-conical and a Berkovich indenter tip at different positions of its cross-section. Here, it was found that regions at the edge of the sample have a lower elastic modulus than regions in the middle of the cross-section. This agrees well with polarized light microscopy results, which reveal a skin layer with less crystallinity close to the sample edge. Therefore, the NI measurements in this edge zone result in a lower elastic modulus compared to the more crystalline middle of the cross-section. In summary, semi-crystallinity influences the NI results obtained for POM and the mean of the elastic modulus distribution over the cross-section of the POM sample is in good agreement with macroscopic compression test results.

Mechanical characterization of boron carbide single crystals

AbstractOut‐of‐plane anisotropy in the mechanical response of boron carbide was studied by performing nanoindentation experiments on four specific crystallographic orientations of single crystals, that is, , , , and . For each orientation of the single crystals, in‐plane variations of indentation modulus and hardness were also studied by monitoring the relative rotation between the crystal surface and a Berkovich indenter tip. A significant out‐of‐plane anisotropy in indentation modulus was observed with ~80 GPa difference between the highest and lowest values. A smaller but measurable out‐of‐plane anisotropy in indentation hardness was also observed. In‐plane anisotropy, on the other hand, was found to be significantly influenced by the scatter in the data and geometrical imperfections of the indenter tip. Investigations of indentation pop‐in events suggested that deformation is entirely elastic prior to the first pop‐in. Furthermore, quasi‐plastic flow along the orientation of the single crystals was found to be more homogeneous than the other tested orientations. For select indents, cross‐sectional transmission electron microscopy (TEM) of the indented regions showed formation of a quasi‐plastic zone in the form of lattice rotation and various microstructural defects. The quasi‐plastic zone grew in size with increasing the indentation depth. The TEM observations also suggested the crystal slip to be a potential mechanism of quasi‐plasticity and a precursor for formation of amorphous bands that could eventually lead to cracking and fragmentation. The proposed failure mechanism provides valuable insights for calibrating constitutive computational models of failure in boron carbide.

Alteration in micro-mechanical characteristics of wellbore cement fracture surfaces due to fluid exposure

The stiffness of wellbore cement fracture surfaces was measured after exposing to the advective flow of nitrogen, silicone oil, and medium sweet dead crude oil for different exposure periods. The test specimens were extracted from fractured cement cylinders, where the cement fracture surfaces were exposed to the different fluids up to 15 weeks. A nanoindenter with a Berkovich indenter tip was used to measure load-indentation depth data, which was used to extract the elastic modulus (E) and nano-hardness (H) of the cement fracture surfaces. A reduction in the elastic modulus compared with an unexposed specimen were observed in all the specimens. Both elastic modulus and nano-hardness for the specimens exposed to silicone oil were lower than specimens exposed to nitrogen gas and varied with the period of exposure. The elastic modulus and nano-hardness of the specimens exposed to crude oil were the lowest with a significant decrement with the exposure period. The frequency distribution of the nanoindentation measurements shows that the volume-fraction ratio of the two types of cement hydrated nanocomposites for both the unexposed and test specimens is about 70:30%. Phase transformation beneath the indenter is observed for all of the specimens, with more obvious plastic deformation in specimens exposed to crude oil. Analytical measurements (SEM, EDS, FT-IR, and XRD) on exposed cement fracture surfaces reveal different levels of physical and chemical alteration that are consistent with the reduction in stiffness measured by nanoindentation. The study suggests that cement stiffness will decrease due to crude oil exposure, and the fracture will be sensitive to stress and pore pressure with time.

Phase Transformation in Germanium-on-Insulator (GeOI) Films Using Raman Spectroscopy and Nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the SMART CUT wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the published SOI literature and bulk germanium results obtained by Liu et al., 2020 [1] and SOI films results obtained by Miller et al., 2007 [2]. From our measured hardness results of 8 to 10 GPa for very thin single crystal Ge(100) films compared to 13.1 GPa for Bulk Ge (100), one can conclude that the thinning of crystalline Ge films leads to a considerable softening of the crystalline Ge resulting in less brittle robust films. Similarly, our measured modulus of 130 to 140 GPa for thin Ge (100) films compared to the reported bulk Ge (100) value of 157 GPa corroborates these research findings. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicate bands of metastable Ge phases @ 220 cm-1, 195 cm-1 and 184 cm-1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm-1, 399 cm-1 and 430 cm-1.Apart from the well documented electrical advantages of GeOI, the reported research on the nanomechanical properties reveals considerable softening and flexibility of the very thin exfoliated single crystal Ge films that promise added benefits and further implications for enhancing potential device application of GeOI wafers for process integration.1.] N. Liu, X-J Yang, Z. Yu, and L. Zhao, Trans. Nonferrous Met. Soc. China, 30, 181 (2020).2.] N. Miller, K. Tapily, H. Baumgart, G.K. Celler, F. Brunier, and A.A. Elmustafa, Mater. Res. Soc. Symp. Proc. Vol. 1021 © 2007 Materials Research Society, 1021-HH05-24. Boston, Massachusetts. (2007).

Assessment of small-scale deformation characteristics and stress-strain behavior of NiTi based shape memory alloy using nanoindentation

The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated.

Nanomechanical Characterization for Cold Spray: From Feedstock to Consolidated Material Properties

Cold gas-dynamic spray is a solid-state materials consolidation technology that has experienced successful adoption within the coatings, remanufacturing and repair sectors of the advanced manufacturing community. As of late, cold spray has also emerged as a high deposition rate metal additive manufacturing method for structural and nonstructural applications. As cold spray enjoys wider recognition and adoption, the demand for versatile, high-throughput and significant methods of particulate feedstock as well consolidated materials characterization has also become more notable. In order to address the interest for such an instrument, nanoindentation is presented herein as a viable means of achieving the desired mechanical characterization abilities. In this work, conventionally static nanoindentation testing using both Berkovich and spherical indenter tips, as well as nanoindentation using the continuous stiffness measurement mode of testing, will be applied to a range of powder-based feedstocks and cold sprayed materials.

Evaluation of the mechanical properties of germanium-on-insulator (GeOI) films by Raman spectroscopy and nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the Smart Cut™ wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the literature published Silicon-on-Insulator and bulk germanium results. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicates bands of metastable Ge phases @ 220 cm−1, 195 cm−1, and 184 cm−1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm−1, 399 cm−1, and 430 cm−1.

Assessment of Small-Scale Deformation Characteristics and Stress-Strain Behavior of NiTi Based Shape Memory Alloy Using Nanoindentation

The study is focused on appreciating the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress-induced-martensitic-transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is appreciated.

On the uniqueness of intrinsic viscoelastic properties of materials extracted from nanoindentation using FEMU

Instrumented nanoindentation is widely used to extract the material properties from the measured force-displacement curves. In this work, the uniqueness/non-uniqueness of the intrinsic viscoelastic properties of materials determined by nanoindentation during load-unload tests is investigated. A four-parameter viscoelastic law with constant Poisson's ratio is used to model the mechanical behavior of a polymer material and a 2D-axisymmetric Finite Element Model (FEM) is used to simulate the nanoindentation test. Firstly, a nanoindentation experimental triangular load-unload test is performed on a bulk sample of polypropylene (PP) with a Berkovich indenter tip at a depth rate of 1000 nm/min. The values of the four material parameters are estimated by the Finite Element Model Updating (FEMU). The numerical results can accurately fit the experimental data. However, several quasi-solutions are shown to exist. These load-unload data allow to identify only three viscoelastic parameters if the Poisson's ratio is known. Secondly, the effect of nanoindentation depth rate, loading type (triangular, trapezoidal, exponential, sinusoidal) and apex angle is numerically investigated using an identifiability index based on the conditioning of the inverse problem. We show a correlation between the identifiability index and the energy dissipated by the material during the tests. The extraction of all material parameters remains impossible using a single test. Finally, some combinations of several nanoindentation triangular tests and indenter tip angles are also investigated. We show that a dual nanoindentation technique (cube corner and Berkovich tips) with triangular load-unload tests is an interesting combination to reliably extract all the viscoelastic parameters, provided that plasticity is taken into account. This result illustrates the interest of using this numerical identifiability index to design nanoindentation experiments to ensure the robustness of the intrinsic viscoelastic properties extraction.

Roughness of Deposited Carbon-Based Coatings and Its Statistical Characteristics at Nano and Microscales

Topography of surfaces may influence many processes in tribology including friction and adhesion. Its influence is usually taken into account in various statistical models of rough surfaces. Most of these models are based on an explicit or implicit assumption of normality of the asperity heights or similar assumptions that involve Gaussian distributions. Recently it has been shown that the height distribution of surfaces prepared by grinding are not Gaussian at both nano and micro-scales, while topography of epoxy resin replicas of polishing papers having nominal asperity sizes up to several micrometers, was Gaussian. Here we study roughness of carbon-based coatings deposited by direct current pulsed magnetron sputtering with and without substrate bias voltage at micro and nano-scale. Hardness measured using a Berkovich indenter tip gave 43 (biased) and 14 (non-biased) GPa, respectively. First the heights of the nano-asperities were determined by AFM (Atomic Force Microscopy). Then the heights of the micro-asperities were measured by a profilometer (a stylus). Finally the same regions measured by stylus were again studied by AFM. Standard statistical parameters of surfaces are determined at each scale. It has been also shown that the stylus measurements did not cause plastic deformations of the harder (biased) sample because the distributions of heights at nano-scale were the same. Using the experimental data obtained, the assumption of the normal distribution for the roughness heights has been studied by application of various modern tests of normality. Measurements of surfaces by stylus and by AFM with the 117 nm steps showed that the surfaces satisfy the assumption of normality of the heights. However, further studies with the 10 nm AFM steps showed that the roughness of the non-biased sample is not normal. Hence, the applicability of the standard statistical models of adhesive contact between rough solids to the non-biased sample may be questionable.

(Invited) Mechanisms of Stress Relaxation and Intensification at the Lithium/Solid Electrolyte Interface

Under electrochemical cycling, stress intensification and relaxation within small volumes at the lithium/solid-state electrolyte (SSE) interface are thought to be critical factors contributing to mechanical failure of the SSE and subsequent short-circuiting of the device. Recent hypotheses independently proposed by Porz et al.,[1] Herbert et al.[2,3] and Wang et al.[4] consider the relationship between stress intensification and local variations in the current density. These arguments are examined in light of nanoindentation studies performed in high-purity vapor deposited lithium films at indentation depths less than 1 μm.[2, 3] These experimental studies examine how lithium responds to stress and stress gradients at length scales and strain rates relevant to the observed SSE failure. Applying the findings of the nanoindentation studies, we present a mathematically tractable and transparent model that relates a two-dimensional state of stress at the lithium/SSE interface to the average current density, the length scale of an interfacial defect and lithium diffusional creep and/or dislocation plasticity stress relaxation mechanisms. Furthermore, the approach presents a common framework to compare and contrast the most recent hypotheses of SSE stress development.[1-4] Accordingly, the model addresses interfacial defects in the form of local deviations from a flat interface (non-planar) and local variations in the overpotential and/or exchange current density (planar interface). Through this simple elastic perturbation model, we describe a defect length scale that is too large for effective Nabarro-Herring diffusional creep relaxation (diffusion length is too long), but too small for efficient dislocation multiplication (insufficient room to physically operate the source), suggesting the possibility of large, deleterious stresses at the lithium/SSE interface. In this instance, the properties of the SSE may become critical in relieving pressure and mitigating the stress intensification enabling mode I or mixed-mode fracture of the SSE. Such concerns have motivated a nanoindentation creep study of Lipon performed with a diamond Berkovich indenter tip. The indentation study shows that Lipon does have a stress relaxation process with a stress exponent, n, of approximately 20, which is found to be effectively independent of temperature and length scale over the respective ranges of 25 to 60 °C and 400 to 600 nm of indentation depth. The uncommonly high magnitude of n suggests molecular units of Lipon (3 Li+ and a unit of PO4 -3) are capable of local rearrangement by short-range diffusion and, thereby, potentially capable of contributing directly to stress relief at the interface. This suggests Lipon may be less susceptible to failure by fracture. [1] L. Porz, T. Swamy, B.W. Sheldon, D. Rettenwander, T. Frömling, H.L. Thaman, S. Berendts, R. Uecker, W.C. Carter, Y.-M. Chiang, Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes, Advanced Energy Materials 7(20) (2017). [2] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow, J. Mater. Res. 33(10) (2018) 1347-1360. [3] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow, J. Mater. Res. 33(10) (2018) 1361-1368. [4] M. Wang, J. Wolfenstine, J. Sakamoto, Temperature dependent flux balance of the Li/Li7La3Zr2O12 interface, Electrochimica Acta 296 (2019) 842-847.

Enhancing adhesion strength of a-C:H:Cu composite coatings on Ti6Al4V by graded copper deposition in a rf-PVD/PECVD hybrid process

In the medical field, poor coating adhesion often results in the limitation of the application of coating systems on implant devices. This limitation indicates a need for the development of improved techniques and methods to overcome this issue. In this paper, the use of graded copper deposition to enhance the adhesion strength of a-C:H:Cu composite coatings for biomedical applications grown by a rf-PVD/PECVD hybrid process is examined. In situ OES measurements give deeper insight in the amount of Cu(I) ions in the process plasma during deposition process which is used to control XCu in the a-C:H:Cu coatings cross section. With this knowledge, the critical load of a-C:H:Cu coatings can be enhanced from 50 mN to >300 mN depending on overall XCu. The influence of substrate bias and Cu mole fraction (XCu = (4 … 84) %) on the mechanical and structural properties of hydrogenated amorphous carbon films (a-C:H) is investigated. Polished Ti6Al4V and Si were used as substrate materials. The coating's microstructure was determined by STEM imaging and X-ray diffraction, while mechanical and adhesion properties were examined by nanoindentation and nano scratch tests using a Berkovich indenter tip. The nanocomposite coatings consist of Cu nanoparticles embedded in an a-C:H matrix. Their morphology changes to a columnar structure with Cu nanorods for high XCu when applying a substrate bias voltage of −50 V. For Cu free a-C:H coatings, a maximum hardness of 13.6 GPa was measured which is lowered down to 4 GPa for a-C:H:Cu coatings with XCu = 84%. A significant strengthening of the coatings is observed when applying a subsequent substrate bias voltage of −50 V, which further enhances scratch resistance.

Estimating fracture toughness of C–S–H using nanoindentation and the extended finite element method

In this paper, an approach to integrating nanoindentation testing and finite element simulations is introduced to compute the fracture toughness of cementitious materials. Calcium silicate hydrate (C–S–H) was synthesized using the standard procedure of mixing calcium oxide (CaO) and silicate (SiO2) at a mixture ratio of 1.5. C–S–H powder was filtered, dried to a relative humidity of 11%, and then compacted at 400 MPa. Nanoindentation tests incorporating dwell time were performed on polished C–S–H specimens using a Berkovich indenter tip. The reduced elastic moduli of the C–S–H specimens were extracted from the nanoindentation measurements. Viscoelastic and viscoelastic-plastic finite element models with creep and cracking capabilities were developed to simulate the nanoindentation tests and to extract the fracture energy. The viscoelastic-plastic model utilized the extended finite element method (XFEM) to describe cracking and evaluate the cracking surface of C–S–H. The analysis showed that the proposed approach could fairly predict the fracture energy release rate and thus fracture toughness of C–S–H. The calculated fracture toughness was in agreement with the fracture toughness values reported in the literature.