Defects Population Research Articles

Study of defects population and contaminations in silica/tantala coated mirrors

This article presents a study on the performance degradation of amorphous silica mirrors coated with silica/tantala, commonly utilized in gravitational wave interferometry measurements. The primary objective is to understand the factors contributing to the reduced performance, specifically the heightened noise observed after prolonged use of the mirrors in a vacuum. In this article, SiO2/Ta2O5 multilayered coated sample mirrors underwent analysis through x-ray photoemission spectro-microscopy, atomic force microscopy, and Raman spectroscopy, before and after exposure to a contaminating vacuum environment. Our findings revealed the possible correlation between the deterioration in mirror performance and an upsurge in contaminant carbon and defect populations within the silicon dioxide structure.

Selective Generation of Luminescent Defects in Hexagonal Boron Nitride

AbstractSingle photon emitters from atomic defects in crystals like hexagonal boron nitride (hBN) are vital for quantum technologies. Although various techniques are devised to obtain defects emission in hBN, simultaneous control over defects position, type, and emission spectrum has not been achieved yet. Here, ion implantation with 12C, 20Ne, and 69Ga are used to create a composite defects population with emission ≈820 nm. The correlation of Raman and photoluminescence (PL) spectroscopy helps to identify the defects’ type. After selecting Ga as the ion species yielding the maximum emitter brightness, a strategy based on thermal annealing is developed to modify the composition of the induced defects. This results in an emitter ensemble with selected spectral properties, even when starting from different implantation conditions. Specifically, thermal annealing induces a defect transmutation from one type to another, shifting the emission wavelength from 820 to 625 nm. Moreover, sample patterning is combined with focused ion beam implantation and subsequent annealing in an efficient method to deterministically set the defects position as well as the PL spectral composition. These results offer a practical avenue to achieve in situ positioning and tuning of ensembles of emitters in hBN, promising for quantum information and sensing applications.

Effect of re-lasing on the fatigue properties of 316L Stainless Steel Produced by laser powder bed fusion

Despite the great success of the metal additive manufacturing process by Laser Fusion on Powder Bed (L-PBF), this technique still lacks maturity in several areas. Among its major challenges, the low fatigue strength of the L-PBF parts is caused, among other features, by two main types of defects. Firstly, the L-PBF process generates a high surface roughness, mainly linked to the phenomenon of partial powder melting on the part surface. Secondly, internal defects are created during the building process and are randomly distributed in the material. The present study investigates the effect of a second lasing on the improvements of the surface roughness, defects population as well as fatigue properties of L-PBF parts. Based on preliminary experiments, two re-lasing strategies were selected. A high cycle fatigue campaign was subsequently carried out with a stress ratio of R= -1 on samples with or without re-lasing, in the as-built state or after polishing. Specimens printed in Net-Shape showed even after re-lasing and polishing, a much lower fatigue strength than those machined in the bulk after additive manufacturing. In addition, results show that the tested re-lasing conditions has a beneficial effect on the quasi-static and fatigue strength of the 316L obtained by L-PBF.

Effect of Surface Modification on Structural and Thermal Properties of Nanocarbons of Different Dimensionalities

Multi-walled carbon nanotubes and graphite nanoplatelets were functionalized via acid treatment to overcome the problem of agglomeration. Fourier transform infrared spectroscopy showed the chemical modification of the nanocarbons while the general relationship between the chemical treatment and the defects population was analyzed by Raman spectroscopy. The information regarding the mass loss and impurities is obtained from the thermogravimetric analysis. X-ray diffraction showed the effect of acid treatment on the physical states of the nanocarbons including the crystalline texture. The comparative high interlayer distance in graphite suggested that graphite particles are exfoliated into sheets of graphene by this technique with smaller particle sizes. The thermogravimetric analysis confirmed the complete removal of impurities in the case of multi-walled carbon nanotubes (MWCNTs) and about 20 % of impurities as seen in oxidized graphite attributable to the presence of residual manganese that might have been introduced during the functionalization process. Moreover, the thermal stability was also observed well in the case of MWCNTs with lesser impurities left. Overall, two different nanocarbons with well-structured chemical modifications were obtained with a variation in the feasibility of functionalization.

Dynamic and kinetic properties of point defects in γ U-10Mo: a molecular dynamics study

Controlled defects population and their movement constitute the foundation for describing micro-structural evolution in any material systems for nuclear applications. Molecular dynamics (MD) simulations were performed to study temperature (1200–1400 K) dependent displacement cascades with a U primary knock on atom (pka) carrying 2 keV kinetic energy in γ U-10Mo. We observed a strong impact of the temperature on the population of point defects. The impact of kinetic energy of the pka on the number of survived and maximum point defects is addressed carefully where an increase in the number of defects led to an increase in the energy. Nudged elastic band (NEB) calculations were involved to calculate the activation energies of self-interstitial Mo for different intra and inter-planar transitions. The transitions between 〈011〉 to 〈011〉, 〈001〉 to 〈011〉, 〈001〉 to 〈111〉, 〈111〉 to 〈001〉 and 〈111〉 to 〈110〉 Mo-Mo dumbbell configurations were explored. The impact of applied strains on the activation energy in each case of the transitions was investigated where irregular behavior in the energy with rise in the strain was observed.

Manipulating hot carrier behavior of MAPbBr3 nanocrystal by photon flux and temperature

The temperature-dependent time-resolved spectroscopies are employed to investigate the hot carrier characteristics in MAPbBr3 nanocrystals (NCs). Simultaneously, the absorption and photoluminescence data exhibits that MAPbBr3 NCs pass through cubic phase and tetragonal phase in turn as the temperature drops. As the phase transition from the cubic phase to the tetragonal phase, the band gap narrows, the shallow defects population that participates in the carrier dynamics and the Longitudinal-optical phonon energy (ELO) decreases. We uncovered that the hot carrier can be induced in the MAPbBr3 NCs through changing the photon flux and the temperature. Moreover, the hot carrier cooling time and the carrier heating effect in tetragonal phase improved in comparison with those in cubic phase, owing to the influence of bandgap, shallow defect and ELO as mentioned above. These results can help us to further understand the hot carrier characteristics in the perovskite MAPbBr3 NCs for designing and optimizing the perovskite-based optoelectronic devices of high performance.

Ductile damage of AA2024-T3 under shear loading: Mechanism analysis through in-situ laminography

The mechanisms leading to fracture of aluminum alloy AA2024-T3 under shear loading are investigated via X-ray synchrotron laminography. A 1mm-thick flat double-gage section shear specimen is loaded inside a synchrotron X-ray beamline. The microscale defects population is reconstructed on six loading steps as well as on the broken specimen. The material exhibits an initial void volume fraction of 0.7% as well as a high concentration of large Cu-rich intermetallic particles. Using 2D Digital Image Correlation of projected volume data, based on the void contrast, it is possible to track the evolution of both types of mesoscale defects throughout the loading and to correlate the deformation mechanisms with the local strains. Upon mechanical loading, pre-existing voids rotate and elongate following the deformation of the aluminum matrix. The intermetallic particles fail at early loading stages in a brittle manner, leading to the nucleation of voids normal to the maximum principal stress direction. The newly-created voids continue to grow during the subsequent loading steps impeded by the fragmented particles, eventually forming micro-cracks. A fracture mechanism is proposed based on these observations and assessed with a representative volume element simulation, pointing towards the formation of a shear localization band during the final loading step.

Irradiation-assisted stress corrosion cracking susceptibility and mechanical properties related to irradiation-induced microstructures of 304L austenitic stainless steel

304L SSs are used for structural components of Light Water Reactor (LWR) nuclear power plants and have been shown to face Irradiation Assisted Stress Corrosion Cracking (IASCC). This degradation phenomenon has been of great concern regarding structural integrity of reactors for lifetime extension of LWRs but is still not completely understood. This study evaluates the evolution of mechanical properties (hardening) and susceptibility to stress corrosion cracking regarding different radiation induced microstructures. These microstructures were chosen to explore representative conditions encountered in Pressure Water Reactor (PWRs) conditions but also the effect of various populations of cavities as extreme case of those reported at higher flux and temperature. Irradiation microstructures are produced by heavy ions irradiation under two different temperatures (450 °C and 600 °C) and with or without helium implantation. The evolution of microstructure, hardness and cracking susceptibility are characterized after irradiation. Calculations based on defects population are found to be in agreement with hardening measurement for irradiations with no helium implantation only. Cracking susceptibilities are characterized after slow strain rate tests performed at 4% plastic strain under PWR environment. Cracking susceptibility changes for different irradiations and quantitative assessment of the effect of irradiation microstructure on IASCC phenomenon are discussed.

Effects of processing parameters on 3D structural ordering and optical properties of inverse opal photonic crystals produced by atomic layer deposition

AbstractVertical convective self‐assembly has been extensively used for the preparation of direct photonic crystals, which can be later infiltrated with a more stable material, such as oxide ceramics, by atomic layer deposition. However, the relationship between the self‐assembly parameters of the direct photonic crystals and the optical properties of the inverse opal photonic crystals remains elusive. In this work, the effect of different experimental parameters on the 3D structure and the density of defects of polystyrene direct photonic crystals produced by vertical convective self‐assembly was assessed. Self‐assembly was investigated using deionized water as media with polymer particles’ concentrations up to 2 mg/mL; temperatures of 40, 50, and 80°C; and relative humidity of 45%, 70%, and 90%. The 3D structure of the resultant direct photonic materials was characterized by the combination of scanning electron microscopy and image analysis, and their optical properties were assessed by reflectance measurements. These results were correlated with the performance of oxide‐based inverse opal photonic crystals produced by the controlled infiltration of the former direct photonic crystals by atomic layer deposition (ALD). It was found that the thickness increased with the concentration of polystyrene particles, while the photonic structure ordering is dependent on the synergy between humidity and temperature. Results also showed higher defects population with increasing evaporation temperature and decreasing relative humidity.

Isothermal fatigue damage mechanisms at ambient and elevated temperature of a cast Al-Si-Cu aluminium alloy

Cast aluminium alloys have a key position as structural materials for the automotive industry combining good mechanical performance, excellent castability and thermal conductivity, making them a suitable choice for engine parts such as cylinder heads. In this paper, the low-cycle fatigue behaviour of a cast aluminium alloys is investigated at ambient and elevated temperatures by the use of in situ optical surface crack monitoring, secondary electron microscopy and X-ray tomography which are combined to study the fatigue mechanisms. The observations highlight the effect of defect distribution and temperature on the crack propagation path and in particular the change of the role of eutectic regions as the temperature increases. In addition, based on the defects population analyses by tomography, a fatigue crack initiation model is proposed to consider both temperature and defect effects on the total low-cycle fatigue life and fatigue crack growth rates.

Low-level laser therapy as an adjunct to connective tissue graft procedure in the treatment of gingival recession defects: A systematic review and meta-analysis.

The aim of this study was to systematically evaluate the effectiveness of low level laser therapy (LLLT) as an adjunct to connective tissue graft (CTG) procedure for the treatment of gingival recession (GR). The addressed PICO question was; "In patients with Miller Class I or II recession defects (Population), what is the effect of LLLT as adjunct to CTG (Intervention) in comparison to CTG alone (Comparison) on gingival recession depth (Outcome)" Electronic databases were searched up to December 2017. Primary outcomes included gingival recession depth (GRD), whereas secondary outcomes were width of keratinized tissue (WKT) and relative clinical attachment level gain (RCAL). The weighted mean differences (WMD) of outcomes and 95% confidence intervals (CI) for each variable were calculated using random effect model. Four randomized clinical studies were included. Two studies showed significantly greater improvements with LLLT whereas, 2 studies showed comparable outcomes between LLLT and CTG group. Considering the effects of adjunctive LLLT as compared to CTG, a high degree of heterogeneity for GRD (Q value = 9.40, P = .02, I2 =68.11%) and WKT ((Q value = 16.04, P = .001, I2 =81.31%) was noticed among both the groups. Meta-analysis showed a statistically significant GRD (WMD= -0.61, 95% CI= -1.23 to 0.004, P = .05) for LLLT + CTG treatment versus CTG alone only. LLLT improves clinical and patient-centered outcomes of CTG procedures for the treatment of GR remains debatable. However, due to the small number of included studies and high heterogeneity in the laser parameters, precautions must be exercised when interpreting the results of the present systematic review. Gingival recessions in dentistry are of major esthetic concern. Minimal gingival recessions can be treated by flap operations, but the predictability and stability of the outcomes is debatable. In the present review, low level laser therapy (LLLT) adjunct to connective tissue graft (CTG) depicted a significant improvement in the predictability and stability of root coverage outcomes compared with CTG alone.

Fragmentation of an advanced ceramic under ballistic impact: Mechanisms and microstructure

In this paper, the impact-induced fragmentation of a commercially available hot-pressed boron carbide is explored. Fragmentation has been noted previously by many authors to be important in the impact performance of advanced ceramics, and so this paper seeks to provide some of the first near-complete and detailed measurements of individual fragment size and shape distributions available in the literature. Fragment size and shapes are quantified using methods developed in previous papers by the authors, and results reveal that two distinct fragmentation mechanisms exist as a consequence of the impact failure of boron carbide: one mechanism that creates small fragments that is associated with the coalescence of fractures originating from carbonaceous defects in the material, and one that creates larger fragments that is associated with structural failure (e.g., radial and circumferential cracking). While these mechanisms are similar to those noted for uniaxial compressive failure, results presented here highlight the importance of fragment shape as a consequence of impact failure. Namely, results indicate that both blocky and shard fragments are formed during impact into a boron carbide plate. Blocky and shard fragment types span across both the small and large fragmentation mechanisms. Using Scanning Electron Microscopy, blocky fragments were found to be associated with the predominant growth of cracks parallel to the impact direction, while shard fragments contain fracture surfaces that are associated with crack growth and coalescence in a direction perpendicular to the impact direction. The shards are, thus, believed to be a consequence of structural bending. No amorphous features were found on any blocky or shard fragments observed in this study (determined using Raman Spectroscopy), suggesting brittle fracture may be the dominant mechanisms that creates the shard fragments. Altogether, the implications of these results is that one can control fragment size and shape by controlling the carbonaceous defects population in boron carbide. This should help in the design of next-generation advanced ceramics for personal protection.

Steps roughening in thermal relaxation and low-coverage growth of sloped Pt(110) and Ir(110) surfaces: A numerical study

The dynamical roughening of [001] steps on sloped Pt (110) and Ir (110) surfaces is investigated by kinetic Monte Carlo simulations. Our model includes deposition, diffusion and fully reversible aggregation on these surfaces with both anisotropic barriers and anisotropic attachment. The barriers for the diffusion processes have been calculated by means of classical molecular dynamics simulations where both metals are modeled by realistic many-body potentials. The roughness is evaluated through calculations of the step width in thermal relaxation of the surface and low-coverage growth conditions. Results indicated a non-trivial behavior of the width in time during relaxation. In growth, power-law behavior is recovered for both metal surfaces. Defects population on terraces is investigated through calculations of adatom and island densities. It is found that at very low temperature (T = 200K for Pt and 400K for Ir and below), a power-law behavior with the growth time is got. Beyond, fluctuations in generated data become important and do not allow to correctly access the true trend of both quantities. Their behavior with the diffusion length at low temperature is singled out.

Patterning and pattern selection in a surface layer: Feedback between point defects population and surface layer temperature variations

Abstract We study dynamics of pattern formation in a prototype system of nonequilibrium point defects in thin foils under sustained nonequilibrium conditions. A reaction–diffusion model describing spatio-temporal behaviour of both vacancy population and local temperature of a surface layer is used. It is shown that pattern selection processes caused by coupling between defect population and local temperature of a surface are realized. Associated oscillatory dynamics of main statistical moments of both vacancy concentration field and surface layer temperature is analysed in detail. It is found that during the system evolution spatial distribution of local temperature variations of the surface layer relates to vacancy population distribution. It is shown that the mean size of vacancy clusters (from 30 nm up to 300 nm) evolves in oscillatory manner due to pattern selection processes. Morphology of defect complexes can be controlled by defects generation rate.

Geometrical Size Effect in High Cycle Fatigue Strength of Heavy-walled Ductile Cast Iron GJS400: Weakest Link vs Defect-based Approach

Fatigue strength is known to decrease with increasing dimension of the component. This is due to a technological size effect, related to the production process, and to a geometrical size effect, due to a higher probability of finding a large defect. To investigate the latter, an heavy-walled component made of Ductile Cast Iron (DCI) has been trepanned and a fatigue test plan has been carried out using 4 different specimen geometries. An attempt has been made to relate the resulting fatigue strength using a weakest-link approach based on the effective volumes and surfaces. This approach seems to work well only in cases of different specimen's lengths. Some of the fracture surfaces were analyzed by means of SEM and the initiating defects were identified and measured. An approach in which the defects population can be randomly distributed in the specimen has been tried. Virtual fatigue tests have been carried out by considering pure propagation of the worst defect. The resulting fatigue curves showed that this approach is promising but needs further description of the initiation phase.

Geometrical size effect in high cycle fatigue strength of heavy-walled Ductile Cast Iron GJS400: Weakest link vs. defect-based approach

Fatigue strength is known to decrease with increasing dimension of the component. This is due to a technological size effect, related to the production process, and to a geometrical size effect, due to a higher probability of finding a large defect. To investigate the latter, an heavy-walled component made of Ductile Cast Iron (DCI) has been trepanned and a fatigue test plan has been carried out using 4 different specimen geometries. An attempt has been made to relate the resulting fatigue strength using a weakest-link approach based on the effective volumes and surfaces. This approach seems to work well only in cases of different specimen's lengths. Some of the fracture surfaces were analyzed by means of SEM and the initiating defects were identified and measured. An approach in which the defects population can be randomly distributed in the specimen has been tried. Virtual fatigue tests have been carried out by considering pure propagation of the worst defect. The resulting fatigue curves showed that this approach is promising but needs further description of the initiation phase.

Lifetime assessment of engineering thermoplastics

The life expectancy of thermoplastics in durable applications varies from about 10years to 50 and even 100years in certain cases. It calls for an accelerated testing of material and structures. The challenges of accelerated testing for lifetime are (a) to reproduce the mechanisms of field failures and (b) to develop a reliable procedure for extrapolation of a relatively short test data into long-term service conditions. Acceleration of fracture by high stress level turns to be inadequate, since the fracture mechanisms change with stress level. Acceleration of testing for lifetime by elevated temperature is the most widely used technique at the present. This paradigm, however, faces a problem associated with the changes in the mechanism and kinetics of slow crack growth (SCG). At a certain combination of load and temperature, a transition from a continuous SCG to discontinuous, stepwise crack propagation has been recorded. Optical and scanning electron microscopy observations suggest that the change of SCG mechanisms is closely related to the material ability to form in front of the growing crack a stable process zone that consists of single or multiple crazes and/or shear bands. The crack acceleration in the continuous growth mode is observed to be significantly higher than that in stepwise propagation. Such changes in the mechanism and kinetics of SCG are associated with a transition from a ductile to brittle behavior of microfibers within the process zone. It is referred to as ductile–brittle transition of the second kind (DBT2) based on a resemblance with well-known ductile–brittle transition in dynamic impact resistance. DBT2 is presented in form of SCG mechanisms map in temperature–stress intensity factor coordinates. SCG mechanism map implies certain limitations for extrapolation of conventional temperature accelerated test data to the service conditions of plastic components. An alternative to conventional accelerated testing approach to evaluate lifetime of plastics structures is proposed in this paper. It consists of three steps. The first is a characterization of the defects population that may be responsible for fracture initiation. Formulation of constitutive equations of fracture process based on specially designed tests is the second step. Numerical simulation of fracture process using constitutive equations developed within the second step and evaluation of the lifetime of plastic structure is the third step. A validation testing of the proposed program is required.

Evaluation of the fatigue defect population in an elastomer using X‐ray computed micro‐tomography

AbstractAs elastomeric materials are heterogeneous by nature, their fatigue behavior is strongly driven by the initiation and the growth of cavities. In this study, X‐ray micro‐tomography is used to describe the fatigue mechanisms at a microscale. This non destructive method has already been widely applied to elastomeric materials to control the fillers size and dispersion or to analyze the cavitation induced under high hydrostatic pressure fatigue loading, for example. Here, this technique is used with a good resolution to analyze the evolution of the defects population during a fatigue campaign on hourglass shaped axisymmetric specimens. The initiation and propagation mechanisms are clearly shown on 3D observations, and the influences of the maximum principal strain and of the number of cycles on several parameters (size repartition, porosity, and defect volumic density) are investigated. A scenario for the fatigue damage evolution is proposed and some fatigue initiation criteria are finally discussed, using the results obtained at the microscopic scale. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

X-ray computed [formula omitted]-tomography: a tool for the characterization of fatigue defect population in a polychloroprene rubber

As elastomeric materials are heterogeneous by nature, their fatigue behavior is strongly driven by the initiation and the growth of cavities. The explanations on the microscopic mechanisms are usually supported by SEM observations. SEM is a 2D destructive technique that can induce artifacts and is not suitable to analyze 3D phenomena. Moreover, the time needed to observe a single slice prevents from scanning a whole massive sample. In this study X-ray micro-tomography is used. This non destructive method has already been widely applied to elastomeric materials to control the fillers size and dispersion or to analyze the cavitation induced under high hydrostatic pressure fatigue loading, for example. Here, this technique is used with a good resolution to analyze the evolution of the defects population during a fatigue campaign on hourglass-shaped specimens. The initiation and propagation mechanisms are clearly illustrated on 3D observations, and the influences of the maximum principal strain and of the number of cycles on several damage parameters (size repartition, porosity, defect volumic density) are investigated. A scenario for the fatigue damage evolution is proposed and some fatigue initiation criteria are finally discussed with the help of the micro-structural measurements.

Fast characterization of high-cycle fatigue properties of a cast copper–aluminum alloy by self-heating measurements under cyclic loadings

Even if casting offers the possibility of producing complex shaped parts in a few operations, this process introduces inevitably casting defects such as shrinkage cavities which are harmful regarding fatigue resistance. In this work, this initial defects population is taken into account by indirect approach, i.e. without describing size and geometry of pores, to characterize rapidly High-Cycle Fatigue (HCF) properties of cast materials by the aid of self-heating measurements under cyclic loadings. A probabilistic multiaxial two-scale model is proposed to analyze the self-heating measurements in order to predict S/N curves. It is assumed that fatigue damage is due to microplasticity that induces a dissipative energy which is related to the temperature increase of the studied specimen under cyclic loadings for low stress amplitudes. To describe microplastic activity, a Poisson distribution of elasto-plastic sites within an elastic matrix is considered. In this paper, the identification procedure has been performed on the particular case of a sand cast copper-aluminum alloy used for casting marine propellers. The validation of the proposed approach concerns the prediction of classical fatigue results. The results of this study show a good matching between model predictions and experimental fatigue data, including scattering.