Hydrogen Effusion Research Articles

Evolution of interfacial nanostructures with temperature governing fretting wear in diamond-like carbon films

Diamond-like carbon (DLC) films are capable of offering low friction coefficient and high wear resistance when rubbed in both dry and lubricated interfaces. However, the inherent mechanisms governing fretting wear in DLC films with temperature are still not well comprehended. Herein, atomic-scale analysis was conducted to unveil the relationship between interfacial tribofilms and the fretting wear behaviors. The results emphasize the determining role of the interaction between the tribo-sintered tribofilms and the transferred carbon tribofilms, as well as the structural evolution in the DLC film with temperature. The room temperature (RT) fretting experiment establishes a bilayer nanostructured tribofilm with a C-rich transfer layer on a Fe-rich tribo-sintered layer. As the fretting temperature elevates the graphitization degree of the tribofilms exacerbates, surface oxidation and hydrogen effusion arise in the DLC film, resulting in higher friction coefficients and wear rates. Chromium outward diffusion at 500 °C establishes a Cr-rich interlayer in the tribofilm, which forms interfacial chromium-carbon bonds, promotes adhesive carbon transfer, and leads to the sharp increase of the friction force and wear rate. These findings provide new insights into the fretting wear mechanisms, and provide guidance for the application of DLC films in elevated-temperature fretting scenarios such as aero-engines tenon and spline connections.

Exploring the Effect of Microstructure and Surface Recombination on Hydrogen Effusion in Zn–Ni‐Coated Martensitic Steels by Advanced Computational Modeling

Exploring the Effect of Microstructure and Surface Recombination on Hydrogen Effusion in Zn–Ni‐Coated Martensitic Steels by Advanced Computational Modeling

Exploring the Effect of Microstructure and Surface Recombination on Hydrogen Effusion in Zn–Ni‐Coated Martensitic Steels by Advanced Computational Modeling

Ultrahigh‐strength steel (UHSS) structures are plated with Zn–Ni coatings because of their excellent corrosion resistance properties, but the plating process is accompanied by the production of hydrogen. The presence of hydrogen in steel results in hydrogen embrittlement. Hence, during the production of UHSS parts, dedicated outgassing steps are employed to remove the diffusible hydrogen from the steel. In a production environment, the real effect of the outgassing process and the outgassing efficiency is unknown for parts coated with Zn–Ni. Hence, a finite element model is developed to capture the evolution of the hydrogen concentration profile in coated UHSS parts during outgassing to study the influence of coating morphology and microstructural features of steel. In order to develop the geometry of the model, scanning electron microscope images are analyzed to understand the microstructure and morphology of the coating. Numerical samples are generated by combining different coating morphologies with steel substrates of varying microstructural features to attain a series of samples with varying features. The results of the outgassing simulations clearly demonstrate the major role of the coating morphology on the hydrogen flux.

Evaluation of microstructural and environmental effects on the hydrogen uptake and desorption in high-strength carbon steels: A thermal desorption spectroscopy study

The hydrogen uptake in three high-strength carbon steels is studied after exposure to three different environments: an aqueous solution with CO2 bubbling, CO2/H2S bubbling, and cathodic charging without external gassing. The hydrogen uptake is substantially higher in CO2/H2S environment compared to CO2 environment for all materials. The lamellar cementite morphology absorbs higher hydrogen than the material with spheroidite microstructure with similar carbon content. The corrosion layer formed on the steels in CO2 environment strongly affects the hydrogen effusion and the thermal desorption spectroscopy spectrum in all materials, while the corrosion layer formed in CO2/H2S environment does not show this effect.

Hydrogenation of p+ poly-Si by Al2O3 nanolayers prepared by atomic layer deposition

Polysilicon (poly-Si) passivating contacts have enabled some of the highest lab-scale crystalline silicon (c-Si) solar cell conversion efficiencies, largely due to their excellent surface passivation quality, which can be aided by means of hydrogenation treatments. One frequently applied method is to use hydrogen-rich capping layers, such as Al2O3 or SiNx, which can provide hydrogen to the poly-Si/SiOx/Si interface upon annealing. In this work, the effect of the deposition conditions of the Al2O3 layers, fabricated by various types of atomic layer deposition (ALD), on the hydrogenation of p+ poly-Si is investigated, and it is compared to the direct passivation of c-Si by the same Al2O3 layers. It is found that excellent hydrogenation of the p+ poly-Si by Al2O3 could be reached by a wide range of ALD conditions, but higher annealing temperatures were required as compared to passivation of c-Si. The different ALD conditions result in Al2O3 layers with varying refractive indices, O/Al ratios, and hydrogen content, although these material properties become highly similar upon annealing. Furthermore, the p+ poly-Si layer appears to act as a reservoir for hydrogen, which alters the hydrogen effusion profiles of the Al2O3 layers. The results show that a wide range of Al2O3 layers are highly suitable for hydrogenation of p+ poly-Si and provide more insight into the hydrogenation process of poly-Si passivating contacts.

Scanning Kelvin Probe for Detection in Steel of Locations Enriched by Hydrogen and Prone to Cracking

Hydrogen, due to corrosion processes, can degrade high strength steels (HSS) through embrittlement and stress corrosion cracking mechanisms. Scanning Kelvin probe (SKP) mapping of surface potential was applied, to visualize the locations with an increased subsurface concentration of hydrogen in mild steel and martensitic HSS. This work can help to determine the reasons behind hydrogen localization in a steel microstructure, leading to embrittlement and hydrogen-assisted cracking. Cathodic charging was used to insert hydrogen, which decreased the steel potential. Hydrogen effusion in air passivates steel, increasing the potential of HSS and mild steel. The passivation of steels was monitored depending on different conditions of cathodic pre-charging and the amount of absorbed hydrogen. The SKP could determine the area of diffusible hydrogen and the area of cracks. In addition, low potential locations linked to the hydrogen trapped in the deformed HSS microstructure were also determined, which delayed the steel passivation. Mild steel showed a uniform potential distribution related to interstitial hydrogen, without potential extremes attributed to locally accumulated hydrogen. Thus, SKP sensing can detect locations containing increased concentrations of hydrogen and sensitive to steel cracking.

Tuning of solid-to-solid structural transitions in amorphous carbon films by optical pumping and chemical modification

Amorphous carbon (a-C) attracts great attention in tribology research and thin film technologies due to its versatile properties. However, high temperatures and mechanical stresses may cause significant changes in the structural ordering of the a-C network. We present an optical method to initiate structural ordering and to probe solid-to-solid structural transitions of element modified a-C films. A pulsed pump laser introduces heat into the film in a controlled manner, while a second laser probes confocally the first- and second-order Raman scattering signatures of the a-C network. For low pump power, the number of defects and non-sixfold aromatic rings is reduced. A further increase in the laser power leads to sharply evolved changes in the Raman scattering features, indicating a transition from a-C to defected graphite and an effusion of hydrogen. Moreover, graphite-dominant defect relaxation and an enhancement in hexagonal lattice areas occur and, in turn, activate second-order Raman scattering lines. A rising laser power subsequently results in progressive graphitization. Chemical modification of the films with Si or Cu enhances their thermal stability and even shifts the upper thermal limit of the film ablation, while the a-C:W film demonstrates a more efficient enrichment of nanocrystalline graphitic clusters.

Silicon–Hydrogen Bonding Configuration Modified by Layer Stacking Sequence in Silicon Heterojunction Solar Cells

Recent improvements in highly efficient crystalline silicon (c-Si) solar cells have relied on surface passivation. Hydrogen plays a crucial role in the surface passivation of silicon heterojunction (SHJ) solar cells because Si–H bonds passivate dangling bonds in amorphous silicon and on the c-Si surface. In this work, we demonstrate that the Si–H bonding configuration is modified by layer stacking sequence for SHJs. The quality of surface passivation strongly correlates with low-temperature hydrogen effusion from the SHJ structure. Our results show that the deposition of doped layers on intrinsic amorphous silicon supplies additional hydrogen to the amorphous/crystalline heterostructure. Moreover, the deposition of a p-layer modifies the microstructure of the intrinsic layer underneath, whereas depositing an n-layer does not induce structural changes. We suggest that the low-temperature hydrogen effusion characteristics can be used as a sensitive indicator for examining the passivation quality of SHJ solar cells.

Mechanical load induced hydrogen charging of retained austenite in quenching and partitioning (Q&P) steel

This work investigates the effect of a constant load on hydrogen diffusion through a Q&P steel containing metastable retained austenite by combining electrochemical hydrogen permeation and thermal desorption spectroscopy. Material samples are placed under different external loading conditions, ranging from 50% to 125% of the yield stress. The permeation transients indicate that hydrogen diffusion is delayed under all stressed conditions, even at stresses in the elastic regime, with the delay increasing with the applied load. From thermal desorption spectroscopy performed on the same specimens after the permeation test, it appears that the samples tested under load show a high temperature peak, which is not present in the unloaded sample. Further differential scanning calorimetry analysis confirms that the high temperature peak is related to retained austenite and is a result of hydrogen effusion and hydrogen release due to transformation of the retained austenite.

Temperature mitigates the hydrogen embrittlement sensitivity of martensitic steels in slow strain rates

The joint effect of temperature and strain rate on hydrogen embrittlement properties of martensitic steel was investigated. At 50 °C, the elongation loss first increases and then, decreases with decreasing the strain rate. It was firstly reported that at the low strain rates, hydrogen embrittlement susceptibility was mitigated by temperature due to an increase in the hydrogen effusion to the surface of the material and the release of a significant amount of hydrogen before the yield point by temperature effect. At 25 °C, elongation loss increase with decreasing the strain rate, since in lower strain rates, the hydrogen can interact with mobile dislocations, which finally leads to H-induced fracture. The Kurdjumov-Sachs relationship between martensite and austenite was established, and prior austenite EBSD micrographs were obtained. The intergranular crack was observed which can be related to acting prior austenite grain boundaries as an H-induced crack path.

Atomic-layer-deposited Al-doped zinc oxide as a passivating conductive contacting layer for n+-doped surfaces in silicon solar cells

Stacks consisting of an ultrathin SiO 2 coated with atomic-layer deposited (ALD) zinc oxide (ZnO) and aluminum oxide (Al 2 O 3 ) have been shown to yield state-of-the-art passivation of n -type crystalline silicon surfaces. The distinguishing aspect of this novel passivation stack is the very conductive nature of the passivating ZnO layer. In this work, it is demonstrated that such a stack can provide additional functionalities relevant for silicon solar cells. Specifically, it is shown that the conductive and transparent stacks can passivate textured and n + -diffused silicon surfaces and that they can form an Ohmic contact to n + -diffused surfaces with a low contact resistivity, provided the ZnO is Al-doped. The Al 2 O 3 capping layer has previously been shown to be crucial in the passivation mechanism by preventing the effusion of hydrogen during annealing. Here, it is demonstrated to enable a significant improvement in both the transparency and lateral conductivity of the ZnO upon annealing as well, up to a level typically only attainable by In-based transparent conductive oxides. It is furthermore shown that the passivation of the stacks is thermally stable up to 500–600 o C, depending on the preparation method for the interfacial SiO 2 . Together, these properties make the presented stack an interesting building block for crystalline silicon solar cells, with possibilities for integration as passivating front contact in Passivated Emitter and Rear Cell (PERC)-like solar cells, e.g. as bottom cell top contact in silicon-perovskite tandem cells, as well as a conductive hydrogenation source for poly-Si passivating contacts. • SiO 2 /ALD ZnO:Al/Al 2 O 3 stacks yield excellent passivation of n -type, textured c -Si. • Contact resistivities as low as 15 mΩcm 2 on n + -doped silicon surfaces were obtained. • Doping of the c-Si and ZnO:Al is crucial for obtaining a low contact resistivity. • ALD ZnO:Al can be made highly conductive and transparent by capping and annealing. • The stack has potential in PERC, poly-Si contact and c -Si/perovskite tandem cell.

Understanding Light- and Elevated Temperature-Induced Degradation in Silicon Wafers Using Hydrogen Effusion Mass Spectroscopy

Hydrogen has been long known for its ability to passivate defects in silicon devices. However, multiple recent studies on understanding the mechanism behind light- and elevated temperature-induced degradation (LeTID) have proposed that hydrogen plays an important role in this degradation mechanism. Despite its important role in photovoltaic applications, the quantitative assessment of hydrogen is difficult and seldom reported. In this work, we applied hydrogen effusion mass spectroscopy to quantify the hydrogen released from hydrogenated silicon nitride (SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H) and atomic layer deposited (ALD) aluminum oxide (AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) dielectric films at elevated temperatures. We demonstrate that the amount of hydrogen effused from these layers strongly correlates with the extent of LeTID observed in the multicrystalline silicon wafers passivated with these monolayers and their stacks. It is shown that the hydrogen effusion scales linearly with the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H thickness, similar as the extent of LeTID. The effusion measurements on the AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H stack revealed that the presence of the AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film modifies the total amount of hydrogen that is effused, whereas it was found to slow the hydrogen in-diffusion. This result is consistent with the LeTID extent determined after contact firing where ALD AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layers were found to act as a hydrogen diffusion barrier, strongly reducing LeTID when placed in between c-Si and SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H and increasing LeTID when placed on top of SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H.

Hydrogen density-of-states distribution in β-Ga2O3

Hydrogen bonding in β-Ga2O2 is investigated using hydrogen effusion measurements. The samples were grown by plasma-assisted pulsed laser deposition. With increasing deposition temperature, the total H concentration decreases from 8.1 × 1019 to 9.2 × 1018 cm−3. The dependence of the hydrogen chemical potential, μH, on the H concentration is derived from the effusion spectra and subsequently used to determine the H density-of-states distribution. β-Ga2O2 deposited at Tdep ≤ 723 K exhibits a needle-like peak in the H density-of-states distribution at ≈1.8 eV below the H transport states. With increasing Tdep, the H density-of-states changes and two broad peaks emerge that are located at ≈1.7 and 2.4 eV below the H transport states for Tdep ≥ 873 K.

High-Temperature Tribological Performance of Al2O3/a-C:H:Si Coating in Ambient Air

The study investigates thermal stability and high temperature tribological performance of a-C:H:Si diamond-like carbon (DLC) coating. A thin alumina layer was deposited on top of the a-C:H:Si coating to improve the tribological performance at high temperatures. The a-C:H:Si coating and alumina layer were prepared using plasma-activated chemical vapour deposition and atomic layer deposition, respectively. Raman and X-ray photoelectron spectroscopy were used to investigate the structures and chemical compositions of the specimens. The D and G Raman peaks due to sp2 bonding and the peaks corresponding to the trans-polyacetylene (t-Pa) and sp bonded chains were identified in the Raman spectra of the a-C:H:Si coating. Ball-on-disc sliding tests were carried out at room temperature and 400 °C using Si3N4 balls as counter bodies. The a-C:H:Si coating failed catastrophically in sliding tests at 400 °C; however, a repeatable and reproducible regime of sliding with a low coefficient of friction was observed for the Al2O3/a-C:H:Si coating at the same temperature. The presence of the alumina layer and high stress and temperature caused structural changes in the bulk a-C:H:Si and top layers located near the contact area, leading to the modification of the contact conditions, delivering of extra oxygen into the contact area, reduction of hydrogen effusion, and suppression of the atmospheric oxidation.

Investigation of Thermal Stability Effects of Thick Hydrogenated Amorphous Silicon Precursor Layers for Liquid‐Phase Crystallized Silicon

The thermal stability of thick (≈4 μm) plasma‐grown hydrogenated amorphous silicon (a‐Si:H) layers on glass upon application of a rather rapid annealing step is investigated. Such films are of interest as precursor layers for laser liquid‐phase crystallized silicon solar cells. However, at least half‐day annealing at T ≈550 °C is considered to be necessary so far to reduce the hydrogen (H) content and thus avoid blistering and peeling during the crystallization process due to H. By varying the deposition conditions of a‐Si:H, layers of rather different thermal stability are fabricated. Changes in the surface morphology of these a‐Si:H layers are investigated using scanning electron microscopy and profilometry measurements. Hydrogen effusion, secondary‐ion mass spectrometry (SIMS) depth profiling, and Raman spectroscopy measurements are also carried out. In summary, amorphous silicon precursor layers are fabricated that can be heated within 30 min to a temperature of 550 °C without peeling and major surface morphological changes. Successful laser liquid‐phase crystallization of such material is demonstrated. The physical nature of a‐Si:H material stability/instability upon application of rapid heating is studied.

A critical study on different hydrogen plasma treatment methods of a-Si: H/c-Si interface for enhanced defect passivation

Defect engineering of amorphous-crystalline silicon interface is a critical aspect for high efficiency silicon heterojunction solar cells. In this work, we have done a comparative study of different hydrogen plasma treatment (HPT) methods namely Pre, Post and Intermediate (Inter) HPT using radio frequency plasma-enhanced chemical vapor deposition (rf-PECVD) to understand the passivation mechanism to obtain state of the art passivation. It was observed that Inter and Post HPT can give high minority carrier lifetime on both p and n type wafers and has lesser interface defect states. The analysis from Fourier Transform Infrared Spectroscopy and high resolution Transmission Electron Microscopy shows that these HPT methods results in higher bulk disorder yet atomically sharp and superior interface due to hydrogen diffusion which saturates the interface dangling bonds. While a Pre-HPT results in epitaxial growth leading to poorer performance inspite of lowest dihydrides in the bulk which further highlights the importance of the interface. We have demonstrated Inter and Post HPT of ultrathin (5 nm) a-Si:H films which could give surface recombination velocity as low as 13 cm/s. The degradation due to hydrogen effusion in these HPT a-Si:H films has also been studied.

Hydrogen effect on the passivation and crevice corrosion initiation of AISI 304L using Scanning Kelvin Probe

Scanning Kelvin Probe was applied to study passivation of AISI 304L stainless steel after cathodic polarisation. The rate of passivation in air decreased as a function of duration and current density. X-ray Photoelectron Spectroscopy showed enrichment of the surface film by hydroxides of Fe (II) that was the result of hydrogen effusion from the bulk. SKP measured a decreased potential drop in the passive film. Pre-polarisation accelerates the crevice corrosion of steel in presence of chlorides. Using SKP mapping, increased hydrogen sub-surface concentration and lower level of passivity was observed in anodic zones of the crevice.

Hydrogen Assisted Fracture of 30MnB5 High Strength Steel: A Case Study

When steel components fail in service due to the intervention of hydrogen assisted cracking, discussion of the root cause arises. The failure is frequently blamed on component design, working conditions, the manufacturing process, or the raw material. This work studies the influence of quench and tempering and hot-dip galvanizing on the hydrogen embrittlement behavior of a high strength steel. Slow strain rate tensile testing has been employed to assess this influence. Two sets of specimens have been tested, both in air and immersed in synthetic seawater, at three process steps: in the delivery condition of the raw material, after heat treatment and after heat treatment plus hot-dip galvanizing. One of the specimen sets has been tested without further manipulation and the other set has been tested after applying a hydrogen effusion treatment. The outcome, for this case study, is that fracture risk issues only arise due to hydrogen re-embrittlement in wet service.

Metastable precursor structures in hydrogen-infused super duplex stainless steel microstructure – An operando diffraction experiment

We report the evolution of metastable precursor structures during hydrogen infusion in the near-surface region of a super duplex stainless steel. Grazing-incidence x-ray diffraction was employed to monitor, operando, the lattice degradation of the austenite and ferrite phases. Electrochemical hydrogen charging resulted in the splitting of the diffraction peaks of the austenite phase, suggesting the evolution of a metastable precursor structure. This may be explained by the formation of quasi-hydrides, which convert back into the austenite parent structure during hydrogen effusion. The ferrite showed less lattice deformation than the austenite and no phase transformation.

Influence of the microstructure of Zn–Ni coatings on hydrogen effusion characteristics

This study reports about the influence of the microstructure of electroplated Zn–Ni alloy coatings, especially the extent and characteristic of existing microcracks within, on hydrogen effusion characteristics at room temperature (RT) and 200 °C. For comparison, the behavior of an electroplated Zn coating was also investigated. The microstructures of the Zn and Zn–Ni coatings were examined by means of scanning electron microscope (SEM) and optical microscope (OM). These results were correlated with the hydrogen effusion profiles, which were determined by using a carrier gas hot extraction (CGHE) technique. In addition, a silver decoration method was applied to visualize the hydrogen effusion paths within the coatings. First both Zn–Ni electroplating processes exhibit less hydrogen absorption compared to Zn. In addition the microstructure of the Zn–Ni coatings, especially the characteristic and magnitude of the microcracks, significantly influences the hydrogen effusion characteristics. By using the silver decoration method according to (Schober in MTA 14:2440-2442, 1983) it could be proofed that for Zn–Ni hydrogen primary effuses along the microcracks and the interfaces of pustules within the coating. But the microcracks seem to promote the hydrogen effusion much more than the interfaces of the pustules.