Surface Oxidation Research Articles

Exceptional hardness in multiprincipal element alloys via hierarchical oxygen heterogeneities

Refractory multiprincipal element alloys (RMPEAs) are potential successors to incumbent high-temperature structural alloys, although efforts to improve oxidation resistance with large additions of passivating elements have led to embrittlement. RMPEAs containing group IV and V elements have a balance of properties including moderate ductility, low density, and the necessary formability. We find that oxidation of group IV-V RMPEAs induces hierarchical heterogeneities, ranging from nanoscale interstitial complexes to tertiary phases. This microstructural hierarchy considerably enhances hardness without indentation cracking, with values ranging between 12.1 and 22.6 GPa from the oxide-adjacent metal to the surface oxides, a 3.7 to 6.8× increase over the interstitial-free alloy. Our fundamental understanding of the oxygen influence on phase formation informs future alloy design to enhance oxidation resistance and obtain exceptional hardness while preserving plasticity.

Chemistry of Formate and Water Ligands on Metal Oxide Cluster Nodes of Metal-Organic Framework hcp Hf-UiO-66: Keys to Understanding Reactivity of Paired μ2-OH and Defect Sites.

Many metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters, and ligands that have been observed on these nodes include formates, acetates, water, hydroxyl groups, and others, all of which are potentially important in affecting reactivities for applications in separations, catalysis, and sensing. Formate is a common node ligand, arising from formic acid used as a modulator and from N,N-dimethylformamide used as a solvent in MOF syntheses. Yet only little work has been reported characterizing the reactivities of node formate ligands. Infrared spectra reported here show that formate bonds to two types of sites on the paired Hf6O8 nodes of hcp UiO-66, namely, defect and μ2-OH sites. Quantifying the number of formate ligands by 1H NMR spectroscopy of digested samples showed an almost equal number of formate ligands on the two sites, indicating the likelihood that they neighbor each other. These formate ligands interact with water molecules, reversibly switching their bonding from bidentate to monodentate. The formates on μ2-OH sites of hcp Hf-UiO-66 interact much more strongly with water than those on defect sites of the same node, and both interact more strongly than isolated defect sites of Hf-UiO-66. Correspondingly, the catalytic activities of hcp UiO-66 determined as turnover frequencies on each site are approximately twofold higher than those on UiO-66, bolstering the inference that methanol dehydration is catalyzed by a node defect site and a neighboring node μ2-OH site. The results show how MOFs, with their well-defined node structures, provide unprecedented opportunities to understand details of reactivities and catalysis on metal oxide clusters, in contrast to bulk metal oxide surfaces.

Dynamic assessment of titanium surface oxides following mechanical damage reveals only partial passivation under inflammatory conditions

Motivated by clinical problems of titanium implant degradation, we developed a workflow that enabled assessment of surface oxide dynamics as a function of clinical interventions and inflammation conditions. We found that mechanical damage led to decrease of stoichiometric TiO2 ratio in the passivation oxide film and further resulted in accelerated degradation under inflammatory anaerobic conditions. This method can be employed for the assessment of surface oxides to monitor implant safety.

Carbon nanotubes perpendicularly grown on graphene oxide nanosheets derived from metal-organic frameworks: Synergistic reinforcement of poly(l-lactic acid) scaffold

The construction of the nanohybrid composed of two carbonaceous nanomaterials is a promising strategy to inhibit their aggregation, and effectively exert the advantages of their excellent mechanical properties as reinforcement for polymers. Here, carbon nanotubes (CNTs) were in situ perpendicularly growing on the surface of graphene oxide (GO) through metal-organic frameworks (MOF)-derived strategy by chemical vapor deposition (CVD), aiming to facilitate their dispersion on poly(l-lactic acid) (PLLA) bone scaffold fabricated by selective laser sintering (SLS). Specifically, GO provided nucleation sites for the growth of MOF, and worked as the templates when CNTs grew, in which MOF provided catalysts and carbon sources simultaneously. The precursor was heated to 550 °C and kept for 2 h, then heated to 900 °C and kept for 2 h before cooling. The obtained nanohybrid (GO@CNT) exhibited a superior dispersion state than the pure GO in the scaffold at the same loading, especially when the loading was 0.75 wt%. Adding 0.75 wt% GO@CNT endowed the scaffold with a remarkable increment in tensile strength and compressive strength of 13.36 MPa and 24.72 MPa with enhancement of 55.35% and 18.85%, respectively, compared to the scaffold containing 0.75 wt% GO. A crack extension model combined with experiment results was proposed to better understand reinforcement mechanisms. Additionally, the scaffold containing GO@CNT exhibited benign cytocompatibility with a cell-spreading area of 86.31% and cell density of 564 cells/mm2 after culturing for 5 d according to the results of cell adhesion and immunofluorescence tests, making it a promising candidate for bone defect repair.

Friction Coefficient Evolution of Si3N4 Binary Coating with a Stoichiometric Ratio of 57/43

Friction coefficient depends on various factors or surface characteristics during tribological testing, and this friction coefficient can be modified by altering the properties of one of the two contacting surfaces. It is crucial to monitor the friction coefficient continuously, not only at the conclusion of the test. This research examined the evolution of friction coefficient of silicon nitride (Si3N4) coating and H13 steel over different sliding distances (250, 500, 750, 1000 m). The study assessed surface wear and oxidation through three-dimensional profilometry and SEM/EDX. The findings indicated a reduction in friction coefficient by 22%, a decrease in wear rate by 88%, and a reduction in wear volume by 87% when comparing the silicon nitride coated steel to the uncoated steel. Furthermore, the changes in friction coefficient provided insights into the timing of the complete fracture of the hard coating.Graphical abstract

Study on corrosion behavior Ⅱ of hot-dipped Zn-6Al-3Mg alloy coating: Surface oxide layer and ternary eutectic corrosion

The oxide layer and ternary eutectic of Zn–6Al–3Mg alloy coating have an important effect on its corrosion behavior and corrosion resistance, but the influence of oxide layer on the corrosion behavior of the coating and the internal corrosion mechanism of ternary eutectic structure are still unclear. In this study, the oxide layer and ternary eutectic of the coating were characterized in detail, and the corrosion behavior of the oxide layer and ternary eutectic in the environment containing Cl were studied based on first principles of density functional theory. The results show that the surface oxide layer can initially prevent the erosion of Cl-, MgZn2 in ternary eutectic preferentially corrodes after the oxide layer dissolves, corrosion products are formed.

Design of air-stabilized Mg-Sc alloy with enhanced hydrogen storage properties via in-situ formation of Sc-hydride in Sc dissolved phase

Severe surface oxidation and slow kinetic rates are the main obstacles which limit the industrial application of Mg-based hydrogen storage alloys. In this work, an attempt is carried out to design and develop the highly active and air-stabilized Mg-Scx (x = 1, 2 and 3 at.%) alloys via an inexpensive and efficient method. The Sc atoms are completely dissolved in the Mg matrix to form single-phase solid solution alloy. At 325 °C and 2.0 MPa hydrogen pressure, Mg-Sc2 alloy exhibits the rapidest absorption rate with the hydrogen capacity of 6.25 wt%. The synergism of lattice distortion induced by Sc-doping and the in-situ formed ScH2 nanoparticles enhance the de/hydrogenation kinetics. After exposed to air for 800 h, almost no MgO can be detected and the hydrogen storage capacity is only reduced by 0.11 wt%, maintaining the initial total capacity more than 98 %. Sc can protect Mg-based alloys from O2 and H2O pollution, which is ascribed to the formation of an oxygen atoms exclusion zone around the solute atoms caused by valence electrons transfer.

Facile synthesis of novel acidic modified magnetic graphene oxide and its application in the green synthesis of pyrimido[4,5-b]quinolines

This study aimed to create an innovative acidic nano catalyst capable of producing pyrimido[4,5-b]quinolines under environmentally friendly conditions. To achieve this objective, 1,3-benzenedisulfonyl amide (BDSA) was immobilized onto the surface of magnetic graphene oxide (GO/Fe3O4@PTRMS@BDSA@SO3H), and its surface was acidified using chlorosulfonic acid. The synthesized catalyst's structure was thoroughly examined and verified through various analyses, including FTIR, EDX, elemental mapping, FESEM, XRD, TGA, and DSC. This novel nano catalyst exhibited exceptional activity and selectivity in synthesizing pyrimido[4,5-b]quinoline derivatives under solvent-free conditions, at low temperatures, and with high efficiency. Its catalytic effectiveness stemmed from features such as easy and eco-friendly synthesis methods, abundant accessible catalytic sites, a high surface area, remarkable selectivity, and facile separation from the reaction medium. Additionally, the catalyst proved to be cost-effective, safe, scalable, and reusable for up to four times.

Decoding the surface of a complex oxide.

Atomic force microscopy reveals the elusive structure of the aluminum oxide surface.

The shear bond strength of orthodontic brackets after surface conditioning of Zirconia, Ceramic and E. max models

The current study aims to evaluate the shear bond strength (SBS) of metal orthodontic brackets after surface conditioning by hydrofluoric acid (HF) and aluminum oxide air abrasion (Al2O3) and using two prime types and to determines the adhesive remnant index (ARI). We compared the bond strength of different types of models (full glazed zirconia, ceramic faced zirconia and E. max faced zirconia) with different surface conditioning methods for attaching metal orthodontic brackets. The study used 60 models for each material, divided into subgroups based on the type of prime material and surface preparation method. The bond strength was measured using a universal testing machine and the results were analyzed using statistical software. Shear bond strength of the Aluminum oxide was statistically (p<0.05) higher than the hydrofluoric acid and control groups in all groups (full glazed zirconia, ceramic faced zirconia and E. max faced zirconia). The Assure® Plus primer with Aluminum oxide surface treatment give rise to the highest shear bond strength than 3M™ Transbond™ XT, while there was no significant difference between Assure® Plus and 3M™ Transbond™ XT primer for both control and hydrofluoric acid groups. The highest shear bond strength was obtained with Aluminum oxide conditioning method for all types of ceramic materials used in this study.

Revealing the environmental-assisted degeneration mechanism of an Al-containing dual-phase high entropy alloy in supercritical water

A dual-phase high entropy alloy (DP-HEA) containing 7.82 wt% Al was synthesized to withstand corrosive supercritical water. The long-term corrosion and quasi-static tensile tests reveal superior resistance to such environmental-assisted degeneration. The outstanding corrosion resistance can be attributed to the high aluminum content in B2 phase, which forms a continuous Al2O3 layer for protection, while the FCC phase is selectively corroded. Under external stress, stress corrosion cracking (SCC) initiates from surface oxide and penetrates uncompact mixed Cr/Al-rich oxide in deteriorated FCC regions or corroded phase interface, though network-shaped B2 can inhibit SCC propagation, which supports the brittle film rupture model.

Water Vapor Adsorption-Desorption Hysteresis Due to Clustering of Water on Nonporous Surfaces.

Water vapor is continuously adsorbed onto and desorbed from all kinds of surfaces depending on changes in relative humidity. Adsorption-desorption hysteresis of water that occurs on various nonporous surfaces and extends down to low relative humidities has been reported for decades, but remains unexplained. Here we show experimentally that such hysteresis is a common phenomenon on metal oxide and mineral surfaces and can be divided into two distinct categories based on the wettability of the adsorbent surface. Type I hysteresis occurs on more hydrophobic surfaces and is associated with adsorption isotherms that behave rather linearly as water saturation is approached, whereas type II hysteresis occurs on more hydrophilic surfaces and is associated with adsorption isotherms that curve steeply upward close to saturation. Our model calculations strongly indicate that adsorption in both types occurs cluster-wise, and the type I hysteresis is caused by contact angle hysteresis, while type II hysteresis is associated with film formation close to saturation. The understanding of water vapor adsorption and desorption mechanisms may be key for explaining and quantifying physical and chemical interfacial phenomena in atmospheric and industrial environments.

Investigation on Be, Mg, Ca, B, Ga, C, Si, and Ge atoms doped on Al (111) surface and their effect on oxygen adsorption: A first-principle calculation

Oxygen corrosion of the aluminum alloy has a substantial impact on the durability of engineering materials and equipment. To better understand how various alloy elements affect oxygen corrosion, we used first-principle DFT method to study the role of each given dopant atom (Be, Mg, Ca, B, Ga, C, Si, and Ge) in the formation of surface oxide film on the Al (111) surface, in the case replacing a surface Al atom. Our calculations show that the electron gain or loss abilities of these alloy elements differ from that of Al metal, with Be, B, C, and Si atoms tending to penetrate into the surface, Mg, Ca, and Ga atoms protruding, and Ge atom just embedded into the Al (111) surfaces. Additionally, the type of doping element at the Al (111) surface can greatly affect the O2 absorption, predicting that Ca-doped surface absorb the least amount of oxygen, followed by Si-doped surface, with Be-, Mg-, B-, Ga-, C-, and Ge-doped surfaces showing higher O2 absorption. Based on adsorption energy (Eads) and partial density of states (PDOS) analysis, we conclude that the doped Al (111) surfaces can make O2 adsorption different owing to the difference in electronic properties of the doping atoms. These insights draw a greater knowledge of the role of alloy elements in the oxygen corrosion of Al alloys, enabling guidance for designing more corrosion-resistant materials.

Preparation of Au/α‐Fe2O3 Electrode for the Detection of Dopamine

AbstractIn this study, Au/α‐Fe2O3 sensing material was synthesized and used as the working electrode for dopamine sensing. The Au/α‐Fe2O3 composites were prepared with various loadings (0.5 %, 1 %, and 3 %) to enhance their sensing properties. All three composites were tested for dopamine sensing, with 1 % Au/α‐Fe2O3 showing the best response. The Au/α‐Fe2O3 composites were characterized using XRD, TEM, and SEM to confirm successful doping of Au on the iron oxide surface. The CV plot obtained by plotting Cyclic Voltammetry (CV) vs. dopamine (DA) was linear (R2=0.9872) for DA concentrations ranging from 6.25 μM–200 μM on 1 % Au/α‐Fe2O3. Similarly, the DPV plot between DPV vs. DA was linear (R2=0.9895) for the same DA concentration range on 1 % Au/α‐Fe2O3, with a detection limit of 0.19 μM. Interference and stability experiments were conducted with dopamine (DA), ascorbic acid (AA), and uric acid (UA). The results showed that DA sensing was not affected by the other two substances for up to 14 days. The signal results indicated that the current intensity remained at 90.75 % of the original, proving that this material has good stability.

Surface Degradation of Mg2X-Based Composites at Room Temperature: Assessing Grain Boundary and Bulk Diffusion Using Atomic Force Microscopy and Scanning Electron Microscopy.

Practical application of thermoelectric generators necessitates materials that combine high heat-to-electricity conversion efficiency with long-term functional stability under operation conditions. While Mg2(Si,Sn)-based materials exhibit promising thermoelectric properties and module prototypes have been demonstrated, their stability remains a challenge, demanding thorough investigation. Utilizing atomic force microscopy (AFM) and scanning electron microscopy (SEM), we investigate the surface degradation of a composite material comprising Si-rich and Sn-rich Mg2(Si,Sn) solid solutions. The investigation reveals a pronounced dependence of stability on Sn content, with the Sn-rich phase Mg2Si0.13Sn0.87 displaying the formation of a nonprotective oxide layer. Subsequent AFM measurements provide evidence of dominating grain boundary diffusion of loosely bound Mg, compared to bulk diffusion, observed within a few days, ultimately resulting in a complete surface oxidation of the Sn-rich phase within several weeks. On the other hand, Mg2Si and Si-rich Mg2Si0.80±0.05Sn0.20±0.05 remain stable against Mg diffusion to the surface even after prolonged exposure. Comparison with previous investigations confirms that the degradation rate is found to be highly dependent on the Sn content, with markedly higher rates observed for x = 0.87 compared to x = 0.70 in Mg2Si1-xSnx. These findings contribute to a better understanding of the stability challenges associated with Mg2(Si,Sn)-based materials, essential for the development of robust thermoelectric materials for practical applications.

Fracture Mechanisms and Crack Propagation in Monolayer Ti3C2Tx under Nanoindentation: The Influence of Surface Terminations and Vacancy Defects.

Monolayer MXenes are a novel class of two-dimensional transition metal carbides/nitrides with fascinating physicochemical properties. Despite recent advances in the study of MXenes' mechanical properties, a comprehensive understanding of the fundamental physical mechanisms that affect fracture due to surface terminations and vacancy defects in MXenes under nanoindentation remains largely unexplored. Here, we address this gap using molecular dynamics simulations and nanoindentation theory to investigate the effects of surface terminations and vacancy defects on the fracture behavior of Ti3C2Tx MXenes. By inducing the rupture of monolayer MXenes through nanoindentation, we find that bare Ti3C2 exhibits brittle fracture behavior. The presence of surface terminations and vacancy defects reduces the load-carrying capacity and flexibility of MXenes. Interestingly, surface terminations increase the stiffness of the structure, while vacancy defects have the opposite effect. We also find that high concentrations of surface oxidation impart ductile fracture characteristics to MXenes and increase the maximum crack length at failure. Additionally, defects exceeding the critical concentration can effectively prevent brittle crack propagation by causing frequent crack deflection and blunting crack tips. Combining these findings, we propose a new strategy to synergistically enhance the fracture toughness of MXenes through high concentrations of surface oxidation and vacancy defects exceeding the critical concentration without significantly affecting strength and stiffness, thereby avoiding catastrophic failure in MXene monolayers due to brittle fracture. This work provides fundamental insights into the mechanical properties and fracture mechanisms of monolayer MXenes.

Iridium Surface Oxide Affects the Nafion Interface in Proton-Exchange-Membrane Water Electrolysis

Iridium Surface Oxide Affects the Nafion Interface in Proton-Exchange-Membrane Water Electrolysis

Effect of Powder Reuse on Powder Characteristics and Properties of DED Laser Beam Metal Additive Manufacturing Process with Stellite® 21 and UNS S32750

In this work, the influence of powder reuse up to three times on directed energy deposition (DED) with laser processing has been studied. The work was carried out on two different gas atomized powders: a cobalt-based alloy type Stellite® 21, and a super duplex stainless steel type UNS S32750. One of the main findings is the influence of oxygen content of the reused powder particles on the final quality and densification of the deposited material and the powder catch efficiency of the laser deposition process. There is a direct relationship between a higher surface oxidation of the particles and the presence of oxygen content in the particles and in the as-built materials, as well as oxides, balance of phases (in the case of the super duplex alloy), pores and defects at the micro level in the laser-deposited material, as well as a decrease in the amount of material that actually melts, reducing powder catch efficiency (more than 12% in the worst case scenario) and the initial bead geometry (height and width) that was obtained for the same process parameters when the virgin powder was used (without oxidation and with original morphology of the powder particles). This causes some melting faults, oxides and formation of undesired oxide compounds in the microstructure, and un-balance of phases particularly in the super duplex stainless steel material, reducing the amount of ferrite from 50.1% to 37.4%, affecting in turn material soundness and its mechanical properties, particularly the hardness. However, the Stellite® 21 alloy type can be reused up to three times, while the super duplex can be reused only once without any major influence of the particles’ surface oxidation on the deposited material quality and hardness.

Excellent low temperature superplasticity and its deformation mechanism in nano/ultrafine grained Fe–17Cr–6Ni stainless steel

Superplastic deformation typically occurs in non-ferrous metals at high temperatures, which results in severe surface oxidation and high energy consumption. In this study, we designed and manufactured a nano/ultrafine-grained stainless steel with a dual-phase microstructure that exhibits excellent low-temperature superplastic deformation capability. A maximum tensile elongation of approximately 500% was achieved when the tensile test was conducted at 700 °C with an initial strain rate of 5 × 10⁻⁴ s⁻1. Even after a 500% tensile elongation, the austenite grains in the gauge section of the tensile specimen still maintained an equiaxed grain shape, and the texture also weakened significantly, indicating that grain boundary sliding and grain rotation dominated the deformation process during superplastic flow. The outstanding superplasticity is mainly attributed to the dual-phase microstructure composed of nano/ultrafine austenite grains and martensite.

The negative differential resistance of nitrogen implanted TiO2

The microstructure and negative differential resistance (NDR) effect of nitrogen implanted rutile TiO2 were investigated by measuring the XPS, Raman spectra and current voltage curves. It was found that the light illumination has large influence on the NDR effect. Under the illumination of 60 mW laser light, a large NDR with a small electric field (1250 V/cm) is obtained. This electric field is about three orders smaller than that reported in literature (1×106 V/cm). The electric field induced tunneling is the possible mechanism of electric transport at higher field region. The NDR is thought to be related to the light and nitrogen dopant induced reaction including the destroying of water, the scavenging of electron, and the surface oxidation transform of non-stoichiometric TiO2−x to stoichiometric insulating state. The results of this paper are not only useful in understanding the mechanism of NDR, but also useful in providing an effective method in manipulation NDR.