W. Aperador

High Electrocatalytic Response of a Mechanically Enhanced NbC Nanocomposite Electrode Toward Hydrogen Evolution Reaction

Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to replace scarce and commercially expensive platinum electrodes. Thin-film electrodes of metal carbides are a promising alternative due to their reduced price and similar catalytic properties. However, most of the studied structures neglect long-lasting chemical and structural stability, focusing only on electrochemical efficiency. Herein we report on a new approach to easily deposit and control the micro/nanostructure of thin-film electrodes based on niobium carbide (NbC) and their electrocatalytic response. We will show that, by improving the mechanical properties of the NbC electrodes, microstructure and mechanical resilience can be obtained while maintaining high electrocatalytic response. We also address the influence of other parameters such as conductivity and chemical composition on the overall performance of the thin-film electrodes. Finally, we show that nanocomposite NbC electrodes are promising candidates toward HER and, furthermore, that the methodology presented here is suitable to produce other transition-metal carbides with improved catalytic and mechanical properties.

Robust tribo-mechanical and hot corrosion resistance of ultra-refractory Ta-Hf-C ternary alloy films

In this work we report the hot corrosion properties of binary and ternary films of the Ta-Hf-C system in V2O5-Na2SO4 (50%wt.-50%wt.) molten salts at 700 °C deposited on AISI D3 steel substrates. Additionally, the mechanical and nanowear properties of the films were studied. The results show that the ternary alloys consist of solid solutions of the TaC and HfC binary carbides. The ternary alloy films have higher hardness and elastic recoveries, reaching 26.2 GPa and 87%, respectively, and lower nanowear when compared to the binary films. The corrosion rates of the ternary alloys have a superior behavior compared to the binary films, with corrosion rates as low as 0.058 μm/year. The combination and tunability of high hardness, elastic recovery, low nanowear and an excellent resistance to high temperature corrosion demonstrates the potential of the ternary Ta-Hf-C alloy films for applications in extreme conditions.

Monitoreo de la Corrosión de Aceros Embebidos en Concretos obtenidos a partir de Subproductos Industriales

Corrosion of ASTM A 706 structural steel embedded in concrete obtained from by-products as steel slag and fly ash alkali activated was evaluated. Characterization techniques such as open circuit potential, linear polarization resistance and electrochemical impedance spectroscopy in a solution of sodium chloride 3.5% were employed. The steel-concrete interface was characterized by transmission Mossbauer spectroscopy at room temperature and X-ray diffraction. The results obtained by the different characterization techniques were compared with values obtained for Portland cement concrete. The corrosion products identified are magnetite (Fe3O4), wuestite (FeO) and goethite (α-FeOOH).

Monitoreo mediante EIS del acero embebido en un concreto de escoria activada alcalinamente expuesto a carbonatación

In this work the technique of impedance spectroscopy electrochemistry (EIS) was used to evaluate the effects of carbon dioxide on the corrosion of ASTM A 706 structural steel embedded in concrete with alkali activated slag (AAS), AAS concrete is a mixture of ground granulated slag, fine and coarse aggregates and alkaline solution (sodium silicate) in the amount required for the concrete mix. The study was conducted in comparison with specimens exposed to natural conditions with a low concentration of CO2 (0.03% CO2). The carbonation of the concrete was obtained through accelerated carbonation under controlled conditions (3% CO2, 65% of relative humidity and 20oC of temperature). The data of Impedance in the middle frequency region 1mHz - 100KHz. A high frequency response was found using the interface-specific exposure and low frequency response corresponding to the specific interface – steel. EIS was used to established the ability to achieve passivation of steel embedded in AAS concrete in natural and accelerated environmental conditions.

Enhancement of the Pitting Corrosion Resistance of AISI 316LVM Steel with Ta-Hf-C/Au Bilayers for Biomedical Applications

Tantalum carbide (TaC), hafnium carbide (HfC), and Ta-Hf-C mixed coatings with and without a gold (Au) interlayer were deposited on 316LVM steel substrates by the magnetron cosputtering technique in order to improve the corrosion resistance of steel substrates in a simulated biological fluid. To study the effect of the gold interlayer on pitting corrosion, the different systems were placed in contact with Ringerźs solution at pH 7.4 and a temperature of 37°C. The electrochemical properties of the coatings were determined using polarization curves. Subsequently, the surface morphologies were observed using scanning electron microscopy (SEM) in order to analyze the corrosion processes on the different surfaces. The gold interlayer was found to significantly improve the electrochemical properties of the system, showing a decrease in the pitting corrosion and deterioration rate, while it is expected that the binary and ternary carbides provide mechanical stability to the bilayers protecting the gold.

Evaluación del Desempeño de Buriles con Recubrimiento Monocapas de Nitruro de Hafnio en el Proceso de Mecanizado

espanolRecubrimientos monocapa de nitruro de hafnio fueron depositados en buriles de acero rapido, mediante tecnica de deposicion fisica en fase vapor mediante pulverizacion con magnetron (magnetron sputtering). Se uso buriles sin recubrir como sistemas de referencias, en el mecanizado por arranque de viruta del acero AISI1020, midiendo las temperaturas generadas. Se caracterizo la resistencia al desgaste de las herramientas evaluandolas mediante microscopia electronica de barrido, y las superficies obtenidas en el acero mecanizado mediante un rugosimetro. Se evidencio que el uso de recubrimientos monocapa de nitruro de hafnio en buriles de acero rapido disminuye el desgaste adhesivo de la herramienta y la transferencia de energia generada durante el proceso de corte. Ademas, el recubrimiento ofrece ventajas en su aplicacion en el corte por arranque de viruta de metales ferrosos, al aumentar la vida util de la herramienta, mejorar la calidad del producto, y disminuir los tiempos y costos del proceso. EnglishHafnium nitride monolayer coatings were deposited as thin films on high-speed steel by magnetron sputtering physical vapor deposition. Uncoated high-speed steels ASSAB 17 were used as reference systems, for machining on AISI1020 steel by chip removal, measuring the temperature during the process. Wear resistance of the tools was characterized by using scanning electron microscopy, and through of the surfaces of the steel using a roughness tester. It was evidenced that the use of monolayer hafnium coatings on uncoated high-speed steel reduces the adhesive wear of the tool and the energy transfer during the cutting process. Also, hafnium nitride coating offers advantages in its application to chip removal process of ferrous metals, by increasing the life of the tool, improving the quality of the work piece, and reducing cutting process time and cost.

Tribological and wear behavior of HfN/VN nano-multilayer coated cutting tools

Wear and tribological behavior of [HfN/VN]n multinanolayers deposited via magnetron sputtering has been exhaustively studied in this work. Enhancement of both hardness and elastic modulus up to 37 GPa and 351 GPa, respectively, was observed as bilayer periods in the coatings were decreased. The sample with a bilayer period (Λ) of 15 nm and bilayer number n = 80, showed the lowest friction coefficient (∼0.15) and the highest critical load (72 N), corresponding to 2.2 and 1.38 times better than those values for the coating deposited with n = 1, respectively. Taking into account the latest results of tungsten carbide (WC) inserts were used as substrates to improve the mechanical and tribological properties of [HfN/VN]n coatings as a function of increased interface number and to manage higher efficiency of these coatings in different industrial applications, like machining and extrusion. Their physical, mechanical, and tribological characteristics were investigated, including cutting tests with AISI 1020 steel (workpiece) to assess wear as a function of the bilayer number and bilayer period. A comparison of the tribological properties revealed a decrease of flank wear (approximately 24%) for WC inserts coated with [HfN/VN]80 (Λ =15 nm), when compared to uncoated tungsten carbide inserts. These results demonstrate the possibility of using [HfN/VN] multilayers as new coatings for tool machining with excellent industrial performance.

Evidence of Electrochemical Resistance on Ternary V-C-N Layers

In this research, vanadium carbo-nitride (VCN) coatings were synthesized via physical vapor deposition with the aim to determinate the electrochemical behavior of the VCN layers on industrial steel substrates. The VCN coatings, deposited at various negative bias voltage were characterized by X-ray diffraction (XRD), exhibiting the crystallography orientations corresponding to a mix of VCN, VC and VN phases while the X-ray photoelectron spectroscopy (XPS) measurements were used to determinate the chemical composition of the metallic carbon-nitride materials. By using electrochemical impedance spectroscopy (EIS) and Tafel curves, it was possible to estimate the electrochemical behavior of the VCN coatings in a sodium chloride (NaCl) solution. Moreover, scanning electron microscopy (SEM) was performed to analyze morphological and chemical surfaces changes on the VCN layer due to the reaction in NaCl/VCN/steel surface interface. The electrochemical behavior of the VCN coatings in relation to the uncoated AISI 8620 steel showed a reduction of 98% in the corrosion rate, indicating that the applied VCN coatings may be a promising material for industrial applications.

Characterization of the Micro-Abrasive Wear in Coatings of TaC-HfC/Au for Biomedical Implants

The object of this work was the deposition of a Ta-Hf-C thin film with a gold interlayer on stainless steel, via the physical vapor deposition (PVD) technique, in order to evaluate the properties of different systems subjected to micro-abrasive wear phenomena generated by alumina particles in Ringers solution. The surface characterization was performed using a scanning electron microscope (SEM) and atomic force microscope (AFM). The crystallographic phases exhibited for each coating were obtained by X-ray diffraction (XRD). As a consequence of modifying the composition of Ta-Hf there was evidence of an improvement in the micro-abrasive wear resistance and, for each system, the wear constants that confirm the enhancement of the surface were calculated. Likewise, these surfaces can be bioactive, generating an alternative to improve the biological fixation of the implants, therefore, the coatings of TaC-HfC/Au contribute in the development of the new generation of orthopedic implants.

Electrochemical Performance Estimation of Anodized AZ31B Magnesium Alloy as Function of Change in the Current Density

The anodized AZ31B magnesium alloys were synthesized via electrodeposition processes. The aim of this work was to determine the electrochemical behavior of magnesium alloys by using anodized alloys as a protective coating. The anodized alloys were characterized by x-ray diffraction, exhibiting the crystallography orientation for Mg and MgO phases. The x-ray photoelectron spectroscopy was used to determine the chemical composition of anodized magnesium alloys. By using electrochemical impedance spectroscopy and Tafel curves, it was possible to estimate the electrochemical behavior of anodized AZ31B magnesium alloys in Hank’s balanced salt solution (HBSS). Scanning electron microscopy was performed to analyze chemical changes and morphological surface changes on anodized Mg alloys due to the reaction in HBSS/anodized magnesium surface interface. Electrochemical behavior in HBSS indicates that the coatings may be a promising material for biomedical industry.