Pulsed DC Plasma Research Articles

Influence of Pulse Parameters on the Microstructure of Ceramic Oxide Coatings Produced on Al Alloy by Pulsed DC Plasma Electrolytic Oxidation

Influence of Pulse Parameters on the Microstructure of Ceramic Oxide Coatings Produced on Al Alloy by Pulsed DC Plasma Electrolytic Oxidation

Improvement of the Wear and Corrosion Behaviors of DLC/oxynitriding Duplex-treated PM60 High-speed Steel <i>via</i> Various Power Densities of DC-pulsed Plasma Enhanced CVD

Improvement of the Wear and Corrosion Behaviors of DLC/oxynitriding Duplex-treated PM60 High-speed Steel <i>via</i> Various Power Densities of DC-pulsed Plasma Enhanced CVD

Influence of acetylene precursor diluted with argon on the microstructure and the mechanical and tribological properties of a-C:H films deposited via the modified pulsed-DC PECVD method

Abstract Amorphous hydrogenated carbon (a-C:H) films were deposited employing an asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition (PECVD) system and an active screen that worked as an additional cathode. The influence of the acetylene (C 2 H 2 ) precursor diluted with argon on the microstructure and the mechanical and tribological properties of a-C:H films were studied. Raman spectroscopy was used to estimate the film's atomic arrangements and the hydrogen content of the films. A profilometer was used to evaluate the total stress through measurement of the change in the substrate curvature, and nanoindentation experiments allowed the determination of the hardness and the elastic modulus of the films. The friction coefficient and the wear rates of the films were determined using a tribometer in unlubricated sliding friction experiments, while the critical load of failure was determined via a classic scratch test. In order to improve the adhesion of the a-C:H films to AISI 304 and AISI 1020 steel substrates, a thin amorphous silicon interlayer was deposited as an interface between the substrates and the films, using silane as a precursor. The results showed that the atmosphere of acetylene diluted by argon induced modifications in the properties of the a-C:H films. Hard, adherent, low-stress, and high wear resistant a-C:H films were deposited on steel substrates by means of a combination of a modified asymmetrical bipolar pulsed-DC PECVD system and acetylene–argon atmospheres.

Effect of hexane precursor diluted with argon on the adherent diamond-like properties of carbon films on steel surfaces

Various precursors of diamond-like carbon (DLC) growth have been studied in order to find the best set of mechanical and tribological proprieties. Gaseous hexane (C6H14) stands out as an option because of its high vapor pressure and the fact that it accepts dilution with other hydrocarbons or even nanoparticles, forming convenient colloidal substances. Therefore, in the present paper, hexane will be studied using argon as an inert additional gas, in order to determine the mechanical and tribological properties and microstructure of DLC films deposited via a modified and asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition system. The addition of argon to the hexane precursor atmosphere is expected to increase the ratio of ion to neutral radicals on the surface of the growing film without changing the H/C ratio of the gas mixture. The film's microstructure and the hydrogen contents were probed by means of Raman spectroscopy. The internal stress was determined through measurement of the change in the substrate curvature by means of a profilometer, while nanoindentation experiments showed the hardness of the film. The adhesion of the films was evaluated via the scratch test. In order to overcome the low adhesion of these films on AISI 304 stainless steel surfaces, a silicon interlayer, obtained by using low-energy ion implantation and silane as a precursor gas, was used. The results show that the precursor hexane atmosphere diluted with argon induces modifications in the properties of the films when a high quantity of argon is used. The importance of the effect of ion bombardment during film growth on the properties of the films was confirmed.

Influences of Processing Time and Discharge Current Density During Pulsed Plasma-Oxidizing process of AISI 316L

The influences of discharge current density and oxidation process duration on surface morphology, crystalline phase composition, and electrochemical corrosion performance of pulsed plasma-oxidized AISI 316L stainless steel were studied. Analysis of results shows that samples treated through DC pulsed plasma oxidation (0.5 mA/cm2, 60 min) exhibit corrosion current values of 2.0 × 10−4 mA/cm2 which represent one eighth of the corrosion current obtained for untreated samples and one half that of chemically passivated samples. Scanning electron microscopy, grazing incidence x-ray diffraction, and Raman analysis show that the improved corrosion performance of oxidized samples could be attributed to morphological changes of the oxidized layer which is composed of hematite and magnetite iron oxides. Corrosion performances of DC plasma-oxidized samples suggest that through this technique, it is possible to obtain comparable or even better corrosion performance than techniques, such as inductively coupled plasma-oxidizing process, but without the inherent higher instrumental requirements of radiofrequency plasma processes.

Improving the Abrasive Wear Resistance of a Microalloyed Steel by Plasma Nitriding

The aim of this work was to study the micro-abrasive wear resistance of an API 5L X-70 microalloyed steel plasma nitrided under different conditions of time and temperature. Pulsed DC plasma nitriding experiments were performed under a treatment atmosphere of 10%N2 + 90%H2, at temperatures of 410, 440 and 470 °C and for nitriding times of 1, 3 and 5h. The results show that plasma nitriding performed at 440 °C and 1h led to the formation of a compound layer constituted mainly of ε-Fe2-3N nitride and a diffusion zone with large needle-like nitride, offering the highest wear resistance. The amount of γ’- Fe4N phase was found to increase with the plasma nitriding time, decreasing the wear resistance of the material.

Deposition of amorphous hydrogenated carbon films on steel surfaces through the enhanced asymmetrical modified bipolar pulsed-DC PECVD method

Deposition of amorphous hydrogenated carbon (a-C:H) films employing an asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition (PECVD) system, an active screen that worked as an additional cathode represented a step forward for thin film growth by using lower pressure and higher plasma density than the conventional PECVD system. Acetylene gas was used as a precursor. In order to overcome the low adherence of the a-C:H films on steel substrates, an amorphous silicon interlayer was deposited as an interface between the substrate and the films, using silane as a precursor. The films were analyzed according to their microstructural, mechanical, and tribological properties as a function of the substrate bias voltage. The film's atomic arrangements and the hydrogen content of the films were estimated by means of Raman spectroscopy. The total stress was evaluated through the measurement of the substrate curvature, using a profilometer, while nanoindentation experiments helped determine the films' hardness and elastic modulus. The friction coefficient and the wear rates of the films were determined using a tribometer in unlubricated sliding friction experiments, while the critical load of failure was determined by a classical scratch test. The results showed that the use of the employed experimental setup and an amorphous silicon interlayer improved the a-C:H films' deposition onto steel substrates, producing good adhesion, low compressive stress, and a high hardness. The composition, microstructure, and mechanical and tribological properties of the films were dependent on the applied bias voltage. These results suggest that a combination of a modified pulsed-DC PECVD system, the use of an active screen as a cathode, and acetylene as a precursor gas for growing a-C:H films may represent a new and useful alternative for mechanical and tribological applications.

Hydrogenated amorphous carbon thin films deposition by pulsed DC plasma enhanced by electrostatic confinement

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with attractive properties such as high hardness, low friction, chemical inertness and high wear resistance. In this work, hydrogenated amorphous carbon (a-C:H) thin films were deposited using a plasma enhanced chemical vapor deposition technique by pulsed DC plasma with a simple, low-cost and efficient arrangement of multi-cathodes and multi-anodes in order to enhance the plasma by electrostatic confinement. The samples were characterized by Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Elastic Recoil Detection Analysis, Raman Spectroscopy, and nanoindentation measurements. a-C:H thin films show an homogeneous hydrogen profile along the films deposited at −600V and −800V and variable working pressure. According to Raman spectra, both the ID/IG ratio and the G-peak position increase at higher voltages (more remarkable dependence) and lower working pressures (less remarkable dependence). Moreover, the hardness depends on working conditions such as power supply voltage and total working pressure. The electrostatic confinement enhances the deposition rates of a-C:H thin films up to values of 0.9μm·h−1, which is almost double than those previously published by pulsed DC plasma in similar conditions. The Raman spectra follow the stage 2 of an established model and this structure transition may explain the hardness behavior.

The production mechanisms of OH radicals in a pulsed direct current plasma jet

The production mechanism of OH radicals in a pulsed DC plasma jet is studied by a two-dimensional (2-D) plasma jet model and a one-dimensional (1-D) discharge model. For the plasma jet in the open air, electron-impact dissociation of H2O, electron neutralization of H2O+, as well as dissociation of H2O by O(1D) are found to be the main reactions to generate the OH species. The contribution of the dissociation of H2O by electron is more than the others. The additions of N2, O2, air, and H2O into the working gas increase the OH density outside the tube slightly, which is attributed to more electrons produced by Penning ionization. On the other hand, the additions of O2 and H2O into the working gas increase the OH density inside the tube substantially, which is attributed to the increased O (1D) and H2O concentration, respectively. The gas flow will transport high density OH out of the tube during pulse off period. It is also shown that the plasma chemistry and reactivity can be effectively controlled by the pulse numbers. These results are supported by the laser induced fluorescence measurements and are relevant to several applications of atmospheric-pressure plasmas in health care, medicine, and materials processing.

Flow Control Over a Circular Cylinder Using Pulsed Dielectric Barrier Discharge Actuators

Abstract Immersed bodies such as struts, vanes, and instrumentation probes in gas turbine flow systems will, except at the lowest of flow velocities, shed separated wakes. These wakes can have both upstream and downstream effects on the surrounding flow. In most applications, surrounding components are designed to be in the presence of a quasi-steady or at least nonvariant flow field. The presence of unsteady wakes has both aerodynamic and structural consequences. Active flow control of wake generation can therefore be very valuable. One means to implement active flow control is by the use of plasma actuation. Plasma actuation is the use of strong electric fields to generate ionized gas that can be actuated and controlled using the electric fields. The controlling device can be based on AC, DC, or pulsed-DC actuation. The present research was conducted using pulsed-DC from a capacitive discharge power supply. The study demonstrates the applicability of, specifically, pulsed-DC plasma flow control of the flow on a circular cylinder at high Reynolds numbers. The circular cylinder was selected because its flow characteristics are related to gas turbine flowpath phenomena, and are well characterized. Further, the associated pressure gradients are some of the most severe encountered in fluid applications. The development of effective plasma actuators at high Reynolds numbers under the influence of severe pressure gradients is a necessary step toward developing useful actuators for gas turbine applications beyond laboratory use. The reported experiments were run at Reynolds numbers varying from 50,000 to 97,000, and utilizing various pulse frequencies. Further the observed performance differences with varying electric field strengths are discussed for these Reynolds numbers. The results show that flow behaviors at high Reynolds numbers can be influenced by these types of actuators. The actuators were able to demonstrate a reduction in both wake width and momentum deficit.

Effect of Temperature and Gas Flux on the Mechanical Behavior of TiC Coating by Pulsed DC Plasma Enhanced Chemical Vapor Deposition

Effect of Temperature and Gas Flux on the Mechanical Behavior of TiC Coating by Pulsed DC Plasma Enhanced Chemical Vapor Deposition

Plasma Treatments for Metallic Surface Modification to Obtain Highly Adherent Diamond-Like Carbon Coatings

The obtained results of the deposition of hard and adherent diamond-like carbon (DLC) films on four metal substrates, stainless steels AISI 304 and ASTM F138, carbon steel AISI 1020, and Ti6Al4V alloy, are presented. The DLC films were deposited using a modified pulsed-dc plasma enhanced chemical vapor deposition technique. With the aim to improve the film adhesion, surface modification like nitriding, carbonitriding, and carburizing were used previously to DLC film depositions. Strong DLC films' stress dependence of the plasma treatments was observed. The stress reduction suggests an increase in the DLC films adherence to metallic substrate. Tribological tests also confirmed these results.

Numerical and experimental study on a pulsed-dc plasma jet

A numerical and experimental study of plasma jet propagation in a low-temperature, atmospheric-pressure, helium jet in ambient air is presented. A self-consistent, multi-species, two-dimensional axially symmetric plasma model with detailed finite-rate chemistry of helium–air mixture composition is used to provide insights into the propagation of the plasma jet. The obtained simulation results suggest that the sheath forms near the dielectric tube inner surface and shields the plasma channel from the tube surface. The strong electric field at the edge of the dielectric field enhances the ionization in the air mixing layer; therefore, the streamer head becomes ring-shaped when the streamer runs out of the tube. The avalanche-to-streamer transition is the main mechanism of streamer advancement. Penning ionization dominates the ionization reactions and increases the electrical conductivity of the plasma channel. The simulation results are supported by experimental observations under similar discharge conditions.

Adherent amorphous hydrogenated carbon coatings on steel surfaces deposited by enhanced asymmetrical bipolar pulsed-DC PECVD method and hexane as precursor

In order to overcome the high residual stress and low adherence of amorphous hydrogenated carbon (a-C:H) films on steel substrates, a thin amorphous silicon interlayer was deposited as an interface between the substrate and a-C:H films produced by using hexane vapor as a precursor gas. Amorphous silicon interlayer and a-C:H films were grown by employing a modified and asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition (PECVD) system, using silane and hexane atmospheres, respectively. The a-C:H films were analyzed according to their microstructure, and mechanical and tribological properties as a function of self-bias voltage. The chemical composition and hydrogen content of the a-C:H films were estimated by means of Raman scattering spectroscopy. The total stress was evaluated through the measurement of the substrate curvature, using a profilometer, while nanoindentation experiments helped determine the films' hardness. The friction coefficient and critical load were determined by using a tribometer. The corrosion resistance was evaluated by electrochemical potentiodynamic polarization techniques on a 3% solution of sodium chloride (NaCl). The results showed that the use of the amorphous silicon interlayer, deposited by low energy ion implantation, improved the a-C:H film deposition onto steel substrates, producing good adhesion, low compressive stress, and a high hardness. The composition, the microstructure, and the mechanical and tribological properties of the films were strongly dependent on the self-bias voltages. These results suggest that a combination of a modified pulsed-DC PECVD system and hexane as a precursor gas for growing a-C:H films may represent a good and new alternative for coating scaling for mechanical and tribological applications.

Plasma-assisted synthesis and high-resolution characterization of anisotropic elemental and bimetallic core-shell magnetic nanoparticles.

Magnetically anisotropic as well as magnetic core–shell nanoparticles (CS-NPs) with controllable properties are highly desirable in a broad range of applications. With this background, a setup for the synthesis of heterostructured magnetic core–shell nanoparticles, which relies on (optionally pulsed) DC plasma gas condensation has been developed. We demonstrate the synthesis of elemental nickel nanoparticles with highly tunable sizes and shapes and Ni@Cu CS-NPs with an average shell thickness of 10 nm as determined with scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. An analytical model that relies on classical kinetic gas theory is used to describe the deposition of Cu shell atoms on top of existing Ni cores. Its predictive power and possible implications for the growth of heterostructured NP in gas condensation processes are discussed.

The hydro-abrasive erosion wear behavior of duplex-treated surfaces of AISI H13 tool steel

The influence of duplex surface treatments consisting of a DC-pulsed plasma nitriding process and subsequent coatings of CrN and TiAlN deposited by physical vapor deposition (PVD) on AISI H13 tool steel was studied in this article. The treated samples were characterized using metallographic techniques, SEM, EDS, and microhardness methods. Hydro-abrasive erosion wear tests were performed in a specifically designed wear tester in which the samples were rotated in a wear tank containing a mixture of distilled water and ceramic abrasive chips with a fixed rotational speed. The wear rates caused by the abrasive particle impacts were assessed based on accumulated weight loss measurements. The worn surfaces were also characterized using optical microscopy, SEM, and EDS. Microhardness measurements indicated a significant increase in the surface hardness of the duplex-treated samples. The surfaces of the samples with the TiAlN coating were approximately 15 times harder than that of the untreated samples and 3 times that of the plasma nitrided samples. Hydro-abrasive erosion wear results showed that the duplex surface treatments, especially the CrN coating, displayed the highest erosion wear resistance.

Mechanical behavior of TiN/TiC multilayer coatings fabricated by plasma assisted chemical vapor deposition on AISI H13 hot work tool steel

In the present paper, mechanical properties of multilayer coatings were investigated. Titanium nitride/titanium carbide (TiN/TiC) multilayer coatings were prepared on the surface of AISI H13 hot work tool steel by the pulsed-DC plasma assisted chemical vapor deposition (PACVD). The microhardness, the toughness, the wear loss and the friction coefficient were measured by the Knoop and the Vickers hardness test and the pin on disk wear test. The optical microscopy was also used to examine worn surfaces and to characterize wear mechanisms. The field emission scanning electron microscopy (FE-SEM) and the grazing incidence X-ray diffraction (GIXRD) were applied to study the crystalline structure of multilayer coatings. Multilayer coatings with 10 layers exhibited lower friction coefficient (as 0.2), lower wear loss and higher toughness (17.7MPa·m1/2) than those values in other multilayer coatings. Increasing the number of layers increased the microhardness, the fracture toughness and the wear resistance with respect to single-layer coatings.

Improvement of the Tribological Properties of DLC/oxynitriding Duplex-treated AISI H13 Alloy Steel

In this study, diamond-like carbon (DLC) films were prepared by DC-pulsed plasma CVD after the oxynitride treatment of AISI H13 tool steel. In order to investigate the tribological properties of DLC films, a Raman spectroscopy analysis, wear test, adhesion and roughness tests were performed. The main parameters of the DC-pulsed plasma CVD process includes various pretreatment times of argon plasma (15, 30, 45 and 60 min). Experimental results showed that an oxynitride layer and the DLC films could be completely obtained after DLC/oxynitriding duplex treatment. The duplex coating layers had optimal adhesion (critical load reached to 10.65 N) and wear properties after DC-pulsed plasma was CVD treated via a low pulse voltage (–1.5 kV), pretreatment times of the argon plasma were 15 min and the substrate temperature was kept at 40°C. Meanwhile, the optimal DLC/oxynitriding duplex treated specimens possessed the lowest wear volume (2.25 × 10 –3 mm 3 ) and a lower friction coefficient (0.06).

REDUÇÃO DIRETA DE PÓS DE HEMATITA POR PLASMA FRIO DE HIDROGÊNIO

The goal of this work was to study the reduction kinetics of hematite powders (Fe2 O3 ) using cold hydrogen plasma as reducing agent. Reduction experiments were carried out in a DC pulsed plasma reactor, under hydrogen flow-rates of 300 cm3 /min, at pressure of 400 Pa, times from 30 to 120 minutes and temperatures of 320, 340, 360 and 380°C. Fe2 O3 powders after reduction experiments were characterized by X-ray diffraction, weight loss of oxygen (gravimetric analyses) and light microscopy. The results show that using a reduction temperature of 380°C after 120 min allows obtaining α-iron with a reduction fraction of about 0.93. The powder particles are transformed into two steps: Fe2 O3 →Fe3 O4 →Fe-α. The apparent activation energy experimentally established for the reduction of Fe2 O3 is about 98.4 kJ/mol

Nitrogen diffusion in different microstructures of plasma nitrided CK45 steel

In this study, the nitrogen diffusion in different microstructures of CK45 steel was studied. Different microstructures were achieved by different heat treatment cycles as annealing, normalising and quench tempering. After heat treatment, plasma nitriding was performed at 550°C temperature, nitriding times of 5 h, in a gas mixture containing 25 vol.-%H2/75 vol.-%N2 and DC pulsed plasma nitriding unit. The surface, cross-section and the thickness of diffusion and white layers of specimens were studied in terms of optical and scanning electron microscopy. Surface phases have been studied by X-ray diffraction analysis. Results showed that by normalising the samples, higher diffusion depths could be achieved and the white layer thickness would be also higher. Other results showed that core hardness of quench tempered specimens could decrease after plasma nitriding due to being subjected to nitriding process temperature.