J.G. Han

Surface treatment of metals using an atmospheric pressure plasma jet and their surface characteristics

Abstract We have treated the surfaces of Al, SUS and Cu metals using an atmospheric-pressure plasma jet generated by nitrogen and oxygen gases under the atmospheric pressure at room temperature. The plasma ignition occurred by flowing mixed gases between two coaxial metal electrodes, and the voltage was applied with impulse type and 16–20 kHz frequencies. The treated surfaces were basically characterized by means of a contact angle analyzer for the activation property on their surfaces. From the results of XPS, FE-SEM, OES and AFM, we could confirm that the main phenomena such as the reactive etching and oxidation were observed on their surfaces as well as even the aggregation of particles by the activated atoms, radicals and metastable species in the plasma space. However, all treated surfaces contained only oxygen and carbon without nitrogen, even though the excited nitrogen species were generated in the plasma due to its higher reactivity than oxygen ones observed in the OES data. The aging effect on the duration time of the surface energy, moreover, was also studied because of the production cost on the industrial applications in addition.

A study on corrosion resistance characteristics of PVD Cr-N coated steels by electrochemical method

The corrosion behavior of Cr-N coated steels with different phases (α-Cr, CrN and Cr2N) deposited by cathodic arc deposition on AISI H13 steel was investigated in a 3.5% NaCl solution at ambient temperature. Potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were the techniques applied to characterize the corrosion behavior. It was found that the CrN coating had a lower current density from potentiodynamic polarization tests than others. The porosity, corresponding to the ratio of the polarization resistance of the uncoated and the coated substrate, was higher in the Cr2N coating than in the other Cr-N coated steels. EIS measurements showed, for most of the Cr-N coated steels, that the Bode plot presented two time constants. Also, the Cr2N coating represented the characteristic of Warburg behavior after 72 h of immersion. The coating morphologies were examined in planar view and cross-section by SEM analysis and the results were compared with those of the electrochemical measurement. The CrN coating had a dense, columnar grain-sized microstructure with minor intergranular porosity. From the above results, it is concluded that the CrN coating provided a better corrosion protection than the other Cr-N coated steels.

Effects of the thickness of Ti buffer layer on the mechanical properties of TiN coatings

In order to improve the impact adhesion property of TiN hard coating maintaining their hardness and wear resistance, the ductile Ti buffer layer which could absorb impact energy was inserted in the middle of TiN film as well as the Ti interlayer on substrate. These TiN films were deposited using cathodic arc ion plating with various thickness of Ti buffer layers between 0.24 and 0.75 μm. The result of X-ray diffraction analyses showed that the preferred orientation of TiN is (1 1 1). The Ti interlayer grown on the substrate has the preferred orientation of (1 1 1). However, the Ti buffer layer grown on the TiN layer has a (0 0 2) plane, which establishes the epitaxial relationship with the TiN(1 1 1) plane. The elastic modulus and hardness of Ti buffer layer with 0.48 μm thickness were measured to be approximately 220 and 8.2 GPa, respectively, which were higher than values (165, 6.5 GPa) of the Ti interlayer. The results of impact adhesion test under the impact load of 2.5 kgf showed that the indentation cavity volume and circular cracks of TiN coating were greatly reduced with increasing the thickness of Ti buffer layer. This tendency can be attributed to the effective absorption of impact energy by the Ti buffer layer.

Surface modification for hydrophilic property of stainless steel treated by atmospheric-pressure plasma jet

Abstract Surface of a stainless steel has been modified by atmospheric-pressure plasma jet method at room temperature. The impulse voltage is applied to ignite a plasma discharge using high purity (99.999%) reactive gases: N 2 and O 2 . The treated stainless steel is characterized by the activation property of the surface using a contact angle analyzer. Surface energy for the treated stainless steel is increased remarkably when compared to the untreated surface. From the results of X-ray photoelectron spectroscopy and atomic force microscopy, we could confirm that the main functional groups, causing the change in hydrophilic surface were generated under the surface reactions caused by reactive etching and oxidation of ions and activated species in the plasma. In addition, the aging effect during the duration of the hydrophilic property is also studied to investigate the production cost for the industrial applications.

Localized corrosion mechanisms of the multilayered coatings related to growth defects

Abstract Multilayered WC–Ti 1− x Al x N coatings were deposited on AISI D2 steel using cathodic arc deposition method. These coatings contain structural defects such as pores or pinholes. Thus, the substrate is not completely isolated from the corrosive environment. These growth defects in the coatings are detrimental to corrosion resistance of the coatings used in severe corrosion environments. The localized corrosion of the coatings was studied in deaerated 3.5 wt.% NaCl solution using classical electrochemical technique (potentiodynamic polarization test). Coating characteristics were examined by means of glow discharge optical emission spectroscopy, scanning electron microscopy, auger electron spectroscopy and transmission electron spectroscopy. The porosity was calculated from a result of potentiodynamic polarization test of the uncoated and coated specimens. The calculated porosity is higher in the WC–Ti 0.6 Al 0.4 N than others, which is closely related to the packing factor. The positive effects of greater packing factor act on inhibiting the passage of the corrosive electrolyte to the substrate and reducing the localized corrosion kinetics. From the electrochemical tests and surface analyses, the major corrosion reaction of coatings is caused by defects (pores, pinholes and crevices), coating delamination and galvanic effect between the droplet and the coating.

SSS detection method for initial cell search in 3GPP LTE FDD/TDD dual mode receiver

In 3GPP Long Term Evolution (LTE) system, initial cell search procedure comprises two steps using primary synchronization signal (PSS) and secondary synchronization signal (SSS). This paper presents SSS detection method for second step of initial cell search. In general, coherent detection using estimated channel frequency response (CFR) at PSS is used for SSS detection, but performance degradation is occurred by difference of channel between PSS and SSS in time division duplex (TDD) mode. Thus we propose non-coherent detection using partial correlation and differential correlation. Moreover, we consider cyclic-prefix (CP) detection algorithm that uses CP and its repetition part of data symbol, and we investigate influence of CP detection in SSS detection process.

High-rate deposition of plasma polymerized thin films using PECVD method and characterization of their optical properties

Abstract Plasma polymerized organic thin films were deposited at temperature in the range of room temperature to 693 K by plasma enhanced chemical vapor deposition method using the thiophene (C4H4S) precursor. Radio frequency (RF) with 13.56 MHz was applied and the RF power was changed in the range from 30 to 100 W with a gas ratio of Ar:H2=1:1. The as-grown organic thin films were basically characterized with ellipsometry, UV–Vis spectroscopy, and photoluminescence (PL) measurements. In order to compare the difference of optical properties of the plasma polymerized organic thin films, the effects of the RF power and deposition temperature on their structural properties were mainly studied in this work. UV–Vis spectra showed an energy band gap shift from 3.78 to 4.02 eV with increasing RF power and quite high optical transmittance up to 95%. During CVD, moreover, the plasma diagnostics were in situ carried out by using optical emission spectroscopy that showed a strong dependency of intensity on RF powers. To check a possibility of optical device application, PL measurements were also carried out. From the PL data, we could obtain the maximum PL emission intensity at approximately 550 nm wavelength from a plasma polymerized organic thin film grown at room temperature and RF power of 70 W with gas ratio of Ar:H2=1:1. The maximum growth rate is obtained to be 115 nm/min.

Corrosion behavior of PVD-grown WC–(Ti1−xAlx)N films in a 3.5% NaCl solution

Abstract WC–(Ti 1− x Al x )N coatings of stepwise changing Al concentration (WC–Ti 0.86 Al 0.14 N, WC–Ti 0.72 Al 0.28 N, and WC–Ti 0.58 Al 0.42 N) were deposited on AISI 1045 substrate by high-ionization sputtered PVD method. The Al concentration could be controlled by using evaporation source for Al and fixing the evaporation rate of the metals (WC alloy and Ti). The corrosion behavior of WC–(Ti 1− x Al x )N coatings in deaerated 3.5% NaCl solution was investigated by electrochemical corrosion tests and surface analyses. Particular attention was paid to the effects of Al target power density on the film properties related to the corrosion behavior. The measured galvanic corrosion currents between coating and substrate indicated that WC–Ti 0.72 Al 0.28 N coating showed the best resistance of the coating tested. The results of potentiodynamic polarization tests showed that this coating had passivation and lower porosity. This indicated that this coating is effective in improving corrosion resistance. In electrochemical impedance spectroscopy, the WC–Ti 0.72 Al 0.28 N coating showed one time constant loop and the increased polarization resistance of coating relative to other samples. The better corrosion performance of WC–Ti 0.72 Al 0.28 N coating is due to the modified compactness, porosity and adhesion of the coating layer.

The effect of Al composition on the microstructure and mechanical properties of WC–TiAlN superhard composite coating

Abstract WC–Ti (1− x ) Al x N nc-films were deposited on WC–Co and Si substrates using a multi-cathode arc ion-plating system. The microstructure and mechanical properties of the films were investigated to find out the nanostructured film growth mechanism. The microstructure of the WC–Ti (1− x ) Al x N films depend on the Al concentration ( x ). With increasing Al in the film, the interfaces between WC and TiAlN layers loose their coherency and WC–Ti 0.37 Al 0.57 N films show a completely nanocrystalline structure with a grain size of 10 nm, which is in agreement with the superlattice period (λ). The residual stress in WC–Ti (1− x ) Al x N films was independent of the x value and measured to be approximately 6.5 GPa. This high stress of the films was reduced to a value of 4.7G Pa by introducing Ti–WC buffer layers periodically with a thickness ratio ( D buffer / D nc- ) of 0.8. When the D buffer / D nc- ratio was 0.3, film adhesion strength achieved a maximum value of 45.5 N while at higher D buffer / D nc- ratios than 0.3 the film adhesion strength decreased to 25 N. The microhardness of WC–Ti (1− x ) Al x N film was measured to be in the range of 38–50 GPa. The highest value of film hardness was obtained from the nanocomposite film of WC–Ti 0.43 Al 0.57 N. In the X-ray diffraction analysis (XRD) analysis, the Ti 0.43 Al 0.57 N film exhibited the same structure as the superhard ( H ≥40 GPa) phase, which exhibits only TiAlN(111) and (200) reflections. Transmission electron microscopy (TEM) analysis also showed that WC–Ti 0.43 Al 0.57 N film was composed of very fine (∼10 nm) nanocrystalline grains. So, we believe that the nanocrystalline microstructure of the film is of fundamental importance for the dramatic enhancement of film hardness. The plastic deformation resistance factor (H 3 /E 2 ) of WC–Ti (1− x ) Al x N films was calculated to be in a range of 0.27–0.46.

Duplex plasma surface treatment process on mild steel and high alloyed tool steel

Duplex plasma surface treatment by calorizing and plasma nitriding was carried out on mild steel and high alloyed tool steel to improve their elevated temperature wear properties. Effects of calorizing on the plasma nitriding behavior of the steels and the characteristics of the duplex-treated specimens were analyzed using SEM, EDS, XRD, and a microhardness tester. Calorizing on mild steel created a FeAl compound layer of approximately 120 μm thickness, resulting in the improvement of the surface hardness by a factor of 2. Subsequent plasma nitriding increased the surface microhardness above 1200 Hv (0.1 kgf) and the nitriding depth was of the order of 80 μm. The high alloyed tool steels showed a decrease in microhardness in the Al-diffused layer upon calorizing. The subsequent plasma nitriding, however increased the hardness up to 1280 Hv and the nitriding depth of over 80 μm was obtained. The room temperature wear resistance of the duplex-treated steels was slightly better than that of the nitrided one, but there was considerable improvement in the high temperature wear resistance at 500°C in the duplex-treated steels when both the wear volume and the weight change due to oxidation were considered.