فصلنامه علوم و مهندسی سطح ایران
پیاپی 55 (بهار 1402)

In this study, the effects of F-doped diamond-like carbon (F-DLC) thin films on corrosion resistance of the cobalt-chromiummolybdenum (CoCrMo) alloy substrate were investigated. For this purpose, deposition of F-DLC thin film on CoCrMo substrates was carried out through radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) method. Methane, argon and CF4 gases were used to deposition of F-DLC thin film. TiN intermediate layer was used to improve the adhesion of F-DLC thin film on CoCrMo substrate. Corrosion behavior of the samples have been determined using electrochemical impedance and polarization tests. Electrochemical measurements indicated that the corrosion resistance of CoCrMo substrates increases to a great extent by deposition of F-DLC thin film, so that the corrosion potential and corrosion current decreased from -0.396 volt to -0.057volt and from 5.55×10-6 to 1.288×10-7 µA, respectively. Raman spectroscopy, as a nondestructive method to characterize carbon-based materials, was used for the structural characterization of DLC and F-DLC films. The results demonstrated that higher concentration of sp2 bonding and more graphitic structure in the F-DLC films, which could lead to lower intrinsic stress in these films, compared to those of the DLC coatings.

In this study,the surface structure,corrosion behavior of Monel 400 alloy are  comparatively investigated after high energymicro-shot peening process with its solution-annealed counterpart. To the aim of this investigation, microhardness, roughness measurements ,grazing-incidence X-ray diffraction  (GI-XRD),field-emission scanning electron  microscopy (FE-SEM), and transmission electron microscopy (TEM) were employed  to characterize the surface layers of high energy micro-shot peened andsolution-annealed samples. Based on the results,the process decreases the surface grain size from 30±4 µm to 76±5 nm,andincreases the density of dislocation structures on the surface. Moreover,electrochemical corrosion tests were carried out in HClsolution to compare the corrosion behavior of solution-annealed,micro-shot peened surfaces. Based on the results of thecyclic potentiodynamic polarization test,the micro-shot peened sample shows a higher initial corrosion current density at theexpense of a lower passivity current density,as compared with the solution-annealed one. The repassivation potential for themicro-shot peened sample is about +150 mV higher than the solution-annealed sample,which is a proof of the higher resistanceof this sample against localized corrosion. Based on the fitting of the data obtained from the electrochemical impedancespectroscopy,the polarization resistance of the passive film in the micro-shot peened sample is more than twice of this one forthe solution-annealed sample.

Effect of RF power on the structural, electrical and optical properties of cupric oxide (CuO) thin films was investigated. The films were deposited using a magnetron sputtering system powered by radio frequency source. The effect of applied power on the properties of the films was studied. X-ray diffraction analysis (XRD), energy dispersive X-ray spectroscopy (EDX) and scanning electron microscope (SEM) were used in order to study the structural properties and surface morphology and chemical composition of the deposited films, respectively. Simultaneous optical emission spectroscopy (OES) analysis has also been used to monitor the plasma during the deposition process. The results showed that the ionization in the plasma volume was to the extent that copper atoms were excited and even ionized in all three powers. Moreover, as the power increases, ionization was enhanced which resulted in enhancement of both the sputtering process and oxidation reactions. As a result, the deposition rate and the concentration of oxygen in the composition of the films were increased. The results of the Hall effect electrical analysis showed that the grown CuO thin films were p-type semiconductors. The investigation of increasing RF power from 25 to 100 W showed that the direct band gap of the films decreased from 2.22 to 2.17 eV and resistivity was also reduced from 2.93 kΩ.cm to 14.8 Ω.cm.

The purpose of this research is to investigate the electrochemical and antibacterial behavior of calcium-phosphate coating formed by the plasma electrolytic oxidation (PEO) method using an electrolyte containing Na3PO4.12H2O, KOH, and Ca3(PO4)2 salts on pure titanium. For this purpose, the surface structure of the coating was evaluated by scanning electron microscopic images and its chemical composition by Raman spectroscopic analysis. The electrochemical behavior of the coating was studied in simulating body fluid (SBF) using electrochemical impedance spectroscopy and potentiodynamic polarization tests. The antibacterial behavior of PEO coatings was evaluated under two conditions without irradiation and under ultraviolet radiation and against gram-positive Staphylococcus aureus bacteria. The results showed that the formed PEO coating had a porous structure and included the anatase crystalline phase. After 7 days of exposure of the samples to SBF solution, the values of polarization resistance (Rp) of pure titanium substrate and PEO coating were 0.097 and 0.263 MΩ.cm2 , respectively. Therefore, by modifying the surface of pure titanium with the help of the PEO coating process, its corrosion resistance increased by about 2.7 times. After exposing the PEO coatings to ultraviolet radiation, their antibacterial behavior was improved due to the production of reactive oxygen species (ROS) by the coatings and as a result damage to Deoxyribonucleic acid (DNA), proteins, and other intracellular components and finally the death of bacteria.

The surface modification of St37 steel irradiated with two different plasma regimes was investigated and compared. At first, using a plasma focus device, the interaction of high-energy nitrogen ions, colliding with the specimen in a pulsed manner (in nanosecond time scale) was studied. Then, the interaction of nitrogen ions with the specimens in the glow discharge plasma regime was investigated. Using SEM, EDS and XRD Analyses the morphology changes of the samples were studied. The SEM images show that small, dense and abundant bubbles (about 100 nm) were created by the pulsed plasma source on the surface of the specimens. On the other hand, the glow discharge plasma has created blisters with a size between 0.5-2 micrometers on the surface of the samples. The XRD results indicate the chemical reaction of nitrogen ions with iron in the glow discharge plasma regime. Also, the Vickers hardness test results illustrated that the hardness of the samples increased significantly.

In this research, transparent multi-layer graphene nanocoating was used to protect copper against oxidation at low temperature and the role of this coating on maintaining brightness and preventing copper color change was evaluated. For this purpose, multilayer graphene coatings were applied on copper substrate using chemical vapor deposition (CVD). The multilayer graphene coating before and after the oxidation test was characterized using Raman analysis, optical and scanning electron microscopes. In addition, the color changes in the CIE-L*a*b* color space and the reflectance percentage in the range of the visible spectrum were studied. The results showed that the transparent multilayer graphene coating can maintain the appearance properties of copper after thermal oxidation for up to 4 hours. The results of the present research show that the multilayer graphene coating is effective in protecting the oxidation and as a result maintaining the color of copper. In addition, the results of this research showed that graphene oxidation starts and spreads from the defects and graphene grains boundaries.