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Rehman, Zeeshan Ur,Koo, Bon Heun American Scientific Publishers 2018 Science Of Advanced Materials Vol.10 No.1
<P>A two-step PEO (Plasma electrolytic oxidation) technique was applied to produce coatings on AZ91D magnesium alloy. AZ91D samples were oxidized in an alkaline silicate electrolyte in the first step; then the specimens with the as-prepared coatings were transferred into the K2ZrF6-Na2SiO3 center dot 10H(2)O based electrolyte solution for further oxidation in the second step. Phase composition and distribution of elements in the PEO coatings were investigated by EDS and XRD technique. The result explored that the coatings after second step contained Zr, Si and Mg based complexes. From the SEM analysis, it was found that the inner layer was damaged with the second step coating however the top layer of the coating was highly crystalized and non-defective. Due to the high crystallinity and Zr based complexes in the coatings, hardness and the anticorrosion properties of the coatings were highly improved. The highest hardness and corrosion resistance of the second step coating was found to be 1341.7 HV and 46.86x10(3) (Omega/cm(2)) respectively.</P>
Ur Rehman, Zeeshan,Lee, Dong-Gun,Koo, Bon Heun Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.10
In this work, ceramic coatings were prepared on Al7075 aluminum alloy using microarc oxidation (MAO) process in a silicate-fluoride based electrolyte solution. The effect of $OH^-$ concentration, by adding NaOH to the solution on the microstructural and mechanical properties of the coating was investigated. Surface morphology and cross sectional view of the coating was analyzed using SEM while XRD was used to examine the phase compositions of the coatings. From XRD ${\alpha}-Al_2O_3$ phase was found to be increased by adding NaOH to the electrolyte. Thereby, the hardness and the wear properties of the MAO coatings were found to be superior to those of the coatings prepared without NaOH addition or with amount maximum than 2 g/l NaOH. Moreover, the morphology of the coatings was transformed form nodule-based cluster to crater based structure with the addition of NaOH to the MAO electrolyte solution.
Ur Rehman, Zeeshan,Jeong, Yeong Seung,Koo, Bon Heun Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.10
MAO ceramic coatings were prepared on AZ61 magnesium alloy for various processing times ranging from 5 to 60 min, in an electrolyte solution based on silicate-fluoride. The mechanical, electrochemical and, microstructural properties and the phase compositions of the coating layers were investigated. In this work, unlike previous studies, coatings with high amounts of the $Mag_2SiO_4$ phase were formed which contained small amounts of MgO and $MgF_2$ at a processing condition of 30 min. A microstructural analysis revealed that the porosity of the coatings was reduced considerably with an increase in the processing time, together with a change in the pore geometry from an irregular to a spherical shape. Potentiodynamic polarization and mechanical testing results showed that the coatings acquired after a processing time of 30 min were superior to all of the others.
Zeeshan Ur Rehman,Ye-Tang Pan,David George Churchill,구본흔 한국화학공학회 2023 Korean Journal of Chemical Engineering Vol.40 No.4
A detailed comparison of the bio- and synthetic polymers based layer by layer (LBL) coatings on a singular cotton fabric substrate was carried out. The growth of the deposited layers and subsequent properties, that could govern the fire protection of the substrate, were studied using low voltage-scanning electron microscope (LV-SEM) and thermal analysis techniques. From the 2D and 3D microstructural evaluation, uniform layered structure, agglomerated/precipitated structures and inter-fibers connecting layered-structure were explored, more thoroughly for chitosan and alginate based (CHI-2, ALG-2) biopolymer coatings, whereas the microstructure of synthetic polymers-based coatings ammonium polyphosphate and Polyvinyl Alcohol (APP-2 and PVA-2) showed only uniform layers. From the microthermal evaluation (TGA and MCC), a two-step degradation was recorded for all samples with a higher char residue recorded for APP-2 (TGA, ∼39.2%, at 600 °C, MCC ∼33.0% at 800 °C) followed by CHI-2 (TGA, ∼12.5% at 600 °C, MCC, ∼8.92% at 800 °C) suggesting superiority of the APP-2 based deposition over that of CHI-2. In addition, the same pattern of dominance was found consistent with the peak heat release rate (PHRR) values and total heat release (THR) values for APP∼ (85.1 W/g, 6.53 KJ/g), and CHI-2∼ (227.74 W/g, 17.17 KJ/g), respectively. However, from the VFT analysis, both samples were found to have comparable properties, in terms of structural integrations, char residue, and flame resistance.