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Afzal M.,University of Engineering and Technology Lahore | Ahmed F.,University of Engineering and Technology Lahore | Khan A.N.,Institute of Industrial and Control System | Anwar M.Y.,University of Engineering and Technology Lahore | And 2 more authors.
International Journal of Materials Research | Year: 2015

In the present research, WC-12 %Co cermet coatings were deposited on AISI-321 stainless steel substrate using air plasma spraying. During the deposition process, the standoff distance was varied from 80 to 130 mm with 10 mm increments. Other parameters such as current, voltage, time, carrier gas flow rate and powder feed rate etc. were kept constant. The objective was to study the effects of spraying distance on the microstructure of as-sprayed coatings. The microscopic analyses revealed that the band of spraying distance ranging from 90 to 100 mm was the threshold distance for optimum results, provided that all the other spraying parameters were kept constant. In this range of threshold distance, minimum percentages of porosity and defects were observed. Further, the formation of different phases, at six spraying distances, was studied using X-ray diffraction, and the phase analysis was correlated with hardness results. © Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282. Source


Qureshi I.N.,National University of Sciences and Technology | Shahid M.,National University of Sciences and Technology | Nusair Khan A.,Institute of Industrial and Control System | Durrani Y.A.,University of Engineering & Technology, Taxila
Journal of Thermal Spray Technology | Year: 2015

Thermal barrier coating (TBC) systems were produced by air plasma spraying system on nickel base superalloy. These coatings were composed of a Y2O3-stabilized ZrO2 topcoat and a CoNiCrAlY bondcoat and are known as standard TBC. In this paper, standard TBC samples were compared with TiN-modified bondcoat TBC samples. Titanium nitride was deposited by utilizing a physical vapor deposition technique. Both TBC systems were exposed to high temperature in the presence of corrosive salts, i.e. a mixture of V2O5 and Na2SO4 (50:50) for 50 h. It was observed that the TiN-modified samples showed better results in terms of oxidation resistance and delamination. The formation of Cr2Tin−2O2n−1 phases at the interface of the topcoat–bondcoat, in TiN-modified samples were found to enhance the thermal and oxidation properties of the TBC. © 2015, ASM International. Source


Qureshi I.N.,National University of Sciences and Technology | Shahid M.,National University of Sciences and Technology | Nusair Khan A.,Institute of Industrial and Control System
Journal of Thermal Spray Technology | Year: 2016

Yttria-stabilized zirconia thermal barrier coating along with CoNiCrAlY bondcoat was deposited using air plasma spray on Inconel-X750 superalloy. The coated samples were exposed at 950 °C in a mixture of Na2SO4 and V2O5. The exposed specimens were investigated using XRD and SEM. The formation of spinel and perovskite structures was revealed at the interface of topcoat and the bondcoat. Further, the chemical composition profile of all samples helped to analyze the diffusion behavior of different constituent elements of bondcoat and substrate. XRD analyses of the samples confirmed the phase transformation of the tetragonal zirconia into monoclinic zirconia and yttrium vanadate. The shift of high angle peaks indicated lattice distortion, which was directly related to the stresses in the coating. © 2015, ASM International. Source


Qureshi I.N.,National University of Sciences and Technology | Shahid M.,National University of Sciences and Technology | Khan A.N.,Institute of Industrial and Control System
Acta Physica Polonica A | Year: 2015

Thermal barrier coatings (TBCs) are widely used for industrial and aero turbines. The use of residual fuel oil is well known due to economic reasons, which causes hot corrosion. Hot corrosion over extended exposures reduces durability. Therefore, there is a requirement to develop new design approaches for TBCs in order to operate under hot corrosion environment. In this paper, the effect of bond coat thickness on the hot corrosion resistance was studied. Hot corrosion test were carried out in 50 wt:% Na2So4+50 wt:% V2O5 molten salt at 950 °C for 50 hours. The characterizations of the coatings included X-ray diffraction analysis, scanning electron microscopy and optical microscope. Results indicated that TBCs with thick bond coat exhibited superior hot corrosion resistance to the TBCs with conventional bond coat. Source


Rauf M.M.,National University of Sciences and Technology | Shahid M.,National University of Sciences and Technology | Nusair Khan A.,Institute of Industrial and Control System | Mehmood K.,Institute of Industrial and Control System
Journal of Materials Engineering and Performance | Year: 2015

Air plasma-sprayed Ni-20Cr coating on stainless steel (AISI-304) substrate was re-melted using CO2 laser to remove the inherent defects, i.e., porosity, splat boundaries, and oxides of air plasma-sprayed coating. The (1) uncoated, (2) air plasma-sprayed, and (3) laser-re-melted specimens were exposed to cyclic oxidation at 900 °C for a hundred cycles run. The oxidation products were characterized using XRD and SEM. Weight changes were determined after every 4th cycle; Uncoated samples showed severe oxidation indicated by substantial weight loss, whereas air plasma-coated samples demonstrated noticeable weight gain. However, oxidation resistance of laser-cladded samples was found to be significantly improved as the samples showed negligible weight change; porosity within the coating was minimized with an improvement in interface quality causing reduction in delamination damage. © 2015, ASM International. Source

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