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Development of a graphene doped nickel-base superalloy shaft and air-foil bearing coating material for high-speed machinery.
Khoele, Khotso
Khoele, Khotso
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Abstract
Utilization of high-speed machinery (HSM) is globally increasing due to their significant impact of increasing productivity and lowering production costs on their area of applications. However, air-foil bearings (AFBs), which are at the center of HSM’s functionality, have poor damping properties, low load capability and complexity of frictional wear during start-up and ending periods. These phenomena occur between the shaft of HSM and top-bump foils of the AFBs. Furthermore, an application of Inconel 718 nickel-based super alloy as a shaft material on HSM leads to decreased mechanical properties at higher temperature applications. Hence, it became necessary to improve mechanical properties of Inconel 718 nickel-based superalloy and develop a novel coating system for the air-foil bearings as well. Both pure Inconel 718 and Inconel 718 doped with graphene nanoplatelets (GNPs) samples were cut and sectioned to 25 × 20 × 10 mm for mechanical properties and corrosion studies. Mechanical properties were characterized by Instron 3384 testing machine. Universal Tribometer s/n : RTEC2441, USA was utilized for tribological measurements at room temperature. Surface morphologies were examined by Scanning electron microscopy (SEM). Mechanical properties were carried out between pure Inconel 718 nickel-based super alloy and Inconel 718 doped with GNPs, and corrosion behavior on both materials was characterized in 0.11 mol/L. On foils, bare
and coated foils were supplied on rectangular dimensions of 15 × 15 × 5mm. Corrosion studies on were carried out from three different techniques. The first technique was the open circuit potential (OCP). Herein, the samples were inserted in the media till steady state conditions were obtained. Potentiodynamic polarization (PDP) was conducted to ascertain characteristics of oxide layers forming on foils in conduct with engaged media. Finally, electrochemical impedance spectroscopy (EIS) was engaged to examine the quality of oxides layers forming on foils. Prior and post corrosion measurements, foils’ surfaces were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion curves showed noble degradation on Inconel 718 doped with GNPs. Mechanical properties with higher hardness and young modulus values were noted from the modified Inconel 718. Higher abrasive wear index and lowerer specific wear rate were also discovered from the modified inconel 718. In terms of friction coefficient, lower friction coefficients were noted under all conditions from the modified Inconel 718. Nonetheless, an increase of the load proved to affect properties of tribological oxide layer on both pure and modified Inconel 718. Better surface morphologies were also noted from the surface of the modified Inconel 718. From the foils’ investigations, corrosion results revealed designated F2 coated foil to be superior in terms of better corrosion resistance properties, while F3 posed the most inferior characteristics. Analysis from SEM and XRD also corroborated corrosion results. Higher hardness and young modulus values were recorded from coated foils. On the other hand, specific wear rates were relatively low from coated foils. Furthermore, abrasive wear index was higher from the F2 coated foil. Friction coefficients were also lower from the coated foils under all engaged tribological conditions. Highly ruptured SEM morphologies were noticed from the bare foil. Nonetheless, mild and negligible aggressions were discovered from the surfaces of nano-coated foils. Overall, coated foils possessed relatively higher hardness and Young Modulus values. Specific wear rates were relatively low from coated foils. Furthermore, Friction coefficients were lower from the coated foils under all engaged tribological conditions. All coating materials show an effectiveness in terms of providing cushion of hydro-dynamic oxide layers during tribological measurements. On industrial application, it is believed that oxide forms on F2 could act as a buffer between shafts of high-speed machinery and the AFBs .
Description
Submitted in partial fulfillment of the requirements for the degree, Doctor of Engineering: Metallurgical Engineering in the Department of Chemical, Metallurgical and Materials Engineering Faculty of Engineering and the Built Engineering at the Tshwane University of Technology.
Date
2024-08-05
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Tshwane University of Technology
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Keywords
Air-foil bearings, Tribology, SEM, Nano-composites, Inconel 718, Foils, Graphene Nanoplatelets
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CC0 1.0 Universal