Deposition of Fly Ash+Bauxite Coatings on Metals Using Nitrogen and Hydrogen Plasma

Abstract

The technological progression in the modern time has been not only a boon for enhancing human life but also a bane to produce a huge amount of (industrial) wastes, which has caused immense concern regarding its utilization and in avoiding environmental threats viz. pollution etc. Coal-fired power plants are the primary source of fly ash generation. In the past few years, fly ash has been utilized for multiple purposes, viz., mines filling, making bricks, cement, etc. The present research aims at the utilization of fly ash as more valuable substance, i.e., as a ceramic coating substance for the improvement of thermal spray coatings for various applications. The plasma spray technology is advantageous to prepare value-added products from low-grade-ore and also to deposit ceramics, metals, and a combination of these producing approximately a homogenous composite coating with the required microstructure and on an array of substrates to provide tailor-made properties. In the present work, bauxite mineral with different proportions (10% and 20%) is added to fly ash for the development of wear resistance ceramic coatings on SS 304 and Ni superalloy substrates. The three coating compositions prepared are, fly ash (hereafter referred to as C 1), fly ash + bauxite 10% (hereafter referred to as C 2), and fly ash + bauxite 20% (hereafter referred to as C 3). Overlay plasma spray coatings are deposited using N2 (hereafter referred to as PSN2) and H2 (hereafter referred to as PSH2) as secondary gases, varying the input power level of the plasma torch. The maximum deposition efficiency of 23.74% is achieved with C 3, 25 kW PSH2 coating, and a minimum deposition efficiency of 9.21% is observed with C 1, 10 kW PSN2 coating. The coating thickness also indicated similar kinds of results, i.e., a maximum of 260 μm with C 3, 25 kW PSH2 coating, and a minimum of 97 μm with C 1, 10 kW PSN2 coating. From the surface and interface morphology, it can be observed that bauxite addition to fly ash does not favour in homogenous melting in PSN2 coating, whereas in PSH2 coating bauxite addition prompts particle melting and coagulation/agglomeration significantly. This results in minimum surface roughness of 9.002 μm with C 3 25 kW PSH2 coating and a maximum 17.4571 μm with C 3 20 kW PSN2 coating. Magnetite, along with cristobalite and quartz phases, are detected with C 2 and C 3 PSN2 and PSH2 coatings. Adhesion strength decreases with bauxite addition in PSN2 coatings, whereas increases for PSH2 coatings. The porosity levels of the coatings decrease with an increase in power level; after an optimum power level, it increases in both cases. The microhardness of the deposited coatings on the SS 304 substrate is higher than that of coatings on Ni superalloy. Maximum hardness of 598 HV is measured with C 3 25 kW PSH2 coating. Further, an erosion wear test is conducted to evaluate the wear behaviour of the coatings. This piece of research work will be beneficial for the use of industrial waste and ore minerals for high-valued applications.