Surface modification of the metal components involved in heat transfer is one of the passive methods used to enhance their heat transfer efficiency. Coatings with micro- or nano-scale features are widely employed to modify surfaces due to their enhanced surface area, which increases the number of boiling nucleation sites, leading to the generation of a higher number of bubbles. In geothermal power generation, due to the aggressive nature of the working fluid and chemistry of the geothermal brine, ceramic coatings are preferred. Along with having good compressive strength and hardness, ceramic coatings can protect the surface against corrosion, oxidation and wear. The coating deposition method has a specific impact on the coating microstructural characteristic as well as performance, which define its application. Among these, plasma-spraying has been widely used to deposit ceramic materials, due to its high jet enthalpy and the excellent corrosion and wear resistance of the obtained coatings. In this work, solution precursor plasma sprayed coatings of TiO₂/Al₂O₃ composite were developed on carbon steel for boiling heat transfer applications in geothermal heat exchangers. The effect of plasma current and stand-off distance variation on the microstructural properties of coated samples were assessed. The morphology of the final coatings was compared using SEM/EDX. Further, the surface wetting property was analyzed through a drop sample analyzer and found that with spray parameters the water contact angle and diiodomethane contact angle fluctuate.

Geothermal heat exchangers, due to the continuous contact with the aggressive environment such as humidity, dissolved ions and non-condensable gases within the geothermal fluids, temperature fluctuations etc. are always at risk of corrosion and scaling. This greatly affects the lifetime and efficiency of the geothermal power plant as a whole. Hence, to protect the material and enhance the heat transfer ability, innovative coatings are used. Different active, passive and compound techniques such as flow disruption, surface roughness, out of plane mixing, fluid additives, channel curvature, etc. are used to improve the heat transfer performance [1]. Surface modification, relying on alteration of the surface chemistry and topography, is one of the extensively used heat transfer amplification method. Generally, coatings with micro- or nano-scale features are used, which multiplies the boiling nucleation sites due to the enhanced surface area [2]. Coatings deposited through thermal spray are widely favored because of being time- and cost-efficient and their robust nature. Plasma spray is one such technique that provides the flexibility to deposit a wide range of materials on different substrates such as metals, glass, fabrics, polymers, etc. Conventionally, powder-based plasma spray is used to deposit metal oxides, however, due to the issues like agglomeration, clogging, feedstock preparation, research as shifted towards liquid-based feedstocks (suspensions/solutions). Along with resolving the handling issues, liquid feedstock-based plasma spray introduce micro- and nano-features such as columns, lamellae, pores, vertical cracks, etc. into the deposited coatings [3]. The coating microstructures can be controlled by varying different parameters involved in spraying such as plasma source, injection type, solvent type, standoff distance, solid concentration, particle size, etc. [3]. Once injected, the suspension or solution precursor interacts with the plasma jet and due to kinetic and thermal energy exchange, hits the substrate into a molten or semi-molten state. Depending on the droplet size and …