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Top1. Introduction
Since the inception of electroless coating, among all the variants, electroless nickel-phosphorous (Ni-P) coatings have been globally accepted because of its salient features such as uniform deposition, excellent corrosion resistance with high hardness and superior wear resistance (Sudagar et al., 2013; Sahoo and Das, 2011; Loto, 2016; Panja et al., 2014; 2015; 2016; Panja and Sahoo, 2014a; 2014b; 2014c; 2015a; 2015b). The hardness of as-plated Ni-P coating varies inversely with phosphorus content, and for heating temperature, maximum value attained at 400°C. Further, enhance in heat treatment temperature liable for diminishing hardness value (Keong et al., 2003). Ni-P specimens that are heat-treated at 400°C for 1h (Staia et al., 1996) proven to provide maximum hardness as well as maximum intensity of resistance against wear. The hardness and wear resistance of the coating described above can further be increased by the incorporation of second phase particles in the same matrix. Some of the particles which are responsible for doing so are titania, alumina, SiC, B4C, diamond, etc. (Sharma and Singh, 2013). Apart from that, few soft particles such as WS2, MoS2, PTFE (poly tetra fluoro ethylene), and graphite (Wu et al., 2006) are responsible for providing lubricating action as well as to improve corrosion performance of electroless nickel coatings. The details on wear and corrosion resistance of electroless composites have been discussed by Agarwala and Agarwala (2003) and Balaraju et al. (2003). Gadhari and Sahoo (2016) made a detailed discussion on the preparation of nickel-based composite coating through the incorporation of various hard/soft particles (micro/nano size) in the Ni-P matrix for better tribological and mechanical properties. Under the variety of hard particles available, Al2O3 is the most significant, because of its high thermal stability and inertness (Vasudev et al., 2020; Prashar et al., 2020) as well as higher elastic modulus (Aal et al., 2007). Various studies related to the mechanism of formation of Ni-P-Al2O3 composite coating are available (Alirezaei et al., 2012; Novak et al., 2010; Zhou et al., 2008). An investigation on the impact of heat treatment on the microhardness of electroless Ni-P composite coating (Apachitei et al., 1998) demonstrated that the particles adhered into the Ni-P lattice increase the hardness of the deposit and such composite coatings are a lot harder than the Ni-P deposit. After heat treatment at 400°C, the hardness of both Ni-P and composite coatings enhances, but composite coatings indicated higher hardness in contrast to the electroless Ni-P deposits. A comparative study concerning the corrosion performance of Ni-P and Ni-P-Al2O3 coating is also performed, but there is a shortage of studies related to the wear performance of the same (Balaraju and Rajam, 2006). The effects of various concentrations of Al2O3 particles and heating temperature on tribological, as well as corrosion performance has been evaluated by Gadhari and Sahoo (2017). They found that 10g/l concentrations of alumina particle with heat treatment at 400°C provided excellent results among the range of parameters chosen for their experimental work. Another study considering Ni-P-Al2O3 nanocomposite coatings shows better hardness, wear, and corrosion resistance in contrast to Ni-P deposits (Makkar et al., 2015).