This chapter aims to discuss the evolution of electroless nickel coatings with respect to their tribological behavior with special emphasis on their applicability at high-temperature-based applications. Electroless nickel coatings have tremendous potential as anti-wear and anti-friction coatings under ambient condition. The investigation of their tribological properties at high temperatures is relatively new. At demanding conditions, most conventional lubricants lose their properties and hence the use of self-lubricating coatings is unavoidable. Due to high melting point of nickel and oxidation resistance, electroless nickel-based coatings may prove to be suitable candidate at high temperatures. A review and analysis of the tribological characteristics of electroless nickel coatings especially at high temperatures is therefore necessary and has been reported in this chapter. Future research directions for the improvement of coating properties at high temperature are also identified from the review.
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Reduction of friction and wear of machine parts where tribological contact exists by the application of surface coatings is being widely investigated by researchers since excellent surface properties may be achieved without altering the bulk properties of the material. Amongst several popular coating techniques, the electroless method has gained immense importance since overwhelming results have been achieved by the appropriate control of coating bath parameters. High hardness, wear resistance, corrosion resistance, low coefficient of friction (COF) and uniform deposition are the key properties of electroless nickel (EN) coatings (Sahoo & Das, 2011). Since it is an autocatalytic process and avoids the use of electricity a wide variety of substrates may be coated with ease. In general, an electroless bath comprises of a source of nickel ions, reducing agent, complexants, buffers, stabilizers and surfactants (Loto, 2016). Appropriate control of the bath parameters lead to enhanced corrosion resistance as well as tribological behavior. EN coatings finds use in aerospace, automobile, chemical, electrical, electronics, textile, food processing, mining machinery, etc. industries due to their outstanding properties compared to the electro-deposited ones (Sudagar et al., 2013).
EN coatings have evolved over the last few decades as an advanced material with tremendous potential especially in cases where tribological contact takes place under aggressive environments. The coatings may be classified as pure nickel, alloy and composite coatings (Sudagar et al., 2013; Sahoo & Das, 2011; Agarwala & Agarwala, 2003; Mukhopadhyay et al., 2016a). Pure nickel coatings were manufactured by using hydrazine as reducing agent which has been discontinued lately due to the associated hazards (Sudagar et al., 2013). EN alloys are obtained mainly from sodium hypophosphite and sodium borohydride based baths resulting in formation of Ni-P (Sahoo, 2009; Mukhopadhyay et al., 2016b, 2016c; 2017a; Sahoo and Roy, 2017) and Ni-B (Krishnaveni et al., 2005; Vitry et al., 2010, 2012a, 2012b; Vitry & Bonin, 2017a, 2017b; Mukhopadhyay et al., 2016a, 2016d; Duari et al., 2016a) coatings respectively. Several metals could be alloyed with the binary coatings such as tungsten (Balaraju & Rajam, 2009; Balaraju et al., 2012; Palaniappa & Seshadri, 2008; Mukhopadhyay et al., 2016e, 2016f; Duari et al., 2017), copper (Balaraju & Rajam, 2005; Yu et al., 2001; Liu et al., 2015; Sahoo & Roy, 2017; Duari et al., 2016b), molybdenum (Vargas Mendoza et al., 2006; Balaraju et al., 2014), tin (Popoola et al., 2016; Balaraju et al., 2007), etc. Depending upon the alloying element, the properties of EN coatings such as hardness, corrosion/wear resistance, COF, magnetic properties etc. may be improved. EN composite coatings are obtained by the incorporation of hard ceramic or soft self lubricating particles along with the binary variants. These particles include Al2O3, TiO2, SiC, diamond, ZnO, SiO2, hBN, Si3N4, B4C, PTFE, MoS2, WS2, etc (Agarwala & Agarwala, 2003; Gadhari & Sahoo, 2016). Recent developments include the development and exploration of properties of carbon nanotubes (CNTs) co-deposited in EN coatings (Sudagar et al., 2013). With such a wide range of the deposits and its flexibility to coat even non-conducting substrates, EN coatings have provided an effective solution to a variety of engineering problems starting right from reduction of friction, wear and corrosion to decorative purposes. Although sodium hypophosphite based coatings enjoy a dominant share in the market, recent investigations are exploring the properties of sodium borohydride based coatings due to their higher hardness, wear resistance and anti-friction properties. Needless to say, with such a wide range of applicability, EN coatings are increasingly replacing chromium due to its environmental hazards.