The present chapter aims to determine optimal tribo-testing condition for minimum coefficient of friction and wear depth of electroless Ni-P, Ni-P-W and Ni-P-Cu coatings under lubrication using grey relational analysis. Electroless Ni-P, Ni-P-W and Ni-P-Cu coatings are deposited on AISI 1040 steel substrates. They are heat treated at suitable temperatures to improve their hardness. Coating characterization is done using scanning electron microscope, energy dispersive X-Ray analysis and X-Ray diffraction techniques. Typical nodulated surface morphology is observed in the scanning electron micrographs of all the three coatings. Phase transformation on heat treating the deposits is captured through the use of X-Ray diffraction technique. Vicker's microhardness of the coatings in their as-deposited and heat treated condition is determined. Ni-P-W coatings are seen to exhibit the highest microhardness. Friction and wear tests under lubricated condition are carried out following Taguchi's experimental design principle. Finally, the predominating wear mechanism of the coatings is discussed.
TopIntroduction
Surface modification techniques via the application of coatings have received considerable attention in a quest to achieve enhanced physical, mechanical, electrical and tribological properties of industrial products. Amongst several techniques, deposition of nickel alloy and composite coatings by the electroless method has been widely accepted and investigated by researchers due to their excellent mechanical and tribological properties (Sahoo & Das, 2011; Sudagar et al., 2013; Loto, 2016). A key feature of the coatings is the deposit uniformity i.e. intricate parts receive equable deposition like the exposed ones. Moreover, the use of electricity is not involved allowing deposition over different substrates with proper surface activation (Mukhopadhyay et al., 2016a; 2017a). This led to improvement in properties of magnesium alloys, glass/carbon fiber reinforced composites, etc. by the deposition of EN coatings (Correa et al., 2017; Balaraju et al., 2016; Zuleta et al., 2017). Therefore, electroless deposited nickel coatings find wide usage in electrical, electronics, chemical, aerospace, automobile, printing and textiles industries (Agarwala & Agarwala, 2003; Gadhari & Sahoo, 2016a).
Electroless nickel (EN) coatings are broadly classified as pure nickel, alloy and composite coatings. Pure nickel (99%) is obtained from hydrazine baths and has been discontinued lately due to the associated hazards. EN alloy coatings are primarily obtained from sodium hypophosphite and sodium borohydride based baths leading to the formation of Ni-P (Hur et al., 1990; Sahoo, 2009) and Ni-B (Das and Sahoo, 2011; Duari et al., 2016; Mukhopadhyay et al., 2016b; 2016c; 2017b; 2017c) coatings respectively. Electroless Ni-P coatings have excellent anti-wear, anti-friction and anti-corrosion properties (Staia et al., 1996; Sahoo, 2009; Mukhopadhyay et al., 2016d; 2017a; Panja & Sahoo, 2014; 2015; Panja et al., 2016). Ni-P coatings are also being investigated as potential candidates for high temperature applications (Kundu et al., 2016; Masoumi et al., 2012; Ghaderi et al., 2016). The tribological properties are enhanced on heat treatment and the incorporation of composites like Al2O3 (Gadhari & Sahoo, 2016b), TiO2 (Gadhari & Sahoo, 2016c), SiC (Franco et al., 2016), MoS2 (Li et al., 2013), WS2 (Sivandipoor & Ashrafizadeh, 2012), PTFE (Der Ger et al., 2003), SiO2 (Sadeghzadeh-Attar et al., 2016), diamond (Reddy et al., 2000), etc. Nitriding improves the adhesion of the coatings with the steel substrate (Soares et al., 2017). Duplex and multi-layered coatings are also observed to improve microhardness, corrosion resistance and tribological behavior of EN coatings (Song et al., 2017; Vitry & Bonin, 2017; Liu et al., 2016). Usually, the deposits in their as-plated condition may be a mixture of amorphous and nanocrystalline phases or completely amorphous depending on the amount of P. The crystallinity of the coatings increases with an increase in the P content while the microhardness decreases (Czagány et al., 2017). Heat treatment leads to the precipitation of crystalline nickel and its phosphides. These hard phases improve the microhardness and wear resistance. The wear mechanisms reported for Ni-P alloy and its composite variants are adhesive, abrasive, spalling, fatigue, etc. depending on the test conditions.