Taguchi -Grey Based Optimization of Friction and Wear of Electroless Ni-P Coatings in Acidic Environment

Introduction

Most of the engineering components undergo rubbing action due to which wear takes place on the surface of the components and become useless after a certain period. Life and performance of these engineering components can be extended by applying hard coatings over the surface of the components. Among the various metallic coating method based on aqueous solutions, most of metals are electroplated since electroplating is technically straightforward and cheap than electroless deposition. Though, the importance of electroless deposition in industrial application can’t be denied mainly for copper, nickel, and some nickel based alloys. The electroless plating deposition was introduced by Brenner and Riddell (1946, 1950). Electroless Ni-P (EN) coating is an autocatalytic deposition of a Ni-P alloy from an aqueous solution onto a substrate without the application of electric current. It has got the ability to deposit uniformly on uneven surfaces or geometries. It provides a deposit that follows all the contours of the substrate without building up at the edges and corners (Riedel, 1991). EN coatings have found extensive use in surface engineering due to their excellent mechanical, physical, electrical, corrosion and tribological properties (Sahoo & Das, 2011; Sudagar, Lian & Sha, 2013; Agarwala & Agarwala, 2003). Electroless nickel coatings are used in different areas such as aerospace, aviation, automotives, oil and gas processing, food processing, microelectronics, radio electronics, computer engineering, chemical processing, textiles, machinery, mining and metallization of plastic etc (Sahoo, 2008). The mechanical and tribological properties of EN coatings can further be improved by the incorporation of hard particles, heat treatment (Ashassi-Sorkhabi & Rafizadeh, 2004; Keong, Sha & Malinov, 2002) and dry lubricants (Straffelini, Colombo & Molinari, 1999; Zhao, Liu, Muller-Steinhagen & Liu, 2002; Huang, Zeng, Annergren & Liu, 2003). The properties and microstructures of EN coatings depend on the amount of phosphorous alloyed in the deposit (Hur, Jeong & Lee, 1990). EN coating provides natural lubricity due to the presence of phosphorous content and it is further improved by annealing (Ramalho & Miranda, 2005; Novak, Vojtech & Vitu, 2010). Heating the coating at 400^oC for 1 h, results in highest hardness (Staia, Castillo, Puchi, Lewis & Hintermann, 1996; Wang, Gao, Xu & Xue, 2006). As harder materials generally encounter lesser wear, heat treated EN coatings are found to be more wear resistant than the as deposited ones. EN coating is also found to be good anti-corrosive properties due to the present of phosphorus content (Bai, Chuang & Hu, 2003; Nava, Davalos, Martinez-Hernandez, Manriquez, Meas, Ortega-Borges, Perez-Bueno & Trejo, 2013). Anti-corrosive properties of EN coating increase with increase in phosphorus content (Bigdeli & Allahkaram, 2009). In present day, EN coatings are widely used for corrosion protection application in a variety of environments viz. marine applications, mining industry, mineral processing, chemical industry, petrochemical industry, slurry handling and chlor-alkali industry etc. They act as barrier coatings, protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. However, only EN coating with high P coating is found to offer excellent protection against corrosion (Sankara Narayanan, Baskaran, Krishnaveni & Parthiban, 2006; Gawrilov, 1979). Due to the dynamics involved, these machineries would require protection against both wear and corrosion simultaneously as they undergo tribo-chemical interactions. Wide application of EN coating was used as corrosion resistance materials in acidic environment (Parkinson, 1997; Mallory & Hajdu, 1990), especially in chemical and petrochemical industries.