Study of Hardness and Corrosion Resistance of Electroless Ni-P-TiO2 Composite Coatings

Introduction

Electroless coating is the method of plating metal by chemical method instead of electrical means. In this method specimen to be coated is immersed in the chemical bath with suitable bath temperature. Such coatings are successfully applied to both ferrous and non-ferrous surfaces of any geometry. The failure of machine components in the industries is mainly due to corrosion and wear. It increases the downtime, maintenance cost and the replacement cost, which ultimately affects on economy of the industries. Both corrosion and wear are surface phenomena and occur on the surface of the components. Electroless nickel (EN) coatings are best suitable to protect them against corrosion and wear. Such coatings are extensively used in industries due to their excellent mechanical, tribological, soldering and brazing properties (Sahoo and Das, 2011; Sahoo, 2008a-d). Pure nickel (black nickel), alloy and poly alloy coatings, composite coatings, and nano coatings are the basic types of electroless coatings (Sudagar et al., 2013).

Incorporation of fine inert second phase particles in the electroless coating is known as electroless composite coating. The EN composite coatings which contain soft particles such as MoS₂ (Mohammadi et al., 2010), PTFE (Ramalho and Miranda, 2005; Omidvar et al., 2008; Zhao and Liu, 2005a), HBN (Zhang et al., 2008), WS₂ (Sivandipoor and Ashrafizadeh, 2012) and graphite are known as lubricated composite coatings. Similarly, the EN composite coatings which contain hard particles such as, Al₂O₃ (Alirezai et al., 2004; Sharma and Singh, 2011; Hamdy et al., 2007; Gadhari and Sahoo, 2014), TiO₂ (Wang et al., 2000; Hosseini and Bodaghi, 2013; Novakovic et al., 2006), B₄C (Araghi and Paydar, 2010; Vaghefi and Saatchi, 2003), SiC (Allahkaram et al., 2011; Malfatti et al., 2005; 2009; Zarebidaki and Allahkaram, 2011), Si₃N₄ (Balaraju et al., 2010; 1998; Ramesh et al., 2009; Krisnaveni et al., 25), ZrO₂ (Stankiewicz and Szczygiel, 2012; Stankiewicz et al., 2012) and diamond (Bozzini et al., 2001; Jappes et al., 2009; Xu et al., 2005; Reddy et al., 2000) are known as hard composite coatings. The as-deposited coatings with higher phosphorus content have higher corrosion resistance. In composite coatings, the phosphorus content decreases in the presence of composite particles. The EN coating with amorphous structure (as-deposited coatings) has higher corrosion resistance compared to crystalline structure (heat treated coatings) (Zarebidaki and Allahkaram, 2012). Leon et al. (2010) have observed improvement in corrosion resistance of heat treated (400°C) Ni-P- and Ni-P-Al₂O₃ composite coatings compared to as-deposited coatings. In some cases, it is observed that corrosion resistance of composite coatings increased due to large grain size, which reduces the porosity of the coating. To get outstanding properties of the composite coatings, the second phase particles must be uniformly distributed during the deposition process otherwise several defects are formed due to agglomeration of second phase particles. In composite coatings, micro cracks are formed due to presence of second phase particles, which results in decrease in corrosion resistance.