Optimization of Friction and Wear Properties of Electroless Ni-P-Al2O3 Composite Coatings

Introduction

In industries machine components, tools and other equipments are mostly related to sliding and rolling type of motion in between them. In such situations the components or tools may be failed due to lower hardness and wear resistance. Hence it is essential to improve the hardness, wear resistance and frictional resistance of said components to increase their life. In present days coating plays vital role in surface engineering techniques, as these are well suitable for excellent wear resistance, frictional resistance, erosion, corrosion resistance and higher hardness instead of protecting the base metal from environmental changes. Electroless coating or autocatalytic coating is defined as the deposition of a metallic coating by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. The main advantages of these coatings are, these are having uniform and smooth thickness irrespective of complicate shape and sharp edges of base metal and it can be possible to coat nonferrous or electrically non conductive material. This process depends on the existence of a reducing agent, which reacts with metallic ions to deposit metal. When the substrate is dipped in the electroless bath, it develops potential. Due to this negative and positive ions are attracted towards the surface of the specimen and release the energy charge transfer process. Electroless bath consist of source of metallic ions, source of reducing agent, complexing agent, wetting agent, stabilizer, and pH controlling agent. The electroless Ni-P coating having more than 7% phosphorus has an excellent corrosion resistance. Therefore these coatings are used in industries which have corrosive environment like as mining, chemical, textile, oil and gas etc. Similarly the composite coating made by using hard/soft particles are having higher hardness, corrosion resistance, wear resistance and frictional resistance, hence these are widely used in variety of industries like as chemical, automobile, textile, mechanical and aerospace etc. (Balaraju et al., 2003). Basically electroless coating are classified into four group viz., alloy and poly alloy coatings, composite coatings, nano coatings and pure nickel and black coatings (Sudagar et al., 2013).

Electroless nickel composite coatings are formed by incorporating fine inert soft/hard particles into electroless nickel coatings. In these coatings second phase particles are appropriately wrapped by Ni-P layer during metal deposition process. Soft particles like as PTFE. MoS₂, HBN, and graphite are strongly recommended for composite coatings which are having higher corrosion resistance and lubricity. Whereas hard particles SiC, WC, Al₂O₃, Si₃N₄ CeO₂, TiO₂, ZrO₂, and diamond etc. are recommended to increase the hardness, wear resistance, corrosion resistance, and frictional resistance (Sharma and Singh, 2013). In fact the properties of electroless nickel composite coating are depending on amount of hard/soft particles embedded in the coating. To obtain excellent properties of composite coatings, the second phase particles must be uniformly distributed during deposition process otherwise due to non-uniform distribution of particles numerous defects are formed owing to segregation and agglomeration of composite or nano particles in the electroless bath (Sahoo and Das, 2011). After heat treatment at 400ºC or more for one hour, the mechanical and tribological properties of composite coatings are improved impressively (Apachitei et al., 2001). Compare to various hard particles available for composite coating, Al₂O₃ is the most important due to its high elastic modulus, strength retention at high temperature, and high wear resistance (Abdel Aal et al., 2007). By and large, the existence or availability of composite particles in the electroless Ni–P composite coatings depends on impingement of particles and their holding time on the on the surface of the coating (Balaraju et al., 2006). Alirezaei et al. (2004) have found that the deposition rate, deposition percentage of particles, roughness and hardness of Ni–P–Al₂O₃ composite coatings depend on the concentration of alumina particles in the electroless bath and also confirmed that hardness and average roughness are increased with Al₂O₃ particles content.