WEAR AND CORROSION BEHAVIOUR OF ELECTROLESS Ni-P BASED NANOCOMPOSITE COATINGS

Abstract

Wear and corrosion resistance is counted as the basic requirements for reliable and efficient service life of the engineering system. The purpose of conducting wear and corrosion tests of the coatings is to understand behavior of a particular family of materials under tribological applications and corrosive environments. Nickel based dispersion coatings are used in multitude applications where corrosion and wear resistance is required. Electroless coating technology is a controlled chemical reduction process which has emerged as a leading growth area in surface engineering, metal finishing. Here the driving force is supplied by chemical reducing agent in solution (sodium hypophosphite). The reaction continues as long as the surface remains in contact with the bath solution or solution gets depleted of metallic ions. Electroless nickel coatings are widely used in different industries because of their physical properties, such as good hardness, coating uniformity, wear and corrosion resistance, and capability of depositing on either conductive or nonconductive substrates. Nanocomposite coatings are the ones in which either the thickness of coatings or size of second phase particles that are embedded into the matrix is of nanosized. EL composite coatings can be developed when either the second phase(s) can be added during the coating or the second phase(s) can be generated to nucleate and grow within the bath. The incorporation of nanosized ceramic particles like alumina, zirconia and titania in Ni-P matrix offers many advantages in terms of hardness, friction coefficient, chemical inertness, and thermal stability of the resulting composite coatings. Hence a quest for improved wear and corrosion performances has led many researchers to develop and investigate either newer variants of electroless nickel based nanocomposite coatings or provide modification in the coating process parameters. Moreover, due to complexity of the structure, wide variety of wear and corrosion apparatus, distinct environment, substrate and counterface medium used during the wear and corrosion tests, there is always a need regarding the systematic studies of wear and corrosion behavior of Ni-P-X nanocomposite coatings. Therefore, an effort to develop EL N-P-X [X= Al2O3, ZrO2 and TiO2] nanocomposite coatings by conventional and non-conventional means on mild steel substrate for wear and corrosion applications has been made in this thesis. The objective of the present work is to provide a better understanding of preparation methods used for the synthesis of nanoparticles incorporated in Ni-P matrix. This study also aims to develop EL Ni-P-X [X= Al2O3, ZrO2 and TiO2] nanocomposite coatings through conventional and non-conventional methods. These developed coatings are characterized, ii heat treated and subjected to wear, friction and corrosion tests and compared with Ni-P alloy coatings. The thesis is divided into nine chapters. Chapter 1 presents an introduction to the electroless Ni-P and Ni-P-X coating technology for enhancing the service life of components used in various industries. It also gives an insight into the reinforcements used along with their synthesis routes. This chapter also provides a brief idea about the need, aim and scope for the present work. Chapter 2 outlines a brief review of literature, discussing the advantages, mechanisms, condition required and limitations of EL Ni-P coating technology. The significance of EL Ni-P-X nanocomposite coatings to yield desired properties is also featured. It also discusses the beneficial aspects of reinforcements like Al2O3, ZrO2 and TiO2 into Ni-P matrix and a brief survey on their preparation modes are outlined. An extensive survey on studies where Al2O3, ZrO2 and TiO2 particles are incorporated in Ni-P matrix especially for wear and corrosion applications has also been presented. All these lead to formulation of problem for undertaking the present work. The proposed formulation of the problem is discussed in chapter 3. It defines the scope of present work based on literature review and the adopted methodology and the gaps of information found in the area of development of EL Ni-P-X [X=Al2O3, ZrO2 and TiO2] nanocomposite coatings for wear and corrosion resistance applications. Experimental details including raw materials used, experimental set-up, equipments used, process parameters employed in synthesis of Al2O3, ZrO2 and TiO2 nanoparticles by two approaches are discussed in chapter 4. The details including the development of EL Ni- P based nanocomposite coatings by two different means are also featured. The characterization techniques along with details of mechanical and physical properties tested are also presented in this chapter. Chapter 5 describes the synthesis of nanosized Al2O3, ZrO2 and TiO2 powders using two different approaches namely Bottom up and Top down. The chemical precipitation method was employed to synthesize Al2O3, ZrO2 and TiO2 nanoparticles. The thermal studies of the dried powders were carried out. Al2O3, ZrO2 and TiO2 nanoparticles of average particle size of 12nm, 7nm and 10nm were synthesized by this bottom up approach. Al2O3, ZrO2 and TiO2 nanoparticles were also prepared separately by mechanical milling in high energy ball mill at 250 rpm for 40h. The variation of particle size with respect to different extents of milling (5h, 10h, 20h, 30h and 40h) was also studied. Increase in milling time lowered the particle size and formed amorphous phases. The Al2O3, ZrO2 and TiO2 iii nanopowders of size range 15nm, 21nm and 39nm respectively were achieved after 40h of milling. The prepared nanopowders ‘X’ were used as the reinforcement for development of Ni-P-X [X= Al2O3, ZrO2 and TiO2] nanocomposite coatings as discussed in the chapter 6. Chapter 6 features the development of electroless Ni-P-X [X= Al2O3, ZrO2 and TiO2] nanocomposite coatings by conventional and non-conventional means where nanosized chemically prepared and milled powders are employed as reinforcements in conventional route. The optimal concentration of ‘X’ particles for sound Ni-P-X coatings in non-conventional route is determined. Its heat treatment effect (400oC for 1h) on morphology, phase and hardness studies are analysed with respect to Ni-P alloy coatings. The results revealed that the codeposition of ‘X’ nanoparticles into Ni-P matrix by conventional route results in surface modification of the coatings with presence of nodular protrusions as analysed by FESEM micrographs. A typical globular type morphology with no clear ‘X’ [X= Al2O3, ZrO2 and TiO2] particles can be observed on the surface of the insitu Ni-P-X nanocomposite coatings. The existence of nanosized ‘X’ particles can be confirmed by virtue of EDAX, X-ray mapping and XRD analysis. AFM studies depicts that nonconventional Ni-P-X nanocomposite coatings have relatively smooth surface over the conventional Ni-P-Al2O3 coatings. The thickness of the developed coatings is ~ 30μm showing good adhesion between the mild steel substrate and the coating. The optimum ‘X’ concentration is found to be 15g/l for sound non-conventional EL Ni-P-X coatings with higher hardness. All developed Ni-P-X nanocomposite coatings exhibits higher hardness than Ni-P deposits due to the reinforcement of hard second phase ‘X’ in Ni-P matrix and achieves maximum on heat treatment due to the precipitation of Ni3P phase. AFM studies reveal that the roughness of the coatings decreases after heat treatment. Non-conventional Ni-P-X nanocomposite coatings attained highest hardness over the two conventionally developed coatings. However, the conventional coatings containing milled nanopowders exhibited higher hardness over chemically prepared particles on account of its better adhesion on Ni-P surface. Chapter 7 discusses the results obtained on the friction, wear and corrosion resistance of EL Ni-P-X [X= Al2O3, ZrO2 and TiO2] nanocomposite coatings developed by conventional and non-conventional means and compared with Ni-P alloy coatings. The effect of loads (1, 1.5 and 2N) and sliding velocities (0.1 and 0.2m/s) on the specific wear rate of EL Ni-P and Ni-P-X nanocomposite coatings in heat treated condition are discussed. The results indicates an improvement in wear resistance and lower coefficient of friction for all EL Ni-P-X nanocomposite coatings with respect to Ni-P alloy coatings, both at different iv loads and sliding velocities owing to its higher hardness. Mild adhesive wear is the predominant mechanism taking place during wear for all Ni-P-X nanocomposite coatings. The electrochemical polarization curves for EL Ni-P and Ni-P-X nanocomposite coatings in ‘as coated’ and ‘heat treated’ conditions under different environments (pH-4.5 & 8) are analysed. EL Ni-P-X nanocomposite coatings are found to have a higher corrosion potentials (Ecorr) and lower corrosion currents (Icorr) than those of Ni-P coating, indicating an improved corrosion protection in both acidic (pH-4.5) and alkaline (pH-8) atmosphere. The heat treatment also enhances the corrosion resistance due to formation of compact, smooth and homogenous coatings and the trend is followed in both acidic and alkaline environments. The lower wear rate, friction coefficient and corrosion rate is showed by EL Ni-P-X nanocomposite coatings developed by insitu route compared to conventional ones. However, the inclusion of milled nanoparticles on Ni-P surface imparts slightly lower friction coefficient and wear rate but higher corrosion rate with respect to chemically prepared nanoparticles. The corroded surfaces are analysed with the help of FESEM micrographs. Comparatively, better corrosion resistance is observed in alkaline (pH-8) atmosphere as compared to acidic (pH-5) one. Chapter 8 draws the conclusion from this work and contribution made in this thesis on development of Ni-P-X [X=Al2O3, ZrO2, TiO2] nanocomposite coatings with respect to its hardness, wear, friction and corrosion characteristics. The scope for the future work on the area of EL Ni-P based nanocomposite coatings is suggested in Chapter 9.