SYNTHESIS AND TRIBOLOGICAL CHARACTERISATION OF Ni-P BASED ELECTROLESS COMPOSITE COATINGS

Abstract

Non-crystalline solids like amorphous metallic alloys have emerged as an important class of novel industrial materials, which has received considerable attention of scientific community in the recent years. Amongst the various techniques used to prepare amorphous solids, electroless deposition is an important method, established over the years. In electroless coating technique, the alloys with or without external reinforcement are deposited in the form of a uniform film on a catalytic surface by a reduction reaction. Since the inception of electroless coating by Brenner and Riddel in 1946, it has become a subject of intensive research and during the past decade, emphasis has shifted to the studies of its properties and applications. The simplicity of the technique has credited many industrial usage of this method. This valuable process can coat not only electrically conductive materials including graphite but also fibers/fabric and insulators like plastics, rubber etc. The co-deposition of externally added particulate matter within the growing film has led to a new generation of electroless composite coatings, many of these coatings possess excellent wear and corrosion resistance. Relatively higher hardness of electroless coatings obtained inherently, could be further enhanced by suitable heat treatment which has made this coating a suitable candidate for many applications involving wear. Despite the various materials in use - alloys, ceramics and polymeric composites, employed for wear resistant applications, much of the wear research being conducted is directed towards coatings. In the face of development of new wear resistant coatings, electroless coatings have succeeded to retain their place in wide ranging wear resistance applications. It may not be out of place to list a few important industrial applications of electroless nickel based coatings to combat wear as in ball studs, differential pinion ball shafts, disc brake pistons, knuckle pins, piston heads, bearing journals, hydraulic actuator splines, gyro parts, sun gears, loom ratchets, tank turret bearings, rotor blades, stator rings, precision tools etc. Generally, co-depositing particles in electroless nickel and particle concentration in deposits depend on various factors like bath composition, particle ii characterisation, operating condition etc. Each combination between a certain type of particles and the Ni-P matrix can lead to a new set of properties, therefore, a multi-criteria decision is involved in the selection of appropriate coating system for a specific substrate. The electroless coatings offer a unique advantage of tailoring the desired properties by selecting the composition of alloy/composite/metallic coating and suitable heat treatment to suit the specific requirement. The electroless composite coatings possesses some unique features like uniform deposition, high hardness, amorphous and microcrystalline nature in as-deposited condition, improved hardness and adhesion by suitable heat treatment etc. Also, Ni-P coated graphite provides better wettability characteristics with molten aluminium alloy. The present investigation is aimed to develop an electroless composite coating by in-situ co-precipitation of alumina and zirconia followed by their co-deposition into Ni-P matrix and analyse the friction and wear behaviour of the newly developed electroless Ni-P-X (X = ZrO2-A1203-A13Zr) composite coated steel and aluminium under dry sliding condition using a pin-on-disc wear testing machine in the specific context of coating composition and heat treatment. Also the work is aimed to coat the carbon fabric using the electroless technique and assess the feasibility of reinforcement of coated fabric into aluminium alloy. In the present study, synthesis and characterisation of a new electroless Ni-P based composite coating by in-situ co-precipitation of A1203 and ZrO2 followed by their co-deposition along with Al3Zr on three different substrate materials viz., commercial aluminium, low carbon (0.14%) steel and carbon fabric have been carried out. The coating characterisation in terms of qualitative and quantitative analysis has been dealt with. Steel and aluminium pin wear samples of cylindrical shape, having flat surface with rounded corner have been coated and then subjected to wear study in different conditions, i.e., as-coated and heat treated. Steel substrates were coated with Ni-P-X and Ni-P followed by wear testing in as-coated, heat treated at 400°C for 1 h and 400°C for 2 h conditions whereas aluminum substrates were coated with Ni-P-X followed by wear testing in as-coated, heat treated at 400°C for 1 /7 and 250°C for 12 h conditions. The friction and wear behaviour of above types of pins has been analysed and compared with those iii observed for uncoated substrate material of normalised steel and commercial aluminium. The present work also focuses on electroless Ni-P-X and Ni-P coatings on carbon fabric as substrate and the feasibility of reinforcement of coated carbon fabric into aluminium alloy by use of conventional casting method. Chapter 1 underscores the salient features of the electroless nickel based coatings in terms of their industrial applications and scientific research. Chapter 2 is a critical review of literature explaining the different electroless coating systems, bath types, coating mechanism, composition and properties of coatings by various experimental techniques. The effect of different coating parameters like concentration of nickel sulphate, concentration of sodium hypophosphite, bath temperature, pH, bath loading area and time has been summarized. Structure and crystallisation kinetics in electroless Ni-P coatings have been outlined. The present day knowledge of coating tribology and contact mechanisms in general and wear studies on electroless nickel based coatings in particular has been summarized on the basis of available literature. The limited knowledge on the coating composition and wear behaviour of both the electroless alloy and electroless composite coating systems has been highlighted as these variables have the relevance to the present investigation. The knowledge base concerning electroless coated carbon fibers, vitreous carbon and wettability characteristics has been outlined. In the end of this chapter, the formulation of problem is presented. Chapter 3 deals with the experimental techniques for electroless coatings and their characterisation as adopted in the present investigation.. The details of the electroless coating setup, plating bath and co-precipitation reaction used, have been described. The selection of coating parameters has been carried out based on maximum coating weight achieved. Synthesis of electroless Ni-P and Ni-P-X on three different substrate materials such as aluminium, normalised steel and carbon fabric are outlined in respect of pretreatment and surface preparation for coating. The method followed for the chemical analysis of coatings has been discussed. The experimental details for the heat iv treatment carried out and the hardness tests performed have been described. The X-ray diffraction analyses of the co-precipitated powder, as-coated and heat treated electroless coatings, uncoated and coated carbon fabric and the wear debris generated during dry sliding wear tests have been carried out to identify the phase constituents following the method described. The procedure used to confirm the phase constituents in coatings by TEM has been outlined. The procedure of sample preparation and make/model of different tools used in metallographic studies conducted have been summarized in respect of the qualitative and quantitative analyses of the coating. The different characterisation tools used in the present investigation viz., Optical microscope, SEM, TEM, X-ray diffraction, EPMA (both qualitative and quantitative), DSC etc., also have been summarised. The principle used for non-isothermal studies by DSC also fbrms a part of this chapter. Dry sliding wear tests carried out for different loads on electroless coating—substrate systems and the substrate have been elaborated. The details of the pin-on-disc wear testing machine, pin shaped specimen and the counterface of En-32 steel hardened to HRC 62-65 used in the study have been provided. In determination of friction and wear of normalised steel (NS), Ni-P-X as-coated steel (AC1), Ni-P-X coated and heat treated at 400°C for 1 h (I IT,-A), Ni-P-X coated and heat treated at 400°C for 2 h (HT2-A), Ni-P as-coated steel (AC2), Ni-P coated and heat treated at 400°C for 1 h (HT1-B) and Ni-P coated and heat treated at 400°C for 2 h (HT2-B) three normal loads viz., 29.4, 34.3 and 39.2 N have been used whereas for commercial aluminium (AL), Ni-P-X as-coated aluminium, Ni-P-X coated and heat treated at 400°C for 1 h (HTI-C) and Ni-P-X coated and heat treated at 250°C for 12 h (HT2-C) four normal loads viz., 4.9, 7.4, 9.8 and 12.3 N have been used. All the tests were carried out at constant sliding speed of 0.5 m/s and in the relative humidity range of in-between 75-80 %. Wear surfaces of the pin after dry sliding have been examined under SEM while both the optical microscope and the SEM are used for examination of the wear debris generated during dry sliding wear tests. Procedure used for tensile testing of uncoated carbon fabric, Ni-P-X as-coated and heat treated at 400°C for 1 h, Ni-P as-coated and heat treated at 400°C for 1 h has been described. The method followed for reinforcement of uncoated and Ni-P-X coated carbon fabric in aluminium by casting and their examination under SEM and EPMA X-ray mapping has been outlined. Chapter 4 describes the synthesis and characterisation of Ni-P-X and Ni-P based electroless coatings in as-coated and heat treated conditions. In order to optimize the bath parameters for the new composition of the electroless bath used for synthesis of Ni-P-X coatings, selection of electroless bath temperature and pH for coating has been carried out by using an experimental design technique. Further, an attempt has been made to explain the process in terms of selection and optimization of coating condition, including bath loading factor and coating time. With the acquired data, an empirical model for the variation of coating weight, Win mg, with time, 1, of Ni-P-X deposition is as given below. W=k11") (1) where, k1 is the coefficient and /// is an exponent. The coefficient, k1 significantly changes with area immersed for the coating but the exponent iii does not change much with the area. The average value of ni has been found to be 0.73. The coefficient, k1 as a function of area is given by k1 = 0.59 AS 0.96 (2) where, A, is the area of the substrate in cm2. The coating weight for composite coating has been experimentally found to be saturating after coating time of about 45 min which is irrespective of the surface area immersed in the electroless composite coating bath used in the present work. The reproducibility of the coating has been assessed by the repeated deposition of Ni-P-X and Ni-P on different pin shaped substrates used. The coating rate for composite coating under consideration is about 1.50 times greater than that of Ni-P alloy coatings for similar operating conditions. The co-precipitated powder that settled at the bottom of the electroless bath has been subjected to microscopic examination and X-ray analysis and the results reveal that the powder consists of very fine particles and a few large agglomerates vi of oxides of aluminium and zirconium. Characterisation of as-coated Ni-P-X coatings are carried out by metallographic studies of different coatings. The coating mechanism has been studied in terms of globule sizes and its frequency of distribution. The electroless coating has nucleated at several isolated sites and have grown both laterally and vertically to cover the entire surface of the substrate. The deposition of hemispherical islands growing gradually to cover the substrate area by repeated nucleation and lateral growth has been observed. This mechanism has been observed for both the Ni-P alloy and the Ni-P-X composite coating for all three substrate materials under this study. Uniform layer of coating and its thickness obtained have been confirmed by SEM micrographs of the cross section of coating. The results of qualitative and quantitative analyses of the coatings by different analytical tools viz., XRF, X-ray diffraction, SEM-EDX, TEM, EPMA and DSC have been discussed. Qualitative studies depicted the uniform distribution of the second phase particles such as ZrO2, A1203, and A13Zr in the composite coatings. The particles co-deposited in electroless Ni-P-X composite coating by using an in-situ co-precipitation reaction are distributed in nanosized particles and a few regions consisting of relatively coarser agglomerates of these particles. The as-coated deposits show the typically microcrystalline nature of the film. Quantitative study reveals that the presence of second phase particles result in a little decrease in phosphorus content. The electroless Ni-P-X composite coating primarily consists of Ni/Ni,Py phases and has the crystallization temperature of around 375°C. From the DSC studies, the non-isothermal kinetics of transformations in electroless Ni-P and Ni-P-X coatings have been observed for different heating rates of 10, 15, 20 and 25 °C/min and Avrami exponent (n) and activation energy k.hno/) are calculated by using Kissinger method. The growth dimension for composite coating is observed to be marginally less than that for the Ni-P coatings. The as-coated specimens with different substrates are subjected to heat treatments. The coated steel samples are heat treated at 400°C for 1 h and 2 h whereas coated aluminium samples are heat treated at 400°C for 1 h and 250°C for 12 h. The micro and macro hardness data for coatings in different conditions: as-coated and heat treated, have been presented. The heat treatment applied to substrate along with coating at 400°C for 1 h experienced the vii maximum hardness, which is about twice the hardness of as-coated specimens and this is attributed to the formation of Ni3P phase. The presence of A1203, AI3Zr and ZrO2 particles, which are embedded in the Ni-P matrix, also contributes to improvement in hardness. The tribological behaviour of electroless Ni-P-X and Ni-P coatings on steel substrate in different conditions, i. e., as-coated, heat treated at 400°C for 1 h and heat treated at 400°C for 2 h under different normal loads of 29.4, 34.3 and 39.2 N are studied in terms of dry sliding friction and wear against counterface of case hardened steel and reported in chapter 5. The primary objective of the study is to understand the effect of the coating and load on the friction and wear behaviour of coated steel substrate in both the as-coated and the heat treated conditions. The results have been discussed to develop a coherent understanding of the tribological behaviour in terms of its correlation with the coating composition and hardness. The cumulative volume loss has been considered as basic data for wear performance of pin type samples. The cumulative volume losses for different types of pins have been observed to increase linearly with increasing sliding distance. The cumulative volume loss has also been observed to increase with increasing applied normal load. The Ni-P-X coating on steel substrate has been found to result in significant improvement in the resistance to wear in terms of cumulative volume loss and wear rate. A further improvement in the resistance to wear is observed after heat treating the coated steel samples. The wear rate increases with increasing load but that decreases with increasing hardness of the pin. Therefore, the wear rate variations are primarily associated with the change in the real area of contact at different normal loads and hardness of the pin. The coefficient of friction is found to fluctuate about its mean value and then stabilizes for almost all types of wear pins used. The average coefficient of friction has been observed to decrease with increasing applied load. The examination of worn surfaces under SEM has been conducted to observe the nature of wear tracks and the formation of transfer layer. The micrographic studies and X-ray diffraction analysis of wear debris collected during wear tests, have been carried out to understand the mechanism of wear. The wear debris particles collected during the test of normalised steel substrate and electroless Ni-P-X and Ni-P as-coated steel pins, are relatively fine but those for coated and heat treated pin specimens are of coarser flake shapes. Fine debris of oxide contains the oxide agglomerates and the coarse and flaky particles of transfer layer must have resulted due to flaking off the transfer layer. The tribological behaviour of Ni-P-X coatings on aluminium substrate in different conditions, i. e., as-coated, heat treated at 400°C for 1 h and heat treated at 250°C for 12 h have been studied in terms of dry sliding friction and wear against counterface of case hardened steel for different normal loads of 4.9, 7.4, 9.8 and 12.3 N as reported in chapter 6. An attempt is made to understand the effect of load on the dry sliding wear and friction for electroless coatings in as-coated and heat treated conditions in terms of cumulative volume loss, wear rate and coefficient of friction. The results have been compared with those obtained for the aluminium substrate. The debris particles collected have been studied by micrographs and X-ray diffraction analysis. The tribological behaviour of these systems with aluminium substrate has been explained in the context of coating composition and hardness. The wear rates of uncoated aluminum substrates are significantly higher than that observed for electroless Ni-P-X coated aluminium. The heat treatments to the coated aluminium pins further improved the resistance to wear. Coating composition, normal load applied and hardness of both the coating and substrate influences the wear rate of the coated substrate. in the dry sliding wear of aluminum substrate and as-coated electroless Ni-P-X aluminium, the debris particles are relatively fine which may primarily formed due to the oxide layer formed on the sliding surface. The debris of the latter contains a few particles and larger agglomerates of oxide. The debris from heat treated coatings are coarse and flaky particles of transfer layer which result from the flaking off the transfer layer of oxide on the sliding surface. Carbon fibers play an indispensable role in the manufacturing of reinforcement metal matrix composite (MMC) and Ni-P-X coatings can facilitate the reinforcement to have better usage in fabric reinforced into aluminium alloys. Therefore, an attempt is made to study the tensile behaviour and feasibility of reinforcement of Ni-P based coated carbon fabric into aluminium matrix and described in chapter 7. The primary objective of this ix segment of investigation is to apply electroless Ni-P-X and Ni-P coatings on carbon fabric and their characterisation in terms of uniformity, composition and phase constituents present. The characteristics of the coated fabric have been compared with those of the coatings on metallic substrates of aluminium and steel as explained in chapter 4. The uniformity of the coated fibers with about 0.8 ,urn thickness has been revealed by microscopic studies. The ultimate tensile strength (UTS) of the coated fabric in as-coated and heat treated (400°C for 1 h) conditions have been compared with that of uncoated carbon fabric. A little improvement in UTS on coating has been found but after heat treatment, a significant increase in UTS is observed. The feasibility of reinforcement of aluminium matrix by the coated carbon fabric using conventional casting technique with the help of specially designed carbon cloth holder in the mould is demonstrated. The micrographs of the carbon fabric zone into aluminium matrix from castings with and without coating on the fabric samples have been shown. The X-ray mapping under EPMA has been used for assessing the different elements near the interface that revealed chemical reaction at the interface between coating layer on the fabric and aluminium matrix. The major conclusions on the basis of above experimental studies have been enumerated in chapter 8. In-situ co-precipitation of alumina—zirconia within the conventional electroless nickel bath can be used for synthesis of electroless Ni-P-X composite coating. This type of coating results in submicron and nanosized islands of the deposits consisting the second phases within the globular structure of Ni-P matrix. In certain regions, clusters of the particles are also observed. The Ni-P-X composite coating is expected to have better consistency in composition and relatively higher coating rate when compared to those in Ni-P coatings carried at the same conditions. Uniform and well adherent electroless Ni-P-X coating can be applied successfully on aluminium, steel and carbon fabric substrates. The Ni-P-X coated steel and aluminium samples exhibit substantially lower cumulative wear volume and wear rates, against hardened steel counterface during dry sliding at 0.5 m/s sliding speed, in the load ranges of 29.4-39.2 N and 4.9-12.3 N respectively when compared to corresponding substrates. The wear rates further decrease due to the heat treatment of coated samples. Operating wear mechanism is primarily oxidative under the given conditions during dry sliding in both the as-coated Ni-P-X and the heat treated after coating the samples. Incorporating ceramic particles like X (Zr02-A1203-A13Zr) in Ni-P coating has the potential of further enhancing wear resistance if it could be incorporated in a relatively larger amounts but the coefficient- of friction may increase. The segment of the investigation dealing with the Ni-P-X and Ni-P coatings on carbon fabric reveals that the reinforcement of coated fabric in aluminium melt is feasible. xi