Effect of Electroless Nickel Coating on Wear Rate of EN8 Steel

Electroless Nickel coating has emerged as a major breakthrough in the development of novel materials with excellent tribological properties. In the present work Ni-P coating has been performed over EN8 steel specimens for an hour. Mechanical and tribological character was determined for the coated samples. Tribological tests were performed for various loads and the wear behavior of coated steel has been analyzed. The coated samples were found to be harder with improved frictional characteristics like lesser wear under similar working conditions.


Introduction
A Number of design applications involves wear and tear process, where parts or joints are subjected to relative motion. Wear can be considered as an undesired removal of material due to mechanical action which cannot be eliminated completely. However, in most of the cases alloys and composites are developed which possess low friction and low wear properties under dry sliding conditions against smooth metallic counterparts [1]. EN8 is an unalloyed medium carbon steel which is a popular choice in industry from the automotive trade to wider general engineering applications. Electroless Nickel-Phosphorus plating process is an industrial chemical plating process where a uniform layer of Nickel-Phosphorus alloy is deposited on the surface of the solid substrate. As the name suggests the electroless plating 4 does not require the passing of electric current [2]. EN coatings have a wide range of industrial applications including fluid handling valves & cylinders, medical equipment, owing to their excellent mechanical, electrical, corrosion and wear resistance properties. The electroless bath consist of a solution of metal ions, complex agents, reducing agents and stabilizers operating in a specific metal ion concentration, temperature and pH ranges [3]. The superior quality deposition improves the mechanical and microstructural properties of base metal. With a refined microstructural arrangement on the top surface, the cladding material exhibits outstanding properties like withstanding high temperature, impact loads and high wear resistance.
Various Electroless Nickel plating processes like Ni-P, Ni-B and pure Ni coating can be achieved based on the reducing agent used in the coating bath. Electroless plating process involving Hypophosphite as the reducing agent has received commercial success because of various factors such as low cost, ease of control and good corrosion resistance. Even though, electroless Ni-P deposits give satisfactory performance in many cases, enhancing their performance to suit different end uses warrants further development [4]. Many literatures depict a fact that electroless coating process is to impart smoothness and hardness to the surface for application areas requiring excellent tribological characteristics. The extent of friction and wear is highly responsible for limiting the life of several important links and joints in automotive and aerospace industry. The friction behavior of Electroless Ni-P coated materials can be modified through various surface treatments and incorporation of particles [5]. To improve the quality of deposition, complex agents such as Citrates, Phosphate and Succinate have been used in several experimental works reported so far [6,7,8]. However, there is very less reports on the effect of loading conditions on the coating behavior in tribological perspectives. Therefore, more experiments are needed to grow confidence on the behavior of coated medium carbon steels. In the present work electroless Ni-P coating has been performed on EN8 steel specimens to assess the mechanical and tribological behavior of coated steel samples under dry sliding conditions. The tribological characteristics has been experimentally analyzed for various applied loads on the specimens.

Methodology
Electroless coating can be considered as a 'self-reduction' process, which is primarily based upon the auto-catalytic reduction process of metal ions in an aqueous solution which contains sodium hypophosphite, NaH 2 PO 2 as a chemical reducing agent [9]. The main function of stabilizer is to prevent the decomposition of plating bath solution. Sodium acetate and Sodium hydroxide are used as buffer which control the pH and to obtain a uniform coating. Two types of baths have been mainly used for depositing Ni-B, Ni-P alloy which are acidic and alkaline baths depending on the reducing agent used. The commonly used Reducing agent of EN coating are Sodium hypophosphite for Ni-P Deposition and Sodium borohydride for Ni-B Deposition. Hypophosphite baths are the most common type of electroless nickel bath because of their higher deposition rates, increased stability and greater simplicity of bath control. The various bath components and their function are described in Table 1.

Coating Deposition
Electroless Ni-P coating is deposited on mild steel of two types: square and cylindrical. Square specimens are used for microstructural characterization related studies while the cylindrical ones are used for the friction and wear tests. The basic experimental setup is depicted in Fig. 1 Initially, specimens are mechanically cleaned to remove foreign matter or any kind of corrosion product. After that, remaining organic particles of steel specimen are cleaned off by rinsing with distilled water. Then the specimen is submerged in con. HCL for 15 seconds. After that the specimen is submerged in Palladium Chloride for 15 seconds at 55°C. Palladium chloride activation is given to each of the samples prior to the deposition to have a good initiation of the coating onto the mild steel substrate. After that the temperature of the heater is set to 82°C and the deposition of substrate material is to be done. The deposition was carried out for 1 hour at a stretch before the bath shows signs of decomposition.

Measurement of frictional parameters
Friction and wear characteristics of the EN-coated specimens are studied with pin on disk wear testing apparatus. The wear tests are performed against Coated cylindrical samples under dry, non-lubricated condition at room temperature. The experimental instrument includes a rotating specimen with disk shape and a cylindrical stationary specimen. The stationary pin specimen holder is attached to a lever arm that has a pivot. Addition of weights produces a test force proportional to the mass of the weight applied. The loading lever ¶s deflection corresponding to the friction force of the specimen plate is measured by a displacement sensor. The friction force and vertical displacement of pin are automatically measured in the pin-on-disk machine using a load cell and a linear variable differential transformer. A variable speed motor rotates the disk at different speeds and is mounted in such a manner that its vibration does not affect the test. In the present work the normal load applied on the disk through the pin are 20N, 30N, 40N, 10N respectively. No lubrication was applied during the tests to estimate the maximum wear rates at minimal or no lubrication conditions. The friction and wear tests are done setting the track diameter of the disk is 80 mm with a rotating speed of 60 RPM in room temperature. Each test was performed for 10 minutes of duration. The friction force values are acquired periodically along the experiment through a data acquisition system into a computer for further processing.

Result and Discussion
(a) (b) Figure 3. Pictures of test samples before and after coating (a) Cylindrical Sample; (b) Square Sample. Figure 3 shows the pictures of test samples before and after coating. As the samples are dipped into the solution with cotton threads, there are certain marks of the thread where the coating has deposited. The hardness of the samples was tested with the applied load of 60 N before the coating. After the coating the microhardness was measured for the coated samples. The comparison as reported in Table 2 shows that the coating process has improved the hardness of EN8 steel specimens. Similar results have been reported in several literatures [11,12] and it has been found that hardness and wear resistance can be increased with heat treatment.   Table 3 shows comparison of results for the variation Coefficient of friction at different applied loads at track rotational speed of 60 RPM. It also shows the variation of wear rate of electroless Ni-P coated samples before any without any specific heat treatment. From the results it can be noticed that the Coefficient of friction is decreased and the wear is increased. Fig. 4 illustrates the variation of Coefficient of friction with the applied load. It can be noticed that is Coefficient of friction consistently decreased with the increasing load up to 30 N load. However, there has been a slight increase in Coefficient of friction when 40 N load was applied as observed in several experimental works reported in the literature [3,13]. Micro-structural properties and crystallization behavior of EN deposits highly depend upon the phosphorus content in the coating which attributes to their response to varying loads.  Electroless nickel coatings are susceptible to major adhesive wear under dry sliding contact, because the low interfacial and high surface free energies of this material combination produce highly compatible surfaces. In figure 5, the measured rate of wear from the data acquisition system has been plotted with the applied load to check the wear resistance capabilities of coated steel. It is observed that the wear is increased with increased application of normal load in a decreasing rate till 30 N. However, there is a sudden rise in wear when the load reaches 40 N as it measures 496.58 micron. It indicates the change in wear resistance over a particular load for the current thickness of coating. Such occurrence can be attributed by the presence of wear debris on the rotating disc which have affected the wear results for the particular set of experiment. When compared with uncladded EN8 samples, the coated samples always show better wear resistance as found in the present work and also reported in several literature [14]. The solution to the severe wear can be achieved through a controlled heat treatment by inducing crystallization of amorphous nickel to Ni 3 P [15]. Ni-P coated samples tested under brine environment [16], also confirms the present trend of frictional parameters reported in the present work.

Conclusion
The present work demonstrates electroless Ni±P coating process on EN8 steel substrate. The mechanical and tribological behavior of the coating has been analyzed in the perspective of its applications. The property changes R൵HUHG E\ Ni-P coatings has proved the way for their use in numerous industrial applications. The Hardness of the coated sample were found higher when compared with noncoated samples. The Wear and Friction characteristics were determined for varying loading conditions. It was found that on average the coefficient of friction decreases with increasing load. Electroless nickel in the as-deposited state exhibits severe adhesive wear in dry sliding contact. The rate of wear was observed to be proportional with applied load in the tribometer testing )XUWKHU GHYHORSPHQWV LQ WKLV ¿HOG ZLOO VHFXUH D prominent place for these coatings in the surface engineering of metals and alloys.