Elsevier

Materials & Design

Volume 30, Issue 4, April 2009, Pages 1341-1349
Materials & Design

Technical Report
Wear behaviour of electroless Ni–P coatings and optimization of process parameters using Taguchi method

https://doi.org/10.1016/j.matdes.2008.06.031Get rights and content

Abstract

This paper reports an experimental study of wear characteristics of electroless Ni–P coatings sliding against steel. Coating process parameters are optimized for minimum wear based on L27 Taguchi orthogonal design with four process parameters, viz., bath temperature, concentration of nickel source solution, concentration of reducing agent and annealing temperature. It is observed that annealing temperature and bath temperature have the most significant influence on wear characteristics of electroless Ni–P coating. The interaction of bath temperature and concentration of nickel source solution has some significant influence on wear. The surface morphology and composition of coatings are also studied with the help of scanning electron microscopy, energy dispersed X-ray analysis and X-ray diffraction analysis. The wear mechanism in EN coatings is found to be abrasive wear.

Introduction

Electroless Ni–P (EN) coatings, first developed by Brenner and Riddell [1], have found extensive use in mechanical, chemical and electronic industries [2] due to their excellent mechanical, physical, electrical, corrosion, and tribological properties. Another advantage is that EN coatings can be applied to a variety of substrate materials and uniformly on intricate part geometries. Electroless Ni–P coating is an autocatalytic deposition of a Ni–P alloy from an aqueous solution on a substrate without application of electric current. Thus, it differs from the conventional electroplating processes that depend on an external source of direct current in order to reduce nickel ions in the electrolyte to nickel metal on the substrate. The electroless bath typically comprises an aqueous solution of metal ions, complexing agents, reducing agents and stabilizers, operating in a specific metal ion concentration, temperature and pH ranges. The rate of deposition and properties of coatings depend on a number of factors such as the type and concentrations of the reducing agent, stabilizer, pH of solution, the temperature of the bath, etc. Since the discovery of EN coatings, the properties and structures of such coatings have received considerable research attention. The properties and microstructures of EN coatings depend on the amount of phosphorous alloyed in the deposit [3], [4], [5], [6], [7]. The post-deposition heat treatment influences the properties by altering the microstructures [5], [6], [7], [8]. The mechanical and tribological properties of these coatings can further be improved by the incorporation of hard particles [8], [9] and dry lubricants [9], [10], [11]. Friction and wear being important properties of EN coatings, these have also been the focus of attention of a number of researchers [12], [13], [14], [15], [16], [17] over the years. In general, the friction coefficient of electroless Ni–P coating decreases with increase in load. The friction coefficient lies within the range of 0.15–0.35 when tested under the 15–60 N loading conditions [12]. The friction study of electroless Ni–P coating with a ramp apparatus concluded that coatings with high phosphorus content have higher friction coefficient than comparing to medium or low phosphorus electroless coatings [14]. The friction coefficient of electroless coating when tested under low loads was found to be as high as 0.7 [12]. Wear performance of electroless Ni–P coating is greatly enhanced with heat treatment, the treatment performed at 400 °C for 1 h having the highest hardness [12]. In a recent study [18], tribological behaviour of EN coatings under lubricated contacts using bio-lubricants has been investigated.

Despite a good number of studies on EN coatings particularly on tribological properties, an extensive review of the literature reveals that no report is available on optimization of the process parameters of EN coating to have minimum wear characteristics. The present study deals with the application of Taguchi method to determine the suitable coating process parameters for optimum wear behaviour in electroless Ni–P coatings. Ni–P coatings are applied on mild steel (AISI 1040) specimens based on Taguchi orthogonal design with four design parameters, viz., bath temperature, concentration of nickel source solution, concentration of reducing agent and annealing temperature as independent variables. A multitribotester is used to evaluate wear behaviour of these coatings. Taguchi analysis is employed to identify optimum combination of coating parameters that yields optimum (minimum) wear coefficient. A confirmation experiment verifies the optimal parameter combination as predicted by Taguchi analysis. Analysis of variance is carried out to observe the level of significance of factors and their interactions. The surface morphology and composition of EN coatings are studied with the help of scanning electron microscopy, energy dispersed X-ray analysis and X-ray diffraction analysis. The present study incorporates two new considerations: optimization of EN coating process parameters based on wear characteristics and inclusion of annealing temperature as an independent process variable.

Section snippets

Taguchi method

Taguchi technique [19], [20] is a powerful tool for design of high quality systems based on orthogonal array experiments that provide much-reduced variance for the experiments with an optimum setting of process control parameters. It introduces an integrated approach that is simple and efficient to find the best range of designs for quality, performance and computational cost. This method achieves the integration of design of experiments (DOE) with the parametric optimization of the process

Coating deposition

Mild steel (AISI 1040) specimens of size 20 mm × 20 mm × 8 mm are used as the substrate material for the deposition of the electroless Ni–P coating. Square shaped specimens are prepared very carefully for the deposition of the electroless Ni–P coating. Shaping, parting and milling processes are used accordingly for the preparation of the samples. The samples are then subjected to surface grinding process. The samples are mechanically cleaned from foreign matter and corrosion products. After that,

Analysis of signal-to-noise ratio

The traditional method of calculating the desirable factor levels is to look at the simple averages of the results. However, it does not capture the variability of the results within a trial condition. Thus, signal-to-noise (S/N) ratio is used considering wear as the performance index. The S/N ratio for wear is calculated using LB (lower-the-better) criterion and the same is expressed as:S/N=-10log1ny2where y is the observed data and n is the number of observations. Table 4 shows the

Conclusions

Taguchi orthogonal array is employed to optimize the coating process parameters with respect to wear behaviour of electroless Ni–P coatings sliding against steel. It is seen that annealing temperature and bath temperature have the most significant influence in controlling wear characteristics of electroless Ni–P coating. The interaction of bath temperature and concentration of nickel source solution has some significant influence. The optimal coating parameter combination for minimum wear is

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