Synthesis of functionally graded nano Al₂O₃–Ni composite coating by pulse electrodeposition

Highlights

• We used changing of pulse parameters during electrodeposition for the first time to synthesis a functionally graded Ni–Alumina composite coating.

• The duty cycle shows strongest influence on the fabrication of functionally graded coatings.

• Changing of the duty cycle from 90% to 10% at fixed frequency of 10 Hz is the optimum condition for production of functionally graded Al₂O₃–Ni coating.

Abstract

The main goal of this research is the synthesis of functionally graded nickel–nano Al₂O₃ composite coatings by using pulse deposition in which the amount of the embedded nano alumina particles changes in the cross section of the composite. For producing functionally graded nanocomposite coatings by pulse electrodeposition under ultrasonic agitation, frequency and duty cycle changes can be applied and in this research, the influence of both parameters has been studied. Microstructure of these coatings investigated by SEM, EBSD and XRD methods. By changing the duty cycle from 90% to 10% at different frequency, the microstructure of the coatings did not change significantly. These coatings showed ductile structure with {0 0 1} texture at primary layers and small randomly oriented grains at final layers. The most important factor affecting the microstructure of the nickel matrix was the average current density and incorporation of nano alumina particles does not have significant effect on the microstructure. The optimum condition for production of functionally graded nano Al₂O₃–Ni coating was changing of the duty cycle from 90% to 10% at fixed frequency of 10 Hz.

Introduction

Metal matrix composite coatings (MMCs) can show unique physical, mechanical and chemical properties [1], [2], [3]. This makes them promising low cost advanced materials which can be produced by means of electrocodeposition. The advantages of the electrochemical fabrication method compared to other coating methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and powder metallurgy include more homogenous distribution of particles, reduced waste materials and ability of continuous processing [4]. However, the dispersion of the particles (particularly for nano particles) in a common plating bath can be problematic. In most cases agglomeration and sedimentation of the particles take place which makes successful co-deposition difficult.

Functionally graded material (FGM) is a novel engineering system developed in the mid-1980s in Japan. In these materials some characteristics like composition and structure gradually change, resulting in material properties changes [5]. It is already known that functionally graded materials may reduce delamination and spallation of coatings [5], [6], [7]. In the simplest FGMs, two different type of structures change gradually from one to the other. The materials can also change in a discontinuous way in a stepwise gradation. So far graded coatings have been produced by various methods such as PVD, CVD, thermal plasma spray and electrodeposition [7]. By using electroplating method, the graded coatings can easily be manufactured by changing the process variables like the current density, rate of stirring and particle loading in the bath. Examples of such graded coatings include a nickel deposit reinforced with SiC microparticles [5], [6], [8], [9] and Al₂O₃ nanoparticles [7], [10], [11].

Among these parameters the imposed current is one of the most important ones, having a great influence on particle content in the composite coating and consequently on coating properties [3], [4], [12]. Different types of currents like direct current (DC), pulsed current (PC) and pulsed reversed current (PRC) have been employed [4], [12], [13], [14], [15], [16], [17], [18]. PC has been proved to be a useful tool for designing metallic coating properties and a considerable number of theoretical models have been introduced to explain the way pulse current alters the process of electrodeposition [13]. Landolt and Marlot reviewed the effect of PC on microstructure and composition of metallic coatings [13] and recently, many other researchers were employing PC and PRC for production of the composite coatings. Pulsed electrodeposition is employed to produce many different composite coatings like Ni/W–Al₂O₃ [14], Ni–SiC [15] and Ni–TiO₂ [3], [12], and results have shown a possible control of particle content in the coating by using PC [3], [15], [17] or PRC [17] and also enhancement of the uniformity of particle dispersion in the coating. However, the effect of pulse electroplating parameters on incorporation rate and properties of the coating is not always reported the same. For example, in the case of Ni–Al₂O₃ composite coating, the authors have reported contradictory behaviors by using PC [4], [14], [18].

The main goal of this research is the synthesis of functionally graded nickel–nano Al₂O₃ composite coatings in which the amount of the embedded nano alumina particles changes in the cross section of the composite. Such composite coatings already were produced by other methods [7], [9], but this is the first time that pulse parameters during electrodeposition process will be used to manufacture functionally graded composite coatings.