Elsevier

Applied Surface Science

Volume 353, 30 October 2015, Pages 615-627
Applied Surface Science

Comparison microstructure and sliding wear properties of nickel–cobalt/CNT composite coatings by DC, PC and PRC current electrodeposition

https://doi.org/10.1016/j.apsusc.2015.06.161Get rights and content

Highlights

  • The Ni–Co/MWCNTs deposited on the Cu substrates with DC, PC and PRCelectro co-deposition.

  • Current type has changed structure and tribological properties.

  • Hardness and wear resistance found best in the PRC coated nanocomposites.

  • Ni–Co alloy showed delamination wear and MWCNTs co-deposition changed wear mechanisms.

Abstract

Nickel–cobalt (Ni–Co) alloys and Ni–Co/multiwalled carbon nanotube (MWCNT) composite coatings were prepared under direct current (DC), pulse current (PC) and pulse reverse current (PRC) methods. The effect of different deposition currents on the surface microstructure, crystallographic structure, microhardness, and reciprocating sliding wear behavior were investigated. MWCNT co-deposition caused to modify Ni–Co surface morphology, decrease in grain size, and increase in surface roughness, since MWCNTs effected the deposition mechanisms of Ni–Co alloy. The nanocomposite coatings deposited using PC and PRC deposition exhibited significant improvement in microhardness and wear resistance due to unique enhanced reinforcement of MWCNTs in Ni–Co coatings. Reciprocating sliding wear tests evidenced that co-deposition of MWCNTs provided effective load bearing ability and self-lubrication between the friction surfaces. However, the friction coefficient increases for all the nanocomposites produced with DC, PC and PRC methods showed to be increased. In the Ni–Co alloy coatings, the predominant wear mechanisms was delamination caused by fatigue micro cracking whereas in the MWCNT co-deposited composites wear mechanism showed abrasive grooves and plastic deformation due to decreased real contact area.

Introduction

Surface properties are directly responsible for the performance of engineering materials because most of the failures such as friction, wear, corrosion and fatigue often take place on the material surface. Metal matrix composites are favored with superior properties compared to unreinforced metals such as higher specific strength, dimensional stability, higher elevated temperature stability and fatigue resistance. In addition, compared to their unreinforced metals and alloys, they are favored with higher stiffness and strength, higher service temperature, higher tribological and magnetic properties [1], [2], [3]. Since metal-matrix composite coatings usually have significantly improved mechanical strength, wear resistance and corrosion resistance, and desired chemical and biological compatibilities than the alloy coatings, it is essential and feasible to improve the comprehensive properties of metal matrices by introducing third phase reinforcements [4], [5], [6].

Nanoparticles are always used to be incorporated in metal and metal alloy deposits to enhance their physical and mechanical properties [7]. Nanocomposite coatings, where nano-sized ceramic particles are incorporated into metal matrix, have been widely investigated for various applications such as wear resistant and tribological coatings [8]. Among various process technologies for nanocomposites, electrodeposition has advantages such as cost-effectiveness relative to spray and sputtering processes [9]. Conventionally, nano powders such as alumina, silicon carbide, and diamond were used as reinforcements for Ni-based nanocomposite coatings [10]. Interest in electro co-deposition of nickel-based composite coatings has increased in recent years due to their unique combination of wear, magnetic, electrical, and corrosion properties [11].

It is demonstrated that simultaneous co-deposition of nickel and cobalt cations produces Ni–Co alloy coatings with better hardness, superior wear and corrosion resistance in comparison with Ni coatings [12]. In addition, specific magnetic properties of nickel–cobalt alloys have made them a permanent candidate for recording head material in computer hard drive industries and micro-electrical mechanical systems [13], [14], [15]. Owing to their high hardness, smooth surface and anticorrosion resistance Ni–Co alloy coatings are considered to apply for automobile, aerospace and other industrial fields. However, the development of science and technology requires improving the coated surface properties to a higher level, and the composite co-deposition technique has been considered as an effective way to improve the performances of the coatings due to excellent physical and mechanical properties of the composite coatings [16]. It has been reported that electrochemical embedding of finely dispersed rigid reinforcements in metallic matrices imparts exceptional advantages in terms of superior mechanical properties and better corrosion resistance as compared to pure metal and alloy coatings [17].

Recently, owing to high strength, elasticity and modulus, carbon nanotubes (CNTs) have been the focus as the incorporating filler to endow the composite coatings with functional properties such as high hardness elastic modulus, good flexibility, and unique conductivity together with minimal coefficient of friction and excellent wear resistance [18], [19]. In spite of the their ultimate reinforcing phase effect of the CNTs, the main drawback of CNTs is their processing difficulty due to strong van der Waals interactions that cause agglomerates. This reduces the aforementioned properties as well as the integration as a second phase in composites [16].

Electrodeposition process is known one of the easiest and economical superior techniques to provide coatings by using different metals their metal matrix composites [20]. Among the electrodeposition processes using direct current (DC) is known most practical and easy method to deposit pure metals, alloys and their composites. In recent years, many researchers have investigated the preparation and properties of composite coatings obtained by electrodeposition, which mainly focus on electrodeposition prepared by PC and PRC methods [21]. These two new current methods provide to increase the deposition rate and consequently lead to improve microstructural, mechanical and corrosion properties compared with the classical DC method [22].

As stated before, Ni–Co alloy has been widely used as the recording head materials for computer hard drive industries. In the case of micro-electrical mechanical system (MEMs), the magnetic layer thickness can vary from a few nanometers to a few millimeters, depending on the applications. The magnetic thin films must also have good tribological properties. The well-dispersed MWCNTs in a Ni–Co matrix cannot only enhances the mechanical properties, but also would be a necessity for the use as the composite materials in micro devices. In spite of some significant number of work were published on the deposition of metal matrix composites with DC, PC and PRC techniques [20], [23], to the best of our knowledge there is no systematic comparative investigations on the microstructural and tribological properties of the Ni–Co/MWCNT nanocomposites deposited with DC, PC and PRC methods. In this current work, we have aimed to present comparative investigations for Ni–Co alloy and Ni–Co/MWCNT nanocomposites. The Ni–Co based alloys and nanocomposite depositions are targeted to increase the wear resistance of the Ni–Co coatings for possible applications next generation micro devices.

Section snippets

Chemical oxidation of carbon nanotubes

MWCNTs over 1.0 μm in length with the outer diameter of 50 nm were purchased from Arry Nano (Germany). Since the mechanical properties of the MWCNT reinforced nanocomposites are primarily determined by the interfacial bonding strength with the matrix material, chemical oxidation of the MWCNT sidewalls and tips could be utilized to increase homogeneous distribution and therefore strength and load bearing ability of the nanocomposite depositions. Chemical oxidation of MWCNTs was carried out with a

XRD analysis

The XRD patterns for Ni–Co alloy and Ni–Co/MWCNT nanocomposite coatings deposited by DC, PC, and PRC methods are presented in Fig. 1a. The XRD patterns show typical peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) crystallographic planes of Ni–Co alloy coatings. Some low intensity peaks (are also appear,) which belong to the free Co. XRD patterns do not show the peaks corresponding to the MWCNTs for the nanocomposite coatings, because of the low amount of the dispersed MWCNT reinforcement. For the

Conclusions

Ni–Co alloy coatings revealed polyhedron like grains with the DC method, and the grain morphology changed from polyhedron like to the equiaxed spherical grain in the PC and PRC methods with decreasing grain size.

All the Ni–Co alloy coatings produced with DC, PC and PRC methods exhibited preferential growth in (2 2 0) plane. Introducing the MWCNTs resulted in predominant growth in the (1 1 1) planes. The crystallite sizes were determined between 17.64 and 56.09 nm.

For the three studied

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