**1. Introduction**

Materials are a fundamental pillar in engineering technology. The materials' electrochemical, thermal and mechanical interaction begins on the surface. However, material surfaces face the constant threat of wear and corrosion resulting in massive losses in industry. The use of surface enhancement technology to prevent or mitigate the loss has therefore become inevitable. Over the last few decades, major scientific development in the fields of metallurgy and materials has occurred, giving rise to new engineering materials with superior properties. Developing suitable processing to produce desired materials is complex and requires altering the inherent properties of the materials. Consideration is made of the overall economic perspective and its environmental impact.

Electrodeposition is an electrochemical process that is used to modify the surface structure. Use of electrodeposition in surface engineering can be traced to nearly 200 years ago based on some hypotheses [1]. The galvanic cell, invented in early 1800- s, paved way for use of electric current to produce coatings as a more cost-effective technique. The electrodeposition technique possesses an edge over other coating techniques due to several reasons [2]:


Electrodeposition is based on the principle that a layer of coating, either single or multilayer, forms as a result of the electrode–electrolyte electrochemical reactions occurring leading to electrodeposition of ions contained in electrolyte. It utilizes the properties of materials in their metastable state as a result of reduction of the grain size on the nano scale. In such a state, the grain boundaries contain a proportion of atoms that is higher or equivalent to those inside the grains [3]. These new types of materials consisting of nanoparticles are referred to as nanocomposite materials and they exhibit superior properties compared to traditional grain sized materials and sometimes offer completely new properties altogether. As a result, nanocomposite materials with grain sizes below 100 nm have received considerable attention from scientists and researchers all over the world. Several new electrodeposition synthesis methods have been devised over the years to increase production efficiency of new materials and minimize cost. This has ranged from using the basic direct current set ups to more ambitious procedures such as: pulse plating, jet electrodeposition, and the pulse reversal current technique [4–6].

Pure Ni is one of the most widely used alloying metals in the world owing to its superior corrosion and wear resistance [7]. Electrodeposited Ni coatings have uses in decoration, functional uses, as well as in engineering for surface protection [8]. Superplasticity in metals (materials exhibiting increased elongations to failure of >500%) is dependent on elevated testing temperatures and fine grain sizes of <10 μm [9]. Prasad and Chokshi [9] reported that electrodeposited nanocrystalline Ni is characterized by good superplasticity properties and is used in studying the phenomenon in electrodeposited metals owing to the ease of synthesizing coatings with small grain sizes. Ni coatings exhibit improved resistance to localized corrosion and this makes them perfect for use as anti-corrosive coatings [2]. Furthermore, research shows that thin films of different materials coated with epitaxial thin Ni coatings of a few nanometers have similar hardness to bulk nickel, such that the wear resistance of micro/nano electro mechanical system devices can be greatly improved if coated with a thin Ni coating [10]. Studies have shown that nano-sized nickel can aid the diamond yielding process. Ni atoms have a three dimension absent state which serves to attract electrons in the carbon fullerenes. This in turn causes the sp2 fullerene to be transformed into a diamondlike sp3 structure. This transformation can also be attributed to high reactivity which effectively aids the change at an impulse during shock wave loading [2].

Ni based alloy coatings can also be produced using electrodeposition. Choice of the alloying metal depends on the properties desired, which can range from good electrical conductivity, good wear and corrosion resistance, soft magnetic properties to special optical properties. Over the years, many different types of metals have been electrodeposited with Ni to form alloys: Co, Fe, Cu and W.

Ni–Co is one such alloy. There exist several different synthesis techniques for Ni–Co alloy coatings and they include: radio frequency magnetron sputtering [11], electrodeposition [12] and vacuum evaporation [13,14]. The electrodeposition technique has several advantages over the other two methods and they include: low cost, simplicity, scalability and manufacturability [15]. Furthermore, electrodeposition can be used to grow a wider range of materials.

Research shows that electrodeposited Ni–Co alloy coatings exhibit better properties compared to pure Ni coatings [12]. Ni–Co alloy exhibits higher hardness, better adhesion, excellent magnetic properties, high wear and corrosion resistance as well as good stability at high temperature [16–19]. Wang et al. [20] researched the effect of cobalt content on mechanical and microstructural properties on Ni–Co alloy coatings. It was reported that Ni–Co alloy coatings exhibited approximately double the microhardness when compared to pure Ni coatings. It was also reported that the Ni–49Co coatings exhibited a decreased rate of wear in comparison to pure Ni coatings. Hassani et al. [21] researched on low temperature superplasticity of nanocrystalline electrodeposited Ni–Co alloy with an average grain size of 20 nm. It was reported that a maximum elongation of 279% at a temperature of 773 K was

obtained. Ni–Co alloys are considered to be the best suited materials for replacing hard chromium [22]. Research shows that microhardness in Ni–Co alloys increases gradually with increase in Co content up to an optimum level, after which the microhardness decreases with further increase in Co content [16].

To further improve the properties of Ni–Co alloy coatings, nanoparticles have been suspended in electrolyte and they become embedded into the electro-formed solid phase layer during electrodeposition [23]. In such materials, the inherent properties of the nanoparticles have been found to significantly influence the overall properties of the nanocomposite coatings. Many different types of nanoparticles have been electrodeposited with Ni–Co alloy including SiC, Al2O3, SiO4, ZrO2, Cr2O3, Si3N4 and TiO2 [24]. These nano particles used in electrodeposition can be classified as either hard materials or soft materials depending on the desired properties. Soft materials such as graphite offer properties which include lower the friction coefficient to reduce wear and the coefficient of friction between shearing surfaces [25]. Hard materials such as Al2O3 improve the microhardness and wear resistance of surfaces [26]. The matrix phase microstructure coupled with nanoparticle content and distribution in the metallic matrix phase significantly influences the properties of nanoparticle reinforced Ni–Co matrix nanocomposite coatings. From past research, it is clear that addition of nanoparticles has served to improve the properties of Ni–Co coatings. In some instances, however, addition of Co has been observed to improve the overall properties of Ni-nanocomposite coatings such as hardness and residual stress [27]. Addition of Co2<sup>+</sup> in Ni/diamond plating baths has been seen to greatly improve the deposition of diamond in the coatings resulting in higher diamond content, enhance bonding between matrix and particles, as well as more uniformly dispersed particles in the metal matrix, resulting in improved wear resistance and hardness properties [28]. The properties of Ni–Co alloys and their composite coatings have also been previously reported on by Karimzadeh A et al. [29]. This review aims takes a different approach to the wide research field of Ni–Co and Ni–Co-based composite coatings. Factors such as the effect of electrode orientation and forces existing in the electrolyte bath on the deposition process have been considered. Moreover, additives such as boric acid have been extensively explored, and the intricate working of the Watts solution has been more deeply discussed for easier understanding. Properties such as adhesion between substrate and coating have been extensively discussed, comparisons between techniques drawn, and recommendations for further adhesion improvement have been presented.
