Effects of Electroplating Characteristics on the Coating Properties

Electroplating parameters that can be listed as bath temperature, pH of the bath, current density, surfactant addition or type, coating thickness must be controlled during the dep‐ osition process since they determine the properties of the coating. However, it is difficult to manage the effects of this high number of parameters including their interaction ef‐ fects. At this point, fractional factorial design that is a statistical method steps in that have the advantage of evaluating the influences and the complex variable interactions of pa‐ rameters with a plausible number of experiments. In the design low and high values must be attributed to the parameters before the experiments and these values are selected according to the solution used. There are suitable plating conditions (written in hand‐ books) for each bath without particle addition and low – high values can be chosen be‐ tween these ranges or just below or above them. For instance, the temperature range is 40-60°C, current density range is 2-7 A/dm 2 and pH range is 3.5-4.0 for nickel electroplat‐ ing [36]. Besides the coating property, the electroplating parameters influence the hydro‐ gen evolution reaction that is a side reaction that takes place at the cathode and may lead to morphological problems on the coated surface. The aim of this chapter is to provide information about how the parameters affect the amount of particles in the deposit. Be‐ cause the reason of adding the particles to the bath is to improve the matrix properties by the particles emerged. So the more particles present and disperse in the coating, the more they will contribute to the coating property. Another important effect is the internal stress that led to departure of the deposit.


Introduction
The electroplating method can be used to produce composite coatings by adding particles in the metal plating bath. The particles are trapped in the coating during deposition. Composite coatings are composed of an electrodeposited metal matrix and dispersed solid particles. The metal powders, metal alloy powders and metal oxide powders of Al, Co, Cu, In, Mg, Ni, Si, Sn, V, Zn and nitrides of Al, B, Si and C (graphite or diamond) and carbides of B, Bi, Si, W and MoS 2 and organic materials such as polytetrafluoroethylene (PTFE) and polymer spheres are used as the particles [40].
The main application areas of the composite deposition are electronic, biomedical, telecommunication, automotive, space and consumer applications where high strength, equiaxed micro-components are required.
The electroplating parameters must be controlled during electrodeposition since they have significant effect on the coating properties, deposition efficiency and hydrogen evolution. Besides the current density, pH of the bath, bath temperature that places among the main electroplating parameters [36], there are more parameters including the type, the size, the concentration of the particles [37] and the addition of surfactants and their types [20]. The main properties of the coating can be listed as the corrosion resistance in TiO 2 -nickel coating [38], tribological properties in MoS 2 -nickel coating [4], mechanical properties in TiO 2 -nickel coating [38], internal stress in MoS 2 -nickel coating [9], texture in chromium -carbon deposit [39], particle content of the deposit [24] are severely affected by the electroplating parameters.
The aim of this chapter is to emphasize on the electroplating parameters, their effects and interaction effect on the coating properties. Particle incorporation in the deposit is an important property that must be analysed since composite electroplating aroused in order to improve the matrix properties with particle addition. Another important property is the internal stress that must kept at minimum levels not to disrupt the deposit.

Current density
Electrodeposition process consists of two steps that are nucleation, growth mechanisms and thickening of the primary layer. The nucleation is enhanced by high current density unlike the growth process [1]. Thus, smaller grain sizes are observed at higher current densities due to the increase in the nucleation rate [2]. On the other hand, high current density increases pH in the vicinity of the electrode during the reduction process that creates a competition between metal deposition and hydrogen gas [6] [3]. Hydrogen evolution contributes to the internal stress in the deposit [4] [5]. Therefore, it is important to figure out the current density values at which hydrogen evolution does not occur together with metal deposition [4].
The current density also affects the particle content in the deposit. The particle (WC) content of the coating increases linearly with an increase in the current density from 0.1 to 0.5 A/dm 2 regardless of the particle diameters [7]. On the other hand, Kuo [6] claims that the particle (MoS 2 ) content in the deposit decreases when the current density increases from 4 to 8 A/ dm 2. According to the results of Hu and Bai [41], increasing current density generally increased the particle content but the effect of current density depends on pH. Figure 1 shows that when the current density (B) is increased, the particle content is increasedwhen pH (C) is 1 and decreased when pH is 5 [41].  Another effect of increasing current density from 0.15 to 5 A/dm 2 is increasing the internal stress [8] since the stress stems from the residual stresses. However significant effect cannot be detected when the current density increased from 1.2 to 4.8 A/dm 2 [9] because the effect of current density on the internal stress depends on the electroplating solution composition. Figure 2 shows the effect of changing only one component amount of the solution on the current density dependence of the internal stress [42]. Moreover the effect of the current density depends on pH.

pH of the bath
The effect of pH on particle incorporation is dependent on the nature of the particles. For instance, when MoS 2 particles are used as the incorporate particles, the effect of pH value on the particle content in deposit was insignificant. Nevertheless it can be concluded that increasing pH decreased the amount of particles present in the coating [6]. However, incorporated particles are significantly decreased when pH is below 2 in Al 2 O 3 -Ni coating pair [11].
In addition, decreasing pH is preferred to manage the internal stress. Low pH values, less than 5, are selected to obtain acceptable stress levels [10]. Increase in pH of the solution may lead to discharge of hydroxyl ions instead of nickel dissolution and oxygen evolution [10] resulted in high internal stresses. It is also concluded that the internal stress was increased when the pH was increased from 2 to 4 in MoS 2 -Ni system [12].

Bath temperature
The effect of the temperature on the particle content in the deposit depends on the type of the particle. There is a small increase in the particle (MoS 2 ) content in coating with increasing the temperature 30, 40, 50 respectively [6]. It was mentioned that the temperature has an insignificant effect. For instance, no effect of temperature was detected in BaCr 2 O 4 -Ni [13] and Al 2 O 3 -Ni [14] coating couples. Because, the applied voltage is the main parameter that directly affects the activity of the reaction. In addition, Ni deposits more efficiently with increase in temperature. On the other hand, the influence of the temperature was reported as positive up to certain point. After that point, the amount of particles decreases with increasing the temperature.
General trend on the effect of the bath temperature on the internal stress is positive, meaning that increasing temperature decreases the internal stress. On the other hand, according to some of the studies the stress is more influenced by the current density regardless of the temperature [10].
Another advantage of the high temperature is the polarization effect. It is known that concentration polarization is the component of the polarization which is due to the change in the electrolyte concentration that stems from the current flow through the electrode -solution interface. So, the electrochemical cell potential difference deviates from its equilibrium value. Concentration polarization is decreased by increasing temperature because diffusion layer thickness gets smaller and ionic diffusion increases.
On the other hand, high temperature values increases energy consumption and supply heat for bath evaporation. Furthermore, thermal stresses will arrive at high processing temperatures and can become a serious problem especially when the coating and substrate have different thermal expansion coefficients. Therefore, an optimum plating temperature must be preferred to satisfy energy consumption and the coating quality.

Particle type
It is a common fact that if the amount of the particles in the solution is increased, the particle content in the deposit will also increase up to a certain point. However the type of the particle also acts as an important parameter. Both of the conductive and non-conductive particles have their own advantages against each other. Because conducting particles (molybdenum disulfide, chromium carbide, zirconium diboride, graphite) attracted to the cathode then act as depositing sites which resulted in dendritic growth [15]. Despite the advantage of easily attraction of conducting particles to the cathode, selective deposition on the conducting sites led to increased surface roughness. On the other hand, non-conductive particles end up with smoother deposit surfaces with low porosity [15].
Mechanical properties can be changed by the type of the incorporated particles. For instance, PTFE in Ni matrix increases wear resistance [16], MoS 2 incorporated with Ni decreased the coefficient of friction [9], Al 2 O 3 and SiC dispersed in Cu increased the microhardness of the coatings [17]. Moreover, wear and corrosion resistance can be improved by the addition of silicon carbide nano-particles [18] [19].

Particle size
Small particle sizes can be agitated easily and led to an increase in the particle concentration in the deposit during composite electroplating. For example, the amount of particles is increased by decreasing the particle size in Ni/SiC system [21]. Furthermore, the effect of particles in decreasing the friction coefficient is more effective when the particle size is decreased in Ni-MoS 2 system [9].

Particle concentration
Generally, increasing the particle concentration in the bath increases the weight percentage of particles in the deposit up to a certain point [9] [22] [23]. That point can be thought like the saturation point. There is a rapid increase in the particle amount in the low particle concentration regions whereas a slight increase occurs in the high concentration regions. The collisions between particles and cathode determine the codeposition of the particles and they are diminished in the high MoS 2 concentration region resulting in slight increase or decreased particle amount in the growing metal deposit [6].

Surfactant addition / type
Coating performance can be developed by the addition of the surfactants like (cetyltrimethylammonium bromide (CTAB), sodyumdodecyl sulfate (SDS), and saccharine [20]. The advantage of adding surfactants is their dispersing effect of particles. Thus the property of the particles will be uniform on the surface. Surfactants adsorb on the particles and favor the distribution of the particles [24].
Surfactants are indispensable especially for the hydrophobic particles (fluorographite, MoS 2 ) to be dispersed in the electroplating solution. Surfactants like sodium lauryl sulphate enhanced the electrostatic adsorption of suspended particles on cathode surface by increasing their positive charges [25]. Similarly, azobenzene (AZTAB) promoted particle deposition into the nickel matrix by their more positive reduction potential than that of nickel [26]. Another surfactant, cetyl trimethyl ammonium bromide (CTAB) has an advantage of increasing the volume percentage of SiC in the deposit besides homogeneous and non-agglomerated distribution of particles in SiC-nickel composite coatings [27].
Further advantage of the surfactants is suppressing the hydrogen evolution reaction. For example, saccharin which is an anionic surfactant is an effective way to overcome the hydrogen evolution problem [34].
The surfactants can be grouped under two main headings which are anionic and cationic surfactants according to their charges. Cationic surfactants increased the particle incorporation in the coating [28] [29] [30] [31]. Anionic surfactants may have positive or negative effect on the codeposition efficiency of the particles depending on the particle type and bath solution. For instance, SDS which is an anionic surfactant did not affect the codepositon of particles [31]. On the other hand, cationic surfactants have the advantage of adsorbing on the particles that have negative surface charge [29]. Therefore, a net positive charge was formed by the adsorption of cationic surfactants that inhibited the formation of particle clusters and led to more stable particle suspension in the bath. Moreover, this positive charge improved the tendency of particles to move towards cathode and increased the amount of particles in the deposit [29] [30]. For instance, addition of cationic surfactant benzyl ammonium salts (BAS), increased the amount of MoS 2 codeposition [32]. In addition, BAS adsorbed on MoS 2 particles decreased the conductivity of the particles and resulted in homogeneous deposition of nickel and MoS 2 particles [32] [15]. On the other hand, it was stated that anionic surfactants in the electrolyte can give particles a negative charge and make them to move towards the substrate [35].
The disadvantage of the surfactants may occur if there are unabsorbed free surfactants, because they could lead to stress development and brittleness in the deposit [33]. Since the amount of incorporated surfactant is generally very small, their undesirable effects may be ignored [33]. However, increasing surfactant (CTAB) amount caused an increase in the internal stress due to the high possibilty of embedded CTAB in the nickel matrix [32].

Fractional factorial design
2 n-1 fractional factorial design is a statistical design that can be used to identify the effects of n electrodeposition variables on the coating properties with the reasonable number of experiments. In addition, the interaction effects of the parameters can be analyzed by the help of fractional factorial. The property of the coating in other words the response value in the program is generally taken as the amount of particles in the deposit during composite coatings. For instance, in the study of Hu and Bai [41], phosphorus content in the deposit was taken as the response value and temperature, current density pH, NaH 2 PO 2 H 2 O concentration of the solution and agitation rate were taken as the electroplating parameters. Another most commonly used response value during composite coating is internal stress. Electroplating parameters were MoS 2 particle concentration, temperature, pH, current density and coating thickness where the response value is the internal stress in the study of Saraloglu Guler et. al. [9]. Other response values can be listed as friction coefficient, corrosion resistance, wear resistance, hardness which are the properties obtained by particle addition so increased amount of particle content in the deposit will have a positive effect on these values. The hydrogen evolution reaction must also be considered during this selection. The effects of the electroplating parameters on hydrogen evolution reaction can be studied before the composite deposition in order to determine the current density range where H 2 is not simultaneously discharged with Ni plating [12].
Fixed limit values that are said to be low (-1) and high (1) levels are selected for the electroplating parameters in fractional factorial design. Table 1 shows the parameters and their low and high fixed limit values for levels of fractional factorial design.