**7. Surfactant addition / type**

Coating performance can be developed by the addition of the surfactants like (cetyltrime‐ thylammonium 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, MoS2) 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 homogene‐ ous 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 incorpora‐ tion 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 codeposi‐ ton 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 MoS2 codeposition [32]. In addition, BAS adsorbed on MoS2 particles decreased the conductivity of the particles and resulted in homogeneous deposition of nickel and MoS2 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].
