**2. Materials and methods**

with more environmentally friendly technologies. The development of "clean" technologies in the electroplating industry is an essential task required and initiated by environmental protection laws worldwide [1]. From an environmental point of view, chromium (Cr) electro‐ deposition, which occurs in a wide range of industrial applications in the automotive, aerospace, mining, and petrochemical fields [2], is undoubtedly one of the most damaging electrodeposition processes. In environmental regulations, chromic acid (CrO3), which is mainly used in hard Cr plating, has been recognized as both highly toxic and carcinogenic and was identified by the U.S. Environmental Protection Agency (EPA) as one of 17 high-priority toxic chemicals [3]. Consequently, the use of hexavalent chromates requires special waste disposal methods and expensive breathing apparatus, and exhaust systems must be employed to address emissions during processing. For these reasons, substitute materials and new designs have been under study for many years. Alloy electrodeposition alternative systems including Ni-W, Ni-P, and Co-W have been considered to replace conventional hard Cr deposition [4,5]. Unfortunately, it is challenging to replace Cr because of its comprehensively favorable material properties, including high hardness, low friction coefficient, and excellent wear and corrosion resistance. A possible approach for the preparation of Ni-based alloy coatings as an alternative to hard Cr is to introduce the new concept of functionally graded deposits (FGDs), which originally evolved from the application of functionally graded materials (FGMs) in which a property gradient arises from position-dependent chemical composition, microstructure, or atomic order [6,7]. Wang et al. [8] found that Ni-P deposits that were heat treated at 400 °C exhibited more than two orders of magnitude higher corrosion resistance than hard Cr deposits. It was found in our previous research that the hardness of Ni-P alloys after heat treatment at 500 °C is close to that of conventional hard Cr [9] and that

Alternatives such as composite coatings have been investigated in recent years. A study on Ni-P-Si3N4 composite deposition revealed that increasing the Si3N4 content in the deposit greatly increases the hardness and that the wear resistance of the Ni-P-Si3N4 composite deposit is four times higher than that of the Ni-P deposit [10]. Other reports [11,12] have concluded that the addition of hard microceramic particles (SiC, Si3N4, Al2O3, WC, B4C, BN, CNTs) to the

An important condition to enhance the hardness of the obtained coatings is by using particles with an average size of less than 1 μm evenly distributed on the surface. In these sense, Guo et al. [13] showed that the presence of carbon nanotubes (CNTs) in the composite coatings improves toughness, strength, and corrosion resistance of the coatings. Likewise, the addition of boron nitride (BN) particles (0.5 μm) to Ni coatings was studied by Pompei et al. [14], and the results showed that Ni-BN coatings present more hardener and wear resistance than those with neat Ni. Similarly, Malfatti et al. [15] found that the incorporation of SiC particles in Ni-P coatings results in higher polarization resistance (lower electrochemical activity) compared to coatings containing only the metallic matrix (Ni-P) for heat-treated specimens. This behavior was associated with a lower superficial active area produced by the nonconductive SiC

they exhibited better wear resistance than hard Cr.

124 Electrodeposition of Composite Materials

particles.

metal matrix can improve its hardness and wear resistance.
