*3.3.2. Results*

**3.3. Effects of heat treatment on the tribological properties of electrodeposited Ni-P-SiC**

Over the last two decades, a variety of surface engineering processes have been developed to enhance the wear resistance, hardness, and corrosion performance of materials. Today, Ni-P alloys are widely used in the aerospace, automotive, and electronic industries because they possess a high degree of hardness, wear resistance, and corrosion resistance, as well as a low friction coefficient [25–28]. In this regard, Malfatti et al. [29] found that the transition from crystalline to amorphous structures occurs progressively over the range of several atomic percent of P and that as-deposited Ni-P coatings are amorphous when the P content exceeds 15 at.%. In contrast, the amorphous alloys can be crystallized via heat treatment, followed by decomposition to nickel phosphide (Ni3P) and face-centered cubic (fcc) Ni crystals at temper‐ atures above 350 °C [30]. The tribological characteristics of the Ni-P coatings can generally be improved via an appropriate heat treatment [31–33], which can be attributed to precipitation of fine Ni crystallites and hard intermetallic Ni3P particles during the crystallization of the amorphous phase [34]. In this respect, the wear resistance of the Ni-P alloys increases after heat treatment [35]. Moreover, Wang et al. [8] recently showed that Ni-P electroless coatings heat treated at 400 °C exhibited corrosion resistances of over two orders of magnitude better

The aim of this section was to study the effects of heat treatment on the physical properties of electrodeposited Ni-P-SiC coatings, including their crystalline structure, hardness, and

Ni-P-SiC electrodeposits were obtained from a modified Watts Ni bath (containing 0.2 M NaCl, 0.65 M NiSO4⋅6H2O, 0.75 M NiCl2⋅6H2O, 0.1 M H3BO3, 0.1 M H3PO3 [36]) + 0.084 mM DTAB + 0.02 g mL-1 SiC, with a pH of 1.5 adjusted with 1 M HCl) (solution S1). These solutions were prepared immediately prior to each experiment using deionized water (18 MΩ cm) and

The Ni-P-SiC coatings were obtained via electrodeposition of solution S1 under galvanostatic conditions. The coatings were then annealed in air for 60 min at one of four temperatures: 300

An atomic force microscope (AFM) (Digital Instruments, Mod. Nanoscope E) was used in contact mode to image the deposited Ni-P-SiC on the steel substrate. These measurements were performed in air (ex situ) using silicon nitride AFM tips (Digital Instruments). All images were obtained at 2 Hz and are represented in the so-called height mode, in which the highest

The deposited phases were identified via X-ray diffraction (XRD) using a Bruker diffractometer (Mod. D8 Advance) (Bragg-Brentano arrangement) with CuKα radiation (λ = 1.54 Å). The

analytical-grade reagents of the highest purity available (Sigma-Aldrich).

range of 2θ from 40° to 95° was recorded at a rate of 0.2° s-1.

**composites**

134 Electrodeposition of Composite Materials

than hard Cr deposits.

resistance to wear.

*3.3.1. Materials and methods*

°C, 400 °C, 500 °C, or 600 °C.

portions appear brighter.

#### *3.3.2.1. Electrodeposition of Ni-P-SiC composites*

Hull cell tests were performed using an S solution + 0.084 mM DTAB + 0.02 g/mL SiC, an applied current of 1 A, and a test time of 12 min. Each test was repeated three times. The typical behavior obtained for this system is shown in Figure 13.

**Figure 13.** Surface morphology of Ni-P-SiC composite deposited by a Hull cell, *i* = 1 A, *t* = 12 min, obtained from solu‐ tion S + 0.084 mM DTAB + 0.02 g mL-1 SiC.

Figure 13 indicates that homogeneous coatings can be obtained when current densities (*j*) between 0.021 and 0.003 A cm-2 are applied. These results demonstrate that 20 μm thick Ni-P-SiC coatings can be obtained by applying 0.021 A cm-2 for 20 min. The obtained coating is shown in Figure 14. Ni-P-SiC coatings obtained through the parallel plate technique are of metallic appearance, are well attached, and exhibit metallic luster.

**Figure 14.** Surface morphology of Ni-P-SiC composite obtained from an S solution (= 0.2 M NaCl, 0.65 M NiSO4⋅6H2O, 0.75 M NiSO4 6H2O, 0.1 M H3BO3, 0.1 M H3PO3) + 0.084 mM DTAB + 0.02 g mL-1 SiC and *j* = 0.021 A/cm2 for 20 min.

#### *3.3.2.2. Thermal treatment*

The Ni-P-SiC composite coatings obtained in the previous section were thermally annealed at different temperatures: 300 °C, 400 °C, 500 °C, and 600 °C for 60 min. Each test was repeated three times for each temperature set point.

Figure 15 shows the Ni-P-SiC plates after annealing. Annealed Ni-P-SiC coatings exhibit evident changes in surface morphology with respect to their nonannealed counterparts (see Figure 14).

**Figure 15.** Surface morphology of Ni-P-SiC composite coatings after thermal annealing at (a) 300 °C, (b) 400 °C, (c) 500 °C, and (d) 600 °C.

#### *3.3.2.3. Glow discharge spectroscopy characterization*

Figure 16 shows the typical concentration profile of a Ni-P-SiC coating after thermal annealing. The oxygen found on the coating surface is associated with superficial oxidation. After removing the oxide layer, a constant composition of Ni and P is observed through the entire coating thickness (24 μm). The Si concentration, however, is lower in the upper layers of the coating (5 to 22 μm) and increases in the deeper layers (22.5 to 25 μm). A similar behavior was observed for all the analyzed coatings.

**Figure 16.** GDS elemental distribution profiles of a Ni-P-SiC coating after thermal annealing and electrodeposited un‐ der galvanostatic conditions (*j* = 0.021 A/cm2 , *t* = 20 min) in solution S + 0.084 mM DTAB + 0.02 g mL-1 SiC.

Table 1 shows the variation of SiC content in the Ni-P-SiC coating matrix after thermal annealing at different temperatures. A decrease in Si content is observed on thermally treated coatings. An approximate 57% loss of Si was observed in the range of 300 °C to 500 °C and increased to 70% when the coating was annealed at 600 °C. This observed behavior could be related to the detachment of SiC particles from the coating matrix during thermal annealing, which is corroborated by composition profile analysis. GDS composition profiles show that the SiC composition remains constant inside a certain range of the coating thickness but increases at the deepest point. This indicates that SiC particle detachment occurs in the upper and middle coating layers.


**Table 1.** Atomic percentages of Si in the Ni-P-SiC composite coatings after 60 min thermal annealing at different temperatures.

#### *3.3.2.4. XRD characterization*

**Figure 14.** Surface morphology of Ni-P-SiC composite obtained from an S solution (= 0.2 M NaCl, 0.65 M NiSO4⋅6H2O,

The Ni-P-SiC composite coatings obtained in the previous section were thermally annealed at different temperatures: 300 °C, 400 °C, 500 °C, and 600 °C for 60 min. Each test was repeated

Figure 15 shows the Ni-P-SiC plates after annealing. Annealed Ni-P-SiC coatings exhibit evident changes in surface morphology with respect to their nonannealed counterparts (see

**Figure 15.** Surface morphology of Ni-P-SiC composite coatings after thermal annealing at (a) 300 °C, (b) 400 °C, (c) 500 °C,

Figure 16 shows the typical concentration profile of a Ni-P-SiC coating after thermal annealing. The oxygen found on the coating surface is associated with superficial oxidation. After removing the oxide layer, a constant composition of Ni and P is observed through the entire

for 20 min.

0.75 M NiSO4 6H2O, 0.1 M H3BO3, 0.1 M H3PO3) + 0.084 mM DTAB + 0.02 g mL-1 SiC and *j* = 0.021 A/cm2

*3.3.2.2. Thermal treatment*

136 Electrodeposition of Composite Materials

Figure 14).

and (d) 600 °C.

three times for each temperature set point.

*3.3.2.3. Glow discharge spectroscopy characterization*

Figure 17 shows the XRD patterns for the Ni-P-SiC coatings without heat treatment and with heat treatment (60 min) at different temperatures. Without heat treatment and at annealing temperatures below 400 °C, only a small broad peak appears in the XRD patterns, suggesting an amorphous structure without phase transition. However, when the heat treatment tem‐ perature was close to 500 °C, the structure became crystalline and the XRD pattern shows new sharp peaks corresponding to crystalline fcc Ni (JCP2 04-0850) and Ni3P (JCP2 89-2743). The transition can be related to the crystallization of neat Ni and the consecutive precipitation of Ni3P from the supersaturated Ni-P solid solution [30,13,37-38]. Studies of similar systems [39– 41] have established that amorphous Ni-P alloys are less dense than crystalline Ni-P alloys, and as consequence, the transition from amorphous to crystalline structure is accompanied by a volume contraction [13]. In agreement with this statement, after the thermal treatment at 500 °C of the Ni-P-SiC composites, a signal corresponding to SiC particles appears.

**Figure 17.** XRD patterns for Ni-P-SiC coatings electrodeposited onto AISI 1018 steel and heat treated at different tem‐ peratures. Ni (JCP2 04-0850) and Ni3P (JCP2 89-2743).
