*3.3.2.5. AFM characterization*

AFM in contact mode was used to obtain images of the Ni-P-SiC coatings both with and without heat treatment for 60 min at different temperatures. Figure 18 shows the AFM images obtained from Ni-P-SiC coatings thermally treated at different temperatures: without thermal annealing, 300 °C, 400 °C, 500 °C, and 600 °C. When the coating was treated at 300 °C (Figure 18b), an amorphous structure containing some crystals was observed; these crystals are associated with the initial formation of the Ni3P species. At 500 °C (Figure 18d), a larger quantity of crystals associated with the formation of the Ni3P species was observed in addition to smaller crystals corresponding to fcc Ni. Finally, at 600 °C (Figure 18e), the entire surface was covered with Ni3P and Ni crystals. These results confirm the phase transition and formation of the Ni3P and fcc Ni species observed by XRD.

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

**Figure 17.** XRD patterns for Ni-P-SiC coatings electrodeposited onto AISI 1018 steel and heat treated at different tem‐

AFM in contact mode was used to obtain images of the Ni-P-SiC coatings both with and without heat treatment for 60 min at different temperatures. Figure 18 shows the AFM images obtained from Ni-P-SiC coatings thermally treated at different temperatures: without thermal annealing, 300 °C, 400 °C, 500 °C, and 600 °C. When the coating was treated at 300 °C (Figure 18b), an amorphous structure containing some crystals was observed; these crystals are associated with the initial formation of the Ni3P species. At 500 °C (Figure 18d), a larger quantity of crystals associated with the formation of the Ni3P species was observed in addition to smaller crystals corresponding to fcc Ni. Finally, at 600 °C (Figure 18e), the entire surface was covered with Ni3P and Ni crystals. These results confirm the phase transition and

peratures. Ni (JCP2 04-0850) and Ni3P (JCP2 89-2743).

formation of the Ni3P and fcc Ni species observed by XRD.

*3.3.2.5. AFM characterization*

138 Electrodeposition of Composite Materials

°C of the Ni-P-SiC composites, a signal corresponding to SiC particles appears.

**Figure 18.** AFM images of Ni-P-SiC electrodeposited onto AISI 1018 steel under galvanostatic conditions (*j* = 0.021 A/cm2 , *t* = 20 min) from solution S + 0.084 mM DTAB + 0.02 g mL-1 SiC and heat treated at different temperatures: (a) without thermal annealing, (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

#### *3.3.3. Tribological properties*

#### *3.3.3.1. Microhardness of Ni-P-SiC coatings*

Figure 19 shows the surface microhardness of the Ni-P-SiC composites as a function of the annealing temperature. When the annealing temperature was less than 400 °C, a slight increase in the hardness values is observed; however, when the heat treatment was in the range of 400 °C to 500 °C, the hardness value change significantly (from 1057.2 HV to 1453.4 HV). This increase in hardness was associated with a structural change due to the formation of hard intermetallic Ni3P particles within the Ni-P-SiC coating. The obtained hardness value of the Ni-P-SiC composites at 500 °C (1453.4 HV) is greater than that of a hard Cr coating, which a microhardness value of 1020 HV [24]. Finally, at annealing temperatures above 500 °C, the hardness of the Ni-P-SiC coating decreased sharply. At higher temperatures, the coating began to soften because the Ni3P particles conglomerated, reducing the number of hardening sites. This process also removes P and SiC from the alloy, producing a separate phase of soft Ni within the matrix and further reducing the bulk hardness.

**Figure 19.** Hardness of the electrodeposited Ni-P-SiC coatings after a 60-min heat treatment.

#### *3.3.3.2. Wear resistance*

Figure 20 shows the wear volume of the electrodeposited Ni-P-SiC coatings as a function of the annealing temperature. Once the coating began to harden to approximately 400 °C, the decrease in the wear volume was small. When the coating was heat treated at 500 °C, the wear volume decreased sharply (i.e., the wear resistance increased). The coating that was heat treated at 600 °C contained cracks in its surface that could negatively affect its abrasion resistance.

**Figure 20.** The wear volume of the Ni-P-SiC coatings after a 60-min heat treatment at different temperatures.

#### *3.3.3.3. Friction coefficients*

*3.3.3. Tribological properties*

140 Electrodeposition of Composite Materials

*3.3.3.2. Wear resistance*

resistance.

*3.3.3.1. Microhardness of Ni-P-SiC coatings*

within the matrix and further reducing the bulk hardness.

**Figure 19.** Hardness of the electrodeposited Ni-P-SiC coatings after a 60-min heat treatment.

Figure 20 shows the wear volume of the electrodeposited Ni-P-SiC coatings as a function of the annealing temperature. Once the coating began to harden to approximately 400 °C, the decrease in the wear volume was small. When the coating was heat treated at 500 °C, the wear volume decreased sharply (i.e., the wear resistance increased). The coating that was heat treated at 600 °C contained cracks in its surface that could negatively affect its abrasion

Figure 19 shows the surface microhardness of the Ni-P-SiC composites as a function of the annealing temperature. When the annealing temperature was less than 400 °C, a slight increase in the hardness values is observed; however, when the heat treatment was in the range of 400 °C to 500 °C, the hardness value change significantly (from 1057.2 HV to 1453.4 HV). This increase in hardness was associated with a structural change due to the formation of hard intermetallic Ni3P particles within the Ni-P-SiC coating. The obtained hardness value of the Ni-P-SiC composites at 500 °C (1453.4 HV) is greater than that of a hard Cr coating, which a microhardness value of 1020 HV [24]. Finally, at annealing temperatures above 500 °C, the hardness of the Ni-P-SiC coating decreased sharply. At higher temperatures, the coating began to soften because the Ni3P particles conglomerated, reducing the number of hardening sites. This process also removes P and SiC from the alloy, producing a separate phase of soft Ni

> Figure 21 shows the characteristic profile of the coefficient of friction of the Ni-P-SiC coatings without and with heat treatment at different temperatures. When the Ni-P-SiC coatings were treated at temperatures above 200 °C, the coefficient of friction rapidly reached equilibrium. After 1000 cycles, coatings treated at 500 °C had the lowest friction coefficient.

**Figure 21.** Frictional coefficient for the Ni-P-SiC coatings after a 60-min heat treatment at different temperatures.
