Effect of SiO₂ Nanoparticles Addition on Tribological and Electrochemical Behaviors of Ni–P–MoS₂ Multi-Component Coatings after Heat Treatment

Abstract

The tribological and electrochemical properties of various nanocomposite coatings of Ni–P–MoS₂–SiO₂ have been investigated. Such coatings were made by the electroless method and then heated at 400°C for 1 h. The amount of SiO₂ nanoparticles was a factor which changed Ni–P–MoS₂ coatings characteristics. In addition to microstructure evaluations and the phase detection, the microhardness, the friction coefficient, and wear behavior were studied for various coatings. Field emission scanning electron microscopy images demonstrated that SiO₂ nanoparticles were distributed uniformly in the Ni–P–MoS₂ matrix. The obtained results showed that when the concentration of SiO₂ nanoparticles in the deposition bath increased from 5 to 20 g/L, then the microhardness and the wear resistance increased as well. Besides, the friction coefficient reached the lowest value of 0.05. For nanocomposite coatings, the ratio of the friction coefficient to the square hardness was a proper parameter which could predict the wear behavior. Additionally, energy dispersive spectroscopy results showed that the content of SiO₂ nanoparticles in such coatings increased from 4.9 to 11.4 wt %. Corrosion tests demonstrated that the best corrosion resistance was observed for the nanocomposite coating when the concentration of SiO₂ nanoparticle was 20 g/L.

INTRODUCTION

Electroless Ni–P coatings have been extensively investigated recently due to their excellent mecha-nical and tribological modes of behavior. Such coatings are widely used in various applications such as mechanical, chemical, and electronic industries [1]. It was reported that different characteristics of electroless coatings are usually enhanced by post-heating. Moreover, the corporation of ceramic nanoparticles (e.g. TiO₂ [2], SiO₂ [1, 3–5], SiC [6] and Al₂O₃ [7]) and solid lubricant materials (e.g. graphite [8], MoS₂ [9–12], PTFE [13], and graphene oxide [14]) in the Ni–P matrix, led to enhanced properties of such composite coatings. In the following paragraph, some of these studies related to reinforcement agents of SiO₂ nanoparticles and MoS₂ particles in Ni–P coatings have been reviewed.

Carbon steel was used as substrate for deposition of Ni–P–MoS₂ coatings [9] and tribological charac-teristics of such composite coatings were investigated at elevated temperatures up to 600°C. Corrosion properties of those coatings were also studied [10], as well as their various wear parameters such as the type of the contact, the wear load, the duration, and the substrate type [15]. It was found that the addition of SiO₂ nanoparticles into Ni–P coatings increased their toughness, hardness, and wear resistance [1]. The corrosion resistance of various Ni–P–SiO₂ nanocomposite coatings on aluminum and magnesium substrates was also reported [1, 4]. The investigation results showed that the corrosion resistance increased due to a lower porosity content for these coatings regarding Ni–P coatings. It was also demonstrated that SiO₂ nanoparticles in the Ni–P coating enhanced its mechanical properties [5].

Moreover, some particles used as solid lubricant materials such as PTFE and MoS₂ particles could reduce the friction coefficient of Ni–P coatings; however, hard nanoparticles such as Al₂O₃, and SiO₂ would decrease the wear rate of such coatings and influence their strength [8, 15, 16]. Thus, recently, the hybrid multi-component coatings which contained both lubricating and hard particles have attracted plenty of scientific interest. For example, it was demonstrated that Ni–P–PTFE–SiC coatings had a combination of advantages of a lower friction coefficient over Ni–P–PTFE coatings and a higher load-bearing over Ni–P–SiC coating in wear testing [17]. In addition, Ni–P–SiC coatings showed the highest hardness and Ni–P–PTFE coatings had the lowest friction coefficient. The wear resistance and the corrosion behavior of Ni–P–PTFE–Al₂O₃ nanocomposite coatings were reported in [18]. Their results demonstrated that such multi-component coatings exhibited a significant degree of corrosion resistance compared to that of Ni–P coatings. Other researchers developed a novel electroless plating method to deposit Ni–P–Al–ZrO₂ nanocomposite coatings [19]. It was found that such nanocomposite coatings showed higher hardness and a lower friction coefficient than Ni–P coatings.

Therefore, due to limited number studies of Ni–P–MoS₂–SiO₂ nanocomposite coatings, in the present research, a possibility of incorporating SiO₂ nanoparticles in Ni–P–MoS₂ matrix was investigated. Wear properties and the electrochemical performance of electroless nanocomposite coatings were evaluated. In addition, the comparison of microstructure and the phase detection of such coatings with respect to Ni–P coatings were done after the heat treatment process. The concentration of SiO₂ nanoparticles in the deposition bath was also introduced as a parameter that influenced the properties of nanocomposite coatings. It is worth noting here that SiO₂ nanoparticles, as a strengthening phase, and MoS₂ particles, as a solid lubricant material, could enhance the mechanical and corrosion properties of Ni–P coatings.

MATERIALS AND METHODS

Preparing Multi-Component Coatings

Ni–P–MoS₂–SiO₂ nanocomposite coatings were prepared by an electroless method. The AISI-st37 steel plates (20 × 30 × 2 mm³) were used as substrates. Details of the chemical composition of Ni-P baths are shown in Table 1.

Table 1.   Chemical composition of Ni-P deposition baths

The baths pH was at 4.5–4.9 and it was kept constant by addition of a certain amount of an ammonia solution during the deposition process. The temperature range of the deposition process was about 81–86°C. The concentration of MoS₂ particles in the deposition baths was 10 g/L to achieve the best properties [11]. The average size of MoS₂ particles was 2 μm. Concentrations of SiO₂ nanoparticles were 5, 10, and 20 g/L in the deposition baths. This concentration range was also used in [20]. The average size of SiO₂ amorphous nanoparticles was 40 nm. Bath compositions for various specimens are listed in Table 2.

Table 2.   Specimen’s names and particles concentrations in deposition baths

Since various parameters: the treatment process, the Ni-P bath composition, the temperature, pH, and the agitation, affected coating characteristics such as the growth rate, the porosity content, the morphology, and physical properties [3], all these parameters were taken as constant for all coatings investigated in this paper.

Before the deposition process, preparation steps for steel substrates were as follows: polishing to 2500 grit, chemical degreasing by acetone at 25°C for 5 min, and pickling in hydrochloric acid 50% for 3 s. The deposition process for all specimens was 60 min. MoS₂ particles and SiO₂ nanoparticles were suspended in the deposition baths by using the sodium dodecyl sulfate surfactant and were stirred by the ultrasonic agitation for 10 min. It is to be noted here that the bath agitation by the magnetic stirrer was also done during deposition in order to achieve a higher rate of convection and diffusion of ions. In this situation, ions and other particles approached better to the specimen surface [20]. Finally, after the deposition step, the heat treatment process was conducted in a resistance furnace at 400°C for 1 h. A similar process was suggested elsewhere to gain better mechanical and tribological properties [9, 13]. The rate of the heating process was about 10°C/min.

Characterization of Multi-Component Coatings

Morphologies of coatings were analyzed by the optical microscopy (OM, Olympus model) and the field emission scanning electron microscopy (FESEM, MIRA3-TSCAN). Various phases in multi-component coatings were detected using a Bruker D8 advanced phase X-ray diffractometer (XRD) with CuKα radiation. The scanning range was from 10° to 90°. In addition, the mean grain size of various phases was determined according to ASTM E112-06. The Scherrer equation from XRD patterns was applied to measure the size of crystallites for various phases. The full-width at half-maximum of peaks used according to Eq. (1):

$$L = \frac{{k{\lambda }}}{{{{B}_{{\text{m}}}}\cos {\theta }}},$$

(1)

where L is the grain size, k is the dimensionless shape factor (with the value of 0.9), λ is the wavelength of X-ray, B_m is the full-width at half-maximum of the Bragg peak, and θ is the Bragg reflection angle [19].

The microhardness for all coatings was measured by a Vicker microhardness tester. The applied load value was 10 g. Three measurements were used to report the mean microhardness value for each specimen. Wear tests were conducted by a pin-on-disk wear tester at a velocity of 0.1 m/s. The applied load was 6 N, and the wear distance was 500 m. There was no lubricant used during wear testing. The friction coefficient for all samples was recorded automatically by a computer attached to the wear tester. The used pin was an alloy steel with a hardness of 711 VHN. The worn surfaces of various specimens were analyzed by the scanning electron microscopy (SEM, Philips-XL30).

The polarization analysis of various multi-component coatings was carried out by an Autolab-OGF500 potentiostat. A saturated calomel electrode (SCE) and a platinum foil were used as reference and counter electrodes, respectively, in the corrosive solution of 3.5 wt % NaCl. Such tests were conducted at room temperature at pH of 7.8–8. The potential range was from –0.8 to –0.1 V vs SCE potential with a scanning rate of 1 mV/s. Prior to the electrochemical test, all samples were stabilized to measure the open circuit potential (OCP). The polarization resistance (R_p) is calculated from equation 2 [22]:

$$\frac{1}{{{{R}_{{\text{p}}}}}} = 2.303{{i}_{{{\text{corr}}}}}\left( {\frac{1}{{{{{\beta }}_{{\text{c}}}}}} + \frac{1}{{{{{\beta }}_{{\text{a}}}}}}} \right),$$

(2)

where i_corr is the corrosion current, β_a is the anodic slope, and β_c is the cathodic slope. The electro-chemical impedance spectra tests were done in a frequency range of 10 kHz to 0.01 Hz, with an applied AC signal of 10 mV. The potential range was about 600 to 250 mV from the OCP. The solution temperature was about 25°C. The Z View software was utilized to analyze the required EIS data.

RESULTS AND DISCUSSION

Since characteristics of coatings depended on various factors such as the particle size, the particle concentration in the electrolyte, the deposition process time, and the dispersion method of particles in the electroless bath [7], most of these parameters were taken as constant in this paper. All coatings deposited on steel substrates were matt in appearance, of the grey color.

OM and SEM Evaluations and Thickness Measurements

Figure 1 illustrates the effect of MoS₂ particles and SiO₂ nanoparticles presence on the surface homo-geneity of Ni–P coatings. As shown in Fig. 1a, the Ni–P coating was uniform on the steel substrate. Thus, the surface of sample-0-0 was smooth and had the lowest nodules. In contrast, the Ni–P–MoS₂ coating (as shown in Fig. 1b) had a rough surface with large nodules. Such discontinuity was due to the preferential deposition of nickel atoms on conductive surfaces of MoS₂ particles when they were attached to the substrate. When SiO₂ nanoparticles were added to the deposition bath, the surface roughness decreased with respect to that with Ni–P–MoS₂ coatings (sample-10-0). Moreover, when the SiO₂ nanoparticles concentration increased from 5 to 20 g/L in the deposition bath, surface nodules decreased largely. It was due to a lower corporation of MoS₂ particles embedded in the matrix. It was found that more nucleation sites would be available on the surface when more nanoparticles were in the deposition bath [11]. Thus, the surface roughness was reduced by increasing the amount of SiO₂ nanoparticles.

Fig. 1.

Top view of OM images of various specimens: (a) sample-0-0; (b) sample-10-0; (c) sample-10-5; (d) sample-10-10; and (e) sample-10-20.

Figures 2a–2e demonstrate the top view of FESEM images of various coatings. Figure 2a shows small nodules with the mean size of 5 μm observed on the surface of the coating. This confirmed special growth mechanism of Ni–P coatings reported elsewhere [12]. Figure 2b shows that the surface of the Ni–P–MoS₂ coating was the roughest, with large clusters. The size of such clusters reached 8 μm for sample-10-0. Similar to the obtained here results, it was found elsewhere that the Ni–P coating surface was very smooth; however, the addition of other particles caused the surface roughness to be coarser [5]. Some researchers [11] reported the following: when MoS₂ particles were present in the deposition bath, they acted as preferential sites for the deposition of nickel atoms; however, in the absence of such particles, nickel atoms were deposited homogeneously on the surface of substrates, and the homogeneity of the surface would be increased.

Fig. 2.

Top view of SEM images of various specimens: (a) sample-0-0; (b) and (c) sample-10-0; (d) sample-10-5; and (e) sample-10-20 and SEM images of two specimens from the side view, including (f) sample-10-0; and (g) sample-10-10.

Figure 2c illustrates some of the clusters at a higher magnification. The coating morphology in such areas had a cauliflower-like structure. A similar morphology was also reported elsewhere [23]. As shown in Fig. 2d, for sample-10-5, the addition of SiO₂ nanoparticles into Ni–P–MoS₂ coatings increased the surface homogeneity. It was found that resulted from an increase of the density of the coating through the reduction in the porosity content and a raised hardness [3]. Moreover, SiO₂ nanoparticles were distributed uniformly in the Ni–P matrix, and the structure of the coating was very dense and crack-free. Figure 2e shows that when the SiO₂ nanoparticles content increased, they were activated to agglomeration due to a high specific surface area effect. This also resulted in an increase of the solution viscosity and a decrease in the deposition rate [20]. It was found that at a higher concentration of SiO₂ nanoparticles in the deposition bath, more nanoparticles were absorbed by the metal initial surface and then embedded into coatings. It was also reported elsewhere [12, 24]. Consequently, a uniform distribution of clusters for Ni–P–MoS₂–SiO₂ coatings was observed with respect to the Ni–P–MoS₂ coating. The addition of SiO₂ nanoparticles to Ni–P–MoS₂ coatings reduced the surface roughness.

Figures 2f, 2g show cross-sectional images of coatings for sample-10-10 and sample-10-20, respectively. According to Table 2, the deposition time and such conditions of the deposition baths as pH and temperature were constant and the coating thickness increased when SiO₂ nanoparticles were present in the deposition bath. A high active surface of nanoparticles usually provided more sites for nucleation of nickel and phosphorus atoms in the process of deposition [20]. When increasing the concentration of SiO₂ nanoparticles in the deposition bath from 5 to 20 g/L, the thickness of the composite coating decreased from 7.82 to 7.24 μm. Such behavior was attributed to an increase of cationic surfactants in the bath. This was also reported elsewhere [13, 15]. For example, the authors in [13] specified that the cathode surface coverage by surfactants hindered the reduction of nickel ions [13]. Thus, the lowest and the highest thicknesses of coatings were related to sample-0-0 and sample-10-5, with the values of 4.82 ± 0.15 and 7.82 ± 0.15 μm, respectively. Totally, an increase of thickness for nanocomposite coatings was about 34 to 63% comparing to that with Ni–P coating.

XRD and EDS Results

To confirm the effect of various particles in multi-component coatings, the EDS and the XRD techniques were applied and the corresponding results are shown in Figs. 3 and 4, respectively, and in Table 3.

Fig. 3.

Relation of SiO₂ nanoparticles concentration in the deposition bath with SiO₂ content in nanocomposite coatings.

Fig. 4.

XRD patterns for various specimens.

Table 3.   Coating thicknesses and EDS results

As shown in Table 3, by adding MoS₂ particles into the deposition bath, the nickel content increased and the phosphorus content decreased in sample-10-0 comparing to those in sample-0-0, while the presence of SiO₂ nanoparticles decreased the nickel and the phosphorus content in the coating. It is worth noting here that a decrease in the nickel content for coatings by the incorporation of nanoparticles could be explained by the following: nanoparticles did not have an auto-catalytic effect towards nickel and therefore hindered the nickel deposition on the surface and occupied most areas [7]. Moreover, the incorporation of SiO₂ nanoparticles in the deposition bath affected the MoS₂ content in the Ni–P coating. By increasing the content of SiO₂ nanoparticles, the MoS₂ content decreased largely. When the SiO₂ nanoparticles concentration changed from 5 to 20 g/L in the deposition bath, the content of SiO₂ nanoparticles increased from to 4.9 to 11.4 wt % in nanocomposite coatings, as shown in Fig. 3. It is to be noted here that the relation of the SiO₂ nanoparticles concentration in the deposition bath with the content of SiO₂ nanoparticles in the coating was not linear. In addition, there was no significant change in the phosphorus and the nickel contents for various nanocomposite coatings with SiO₂ nanoparticles. It was found that the particle content in the Ni–P coating increased with an increase of SiO₂ nanoparticles concentration in the deposition bath. It reached the highest value of 25 vol % at 10 g/L [25].

As shown in Fig. 4, sharp peaks in XRD patterns indicated that the microstructure of these coatings after the heat treating was the crystalline phase. It was reported elsewhere that the Ni–P coating was the amorphous phase and the heat treatment process caused its crystallization to amorphous phases [15]. The heating process at 400°C for 1 h led to the formation of both the Ni₃P phase and the Ni phase. A similar observation was reported in [3]. Since Ni–P coatings can be categorized as low (1–5 wt %), medium (6–9 wt %), and high (≥10 wt %) phosphorous coatings, it can be stated that all coatings had a high phosphorous content due to the formation of Ni₃P phase. It was noticed that medium and high phosphorous coatings had an excellent combination of the wear and the corrosion resistance [3]. When MoS₂ particles were added to the Ni–P coating, the height of the related peaks for the nickel phase increased in the XRD pattern. A similar observation was also reported in [11]. This was related to the fact that in the Ni–P–MoS₂ coating, the nickel phase preferred to nucleate more around MoS₂ particles compared to the situation with the Fe/Ni matrix. Thus, the height of the nickel phase in planes of {111} and {200} sharply increased. Moreover, the phase of Ni_2 × 82S₂ was observed in a low content after the heating treatment process. Due to the amorphous nature of SiO₂ nanoparticles used in the deposition bath and a low content of such material in coatings, the SiO₂ diffraction peaks did not appear in the XRD patterns for nanocomposite coatings. The presence of more nanoparticles had no significant effect so as to change the XRD pattern. Such observation was also reported in [13].

The size of crystallites for various phases is shown in Fig. 5. The lowest and the highest crystallite sizes for the Ni₃P phase were related to sample-10-0 and sample-0-0, respectively. The highest size of crystallite for the nickel phase was also related to sample-0-0 with a value of 5.8 nm. When particles were added to the deposition bath, the crystallite size of both Ni and Ni₃P phases were reduced by about 34–52%.

Fig. 5.

Sizes of crystallites for Ni and Ni₃P phases.

Microhardness Measurements

As shown in Table 4, the microhardness of the Ni–P coating was 559.8 ± 10.0 Vickers hardness number (VHN) after the heat-treating process. It is to be noted that the hardness of Ni–P coatings depended on the phosphorus content and the heat treatment temperature. The hardness values could vary from 500 to 700 VHN when the content of phosphorus changed from three to twenty of the weight percentage [16]. The obtained results showed that the hardness value increased from 559.8 to 591.2 VHN when MoS₂ particles were added to Ni-P coatings. This behavior was an inconsistency with the soft nature of such lubricating particles. However, an increase in the hardness value could be attributed to lower crystallite sizes (as shown in Fig. 5). Other studies [11, 12] demonstrated that such event also corresponded to the effect of dispersion strengthening caused by particles. Thus, as shown in Fig. 2b, MoS₂ particles, which dispersed uniformly in the Ni–P matrix, increased the hardness of the Ni–P coating via the effect of the dispersion strengthening mechanism. It was shown elsewhere that the Ni–P–MoS₂ coating had the hardness value between 605 to 627 VHN, compared to that of the Ni–P coating of 530 ± 60 VHN [11]. It is to be noted that the heat treatment at 400°C for 2 h increased the hardness value to 795 VHN for Ni–P coatings and 1200 ± 100 for Ni–P–MoS₂ coatings [12]. Moreover, the incorporating of SiO₂ nanoparticles as the disperse phase in the matrix of electroless Ni–P coatings also enhanced the hardness. The presence of nanoparticles resulted in the hindrance of the dislocation movement in the ductile matrix [3, 26]. An increase in the content of SiO₂ nanoparticles in the coating raised the hardness by 40% with respect to that of the Ni–P coating. Thus, the highest hardness was related to sample-10-20, with the value of 787.3 ± 10.0 VHN. The microhardness value for the Ni–P–SiO₂ coating was reported as 670 ± 40 VHN in [27]. It was also demonstrated elsewhere that when the content of SiO₂ was 10 g/L in the deposition bath, microhardness reached 367 VHN, without any heat treatment [29]. The hardness of the Ni–P coating improved when it was heated at 400°C, due to the precipitation of Ni and Ni₃P phases. Such phases prevented grain growth and the mobility of dislocations in the matrix [16, 28].

Table 4.   Hardness and tribological properties

Wear Properties

Since the applied force and the sliding distance had significant effects on the wear properties of Ni–P coatings [16], both parameters were taken as constant for all specimens in the present investigation. The obtained results for wear testing are shown in Table 4. All multi-component coatings demonstrated a higher wear resistance compared to that of the Ni–P coating. When the content of SiO₂ nanoparticles in nanocomposite coatings was higher, the wear resistance increased. Thus, the best coating against the applied force had sample-10-20. In other words, the wear rate decreased for about 5 times for this specimen with respect to that of sample-0-0. These results would be attributed to values of hardness and the friction coefficient.

Figure 6 shows friction coefficient curves vs distance for various coatings during wear testing. The friction coefficient for the Ni–P coating was the highest and reached 0.3. A similar result was also reported in [13]. When MoS₂ particles were added to the Ni–P coating, the friction coefficient decreased from 0.3 to 0.1. This result was due to the fact that during sliding, MoS₂ particles created a thin lubricating layer by the shear force, which caused a reduction in the friction coefficient of coatings. Thus, the friction coefficient of Ni–P coatings after incorporation of MoS₂ nanoparticles decreased greatly [10, 15]. It was found that MoS₂ had a low friction coefficient—under 0.1 in inert atmosphere; however, it increased to 0.3 in the humid air. Such behavior corresponded to the transformation of MoS₂ to MoO₃ [11]. The friction coefficient and the wear rate of the Ni–P–MoS₂ heat-treated coating was lower than of the coatings without heat treatment [15]. It was reported that when the friction occurred inside the lubricant layer, the friction value decreased greatly [30]. The friction coefficient decreased at a higher SiO₂ nanoparticles content in the coating. Therefore, the lowest value for the friction coefficient was related to sample-10-20, and it was 0.05. It was found that the friction coefficient value increased to 0.55 when the Ni–P–SiO₂ coating was heat-treated at 390°C, but the wear resistance decreased [25]. The wear resistance of the composite coating increased with an increase of the PTFE content in the Ni–P coating, although the heat treatment at 400°C changed wear properties of the Ni–P–PTFE coating [13].

Fig. 6.

Friction coefficient curves vs wear distance for various specimens.

To predict the wear behavior of the material, some approaches were reported elsewhere [26, 29, 31]. The wear rate of various coatings could be proportional by the E/H ratio (where E is the elastic modulus of coatings and H is hardness), the FE/H ratio (where F is the friction coefficient), and the F/H² ratio. In this paper, the last parameter was a proper criterion to predict the wear rate of various specimens. In other words, as seen in Table 4, sample-10-20 with the lowest value of F/H² ratio showed the lowest wear rate and the highest wear resistance.

SEM images of worn surfaces for various coatings are shown in Fig. 7. Figure 7a shows the coating which was flaked during wear testing. In addition, some longitudinal grooves along the sliding direction were observed. These grooves were accomplished with a high degree of plasticity, which was characterized as a typical adhesive wear mechanism for the Ni–P coating. The surface of the Ni–P coating was also characterized by the micro-ploughing effect. No pores or pits were observed on the surface. Grooves of the Ni–P coating were deep. Moreover, the racking and spalling were observed on the worn surface, thus revealing the highest wear rate. Hence, the dominated wear mechanism for the Ni–P coating would be the adhesive plus the micro-ploughing mode of the abrasive state. Such behavior was reported also in [16]. The width of the wear track for sample-0-0 was about 1.43 mm (the highest among other specimens).

Fig. 7.

SEM images of worn morphology for various specimens: (a) sample-0-0; (b) sample-10-0; (c) sample-10-5; (d) sample-10-10; and (e) sample-10-20.

As shown in Fig. 7b, there were small ploughs on worn areas without removal of layers due to MoS₂ particles as the solid-lubricant material. It was found that during wear testing MoS₂ particles served as spacers to lower the contact area between asperities of two surfaces. Moreover, the spherical shape of MoS₂ particles could create easier sliding between contacted surfaces [12]. Thus, the worn morphology presented a smoother surface with respect to other specimens. It was found that the width of the wear track for sample-10-0 was similar to that of sample-0-0. SEM images of worn surfaces in Fig. 7c to 7e show that when SiO₂ nanoparticles were added to Ni–P–MoS₂ composite coatings, moderate scratches were observed on the worn surface, which could be characterized as the abrasive wear. Increasing SiO₂ nanoparticles content in the Ni–P matrix increased abrasive scratches on the worn surface. More debris and cracks were created on the worn surface. When the shear strength exceeded that of the material, then cracks nucleated and propagated, resulting in the fracture and the detachment of materials from the surface and creation of wear debris. Therefore, material removal took place by the abrasion mode and by the micro-cutting mechanism for all nanocomposite coatings. Such mechanisms were also suggested in [16]. It is to be noted that hard materials could tolerate high loads without reaching the fracture point even if the resulted failure was brittle naturally [32]. It was found that the width of the wear track for nanocomposite coatings decreased from 1080 to 600 µm by increasing the nanoparticles content in Ni–P–MoS₂ coatings.

SEM micrographs of wear debris for various specimens are presented in Fig. 8. As shown in Fig. 8a, the mean size of the wear debris for sample-0-0 was 10 μm. The size distribution of the wear debris for sample-10-0 was large, as seen in Fig. 8b. The presence of SiO₂ nanoparticles decreased the mean size of the wear debris to 500 nm. Such behavior could justify the highest wear resistance for sample-10-20. Besides, a small size of the wear debris (Fig. 8c) showed that the abrasive wear was the dominant mechanism.

Fig. 8.

SEM micrographs of wear debris for various specimens: (a) sample-0-0; (b) sample-10-0; and (c) sample-10-20.

Corrosion Tests Evaluations

Figure 9 demonstrates polarization curves obtained for various specimens in the 3.5 wt % solution of NaCl. In addition, Table 5 shows the related corrosion parameters, including the polarization resistance (R_p), the corrosion current density (I_corr), the corrosion potential (E_corr), and the reduction percentage in the corrosion current with respect to the substrate (%R) for all specimens.

Fig. 9.

Polarization test results for various specimens (obtained potential vs SCE potential).

Table 5.   Results of polarization tests

It is to be noted that, in this paper, the corrosion rate of the bare steel as substrate of coatings was measured to be about 35.6 µA cm^–2. The corrosion potential and the corrosion current density for sample-0-0 were about –510.5 mV and 15.9 µA cm^–2, respectively. Similar results were also reported in [33]. As shown in Table 5, the corrosion potential of sample-10-0 was more positive than that of the Ni–P coating. Such behavior was caused by increasing the nickel content in the coating, as Table 3 shows. This was also reported in [34]. The corrosion current density of the Ni–P–MoS₂ coating decreased from 15.9 µA cm^–2 to 11.8 µA cm^–2. In addition, by adding SiO₂ nanoparticles in the Ni–P coating, the corrosion potential decreased but then increased with respect to that of the Ni–P coating. Thus, the most positive corrosion potential was related to sample-10-20. The corrosion potential moved in the cathodic direction when the concentration of SiO₂ nanoparticle in the deposition bath increased. It was found that changes in the corrosion potential for various specimens showed the incorporation of SiO₂ nanoparticles in the coating [35]. The lowest value of the corrosion current density 1.4 µA cm^–2 was related to sample-10-20, with the highest value of SiO₂ nanoparticles content in the coating. Thus, increasing the SiO₂ nanoparticle content increased the corrosion resistance in the 3.5 wt % NaCl environment through the reduction in surface roughness. A similar observation was also reported in [4]. A higher corrosion resistance obtained for the Ni–P–SiO₂ coating compared to that of the Ni–P coating was due to the reduction in the effective metallic area available for corrosion reactions in 3.5 wt % NaCl environment [5]. As reported in Table 5, the polarization resistance was 1.39 kΩ cm² for sample-10-20 that was greater than the R_p value of 1.31 kΩ cm² for sample-0-0. Besides, the reduction percentage in the corrosion current density with respect to the substrate was 96.1 for the former and 55.3 for the latter, which demonstrated an improvement in the corrosion resistance. These results indicated that by adding the concentration of SiO₂ nanoparticles to 20 g/L in the deposition bath, the property of the corrosion resistance increased. It was found via the polarization curves that the cathode reaction corresponded to the reduction of dissolved oxygen in the 3.5 wt % NaCl environment [23].

To confirm the obtained results by the polari-zation test and to investigate the inhibition mechanism with more details, EIS test was also used. It is to be noted that EIS is a suitable alternative technique for continuous monitoring [36]. In Fig. 10, the Bode plots and the phase angle plots for various coatings after 1 h and 24 h, as immersion times, are presented. As shown in Figs. 10a, 10c, when the immersion time increased from 1 to 24 h, the absolute impedance (Z) decreased from 100–1000 to 50–400 ohm. In addition, in the frequency range from 10^–1 to 10⁴, the absolute impedance was decreasing continuously. Additionally, the lowest absolute impedance was related to the frequency of 10⁴ for all coatings after both immersion times. The lowest absolute impedance value was that of sample-10-5 when the immersion time was 1 h, which is similar to the polarization test. So, the corrosion rate of nanocomposites coatings decreased when the amount of SiO₂ nanoparticles increased. This was due to lowering surface in-homogeneity, which acted as anodic and cathodic microscopic areas. It was found that nanoparticles could act as diffusion obstacles for the movement of Cl^– ions presented in the aqueous environment [6, 22, 37]. As shown in Fig. 10b, for the immersion time of 24 h, the highest value of the absolute impedance was obtained (especially at higher frequencies) for nanocomposites coatings when the concentration of SiO₂ nanoparticles in the deposition bath was 20 g/L. This was a clear indication that the addition of nanoparticles had a significant influence on the corrosive property of the Ni–P coating due to the changes of the surface homogeneity. In addition, a lower corrosion resistance of Ni–P–MoS₂ nanocomposites with respect to the Ni–P coating would be attributed to a lower amount of phosphorus in the coating, as reported in [33]. It was reported that when nickel atoms dissolved in the corrosive environment, then phosphorus atoms started to react with water to create a film of adsorbed hypophosphite anions, preventing further dissolving.

Fig. 10.

(a) and (c) Bode plots for various samples in 3.5 wt % NaCl solution after 1 h and 24 h, respectively; and (b) and (d) phase angle plots for various specimens after 1 and 24 h, respectively.

Figures 10b, 10d show phase angle plots for various specimens after 1 h and 24 h as immersion times, respectively. It was found that the electrochemical behavior of all coatings changed after the immersion time of 24 h. Such behavior illustrated that different fundamental processes would occur on surfaces after increasing the immersion time, which led to a change in the suggested equivalent circuit models. As shown in Fig. 10b, the lowest phase angle belonged to sample-10-10, and it was about –60°. Besides, all coatings showed a minimum mode in the frequency range from 10–100 Hz. However, at a higher immersion time, such behavior was not observed. Totally, the lowest phase angle for sample-10-20 was about –35°. All coatings showed the same behavior for corrosion properties, as illustrated in Fig. 10d. It was reported elsewhere that SiO₂ nanoparticles in the Ni–P coating improved its corrosion resistance in the 4 wt % NaCl solution for 2 to 4 weeks [3].

The best fit of the experimental measurements for the mentioned equivalent circuit models was obtained by using a constant phase element, Warburg diffusion element (Ws1), solution resistance (R2), and charge transfer resistance of the coating (R1). These models are seen in Figs. 10b, 10d. The related parameters are reported in Tables 6 and 7. Such equivalent circuit models were also represented for Ni–P coatings [37]. It was noticed that a constant phase element was usually used instead of ideal capacitance to consider the non-ideal behavior of the coating due to the surface in-homogeneity, the roughness, and other adsorption effects [37]. As shown in Table 6, the solution resistance for all specimens was between 12 to 15 Ω cm². The highest value of the charge transfer resistance was with sample-10-20.

Table 6.   Fitted data for the related equivalent circuit model in Fig. 10b

Table 7. Fitted data for the related equivalent circuit model in Fig. 10d

Table 7 shows that when increasing the immersion time to 24 h, the solution resistance remained constant with the value of 12 Ω cm² for all specimens. The highest value of the charge transfer resistance was also with sample-10-20. In addition, such resistance decreased with respect to the data in Table 6. The highest and the lowest Warburg diffusion element values were attributed to sample-10-20 and sample-10-5, respectively. Thus, the lowest corrosion resistance was related to sample-10-5 due to a lower crystallite size of various phases with respect to Ni–P coating. Consequently, corrosion properties of such coatings depended on several parameters such as surface roughness, the grain size, and the content of phosphorous and other particles.

CONCLUSIONS

The microstructure, the tribological and electrochemical modes of behavior of Ni–P–MoS₂–SiO₂ multi-component nanocomposite coatings were investigated. The concentration of SiO₂ nanoparticles in the deposition bath was a variable parameter. Major findings after the heat treating at 400°C for 1 h can be summarized as follows:

• SiO₂ nanoparticles were successfully incorporated in Ni–P–MoS₂ coatings. FESEM images showed that such nanoparticles distributed uniformly in the Ni–P matrix; however, when the concentration of SiO₂ nanoparticles increased to 20 g/L, in some areas, they were agglomerated.

• When SiO₂ nanoparticles or MoS₂ particles were absorbed on the surface of substrates, they could supply more new active sites for nucleation. Thus, at the constant deposition time, the thickness of composite coatings would increase for about 34 to 63%, with respect to that of the Ni–P coating.

• The microhardness of Ni–P–MoS₂ coatings increased in the presence of SiO₂ nanoparticles up to a maximum of 787 VHN. In addition, the friction coefficient value decreased to 0.05 when the content of nanoparticles embedded into the Ni–P–MoS₂ coating was about 11.4 wt %. SEM images showed that the wear mechanism changed from the adhesive plus of micro-ploughing abrasive mode for the Ni–P coating to the micro-cutting abrasive mode for nanocomposites coatings.

• The polarization test results in the 3.5 wt % solution of NaCl demonstrated that addition of SiO₂ nanoparticles to 20 g/L in the deposition bath improved the corrosion resistance. The reduction percentage in the corrosion current density with respect to the substrate was about 96.1.

• EIS test results after the immersion time of 1 h depicted that the lowest corrosion resistance was with the nanocomposites coating which contained 4.9 wt % SiO₂ particle content. Such results were due to a higher surface roughness, a lower content of SiO₂ nanoparticles and a smaller crystallite size. In addition, the immersion time of 24 h led to a change in the corrosion behavior of such coatings.

• The corrosion behavior of nanocomposite coatings depended on several factors such as the surface roughness, the grain size, the phosphorous content, the immersion time, and the content of nanoparticles. Thus, the predicting of corrosion properties needed a lot of considerations.