Effect of heat treatment on structures and mechanical properties of electroless Ni–P–GO composite coatings

Abstract

In this work, graphene oxide (GO) was incorporated into a nickel phosphorus (Ni–P) alloy matrix by electroless plating. A series of experiments were carried out to examine the mechanical properties. The surface-heat treatment of Ni–P–GO composite coatings was performed at 200 °C, 400 °C, and 600 °C under vacuum atmosphere, respectively. X-ray diffraction (XRD) results indicated that composite coatings from a mixture of amorphous and crystalline phases transformed into a crystal structure after heat treatment. The microstructures of Ni–P–GO composite coatings were revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The micro-hardness and wear resistance of composite coatings were evaluated using both a micro-hardness tester and wear test apparatus. Additionally, the worn surface of Ni–P–GO composite coatings was analyzed by SEM. The results demonstrated that the heat treatment composite coatings have higher hardness and wear resistance than the composite coatings without heating. Composite coatings perform the best after heat treatment at 400 °C.

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1. Introduction

Electroless Ni–P coating as a new surface treatment technology, has drawn ever increasing attention.¹ The composite coatings and the procedure of electroless plating have developed rapidly. Previous investigations containing particles are added into the electroless composite coatings, such as SiC, Co, Al₂O₃, C and so on.^2–4 Among them, elemental carbon plays an important role. Studies show that Ni–P–graphite⁵ can enhance the wear resistance of composite coatings because of its good self-lubrication. Ni–P–diamond composite coatings have extremely high corrosion resistance and wear resistance due to diamond's hardness.⁶ Carbon nanotubes (CNTs) with high elastic modulus, diameter ratio and strength have been applied to the electroless coating⁷ successfully. In recent years, graphene has been the major focus of researchers due to its superior characteristics,⁸ but it's difficult to prepare the perfect graphene and not easy to be directly applied scientific research.⁹ However, graphene oxide (GO), which owns the similar properties to graphene but contains reactive functional groups, i.e. hydroxyl, carboxyl groups and epoxy, which can provide the possibility for the modification.¹⁰ Thus, GO becomes the best alternative. Currently the study on GO main focus on the functionalized graphene polymer composite material, and metal matrix composites with GO.¹¹ GO has excellent dispersion in water and more uniform distribution in the chemical plating bath, which would make an uniform and dense composite coatings.¹² But as we know, there are no reports about the electroless Ni–P coatings using graphene oxide, which is what we added to the plating bath in our present work to obtain a better performance. Moreover, we explored the application of composite coatings with graphene in electroplating, i.e. graphene–Ni composite coatings by electrodeposition.¹³

In our previous study,¹⁴ we have fabricated microstructure of Ni–P–GO composite coatings on the low carbon steel surface successfully. The Ni–P–GO coatings were approximately 10 μm thick according to the cross-section analysis, and the matrix of the mechanical properties of carbon steel have been improved to some extent, details of which were documented in earlier report.¹⁴ In this report, we focus on the improvement of the performance of the as-plated Ni–P–GO composite coatings based the preliminary study.

We introduced the heat treatment into the Ni–P–GO composite coatings, as its ability to disperse the precipitates, to improve the performance of composite coatings and to meet the increasing needs of industry. However, very limited investigation has been carried out on the effect of heat treatment on the Ni–P–GO composite coatings.

In this paper, we describe findings in the influences of heat treatment on the properties (e.g. structure, morphology, micro-hardness and wear resistance) of the electroless Ni–P–GO composite coatings with different temperatures.

2. Experimental section

The low carbon steel (8 cm × 2.5 cm × 1 cm) was used as the substrate for preparing Ni–P–GO composite coatings. Prior to the codeposition, the substrate was cleaned in acetone ultrasonically, which was then followed by activated in HCl (10%) solution and rinsed in distilled water. To ensure a good dispersion of GO, surfactant is added to the plating bath with the shocks and ultrasonic treatment. The composition and operating parameters of the electroless Ni–P–GO composite coatings are given in Table 1. Electroless Ni–P–GO plating was carried out with a pH 4.8 (adjusted by NaOH) at 88 ± 2 °C for 60 min.

Table 1 Bath composition and plating parameters

Plating bath composition	Concentration
NiCO3·2Ni(OH)2·4H2O	25 g L^−1
NaH2PO2·H2O	16 g L^−1
Sodium acetate	11 g L^−1
Citric acid	15 g L^−1
Lactic acid	27 g L^−1
GO	40 mg L^−1

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After the electroless plating deposition, the heat treatment of composite coatings was performed at 200 °C, 400 °C and 600 °C for 1 h to examine the change in structure, morphology, hardness and wear resistance upon heating to different heat treatment temperature.

The surface morphologies of electroless Ni–P–GO composite coatings were examined using a scanning electron microscopy (SEM; S-3400N from Hitachi). The structure of GO and composite coatings was characterized by X-ray diffraction (XRD) with a diffractometer D/max-2200 V and Cu Kα radiation. The scanning speed and the step size were 2° min⁻¹ and 0.02°, respectively. The micro-hardness of coatings was evaluated on the as-plated surface using the HXD-1000 micro-hardness tester at an applied load of 100 N with a dwelling time of 15 s. Friction and wear tests were conducted on universal macro-tribometer mod (UNMT-1L0).

3. Results and discussion

3.1 GO sheets characterization

GO was synthesized using graphite powder as starting material based on a modified Hummer's method.¹⁵ The transparent and wrinkled structures of the GO presented in the TEM image (Fig. 1a), and the dark areas indicate that GO composed several layers to form the wrinkled parts, while the bright areas reveal the single layer. It can be inferred that the GO prepared are the mixture of single layer and multiple-layer GO sheets.

Fig. 1 TEM image (a) and XPS (b) of the GO sheets.

To further confirm GO, XPS was performed. Fig. 1b shows the high-resolution C1s spectrum of the GO (black line), which consists of two prominent convoluted components arising from C–C/C=C bonds at ∼284.6 eV and C–O (hydroxyl and epoxy) bonds at ∼286.6 eV, while the other minor components resulting from different oxygenated carbon atoms, such as C=O and O–C=O, which shows that GO containing hydroxyl and carboxyl functional groups.

3.2 Morphology and structure of the coatings

3.2.1 Morphology of coatings. Fig. 2 shows the SEM image of topography on the Ni–P coatings and Ni–P–GO composite coatings before (a and b) and after heat treatments with different temperature (c–e) and obvious differences are observed among them.

Fig. 2 SEM images: (a) as-plated Ni–P coatings; (b) as-plated Ni–P–GO composite coatings; (c) heat treatment of 200 °C; (d) heat treatment of 400 °C; (e) heat treatment of 600 °C.

As is clear on the surface morphology of Ni–P coatings, there are homogeneous fine globular structures with good uniformity and dense coverage but the Ni–P–GO composite coatings exhibit coarse cystiform structure due to the code position of GO. Moreover, heat treatment in vacuum atmosphere (b–e) led to the flattering of globules, so the globule seems to be relatively coarser than Ni–P–GO composite coatings. It was also observed that the heat treated Ni–P–GO composite coatings become more compact and uniform, and the surface convex is reduced. Among them, the coating is the most flat at 400 °C (c), however, it turned out that the surface roughness increases when the heat treatment temperature reaches 600 °C (d).

To observe the surface morphology of the coating more visually and clearly, we use the atomic force microscopy (AFM) to analyze the Ni–P–GO composite coatings after different heat treatment temperature (Fig. 3). When heat treatment temperature up to 200 °C, there was no significant change in the coating, but the change of the surface is obvious once the heat treatment temperature reaches 400 °C. There are many ups and downs on the surface, and the processes are prominent. These projections should be densely covered surface cell of Ni₃P phase, and the phase is not a simple distributed embedded within the cellular particles but precipitation particle surface and a compact surface grafting on particles. And the structure of the projection is very special, although the shape is thick, the fluctuation degree is also very big, but the grain is very close to the grain. The change of morphology and the above structure directly affect the performance of the coatings.

Fig. 3 AFM images: (a) as-plated Ni–P–GO composite coatings; (b) heat treatment of 200 °C; (c) heat treatment of 400 °C; (d) heat treatment of 600 °C.

The structure of the Ni–P–GO composite coatings was characterized using Raman spectroscopy to gain a clear insight of GO in the Ni–P coatings. We evaluated the effect of Ni–P on the Raman signal of the corroded coating by choosing HNO₃ of 30 vol% from a time range of 5–10 min. The curled GO could be observed within the composite coatings typically by SEM after the treatment of corrosion (Fig. 4a). There are two obvious characteristic peaks showed by the Raman spectrum of the corroded coating, they are 1349 cm⁻¹ and 1600 cm⁻¹, which are corresponding to the reduced GO's D band and G band (Fig. 4b). With a combination of results of SEM and Raman measurements, GO was proved co-depositing with Ni–P in the process of formation of the Ni–P–GO composite coatings.

Fig. 4 (a) SEM image, (b) Raman spectrum of the HNO₃ treated composite coatings.

3.2.2 XRD analyses after heat treatment. X-ray diffraction patterns of electroless Ni–P–GO composite coatings before and after different heat treatment are shown in Fig. 5. It can be seen that the XRD pattern of as-plated Ni–P–GO composite coatings consist of a mixture of amorphous and crystalline phases, which is different from the pure Ni–P coatings. In previous study,^2,5,16–18 the pure Ni–P coatings show only a single broad peak of Ni (111), which is an evidence of fully amorphous structure. The phenomena may account for the fact that GO sheets doped promote the nucleation of nickel phase by increasing the number of nucleation sites.

Fig. 5 XRD patterns of Ni–P–GO composite coatings before and after different heat treatment temperature.

There was no obvious change in XRD patterns obtained for the samples treated at 200 °C and the structure is still consist of amorphous and crystalline phases, indicating that no phase transition took place at this heat treatment temperature. However, further comparison of the XRD patterns of sample treated at 200 °C and the as-plated sample reveals that there is an increase in the intensity and decrease in broadening of the Ni (111) reflection for the sample treated at 200 °C, indicating an increase in crystallinity. When the heat treatment temperature was increased to 400 °C, it can be observed that the Ni–P–GO composite coatings clearly generated phase transformation (Ni → Ni₃P) after heat treatment.¹⁹ The nickel reflection became sharper and intensity increased. These results are consistent with the studies of some other reports.^16–18 When the temperature up to 600 °C, leading to further sharpening and intensifying of Ni₃P reflections further sharpened, and Ni₃P phase appears more in the composite coatings with high strength.

3.2.3 Micro-hardness and wear resistance. Fig. 6 shows the evaluation of the micro-hardness of as-plated and heat treated coatings with heat treatment temperature in the range of 200–600 °C. It is seen that the micro-hardness of heat treated Ni–P–GO composite coatings were higher than the electroless Ni–P coatings. The micro-hardness of the coatings increased from 416 HV to maximum 1100 HV at 400 °C for 1 h after the heat treatment. Moreover, the micro-hardness of coatings were found significantly increased after heat treatment, which are due to the formation and fine dispersion of the harden Ni₃P phases.²⁰ In general, the micro-hardness of the electroless Ni–P coatings can be enhanced by appropriate heat treatment, which can be attributed to the fine Ni crystallites and hard inter-metallic Ni₃P phase during crystallization of amorphous phase. The micro-hardness increased slowly after a heat treatment of 200 °C, which is due to coatings structure is still amorphous and crystalline mixed structure. But Ni₃P phase began to appear and gradually increased after a heat treatment of 400 °C. The micro-hardness of Ni₃P phase increased by a precipitation hardening phenomenon. This result must be related to the over aging of coatings which resulted in the grain coarsening of Ni and Ni₃P phase.²¹ The presence of Ni₃P phase improves the micro-hardness of the coatings and it can be compared to that of the Ni–P coatings. Heat treatment temperature is too high upto 600 °C, Ni₃P deposition dispersed grew up together, a small number of small particles gradually, instead of spacing of coarse particles, resulting in decrease in micro-hardness.

Fig. 6 The effects of different heat treatments temperature on hardness of Ni–P–GO composite coatings.

Further, by comparing the surface micro-hardness of Ni–P coatings treated at 400 °C and as-plated Ni–P–GO composite coatings, it can be seen that the surface micro-hardness of as-plated Ni–P–GO coatings is higher than that of Ni–P coatings treated at 400 °C. The result indicates that GO doped play a more important role in surface micro-hardness compared with the heat treatment, which is attributed to the dispersion strengthening from dispersion state of GO sheets in the composite coatings.

Fig. 7 shows the effect of different heat treatment temperature on the weight loss. It can be seen that the heat treatment has significant influence to the amount of wear. Moreover, it can be confirmed that there is an optimal heat treatment temperature to maximize the beneficial effects of Ni–P–GO composite coatings over the substrate.

Fig. 7 The relationship between wear mass loss and different heat treatments temperature.

Heat treatment after 200 °C, the wear loss of the heat treated Ni–P–GO composite coatings (0.90 mg) has declined compared with the as-plated Ni–P–GO coatings (0.86 mg). According to the above XRD analysis, the Ni–P and Ni–P–GO composite coatings have just begun the precipitation of Ni and Ni₃P crystal phase, these crystals are conducive to resist outside pressure.¹⁹ In the case of heat treatment after 400 °C, the crystal phase of gathering grows and the ability to resist external pressure gradually increased, thus the abrasion of Ni–P–GO composite coatings is reduced to the minimum (0.48 mg). After heat treatment of 600 °C, the crystal growth is not obvious, and the high heat treatment temperature of the coatings to form certain destruction, thus the wear volume increased compared to the case of 400 °C.

The friction coefficients of Ni–P and Ni–P–GO composite coatings are shown in Fig. 8. It can be seen that the friction coefficient is declining with the heat treatment temperature up to 400 °C. Below 400 °C, the friction coefficient of coatings decreases with increasing temperature, which is beneficial for the wear resistance. However, when the heat treatment temperature is higher than 400 °C, the friction coefficient increased. The change of the friction coefficient is attributed to the presence of GO doped and Ni₃P phase.²² The existence of GO and Ni₃P fills the porosity of the coatings which made the coatings ever dense. So the wear property is enhanced. The lowest friction coefficient accounts to highest micro-hardness attained by heat treated Ni–P–GO composite coatings due to the trapping of GO in the Ni–P coatings and precipitation of Ni₃P phase by heat treatment.²³

Fig. 8 The friction coefficient of Ni–P–GO composite coatings on different heat treatment temperature.

Fig. 9 presents the worn surface of the wear tracks for both Ni–P coatings and Ni–P–GO composite coatings before (a) and after (b–d) heat treatment. In case of Ni–P–GO composite coatings in room temperature (a), the worn surface along the sliding direction is composed of longitudinal grooves and partial irregular pits. The presence of grooves implies the micro-cutting and micro-plowing effect of the counter face, while pits are the sign of ductile fracture. The track is wider than that of heat treated Ni–P–GO composite coatings confirming a lower wear resistance of the former. In the case of heat treatment of 200 °C (b), the electroless Ni–P–GO composite coatings have similar track. But the composite coatings have shallow scratches. For heat treatment of 400 °C (c), scratch the most shallow, no much tearing around. And for the case of 600 °C (d), the worn surface of composite coatings presents slight adhesive wear and the composite coatings spall off resulting from softening of the coating at higher heat treatment temperature.²⁴ It is consistent with the analysis results of micro-hardness.

Fig. 9 The SEM morphology of the worn surfaces: (a) as-plated Ni–P–GO composite coatings; (b) heat treatment of 200 °C; (c) heat treatment of 400 °C; (d) heat treatment of 600 °C.

4. Conclusions

In this study, the structures, tribological property and micro-hardness of the electroless Ni–P–GO composite coatings after heat treatment were studied. Based on the experiment and analysis of results using characterization studies and wear tests, the specific conclusions show that:

(1) SEM and AFM results demonstrate that the codeposition of GO sheets in Ni–P matrix results in more coarse nodular Ni–P–GO gradient coatings compared to the smooth Ni–P coatings. However, the heat treated Ni–P–GO composite coatings become more compact and uniform, and the surface convex is reduced.

(2) As-plated Ni–P–GO composite coatings consist of a mixture of amorphous and crystalline phases, which is different from the pure Ni–P coatings. The new Ni₃P phases are precipitation by heat treatment.

(3) Appropriate heat treatment can also improve the mechanical properties of the Ni–P–GO composite coatings. Micro-hardness improved from 416 HV to 1100 HV (increase in 164.42%) for Ni–P matrix coatings after heat treatment and better wear resistance was observed. The specific the wear volume is decrease by 12.73% and 56.36% for Ni–P–GO composite coatings, in as-coated and heat-treated conditions.

Acknowledgements

This paper is financial supported by the Scientific Research Foundation of SEC (ZZyy15092), SIT (YJ2014-32) and LM (201538).

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