Effect of SiC Contents on Wear Resistance Performance of Electro-Codeposited Ni-SiC Composite Coatings

Abstract

This paper focuses on the wear resistance performance of Ni-SiC composite coatings with various contents of SiC particles. The coatings were characterized via a scanning electron microscope (SEM), X-ray diffractometer (XRD), and transmission electron microscopy (TEM), and the wear behaviors of different coatings were tested. The results show that SiC particle incorporation results in a nanocrystalline metal matrix and nanotwins in nickel nanograins. The microhardness and wear resistance Ni-SiC composite coatings increased with the increasing SiC content. Microhardness was improved due to the grain-refinement strengthening effect and the presence of a nanotwin structure. The dominant wear mechanism was described in two stages: the first stage involves the interaction of SiC particles/the counter ball, and the second stage involves the formation of the oxide film its breaking up into wear debris. A higher SiC content increased the duration of the first stage and slowed down the rate of breaking up into debris, thereby decreasing the wear rate.

1. Introduction

Ni-SiC composites have been investigated to the greatest extent and successfully commercialized for the protection of friction parts, combustion engines, and casting molds in the automotive industry [1,2,3]. The electro-codeposition technique has attracted increasing interests for its ability to produce metal matrix composite coatings, as well as other advantages, such as low cost, high deposition rates, and homogenous particle distribution [4,5,6]. A novel composite plating apparatus, the circulating solution co-deposition (CSD) technique, has been proven to be an effective technology for preparing well-qualified Ni-SiC composite coatings [7,8]. The Ni-SiC composite coating prepared by the new CSD method has uniform microstructure, high microhardness, and excellent corrosion and wear resistance compared to the uncoated steel substrate and pure Ni coating. The magnitude of the benefits from the reinforcement effects of the SiC particles depends on the SiC volume percentage dispersed in the matrix [9,10,11].

In addition, it is reported that numerous growth-type faults have been observed in the Ni matrix of electrodeposited Ni-Al₂O₃ composite coatings, such as dislocations and high-density (111) nanotwins along the [011] zone axis [12]. Such growth faults were also observed in both the nanocrystalline Ni and Ni-Fe alloys produced by eletro-codeposition [13]. This paper presents new results concerning the structure of Ni-SiC composite coatings by describing nanotwins in Ni nanograins during the electrodeposition process.

Based on the presented state-of-the-art, it becomes evident that SiC can notably improve the performance of tribological and wear components. However, little knowledge is available about the tribological potential of Ni-SiC coatings since a systematic study evaluating the effect of the doping influence of SiC particles is still missing.

Herein, Ni-SiC composite coatings with various contents of SiC particles were fabricated in this work, and the friction coefficients and wear rates were tested and studied. The purpose of this study is to investigate the effect of incorporated SiC contents on the wear behaviors of Ni-SiC composite coatings. The wear mechanism has been studied in detail.

2. Materials and Experimental Procedure

2.1. Electrodeposition

Ni-SiC composite coatings were electrodeposited on steel substrates by means of the CSD method from Watt’s bath. The bath compositions and operating parameters for electrodeposition are shown in

Table 1. Overview of the electrodeposition parameters for the preparation of Ni-SiC composite coatings.

Bath Composition	Content (g/L)	Plating Conditions	Parameters
NiSO4·6H2O	400	Current density (A/dm2)	10
NiCl2·6H2O	15	pH	4.0 ± 0.1
H3BO3	35	Temperature (°C)	60
C6H4SO2NNa·6H2O	2.5	Plating time (min)	30
CTAB	0.4	Agitation method	Pump-circulating method and air-stirring method
SiC	20

. Details of the CSD plating system and plating procedures were introduced in an earlier study. Volumetric flow rate was controlled to obtain Ni-SiC composite coatings with different SiC contents. SiC particles with a mean diameter of 5 μm were used in the bath solution. After electrodeposition, the obtained coatings were ultrasonically cleaned for 5 min and then rinsed in distilled water and dried in the air.

2.2. Characterization Techniques

A scanning electron microscope (SEM, Mira3 Tescan field emission SEM, Oxford Instrument, Oxford, UK) was used to investigate the surface and cross-section morphology. Back-scattered electron (BSE) images were more accurate to distinguish SiC and nickel. Image processing (Image-Pro Plus 6.0) was utilized to calculate the SiC volume fraction (vol.%) from the cross-section images. The microhardness was measured by a Vickers hardness indenter (Duramin-A300, Copenhagen, Denmark) with a load of 500 g for 10 s. Each microhardness value included an average of ten measurements. X-ray diffractometry (XRD, XRD-7000, Shimadzu, Kyoto, Japan) with Cu Kα radiation (λ = 1.5418 Å) and with a step size of 0.02°, and the scanning range was 20°–100°. The microstructure of the Ni-SiC composite coating was investigated by a transmission electron microscope (TEM, FEI Tecnai F30, Hillsboro, OR, USA) using conventional bright-field (BF) and dark-field (DF) imaging, high-resolution TEM (HR-TEM) and selected area electron diffraction (SAED).

The wear resistance of the obtained coatings was examined under dry sliding conditions in the air at 25 °C for 30 min with the ball-on-disk method. Wear tests were performed sliding against corundum balls (Ø6 mm; Mohs hardness of 9) under a load of 15 N at a fixed speed of 4.2 m/s. Each test was repeated three times. The weight of each specimen was measured by an electronic balance (BS210S type, 0.1 mg accuracy) before and after each test after carefully cleaning and removing unattached debris. The morphology of worn surfaces of coatings and counter balls were studied by SEM, energy dispersive spectroscopy (EDS), and three-dimensional video microscope.

3. Results and Discussion

3.1. Surface Morphology and Structure of the Ni-SiC Composite Coatings

Figure 1 presents the XRD patterns of Ni-SiC composite coatings with different SiC contents. Two phases of the Ni matrix (JCPDS: 14-0117) and co-deposited SiC particles (JCPDS: 73-1664) were confirmed and no interfacial reaction products were produced. An increase in the SiC content showed no evident effect on the XRD patterns of Ni-SiC composite coatings. It may be associated with the sensitivity of XRD detection. More accurate and precise measurements can be taken to further investigate the variation in the grain sizes of the Ni matrix with increasing SiC contents.

Figure 2 displays the morphology of Ni-SiC composite coatings with a SiC content of 17.8 vol.% and 10.1 vol.%, respectively. The SEM surface images (Figure 2a,b) show that both coatings present uniform and compact structures with no defects, such as voids, cracks, or agglomerates. The surface is rougher for the Ni-SiC composites with a higher SiC content of 17.8 vol.%. From the BSE cross-section images (Figure 2c,d), SiC particles are recognizable in dark contrast and are homogenously dispersed in the Ni matrix throughout the whole film thickness. This is the foundation of high mechanical properties for particle-reinforced metal matrix composite coatings.

The previous research work [14] showed the microstructure of the Ni-19.4 vol.% SiC composite coating, and the SiC particle was closely surrounded by nanocrystalline Ni grains. In this work, a detailed TEM investigation was carried out to reveal the lattice defects within the Ni nanograins in Figure 3. Individual grains possessing even small misorientation angles can be distinguished by their changes in contrast at the grain boundaries, as shown by high magnification bright-field image in Figure 3a. The HR-TEM image (Figure 3b) shows an individual Ni nanograin with a diameter of 15 nm, marked as A. There are nanotwins within nanogrian A. Details of the nanotwin enclosed by a rectangle in Figure 3b are shown in Figure 3c, and the twin boundary is marked by a black arrow. A fast Fourier transform (FFT) pattern (Figure 3d) corresponding to Figure 3c reveals that the nanograin has a [110] zone axis orientation and contains a (111) twinning plane. In addition, the FFT pattern inserted in Figure 3 shows streaks in the [111] direction, i.e., normal to the (111) plane, which is characteristic for the presence of (111) twins. It confirms that the complex “zigzag structure” in Figure 3 is comprised of (111) nanotwins, which are joined together along a [011] zone axis. As no external load acted on the coatings, these nanotwins were considered to be growth faults generated during the coating growth.

The presence of growth twins has been reported in a variety of electrodeposited coatings [15,16]. A probable mechanism for the formation of such growth twins was described by a twinning model based on two-dimensional nucleation on {111} growth planes with a stacking fault. Another interpretation is that growth twins were caused by the far-from-equilibrium nucleation and were additionally promoted by a monolayer of adsorbed hydrogen originating from water impurities during the electrodeposition process. Growth twins are typically formed in face-centered cubic (FCC) metals with low intrinsic stacking fault energy. Several experimental studies and computer modeling results have demonstrated that materials with nanotwin structures exhibit outstanding mechanical properties, such as high microhardness and strengths, along with good deformability, compared with their untwined nanocrystalline counterparts [17,18,19].

3.2. Wear Resistance of the Ni-SiC Composite Coatings

The mechanical properties of particle-reinforced metal matrix composite coatings depend not only on the mechanical properties of the matrix but also depend on the characteristics of reinforcement particles, such as the size and shape, the content and the distribution in the matrix, the bonding strength with the matrix, as well as the mechanical resistance to the applied loads. The variations in microhardness and wear loss of the Ni-SiC composite coatings with the content of SiC particles are shown in Figure 4. As the content of SiC particles increases, the microhardness increases, and the wear loss decreases.

The increase in microhardness of the Ni-SiC composite coatings originates mainly from the grain refinement strengthening effect and the presence of the nanotwin structure. Co-deposition with SiC particles refines Ni grains by inhibiting their growth. The Hall–Patch relation revealed the grain refinement strengthening mechanism. Nanotwins contribute positively to the microhardness of Ni-SiC composite coatings due to the effective blockage of dislocation motions by numerous nanotwin boundaries. A higher SiC content results in finer grain size and higher density of nanotwins and, thereby, the higher microhardness of Ni-SiC composite coatings.

It is noteworthy that a dispersion-strengthened composite is characterized by a dispersion of fine particles with a particle diameter ranging from 0.01 to 1 μm through a dislocation–particle interaction or Orowan-hardening mechanism [12]. Thus, considering the mean size of SiC particles (5 μm) used in this study, the dispersion strengthening effect is unlikely operative.

In general, the incorporation of SiC particles contributes positively to the microhardness and wear behaviors of Ni-SiC composite coatings. SiC particles, as hard dispersoids in the matrix, suppress the propagation of microcracks due to their crack-blunting actions, improve the load-bearing capacity of the composites, and restrain matrix deformation by mechanical constraints. These dispersed particles also weaken the plowing effect and adhesive wear, and they retard the grain growth of the matrix at elevated temperatures to sustain good mechanical properties, which reduces the thermal plastic deformation and scuffing wear at high temperatures caused by frictional heating and sliding. Therefore, the microhardness and wear resistance of the Ni-SiC composite coating increase with the increasing SiC content. The differences in wear behaviors of Ni-SiC composite coatings with different SiC contents can be further verified by the worn morphology, as shown in Figure 4.

Figure 5a shows variations in the coefficient of friction (CoF) with the sliding time for Ni-SiC composite coatings with different SiC contents. CoFs evolve in similar procedures: they dramatically increase at the initial stage, then gradually decrease, and finally reach steady states until the end of the tribo tests. The initial rise in CoFs may be attributed to surface irregularities, such as asperities. However, after a running-in process, the surfaces smoothen due to the elastic and plastic effects of the frictional forces act on the coatings; subsequently, CoFs decrease and reach steady-friction states. The average values of CoFs in steady-friction states are summarized in Figure 5b with respect to the SiC contents. The minimum CoF reaches an intermediate SiC content of 13.9 vol.%. CoF decreases as hard SiC particles act as protrusions on the coating surface, and protect the matrix from severe contacts with counter surface. However, with the increasing SiC content, more particle failures happen, such as being pulled out or fractured, which makes the sliding surface coarse, and the CoF increases as a result.

The width and depth of the wear track can be used to qualitatively compare the wear resistance. Figure 6 presents 3D microtopography images of the wear tracks of Ni-SiC composite coatings with different SiC contents. In comparison, the Ni-SiC composite coating with the highest SiC content of 17.8 vol.% exhibits the smallest amount of material removal with a shallow and narrow dent (Figure 6e), demonstrating the best wear resistance. This is in agreement with the results from a measure of the mass loss in Figure 5.

The shapes and areas of the worn scars of corundum balls vary with the SiC contents in composite coatings, as shown in Figure 7. Sliding against the Ni-SiC composite coatings with the increasing content of SiC particles, the worn scar of the counter ball changes gradually from an elliptical shape approaching a round shape, and the worn area decreases. The worn scar at high magnification of the counter ball (Figure 7f) presents continuous and deep-ploughed grooves, inferring that the counter ball is severely abraded by the counterpart coating during friction. This, in turn, reflects the fact that the studied Ni-SiC composite coatings are relatively excellent and wear-resistant.

The mechanism of the changes in worn shapes of the counter ball is shown with the help of the schematic illustration in Figure 8. The formations of the different worn shapes (elliptical or round) are associated with the relative microhardness of corundum counter balls with respect to the Ni-SiC composite coatings. When the corundum ball is harder than the Ni-SiC composite coating, the hard ball causes a large wear loss for the coating, resulting in a recessed worn track on the coating surface. For mating, the counter ball protrudes into this recessed worn track on the coating surface, leading to the formation of an elliptical worn shape of the counter ball (Figure 8a). Conversely, when the counter ball is less hard than the Ni-SiC composite coating, the counter ball is severely abraded by the hard coating, giving rise to the round worn shape on the counter ball (Figure 8b). It may be interesting to know that when the matching materials are selected properly with the relative microhardness of the tribocouple within proper ranges, the tribocouple will exhibit excellent wear resistance and have long service life for tribological applications.

According to the formation mechanism of the worn scars on counter balls in Figure 8, it is evident that the worn scars on counter balls can give information on the wear resistance of Ni-SiC composite coatings. When the worn scar on the counter ball is elliptical (Figure 7a), the corresponding counterpart is a Ni-10.1 vol.% SiC composite coating with low wear resistance, while when the worn scar on the counter ball (Figure 7e) has an approximately round shape and small worn area, the corresponding counterpart is a Ni-17.8 vol.% SiC composite coating with high wear resistance from which the mass loss is low (Figure 4) and the worn track is shallow and narrow (Figure 6e and Figure 9b).

3.3. Wear Mechanism of Ni-SiC Composite Coatings

To further elucidate the role of SiC particles and the wear mechanism, the morphology of worn tracks for Ni-SiC composite coatings are shown in Figure 9. The worn track for the Ni-10.2 vol.% SiC composite coating is deeper and twice wider than that for the Ni-17.8 vol.% SiC composite coating. It is evidence that SiC content appears to be an important parameter for the wear resistance of Ni-SiC composite coatings. The wear morphology (Figure 9c,d) is characterized by smooth polished surfaces with patches of oxide and small and brittle wear debris, which are typical features of oxidation wear. The presence of the oxide film has been confirmed by EDS analysis, and the corresponding spectra are shown in Figure 9e. The process of oxidation wear is about the formation of oxide film and the breakaway of oxide fragments as wear debris. The oxidation of virgin surfaces restarts, and the whole process is repeated; thereby, oxidation wear advances. The initiation and propagation of microcracks on the oxide film are due to the cyclic nature of the load and the shear stress transmitted to the sub-surface layer. The wear morphology in Figure 9 appears to be the outcome of a homogeneous material system. The good bonding of the densely distributed particles with the matrix contributes to the improved mechanical properties and consequently enhances the wear resistance uniformly.

The oxide traces are visible in the penetrated microgrooves, as indicated by the arrows in Figure 10a, and numerous oxidized grooves join together to form a protective oxide film. The oxidation is stimulated due to the high flash temperature in the plastic zones during friction, and the thickness of an oxide film is mainly proportional to the depth of plastic zones. After numerous passes of the load, microcracks initiate from SiC sites embedded in the oxide film (as marked by the blue arrows in Figure 10b) and then propagate to form wear debris. Such wear characteristics are in agreement with the oxidation wear theory developed by Quinn, who stated that oxidation occurred at the instant the virgin metal encountered against the counterpart; further contacts had little effect on the growth of the oxide but caused the oxide film to crack up and spall off to form wear debris.

The oxidation wear mechanism is illustrated in Figure 11. SiC particles protrude above the intact surface of Ni-SiC composite coatings (Figure 11a). The counter ball will slide on these particles, and a stage of the counter ball/SiC particles interaction takes place, which is called the first stage. During this stage, the most important parameter is the mechanical resistance of SiC particles against the loading imposed by the counter ball and the bonding strength between the matrix and particles. If the particles resist and remain embedded, the duration of the first stage is remarkable; thereby, the matrix will be protected for a long time, accompanied by the high abrasion wear of the counter ball (Figure 11(bI)). If the particles are fractured (Figure 11(bII)) or detached (Figure 11(bIII)) or pressed in (Figure 11(bIV)), the particles will be less efficient at protecting the matrix. The duration before failures of particles determines the protective capability of particles for the matrix. After the first stage, a large amount of the Ni matrix is exposed on the sliding polished surfaces (Figure 11c). A stage in the formation of oxide film and its breaking up into wear debris is called the second stage (Figure 11d,e). The particles entirely included in the oxide layer can be removed together with the wear debris, while the large particles remain in places and protrude above the sliding surface, which is a similar condition to that in Figure 11a. A new cycle of the wear process starts and is repeated.

For the Ni-SiC composite coating with a higher SiC content, the duration in the first stage lasts longer. In addition, SiC particles within the matrix facilitate the interruption of microcrack propagation at the sub-surface region. A high SiC content slows down the breaking-up of the oxide film in the second stage, which accordingly decreases the wear loss. Therefore, an increase in SiC content is beneficial to the improvement of the wear resistance of the Ni-SiC composite coatings.

Several efforts were carried out to create wear-mechanism maps, such as for steel Al-SiC composites [20,21] and Al-Si-SiC composites [22,23,24], to address the difficulties associated with the limited understanding of the wear processes. Much information can be extracted from these wear-mechanism maps about the oxidation wear. The flash temperature at the asperity contacts depends strongly on speed but hardly on load. At a speed of 1m/s, the contact flash temperature (near 700 °C) is sufficient for the oxidation of steels. As for metallic matrix composite groups, when the sliding speed exceeds to about 3 m/s, thermal effects play an increasingly important role in the wear behavior of Al-SiC composites. Therefore, it is acceptable that, at a sliding speed of 4.6m/s, the oxidation wear is the dominant wear mechanism for Ni-SiC composite coatings. In addition, the addition of SiC particles can improve the wear resistance of Ni-SiC composite coatings by expanding the mild wear regime to higher speeds and loads, thereby inhibiting severe wear. SiC particles assist with the retention of an oxide layer on sliding surfaces, which prevents direct contact between Ni-SiC composite coatings and counterparts and keeps wear behavior within the mild wear regime accordingly to improve the wear resistance of Ni-SiC composite coatings.

4. Conclusions

(1)

Micron SiC particles incorporation results in a nanocrystalline metal matrix and nanotwins in nickel nanograins have been characterized by transmission electron microscopy (TEM).

(2)

Microhardness and wear resistance Ni-SiC composite coatings increase with the increasing SiC content. Microhardness is improved due to the grain-refinement strengthening effect and the presence of a nanotwin structure. The superior wear resistance of Ni-SiC composite coatings is attributed to its fine microstructure, high microhardness, and the strong bonding of SiC particles with the matrix.

(3)

The worn scars on the counter balls give information on the wear resistance of Ni-SiC composite coatings. The elliptical worn scar reflects the low-wear resistance of the Ni-10.1 vol.% SiC composite coating. The round and small worn scar on the counter ball corresponds to the shallow and narrow worn track of the Ni-17.8 vol.% SiC composite coating, revealing a low wear rate and excellent wear resistance.

(4)

The dominant wear mechanism is oxidation. Oxidation wear has been described in two stages. A higher SiC content increases the duration of the first stage and slows down the rate of breaking up into debris, thereby decreasing the wear rate.