Review of Nickel-Based Electrodeposited Tribo-Coatings

Transcription

1 Trans Indian Inst Met (2018) 71(2): REVIEW PAPER Review of Nickel-Based Electrodeposited Tribo-Coatings Zeynab Mahidashti 1 Mahmood Aliofkhazraei 1 Naser Lotfi 1 Received: 6 January 2017 / Accepted: 4 July 2017 / Published online: 26 July 2017 Ó The Indian Institute of Metals - IIM 2017 Abstract Among various methods used for protecting the industrial components from wear/abrasion failures, electrodeposition has attracted considerable attention in recent years because of its advantages such as being efficient, accurate, affordable, and easy to perform. In this regard, electrodeposition of nickel-based composite and alloy coatings is an inexpensive method compared with other coating methods such as chemical vapor deposition and physical vapor deposition. Furthermore, nickel-based composite electrodeposition is an eco-friendly substitute for conventional toxic coatings such as hard chrome. Embedding hard particles within the metallic matrix improves the wear resistance by increasing the ductility of the matrix in the contact area, changing the preferred grain growth direction to close-packed directions, and boosting dispersion and grain-refinement strength. In addition, lubricant particles provide superior anti-abrasive behavior because of their non-sticky nature. Several factors affect the incorporation of the particles into the electrodeposited coating and therefore the wear behavior of these coatings is related to different parameters such as current density, bath composition, ph, amount and size of the embedded particles. This review paper provides an overview of the wear behavior of nickel-based electrodeposited coatings including their composites and alloys with the focus on the parameters affecting wear rate, coefficient of friction, hardness, and roughness. & Mahmood Aliofkhazraei maliofkh@gmail.com; khazraei@modares.ac.ir 1 Department of Materials Engineering, Tarbiat Modares University, P.O. Box: , Tehran, Iran Keywords Coating Electrodeposition Microstructure Nickel Tribology Wear 1 Introduction Two surfaces sliding against each other, may cause failure by wear [1]. Continuous removal of surface s material reduces the working life of industrial devices and leads to many economic and energy losses [2]. Engineering applications of metals are limited to surface defects such as wear, corrosion, and fatigue. Surface coatings are usually used as protective films to reduce wear and rupture as a result of which service life and reliability of the device increases [3]. Various surface coating techniques such as plasma spray [4], sol gel [5], high-velocity oxygen fuel [6], CVD [7], PVD [8], and electrodeposition [9] have been utilized to improve surface properties. Among these methods, electrodeposition is an easy and affordable method for precise coating of the substrates of different shapes with composite, alloy, or pure metal films [2, 10, 11]. Accordingly, traditional chrome coatings electrodeposited by Cr(IV) electrolytes are widely applied in car engines, hardwares, and cutting tools considering their high hardness and excellent anti-wear performance [12 16]. However, nowadays some of the environmental protection legislations prevent the use of hexavalent chromium due to Cr(VI) high toxicity and carcinogenicity [17 19]. In order to find a suitable replacement for toxic hexavalent chrome electrodeposition, some studies are conducted on electrodeposition of other metallic tribocoatings. For example, tribological behavior of Co Cr alloys electrodeposited from Cr(III) bath is investigated [20, 21]. Besides, most previous studies focused on the

2 258 Trans Indian Inst Met (2018) 71(2): production of unreinforced Co Cr alloys while codeposition of fine particles inside the electrodeposited metal, as a second phase, leads to hardness, wear, and corrosion resistance of the coatings [20]. Weston et al. [14] examined the wear behavior of Co and Co W coating through wear test against hardened steel and compared the behavior of these two coatings with the sliding behavior of chrome electrodeposited on hardened steel. They confirmed that in the presence of loading, the wear rate of Co W electrodeposition alloy is at least one order lower than non-alloyed coelectrodeposites. In this regard, the Co W wear rate is less than the wear rate of electrodeposits. Conditions and the composition of electrodeposition bath play major roles in determining Co structure and material properties and its coatings. In electrodeposition industry, eco-friendly complexing agents such as citrate, acetate, and organic acid salts are widely used in nickel, copper, and cobalt electrodeposition, and their alloys instead of cyanide pollutant baths [22]. Properties of Metal matrix composites (MMC) mainly depend on their composition and structure [23]. In this regard, the uniform distribution and high amounts of particles participating in metal matrix are essential to improve coating properties [24, 25]. In many cases, the improved performance of coating is mainly because of the changes in growth mode or grain size of the metal matrix [25, 26] rather than the presence of the particles. Particle incorporationisaffectedbyavarietyofdependentvariablessuchascurrentdensity, ph, agitation of electrolyte and composition of the bath, and also size, type, and shape of the embedded particles [27]. The embedded particles during MMC electrodeposition are also affected by pulse plating (PP) [28, 29]. It is observed that PP and pulsereverse plating (PRP) [30, 31] can be applied to increase particles incorporation. In the case of Cu Al 2 O 3 composites, for example, the maximum particle incorporation is obtained when the deposition thickness per cycle approaches the particle diameter. Moreover, it is shown that PRP can be used for the selected incorporation in terms of the size of the alumina nanoparticles inside the copper matrix [32]. In the case of Ni Al 2 O 3 composites, PP results in a more uniform and less agglomerated embedded nanoparticles inside the metallic coating [5]. Some studies have reported the effects of PP and PRP on alumina codeposition with nickel and copper [32 35]. Promising improvements are reported in terms of the amount and distribution of incorporated alumina particles with a pulse time of the order of milliseconds. However, no extensive research is found in the literature on the effect of dependent parameters of PP and PRP such as pulse on-time (t on ), pulse off-time (t off ), peak of cathodic current density (i p ), anodic pulse length (t rev ), and (i an ) peak of anodic current density [27]. Furthermore, the deposition current has a significant effect on the codeposition of metal-ceramic composite coatings. In addition to commonly used direct current methods, pulse current can also be used to prepare a more uniform composition and increase ceramic particle deposition efficiency during codeposition of the metal-ceramic composite coating [36]. In this regard, one of the wear-resistant coatings is nickel based alloy and composite coating. This kind of coating has a wide range of application in different industries such as military, aviation, automotive, marine, instrumentation, medical industries, etc. Fig. 1 shows different examples of the coated components which need a non-expensive wear-resistant coating and can be coated with nickel electrodeposition. The relevant parts, types of the coatings, and the range of thicknesses of the coatings is also mentioned in this figure. According to the literature, different alloying elements and hard particles incorporated in the nickel matrix have various effects on the tribological properties such as hardness, roughness, wear rate and coefficient of friction. These elements can lead to an increase or decrease of the aforementioned properties. A brief summery and usual behavior of electrodeposited coatings is given in Table 1. The incorporation of neutral particles during electrochemical deposition of metal enhances technological and practical concerns. Electrodeposition is applied to produce corrosion and wear resistant coatings for example for automotive applications [37]. The effect of working conditions affecting the quantity of incorporated particles is studied for a wide variety of metal-particle compounds [38]. In this regard, several theoretical models to describe coelectrodeposition process have been suggested. However, up to this point, the effects of the incorporated particles type has not been completely understood. This lack of basic understanding of coelectrodeposition process has made the trial and error procedure as the only way to develop parameters for the deposition of composite films that are industrially appropriate and have a relatively reproducible amount of particle [27]. This paper will therefore critically review electrodeposition of various nickel and Ni-based alloy and composite tribo-coatings and their tribological behavior to highlight emerging trends in this method and highlight future directions for the application of these coatings. 2 Pure Nickel (PN) Based Coatings Alloy or metal coating is among the most convenient approaches to obtain wear resistance [39]. A variety of different coating methods has been proposed for this purpose including PVD [40] or various plasma methods [41]. In this regard, one of the efficient methods is

3 Trans Indian Inst Met (2018) 71(2): Fig. 1 Application of nickel-based wear-resistant coatings and their relevant parts, types of the coatings, and the range of thicknesses of the coatings electrodeposition of nickel and alloys such as nickel phosphorous, nickel tungsten, nickel cobalt, etc. each of them is applied in various chemical, mechanical and electronics industries. Because of the growing need to the service life of industrial components, composite coatings with high wear resistance are of great importance [42]. The electrodeposition of composites is a new, valuable surface technology to achieve metal matrix composites with reinforced particles such as organic and inorganic solid particles within the metal matrix [11, 43 45]. Moreover, the results show that the incorporation of other particles in nickel matrix and nickel alloys depending on the nature of the particles improves surface properties and wear resistance of the coating. In terms of wear, depending on the type of particles used, the resulting composite coatings can be used to achieve a high wear resistant or lubricant coating. Composite coatings with a high wear resistance are used with hard ceramic particles such as WC [46, 47], SiC [48, 49], Al 2 O 3 [50, 51], AlN [52], CNT [53], and diamond [54, 55]. SiC particles are a secondary phase in composite coatings while nickel and nickel-phosphorus are usually used as the matrix for SiC particles. These particles improve the hardness and wear resistance that ultimately reduces the coefficient of friction (COF) of the coating [55].

4 260 Trans Indian Inst Met (2018) 71(2): Table 1 Effect of addition of different particles and alloying elements to nickel-based electrodeposited coating (comparison with pure nickel electrodeposited coating) on their roughness, hardness, coefficient of friction and wear rate (rough estimation) Composite coatings Lubricant particles (graphite, MoS 2, PTFE ) Hardening particles (SiC, Al 2 O 3,..) Alloy coatings Co Mo Mn Fe Cr B W P Cu: Roughness ; : : : : ; ; Hardness : : : : ; : : : ; Coefficient of ; : ; ; : ; : ; ; : friction Wear rate ; ; ; ; ; ; ; ; ; ; :: Increase, ; decrease Figure 2 presents the wear mechanism of nickel-alumina/graphite composite coating. As Fig. 2 indicates, the presence of alumina particles in this coating increases the wear resistance of these coatings [56]. In the presence of graphite particles in the coating, a transfer film is fabricated on the surface of the coating that produces the lubricating surface and along with alumina particles reduces the wear resistance of the coatings. The placement of the particles in the coating is schematically presented in Fig. 3. The mechanism of improving wear properties of nickelbased alloy and the multi-walled carbon nanotube is reported in the literature [57]. Figure 4 presents the placement of multi-walled nanotubes in nickel cobalt alloy coating [57]. Co-deposition of multi-walled nanotubes improves the surface morphology of nickel cobalt alloy. In nickel cobalt alloy coating, the dominant wear mechanism is delamination caused by the micro-crack fatigue while the deposited composite of multi-walled nanotubes present the abrasive groove wearing mechanism and plastic deformation due to the reduced actual contact area. Nickel-based alloys containing refractory metals (molybdenum or tungsten), which are characterized by their high hardness, wear, and corrosion resistance and high temperature, are widely used in industrial applications as they are considered as an important alternative for hard and toxic chromium coatings [58]. The factors influencing the wear properties of composite coatings and pure nickel-based coatings discussed in various articles are addressed below. 2.1 PN-Based Composite Coatings Reinforced with SiC Particles The results of studies on the effect of SiC particle size (micrometer and nanometer) on the wear behavior of Ni SiC composites show that the abrasive wear composite increases with increasing the particle size. It has been confirmed that for a given volume fraction of precipitated particles, Ni SiC nanocomposites have much lower wear drop and higher scrape resistance compared with Ni SiC nanocomposites [59]. The presence of SiC particles in composite coatings increases wear resistance of metal and alloy coatings and nickel specifically [60]. The effect of codeposition of particles on the wear resistance of composite coatings is depicted in Fig. 5 [61]. The area associated with the highest hardness increases the minimal wear drop [61]. In sliding wear tests [62], the volumetric wear loss of nickel-micro-sic composite is three times greater than the volumetric wear loss of nanocomposites with the same volume percentage; namely 24%. Moreover, in order to achieve a 2.2 mg wear loss, only 15 vol% of nano-sic is required while this value is 31% for the microcomposite [61]. Therefore, in composite electrodeposition, the reduced size of particles provides a higher hardness, wear resistance, adhesion as well as a lower roughness. The wear tracks on Ni SiC composite coatings after sliding wear tests (Fig. 6) indicate a black appearance and scratches along the direction of sliding [61]. Fig. 2 Schematic figure of wear test of Ni alumina/graphite composite coating

5 Trans Indian Inst Met (2018) 71(2): Fig. 3 A schematic view of three-stage composite coating: a nickel pre-electrodeposition and alumina and graphite particles electrophoretic deposition, b composite after the deposition of nickel in three phases and c composite after removal of additional particles absorbed on the surface Fig. 4 Ni Co MWCNT composite deposition process model [57] It is clear from Fig. 7 that in a fixed vertical sliding loading, wear coefficient for Ni? SiC nanocomposite coating is dependent on the speed of rotation of the cylinder slightly [63]. This coefficient is excessively high for pure nickel coating especially at the rotation speeds lower than 50 rpm. Part A in Fig. 7 indicates the difference in the COF values for these two types of coatings at low rotation speeds. This can be explained by the surface morphologies of two coatings and suggest that in the pure nickel coating, adhesive wear plays a key role in the complex mechanism of corrosion wear. Part B of Fig. 7 is the magnification of the ending part of the part A while part C presents the rotation speed corresponding to wear coefficient of the specimens in terms of time. When an abrasive material is made of a ductile metal such as Al, Cu, Ni, Fe or any alloy of these elements, the material has a plastic deformation in the contact area under combined compressive and shear stresses. A larger plastic deformation causes a larger wear rate since the wear surface tends to roughen and the protective surface layers are easily destroyed. Scrape surface structures of pure nickel coating present higher roughness parameters. Surface roughness parameters at the center of the scrape (mean value) are shown in Fig. 7 part B (R a = 8.19 lm; R q = 9.67 lm; R p = lm; and R v = lm). The Fig. 5 Effect of codeposition of particles on the hardness and wear resistance of Ni SiC composite coatings [61]

6 262 Trans Indian Inst Met (2018) 71(2): Fig. 6 Wear tracks on Ni SiC nanocomposite coatings after sliding wear tests a photograph showing the wear track on the Ni SiC nanocomposite; b SEM of the wear track of Ni SiC nanocomposite; and c SEM of the wear track of Ni SiC nanocomposite [61] entrance of a harder reinforcement phase to a ductile matrix with a specific volume fraction reduces the ductility of the matrix metal in the contact area and thus the matrix wear can be reduced. Roughness parameters of the scraped surface are three times smaller for the nanocomposite coatings compared to pure nickel coating. The average scrape profile of Ni? SiC coating is shown in Fig. 7 part C (R a = 3.09 lm; R q = 3.07 lm; R p = lm andr v = lm) [63]. Also, the wear behavior has been attributed to the orientation of the crystal. Studies show that Ni SiC micron size composites compared with SiC nanoparticles present a better wear behavior under the DC and pulsed current. Embedding the SiC particles at micron size leads to crystal orientation [211] while SiC nanosize codeposition generates a matrix with full orientation [100] [64]. In addition, the rough connection areas prone to plastic deformation leads to higher and more unstable wear coefficients of pure nickel [65]. Meanwhile, the results of wear tests for composite specimens, including micron and nano-silicon carbide performed at 300 C have shown that Ni SiC deposits have superior wear resistance, considering the compact microstructure and the change of preferred orientation. Pure nickel deposits have a cylindrical structure and a preferred [100] orientation. Codeposition of SiC particles changes the size and orientation of the cylinders and leads to a preferred orientation [110] of nickel grains. Microstructural changes and the presence of SiC microparticles in metal matrix leads to a 51% increase in Fig. 7 A Coefficient of friction of nanocomposite coating (a) and c pure nickel (b) in 0.5M Na 2 SO 4 solution at different speed of rotation and constant vertical sliding load (20N) in the layer B which is the magnification of the end of layer A. Layer C: the speed of rotation of specimen related to the rate of wear coefficient over time. B Average surface roughness profile in the middle of the worn surface and roughness parameters related to pure nickel coating after constant vertical sliding load (20N) at different rotation speeds. C The graph of average surface roughness profile in the middle of the worn surface and roughness parameters of Ni SiC nanocomposite coating after constant vertical sliding load (20N) at different rotation speeds (experimental) [63] microhardness and 63% reduction in COF at 300 C while no improvement occurs in wear resistance at room temperature. Codeposition of nanoparticles make the particles fine grained and results in a significant loss of preferred orientation. Microstructural changes in this case lead to a 67% increase in microhardness and 70% reduction in COF at room temperature and also 88% reduction in COF at 300 C compared to pure nickel deposits [66]. Embedding the hard SiC particles into the ductile nickel matrix reduces the ductility of the materials in the contact area, leading to the reduced matrix wear as well [67]. For example, volume loss caused by wear of magnesium specimen coated with Ni SiC is 8 times lower than that of alloys without coating [68]. Also, other results show that by increasing SiC in the electrodeposition bath, wear and corrosion resistance are increased [69]. Although there is no change in mass conversion observed in the pin in the case of pure nickel, small amounts of the coated material

7 Trans Indian Inst Met (2018) 71(2):

8 264 Trans Indian Inst Met (2018) 71(2): are detected on the friction surface of the pin in the case of Ni/SiC coatings. In this regard, the pull out of embedded SiC particles in the coating can have a reverse abrasive effect [67]. Studies conducted on the Ni SiC h/bn composite with varying amounts of sodium dodecyl sulfate (SDS) and Cetyl trimethylammonium bromide (CTAB) surfactants show that wear rate is decreased by increasing the CTAB amount and reducing SDS inside the electrolyte [70]. This significant reduction in wear rate is related to the synergistic effect of embedding more h/bn particles (solid lubricant) and subsequent reduction of the matrix grain size under appropriate amount of surfactant. Combining different amounts of hard reinforcements and lubricants (SiC and h/bn) in the composite coating reduces the direct contact area between the metal matrix and the counter ball surface, resulting in the improved wear coefficient [70]. The curve of thickness loss of pure nickel and Ni SiC composite specimens during wear can be divided into three parts (Fig. 8) [71]. In part I ( cycles), the behavior of the curves in both specimens is similar to each other. At the end of 1000 cycles, pure nickel coating loss is 1.23 lm while the drop in the thickness of the composite coating is 1 lm. In part II ( ,000 cycles), a serious difference is generated between the two curves. After 10,000 cycles, nickel coating loss is 4.3 lm while the composite coating drop is 2.6 lm. Finally, in part III (10,000 30,000 cycles), the difference between two curves increases and after 21,000 cycles, the difference remains constant while the pure nickel still has a higher value. After 30,000 cycles, the drop in the thickness of pure nickel is 12 lm whereas the drop for composite coating is 9 lm. These results suggest that the wear resistance of the composite coating is more than pure nickel [71]. Figure 9 shows the changes in mass loss versus time for the specimens for which influence of sodium hexametaphosphate (SHMP) and probe sonication (PS) have been investigated [72]. As shown in Fig. 9, Ni SiC PS SHMP coating has the lowest mass loss compared to other Fig. 8 Thickness loss in terms of the number of cycles on pure nickel and Ni SiC composite specimens during wear test [71] Fig. 9 Mass loss changes versus time of sliding for Ni coatings embedded with SiC particles and different additives [72] coatings. This behavior is due to greater incorporation of non-agglomerated SiC particles with smaller size. This coating also has the lowest wear rate that is associated with its less surface roughness [72]. Electrolytic bath sonication is another factor that affects the mechanical and wear properties of Ni SiC composites. Ultrasonic frequency adjustment allows achieving a coating with a higher wear resistance than pure nickel, due to the homogeneous distribution of embedded nanoparticles and ultrafine nickel grains [73]. Wear drop of Ni SiC composite coatings with various dimensions, after sliding against corundum ball in single and dual-axis wear tests are shown in Figs. 10 and 11 [74]. Volumetric wear drop of pure nickel and Ni SiC composite coatings in uniaxial sliding wear tests are almost two times less than that of the two-directional sliding wear tests. This behavior is consistent with the wear data obtained by hard ceramic coatings such as TiN that have much lower wear rate under conditions of a uniaxial sliding test than the dual-axis ones. However, under uniaxial sliding, by increasing the amount of SiC content, nickel composite coatings containing 5 lm SiC particles show a lower wear resistance compared to nickel coating (shown in Fig. 10 with ED). On the other hand, nickel composite coatings containing 0.3 or 0.7 micron SiC particles under uniaxial sliding, wear less than pure nickel. Under dualaxis sliding, the volumetric wear drop in all composite coatings is less than the wear measured for pure nickel coatings under the same conditions (Fig. 11) [74]. The results of analyzing tribological properties of Ni SiC, Ni SiC MoS 2 and Ni SiC graphite coatings at temperatures varying among 25 and 300 C have shown that the COF of Ni SiC coating at room temperature during severe wear is stable, but this stability is reduced by increasing the test temperature [75]. Graphite lubricant

9 Trans Indian Inst Met (2018) 71(2): Fig. 10 Volumetric wear factor under uniaxial sliding on Ni SiC composite coating containing various volume percentages of SiC particles with three different sizes. Pure nickel data are also provided [74] particles and MoS 2 in the coating reduces strong adhesive wear and COF instability at high temperature. Also, the COF at high temperatures decreases in the coatings containing graphite and MoS 2. However, the Ni SiC-graphite composite coating has the best wear behavior at all temperatures [75]. The results of in situ analysis for the combined corrosion-wear degradation of nanostructured Ni SiC coating show that in the absence of friction, the released potential reaches a passive amount after the immersion of the samples in an electrically conductive test solution. When applying the frictional force, the free released potential reaches an active value. Under friction conditions, the measured current can be considered as two parts: A current caused by the wear track area where the passive film is destroyed and the surface is activated, and the other that is related to the surface not subjected to friction which remains in passive mode [76]. Table 2 is a summary of the researches performed on nickel SiC composite coatings. 2.2 PN-Based Composite Coatings Reinforced with Al 2 O 3 Particles Results show that wear properties of Ni Al 2 O 3 composite coatings prepared by the two-stage method are more Fig. 11 Volumetric wear factor under dual-axis sliding on Ni SiC composite coating containing various volume percentages of SiC particles with three different sizes. Pure nickel data are also provided [74] desired than the wear properties of these coatings obtained by the single-stage method. In this regard, the higher resistance of two-stage process is attributed to the higher volume fraction of ceramic particles and also the method by which the particles are embedded in the matrix (Fig. 12) [77]. The two-stage coating leads to the preferential deposition of nickel in the film cavities formed in the first stage. This nickel grows inside and around the grains and therefore the space between ceramic particles of the coating prepared in the first stage fully fills with the nickel deposited in the second stage. Thus, these particles are strongly fixed in and linked with the matrix. Furthermore, by controlling the electric current used in the second stage, the burial of the particles in the deposited nickel layer can be prevented. Since the particles that play as load bearing elements are placed on the surface of the coating prepared by two stage method, these particles react with the abrasive object under the wear condition. In contrast, the particles that are codeposited in the single-stage process are buried in the growing layer. Although some of these particles are placed on the surface, they have loose bonds with the matrix and are removed in the initial runs of the wear test. Therefore, in such coatings, the nickel layer initially interacts with the abrasive object rather than particles. As a result, the abrasive object penetrates deeply into the

10 266 Trans Indian Inst Met (2018) 71(2): Table 2 Summary of the research results performed on PN-based coatings with SiC particle Coating composition Particles percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni SiC 4 10 vol% Reciprocating mm 3 /Nm Ni SiC 25 vol% Pin on cylinder (wear corrosion) mg/ Nm Ni SiC N.R ± 10 Pin on cylinder (wear corrosion) 1 8 lm** Ni SiC Micrometer: Nanometer: vol% Ni SiC Micometer: 2 6% Nanometer: 1 2% (w/ w)% 50 Micrometer: 1000 Nanometer: 50 Micrometer: 348 Nanometer: 385 Reciprocating Micrometer: Nanometer: 1 2 mg* N.R N.R. Pin on disk Volumetric wear factor Micrometer: Nanometer: mm 3 / Nm Ni SiC 6 wt% Ball on disc mg/ Nm Ni SiC 1 4 wt% ± 39 Pin on disk mg/nm Ni SiC h/ BN Ni SiC PS SHMP SiC: hbn: wt% 80 N.R Ball on disc mm 3 /Nm 6.37 wt% Pin on disk mm 3 /Nm M50 steel ball N.R. [194] Alumina [63] Alumina cylinder N.R. [60] N.R. N.R. Micrometer: Nanometer: Corundum ball Micrometer: Nanometer: [61] N.R. [64] Si 3 N 4 ceramic balls N.R. [65] Stainless steel (Grade 100, 60 Rc) [67] Steel ball (SAE52100) [70] Brass pin [72] N.R. Not reported * Wear sliding distance and load counter were not reported ** Wear volume was not reported

11 Trans Indian Inst Met (2018) 71(2): Fig. 12 The schematic of the two stage method composite coating. a Electrophoretic deposits in the first stage; b composite of nickel deposits in the second stage c composite after removal of additional particles absorbed by the surface [77] Fig. 13 Wear kinetics for HIP and Ni Al 2 O 3 composites a HIP composites and b electrodeposited coatings. Wear conditions: 40 m/s velocity; the impact angle 90 ; Temperature: 20 C, with alumina abrasive particles [78] coating that reaches the surface of the codeposited particles. Since these particles are not distributed uniformly and are not kept in the matrix as in the two stage process, they pour out after the removal of the nickel layer on them and act as the new abrasive object [77]. In energy production applications where wear occurs in vertical directions due to the existence of hard particles (such as dust), the following curves (Fig. 13) are obtained for Ni Al 2 O 3 composites prepared by powder and electrodeposition methods [78]. The slopes of these curves present the wear rate under stable condition. In the case of Ni Al 2 O 3 alloys synthesized by the hot isostatic pressing (HIP) powder method, the composite with the highest volume fraction of Al 2 O 3 (45 vol%) presents the highest wear rate while pure nickel has the lowest wear rate. Similar results are obtained for Ni Al 2 O 3 composites prepared by electrodeposition method where the alloy with the highest amount of Al 2 O 3 (39 vol%) shows the highest and pure nickel shows the lowest wear rate. The effect of the volume fraction of Al 2 O 3 on the wear resistance of Ni Al 2 O 3 composite is shown in Fig. 14 [78]. It can be seen that increasing the amount of Al 2 O 3 increases the erosion rate of the composite. Consequently, increasing the hardness of Ni Al 2 O 3 composites reduces their erosion resistance (Fig. 14) [78]. The results show that increasing hardness of the composites by the addition of the second phase particles does not necessarily lead to the increased erosion resistance. The effect of hardness on erosion behavior should be considered in accordance to the microstructure of the cermet materials. Electrodeposited and powdered nickel show a similar erosion resistance unlike the large differences in their grain size (5 and 50 microns, respectively). However, embedding Al 2 O 3 particles into the matrix leads to a dramatic increase in the erosion rate. Thus, Ni grain size effect on erosion resistance is significantly less than the effect of the volume fraction of Al 2 O 3.

12 268 Trans Indian Inst Met (2018) 71(2): Fig. 14 The effect of hardness on the wear resistance of Ni Al 2 O 3 composites fabricated by a powder method, b electrodeposition [78] Al 2 O 3 nanoparticles can also change the nickel surface morphology and the preferred orientation of the plane from (200) plane to (111). The wear rate of the Ni Al 2 O 3 composite coating prepared by sediment co-deposition (SCD) with 10 g/l Al 2 O 3 nanoparticles in the electrodeposition bath is approximately one order lower than that of pure nickel coatings. Besides, the wear resistance for SDC composite is higher than that of conventional electrodeposition (CEP). For pure nickel and nickel-al 2 O 3 nanocomposites, adhesive and abrasive wear mechanisms is suggested, respectively. The immediate addition of SiC and Al 2 O 3 nanoparticles to nickel electrodeposition bath will result in a considerable increase in wear resistance probably because of the dispersion strengthening and grain modification strengthening, the morphological change of Ni matrix, and the preferential grain growth direction from \100[ direction to compact \111[ direction [79]. Ni Al 2 O 3 composite coatings deposited by pulse current have superior wear properties compared to those deposited by direct current due to the increment of the alumina content and homogeneous particle distribution in the pulse electrodeposition coatings. In addition, the larger grain size of the nickel matrix is considered as another beneficial factor. Increased density of the (002) planes lead to the higher ductility of Nickel-based coatings and a growth in the [100] direction, enhancing the plastic deformation energy absorption capacity which prevents the formation of micro-cracks. The combination of high ductility and greater Al 2 O 3 content leads to the better wear properties. However, by increasing the number of particles with current density, no significant change is observed in the wear rate of the nanocomposites formed by pulsed current. This is attributed to the increment of deposited particles in the coating which results in the reduction of plastic deformation property and consequently a failure due to the formation of micro-crack occurs [80]. As mentioned, the pulse frequency significantly affects the preferred orientation of the coating. The results show that this frequency can modify the orientation from a preferred one of (111) to a random one. In this case, the wear behavior of the coatings will be highly different under dry and lubricated sliding wear conditions. The wear resistance under dry condition is reduced by increasing the number of alumina particles which is mainly due to the nickel matrix microstructure and the presence of adhesive wear. However, the wear resistance of the coatings is enhanced by increasing the reinforcement volume content under lubricated conditions because of preventing adhesive wear under lubricated conditions [81]. Ni/nano-Al 2 O 3 composites prepared by electro-brush plating indicate a lower wear coefficient compared to the manually electrodeposited composites because of the improved micro-hardness and also smoother and more compact surface morphology of the composites, which subsequently leads to a larger contact surface area [82]. In addition, corrosion wear resistance of Ni/nano-Al 2 O 3 composites prepared by electro-brush plating is better than that of the nickel based electrodeposition coating. This behavior can be explained by the fact that ceramic nanoalumina grains change the crystallization conditions of the coating; thus, the coating is strengthened due to the formation of fine crystals, increased number of dislocations with high density, and increased grain distribution which leads to its improved wear properties [83]. In the case of CO 2 supercritical baths, although the rate of electrodeposition and the number of solid Al 2 O 3 particles in the composite coating is reduced due to the existence of CO 2 supercritical bath, the microhardness, and wear resistance is increased compared to the coatings formed in the common baths. This behavior is attributed to the formation of carbon solid solution restored from the CO 2 supercritical bath in the nickel matrix, that is an

13 Trans Indian Inst Met (2018) 71(2): important factor in improving the wear resistance of Ni Al 2 O 3 composite coatings [84]. Results have shown that by adding surfactant to a certain amount, composite wear resistance increases and decreases thereafter. Addition of a surfactant to an optimum amount can increase codeposited Al 2 O 3 particles, reduce the agglomeration of the particles, and lead to a uniform distribution of Al 2 O 3 in the nickel matrix that ultimately increases wear resistance. At high concentrations of surfactants, the brittleness of the metal matrix decreases the wear resistance [51]. The results of comparing the wear properties of deposited composites from Watts bath containing alpha alumina, gamma alumina, and a mixture of alpha and gamma alumina particles show that alpha-alumina composite has higher wear properties compared with other two composites, because of the hardness of alpha particles and the softness of gamma particles [85]. The changes in the COF of a counter silicon carbide ball on a composite coating deposited by the 0.01 A/cm 2 of current density indicate that the COF is low at the beginning of the test but it increases as the test proceeds (Fig. 15) [86]. The increased COF is related to the gradual changes in the surface of the coating because of the increase in its plastic deformation and the separation defect caused by the penetration of the carbide ball into the coating. The average coefficients of friction are reported to be 0.5, 0.47, 0.54, and 0.53 for deposited coatings in the 0.005, 0.01, 0.02, and 0.03 A/cm 2 of current densities. Although the average COF of all composite coatings is similar, it decreases as the content of alumina particles increases. The large volume fraction of reinforcement particles in metal matrix reduces the distance between the particles in the coating and thus makes the coating harder and more resistant to the plastic deformation. As a result, Fig. 16 Changes in cumulative weight loss over time for pure nickel coating and Ni Al 2 O 3 composites [87] displacement of reinforcement particles from the coating by carbide ball and reduced plastic deformation increase the wear resistance of the composite coating [86]. Figure 16 shows the changes in cumulative weight loss over time of nickel coating and Ni Al 2 O 3 composites deposited by various amounts of alumina in the electrolytic bath [87]. Relative wear behavior is in agreement with the assumption that the harder coating has a higher wear resistance. Pure nickel coating shows a strong weight loss compared with composite coatings. In addition, the wear resistance of the composite coating is improved by increasing the amount of alumina in the electrolytic bath. Trapping alumina nanoparticles inside the nickel matrix, because of the dispersion strengthening effects, prevents plastic deformation of the matrix under loading. These effects are intensified by increasing the number of alumina in the matrix. These particles also reduce adhesive wear by reducing the direct contact with abrasive materials with nickel matrix during the wear test [87]. Fig. 15 SEM micrographs of coated Ni Al 2 O 3 specimens after tensile test a reference specimen, b the specimen with coating (arrows represent separation of coating from the substrate after the tensile test) [86]

14 270 Trans Indian Inst Met (2018) 71(2): Table 3 Summary of results of research conducted on the PN-based coatings with Al 2 O 3 particles Coating composition Particles percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF Roughness (R a ) (lm) References Ni Al 2 O vol% Reciprocating mm 3 /Nm Ni Al 2 O 3? HPB 8 15 vol% Reciprocating mm 3 /Nm Ni Al 2 O vol% Erosion tester Weight loss 1 24 mg* Ni Al 2 O 3 SiC Al 2 O 3 : 9.2 SiC: 7.7 vol% Grinding Weight loss 0 5 mg/cm 2 * Ni Al 2 O vol% N.R. 80 PC: HVDC: Reciprocating mm 3 / Nm Ni Al2O vol% Reciprocating mm 3 / Nm M50 Steel ball N.R. [88] AISI stainless steel ball N.R. N.R. [51] angular alumina N.R. N.R. [78] 2000 grit SiC waterproof paper N.R. N.R. [79] M50 steel ball N.R. [80] AISI stainless steel ball N.R. N.R. [81] Ni nanoal 2 O 3 N.R Vertical: 6.96 ± N.R. 45 steel N.R. [82] Horizontal: 6.94 ± 0.05 Ni nanoal 2 O 3 N.R Fretting wear test Steel ball N.R. N.R. [83] Wear depth 8 24 lm** Ni Al 2 O 3 Conventional bath: 17.4 Supercritical bath: vol% Ni Al 2 O 3 N.R. 11 a: 40 c: 5 Commercial: Conventional bath: 480 Supercritical bath: 800 a: 483 ± 19 c: 390 ± 9 a? c?d: 350 ± 12 Pin on disk Conventional bath: 78.8 lm Supercritical bath: lm** Pin on disk Wear coefficient: a: c: Diamond N.R. N.R. [84] Brass a: c: a? c?d: N.R. [85] a? c?d: mm 3 /Nm

15 Trans Indian Inst Met (2018) 71(2): Table 3 continued References Hardness (HV) Wear rate Counter body COF Roughness (R a ) (lm) Particle size (nm) Thickness (lm) Particles percentage inside coating Coating composition N.R. [86] Silicon nitride ball Ni Al 2 O wt% Pin on disk N.R. Alumina ball N.R [87] Ni Al 2 O wt% Ball on disk mg* CS 10 wheels N.R. N.R. [110] Ni Al 2 O vol% Taber Abraser model 5131 Weight loss mg* [195] CEP: SCD: CEP: SCD: Stainless steel ball Ball on disc CEP: 6 21 SCD: mm 3 /Nm Ni Al 2 O wt% CEP: 520(5 g/l) 580(10 g/l) SCD: 550(5 g/l) 710(10 g/l) * Wear sliding distance and load counter were not reported ** Wear volume was not reported Compared to composite coatings such as Ni Al 2 O 3, Ni SiC, and Ni ZrO 2, pure nickel coating has the highest mass loss under different deposition conditions (DC, PC, and PRC). Such a high mass loss is a direct result of lower microhardness of the pure nickel coating compared with composite coatings. In addition, the Ni Al 2 O 3 composite coating, which is the hardest among other mentioned coatings, indicates the lowest mass loss. For all coatings, reduction of wear mass from DC deposition conditions to PC and PRC, induced by the enhanced hardness of the coatings is observed (due to the increased amount of reinforcing particles in composite coatings and matrix compared to the pure nickel) [87]. A thorough study has been carried out for optimizing the operating parameters, including hexadecylpyridinium bromide (HPB) surfactant, the amount of particles in the electrolyte, and the current density [88]. Their effects have been investigated on the wear properties of the Ni matrix coating reinforced with alumina nanoparticles. The results show that the composite coating have superior wear properties than pure nickel because of the hard nature of reinforcement particles and strengthening effect with the incorporation of alumina particles. Reinforcement particles not only reduce direct contact between the metal matrix and metal ball during the sliding test but also causes famous Orowan effect in the matrix [88]. It has been reported that the hardness of Ni Al 2 O 3 composite codeposited with naphthalene-1,3,6-trisulfonic acid and tri-sodium additives is increased. Thus, it can be stated that these coatings are suitable for wear resistance. This improved wear resistance is probably because of the more particles embedded in the nickel matrix [89]. A brief summary of the researches conducted in the field of nickelbased composite coatings reinforced with Al 2 O 3 particles is given in Table 3. In addition to Al 2 O 3 and SiC, another oxide, carbide, non-oxide and non-carbide particles are used as the matrix phase reinforcement and promoter of wear properties. For example, rare-earth oxides are used extensively in electrical, material, and chemical engineering. Cerium oxide (CeO 2 ) is one of the materials that is considered as a successful option in wear resistance coatings. Molybdenum disulfide (MoS 2 ) is a useful solid lubricant, particularly for vacuum space applications [90]. Its lubrication properties are associated with the layer structure and its inner core planes are located in individual crystallites where cutting is performed easily. Carbon black (CB) has levels of graphite structure and materials containing graphite that are suitable for lubrication [91]. Other hard particles such as WC, SiO 2, and diamond have wear resistance applications. Some of the Ni based composites containing different oxide and non-oxide particles are mentioned in the following.

16 272 Trans Indian Inst Met (2018) 71(2): Fig. 18 Wear resistance of Ni CeO 2 composite coatings obtained at the current density of 8 A/dm 2 with different CeO 2 nanoparticle concentrations [103] Fig. 17 Optical micrographs of wear tracks obtained from a pure nickel coated and b Ni Fe 2 O 3 coated surfaces. Test conditions: the applied load: 15N, sliding distance: 1000 m, test disc rotation speed: 700 rpm, abrasive material was stainless steel ball [92] 2.3 PN-Based Composite Coatings Reinforced with Other Oxide Particles Figure 17 presents the micrograph of wear tracks of Ni Fe 2 O 3 composite coatings [92]. The micrographs show that wear track generated on the composite coating is thinner than the pure nickel coating. The difference observed in the width of wear tracks between the composite and pure coating shows that the composite coating prevents the expansion of wear due to the hardening the composite matrix by trapped Fe 2 O 3 particles compared with the nickel matrix without the particles [92]. Results show that nickel coatings containing nano cylindrical titanate act as a cross-linked and mesh-like matrix such that they improve dispersion strengthening mechanism against external loading. The nanocomposites reinforced with nano cylindrical titanate compared with nickel coatings containing titanium dioxide particles with irregular shapes can (a) reduce surface friction by about 22% against a spherical diamond tip; (b) enhance wear resistance by about 29% in a body slurry abrasion test; (c) increase coating hardness by 10.2%; and (d) increase the elasticity module by about 26% [9]. Increasing TiO 2 nanoparticles enhances the wear resistance of the composite coatings [93 95]. These nanocomposite coatings have a smoother and denser surface, more hardness, and lower wear rate than the pure nickel coating [96]. Moreover, the composite coating containing anatase titania particles have more improved wear resistance than the coating containing commercial titania and rutile titania particles [97]. Additionally, the nickel titania composite coating prepared in lower current have better wear resistance than the ones prepared at higher currents. The films deposited at lower current densities indicate stronger preferred orientation, smaller nickel crystallite size and, the highest particle incorporation. Consequently, these coatings have better wear resistance regarding the synergy caused by the refinement and dispersion strengthening [98]. Incorporation of CeO 2 particles in nickel matrix improves wear resistance and microhardness of the coating [99]. Ni CeO 2 presents a lower COF compared to pure nickel. COFs obtained for the two coatings show that they both have a mild adhesive wear of burnishing type [100]. It is suggested that the change in resistance of Ni CeO 2 nanocomposite coating through SCD method obtained by different electrodeposition parameters is related to the incorporation of CeO 2 nanoparticles in the film because these particles provide dispersion and particle strengthening. During the wear process, CeO 2 nanoparticles are gradually reduced from the matrix and carry the load transferred from the matrix, resulting in the improved

17 Trans Indian Inst Met (2018) 71(2): resistance of the Ni CeO 2 composite coating. The increased content of nanoparticles dispersed in the nickel matrix helps to improve wear resistance. In addition, these nanoparticles can be used as a solid lubricant between the contact surfaces and may reduce the wear rate. Load-bearing capacity and effect of CeO 2 particles on the wear rate are closely related to the number of particles in the coating. Thus, the wear resistance of the Ni CeO 2 composite coatings increases as the particle content increases [101, 102]. It is reported that by applying current densities higher than 8 A/dm 2 for deposition of Ni CeO 2 nanocomposites, the concentration of CeO 2 nanoparticles increases, resulting in the improved wear properties. Figure 18 presents the COF and wear mass loss changes versus CeO 2 content in the bath under the mentioned current density and shows an optimized value that beyond this content, the wear properties of composites becomes less desired than those of pure nickel [103]. The superior wear resistance of Ni SrSO 4 composites than that of pure nickel is attributed to the hardness and more resistance to plastic deformation. Also, the chemical neutrality and thermal stability of SrSO 4 particles prevent the oxidizing wear of the coating [104]. Ni Y 2 O 3 composite coatings also show better wear properties and lower COF compared to pure nickel alloys due to the effect of Y 2 O 3 particles in reducing the roughness and making the surface morphology smoother and more compact. Nano-sized Y 2 O 3 particles also favor (200) and (220) planes and provide both particle and dispersion strengthening [105]. Results show that the incorporation of chromium particles increases microhardness and the wear resistance of nickel coatings. The wear resistance of nickel composite coatings containing nanoparticles of chromium is more than that of nickel composite coatings containing chrome microparticles. Codeposition of smaller nanometer chrome particles together with nickel particles reduces the size of the nickel crystallites significantly and increases the hardness of the composite coating because of the grain refinement strengthening and dispersion strengthening, which leads to the improved wear resistance of the Ni Cr nanocomposite coatings [106]. Similar behaviors have been observed for Ni La 2 O 3 composite coatings [107]. Furthermore, the results of wear and friction tests show that wear resistance of the Ni SiO 2 composite coating is also improved compared with that of magnesium alloy and pure nickel coating which is related to the dispersion strengthening and the effect of the particles on reducing the grain size. The main wear mechanisms for composite coating, magnesium alloy, and pure nickel coating is suggested to be abrasive wear, adhesion wear, and exfoliation wear, respectively [108]. Table 4 Summary of results conducted on PN-based coatings with other oxide particles Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Particle size (nm) Thickness (lm) Particles percentage inside coating Coating composition N.R. [101] Ni CeO wt% mg* 404L stainless steel Ni CeO wt% N.R mg* Steel N.R. [102] Ni CeO wt% mg* N.R N.R. [103] Ni Y 2 O wt% N.R MPa UMT 2 sliding wear N.R [105] machine mg* Nanoscale Roughness * Wear sliding distance and load counter were not reported

18 274 Trans Indian Inst Met (2018) 71(2): Also, the wear resistance of the Ni-pumice electrodeposited composite coating under identical conditions is similar to that of Ni SiC electrodeposited composite coating [109]. Incorporation of AZY particles in the nickel matrix leads to more desirable wear properties by wear resistance synergy, better corrosion, and higher microhardness [110]. The results obtained from related studies on nickelbased composite coatings reinforced with other oxide particles are given in Table PN-Based Composite Coatings Reinforced with Non-oxide Particles Codeposition of inorganic fullerene-like (IF) WS 2 nanoparticles in nickel metal matrix modifies nickel growth significantly and produces finer grains that reduce the stacking fault probability and severely changes the crystallographic texture [111]. These microstructural properties simultaneously increase hardness and reduce COF due to the presence of solid lubricant particles of WS 2. Hence, wear resistance is increased especially when the coatings are formed in the presence of ultrasonic stirring [111]. Results of the study on the wear properties of Ni Zr Silicate composite coating show that the wear resistance of the coating is increased by increasing the amount of fine Zr particles (5 10 lm); however, for the coarse particles (40 60 lm), the wear resistance is generally reduced as the content of particles increases which is mainly due to the loosening of the large Zr particles from the nickel matrix acting as three body abrasion wear mechanism [112]. According to the reports, Ni MCP composites (nickel composites in which oil-encapsulated microcapsules are embedded) have a higher wear resistance compared to pure nickel. This wear resistance is related to the synergistic effect of the high microhardness and the low COF. The COF for the Ni MCP composites initially is low due to the release of oil-encapsulated microcapsules, but after 2000s this coefficient increases because of the existing dry microcapsules [113]. Figure 19 illustrates the effect of graphite incorporation in Ni G composite coating on the COF [114]. By increasing graphite content in the coating, COF is reduced. It is also known that graphite is a solid lubricant and when applied on the surface of a metal, indicates non-stitch properties that results in reduction of the COF [114]. Moreover, embedding Cr particles into the nickel matrix also improves microhardness and wear resistance of the nickel coatings. Codeposition of chromium nanoparticles and nickel reduces the size of the crystals effectively and increases the hardness of the composite coating because of grain-refinement strengthening and significant dispersion strengthening. As a result, the wear resistance of the Ni Cr nanocomposite coatings is enhanced [106]. Microhardness, friction and wear behavior of Cr nanocomposites are strongly dependent on the Cr content. For example, microhardness and wear resistance of Ni Cr nanocomposite coatings with Cr level less than 4% are somewhat more than the pure nickel coating while Ni Cr nanocomposite coating with higher Cr content shows more wear resistance compared to the nickel coating. This behavior is probably attributed to the fact that incorporation of Cr nanoparticles in nickel matrix depends on the hardness of composite coating by dispersion strengthening of Cr codeposited nanoparticles [115]. Figure 20 shows the COF curve at 100 g/l loading of Al particles for Ni Al composite coating at 200 C [116]. Similarly, average COFs of 0.58, 0.58, 0.57 and 0.63 are obtained for 25, 50, 100, and 200 g/l Al content, respectively. The results show that by increasing the loading from Fig. 19 The effect of graphite in Ni G composite coating on coefficient of friction [114] Fig. 20 The coefficient of friction curve for Ni Al composite coating versus wear time [116]

19 Trans Indian Inst Met (2018) 71(2): Fig. 21 Microhardness of the Ni coatings by embedding different amounts of Bi in the electrolyte [118] 25 to 100 g/l, no significant change is observed in the coefficients. The COF for the coating with 200 g/l increases up to 0.63 which is probably because of the high volume of the porosities. The results show that increasing the Al particles does not improve the wear performance of the coating at 200 C [116]. Submicron particles of boron nitride can also be codeposited with nickel in sulfamate bath using natural surfactants to achieve a composite coating. Ni BN composite coatings have a higher wear resistance and higher hardness than pure nickel coatings. Both of these properties are increased by increasing the concentration of boron nitride particles in the electrolyte up to 10 g/l [117]. The amount of debris produced during the wear test of Ni Bi composite coating generated by ion discharge electrochemical deposition is lower than that of pure nickel coating. This result is related to the higher hardness of the composites as these two parameters have a reverse relationship according to Archard law. According to this law, under the same friction conditions, the wear rate is inversely proportional to microhardness of materials. Ni Bi alloy has a higher hardness, lower debris, and higher wear resistance because of the strengthening of Ni Bi solid solution and smaller grain size. Results show that, both the volume loss and wear rate for the composite coatings are lower than those of pure nickel coating. According to Archard law, volume loss during sliding wear has a direct relationship with the work done by the friction force and an inverse relationship with the contact surface. Thus, the higher microhardness should provide a lower volume loss (Fig. 21) [118]. Regarding the improved wear behavior of the Ni WC composite coatings with a lower COF (0.34) compared to that of the pure nickel coating (0.62), it is reported that the wear behavior of the these composite coatings is similar to an elasto-plastic contact in which nickel undergoes a plastic deformation and the entity of asperity interaction with WC particles is elastic. Hardening the matrix along with the high modulus of WC phase leads to a lower COF of composite coatings [47]. The friction and wear results of composite coatings containing carbon nanotubes (CNTs) worn on steel surface under loading of 200N and 10 Hz frequency show that CNTs have reduced COF and increased wear resistance of the composite coating. In addition, the COF is reduced as loading shifts up from 50 to 300N in lubricated conditions. The wear rate of Ni CNT composite coating is two and four times less than that of pure nickel coating and carbon steel, respectively [119]. It is also reported that these particles increase the wear resistance of the deposited nickel and reduce the wear rate up to one-third of the nonstrengthened nickel deposited coatings [120]. The COF for the Ni CNTs, composite coating reduces more sharply than pure nickel coating and carbon steel, indicating that CNTs have self-lubrication properties. These materials are located on the composite surface and reduces COF during the wear. The wear properties of Ni CNTs composite coating strongly depends on the composition and structure. CNTs dispersed in nickel matrix prevent the dislocations movement or at least delay them and prevent the plastic formation, improving the wear behavior of the coatings. Furthermore, codeposition of CNTs, can produce many dislocations in the nickel matrix that strengthen the matrix [119]. In pure nickel coatings, no transfer films are formed while Ni CNT coatings have an oxide film which is highly adhesive. It has been suggested that CNTs are effective in the development of transitional oxide film. Thus, the Fig. 22 Coefficient of friction of Ni diamond composite prepared by the flow of 4 A/dm 2 at 50 C and different concentrations of the diamonds in the bath 1, 3 and 5 g per liter. Coefficient of friction of pure nickel coating is also shown for comparison [121]

20 276 Trans Indian Inst Met (2018) 71(2): composite coating Ni CNTs can be used as a new coating with excellent wear properties under the lubrication conditions [120]. Figure 22 presents the friction curves of Ni diamond composite coatings under lubrication conditions [121]. The COF for this composite is about while it is for pure nickel coating. A potential explanation for this behavior is the shear performance of Ni diamond coating which is better compared to that of the pure nickel coating. In addition, the higher diamond content increases the COF, implying that the distribution intensity of the particles embedded in nickel matrix affects the friction coefficient [121]. Low wear rate of these composite coatings is associated with the dispersion strengthening effect resulting from the incorporation of diamond particles [122]. The Ni Si 3 N 4 composite coating has a perfect tribological behavior when lubricated with the ionic liquid. This behavior is somewhat related to the high hardness of electroformed nickel composite coating containing Si 3 N 4 nanoparticles and the tribiochemical reaction between the sliding surface and the lubricant []. The comparison between the tribological behavior of nickel coatings containing inert particles such as SiC, Al 2 O 3, and Si 3 N 4 show that the coating reinforced by Si 3 N 4 has a better wear behavior than the two other composites while no difference is observed among the COF for the three composites [124]. A brief summary of the works conducted in the field of nickel-based composite coatings reinforced with non-oxide particles is given in Table 5. 3 Nickel-Tungsten Alloy Coatings Ni W alloys, in addition to the reported corrosion resistance [124, 125], have a lower wear coefficient in comparison with the pure nickel. Pure nickel coating has weaker wear properties compared to nickel-tungsten alloy coating due to its large grain size and lower hardness [126]. The addition of tungsten to nickel leads to the reduced wear rate. In other words, in Ni W electrodeposition alloy coatings, due to increased hardness of the coating, wear resistance enhances by increasing the tungsten content [127]. Two possible reasons have been proposed for this behavior. Firstly, the higher hardness of Ni W alloys reduces the contact area between the film and the abrasive body. Secondly, tungsten facilitates back-transfer to steel ball and helps form a protective transfer film. It is suggested that increasing W content inside the film forms a protective transfer film on the steel ball and helps to reduce the wear damage of the system [128, 129]. Besides, heattreated specimens of Ni W alloy coatings have superior wear properties than the original specimen. This claim is justified based on empirical observations according to which the surface without heat treatment indicates a plastic deformation. However, no plastic deformation is observed on the heat-treated worn surface at 700 C [130]. Applying heat treatment at 700 C on the alloy with high tungsten (22 at.%) and small grain size (3 nm) leads to the improved hardness and reduced wear resistance. Long-time annealing and increasing annealing temperature increases the grain size, hardness, and wear resistance while the secondary phases are formed in the structure. For alloys with lower levels of tungsten (6 and 13 at.%) and larger grain sizes (13 and 56 nm), further growth in grain size with uniform reduction of hardness is observed [131]. The Taber wear index for tungsten-nickel alloy coating is 14, which is reduced to 10 after heat treatment at 500 C. This index for hard chrome coating is 2.9. Calculations show that, the wear rate of nickel-tungsten coating is 0.34 nm/cycle while it is 0.13 for chromium coating. The COF of nickel-tungsten is less than that of hard chrome because of the low volumetric wear rate of deposits [132]. Additionally, during the wear corrosion of these alloy coatings, fabricated passive layer on the Ni W specimen surface during the polarization test not only stabilizes corrosion but also acts as a lubricant, leading to a reduced wear coefficient by increasing potential [133]. Researchers have also analyzed the effect of the complexing agents, i.e. citric acid and glycolic acid, on nickel-tungsten alloy coatings, properties. The results of surface morphology and extent of the damage suggest that reducing the mass of the coatings formed by glycolic acid is about 45% less than citric acid. SEM images present the surface morphology of worn coatings obtained from the glycolic acid solution (33 wt%) and citric acid (35 wt%) (Fig. 23)[134]. It can be observed that the worn surface of the Ni W specimen with glycolic acid additive show some light furrows while heaves exist on the worn surface of other specimen with citric acid additive, indicating the most serious damage caused by the citric acid. On the other hand, the hardness results show that the glycolic acid alloy coatings have a higher hardness; thus, they have a higher wear resistance [134]. A comparison of the sliding wear behavior of Ni, Ni W and hard chrome shows that Ni 10.5W has the lowest wear rate compared to other cases, despite the fact that hardness of hard chrome is more than that of Ni 10.5W. The wear behavior is attributed to the ductile wear mechanism on the Ni 10.5W coating in contrast to brittle fracture on the hard chrome coating [135]. Table 6 summarizes the researches performed on nickel-tungsten alloy coatings. 3.1 Nickel-Tungsten Based Composite Coatings Reinforced with Particles Diamond particles can be well dispersed within the Ni W matrix. These particles change the chemical composition of

21 Trans Indian Inst Met (2018) 71(2): Table 5 Summary of the results of research conducted on the nickel based coatings with other nonoxide particles Coating composition Particle percentage inside coating Thickness (lm) Particle size Hardness (HV) Wear rate Counter body COF Roughness (R a ) (lm) References Ni WC wt% lm Reciprocating N.R. Ni Cr wt% nm 550 Ball on disk mg/nm Ni MCP N.R lm 390 ± 10 Pin on disk Wear loss 0 30 lm* Ni Al 2 22 wt% Al Particles N.R. 1 lm N.R. Pin on disk N.R. Ni B at % N.R. Amorphous GPa Flat surface ball 0 17 lm* Ni Bi at.% N.R. N.R Reciprocating 3.4 ± ± mm 3 /Nm Ni CNTs N.R. 25 N.R. N.R. Reciprocating mm 3 ** Ni diamond wt% 35 ± 5 45 lm N.R. Ball on disk mg/nm Steel ball commercial SAE grade Si 3 N 4 ceramic balls N.R. [47] N.R. [106] Steel pins [113] Carbide ball N.R. [116] N.R N.R. [184] Ceramic ball N.R. N.R. [118] GG-15 steel ball N.R. [120] WC ball [121] * Wear volume was not reported ** Wear sliding distance and counter volume were not reported

22 278 Trans Indian Inst Met (2018) 71(2): Fig. 23 The morphology of Ni W alloy coatings, a glycolic acid (33 wt%), b citric acid (35 wt%) [134] Table 6 Summary of results of research conducted on the nickel-tungsten alloy Coating composition tungsten content inside coating Thickness (lm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni W wt% N.R Pin on disk mg/ Nm Ni W at.% Pin on disk mm 3 /Nm Ni W wt% Ring on disk N.R. Ni W 40 wt% Pin on disk mm 3 / Nm DIN tool steel N.R. N.R. [127] Hardened steel ball N.R. [129] JIS SKD-11 tool steel Tungsten carbide ball N.R. [130] [132] Ni W at.% mg* Alumina cube [133] Ni W wt% Ball on disk GCr15 steel ball N.R. N.R. [134] mg/nm Ni W wt% Pin on disk mg/ Nm * Wear sliding distance and counter volume were not reported Low carbon steel (AISI 1045) N.R. [196] the Ni W matrix, improve microhardness, and increase its wear resistance [136]. In addition, ZrO 2 particles improve the microhardness and wear resistance of the Ni W alloy coating [ ]. Hardness versus heat treatment temperature results show that by increasing heating temperature to 600 C, the wear resistance of Ni W coatings is increased with and without the incorporation of the particles. Further increase in temperature (up to 700 C) increases wear weight loss. Also, the reduced weight of the composite coating in comparison with alloy coating is less in both heat-treated and non-heat-treated procedures. It is suggested that incorporation of diamond particles in the nickel-tungsten matrix increases wear resistance [140]. Researchers have studied the effect of alumina particles on the wear properties of Ni W/Al 2 O 3 composite coating deposited by pulsed current [141]. Accordingly, three specimens PC-1, PC-2, and PC-3 have been prepared with almost the same content of tungsten while alumina particles for these specimens are equal to 1.59, 0.70, and 0.36 wt%, respectively. Curve of friction for these specimens are presented in Fig. 24 [141]. As shown in Fig. 24, the curve of friction for specimen PC-1 is very smooth and COF is about But, in specimens containing less alumina, COF is not constant and it reaches the maximum value of 0.6. Moreover, it can be observed that when the wear distance of PC-3 specimen reaches the first 240 m, severe fluctuations occur in the curve, and then COF reduces and reaches a fixed amount. This behavior can be attributed to the non-uniform dispersion of alumina particles in the matrix. Alumina particles are concentrated in a small area of the coating near the substrate. Therefore, when the nanoparticles are released from the coating, sliding friction is converted to rolling friction, finally

23 Trans Indian Inst Met (2018) 71(2): results obtained from related studies in the field of Nickeltungsten based composite coatings reinforced with particles are given in Table 7. 4 Nickel Cobalt Alloy Coatings Fig. 24 The curve for the coefficient of friction of the Ni W/Al 2 O 3 composite coating deposited by pulsed current with almost the same content of tungsten and various alumina particles a PC-1, b PC-2 and c PC-3 [141] leading to the reduction of COF and uniform wear curve. It is obvious that COF is reduced in cases where Ni W has a higher alumina content [141]. The COF of Ni W/BN (h) nanocomposite coatings is almost twice lower than that of the Ni W alloy coatings. This behavior is because of the structure of boron nitride particles in which slipping occurs easily among the layered lattice arrangement of BN (h) particles [142, 143]. The The effects of cobalt content on COF of nickel cobalt alloy coatings are presented in Fig. 25 [144]. As shown in this figure, the COFs of pure nickel and Ni Co alloy coatings with cobalt content less than 49 wt% (rich in nickel) are almost the same. By increasing the amount of cobalt, cobalt-rich alloys demonstrate an excellent wear behavior. Under the same wear test conditions, cobalt-rich alloys with cobalt content greater than 81% presents the lowest COF (more than twice lower than nickel and nickel-rich alloys), followed by the Ni 66 wt%co. Moreover, the COF of cobalt-rich alloys are more stable than those of nickelrich alloys. Considering the XRD analysis results, the closeness of COF of the nickel and nickel-rich alloy coatings can be related to the same fcc crystal structure. In case of Ni 66 wt%co alloy, the mixed phase of fcc/hcp with a lower proportion of hcp structure leads to a gradual reduction in the COF. In addition, in case of Ni 81wt% Co alloy, a significant reduction in COF is observed because of the higher proportion of hcp structure. Thus, it can be concluded that reducing cobalt-rich COF by increasing cobalt content is probably due to the restructuring of fcc to hcp. The wear rate changes based on the amount of cobalt and microhardness are presented in Fig. 26 [144]. As it can be observed, Ni Co alloys have a lower wear compared to pure nickel. In addition, the wear rate of Ni Co alloys is gradually reduced as cobalt content increases from 6 to 49 wt%. Clearly, when the cobalt content is less than 49 wt%, a gradual reduction in wear rate occurs by increasing cobalt content due to the increased microhardness from 315 to 462 Hv. As, these alloys show adhesive wear based on the SEM images, Archard s law has been applied in this study. The data on wear rate for Co alloys with cobalt less than 49 wt% is in compliance with Archard s law. Nevertheless, by increasing cobalt, the wear rate of cobalt-rich alloys rapidly decreases, despite the fact that microhardness is reduced as well. This inverse Archard s law may be due to hcp crystalline structure of cobalt rich alloys [141, 144, 145]. On the other hand, under an optimal amount of saccharin additive to nickel cobalt coating, wear resistance increases due to the reduced size of the grain, purer nickel fcc phase structure in nickel cobalt alloy, and more adhesion of the coating to the substrate [146]. Table 8 is a summary of the researches performed on nickel cobalt alloy coatings.

24 280 Trans Indian Inst Met (2018) 71(2): Table 7 Summary of results of research conducted on nickel-tungsten composite coatings with other particles Coating composition tungsten content inside coating Particle percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF References Ni W diamond Ni W diamond wt% 2 22 vol% Ball on disk mg* wt% 21 ± 1 vol% Ball on disk mg* Ni W wt% wt% Ball on disk Al 2 O mg* * Wear sliding distance was not reported Zirconia (ZrO 2 ) Silicon nitride (Si 3 N 4 ) [136] [140] Cr ball [141] 4.1 Nickel Cobalt Based Composite Coatings Reinforced with SiC Particles Fig. 25 Coefficient of friction versus the amount of cobalt in Ni Co alloy coating [144] The addition of SiC nanoparticles as reinforcement to the Ni Co alloy matrix leads to the increased COF [147]. However, for Ni 28Co alloy, this increase is minimal. Increasing COF can be related to the fact that the interaction between particles inside the coated rod and the particles transferred to the disk during sliding causes the SiC nanoparticles to generate greater lateral forces [147]. Wear resistance of nanocomposite coatings is increased by increasing the amount of silicon carbide nanoparticles in electrodeposition bath, which is associated with the refinement and dispersion strengthening of codeposited hard SiC nanoparticles [148]. By reducing the SiC particle size, the volume wear is increased, which is due to the decreasing space between SiC particles in nickel cobalt matrix [149]. In other words, the drop in the wear volume of Ni Co composites reinforced by SiC micron particles is less than that of SiC nanoparticles. The difference in wear behavior is associated with the crystallographic orientation. Coatings with nanoparticles have the dominant orientation (111) while the other ones have the orientation (200) [150]. Research results show that Ni 70Co composite has the optimum wear properties compared to other composites [147]. A brief summary of the researches about nickel cobalt based composite coatings reinforced with SiC particles is given in Table Nickel Cobalt Based Composite Coatings Reinforced with Al 2 O 3 Particles Fig. 26 The wear rate according to the amount of cobalt in Ni Co alloy coating [144] The results show that by increasing alumina amount, the wear properties of microstructure of nickel cobalt/nanoalumina composites prepared by electric deposition of

25 Trans Indian Inst Met (2018) 71(2): Table 8 Summary of results of research conducted on nickel cobalt alloys in the absence of a third particle Coating composition Cobalt content inside coating Thickness (lm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni Co 5 80 wt% Ball on disk mm 3 /Nm Ni Co 0 80 at.% 50 ± Reciprocating mm 3 / Nm Ni Co 7 27 at.% Pin on disk N.R. AISI stainless steel ball AISI stainless steel N.R. [144] Nanoscale Roughness N.R Nanoscale Roughness [197] [146] pulse reverse current are improved. Besides, it is found that coatings prepared by the PRC are more compact and results better wear properties. Alumina hard particles in the coating reduces friction between soft nickel coating and wearing wheel. At the same time, under the normal load, alumina particles have a fixative effect on grain boundaries that limit dislocations. Thus, these particles not only prevent the deformation of the coating but also prevent the ability of the coating against plastic deformation so that the wear resistance of the composite is increased [151]. As an illustrative example, mass loss and hardness reduction results of different coatings including (1) pure nickel, (2) pure cobalt, (3) Ni 16Co alloy, (4) Ni 78Co alloy, (5) Ni 20Co 8.5 vol% Al 2 O 3 and (6) Ni 20Co 8.7 vol% Al 2 O 3 coatings are shown in Fig. 27 [152]. The results show that Ni 20Co 8.7 vol% Al 2 O 3 coating has the lowest mass loss and the highest hardness. These results also indicate that the coating with the highest hardness has the best wear resistance at high temperatures. According to the data obtained, Al 2 O 3 dispersion in Co Ni alloy improves hardness and wear resistance. In addition, wear resistance and hardness depend on the cobalt content. High amounts of cobalt in the coating improve wear resistance and hardness [153]. Table 10 summarizes the researches performed on nickel cobalt composite coating with Al 2 O 3 particles. 4.3 Nickel Cobalt Based Composite Coatings Reinforced with Other Particles Results show that the nanocrystalline graded Ni Co/CoO composite coating under dry sliding conditions has a more satisfactory wear performance and higher lubrication than graded Ni Co alloys. This synthesis is performed by electrochemical precipitation method and cyclic thermal oxidation and quenching. By increasing the thickness of the deposited film, the film composition is converted from nickel-rich area to cobalt-rich area and the film structure is altered from fcc to hcp. The analyses by X-ray diffraction show that the surface film is made of very fine and dense CoO particles with preferred orientation (111). This higher performance is attributed to the graded microstructure of deposits, i.e., the solid lubrication of the CoO-rich surface films [154]. Also, the effect of YZA particles (yttrium oxide stabilized by zirconia and alumina) in nickel cobalt alloys has been analyzed by different amounts of cobalt and wear tests show that the composite containing 38 wt% cobalt have the optimum wear properties because of the presence of hcp phase [155]. Wear properties are improved because of the uniform dispersion of MoS 2 nanoparticles in Ni Co matrix. Figure 28 presents the COF and wear rate of Ni Co alloy coating and Ni Co/MoS 2 composite coating on steel at different loading conditions [156]. It can be observed that the composite coating under the same test conditions is much less and has a lower wear rate. By increasing the loading, COF is reduced and the wear rate is enhanced, implying that the average COF per 1N of load is reduced from 0.23 to 0.15 at 4N probably due to the formation of a lubricant film on the sliding surface [156]. Similarly, because of the uniform dispersion of Si 3 N 4 nanoparticles in the nickel cobalt matrix, the wear properties of Ni Co/ Si 3 N 4 composite are improved. At the same time, hydroxylate silicon oxide formed by tribochemical reaction between Si 3 N 4 nanoparticles and water vapor in the air reduces the COF [157]. Figure 29 presents the COF of Ni Co alloy coating and Ni Co CNT alloy composite coating at different cycles [158]. It is observed that the composite coating under the same test conditions has a lower COF and low wear rate, with COF of both coatings gradually increasing with the increasing sliding cycles. Moreover, the COF of Ni Co CNT composite coating is reduced by increasing normal loading and the sliding rate because of the formation of a conductive lubricant film at the contact surface at the time of sliding. CNTs improve the wear properties and lead to grain refinement and strengthening of the coating, since they are well dispersed in the nickel cobalt matrix [158].

26 282 Trans Indian Inst Met (2018) 71(2): Table 9 Summary of results of research conducted on Ni Co alloy with SiC particles Wear rate Counter body COF References Hardness (HV) Particle size (nm) Thickness (lm) Sic percen tage inside coating Cobalt content inside coating Coating composition [147] EN31hardened Steel (650 VHN) Ni Co SiC wt% wt% Pin on disk mm 3 / Nm SAE52100 steel ball N.R. [148] Ni Co SiC N.R. 1 6 wt% Ball on disk mm 3 /Nm En 24 steel N.R. [149] Ni Co SiC N.R vol% , volumetric wear factor (mm 3 /Nm) [150] Brass pin Nano: Micron: Pin on disk Nano: Micron: mm 3 / Nm Nano: 565 Micro: 300 Ni Co SiC wt% wt% 50 Nano: 25 Micron: 1000 Fig. 27 Mass loss and hardness reduction of different coatings prepared by electrodeposition at 300 C: 1 pure nickel, 2 pure cobalt, 3 Ni 16Co alloy, 4 Ni 78Co alloy, 5 Ni 20Co 8.5 vol% Al 2 O 3 alloy and 6 Ni 20Co 8.7 vol% Al 2 O 3 [152] Furthermore, nickel cobalt composite containing 12 vol% chromium oxide in the cobalt matrix and heattreated at a temperature of C indicates more desired wear resistance considering the formation of a glass-like layer in a cobalt matrix [159]. The result of wear test of composites containing diamond in the Ni Co alloy and pure Ni matrixes under dry wear conditions are shown in Figs. 30 and 31 [160]. It is observed that both type of composites have a lower wear rate than pure nickel probably because of the dispersion strengthening effect of diamond particles. Wear rate of nickel cobalt composite is much less than nickel composite, as the reinforcement particles reduce the direct contact between the matrix and steel ball. Embedding cobalt ions to electrolytic bath leads to the uniform dispersion of diamond particles in the matrix that reduces the direct contact between metal metal (nickel-steel) and, as a result, improves the wear performance. However, in case of nickel matrix, relatively low amount of diamond pours out as agglomerated particles from the matrix after a short time because these particles are not completely encapsulated by the nickel matrix [160]. By reducing the current density during electrodeposition, the wear properties of nickel cobalt matrix composites with diamond particles are enhanced and the wear rate and contact quality of diamonds are optimized in a current density of 5 A/dm 2 [161]. A brief summary of the researches conducted in the field of nickel cobalt based composite coatings reinforced with other particles is given in Table Nickel-Phosphorus Alloy Coatings When the Ni P alloys (10.65 at.% P) deposited by electrodeposition method are heated at C, the hardening of the coating initiates and the wear resistance

27 Trans Indian Inst Met (2018) 71(2): Table 10 Summary of results of research conducted on the nickel cobalt/nano-alumina alloy Coating composition Cobalt content inside coating Al 2 O 3 percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body References Ni Co Al 2 O 3 DC: wt% PRC: wt% DC: wt% PRC: wt% DC: PRC: Ring-on block wear tester DC: 2-3 PRC: mg/ Nm Ni Co wt% vol% N.R face wear test Al 2 O 3 machine mg* Ni Co N.R. N.R Reciprocating wear Al 2 O 3 depths lm** GCr15 steel GCrl5 abrasive wheels [151] [152] X-65 steel [153] * Wear sliding distance was not reported ** Lack of data increases insignificantly. Resistance reduction by increasing temperature is not significant and no noticed changes at wear volume are observed at the temperatures above 500 C. Coating heated at 600 C has cracks on its surface that can affect resistance negatively [162]. However, in a study examining graded Ni P alloys, different results are obtained. A comparison is made between the wear properties of heat treated Ni P gradient deposits at room temperature in which the P content changes along the thickness (6 layers) and the non-graded Ni P coating heat-treated at 400 C through the ball-on-disc test. Also, the wear properties of Ni P heattreated gradient deposits at temperatures ranging from room temperature to 600 C have been compared with those of chrome deposits. The results show that the wear resistance of graded Ni P alloys is almost twice that of non-graded Ni P deposits. In addition, graded Ni P deposits have a better wear resistance and lower COF than hard Cr deposits either at room temperature or under high-temperature conditions. The main explanation for this behavior can be that the graded compounds and structures have effectively prevented the formation and dispersion of cracks along the thickness of Ni P heat-treated deposits during the wear process [163]. In normal loading of mn and at the same thicknesses, the COF of Ni P alloy coatings is reduced by increasing phosphorus content. The increased amounts of phosphorus, which makes the coating structure amorphous and decreases hardness, reduces the friction force related to the failure and asperities deformation [164]. Increasing the content of phosphorus in the film reduces the hardness of Ni P films, leading to the conversion of the coating structure from microcrystal to nanocrystalline/xray amorphous. The morphology of worn surface shows that under such conditions the wear mechanism of Ni P coatings is associated with hardness. By increasing the hardness, the wear morphology is converted from the state that has scratches and abrasions to the mode where metal debris are attached to the coating surface. In other words, the wear resistance of Ni P electroformed alloy increases by increasing the hardness. Hardness significantly affects the wear resistance of Ni P coatings so that the optimal wear resistance of Ni P coatings can be 11 times greater than that of Ni coatings [165, 166]. Ni P deposited alloy coating by electro-brush deposition has an amorphous structure before heat treatment. After heat treating at 450 C for one hour, the coating becomes poly-crystalline of Ni and Ni 3 P. Ni P alloy has an excellent wear resistance and can effectively reduce the COF at 450 C. If the coating time is long enough, the coating will reach the proper resistance and acts well at long high-temperature processes [167]. Comparing the wear properties of two Ni P coatings prepared by fluorine surfactant C 12 EO 8 in the CO 2 supercritical bath, it can be concluded that the alloy with surfactant fluorine has lower surface roughness, justifying the better quality of its wear resistance [168]. In addition to the above comparison, the lubricated sliding wear behavior of Ni P alloy is compared with that of hard Cr deposits, where the results show that the coating has weaker wear properties under lubrication than hard Cr depositions. The main reason for this poor wear properties of the coating as compared to the hard Cr deposits, is the higher possibility of maintaining the lubricant by hard chrome surface structure, as a result of the unique nodular effect of hard chrome in the wear process [169]. Table 12 is a summary of the researches performed on nickel-phosphorus alloy coatings.

28 284 Trans Indian Inst Met (2018) 71(2): Fig. 28 Coefficient of friction and wear rate a Ni Co and b Ni Co/ MoS 2 at different loads [156] 5.1 Nickel-Phosphorus Composite Coatings Reinforced with Particles In Ni P carbon black (CB) composites with the CB particles embedded in the Ni P matrix alloy, both before and after particle embedding, the composite coatings have lower COFs than the alloy. Materials containing graphite, such as CBs that have some graphite structure, are appropriate for lubrication applications. Thus, CB acts as a lubricant between the film and the surface and is abrasive that reduces the COF [170]. Results show that the wear behavior of the Ni P/SiC composite electrodeposited by pulse current (PC) is better than that of direct current (DC). Although the microhardness of the DC electrodeposition layers is higher than that of PC, the trend of SiC content is reversed. During the wear test, high amount of fine SiC particles are removed from the surface by the abrasive object. Such high content also converts frictional movement from pure sliding to partial sliding due to which the frictional force is reduced. In addition, PC deposition coatings may have lower hydrogen in their structure (the residual stress due to the less hydrogen embrittlement) since they disappear in the off- Fig. 29 Variations of coefficient of friction versus sliding cycles for a Ni Co and b Ni Co/CNTs composite coatings/gcr15 steel ball was sliding material [158] Fig. 30 Diamond concentration effect in the bath on the wear rate and hardness of Ni diamond deposits, ph = 3.5; 1 A/dm 2,30 C [160]

29 Trans Indian Inst Met (2018) 71(2): Fig. 31 Effect of cobalt content on hardness and wear rate of Ni Co and Ni Co/diamond deposits. ph = 3.5; 1 A/dm 2, 30 C versus concentration of cobalt in alloy coating (filled circle Ni Co diamond, filled square Ni Co) [160] time of pulsed current and increases the wear resistance of the coatings [171]. The use of pulse current compared with direct current leads to the formation of a more compact coating and improves hardness and the wear behavior [172]. Silicon carbide mixture increases the hardness of Ni P coatings but prevents the adhesion of oxide film formed during the wear process to the contact surface. Annealing the coating reduces the wear rate. During the heat treatment of the coating, the crystalline Ni 3 P phase formed inside the coating s matrix lead to an increase in the hardness of the coating. Also annealing of composite coating causes Ni P matrix to crack around SiC particles [173]. Biaxial ball-on-disc tests under gross slip conditions show that the heat treatment reduces the wear volume loss during abrasion in ambient air for both Ni P and Ni P SiC composite alloys. Ni P heat-treated coating has a lower wear volume loss than the composite Ni P SiC coating. Also, the wear rate in the biaxial sliding test is lower than the rate in the single axial sliding test [174]. The results obtained from related studies conducted on nickel-phosphorus composite coatings reinforced with particles are given in Table Other Nickel-Based Alloy Coatings The Ni/SiC and Ni P/SiC composite films have been prepared and their properties have been compared with those of Ni B/SiC composites. In these films, the content of these particles is controlled and kept similar in all coatings by adjusting the SiC concentration in the baths. In addition to these composites, Ni, Ni P, and Ni B films have also been prepared. Ni P, Ni P/SiC, Ni B, and Ni B/SiC films are heat-treated at 573 K because the Ni B and Ni P films are hardened by heat treatment. In contrast, Ni and Ni/SiC films are not heat-treated because this process has no effect on the hardness of these films. The wear resistance performance of the film is measured by mass loss. The hardness order of these films without heat treatment is as follows: Ni B/SiC [ Ni B [ Ni P/SiC [ Ni P [ Ni/ SiC [ Ni; suggesting that the films with harder matrix present greater hardness. By comparing the hardness and mass loss, the researchers have concluded that films with higher hardness shows better wear resistance performance. To analyze the relationship between hardness and wear resistance performance, the mass loss in terms of hardness is plotted in Fig. 32 [175]. As expected, by hardening the films, the wear resistance performance is also improved. In addition, by comparing the mass loss of the composite films with metals (or alloys) with the same hardness, it can be observed that the wear resistance performance of the composite film is more satisfactory. Probably, the composite film prevents the wear due to the presence of SiC particles embedded in the metal or alloy matrix [175]. According to the research [176], Ni Mo coatings with desirable mechanical properties have been fabricated by pulse plating. This study focuses on current efficiency, the amount of molybdenum, and microhardness of the coating affected by pulse plating parameters. The wear test results show that Ni Mo specimen prepared by pulse plating method have a higher COF in comparison with direct current electrodeposition. This phenomenon is mainly attributed to two reasons: Firstly, PC Ni Mo coatings with high hardness generates high impedance against the shear force applied by the abrasive object (ball) and thus the COF of the coating is higher than that of DC Ni Mo coating. Secondly, these tests are based on as-plated coating with no polishing in which surface roughness is a factor that causes high COF. Nevertheless, compared with DC electrodeposition, the wear index presents a significant improvement for Ni Mo alloy deposited by pulsed-plating method. Adhesive deformation accumulation and scrape abrasion results in slowing of up and down of the frictional force (Fig. 33) [176]. Also, it is reported that at high current density (up to 3.5 A/dm 2 ) it is possible to achieve the best micromechanical and tribological properties for Ni Mo electrodeposition coatings. Compared with chrome coatings, Ni Mo coatings present lower microhardness but they have a lower stiffness (lower elastic module). As a result, the plasticity index of these coatings is similar. The Ni Mo coatings are characterized by wear coefficient which is two times lower than that of chromium coating [177]. According to the literature [178], by increasing the boron content in the plating bath up to 11 at.%, the hardness increases significantly as a result of the precipitation of intermetallic Ni 3 B by the heat treatment. However, COF

30 286 Trans Indian Inst Met (2018) 71(2): Table 11 Summary of results of research conducted on the nickel cobalt alloys with other particles Coating composition cobalt content inside coating Particle percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni Co CoO 0 81 wt% N.R. 100 N.R. N.R. Reciprocating mm 3 /Nm Ni Co YZA 0 85 wt% N.R Pin on disk mm 3 / mm* Ni Co/MoS 2 N.R. N.R N.R. Reciprocating mm 3/ Nm Ni Co/Si 3 N 4 N.R. 0 6 vol% Reciprocating mm 3 /Nm Ni Co CNT N.R. N.R. N.R. N.R GPa Reciprocating N.R. Ni Co Diamond Ni Co Diamond 0 15 wt% vol% , Pin on disk mm 3 /Nm 0 62 wt% N.R. N.R. 106, , Mass loss of Drill mg** Ni Co wt% 5 8 wt% N.R Pin on disk CeO mg** AISI stainless steel ball N.R. [154] N.R N.R. [155] SAE52100 steel ball SAE52100 steel ball SAE52100 steel ball high speed steel N.R. [156] N.R. [157] N.R. [158] N.R [160] Corderite N.R. N.R. [161] N.R. N.R. N.R. [198] * Counter load was not reported ** Wear sliding distance was not reported

31 Trans Indian Inst Met (2018) 71(2): Table 12 Summary of results of research conducted on the nickel-phosphorous, without the third particle Coating composition Phosphorous content inside coating Thickness (lm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni P 8 15 wt% Ball on disk mm 3 / Nm Ni P wt% GPa Reciprocating width of track 5 lm*** Ni P wt% Ring on disk mg/ m** Ni P 9.5 wt% ± ± 30 Volume loss rate mm 3 /min* Ni P N.R Ball on disk N.R. Ni P 5 6 at.% N.R Sliding wear test mm 3 / Nm Ni P wt% Ball on disk mm 3 / Nm Si3N4 ceramic ball N.R. [163] Steel N.R. N.R. [164] ShKh15 ball JIS SKD-11 steel AISI 304 stainless steel N.R. 0.1 [199] N.R [200] Si 3 N 4 ball N,R. [167] Diamond pin Si 3 N 4 ceramic ball N.R Nanoscale Roughness [168] N.R. [169] * Wear sliding distance and counter load were not reported ** Wear load were not reported *** Wear volume were not reported Table 13 Summary of results of research conducted on the nickel-phosphorous alloys with other particles Coating composition Element content inside coating Particle percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF References Ni P- Carbon black at.% wt% N.R. 40 Before: After: Ni P SiC wt% wt% DC: PC: Reciprocating N.R. Ring on disk N.R. Alumina ball JIS SKD11 steel [170] [171] of nickel-boron nanocrystalline alloys is increased from 0.2 to In this regard, the coefficient value of 0.2 is very low compared with that of the similar nickel alloys and chrome electrodeposition. Ni P and Ni W COFs are in the range of while it is 0.3 for chromium. It is suggested that, by increasing the boron content, the wear performance of the alloy is reduced due to more brittle structure and increased COF of the coating [178]. In a similar study conducted on the corrosion and wear performance of Ni B coatings, results show that by increasing the boron content up to 20 at.%, microstructure experiences different changes. At lower amount of boron content (\0.8 at.%), the coating has a nanocrystalline structure which changes to an amorphous-nanocrystalline structure at medium amount of boron content (approximately 10 at.%). The coatings demonstrate X-ray amorphous structures at high boron content (20 at.% and above). The wear resistance and mechanical properties somewhat deteriorates as the structure changes from homogeneous nanocrystalline to heterogeneous amorphous-nanocrystalline but at higher values of B, by more amorphization of Ni B coatings, mechanical properties improve again. As a

32 288 Trans Indian Inst Met (2018) 71(2): Fig. 32 The relationship between hardness and mass loss after wear test of Ni P, Ni P/SiC, Ni B, and Ni B/SiC coatings [175] result, Ni B nanocrystalline electrodeposited coatings with low levels of boron present a higher wear resistance and microhardness and can be useful for various applications [179]. Besides, the addition of CeO 2 or Al 2 O 3 nanoparticles to Ni B matrix leads to a significant improvement in the mechanical properties (hardness and elasticity module) of the coating. This behavior can be explained by the dispersion hardening of the Ni B matrix produced by the presence of these hard particles [180, 181]. Ni B coatings have a good adhesion to the substrate in heat-treated and as-plated modes. Ni B heat treated coatings at 300 C present the lowest wear rate under lubricated sliding conditions and the wear resistance of these coatings is much greater than that of chrome coatings. The excellent wear resistance of these coatings is related to the hard elastic modulus (H/E) ratio [182]. Moreover, the results show that heat treatment at 400 C of Ni B alloy forms nickel boron phases that increases the hardness and as a results, the wear resistance of the coating is improved [183]. Moreover, organic compounds used with the aim of wear properties improvement of the composite coatings prepared by nickel matrix and boron particles have an important effect on increasing the amount of boron in the coating and developing the nickel matrix structure. The wear test results without lubrication are very diversified (Fig. 34)[184]. Ni B composite with the highest boron content (5.7 at.%) has the lowest wear resistance. In addition, a great modification in the wear resistance of the coating formed in the presence of an organic compound additive is observed compared to the coating without additives. However, in the presence of ASB (1-Octanesulfonicacid sodium salt) surface-activating compounds, similar results to that of the coating without additives are achieved. The optimum wear resistance is obtained for a coating with WFK1 ([3-[[(Heptadecafluorooctyl) sulfonyl]amino]-propyl]trimethylammonium iodide) surfactant with the concentration of 0.11 mm [184]. Fig. 33 The average coefficient of friction and abrasion index of Ni Mo alloy coatings electrodeposited by different pulse frequencies, A: 0 Hz, B: 10 Hz, C: 100 Hz and D: 1000 Hz [176] Fig. 34 The dependence of wear without lubricant of Ni B composite coating on the concentration and type of organic additives [184] The wear resistance of Ni P alloys can be improved by embedding tungsten. However, there is no proper understanding of how embedding affects corrosion-wear alloy properties. In this regard, a comprehensive research has been performed on Ni W P alloy and the corrosion-wear properties of this alloy, at the primary or thermally treated modes by ball-on-disc, in deionized water and 3.5 wt% sodium chloride solution. The heat-treated Ni W P alloy specimen at 400 C has the minimum wear rate in deionized water and 3.5 wt% sodium chloride solution. The specimen presents point corrosion and abrasive wear in 3.5 wt% sodium chloride solution. The non-heat-treated specimen in 3.5 wt% sodium chloride solution has a uniform corrosion and the main wear process is corrosive and presents adhesive wear. In both heat-treated and non-heattreated specimens, the wear is created by abrasion. The COF of Ni W P alloy treated in water and sodium chloride solution is shown in Fig. 35 [185]. As shown in the figure, the changes in COF of the specimens in water has more fluctuations. It can also be seen that the COF of all the specimens has an increasing trend in corrosive conditions. The corrosive solution reduces the friction between the

33 Trans Indian Inst Met (2018) 71(2): Fig. 35 The coefficient of friction of heat treated Ni P-W alloys at 400 C and without heat treatment in deionized water and NaCl 3.5 wt% [185] Fig. 36 The effects of heat treatment temperature on the wear substrate and corrosive surface, leading to the reduced COF. On the other hand, because of the chemical reaction occurring at the same time, the surface roughness increases in sodium chloride solution. This mutual interaction of these factors result in complex changes in COF [185]. In addition, to improve the surface properties, the SiO 2 and CeO 2 nanoparticles are codeposited in Ni P W alloy coating. The composite coatings are deposited on the surface of steel with simultaneous deposition of nickel, phosphorus, tungsten, SiO 2 and CeO 2 nanoparticles of electrodeposition bath in which nanoparticles are suspended by fast mechanical stirring. Results show that these nanoparticles are equally dispersed among Ni W P alloy coating and the link between the metal matrix and nanoparticle is compressed. Hence, the codeposition of nickel, tungsten, phosphorus, and nanoparticles lead to the fabrication of a uniform composite coating Ni P W SiO 2 CeO 2. As shown in Figs. 36 and 37, this coating presents superior wear properties compared to Ni W P SiO 2 and Ni W P CeO 2 coatings with a reduced wear from 3.76 to 0.78 mg/(cm 2 h), provided that it is heat-treated for 3 h at 400 C [186]. Also, in another study, the wear rate of Ni W P alloy coating and composite coatings Ni W P SiC has been studied. The results of wear weight loss show that composite coatings have the lowest weight loss and they demonstrate more satisfactory wear resistance than alloy coatings [187]. The Ni P W multilayer coatings deposited on mild steel are made of Ni P W triple layers with the low and high amounts of tungsten. Using the 2-110N vertical loads and the sliding velocity of cm/s, Stribeck curves are obtained. These curves indicate three modes of lubrication (elastohydrodynamic, mixed, and boundary) regimes. According to the results in elastohydrodynamic mode, no wear is observed. In the mixed mode, a mild wear occurs resistance of heat treated Ni W P-SiO 2 /CeO 2 composite coatings after 1 h [186] Fig. 37 The effect of heat treatment on the hardness of the heat treated Ni W P-SiO 2 /CeO 2 composite coating [186] by microcutting mechanism, whereas in the boundary mode, a severe wear occurs by an additional mechanism including single brittle layers fracture of the multilayer coating. Wear resistance additives reduces the wear rate significantly by reacting with the metal surface. If all test parameters remain constant, the wear coefficient is determined by sliding velocity in the mixed mode while the wear volume largely depends on the loading [188]. Ni Mo Co ternary alloy coatings are also considered as an alternative to hard chrome (Fig. 38) [189]. Results show that the wear rates and COF of heat-treated Ni Mo Co alloy coating at 400 C are better than that of hard chrome. The results show that the mass loss of Ni Mo Co alloy coating is lower than that of Ni Mo alloy coating while the

34 290 Trans Indian Inst Met (2018) 71(2): Fig. 38 The coefficient of friction changes with the wear test time for pure nickel, Ni Mo and Ni Mo Co without heat treatment [189] average dynamic COF of Ni Mo Co ternary alloy coatings (0.5) is more than the one for Ni Mo alloy coating (0.3). In other words, embedding a small amount of cobalt to Ni Mo alloy coating improves its wear resistance. In addition, it is observed that the heat-treated specimen at 400 C has more desirable wear properties than the one heat-treated at 600 C. The comparison between the wear properties of ternary nickel cobalt-molybdenum alloy coating with hard chrome shows that the more satisfactory wear properties of this alloy are because of the formation of Ni 4 MoO 2 on heat-treated Ni Mo CO coating [189]. Additionally, nanocomposite coatings reinforced with silicon carbide particles Ni Zn P/nano-SiC have a lower weight loss than Ni Zn SiC coating with the same wear rate [190]. Ni B Zn ternary alloy coatings are widely used for wear applications because of the developed small cauliflower-form packets that retain lubricants and therefore reduce wear [191]. The research results on multilayer Ni Fe and Ni Fe (nano Al 2 O 3 ) coatings show that the wear resistance of the layers prepared by two variable frequencies at fixed duty cycle is more than the layers prepared in two variable duty cycles and constant frequency [192]. The results obtained from related studies about other nickelbased alloy coatings are given in Table Summary, Future Trends and Prospects Nickel based alloys containing other metals such as refractory metals (molybdenum or tungsten) characterized by high hardness, corrosion, wear resistance, and temperature are widely used in industrial applications because they are considered as important alternatives for toxic hard chromium coatings. Moreover, results show that the incorporation of other particles in the nickel and nickel alloy matrix depending on the nature of the particles improves surface properties and wear resistance of the coatings. In the field of wear studies, depending on the type of applied particles, composite coatings can be used for high wear resistance of lubricant coating purposes. Composite coatings with high wear resistance are used by hard particles such as carbides and oxides for example WC, SiC, and Al 2 O 3. According to the studies conducted on the incorporation of oxide, carbide, non-oxide, etc. into the nickel matrix, a harder reinforcement phase is embedded into a ductile matrix, leading to the reduced ductility of the matrix material in the contact surface and the reduced matrix abrasion. Also, some reports have attributed to the differences in wear behavior as a result of incorporation of these particles to the changes in crystallographic directions. Some of these nanoparticles can improve the wear properties by dispersion strengthening and strengthening by grain modification, change of nickel matrix morphology, and the change of preferred grain growth orientation into a compressed direction. Under some conditions such as heat treatment, some intermetallic compounds are created that improve the wear properties through solid solution strengthening. The particles can reduce contact surface between the matrix metal and abrasive object and act as a passive layer that ultimately reduces the COF of the coating. Besides, by increasing particular elements such as cobalt in the nickel matrix, the coating structure changes from fcc to hcp, leading to the improved wear performance of alloy coatings. Different operational conditions such as application of current (DC, PC, etc.), use of surfactant, the number of alloy and composite elements within electrolytic bath, the use of CO 2 bath, the size of hard particles (micrometers/nanometers), heat treatment, and lubricating nature of particles (CNT, etc.) can be effective in the incorporation of second phase elements that ultimately determines the wear properties of the composites or alloy coating. Hard chrome coatings have found a wide application in many industries such as aircraft and oil well equipment because of their remarkable features including high hardness ( Hv), low friction coefficient (0.16 in lubrication mode and 0.21 in dry mode), and high resistance to abrasion and corrosion. However, in recent decades, the changes in health and environmental laws have forced many workshops providing hard chrome and decorative chrome to leave their business. Hard chrome plating that has long been used, usually generates hexavalent chromium ions. Studies have proven its hazardous effect, especially for those who work in chrome plating workplaces, as Cr(VI) ion can lead to lung cancer and other serious issues. Therefore, this type of coating is influenced by the emergence of rival coating processes. Among several attempts made in the synthesis of non-toxic materials, electrodeposition of nickel-based composite and alloy coatings has been one of the most frequently used techniques. It is proven that significant improvements in wear

35 Trans Indian Inst Met (2018) 71(2): Table 14 Summary of results of research conducted on other nickel-based alloy coatings Coating composition element content inside coating Particle percentage inside coating Thickness (lm) Particle size (nm) Hardness (HV) Wear rate Counter body COF Roughness (R a )(lm) References Ni Cr N.R. N.R. 16 N.R. N.R. Pin on disk mg/n m Ni Mo wt% N.R. 40 N.R ± Ball on disk mm 3 / Nm Ni B 2 11 wt% N.R. N.R. N.R Ball on disk mm 3 * Ni B SiC N.R wt% , weight loss mg/n* Ni W P W: 29.9 wt% P: 2.85 wt% Ni W P- W: 4.6 wt% SiO 2 - P: 9.3 wt% CeO2 N.R. 50 N.R Ball on disk mg/n* SiO 2 : 8.2 wt% CeO 2 : 7.1 wt% N.R. SiO 2 :30 CeO 2 : M-2000 abrasion machine mg/ N.cm 2 * Ni P-W N.R. N.R. 25 N.R. N.R. Pin on disk mm 3 / Nm Ni Mo Co Mo: 8.12 at.% Co: 6.67 at.% N.R N.R. 730 ± 30 Pin on disk mm 3 / Nm Steel ball AISI N.R. [201] Silicon nitride [176] Ruby N.R. [178] N.R. N.R. N.R. [175] N.R N.R. [185] N.R. N.R. N.R. [186] CALMAX hardened steel hemispherical mild steel pin N.R. [188] N.R. [189] * Wear sliding distance was not reported

36 292 Trans Indian Inst Met (2018) 71(2): Fig. 39 Estimated usage of nickel-based tribo-coatings in various industries properties of nickel/nickel alloy based coatings can be obtained by incorporation of hard nano- or micro-particles (i.e. oxides, carbides, diamonds, etc.) in the nickel/nickel alloy matrix. The volume percentage of these particles affect the ductility of the coating. By increasing the content of reinforcing phase, material removal as a result of severe wear conditions becomes slow because the particles resist the plastic deformation and provide the nickel matrix with high protection. In contrast, when the content of reinforcing particle increases considerably (above 50 vol%), the metal matrix acts as a binder which binds the ceramic particles together. In this case, the wear behavior of the composite approaches to that of the hard particles. These coatings with a high content of hard particles exhibit superior resistance to erosion and corrosion. Besides, they have the beneficial synergy of toughness and hardness. The ceramic particles supply the erosion resistance while the metallic phase provides the ductility of the coating. There are several applications in which both erosion and corrosion features are concerned, including the offshore components and structures. Prolonging the lifetimes for these parts is of great importance because it reduces maintenance costs and improves the safety situations. The most promising coatings of this kind are nickel-based composite coating with a high percentage of hard nanoparticles. Recently, a study conducted on sediment co-electrodeposition of diamond particles from Ni W plating bath has evolved the existing area of the literature [193]. The uniform distribution of 64 wt% diamond content in the matrix has led to the high hardness of the coating around 2000 Hv. In future, it is expected that, by increasing the degree of hard particles embedment in the electrochemical deposition of nickel coatings, these coatings become an acceptable alternative in abrasion-resistant industrial applications. Figure 39 estimates the usage of components used in various industries that can be replaced by nickel-based composite coatings for improvement of wear properties. References 1. Su F-h, Liu C-s, and Huang P, Appl Surf Sci, 258 (2012) Du L, Xu B, Dong S, Yang H, and Tu W, Wear 257 (2004) Farhat Z N, Mater Charact 60 (2009) Makhtar A K, Yussof H, Al-Assadi H, Yee L C, Sahab A R M, Saad N H, Kasolang S, and Saedon J, Procedia Eng 41 (2012) Luo L, Yao J, Li J, and Yu J, Ceram Int 35 (2009) Bolelli G, Berger L-M, Bonetti M, and Lusvarghi L, Wear 309 (2014) Bose K, Wood R J K, and Wheeler D W, Wear 259 (2005) Grzesik W, Zalisz Z, Krol S, and Nieslony P, Wear 261 (2006) Low C T J, Bello J O, Wharton J A, Wood R J K, Stokes K R, and Walsh F C, Surf Coat Technol 205 (2010) Bertoncello R, Furlanetto F, Guerriero P, and Musiani M, Electrochim Acta 44 (1999) Musiani M, Electrochim Acta 45 (2000) Moussavi G, Jiani F, and Shekoohiyan S, Sep Purif Technol 151 (2015) Yagi S, Murase K, Hirato T, and Awakura Y, J Electrochem Soc 154 (2007) D Weston D, Shipway P, Harris S, and Cheng M, Wear 267 (2009) 934.