tribology of electroless nickel coatings – a review

Materials and Design 32 (2011) 1760–1775

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Materials and Design

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Review

Tribology of electroless nickel coatings – A review

Prasanta Sahoo ⇑, Suman Kalyan DasDepartment of Mechanical Engineering, Jadavpur University, Kolkata 700 032, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 September 2010Accepted 5 November 2010Available online 11 November 2010

Keywords:Non-ferrous metals and alloys (A)Coatings (C)Corrosion (E)Wear (E)

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.11.013

⇑ Corresponding author. Tel.: +91 33 2414 6890; faE-mail addresses: [email protected], psjume

Electroless coating is different from the conventional electrolytic coating as the former does not requireany electricity for its operation. The advantages include uniform coating and also nonconductive mate-rials can be coated. Electroless nickel coatings possess splendid tribological properties such as high hard-ness, good wear resistance and corrosion resistance. For this reason, electroless nickel has found wideapplications in aerospace, automobile, electrical and chemical industries. Quest for improved tribologicalperformances has led many researchers to develop and investigate newer variants of electroless nickelcoatings like Ni–W–P, Ni–Cu–P, Ni–P–SiC, Ni–P–TiO2, and so on. Also the enhancement of tribologicalcharacteristics through modification of the coating process parameters has remained a key point of inter-est in researchers. The technological advancement demands the development of newer coating materialswith improved resistance against wear and tear. Electroless nickel has shown huge potential to fit in thatspace and so the study of its tribological advancement deserves a thorough and exhaustive study. Thepresent article reviews mainly the tribological advancement of different electroless nickel coatings basedon the bath types, structure and also the tribo testing parameters in recent years.

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1. Introduction

In the middle of the twentieth century, a revolutionary methodof coating technique was developed by Brenner and Riddell [1].Popularized as the ‘‘Electroless Coating’’, the method did not re-quire any electricity. Since then it has evolved into a mature sub-ject of research and development today due to its wide range ofapplications. Electroless nickel coatings are the more popular var-iant of electroless coatings which possess some distinct collectionof properties (Table 1). Electroless coatings find their use in almostevery domain. From simple knitting needles to the mighty aero-space applications, their range of applications is continuouslybroadened. Primary uses for electroless nickel coatings are shownin Fig 1. However, the main applications of electroless nickel coat-ings are based on its properties viz. wear resistance and corrosionresistance. Moreover, some recent uses of electroless coating in-clude application in MEMS, electromagnetic interference (EMI),in powder metallurgy, as membrane reactors, minimizing foulingin heat exchangers and reduction of bacterial adhesion.

Few metal coatings applied by electro-coatings can match thethickness uniformity of an electroless nickel finish. Because theseprotective coatings are chemically applied, they create depositsof highly consistent depth across all surfaces, including edgesand complex interior geometries. Each option in the electrolesscoatings family also delivers bonus properties that improve com-

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ponent performance. Electroless coating can be broadly classifiedinto three categories viz. alloy and poly-alloy coatings, compositecoatings and pure metallic coatings.

2. Coatings for tribology based applications

2.1. Alloy and poly-alloy coatings

The incorporation of additional metal elements into the electro-less deposits can be an important means of enlarging the range ofchemical, mechanical, physical, magnetic, and other propertiesattainable. A number of alloys can readily be deposited by combin-ing metals that are independently deposited electrolessly fromsimilar baths; an example being nickel and cobalt from alkalinehypophosphite solutions. Also, and more importantly, certain met-als that cannot themselves be deposited by the autocatalytic mech-anism can be induced to co-deposit with an electrolesslydepositing metal. For tribology based applications several binary,ternary and other poly-alloys have been described in the literaturewhich includes:

� Binary alloys� Ni–P [2–13]� Ni–B [14–19]� Ternary alloys� Ni–P–B [14,20],� Ni–W–P [2,21]

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Table 1Properties of electroless nickel coatings (Source: www.macdermid.com).

Feature Benefit

Excellent corrosion resistant Good coating durabilityHigh hardness Low wear characteristicsLow friction co-deposits available Self-lubricating coatingDeposit uniformity Eliminates post plate finishGood brightness Attractive finishFast plating rate High production outputGood chemical resistance Acts as protective coatingSolderability/weldability/brazeability Functional in many applicationsNon-magnetic/magnetic Magnetic property selectivity

Fig. 1. Primary uses for electroless nickel deposits (Source: www.pfonline.com).

Table 2Electroless nickel bath composition and their functions.

Component Function

Nickel Ion Source of metalReducing agent Source of electronsComplexants Stabilizes the solutionAccelerators Activates reducing agentBuffers Controlling pH (long term)pH regulators Regulates pH of solution (short term)Stabilizer Prevents solution breakdownWetting agents Increases wettability of the surfaces

Table 3Reducing agent for EN plating [68].

Deposit Reducing agent Remarks

Ni–P Sodium hypophosphite(NaH2PO2)

Acid or alkaline bath (2–17% P)

Ni–B Sodium borohydride (NaBH4) Acid or alkaline bathAminoborane (DMAB) Alkaline bath (0.5–10% B)

Only Ni Hydrazine (NH2NH2) Alkaline bath

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� Ni–Cu–P [21–23]� Ni–Co–P [24,25]� Ni–P–Sn [26]� Quaternary alloys� Ni–W–Cu–P [21]

2.2. Emphasis on composite coatings

Although electroless nickel alloy coatings can serve a lot of pur-pose, the quest for improved properties such as higher hardness,lubricity, anti-sticking and anti-wear properties has led to theincorporation of many soft and hard particles in the matrix ofthe electroless nickel. Choice of the particles depends on the spe-cific property that is desired. In the field of tribology, electrolessnickel based composite coatings can mainly be divided into twocategories, i.e., lubricating composite coatings and wear-resistantcomposite coatings, according to the types of the doped inorganicand/or organic particulates. The electroless Ni�P based lubricatingcomposite coatings usually contain co-deposited solid lubricantssuch as WS2, MoS2, PTFE (poly tetra fluoro ethylene) and graphite[27] and they usually have a reduced friction coefficient as com-pared with electroless Ni�P coating. Similarly, the wear-resistantcomposite coatings usually have co-deposited hard particles suchas WC, SiC, Al2O3, B4C and diamond, and they usually have in-creased hardness and wear resistance as compared with electrolessNi�P coating [28].

The preparation of electroless nickel based composite coatingswith excellent comprehensive properties is highly dependent onthe stable dispersion of the nanoparticles in plating bath, other-wise the so-called composite coatings would have non-uniformlydistributed particulates and numerous defects, owing to the segre-gation and agglomeration of the nanoparticles with high surfaceenergy and activity in the plating bath [29]. Fortunately, this canbe conveniently realized by capping the nanoparticles with spe-cially selected surface-modifying agents [28].

Various composites coatings has been developed which include;Ni–P–SiO2 [28,30], Ni–P–Al2O3 [31–38], Ni–P–SiC [27,39–42], Ni–P–B4C [43,44], Ni–P–Diamond [34], Ni–P/cenosphere [45], Ni–P–MoS2 [46,47], Ni–P–PTFE [10,27,48–50], Ni–P–BN [51], Ni/CNT[52], Ni–P–CNT [53,54], Ni–P–WC [55], Ni–P–Si3N4 [42,56], Ni–P–Gr–SiC [57], Ni–P–ZnO [58], Ni–P–K2Ti6O13 whiskers [59], Ni–P–ZrO2 [60,61], Ni–P–PTFE–SiC [62], Ni–P–TiO2 [63,64], Ni–Mo–P/PPS [65], Ni–P–CNT–SiC [66], Ni–P–Cr2O3 [67].

3. Electroless nickel bath characteristics and role of individualcomponents

Electroless plating is an autocatalytic process where the sub-strate develops a potential when it is dipped in electroless solutioncalled bath that contains a source of metallic ions, reducing agent,complexing agent, stabilizer and other components. Due to thedeveloped potential, both positive and negative ions are attractedtowards the substrate surface and release their energy throughcharge transfer process. The components of electroless nickel bathand their functions are given in Table 2.

3.1. Reducing agents

Mainly two types of baths have been used for depositing Ni–Balloy viz. acidic and alkaline baths depending on the reducingagent used which is listed in Table 3.

3.1.1. Sodium hypophosphiteHypophosphite baths are the most common types of commer-

cially used electroless nickel baths due to higher deposition rates,increased stability and greater simplicity of bath control. Themechanism of the electroless Ni–P deposition reactions takingplace is still not well understood. However, the most widely ac-cepted mechanisms are illustrated by the following equations [5].

(a) Electrochemical mechanism, where catalytic oxidation ofthe hypophosphite yield electrons at the catalytic surface whichin turn reduces nickel and hydrogen ions, is illustrated below:

H2PO�2 þH2O! H2PO�3 þ 2Hþ 2e� ð1Þ

Niþþ þ 2e� ! Ni ð2Þ

2Hþ þ 2e� ! H2 ð3Þ

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1762 P. Sahoo, S.K. Das / Materials and Design 32 (2011) 1760–1775

H2PO�2 þ 2Hþ þ e� ! Pþ 2H2O ð4Þ

(b) Atomic hydrogen mechanism, where atomic hydrogen is re-leased as the result of the catalytic dehydrogenation of hypo-phosphite molecule adsorbed at the surface, is illustrated below:

H2PO�2 þH2O! HPO��3 þHþ þ 2Hads ð5Þ

2Hads þ Niþþ ! Niþ 2Hþ ð6Þ

H2PO2 þHads ! H2Oþ OH� þ P ð7Þ

The adsorbed active hydrogen reduces nickel at the surface ofthe catalyst.

ðH2PO2Þ2� þH2O! Hþ þ ðHPO3Þ2� þH2 ð8Þ

Simultaneously, some of the absorbed hydrogen reduces a smallamount of the hypophosphite at the catalytic surface to water, hy-droxyl ion and phosphorus. Most of the hypophosphite present iscatalytic, which is oxidized to orthophosphite and gaseous hydro-gen, causing low efficiency of electroless nickel solutions for alloycoating while the deposition of nickel and phosphorus continues.Several other hypotheses regarding reaction mechanism has beenput forward by various researchers [69].

3.1.2. Sodium borohydrideThe borohydride ion is the most powerful reducing agent avail-

able for electroless nickel plating. The reduction efficiency of so-dium borohydride is much higher than that of dimethylamineborane and sodium hypophosphite [18]. It can provide up to eightelectrons for reduction of some metals as opposed to two electronsthat can be provided by sodium hypophosphite, for the same reac-tion. Besides the high reduction efficiency, borohydride reducedbaths are preferred to dimethylamine borane based baths in termsof cost-effectiveness of operation. However, borohydride ionshydrolyze readily in acid or neutral solutions and will spontane-ously yield nickel boride in presence of nickel ions in the platingbath. Hence, control of pH is important to avoid the spontaneousdecomposition of the bath solution and to decrease the cost ofoperation. If the pH of the solutions is maintained between 12and 14, the formation of nickel boride is suppressed and the reac-tion product is principally elemental nickel. Hence it is generallyused as alkaline baths.

The properties of sodium borohydride-reduced electroless nick-el coatings are often superior to those of deposits reduced withother boron compounds or with sodium hypophosphite. The prin-cipal advantages of borohydride-reduced electroless nickel depos-its are its hardness and superior wear resistance in the as-deposited condition.

The corresponding reactions [70] are as follows:Borohydride oxidizes as in the following reactions during the

electroless deposition:

BH�4 þ 4OH� ¼ BO�2 þ 2H2Oþ 2H2ðgÞ þ 4e� ð9Þ

BH�4 ¼ Bþ 2H2ðgÞ þ e� ð10Þ

Corresponding reduction reactions in the coating bath can beexpressed as follows:

Ni2þ þ 2e� ¼ Ni ð11Þ

2H2Oþ 2e� ¼ 2OH� þH2ðgÞ ð12Þ

Reaction (9) can be considered as the main reaction that drivesthe nickel ion reduction [reaction (11)] due to the release of fourelectrons. Reaction (10) is responsible for the presence of depositedboron in the film structure. Therefore, very roughly weight gain orcoating rate [deposition of metallic nickel: reaction (11)] can be as-

sumed as being determined by reaction (9) and the boron contentof the deposited film can be assumed as being determined by reac-tion (10). If any factor influences the rate of reactions (9) and (10)at a same magnitude, the change in the coating rate must be muchhigher than that in boron content of the deposited film due to thefourfold release of electrons in reaction (9). There may even be acompetition between reaction (9) and reaction (10) because BH�4oxidizes through two different reactions at the same time. Finally,direct reduction of water [reaction (12)] takes place in the coatingbath which is strongly alkaline aqueous solution.

3.2. Complexing agents

There are three principal functions that complexing agents per-form in the electroless nickel plating bath [69] :

� They exert a buffering action that prevents the pH of the solu-tion from falling too fast� They prevent the precipitation of nickel salts, e.g., basic salts or

phosphites.� They reduce the concentration of free nickel ions by forming

meta-stable complexes

Moreover, the complexing agent also influences the reactionmechanism and deposition rate and hence the deposit. Generallyall complexing agents follow an inverted bell shaped curve whentheir concentration is compared with the deposition rate i.e. thereis a certain concentration when the deposition rate is maximum.The deposition rate gradually increases up to that optimum con-centration and then falls. In case of reduction by hypophoshite,the optimum concentration for sodium citrate which acts as thecomplexing agent is about 30 g/l [5]. Ethylenediamine is the popu-larly used complexing agent in case of reduction by borohydride.The optimum concentration for ethylenediamine is found to be90 g/l [70]. Ammonium fluoride improves the deposition rate andthe buffering capability of Ni–P bath [71].

Generally the complexing agents are made of organic acids ortheir salts viz. acetate, succinate, propionate, citrate, etc. The inor-ganic compounds used as complexing agents are the pyrophos-phate anion and the ammonium ion [69].

3.3. Effect of surfactants

Surfactants are wetting agents that lower the surface tension ofa liquid, allowing easier spreading, and lower the interfacial ten-sion between two liquids or a liquid and solid surface. In an elec-troless nickel bath, presence of surfactant promotes the coatingdeposition reaction between the bath solution and the immersedsubstrate surface.

Elansezhian et al. [72] studied the effect of two surfactants viz.sodium dodecylsulfate (SDS) and cetyl trimethyl ammonium bro-mide (CTAB) on the surface topography of electroless Ni–P coating.It was found that surface finish of the coated layer significantly im-proved when the concentration of the surfactant exceeded about0.6 g/l. But at the lower levels of concentration the surface finishwas found to be poor [8] (Fig 2). Also hardness of deposits in-creased with the addition of the surfactants. Also by adding theabove two surfactants during deposition of Ni–P, the phosphoruscontent had gone up resulting in improved quality of the deposits.Particularly it improved the corrosion resistance of the coating.

Influences of anionic surfactant (sodium dodecyl benzene sul-phonate, sodium lauryl sulphate) and cationic surfactant (trietha-nolamine) on deposition of Ni–P–nanometer Al2O3 compositecoatings were investigated. Deposition using cationic surfactant(triethanolamine) showed fast deposition rate, good abrasion resis-tance and uniform dispersion of Al2O3 particles [38]. Cheong et al.

Fig. 2. Average surface roughness of EN deposits vs. concentration of: (a) SDS, (b)CTAB [8].


[73] studied the effect of stabilizer on the properties of magnesiumalloys and observed that addition of stabilizers increased the corro-sion resistance of the coating.

Fig. 3. Role of power density of laser on alloying depth [77].

4. Effect of heat treatment

Heat-treatment is an important factor that affects the thickness,hardness, structure and morphology of deposit [74]. Generallyacknowledged optimal heat treatment regime is 400 �C/1 h as it re-sults in maximal hardness of electroless nickel coatings. The hard-ness increase is attributed to the crystallization of nickel and to theprecipitation of fine particles of Ni3P phase. Use of higher heat-treatment temperatures and longer times leads to the progressivehardness decrease, which can be attributed to the nickel graingrowth and to the phosphides coarsening. In case of electrolessNi–B coatings, heat-treatment (350 and 450 �C for 1 h) results inthe transformation of the amorphous phase to crystalline nickeland nickel boride (Ni3B and Ni2B) phases. Annealing at tempera-tures higher than 450 �C, results in the growth of crystalline nickeland conversion of Ni2B phase to the more stable Ni3B phase [18].

Heat treatment also has profound impact on the corrosion resis-tance of electroless nickel coatings. It is invariably found that asdeposited coatings exhibit the best corrosion resistant propertiesdue to their amorphous structure. But with heat treatment, the

corrosion resistance of the coating gradually decreases. This isattributed to the advent of crystallinity in the coating due to heattreatment. Crystallinity increases the grain boundaries, which formactive sites for corrosion attack [14]. However, a recent study pro-poses a new kind of thermochemical treatment, which is found toproduce better corrosion resistance in electroless Ni–B coatingscompared to normal heat treatment [75]. This effect is believedto come from the chemical surface modification due to nitrogendiffusion.

5. Special treatments of the coating

5.1. Charging and outgassing of hydrogen

The effects of charging and outgassing on friction and wear ofNi–P amorphous and nano crystalline coating have been studiedby Zhou et al. [7]. It is seen that atomic hydrogen enriching onthe surface lubricates the surface in contact, resulting in reducingthe friction coefficient and improving the wear durability. Thewear durability is found to be reversible as it can be restored afteroutgassing.

5.2. Laser treatment

The application of optical engineering in material processinghas already received tremendous acceptance. The range of surfacetreatment processes available goes all the way from transforma-tion hardening, annealing, shock hardening and bending (as pro-cesses which do not involve melting) to processes which involvemelting such as surface melting, surface alloying, surface claddingand those processes which involve some form of photochemistrysuch as laser chemical vapour deposition, laser physical vapourdeposition and stereo lithography. The principal laser techniquesemployed for the production of corrosion resistant metallic sur-faces are laser surface melting (LSM), laser surface alloying (LSA)and laser cladding. LSM is used [76] to improve the friction andwear property of electroless Ni–P coating. The low friction wasassociated with the formation and dispersion of Ni3P crystals thatprevented adhesion at friction. LSM is also employed to improvethe hardness and wear behavior of composite coatings like Ni–P–nanoAl2O3 [31,35]. The transformation of the coating from non-crystalline state to crystalline state is mainly responsible for thisimprovement of the properties. Gordani et al. [77] have appliedLSA to obtain a good metallurgical bonding between aluminium al-loy substrate and Ni–P coating. The effect of alloying depth as afunction of power density of laser is shown in Fig. 3. Both surfacehardness and corrosion resistance is found to increase by this


process. Moreover, the best hardness and corrosion resistance isfound to develop at a laser scanning rate of 37 mm/min [78].

5.3. Ion implantation

The ion implantation is a post deposition surface treatmenttechnique in which ions of a material can be implanted into thecoating, thereby changing its physical properties. Ions of titaniumare implanted in electroless Ni–P coating developed over stainlesssteel in order to improve its corrosion resistance properties [79].The improvement in corrosion behavior is believed to be due tothe formation of TiNi alloy in the implanted layer. For the Ti ion-implanted electroless nickel, the increase of the hardness is re-sulted from the formation of TiNi alloys in the implanted layer,which provides a strain hardening effect via lattice mismatch.The nano hardness of the Ti ion-implanted electroless nickel ismuch higher than those of sputtered TiNi coatings and Ni- im-planted TiNi coatings.

5.4. Nitridation

Nitridation involves heat treatment of a material in the pres-ence of nitrogen atmosphere. Vitry et al. [80] found an increaseof up to 1500HV100 in micro-hardness of Ni–P coatings subjectedto vacuum nitridation.

5.5. Heat treatment in active atmosphere

The electroless Ni–P coatings have been subjected to heat treat-ment under neutral and active atmospheres, among which the la-ter induced enhanced case hardness, wear resistance and lowerfriction coefficient [81].

6. Effect of substrate roughness

Electroless nickel coatings follow the surface profile of the sub-strate on which it is deposited. Hence, the roughness of the sub-strate has a profound impact on the tribological properties of thecoating.

It was found that roughness of the substrate affected the filmcomposition that can be explained as roughening of the GFRP sub-strate [3], resulting in more active sites, which was favorable forthe adsorption of hypophosphite and nickel ions and responsiblefor the autocatalysis reaction:

Ni2þ þ 4H2PO�2 þH2O ! Niþ 3H2PO�3 þ PþHþ þ 3=2H2 ð13Þ

The precipitation of catalytic nickel progressively increasesfavoring phosphorous co-deposition. The phosphorous contentsdecrease when nickel ion concentration increases, on the otherhand, the phosphorous contents are directly proportional to thefraction of the active surface covered by hypophosphite ions [3,82].

The micro-hardness of electroless nickel coatings significantlyincreased in magnitude when the surface roughness of the GFRPand CFRP substrate was P0.3 lm [3,82]. The surface roughnessof electroless Ni–P coatings decreased as surface roughness ofthe CFRP substrate increased and as P content increased. Moreoverit was found that the corrosion resistance of the ENP coatings in-creased with low surface polishing condition of substrate [3,82].Electroless nickel deposits become more compact and defect-freeas the surface roughness of Mg alloy based substrate increases[83]. Mechanical roughening of the substrate can improve theadhesion of coatings as seen by scratching test. Studies showedthat nickel plating on the rough substrate has a higher frictioncoefficient than that on the polished surface [84].

7. Evaluation of the coatings

7.1. Hardness

Hardness is the material property that is of high importance forpractical applications. Surface hardness is especially critical for usein components and semi-finished products in order to control wearand tear processes. This makes hardness a characteristic of materi-als that determines the safety and function of technical systemsand constructions. This is why international research efforts inthe field of hard materials, with its integral importance withinthe industrial value chain, are continuously looking to develop cus-tom hard materials. Electroless nickel has already established as ahard coating for tribology based applications. Typically electrolessnickel deposit is too thin for reliable surface testing, resulting inthe reading being influenced by the substrate. Hence, the hardnessof the coatings is evaluated by applying very low load in micro-hardness testers.

The hardness of electroless Ni–P and Ni–B coatings are depen-dent on the phosphorous and boron content respectively. Howeverthe hardness of electroless Ni–P increases with decrease of phos-phorous content while the hardness of Ni–B increases with the in-crease in boron content of the coating [70] Yan et al. [4] have foundthat the hardness of the coating at first increases with the increasein phosphorous content up to 8% and then decreases (Fig 4). Themaximum hardness attained by the as deposited coating wasfound to reach a value of 910 HV0.1.

Hardness of electroless coating is found to increase with theannealing temperature up to a certain temperature above whichthe hardness is found to decrease [15]. Ni–P, which is a supersatu-rated alloy in as-deposited state, can be strengthened by precipita-tion of nickel phosphide crystallites with suitable heat treatments.The phosphides act as barriers for dislocation movement, therebyincreasing the hardness further. However, the hardness of Ni–Pfilms degrades with excessive annealing due to grain coarsening[2] leading to surface brittleness and enhanced dislocation propa-gation [28].

The principal advantages of borohydride-reduced electrolessnickel deposits are its high hardness and superior mechanical wearresistance in the as-deposited condition. Electroless Ni–B coatingsare more wear resistant than tool steel and hard chromium coat-ings and it can replace gold in electronic industries [14]. Parametricoptimization of the deposition conditions has been carried out toimprove the hardness of Ni–B coatings [15] and the maximumhardness is found to be about 1400 HV0.1.

Nickel–boron coatings on aluminium alloy possess a hardnessof 800 HV0.1. Heat treatments of those coatings under a neutralatmosphere (95% Ar + 5%H2) allow an increase of the hardnesscaused by the crystallization of Ni, Ni2B and Ni3B phases but no dif-fusion at the interface occurs. The maximum hardness is 1300HV0.1. Nano-indentation hardness profiles on treated and un-treated samples show that the hardness stays homogeneous acrossthe coating [17]. The samples of Ni–B coating were then submittedto pure nitrogen vacuum nitridation. An increase of hardness up to1500 HV0.1 was observed for nitrided samples [80]. Krishnaveniet al. [18] studied the relationship between the micro-hardnessof the electroless Ni–B coatings with heat-treatment temperatureand reported two maxima (Fig. 5) in the hardness vs. heat-treat-ment temperature curve, one at 350 �C and the other one at450 �C. Beyond 450 �C, coatings began to soften as a result of con-glomeration of the Ni3B particles and the hardness decreases.

The hardness of the coating increases with the amount of tung-sten in the deposit. This is due to the solid solution strengtheningof the nickel matrix by the dissolved tungsten. Heat treatment ofelectroless Ni–P coatings and electroless Ni–P–B4C compositecoatings results in an increase in the coating hardness, with a

Fig. 5. Change of hardness of electroless Ni–B deposit as a function of heat-treatment temperature [18].

Fig. 4. Effect of phosphorous content on: (a) Micro-hardness of Ni–P coating and (b)Wear volume [4].


maximum hardness being achieved after heat treatment at 400 �Cfor 1 h. The main reason for the increase in hardness is the forma-tion of Ni3P and Ni5P2 hard phases from the amorphous phase [44].Peak hardness of ternary alloys is found to be more at annealingtemperature of 500 �C compared to 400 �C in case of binary Ni–Palloys. It was found that inclusion of ZrO2 particles into the Ni–W–P coatings further increase the hardness of the coating [85]. Ithas been found that the introduction of hard particles leads to in-crease in the mechanical and physical properties of electroless Ni–P alloy coating. Micro-hardness of Ni–P coatings increases with theincorporation of SiC nano particles and reach maximum value afterheat treatment [39]. Electroless Ni–CNTs composite coatings exhi-bit a remarkable hardness value, which is more than Ni–SiC coat-ings. Such a micro-hardness value, displayed by CNTs reinforcedcomposite deposits, is quite remarkable, indicating the efficientload transfer of CNTs in the composite. Ni–P–SiC exhibited highesthardness among Ni–P and Ni–P–Gr coatings [57]. The lamellarstructure of graphite made deformation of the coating easier whenthe coating was pressed and caused the low bearing. It is foundthat micro-hardness of EN–PTFE decreased with the increase inPTFE content [86]. The nano-Al2O3 composite deposit has higher

micro-hardness and better abrasion resistance than that of electro-less Ni–P and micro – Al2O3 composite electroless Ni–P [38]. Co-deposition of WC along with Ni–P coatings increases its hardnessand the hardness increases with the increase in the volume percentof WC in the layer. Increase in hardness is the result of existence ofhard WC particles as a barrier to plastic deformation of Ni–P matrixunder the load, which obstructs the shift of dislocation in Ni–P al-loys [55].

Electroless Ni–P deposits with high hardness and wear resis-tance were also developed without the necessity of subsequentthermal treatment. Various ratios of lactic acid and sodium acetatewere adopted as complexing agents in acidic electroless nickelbaths, with a premise that the ions of nickel were just complexedby the complexing agent. Results indicated that the deposition ratefirst increased and then decreased with increasing atom ratio oflactic acid. When the atom ratio of lactic acid to sodium acetatewas 4:6, the deposition rate reached the maximum, and the coat-ings obtained from such bath had high micro-hardness (820HV)and wear resistance [87].

The addition of the surfactants SDS and CTAB resulted in an in-crease of more than 50% in as-deposited hardness of the Ni–P coat-ing. This is due to the fact that addition of the surfactants results ina mixture of nano crystalline and amorphous structure [27,72].

7.2. Tribological studies of electroless nickel coating

7.2.1. Configurations used in tribo testingTo evaluate the friction and wear behavior of electroless Ni–P

coating, researchers have made use of a variety of tribotesters withdifferent contact configuration. The various contact configurationsemployed are shown in Table 4. From the table, it is apparent thatpin on disc, ball on disc and block on ring are more popular config-uration among the researchers for tribological testing. Althoughmany configurations have been utilized, no attempt has been madeto compare the tribological behavior of the coating encounteredusing different configurations. This may form part of a futurestudy.

7.2.2. Tribological properties addressed7.2.2.1. Roughness characteristics. Roughness is generally an unde-sirable property, as it may cause friction, wear, drag and fatigue,

Table 4Different test configurations.

Configurations Study

Pin on disc [2,13,18,32,33,36,37,42,43,59,88]Ball on disc [7,9,28,31,31,32,47–49,52,53]Block on ring [6,24,27,44,89,90]Ring on ring/disc on disc [4,34]Ring on disc [57,62,91]Taber apparatus [16]Pin in slot [90]Crossed cylinder contact [10]Ball on block [84]Four ball tester [56]Pin and vee tests [19]


but it is sometimes beneficial, as it allows surfaces to trap lubri-cants and prevents them from welding together. Electroless nickeldeposition has become commercially important for finishing steel,aluminium, copper, plastics and many other materials [69]. Elec-troless nickel coatings are very uniform and they follow the surfaceprofile of the substrate rather than just fill the spaces between sur-face asperities. Thus, in general the roughness of the coating doesnot vary much from the roughness of the substrate. However, elec-troless coatings are suspected to have some smoothening effectabove a critical substrate roughness. After plating, electroless Ni–P and Ni–B coatings are found to present a very smooth surface[17,40]. Heat treatments of the coatings caused a further decreaseof average and maximal roughness [2,17]. Kanta et al. [92] ob-served that after plating, the roughness of electroless coatingsare significantly higher. But the dual Ni–P/Ni–B coating is smooth-er than the Ni–B coating. The use of a Ni–P pre-coating is believedto have a leveling effect on this etched surface and produces asmoother top deposit in the end. Co-deposition of copper in Ni–W–P alloy resulted in a quaternary alloy of very smooth micro-structure and showed a reduction of roughness value from 60 to22 nm [21]. Balaraju et al. [93] prepared ternary electroless Ni–W–P coating using sulphate and chloride based baths and it wasfound that roughness value of coating evolved from chloride bathstend to be lower than that developed from sulphate based bath.Presence of surfactants is also found to affect the surface roughnessof electroless nickel coatings. Average roughness value of Ni–Pcoatings is found to reduce at higher concentration (>0.6 g/l) ofsurfactants. This is due to the fact that the amount of nickel parti-cles deposited on the coating surface is increased at higher concen-tration of SDS and CTAB as the contact angle is reduced and thisleads to the better wettability of Ni–P deposit [8]. The co-deposi-tion of relatively coarse particles to form composite coatings arefound to shoot up the roughness of pure Ni–P coatings by about5–7 lm [40].

Majority of the roughness studies use Ra (CLA) in order to char-acterize the surface roughness of the electroless nickel coatings.However, a surface in general is composed of a large number oflength scales of superimposed roughness [94] and normally char-acterized by three different types of parameters, viz., amplitudeparameters, spacing parameters, and hybrid parameters. Ampli-tude parameters are measures of the vertical characteristics ofthe surface deviations and examples of such parameters are centreline average roughness, root mean square roughness, skewness,kurtosis, peak-to-valley height, etc. Spacing parameters are themeasures of the horizontal characteristics of the surface deviationsand examples of such parameters are mean line peak spacing, highspot count, peak count, etc. On the other hand, hybrid parametersare a combination of both the vertical and horizontal characteris-tics of surface deviations and examples of such parameters are rootmean square slope of profile, root mean square wavelength, coreroughness depth, reduced peak height, valley depth, peak area, val-

ley area, etc. Thus consideration of only one parameter such as cen-tre line average roughness is sometimes not enough to describe thesurface quality. Sahoo [95] has employed five different roughnessparameters, viz., centre line average roughness (Ra), root meansquare roughness (Rq), skewness (Rsk), kurtosis (Rku) and mean linepeak spacing (Rsm) in order to study the surface texture generatedin electroless Ni–P coating and minimize the by optimizing theelectroless bath parameters. It was found that concentration ofthe reducing agent and its interaction with concentration of thenickel source solution, have significant influence in controllingthe roughness characteristics of electroless Ni–P coating.

7.2.2.2. Friction behavior and mechanism. The general objective ofapplying coating to a surface for any tribology based applicationis to impart both smoothness and hardness to the surface so thatfriction and wear are reduced. Among the two degrading phenom-ena, friction is a critical factor dictating the efficiency of mechani-cal assemblies that involve sliding surface contact. It is also partlyresponsible for wear, which is often the limiting mechanism of de-vice service life. Thus minimization of the friction of the coating isa vital need. Electroless nickel coatings are in general very smoothand lubricious [96] in nature thanks to their unique nodular micro-structure resembling that of a cauliflower [97]. But different sur-face treatments and incorporation of various particles andelements lead to a change of the conventional microstructure ofthe coating thus resulting in modified friction behavior. Moreoverthe friction characteristics are also dependent on the condition ofthe tribological testing and the counterface material used. Heattreatment in general results in a reduction of the friction coeffi-cient of various electroless coatings compared to the as depositedcoatings. The heat-treated Ni–B deposits are found to present a vir-tually incompatible surface for the hard counterface material andhence results in a reduction of coefficient of friction [18]. Ampli-tude of the friction coefficient oscillation of Ni–P coating after tem-pering is found to be lower than that of Ni–P coating [9]. Heattreatment is also found to decrease the coefficient of friction ofNi–P–ZrO2 coatings [61]. The laser irradiated surface showed lowerfriction coefficient than that of the furnace annealed surface withsimilar hardness [76].

The addition of hard particles viz., B4C and SiC has a tendency toincrease the friction coefficient of electroless nickel coatings as thenatural lubricity of the coatings is lost. The coefficient of friction ofelectroless Ni–P coating is found to increase after addition of B4Cparticles [43,44] which may be attributed to the particles of B4Cseparated during testing and probably imprisoned between thespecimen surface and the pin [43]. The friction coefficient of elec-troless Ni–P–B4C composite coating with 25 vol.% of B4C is approx-imately doubled with increasing the loads from 15 to 60 N [44].Lower value of the friction coefficient may be corresponded tothe presence of surface oxides formed as a result of frictional heat-ing that is more considerable for higher loads. The co-deposition ofSiC particles in Ni–P coating also increases the friction coefficientof the coating and Wu et al. [27] found an increase of about 10%in friction coefficient when compared to pure Ni–P coating. Onthe other hand, friction coefficient showed a drastic reductionwhen soft particles viz.. PTFE, Graphite and MoS2 are introducedin the coating. The incorporation of PTFE [27] and Graphite [57]in Ni–P matrix decreased the friction coefficient markedly andfacilitated the stable state of the whole wear course. In a studyby Wu et al. [27], a decrement as large as 70% was observed forfriction coefficient of Ni–P coating after the introduction of PTFEin its matrix. The PTFE-rich mechanically mixed layer (PRMML)formed on the worn surface is responsible for the good anti-frictionproperties. The SiC particles when mixed with PRMML, plays aload-bearing role in protecting PRMML from shearing easily. Acontinuous supply of PTFE to the tribo-surface is an important

Fig. 7. Variation of friction and wear rate of Ni–P–IF MoS2 with different volumefractions of IF MoS2 at load of 15 N [47].


precondition for the formation of PRMML and the exertion of itsanti-friction properties. The low friction coefficient of Ni–P matrixwith PTFE particles causes a little change of the temperature andfurther ensures the stable state of the whole sliding. The frictionplots of various coatings tested by Wu et al. [57] are given inFig. 6. The relatively high carbon content in the form of graphitewas responsible for the lower friction coefficient Ni–P–Gr com-pared to Ni–P–Gr–SiC coating.

In case of wear of Graphite reinforced Ni–P coatings, a graphite-rich film (GRF) formed on the worn surface of the Ni–P matrix withgraphite which seemed to be responsible for good tribologicalbehavior. A tribolayer is found to cover the worn surface. Since itconsists of materials from the composite and the counter-face aswell as environment, it was denoted as a ‘mechanically mixedlayer’ (MML), graphite-rich mechanically mixed layer (GRMML)here. Because of the softness and lamellar structure of graphite,the micro-hardness of Ni–P–Gr coating was deteriorated and thesubsurface cavities containing graphite particles tended to deformwith the subsurface matrix, squeezing out graphite to the wearingsurface during the sliding process under high pressure. The graph-ite smeared on the wearing surface layer by layer and transferredto the mating surface, changing the test system [57]. The presenceof the GRMML decreased the direct contact area between the test-ing sample and the counterface, therefore improving the wearresistance of the hybrid composite.

Although PTFE promotes lubricity in the coating, Ramalho andMiranda [10] found PTFE not to be effective in the friction reduc-tion. They explained this anomaly by the fact that the phosphoruscontent by itself leads to low friction and the PTFE lubrication ef-fect is not enough to balance the effect of increasing the surfaceroughness induced by the PTFE particles. Hence, the friction coeffi-cient increases.

Friction coefficient of the Ni–P–nano-MoS2 composite coatingdecreased greatly compared to those of Ni–P electroless coatings[46] and friction coefficient of the coatings showed a decreasingtrend with the increase of the volume fractions of IF–MoS2 [47](Fig. 7). Moreover, Ni–P–(IF–MoS2) composite coating displayedexcellent friction properties in vacuum implying the good stabilityof IF–MoS2 in different environments. The favorable effects of IF–MoS2 nanoparticles on the friction properties are attributed totheir unique fullerene-like structure.

Fig. 6. Friction behavior of various Ni–P coatings [57].

Chen et al. [52] studied the tribological behavior of carbon nanotubes (CNT) incorporated electroless nickel coatings and comparedthem with Ni–P–graphite and Ni–P–SiC. They found that Ni–P–CNTshowed the best lubricating property which can be related to theself-lubricating property of the CNTs. CNTs comprise concentriccylindrical layers or shells of graphite-like sp2 – bonded cylindricallayers or shells, where the intershell interaction is predominatelyvan der Waals, can easily slide or rotate each other, leading to alow friction coefficient. Also friction coefficients decrease with in-crease of load from 10 to 30 N. Incorporation of SiO2 nano particlescontributed to the improvement of the friction-reducing ability ofelectroless Ni–P matrix [28]. Lee [82] studied the friction behaviorof Ni–P coatings in lubrication with brine solution and found thatthe strong passivity of the coating acts as a lubricator and reducesthe friction coefficient. Parametric studies involving the variationof bath composition to reduce the friction coefficient of Ni–P coat-ings has also been conducted [98] which revealed that concentra-tion of the nickel source has significant influence in controlling thefriction characteristics of electroless Ni–P coating.

7.2.2.3. Wear resistance and mechanism. One of the unique charac-teristics of electroless nickel deposition is the superior wear resis-tance of the coatings. Theoretically, there is a correlation betweenwear resistance and hardness of a surface. However, the wearproperties of a surface are affected by numerous other parameterssuch as the nature of the applied stress and the surface morphol-ogy. The wear resistance of electroless nickel deposits depends


on both phosphorus content and the type of post heat treatmentapplied. In general, heat treatment increases hardness and hencethe wear resistance of the coating but the grain coarsening at high-er heat-treatment temperatures negatively affect the wear resis-tance of the coating. Laser irradiated coatings showed lowerwear amount than that of as-deposited amorphous alloy and fur-nace annealed coatings [76]. Electroless Ni–B coatings are foundto be more suitable for wear resistant applications consideringtheir higher hardness in as-deposited condition which can be in-creased to a great extent by suitable heat treatment. The Taberwear index of Ni–B coating is found to reduce to an astonishing va-lue of 13 after subjected to heat treatment at 400 �C in an atmo-sphere containing 95% Ar and 5% H2 [16]. The coating’s resistanceto Taber abrasion is dominated by coating properties and not bycoating–substrate adhesion. This means that Ni–B coated alumin-ium can be used instead of similarly plated steel in applicationswhere only wear resistance is required, such as piping and slidingparts [92].

However, scratch test by Vitry et al. [17] shows good adherenceof Ni–B coating to the Al–Mg–Cu alloy substrate. The columnarstructure of electroless Ni–B coatings is found to be useful inretaining lubricants under conditions of adhesive wear [14]. Thewear mechanism in electroless nickel coating is primarily foundto be either adhesive or abrasive or a combination of both. Thewear process of as-coated Ni–B coatings is governed by adhesivewear mechanism with high mutual solubility of nickel and iron(from counterface material). The heat treated coating being harder,exhibit no gross adhesion between the coating and the counterfacematerial [18]. Incorporations in plain electroless nickel are made asusual and some of the engineered coatings are found to exhibit bet-ter wear behavior compared to the parent material. Compared toNi–P coating, Ni–Cu–P can improve the wear resistance of the coat-ing [23]. Inclusion of tungsten in Ni–P coatings showed improvedwear resistance which increases with increasing tungsten content[2]. This could be attributed to the solid solution strengthening ofnickel matrix by tungsten. The mechanism of wear of electrolessNi–P deposit depends on the attractive force that operates betweenthe atoms of nickel from the coating and iron from the counterdisc. A negative wear depth curve is also observed in some cases[2] as a result of the buildup of oxide debris at the interface ofthe coated pin and counter disc. Moreover, this buildup of oxidesacts as a lubricant at the interface, resulting in lower frictionalcoefficient in case of heat-treated coatings. Co-deposition of bothtungsten and ZrO2 particles improves the wear resistance of theelectroless Ni–P-based coatings further [61]. But the effect ofZrO2 particles is dominant and in the quaternary Ni–W–P–ZrO2

coatings they determine the latter’s wear resistance [85]. Hamidet al. [55] reported that co-depositing tungsten carbide (WC) withNi–P matrix improved the abrasion resistance of steel thirteentimes while electroless Ni–P improved the abrasion resistance ofsteel three times. Higher abrasion resistance of Ni–P–WC coatingis attributed to increase of the effective role of Ni matrix for sup-porting and keeping the particles in the matrix so that the coatingcan sustain shear stress, rupture and damage. A particular volumepercent (53 %) incorporation of WC was found to suffer the mini-mum weight loss in wear test. Several other soft and hard particu-lates improve the wear performance of electroless nickel coatingwhich include: alumina [33,36,37], boron carbide [43,44], siliconcarbide [42,57], silicon nitride [56], silicon oxide [28], PTFE[10,27,50] and molybdenum sulphide [47].

Researchers have in general found that annealing at tempera-tures other than 400 �C adversely affects the wear performanceof electroless nickel coatings. This is due to the formation of in-ter-metallic phases that reduce the coating adherence to the sub-strate. Novaík et al. [33] analyzed the wear tracks and found thatabrasion is the major wear mechanism. However due to the forma-

tion of inter-metallic sub-layers, partial coating delamination mayoccur during tribological testing on the coatings annealed at 450 �Cand above. They proposed the use of Al2O3 fiber reinforcement inthe coating which reduced the scaling and increased wear resis-tance of coatings as compared to the non-reinforced Ni–P coatings.This idea may help in the application of electroless nickel as anti-wear coatings in parts subjected to high temperatures. The wearbehavior of Ni–P–Al2O3 composite coatings was tested by intro-ducing model lubricants which contained additives of the anti-wear type and friction modifier type. It was found that the lubri-cant containing zinc dialkyl dithio phosphate (ZDDP) yielded sur-face layers with stable operating parameters. The nickel coatingshad a tribological effect (tribochemical, adsorption and reaction–diffusion) that enhanced the generation of anti-wear layer in ZDDPwith model lubricants [32]. The findings have been confirmed bytribological tests conducted in the macro- and micro- or nano-met-ric scale [36]. Ebrahimian-Hosseinabadi et al. [44] observed a twoway relationship between the wear resistance of Ni–P–B4C and thevolume percent of B4C particles. The wear resistance of the coatingincreased with increase of volume percentage up to 25% abovewhich the wear resistance decreased. They attributed the loweringof wear resistance of Ni–P–B4C coating to the decrease of the effec-tive role of the Ni matrix for supporting and keeping the particlesin the matrix. The introduction of SiO2 nano particles helps to re-tard dislocation propagation of the composite coatings, while crys-tallization of amorphous Ni–P matrix at proper elevatedtemperature leads to changes in the microstructure and henceeffectively increase the hardness and wear resistance of the com-posite coatings [28]. Das et al. [56] developed carbon steel ball-bearings with 4 lm thick coating containing 2.9 wt.% Si3N4 thatcan be used for wear application in pH 10 water for 9 years underthe actual application environment conditions. A comparison be-tween the properties of Ni–P–SiC coatings developed with macroand micro sized SiC particles showed that the former exhibited les-ser wear [42]. Recently, electroless nickel containing PTFE as acomposite material has been used because it is uniform, highlyadherent, hard wearing, dry lubricating, non-galling, has a lowercoefficient of friction and good corrosion resistance properties[50]. Non sticking and good chemical inertness of PTFE help avoidsevere adhesive wear and oxidation wear [27]. A reduction of 2 or-ders of magnitude in wear coefficient is observed by the introduc-tion of PTFE particles in the electroless Ni–P coatings [10]. Thewear resistance of Ni–P/cenosphere composite coatings is foundto be 30% higher in volume than pure composite coatings [45].Potassium titanate (K2Ti6O13) whiskers (PTWs) have been used toreinforce the Ni–P coatings and the wear resistance of the resultingcoating is found to be higher than Ni–P and Ni–P–SiC. The favor-able effects of PTWs on the tribological properties of the compositecoatings are attributed to the super-strong mechanical propertiesand the specific tunneling structures of PTWs. The PTWs greatlyreinforce the structure of the Ni–P-based composite coatings andthereby greatly reduce the adhesive and plough wear of Ni–P–PTWs composite coatings [59]. The hardest material, diamondhas also been co-deposited with electroless Ni–P coatings to in-crease its wear performance [34].

One of latest trends observed in the recent years has been theincorporation of two or more particles having different propertiesinto electroless nickel coatings so that the final coating possessesthe advantages of all the inclusions. Ni–P–Gr–SiC has been devel-oped that showed a combination of the advantages of Ni–P–SiCin high load-bearing and Ni–P–Gr in a low friction coefficient[57]. Ni–P–PTFE–SiC coating combines the advantages of Ni–P–SiC in high load-bearing, wear resistance and Ni–P–PTFE in lowfriction coefficient [27].

The use of carbon nano tubes (CNT) in electroless nickel coat-ings is a recent development. Wear resistance of Ni–CNTs are


found to be much superior to coatings such as Ni–SiC and Ni–Graphite due to improved mechanical properties, self-lubricationand unique microstructure of Ni–P–CNTs composite coatings[52,53]. The factors which influenced the content of carbon nano-tubes in deposits were types of agitation, surfactants and carbonnanotubes concentration in the plating bath. Zhao and Deng [54]showed that Ni–P–CNTs composite coating greatly increased thehardness and wear resistance with the increase of the content ofcarbon nanotubes in deposits. The examination of the wear surfacerevealed that Ni–P–CNT coatings encountered abrasive wear. Veryfew cracks, scales and very fine scratches appeared on the wearsurface but at the same time some small bulges were observedand the abrasion grooves were discontinuous. These demonstratedthat the CNTs dispersed in the nickel matrix block, or at least de-layed the movement of dislocations in the metal nickel and thusinhibited plastic deformation, which improved the wear behaviorof the coatings [52].

Some enthusiasts have also studied the impact of various tribo-logical testing conditions on the wear behavior of electroless nickelcoatings. It is found that rate of wear is strongly dependent on theapplied load and it increases along with increasing load [18] irre-spective of whether specimen is Ni–P coated or Ni–P coated andtempered. But then, load is found to have stronger impact on thewear rate for Ni–P coating as Li et al. [9] have observed that an in-crease of load from 1 N to 1.5 N, magnifies the wear rate by almost10 times for as deposited Ni–P whereas the same magnifies thewear rate by only 1.3 times in case of tempered Ni–P coatings.The optimization of the coating parameters is also done [6] whichreported that annealing temperature and bath temperature havethe most significant influence in controlling wear characteristicsof electroless Ni–P coating. Wear is measured in terms of weightloss or mass loss in most of the studies. Some have employed depthof wear to represent the magnitude of wear encountered. Winow-lin Jappes et al. [34] have introduced and analyzed a novel ap-proach on wear analysis by pasting silver foil on the specimenbefore X-ray diffraction analysis (XRD). For the quality of resis-tance against wear, electroless nickel alloy and composite coatingswere checked for their potential application as lubricious and anti-wear coatings in small arms weapon actions [90].

The friction and wear of NiP and NiP + PTFE electroless coatingswere investigated under lubricated conditions, particularly withbio-lubricants. Stribeck curves were used to compare the perfor-mance of different lubricants [99]. The influence of the differentlubricants under study on the wear amount was investigated bymeasuring the wear scar at the end of each test. The mineral oilwas the best lubricant tested with the smallest specific wear rate.The addition of PTFE particles to Ni–P coating is helpful in bound-ary lubrication conditions. The effect of the PTFE derives from itsintrinsic low friction coefficient against steel, and probably fromthe beneficial effect of the surface pattern created by the effect ofthe PTFE particles on the coating surface. In boundary lubricationconditions, the wear of Ni–P coating occurs by mild abrasion withtypical parallel wear scars along the direction of the moving body[99].

7.2.2.4. Adhesion. Coatings with poor adhesion do not provide goodprotection. The adhesion evaluation of electroless nickel has re-mained an unsolved problem [100] for many years. Most of themethods applied such as pull-off test, ring-shear test, peel test,and three-point bend test give semi quantitative or qualitative re-sults only. In general, electroless nickel coatings have superioradhesion compared with the nickel electro-deposition method.This is due to the existence of a stronger metal-to-metal bondingduring the electroless deposition. However, in case of coatingnon-ferrous alloys such as magnesium and aluminium, the adher-ence of the electroless nickel coatings to the substrate is some-

times poor. Hence, the study of adhesion is mainly concentratedin studies using one of these alloys as substrates [17,101–103].Heat treatment is found to enhance the adhesion between Ni–Pcoating and 7075 aluminium alloy up to a certain temperature(300 �C) above which adhesion decreased [102]. Liu and Gao[103] investigated the adhesion of Ni–P coatings in different vari-ants of magnesium alloys by scratch test and found that adhesionstrength of the coatings on AZ31 and AZ91 magnesium alloys arehigher than that on pure magnesium. The critical load (a measureof adhesion strength) in case of AZ31 alloy reached 13.1 N. Zincat-ing is found to increase the adhesion between electroless nickeland the substrate alloy [101].

7.2.2.5. Corrosion behavior and mechanism. Electroless Ni–P coat-ings are also widely used for corrosion protection application in avariety of environments. They act as barrier coatings, protectingthe substrate by sealing it off from the corrosive environments,rather than by sacrificial action. However, in this respect, only elec-troless Ni–high P coating is effective in offering an excellent pro-tection, whereas electroless Ni–low P and Ni–medium P coatingsare not recommended for severe environments [39]. In general,the corrosion resistance of any alloy depends on the ability to forma surface protective film. Phosphorus can make the corrosion po-tential increase and the corrosion current decrease, and it pro-motes the anodic and cathodic reactions during the corrosionprocess, thereby increasing the anodic dissolution of nickel. Accel-erated corrosion of nickel provides prerequisites for concentratingP and thereby for the formation of Ni3P and NixPy stable intermedi-ate compounds on the surface, which acts as barrier passive film. Itis evident from the literature reports on Ni–P coatings that prefer-ential dissolution of nickel occurs leading to the enrichment ofphosphorous on the surface layer. This enriched phosphorous re-acts with water to form a layer of adsorbed hypophosphite anions(H2PO�2 ). This layer in turn will block the supply of water to theelectrode surface, thereby preventing the hydration of nickel,which is considered to be the first step to form either solubleNi2+ species or a passive nickel film. Hence, the better corrosionresistance obtained for electroless Ni–P and poly-alloy coatings isdue to the enrichment of phosphorous on the electrode surface[11,104].

The corrosion behavior of electroless Ni–P coatings is governedby three principal factors, namely, the degree of amorphous state,extent of internal stress and the percentage of phosphorus content.The combined effect of all of these factors or dominance of any oneof these factors is responsible for the observed corrosion behavior.The difference in corrosion resistance between electroless Ni–lowP and Ni–high P coatings is in particular due to the difference instructure-a crystalline, inhomogeneous structure for the low Pmaterials, and an amorphous, homogeneous one for the high Pmaterials. For this reason, the potentiality of high phosphorous(12–14%) Ni–P coating in marine applications has been investi-gated by Gao et al. [12]. The corrosion investigations were doneby immersing the samples in 3.5% NaCl solutions and by standardsalt spray test. The tests were conducted for a variety of time peri-ods ranging from 0.5 h to 29 days with intermediate periods of 2, 6and 13 days. At the beginning of immersion, a passivation film wasfound to form on the deposit, but it was not integrated. The passiv-ation phenomenon became increasingly obvious as the immersiontime increased. The two time constants obtained from the EIS(electrochemical impedance spectroscopy) spectra also confirmedthe formation of the passivation film. The scanning electron micro-graphs showed that the prepared deposits were amorphous. Butafter a 15 d standard salt spray test, a few pinholes appeared onthe deposit and the weight content of phosphorus on the surfaceof the deposit was higher, which was beneficial to the formationof the passivation film, than it was before the standard salt spray


test, while the nickel content was lower because the dissolvedweight of nickel was greater than that of phosphorus. The resultsfrom potentio-dynamic scan and EIS test showed that passivationfilm formed on the Ni–P deposit after immersion in 3.5% NaCl solu-tions and decreased the corrosion rate of Ni–P samples.

Sankara Narayanan et al. [11] prepared electroless Ni–P gradedcoatings by sequential immersion in three different hypophosph-ite-reduced electroless plating baths (L: low, M: medium, H: high).It was found that when electroless Ni–high P coating forms theouter layer of the graded (LMH) Ni–P coating, the corrosion resis-tance increases. In contrast, when electroless Ni–low P coatingformed the outer layer of the graded (HML) Ni–P coating, the cor-rosion resistance was lower than its counterpart. However, the cor-rosion resistance offered by the latter type of graded coating wasbetter than that of electroless Ni–low P coating, due to the barrierproperties of the underlying Ni–medium P and Ni–high P layers.Heat-treatment of electroless Ni–P graded coatings was likely tomodify the graded layers so that the gradation is lost. Hence theconcept of graded electroless Ni–P coatings was found to be validonly in as-plated condition.

Electroless Ni–B coatings are found to have a lower resistanceagainst corrosion compared to Ni–P coatings [105] but the resis-tance improved significantly by incorporating phosphorous in thecoating to form a Ni–P–B coating [14]. The concept of Ni–P/Ni–Bduplex coatings was found to have a noble electrochemical mannerthan Ni–B coatings [92].

Heat treatment is found to decrease the corrosion resistance ofelectroless coatings invariably. This is attributed to the change ofmicrostructure of the coatings with heat treatment. Electrolesscoatings in as-deposited condition generally exhibit an amorphousstructure which imparts higher corrosion resistance. But, heattreatment induces crystallinity into the deposits, which in turn in-creases the grain boundaries that form active sites for corrosion at-tack [14].

As in the case of friction and wear studies, incorporation of spe-cific elements in the electroless Ni–P coatings is found to increaseits corrosion protect ability. Copper element significantly improvedthe corrosion resistance of Ni–P coatings with the condition thattheir thickness reached a certain range [22]. Presence of copperin Ni–W–P deposit also improved its corrosion performance inboth 3.5% sodium chloride and 1 M hydrochloric acid solutions[21]. Small amount of tin addition can improve the formability ofthe amorphous phase in Ni–P coating. The electrochemical corro-sive experiments of Ni–P–Sn deposits showed that the additionof tin improved the anticorrosion property in NaCl solution, but re-duced the anticorrosion ability in sulphuric acid solution [26].

Although the co-deposition of ceramic particles in Ni–P matrixmay result in improvement of friction and wear characteristics ofthe coating, but the same adversely affects the corrosion perfor-mance of the coatings. Inclusion of B4C [43], Si3N4 [56] and PTFE[86] particles in Ni–P coating decreased its corrosion resistancewhich may be attributed to the defects in the matrix either dueto increased porosity or due to the creation of micro cracks onthe surface by the impregnation of the corresponding particles intothe coating [43]. The defects lead to easy access of the corrodingmedia to the surface resulting in the loss of corrosion protection.The chemical activity of Si3N4 particles in alkaline media had a syn-ergistic effect on the loss of corrosion resistance of Ni–P–Si3N4

composite coatings [86]. The decrease in the anticorrosion abilityof Ni–P–nano MoS2 composite coatings again was the result ofthe formation of microcells around the nanosized MoS2 particles,and the active ion-like Cl� destroyed the surface film and inducedthe corrosion on the inside part of the coating [46]. However, somecomposite coatings displayed good corrosion behavior. Corrosionresistance of Ni–P cenosphere composite coatings was found tobe 77% more than plain Ni–P coatings [45]. Bigdeli and Allahkaram

[39] reported that the incorporation of SiC particles in Ni–P coat-ings offered better corrosion protection which could be ascribedto a reduction in the effective metallic area available for corrosionin Ni–P–nano SiC coating. In contrast to general observation, theyeven found that heat treatment at 400 �C for 1 h significantly im-proved the coating density and structure, giving rise to an en-hanced corrosion resistance for the Ni–P and Ni–P–SiC coatings.The Ni–Mo–P/PPS coating also had good corrosion-resistant perfor-mance and its corrosion rate was reported to be one third of that ofNi–Mo–P in 10% H2SO4 [65].

Balaraju et al. [104] performed a special study of the corrosionbehavior of electroless Ni–P coatings in non-deaerated and deaer-ated conditions and it was observed that no passivation phenome-non occurs in non-deaerated conditions. Corrosion of Ni–P coatingswas also studied in special environments such as in the presence ofcarbon dioxide and it minimized the corrosion of N80 steel [106].Electroless nickel coating has also been used to protect steel rein-forcements embedded in concrete specimen exposed to marineenvironment against corrosion [107].

7.2.2.6. Wear corrosion. Wear and corrosion are found to be quiteinterdependent. In many ways, wear triggers corrosion and a cor-roded surface will not certainly display a good wear resistance.Many modern types of machineries are subjected to extremely cor-rosive environments viz. marine applications, chemical plants, etc.The dynamic parts in these machineries would require protectionagainst both wear and corrosion simultaneously as they undergotribo-chemical interactions. Hence, to address this phenomenonthe term wear-corrosion is employed. Electroless nickel coatingspossess an excellent protection of corrosion and wear corrosionand so are favorably applied to the surface of glass fiber reinforcedplastic (GFRP) substrate for making blades of offshore wind turbineinstallations [3]. Deposition of Ni–P coatings on a carbon fiber rein-forced plastic (CFRP) substrate also increases its wear-corrosionresistance. This wear-corrosion resistance increased as coatingthickness and phosphorous content increased and the polishingcondition decreased [82].

Liu [67] considered the effect of NaOH concentration in med-ium, the content of Cr2O3 in the coating and technology of heattreatment on corrosive wear resistance of a Cr2O3 composite elec-troless plating, which was compared with Ni–P coating. The corro-sive wear of the coating was seen to be linearly dependent onNaOH concentration, and the corrosive wear resistance of the com-posite electroless plating was superior to the Ni–P coating in anyNaOH concentration. When the content of Cr2O3 in coating was8%, the corrosive wear resistance of the composite electroless plat-ing is the best, and the corrosive wear resistance is improved withthe employment of proper technology of heat treatment.

8. Microstructural characterization

Microstructural characterization of the coatings is important asit helps to better understand its macroscopic behavior by observ-ing the changes occurring at the microscopic level of the coating.During the deposition of electroless nickel films, the growth ofthe film starts at isolated locations on the substrate. The wholesubstrate is then covered by lateral growth [5]. Electroless nick-el–boron in general exhibit a nodular structure that resemblesthe surface of a cauliflower (Fig 8). This nodular structure makesthe coating naturally lubricious [97]. The phosphorous and boroncontent mainly control the microstructure and properties of elec-troless nickel coatings. Low-phosphorus coatings show a nanocrys-talline structure as compared to high-phosphorus amorphouscoatings, which is responsible for their enhanced hardness andabrasive wear resistance. Medium-phosphorus coatings exhibit a

Fig. 8. Surface morphology of Ni–B coatings (a) as deposited, (b) annealed at 250 �C, (c) annealed at 350 �C and (d) annealed at 450 �C.


transition structure and a hardness and abrasive wear resistance,which lie in between those of nanocrystalline low-phosphorus.andamorphous high-phosphorus coatings [108]. The coating is amor-phous or microcrystalline in as-deposited phase but turns crystal-line with heat treatment and phosphides and borides areprecipitated. Moreover, the nodules are observed to inflate givingrise to a coarse grained structure (Fig 8). This in turn affects theproperties of electroless nickel coatings viz. hardness and corrosionresistance. The laser alloyed surface is found to consist of fine den-drites which increased with increase in heat input. Formation ofthe dendritic structure was correlated to the fast heating and cool-ing rates encountered in laser alloying process [77].

Incorporations and co-depositions may modify the structure toa great extent that may contribute to the enhanced properties inthe coating. Tungsten inclusion in Ni–P matrix had increased thegrain size from 1.2 to 2.1 nm. Crystallinity had improved furtherdue to the co-deposition of copper in ternary Ni–W–P deposit,resulting in smoother deposit compared to coarse nodular ternarydeposit, with a grain size of 6 nm [104]. However, it was reportedby Balaraju et al. [109] that incorporation of Si3N4, CeO2 and TiO2

particles in Ni–P matrix does not have any influence on the struc-ture and phase transformation behavior of electroless Ni–Pcoatings.

9. Improving the properties of some useful alloys

9.1. Aluminium based alloys

Al-based alloys show several attractive properties, such as lowweight, high specific strength, high thermal conductivity, and rela-tively good corrosion resistance in air. For these reasons, they arepopular construction materials in the automotive and aerospaceindustry. In some applications, however, they suffer from insuffi-cient wear resistance, and several approaches have been adoptedto prolong their life including reinforcement with particles or fibers,increase of Si content, hard physical vapour deposition (PVD), orelectrodeposited Cr coatings. Unfortunately, problems often arisewhen components of complex shapes are coated. These problemsare in particular avoided when using electroless nickel coatings.Many studies have been devoted to the electroless composite coat-ings on steel substrates [32] but little work has been reported onthe surface protection of Al-based alloys [110]. The important differ-ence between steels and aluminium alloys is that aluminium easilyforms a stable passive oxide layer, reducing the adhesion of the coat-ing. Hence, a chemical pre-treatment of aluminium substrateremoving the oxide is necessary, and, sometimes, additional Zn orPd activation is used [111]. Electroless aluminium (1050) alloy


substrates were found to exhibit better resistance against corrosioncompared to steel (ST37) substrates [92].

Composite coatings of Ni–P–Al2O3 and Ni–P–SiC can be used toimprove abrasion resistance of Al–Si castings, particularly, if theyare subsequently heat treated at 400 �C/1 h. Abrasion rate reducesalmost 20 times as compared to the uncoated alloy [40]. Adhesionof coatings to substrate was better in case of Al–Si alloy due to thepresence of silicon in its structure [112]. Al–Si engine components,such as pistons, etc., may be exposed to temperatures above 400 �Cduring operation, a temperature at which electroless Ni–P coatingsexperience a progressive hardness decrease due to grain coarsen-ing effect. To solve this problem, Al2O3 fiber reinforcement wasprovided which increased the wear resistance of the coating butwithout any success at higher temperatures [33]. The possibilityof improving the wear and friction performances of current alu-minium alloys in the bearings region of lightweight external gearpumps has been investigated by applying Ni–P–PTFE coatings[49]. Laser treatment has been employed to improve the surfaceproperties of aluminium alloys coated with electroless nickel(Ni–P) coatings [77].

9.2. Magnesium based alloys

The inherent lightness and high specific strength of magnesiumalloy have brought about increasing attention on its application inautomobile, motorcycle, airplane applications, etc. However, mag-nesium is intrinsically highly reactive (standard potential �2.37 V)and has low hardness. Therefore, it is prone to atmospheric corro-sion, and has poor wear resistance. These are actually the mainobstacles for the application of magnesium alloy in practical envi-ronment. Electroless nickel plating on magnesium alloys is a prop-er protection method because it is environment-friendly, anddeposited nickel has good resistance to corrosion and abrasion.The corrosion resistance of AZ91D magnesium alloy is greatly im-proved by direct-electroless plating which were testified by theimmersion experiment and the potentiodynamic polarizationexperiment in 3.5% NaCl solution [9]. Free corrosion potentialshifts from �1500 mV to �250 mV after AZ91D magnesium alloyis treated by electroless Ni plating. The wear resistance of AZ91Dimproves considerably after Ni-P coating and subsequent temper-ing. Ni–P–B4C coating has been employed on cast magnesium alloyAZ91D in order to improve its wear and corrosion resistance prop-erties. Micro-hardness and hence the wear resistance togetherwith corrosion resistance of the alloy is very much increased [43].

Table 5Results of wear tests under reciprocating motion [24].

Coating 4.5 N Load 9.5 mm dia pins

Number of cyclesrun (1000�)

Wearvolume(mm3)

Wear coefficient(mm3/Nm)

GearKote 12.2 0.041926 2.2E�05TiCN 25 0.009805 2.5E�05Xylar 201� 35 0.036985 6.7E�05NiTuff� on

aluminium30 0.040745 8.6E�05

NiCoTef� onaluminium

30 0.002122 4.5E�07

Ferritic NC + Xylan1014� w/3% WS2

38.5 0.004160 6.9E�07

Xylan 1014� 55 0.006574 7.6E�07NiCoTei� on steel 30 0.006369 1.3E�06Zinc

phosphate + MoS2

35 0.016263 2.9E�06

NanoCEM™ 35 0.040163 7.3E�06

10. Applicability from Tribological point of view

Electroless nickel coatings have found tremendous applicabilitydue to the wide range of properties they exhibit together with theadvantages of using them. The coatings have found widespreadapplications in many industries, such as printed circuit boards,magnetic storage media, microelectronics, radio-electronics, com-puter engineering, aviation, aerospace, petroleum, chemistry,machinery, textiles, automotives and metallization of plastic [3].From the tribological view point, electroless nickel coatings aremainly applied due to their hardness and wear resistance, lubricity(low friction coefficient) and corrosion resistant properties.

Electroless nickel phosphorous coatings have been successfullyapplied on GFRP substrate for making blades of offshore wind tur-bine installations [3]. An offshore wind turbine should be designedso as to be able to withstand fatigue loads and a hostile environ-ment at all times. Depositing Ni–P coatings on GFRP substrateincreases its wear-corrosion resistance besides imparting conduc-tivity to resist against damage by lightning strikes. Electrolessnickel coating has also been used to improve the wear and

wear-corrosion properties of CFRP materials which are frequentlyutilized in the aerospace, aviation and automobile industries dueto their light weight [82]. The potential of Ni–P coatings for marineapplications have been investigated by various researchers [12,89].UHMWPE, an advanced engineering plastic used in making seawa-ter hydraulic drive is found to produce the lowest friction coeffi-cient when sliding against electroless Ni–P coating under seawater lubrication [89]. The suitability of electroless nickel coatings(NiCoTef�) was checked for applying onto the action componentsof hand-held automatic weapons (i.e. those other than the barreland stock) that are subjected to rapid repeated sliding contactand shock loading conditions. The affinity of dirt, dust and finesand to the residual liquid lubricant films on the surface of partscan lead to malfunction and jamming of weapons when operatedin desert or other dusty environments. To avoid this problem,new generations of weapons must make use of ‘‘dry’’ coatings,which can provide low friction between sliding components, corro-sion protection, and can last many cycles. The results of the weartest with different materials are given in Table 5. It was found thatthe components for which the wear life is a higher priority thanhaving the lowest COF, a electroless nickel coating is a fairly goodchoice.

Composite coatings of electroless Ni–P–PTFE were consideredfor improving the performances of aluminium alloys in the bear-ings regions of lightweight external gear pumps [49]. The coatingsexhibited satisfactory friction and wear behavior when slidedagainst steel. However, the presence of PTFE spheres and the hard-ness mismatch with the substrate tends to favor tensile cracking ofthe substrate. Das et al. [56] reported the application of 4 lm thickelectroless Ni–P–Si3N4 on the surface of ferrous based bearingsthat can be successfully used for water lubricated applications inpH 10 water for 9 years under the actual application environment.The excellent wear resistance plus low friction coefficient of theNi–P–Cgraphite–SiC coating is found suitable for applications inmoulds, automobile parts and other fields [91].

Electroplated hexavalent chromium coatings are applied widelydue to their excellent tribological properties. But chromium coat-ings being toxic, due to environmental concerns, an alternative toit is always sought. Klingenberg et al. [25] compared the perfor-mances of both types of coatings and suggested that all of the elec-troless Ni–P, Ni–Co–P and Co–P processes with occluded diamondparticles have the potential to impart the required adhesion, hard-ness and tribological properties, while reducing the environmentalimpact of chromium plating processes [25]. The suitability ofelectroless nickel coating for application in a commercial brake


valve assembly was compared with that of hard chromium coat-ings [13]. It was observed that electroless coating performed sim-ilar to chromium coating displaying moderate friction andinsignificant wear over the sliding distance under examination.Thus, electroless coating could replace chromium coatings whichare much toxic to the environment.

Because of high electrical and thermal conductivity properties,combined with the in situ lubricating ability of graphite-containingcomposites, which mix further with ceramic SiC particles, makethem potential candidates for such applications as advanced highspeed–high load bearing, high speed–high current electricalbrushes, etc. [57].

Among the various metallization processes, electroless metalplating is a preferred way to produce metal–coated fabrics due tothe attractable advantages such as uniformity of coverage, excel-lent conductivity, possibility of metallizing non-conductors andflexibility. Ni–P plating is applied in order to render abrasion resis-tance to EMI shielding fabrics [30]. The abrasion resistance is foundto increase with the incorporation of SiO2 particles in the coatingwithout much change in shielding effectiveness of the fabric.

11. Concluding remarks

From the present review, it can be seen that electroless nickelcoatings have emerged as suitable coatings that can serve as viablereplacements to the conventional electroplating in suitable situa-tions. Their properties such as hardness, low friction, wear resis-tance and corrosion resistance have led to their usage intribological applications. Besides, the uniform deposition and theability to coat any materials have served as an added advantageto their application in various areas. The advantages of modifyingthe properties of electroless nickel coatings by suitable surfacetreatments (heat treatment, laser treatment, etc.) and the incorpo-ration of various elements (copper, tungsten, etc.) and particles(SiC, TiO2, Si3N4, etc.) have been utilized by various researchersto evaluate the suitability of these coatings for various applica-tions. The review reveals that the electroless coatings are mainlyapplied for wear resistance and corrosion resistance applications.The coatings have been found useful in marine environments aswell as for coating of small arms weapons to be used in desertand dusty areas. By observing the expansion possibilities of elec-troless nickel coatings, more advanced tribological application ofthe coatings may be expected.

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tribology of electroless nickel coatings – a review

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