Ni-P coatings electroplating — A reviewPart I: Pure Ni-P alloy

Aleksandra LELEVIC

H2020-MSCA-ITN-2016 SOLUTIONArtia Nanoengineering & Consulting

Athens, Greecea.lelevic@artianano.com

lelevic.aleksandra@gmail.com

July 13, 2018

Abstract

In the electroplating industry Ni-P coatings are extensively employed owing to their excellentproperties which enable substrate protection against corrosion and wear. Depending on their composi-tion and structure, as-plated deposits demonstrate good mechanical, tribological and electrochemicalfeatures, catalytic activity but also beneficial magnetic characteristics. With subsequent thermal treat-ment hardness of Ni-P metal-metalloid system can approach or be even higher than that of hard Crcoatings. The purpose of this paper is to provide a general survey of the research work dealing with theelectrodeposition of Ni-P binary alloy coatings. Proposed phosphorus incorporation mechanisms, Ni-Palloy microstructure before and after thermal treatment, its mechanical, tribological, corrosion, catalyticand magnetic properties are considered, so are the key process variables influencing phosphoruscontent in the deposits and the roles of the main electrolytic bath constituents. Findings on the merits ofemploying pulse plating and fabrication of unconventional (layered and functionally graded) structuresare succinctly explored.

1 Introduction

In the modern world billions of dollars per yearare lost due to machinery overhaul or completebreakdown related to material structural failuresinduced by progressive corrosion, wear, fatigueand rupture. Surface degradation impacts a largenumber of industrial sectors and can account forup to 3-4% of the Gross Domestic Product in de-veloped economies [1, 2]. One way to address anddiminish issues related to material deterioration isto impart on its surface a coating that will mitigatethe influence of the surrounding environment andof the working conditions and prolong the servicelife of the equipment.

For the deposition of coatings a variety of tech-niques exist that can be mechanical, physical, chem-ical and electrochemical in nature [3]. Among them,simple electroplating offers many advantages [4–6].

It involves a process that takes place in the low costelectrolysis cell, in an aqueous solution, at normalpressure and at relatively low temperature, whichmakes it ideal for the industrial scaling up [7–9].Particular appeal of electroplating is in the possibil-ity of customizing appearance and properties of thecoating by modifying the composition of the electro-plating bath and/or the electroplating conditionsand producing a wide range of metallic materialsfrom metals and simple alloys to compositionallymodulated systems and composites. Depositionrates in the order of several tens of micrometers perhour can be routinely achieved [10].

In terms of types of coatings fabricated throughelectroplating, in aerospace, automotive and gen-eral engineering industry functional Cr coatingsare very widely employed owing to their remark-able combination of properties which include: highhardness, good corrosion, wear and heat resistance

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and low friction coefficient [7]. However, the mostcommon commercial Cr electrodeposition processemploys an aqueous bath that contains hexavalentCr. This ion is suspected to be a carcinogenic agentwhich in combination with its strong oxidizing na-ture imposes serious human health and environ-mental concerns [11]. As a consequence, the useof hexavalent Cr has been limited by the EuropeanUnion Restriction of Hazardous Substances Direc-tive [12]. In addition, the typical wetting agent-PFOS (perfluoro-octanesulfonic acid) employed inCr electroplating to reduce the danger of the pro-cess through minimizing the misting generatedduring plating is being prohibited due to its ex-ceptional stability which presents a problem forthe environment [7]. In the light of all mentioned,attention is focused on finding alternatives to Crwhile maintaining the request for excellent func-tional properties of the fabricated deposits.

It has been reported by many authors that cer-tain binary, ternary or quaternary alloy coatingsmay present a suitable replacement for hard Crand may possess an interesting combination offeatures that can culminate even in a possible im-provement in terms of performance characteristics[13–16]. Amongst binary alloys Ni-P has attracteda lot of attention. Vast number of studies havebeen conducted which prove that coatings basedon Ni-P with careful tailoring of composition andstructure can offer smart and adaptive solutionsto a wide range of environmental and workingconditions [17, 18]. Compared to Cr coatings ob-tained from hexavalent Cr baths, effluents resultingfrom Ni electroplating process have conventional,simpler treatment, such as for example processescalled Clean Technologies (based on electrodialy-sis) [19]. Ni-P coatings exhibit many interestingfeatures. They are, depending on the electroplatingconditions and the applied post-treatment, charac-terized by: optimal mechanical properties, goodwear and corrosion resistance, electro-catalytic ac-tivity and favourable tribological features. Withproper customization of composition and structureand subsequent thermal treatment hardness of Ni-P electrodeposits can approach or surpass that ofhard Cr coatings [20]. Ni-P alloy electrodepositsfind applications as protective, functional and deco-rative coatings primarily in automotive, aerospaceand general engineering industries. Notable arethe applications of Ni-P in the fabrication of dec-orative coatings in automotive industry, high pre-cision parts, diffusion barriers, catalytic coatings

for hydrogen evolution, thin film magnetic discs,microgalvanics, etc. [21].

In order to attain the uttermost enhancementof the Ni-P deposit beneficial properties it isparamount to control the coating’s composition andits microstructure. This can be achieved through al-tering electroplating bath composition, depositionconditions and by applying appropriate alloy post-treatment. To go beyond the limits of the propertiesthat can be achieved for this binary alloy system,co-deposition of nano, sub-micron or micron sizeparticles and fabrication of composite coatings isthe centre of attention of the modern electroplat-ing research and innovation efforts. While classicalindustrial demands on hard, homogeneous, wearresistant coatings and corrosion protection are stillvalid, added capacities in terms of: chemical sta-bility, bio-compatibility, microstructured surfacesand functional coatings (lubricant, magnetic) arein high demand. This is where the production ofcomposites but also compositionally modulatedcoatings, characterized by variable configurationacross their thickness, can offer new possibilitiesand new combinations of properties that can beadapted to almost any need and satisfy the increas-ing demands on multi-functionality.

2 Electroplating benefits andprospects

Conventional electroplating possesses many advan-tages in terms of versatility, ease of use and costeffectiveness. The technique has many assets whencompared to physical methods such as magnetronsputtering and chemical methods such as chemi-cal vapour deposition. Physical vapour depositionis a high cost line-of-sight process with possiblypoor throwing power, while in chemical vapourdeposition high operating temperatures may causesubstrate softening [3, 22]. Heat resistance of thesubstrate may very much limit the choice of thedeposition technique, in particular for metals suchas Al, Cu, Mg for which maximum deposition tem-perature is about 100 ◦C. Disadvantages of elec-troplating include mainly problems with currentefficiency, throwing power, substrate adhesion andcoating uniformity [3].

In his work Dini [23] performed a comparisonof coatings fabricated by various deposition tech-niques (PVD, CVD, PS and EP) on the bases of awide range of properties, namely: structure, poros-

2

ity, density, stress, corrosion, adhesion, tribology,fatigue, thermal conductivity, etc. His findings indi-cated that no coating technology provides superiorresults in terms of all considered parameters. Thechoice of coating technique must in fact be madeby taking into consideration a number of factors,such as: substrate compatibility, adhesion, porosity,possibility of repair or re-coating, inter-diffusion,effect of thermal cycling, resistance to wear andcorrosion, whilst simultaneously establishing a sen-sible balance between the obtained benefits and theincurred costs.

Electroplating possesses advantages over tradi-tional electroless plating also, in terms of providinga higher deposition rate in a simpler electrolyteand the added benefit of the possibility to controldeposit’s composition and microstructure by chang-ing the applied current waveform and electroplat-ing conditions [4, 5, 24]. Thick deposits are easilyachieved through electroplating. Amorphous Ni-Palloys can be electroplated up to mm thickness onwidely differing substrates [17]. However, it is stillnecessary to work on improving the electroplateddeposits’ uniformity and on maximizing the pro-cess efficiency when compared to electroless plat-ing. By definition, electroless process must be meta-stable to ensure deposition. Plating in electrolessmode is characterized by solution instability, slowdeposition rate (generally around 10 µm h−1, acidicelectroless baths can plate at about 25 µm h−1), hightemperatures of operation (85 ◦C min for acceptabledeposition rate), difficulties with barrel plating ofsmall parts and high cost of the operation whichis approximately 5 to 10 times higher than for elec-trodeposition [25]. Nevertheless, electroless platinghas excellent throwing power and the advantageof producing deposits of uniform thickness partic-ularly on components with complex shapes [26].In order to achieve the latter in case of electroplat-ing an intricate system of internal anodes and/orshielding is necessary due to the non-uniform cur-rent distribution characterizing this process. Ni-Pdeposits fabricated by electroless deposition arealso reported to be harder and to possess bettercorrosion resistance compared to those obtainedthrough electrodeposition [17].

Electroplating is quite a mature technique andalthough widely applied in industry since the endof the 19th century, currently it is exhibiting stag-nancy in terms of growth owing to the lack oftechnological innovations. However, according toseveral global market studies, electroplating is to

demonstrate an expansion, predominantly in theAsia-Pacific excluding Japan region (APEJ). In themarket report titled “Electroplating Market: GlobalIndustry Analysis and Opportunity Assessment,2016–2026” [27], Future Market Insights foreseethat the global electroplating market is expectedto expand at a compound annual growth rate of3.7% by the end of 2026 reaching worth of US$21 Bn. APEJ will be according to them the fastestgrowing region and the Automotive and Electri-cal & Electronics segments are estimated to collec-tively hold about 65% of the total value share ofthe global electroplating market. On the basis ofmetal type, owing to the growth of the Electrical &Electronics industry segments, copper and nickelare expected to gain significant basis points. Asmajor growth drivers Future Market Insights iden-tify applications across a diverse set of industriesand expected eminent growth in the Asia Pacificregion, while the biggest challenges according tothem will be the rise of stern laws and stringentenvironmental regulations, decelerated economicgrowth in mature markets or markets in a state ofequilibrium and the growing popularity of electro-less nickel plating.

3 Ni-P alloy electroplating

3.1 Phosphorus co-deposition mecha-nism

Ni-P electroplating is predominantly performed inan aqueous electrolytic bath that contains Ni+2 ionsas a source of nickel and a phosphorus oxyacid (orits salt) which acts as a source of phosphorus. Elec-trodeposition process is being driven by the electriccurrent passing between the anode and the cathode(plating substrate) [26]. Standard reduction poten-tials for Ni (-0.25 V) and for P (-0.28 V) are nearto each other facilitating their easy co-deposition.Earlier reviews that deal with the electrodepositionof Ni-P alloy and its composites were devised byBerkh and Zahavi [18] and by Daly and Barry [17].

Two mechanisms are generally proposed in or-der to elucidate the process of phosphorus incorpo-ration during Ni-P alloy electrodeposition, namely:direct and indirect mechanism.

In the direct mechanism it is proposed that Ni+2

ions and phosphorus oxyacid are directly reducedto Ni and P atoms which then form a NixP solidsolution. According to Brenner [28] who first sug-gested this mechanism, phosphorus oxyacid is re-

3

duced directly to phosphorus and phosphorus isco-deposited with nickel owing to the polarizationinvolved in the deposition of nickel assisting thedeposition of phosphorus (induced co-deposition).The main argument against the direct depositionmechanism is the claimed impossibility of obtain-ing phosphorus in aqueous solutions through elec-trochemical methods in the absence of metal ions[29].

According to the indirect mechanism, first pro-posed by Fedotev and Vyacheslavov [30] and sub-sequently by Ratzker et al. [31], phosphorus oxy-acid is initially reduced to phosphine (PH3) in thepresence of H+ ions, formed PH3 is then oxidizedto P in the presence of Ni+2 ions, while Ni+2 is si-multaneously reduced to Ni. Zeller and Landau[32] supported the hypothesis of PH3 formationand suggested its subsequent reaction with Ni+2

producing Ni-P, H+ and a series of phosphorousoxyacids.

Figure 1 gives an overview of the assumed halfreactions describing phosphorus incorporation ac-cording to both proposed routes, direct and indi-rect.

Figure 1: Half reactions relevant to incorporation of phosphorusinto Ni-P alloys. Reprinted with permission from [17]. Copy-right (2003) Taylor & Francis.

Occurrence of indirect mechanism has been cor-roborated by authors who have reported to havedetected phosphine [33–35]. Harris and Dang[33] quantified by chemical analysis PH3 producedduring Ni-P electrodeposition. Formation of thisspecies was also identified by Crousier et al. [29]who performed a cyclic voltammetry study. Zengand Zhou [36] obtained Raman spectra during Ni-Pelectrodeposition that indicated the formation ofNi(PH3)n intermediate species. According to theauthors, this intermediate was then oxidized byNi+2 with the consequent formation of the alloy.

Saitou et al. [34] generated analytical solutionsof kinetic equations for the reactions describingthe phosphorus co-deposition according to both ad-vocated mechanisms. The results predicted depen-dence of the phosphorous content in Ni-P electrode-posits on current density (Figure 2). Phosphorouscontent, as measured by the electron probe micro-analyzer (EPMA), was found to decrease with theincrease in current density which was in agreementwith the solution of kinetic equations correspond-ing to the indirect mechanism of phosphorus co-deposition.

Morikawa and colleagues [37] studied the elec-trodeposition of Ni-P from a citrate bath. Bind-ing energy data obtained from X-ray photoelectronspectroscopy (XPS) indicated the direct reduction ofH2PHO3 to P, implying direct deposition of nickel-phosphide. Detection of phosphine by authors ofprevious studies Morikawa et al. explained withthe possibility of partial reaction of phosphorusatoms on the electrode surface with hydrogen atlow pH values, which in their case was negligibleowing to high pH value of the employed citratebath.

Figure 2: Schematic diagram of phosphorous concentration vs.current density derived from rate equations for the indirectand direct mechanism. Reprinted with permission from [34].Copyright (2003) Electrochemical Society, Inc.

4

Ordine and coworkers [35] found kinetics ofphosphorus incorporation to be strongly affected bythe electrolytic solution pH value. They employedinterfacial pH measurements, cathodic polariza-tion curves and electrochemical impedance spec-troscopy and observed different mechanisms of Niand Ni-P electrodeposition at different degrees ofsolution acidity in the presence of NaH2PO2·H2Oin the electroplating bath.

Sotskaya and Dolgikh [38] conducted electro-chemical investigation of the kinetics regarding ca-thodic reduction of hypophosphite anion. Their re-sults indicated that phosphorus formation proceedsvia two parallel routes: electrochemical and chemi-cal, whose realization depends on the nature of theemployed metal catalyst. According to them atomicphosphorus is formed as a result of the cathodic re-duction of hypophosphite ion. They observed alsothat the Ni-P alloy electro-synthesis takes place inthe region of potentials, which does not correspondto the electro-reduction of hypophosphite anion,thus they concluded that a chemical disproportion-ation reaction of hypophosphite occurs in this case[39].

Bredael et al. [40] proposed that two mecha-nisms are involved in the formation of Ni-P elec-trodeposits, one that is active at low current effi-ciency and leads to formation of amorphous de-posits and the other dominating at high currentefficiencies and leading to formation of crystallineones. Such a claim was corroborated by polariza-tion measurements in which a change of slope ofthe polarization curve was observed, this at theaverage current density corresponding to the tran-sition from amorphous to crystalline structure.

One can infer that making an attempt to delin-eate the phosphorus co-deposition process with Niconstitutes a significant endeavour. Disagreementsexists on whether direct or indirect mechanism canbe assumed, with most of the authors supportingthe latter. Additionally, indications exist that phos-phorus incorporation is not completely straightfor-ward and that several different mechanisms mightgovern its co-deposition depending on the processconditions. In the origin of the incurred difficultieslie the large number of factors directing and influ-encing the deposition process and many differentapproaches applied to define their nature and mag-nitude, all in various electroplating environments.

3.2 Electrolytic bath composition

The properties of the electrodeposited Ni-P coat-ings depend greatly on the composition of the em-ployed electrolytic bath. The majority of nickelplating solutions are based on the ‘Watts’ formula-tion developed by Professor Oliver P. Watts in 1916[41] owing to its simplicity and low cost. Modi-fied Watts electrolyte for Ni-P plating combines thetraditional nickel sulfate, nickel chloride and boricacid with a phosphorus oxyacid which is a sourceof phosphorus. Modified Watts bath containingsodium hypophosphite as a phosphorus source hasalso been reported in many works [35, 36, 42–44].Other than sulfate electrolyte, several other kindsof Ni-P aqueous electroplating baths exist, contin-gent on the nickel source nature mostly used aresulfamate and sulfonate ones.

Nickel source–Most of the studies dedicated tonickel and nickel alloys electrodeposition havebeen restricted to simple sulfate containing or onWatts baths. A wide range of coatings can be de-posited from these versatile and stable electrolytes,thus they still remain a basis for electroplating Ni-based coatings [45].

Sulfamate baths are employed primarily for thepurposes of high speed plating and electroform-ing. Nickel sulfamate possesses high solubility inaqueous solutions, thus higher nickel concentra-tion can be achieved compared to other nickel elec-trolytes which facilitates higher plating rates [46,47]. Ni-P deposits obtained from sulfamate bathsexhibit lower internal stress, good ductility andenable higher current efficiency [4, 47, 48]. Stressvalues of electrodeposits fabricated from sulfamate,sulfate and Watts baths are reported to be approxi-mately 30, 180 and 250 MPa, respectively [48]. Nev-ertheless, several issues need to be addressed whena nickel sulfamate bath is used. Sulfamate ion isstable in neutral or slightly alkaline solutions, how-ever because of the nickel hydroxide precipitationthese solutions are not used at pH values greaterthan 5. Sulfamate hydrolysis reaction has beenfound to proceed at an increased rate at lower pHvalues and at higher temperatures, thus sulfamatebaths need to be operated at lower temperaturesand higher pH values than Watts-type nickel plat-ing solutions. pH values less than 3.0 and temper-atures above 70 ◦C ought to be avoided as nickelsulfamate can hydrolyze to its less soluble form-nickel ammonium sulfate [46, 47]. Incorporation ofammonium and sulfate ions in the deposit can leadto the internal tensile stress augmentation. Sulfa-

5

mate ion additionally tends to decompose at theanode; at insoluble anodes such as platinum and atpassive nickel oxide electrodes, and its decomposi-tion can result in intermediate species which mayaffect the quality of the electrodeposited coatings[46, 47]. However, nickel sulfamate solutions arepreferred in some cases over nickel sulfate solutionsbecause of the superior mechanical properties ofthe fabricated coatings, high rates of deposition andlower influence of variations in pH and current den-sity on the deposit’s quality [49]. Figure 3 showsthe comparison of phosphorus content, current effi-ciency and stress in the obtained Ni-P deposits asa function of phosphorous acid concentration, forthe sulfate and sulfamate electrolytes.

Another interesting electrolyte for Ni-P alloyelectroplating is the one containing methanesul-fonate or methanesulfonic acid. Sknar et al. [50]found the effect of reducing kinetic difficulties ofthe Ni-P alloy electrodeposition to be more pro-nounced when using methanesulfonate than whenemploying sulfate electrolyte. This is owing toweaker buffering properties and lower stability ofnickel acido-complexes of methanesulfonate whichcontribute to the increase of concentration of thepresent electroactive species. Alloys obtained fromsulfate baths possess higher phosphorus contentwhich is explained by the elevated acidity in thenear electrode layer due to stronger buffering prop-erties of the sulfate electrolyte [39]. Methanesul-fonic acid has good electrolytic conductivity andis capable of dissolving many metals as well asacting as a useful medium for dispersion of solidsprior to electrophoretic coating. A diverse rangeof surface coatings and films are available frommethanesulfonic acid electrolytes [51]. Comparedwith known nickel plating baths, such as Wattsbath, methanesulfonate bath can be considered toenable higher current density and result in higherthrowing power. Maximum nickel deposition ratescan be achieved with deposits having low poros-ity, low internal stress and high ductility [51]. Thisorganic acid may be considered as a ‘green’ elec-trolyte since it possesses few environmental, stor-age, transport or disposal problems being readilybiodegradable. Additionally, methanesulfonic acidexhibits high solubility for metal salts such as thoseof Pb and Ag, which are soluble only in a limitednumber of acid electrolytes [52].

Phosphorus source–Source of phosphorus in theNi-P electroplating bath is typically a phosphorusoxyacid or its salt. There exist mainly two kinds of

electrolyte systems for Ni-P electroplating: the onecontaining phosphorous acid and the other contain-ing hypophosphite. Generally, the quality of Ni-P deposits electroplated from electrolyte contain-ing hypophosphite is changeable because the de-posits darken when electroplating time gets longeror current density becomes higher, while the Ni-P deposits electroplated from electrolyte contain-ing phosphorous acid are smooth and bright [5].However, as reported, phosphine is more readilyproduced from hypophosphite than from phospho-rous acid owing to the rate of reduction of H3PO3being the limiting factor in the phosphorous co-deposition in this case [18].

Anode dissolution–If used as a contributingsource of Ni in the bath, nickel chloride has two ma-jor functions: it appreciably increases solution con-ductivity thereby reducing voltage requirementsand it is important in obtaining satisfactory dis-solution of nickel anodes [53, 54]. Due to the in-crease in solution conductivity, plating thicknessand cathodic current efficiency are reported to in-crease with the increase of chloride concentration[44]. In the chloride electrolytes activity of Ni+2

ions is higher than in sulfate electrolyte and metaldeposition potential is lower [55, 56]. However, de-posits obtained from chloride electrolytes with highchloride concentrations are reported to possess dif-ferent texture and higher internal stress comparedto those obtained from sulfate ones [56]. It is possi-ble to operate with zero chloride in the bath if sulfuractivated nickel anodes are used under appropriateoperating conditions, nonetheless a low concentra-tion of nickel chloride (min 5 g L−1) is generally rec-ommended so as to provide reasonable insuranceof satisfactory anode performance [57]. Alterna-tively, nickel bromide that does not increase stressas much as chloride may be used.

Buffering–As most authors claim, boric acid lim-its the effects on the solution pH value resultingfrom the discharge of hydrogen ions and simpli-fies pH control [58]. However, many studies showthat the beneficial influence of boric acid on Ni elec-trodeposition is somewhat complicated and that itseffect stems further from simple buffering, extend-ing to its impact on deposit’s crystallographic struc-ture, morphology, brightness, adhesion, etc. [56].Presence of boron in the bath waste is nonethelessreported to be harmful for the environment. Sincethe enforcement of Water Pollution Control Act inJapan [59] the imposed national minimum effluentstandards have greatly influenced the electroplat-

6

(a) Phosphorus content vs C(H3PO3) (b) Current efficiency vs C(H3PO3) (c) Stress in the deposit vs C(H3PO3)

Figure 3: Comparison of stated parameters’ values versus H3PO3 concentration for the cases of Ni-P alloy electrodeposition fromsulfamate and sulfate baths. Reprinted with permission from [48]. Copyright (2003) Electrochemical Society, Inc.

ing industry. Thus, it became necessary to findmore environmentally friendly solutions for theelectrodeposition of nickel and its alloys.

One of the possible alternatives is proposed inthe terms of substituting boric acid with citric acid.This species can complex free nickel ions in the so-lution influencing the deposition rate but also actsas a buffering agent [60]. Doi et al. [61] found thatthe properties of Ni electrodeposits and resultingcathode current efficiency depend on the employedcitrate bath pH. Electroplating resulted in high cath-ode current efficiency and hard deposits exhibitingcrystal structure of nearly random orientation incase of the bath pH value being between 4 and 6,while when the pH value was 3.5 or less current ef-ficiency was lower and hardness of the obtained de-posits decreased. Dadvand and colleagues [44] re-ported electrodeposition of Ni-P from a citrate bathcontaining sodium chloride, citric acid, nickel sul-fate, ammonia and sodium hypophosphite. Theyfound cathode current efficiency in low currentdensity regions to be twice higher compared tothe one achieved when employing modified Wattsbath. Additionally, obtained deposits exhibitedmore uniform plating thickness. Even though com-plexes form between nickel and citrate ions andcitrate ions as such can adsorb on the cathode sur-face, block active sights for Ni+2 discharge processand thus decrease the plating thickness, authors ofthe mentioned work estimated that this effect wassmall for citric acid concentrations below 20 g L−1.Morikawa and colleagues [37] employed Ni-citratebath and obtained uniform and bright Ni-P elec-trodeposits with high throwing power. H3PO3 wasadded in excess so as to generate a large amountof P atoms on the Ni electrode. Thus according tothem, the bath composition was such that it satis-

fied the requirement necessary to form nickel phos-phide with a stoichiometric composition equivalentto Ni3P in a wide range of current densities. Phos-phorus content in the deposits was found to beconstant around 25 at.% in a broad range of platingconditions.

When it comes to the environmental impact ex-erted by the electroplating process from either ofthe two baths: the one containing boric or the othercontaining citric acid, findings are quite contradic-tory. Takuma et al. [59] applied a life cycle as-sessment method to demarcate the extent of theirinfluence in terms of human toxicity and ecotoxi-city. Results indicated that the newly developedcitrate plating bath exerts higher environmental im-pact compared to the traditional Watts electrolyte,this owing to the release of nickel chelated withcitric acid whose harmful influence overshadowsthe benefit of reduced boron emissions achieved bysubstituting boric acid in the electroplating bath.

In the Brenner type sulfate bath [28, 62], pHbuffering is achieved generally by employing phos-phoric acid (H3PO4) whose slight excess can more-over aid in the H3PO3 oxidation prevention [63].Pillai and colleagues [63] investigated electrode-position of Ni-P alloy from a Brenner type bathcontaining both phosphorous and phosphoric acid.Their observation was that a 100% nickel coatingwithout any phosphorous incorporation was ob-tained when the plating was carried out in the ab-sence of H3PO3 indicating that phosphorous acidis the only electrochemically active phosphorousspecies which acts as a phosphorous source in thegiven solution.

Electroplating baths containing phosphorusand/or phosphoric acid can be very acidic, hencetypically partial neutralization is necessary in or-

7

der to increase the pH value of the solution up toan optimum value [62]. For this purpose differentalkaline compounds can be employed, i.e.: potas-sium or sodium carbonate, potassium or sodiumhydroxide, ammonia. Dadvand et al. [44] advo-cated the benefits of using ammonia for bath neu-tralization, as carbonate salts decrease the solutionconductivity and alkali metal hydroxides promotethe formation of nickel hydroxide complexes.

Additives–Electrolyte additives are another cru-cial factor in the Ni-P electroplating process. Theycan be incorporated in the electrolytic solution inorder to influence grain growth and crystallites ori-entation, to suppress undesirable secondary reac-tions, to promote the co-deposition of alloys, dis-solution of metals, etc. Additives can be groupedaccording to their main purpose as: carriers, surfacewetters, inhibitors or levellers, auxiliary brighten-ers, buffers, conductive electrolytes and chemicalcomplexants [64]. It is important to keep in mindthat these species are also incorporated in the coat-ing during its deposition, hence they may producevarious adverse effects on the deposit’s properties,for example decrease corrosion resistance and ex-acerbate mechanical properties. As such, their useshould be carefully optimized.

Grain refiners/carriers are usually aromatic or-ganic compounds that contain sulfur whose co-deposition with nickel inhibits its grain growth.Most commonly employed grain refiner in Ni-Pelectrodeposition is saccharin. This species is re-ported to effectively reduce the internal stress ofthe fabricated deposits [65], however its use comeswith the concerns related to deterioration of thecorrosion resistance due to incorporation of sulfurin the growing deposit [66]. Saccharin is reportedto prompt the decrease in the phosphorus contentin the coating and to bring an improvement in cur-rent efficiency [67, 68]. This is related to occupa-tion of cathode surface active centres by saccha-rin molecules and its conversion which consumesavailable hydrogen, hence restricts the phospho-rous co-deposition [68].

It is known that properties of Ni electrodepositsdepend on their crystallographic structure. Ni (100)soft-mode texture is associated with deposits thatpossess maximum ductility, minimum hardnessand internal stress [69]. Textural modifications ofthis matrix can be induced by the introduction ofcertain additives [68]. Mixed crystal orientationcan be achieved allowing to fabricate deposits withcontrolled properties.

Table 1 gives an overview of the proposed com-positions of the aqueous baths for Ni-P alloy elec-troplating. Applied deposition conditions andachieved phosphorus content where this informa-tion was available are stated along with some basicobservations.

Non-aqueous baths–All mentioned traditional Ni-P electroplating baths are corrosive and their usepossesses a drawback in terms of generating toxiceffluents. As an alternative to conventional elec-troplating solutions, You and colleagues [70] haveproposed Ni-P electrodeposition from a bath con-taining choline chloride:ethylene glycol (1:2 molarratio) deep eutectic solvent (DES) and NiCl2·6H2Oand NaH2PO2·H2O as nickel and phosphoroussources, respectively. DESs are formed from mix-tures of Lewis or Brønsted acids and bases andcan contain a variety of anionic and/or cationicspecies. Employing ionic liquids in electroplatingis beneficial owing to them being less corrosive,possessing a wider electrochemical window andexhibiting lower hydrogen evolution when com-pared to aqueous baths. As such they can present asuitable alternative for traditional plating solutions[71]. Disadvantages of DESs however include theirlower electrical conductivity, metal salts solubilityand higher viscosity in comparison with aqueouselectrolytes.

It has been proposed that environmental issuesrelated to the use of aqueous baths in Nickel elec-troplating can be mitigated also by performing elec-trodeposition in a supercritical CO2 fluid [80, 81].Chuang and colleagues [82] studied the propertiesand the electrodeposition behaviour of Ni-P coat-ings in emulsified supercritical CO2 in the presenceof suitable surfactants. They observed improvedhardness, wear resistance and surface quality of theobtained coatings compared to deposits fabricatedfrom conventional aqueous electroplating solutions.However, current efficiency from supercritical CO2bath was lower.

3.3 Current efficiency

In accordance with both proposed phosphorus in-corporation mechanisms, direct or indirect, concur-rent reduction of protons at the surface of the grow-ing deposit layer is necessary for the co-depositionof phosphorus with nickel (Figure 1). However,subsequent recombination of formed Hads limitsthe amount of phosphorus that can be deposited.Thus, surface H+ concentration affects both thephosphorus content and the cathode current effi-

8

Table 1: Proposed bath compositions and deposition conditions for Ni-P alloy electroplating from sulfate, sulfamate and sulfonatebaths according to various literature sources. Obtained phosphorus content is stated where applicable and some basic observationsare noted. (jd-current density, t-temperature, ε-duty cycle, f-frequency, CE-cathode current efficiency)

Ref

eren

ceBa

thco

mpo

sitio

nD

epos

ition

cond

ition

sPh

osph

orus

cont

ent

Add

ition

alin

form

atio

n

I:N

iSO

4·6H

2O33

0g

L−

1pH

1.7-

3.0

NiC

l 2·6

H2O

45g

L−

163

◦ CH

3BO

330

gL−

12-

5A

dm−

22-

3%

PH

3PO

30.

225-

2.25

gL−

1

Non

-pit

ting

agen

t0.1

5g

L−

1

Dur

ney,

1984

[72]

II:N

iSO

4·6H

2O15

0g

L−

1pH

0.5-

1.0

NiC

l 2·6

H2O

45g

L−

185

◦ CH

3PO

450

gL−

12-

5A

dm−

212

-15%

PH

3PO

340

gL−

1

NiS

O4·

6H2O

150

gL−

1M

axim

umP

cont

entf

rom

bath

sN

iCl 2

·6H

2O45

gL−

1an

d:I,I

I,III

and

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achi

eved

for

follo

win

gco

ndit

ions

:I:

H3P

O4

50g

L−

180

and

90◦ C

I:35

wt.%

Pfo

r40

gL−

1

H3P

O3

0-40

gL−

1<

10A

dm−

2H

3PO

3an

d80

◦ C(C

E∼

25%

)

Nar

ayan

and

II:H

3PO

40-

200

gL−

180

and

90◦ C

upto

35w

t.%P

II:∼

20w

t.%P

for

125

gL−

1

Mun

gole

,198

5[7

3]H

3PO

320

gL−

1<

10A

dm−

2H

3PO

4an

d80

◦ C(C

E∼

50%

)

III:

H3P

O4

50g

L−

190

◦ CII

I:∼30

wt.%

Pfo

r12

gL−

1

H3P

O3

20g

L−

1<

10A

dm−

2ca

rbon

ate

(CE∼

70%

)N

iCO

3·N

iOH

2·4H

2O0-

15g

L−

1

IV:H

3PO

450

gL−

170

-90◦ C

IV:3

0w

t.%P

for

5A

dm−

2

H3P

O3

20g

L−

15-

40A

dm−

2an

d70

◦ C(C

E∼

25%

)N

iSO

4·6H

2O15

0g

L−

1pH

0.43

-1.0

Tran

siti

onto

amor

phou

sBr

edae

leta

l.,19

93[4

0]N

iCl 2

·6H

2O50

gL−

160

◦ Cup

to20

wt.%

Pst

ruct

ure

at≥

12w

t.%P

H3P

O4

42.5

gL−

12-

150

Adm

−2

H3P

O3

3-70

gL−

1R

DE

750

rev/

min

NiS

O4

0.38

MpH

1.5-

4.5

Max

imum

Pco

nten

t25

at.%

NiC

l 20.

13M

30-9

0◦ C

0-25

at.%

Pfo

r∼

0.5M

H3P

O3,

at60

◦ C,

Mor

ikaw

aet

al.,

1996

[37]

H3B

O3

0.49

M1-

30A

dm−

23

Adm

−2

and

pH3.

5C

itri

cac

id0.

5M(C

E∼

25%

);H

3PO

30-

2MFo

rH

3PO

3>

0.5M

Ple

vels

off

9

Ref

eren

ceBa

thco

mpo

sitio

nD

epos

ition

cond

ition

sPh

osph

orus

cont

ent

Add

ition

alin

form

atio

n

NiS

O4·

6H2O

330

gL−

1pH

1.0

and

4.0

13-2

8at

.%P

(pH

1.0

Com

posi

tion

cont

rolb

yH

uan

dBa

i,20

01[4

2]N

iCl 2

45g

L−

120

and

50◦ C

,NaH

2PO

2·H

2O1M

)si

mul

tane

ous

chan

geH

uan

dBa

i,20

03[7

4]H

3BO

337

gL−

15

and

25A

dm−

24-

12at

.%P

(pH

4.0

ofta

ndjd

;Am

orph

ous

NaH

2PO

2·H

2O0.

5an

d1M

200

and

400

rev/

min

,NaH

2PO

2·H

2O0.

5M)

stru

ctur

efo

r≥

12at

.%P

NiS

O4·

6H2O

150

gL−

1pH

1.8

Prim

ary

nucl

eati

onof

Ni

NiC

l 2·6

H2O

45g

L−

155

◦ C27

at.%

Pfo

llow

edby

Ni-

Pfo

rmat

ion;

Kur

owsk

ieta

l.,20

02[7

5]H

3PO

450

gL−

1-4

20m

V(S

HE)

Gro

wth

driv

enby

NiC

O3

40g

L−

1su

rfac

edi

ffus

ion

ofH

3PO

340

gL−

1N

iand

Psp

ecie

sN

iCl 2

·6H

2O0.

2MpH

3.3

10at

.%P

for

4MN

H4C

lLi

etal

.,20

03[7

6]N

aH2P

O2·

H2

0.1M

25◦ C

2-10

at.%

P(C

E∼

60%

)N

H4C

l0.5

-4M

5-50

Adm

−2

NiS

O4·

6H2O

137

gL−

1pH

1.5

Nan

ocry

stal

line

depo

sit;

NiC

O3

36.5

gL−

170

◦ C0.

5-2.

5w

t.%P

Max

imum

hard

ness

(990

HV

)Je

ong

etal

.,20

03[7

7]H

3PO

32-

3g

L−

11-

3A

dm−

2an

dw

ear

resi

stan

ceaf

ter

Sacc

hari

n5

gL−

1M

agne

tic

stir

ring

heat

trea

tmen

tat3

70◦ C

SLS

0.1

gL−

1

Ni(

SO3N

H2)

2·4H

2OpH

1.0-

2.5

Max

imum

Pco

nten

t14

wt.%

(Ni+2

90g

L−

1 )50

◦ C2-

12w

t.%P

for

ε=0.

1an

df>

100H

zLi

net

al.,

2006

[4]

NiC

l 2·6

H2O

3g

L−

1jp

8A

dm−

2(C

E∼

80%

);C

hen

etal

.,20

10[6

5]H

3BO

340

gL−

110

-500

Hz

Dep

osit

still

crys

talli

ne;

H3P

O3

10g

L−

1ε

0.1-

0.5

Com

pres

sive

inte

rnal

stre

ssSD

S0.

4g

L−

1A

irbu

bblin

g(-

40M

Pa)

NiS

O4·

6H2O

240

gL−

1pH

0.8-

1.8

Max

imum

Pco

nten

tN

iCl 2

·6H

2O28

gL−

145

and

75◦ C

upto

10w

t.%P

for

high

erH

3PO

3

Yuan

etal

.,20

07[5

]H

3BO

330

gL−

14

and

10A

dm−

2co

ncen

trat

ion

and

H3P

O3

2an

d8

gL−

1M

agne

tic

stir

ring

low

erpH

valu

es15

0an

d55

0re

v/m

inN

i(SO

3NH

2)2·

4H2O

pH1.

5-3.

5M

axim

umP

cont

ent

(Ni+2

96g

L−

1 )45

◦ Cfo

rhi

gher

H3P

O3

Cha

nget

al.,

2008

[78]

NiC

l 2·6

H2O

4g

L−

12

Adm

−2

1-4.

5w

t.%P

conc

entr

atio

nan

dH

3BO

332

gL−

1A

irbu

bblin

glo

wer

pHva

lues

H3P

O3

0-4

gL−

1(C

E∼

70%

)W

etti

ngag

ent0

.3m

LL−

1

10

Ref

eren

ceBa

thco

mpo

sitio

nD

epos

ition

cond

ition

sPh

osph

orus

cont

ent

Add

ition

alin

form

atio

n

NiS

O4·

6H2O

170

or33

0g

L−

1pH

0.5-

3.0

Schl

esin

ger

and

NiC

l 2·6

H2O

35-5

5g

L−

160

-95◦ C

Paun

ovic

,201

0[7

]H

3BO

30

or40

gL−

12-

5A

dm−

2

H3P

O4

50or

0g

L−

1

H3P

O3

2-40

gL−

1

NiS

O4·

7H2O

150

gL−

1pH

∼1.

5P

cont

entd

ecre

ases

wit

hN

iCl 2

·6H

2O45

gL−

150

-80◦ C

0-20

wt.%

PH

3PO

3co

ncen

trat

ion

decr

ease

H3P

O4

0-40

gL−

15-

30A

dm−

2an

din

crea

seof

jdan

dt

Pilla

ieta

l.,20

12[6

3]H

3PO

30-

20g

L−

1(u

pto

15g

L−

1 H3P

O3)

;SL

S0.

25g

L−

1D

epos

itam

orph

ous

for≥

9w

t.%P;

Har

dnes

s8.

57G

Pa(4

-7w

t.%P)

,af

ter

anne

alin

gat

400◦ C

max

imal

hard

ness

(12

GPa

)N

iSO

4·6H

2O0.

65M

pH1.

5Tr

ansf

orm

atio

nto

crys

talli

neN

iCl 2

·6H

2O0.

75M

10.6

at.%

Pat

500-

600◦ C

;N

ava

etal

.,20

13[2

0]H

3BO

30.

15M

3A

dm−

2A

nnea

led

depo

sith

ardn

ess

NaC

l2M

990

HV

H3P

O3

0.1M

NiS

O4·

6H2O

0.2M

pH3.

0-4.

0at

surf

ace:

Soft

mag

neti

cch

arac

ter,

H3B

O3

0.00

5M70

◦ C∼

8-12

at.%

Ppl

atin

gco

ndit

ions

depe

ndan

t;A

lleg

etal

.,20

16[7

9]N

aH2P

O2·

H2O

0.1M

-1.3

to-1

Vac

ross

dept

h:D

epos

its

mix

ture

sof

Sacc

hari

n0.

005M

∼4-

6at

.%P

Ni(

P)so

lidso

luti

ons

and

NaC

l0.7

Mam

orph

ous

orN

i 2P

phas

eI:

Ni(

CH

3SO

2)2

1MpH

3.0

Max

Pco

nten

tfor

0.12

MN

aCl0

.3M

60◦ C

NaH

2PO

2an

d2

Adm

−2 :

NaH

2PO

20.

03-0

.12M

2-7

Adm

−2

∼8

(I)a

nd∼

13w

t.%P

(II)

;Sk

nar

etal

.,20

17[5

0]up

to13

wt.%

PII

:NiS

O4

1MP

cont

enth

ighe

rfr

omN

aCl0

.3M

sulf

ate

bath

;H

3BO

30.

7MIn

tern

alst

ress

and

hard

ness

NaH

2PO

20.

03-0

.12M

bett

erfr

omsu

lfam

ate

11

ciency whose magnitude can never reach valuesclose to 100% as long as the phosphorus is co-deposited with nickel.

Cathode current efficiency of Ni-P electroplat-ing is primarily affected by the electrolytic bathcomposition, plating regime, applied current den-sity, temperature, bath pH value and agitation rate.Ross et al. [83] found that the current efficiencydecreases as the deposit’s phosphorus content in-creases and that it depends on the rotation speedof the cathode. In the case of rotating-disc elec-trode (RDE), maximum efficiency was attained forlow phosphorus plating solutions and low rotationspeeds. In general, electrodeposition of high phos-phorus Ni-P coatings proceeds at a lower current ef-ficiency than the electrodeposition of low phospho-rus ones [4, 40]. Naryan and Mungole [73] foundthat the current efficiency decreases with increasingH3PO3 concentration and decreasing temperatureof the sulfate electroplating bath. Similarly, Seoand colleagues [48] observed that with increasingH3PO3 concentration in the sulfamate bath, phos-phorus content in the deposit increases, currentefficiency decreases and stress in the obtained de-posits augments. Morikawa [37] observed that incitrate bath current efficiency exhibits a maximumat intermediate H3PO3 concentrations.

Cathode current efficiency in Ni-P electroplat-ing is generally found to increase with the increaseof current density [25, 84, 85], bath temperature [18]and its pH value [18, 86]. Luke [25] found that thecathode current efficiency does not vary apprecia-bly with current density in the higher range (above∼20 A dm−2) but increases with the increase of cur-rent density at its lower values. Similar trend wasobserved by Toth-Kadar et al. [85]. Li et al. [76]found cathode current efficiency to be lower for alarger total current density. This is owing to thedischarge reaction of H+ ions which results in theevolution of hydrogen gas bubbles. This reactionshares a larger portion of the electrodeposition cur-rent at higher total current densities.

In general, reactions involved in the conversionof phosphorus in the solution into phosphorus inthe deposit can be interpreted differently and thusthe discrepancies in elaborating cathode currentefficiency behaviour may arise.

Cathode current efficiency of Ni-P alloy elec-trodeposition can be as reported significantly im-proved by employing pulse instead of direct cur-rent plating [4]. Benefits of using pulse plating forNi-P deposits fabrication are explored in Section 6.

As mentioned in the previous section, certainadditives such as saccharin are found to improvethe current efficiency of Ni-P electrodeposition [68].

In Ni-P electroplating, anodes are usually madeof nickel and current efficiency of nickel dissolu-tion in additive free solution is approximately 100%(small percentage of the current is consumed onthe discharge of hydrogen ions from water). Hencethere exists a difference in cathode and anode ef-ficiency which leads to nickel ions build up in thesolution and its pH value increase. This problemcan be solved by solution drag-out [87] and by theuse of soluble and inert anodes connected to dif-ferent power supplies, while the current to eachis controlled in order to compensate for the lowcathode efficiency and incurred drag-out losses [7,25].

3.4 Crystallographic structure

Ni-P electrodeposits’ crystallographic structure isstrongly influenced by their phosphorus contentand the employed deposition conditions. Incor-poration of even small amounts of phosphorousin the nickel lattice substantially refines the nickelgrains. In the case of low-phosphorus Ni-P elec-trodeposits, obtained XRD patterns reveal a set ofdiffraction peaks corresponding to face-centred cu-bic (f.c.c.) nickel, i.e.: (111), (200), (220), (311), (222)[88]. Presence of these textures is an indicationof crystallinity of the low-phosphorus coatings inwhich case a material is a supersaturated solutionof phosphorus in f.c.c. nickel [83]. As the phos-phorus content in Ni-P deposits increases, (111)reflection becomes broader while others disappear,as it can be seen on Figure 4. This is indicativeof nickel crystalline structure losing its long-rangeorder due to the increased difficulty of more phos-phorus atoms accommodating into the nickel lat-tice. Co-deposition of phosphorus in octahedralinterstial sites of f.c.c. nickel inhibits the surfacediffusion of Ni atoms and the subsequent crystalgrowth [17]. As the phosphorus content in the de-posit increases the nucleation becomes dominantover nucleus growth and when the critical phos-phorus content is exceeded amorphous structurecharacterized by a short range order of only fewatomic distances is obtained. More colony-like mor-phology is achieved where each colony consists ofnumerous grains with smaller grain size therebymaking the coating brighter and smoother in ap-pearance [63]. Figure 5 shows SEM photographs ofNi-P deposits with varying phosphorus contents.

12

Figure 4: XRD patterns of polycrystalline Ni and nanocrystallineNi-P coatings. Reprinted with permission from [77]. Copyright(2003) Elsevier.

At high-phosphorus content XRD pattern of aNi-P electrodeposit exhibits only a diffuse broadpeak. Hence, an increase in phosphorus contentconverts the microstructure of Ni-P alloys fromcrystalline to amorphous, which results in the de-creased ductility and the increased corrosion re-sistance. It has been hypothesized that adsorbedhydrogen blocks the surface and prevents regularcrystal growth, hence playing a crucial factor in ob-taining N-P deposits with amorphous structure,however amorphous deposits can be fabricatedwith high current efficiencies and low hydrogenevolution thus indicating that the adsorbed hydro-gen does not play a major role [17].

Bredael et al. [89] observed significant broaden-ing of the (111) XRD peak for Ni-P deposits contain-ing more than 9 wt.% of phosphorus. However theystated that the observed effect, other than being asign of the structure becoming amorphous, can alsooriginate from a crystalline material with very finegrains (1nm) and may be due to non-uniform in-ternal stresses and stacking faults. Hence, as theyasserted, it is not possible to exactly distinguishbetween amorphous and microcrystalline structurebased only on XRD patterns. Nonetheless, conven-tionally Ni-P deposits with more than 9 wt.% of

phosphorus are entitled X-ray amorphous.Owing to the variations of the pH value at the

electrode-solution interface inherent to the Ni-Pelectroplating process, phosphorus content variesacross Ni-P deposit thickness, hence its composi-tion and microstructure are not homogeneous (Sec-tion 7). Alkaline solutions are more influenced bythe pH variations compared to acidic ones, thuselectrodeposits produced in this environment areoften characterized by a lamellar structure [79].

Ni-P electrodeposits can be grouped in three cat-egories, alloys with: low (1-5 wt.%), medium (5-8wt.%) and high (above 9 wt.%) phosphorus content[17]. Microstructure-wise, it is possible to fabricatepolycrystalline, microcrystalline, nanocrystalline orfully amorphous Ni-P deposits through electroplat-ing.

Pillai et al. [63] found the structure of Ni-P alloyto undergo transition from crystalline to nanocrys-talline and become amorphous at phosphorus con-tents above 9.14 wt.%. Bredael and coworkers [40]fabricated Ni-P electrodeposits with phosphoruscontents ranging from 0 to 18.0 wt.% and a uni-form composition profile across the samples. Theyobserved that for phosphorus contents above 12wt.% the as-plated Ni-P coatings were amorphous,whereas below this threshold value, which was in-dependent of the plating parameters, crystallineNi-P coatings were obtained. In a subsequent workBredael and colleagues [89] conducted a study withthe goal to exactly identify the percentage of phos-phorus at which the transition from crystalline toamorphous structure takes place. They found thatthe border between these two states can be foundat phosphorus contents between 11.6 and 13.1 wt.%P, with the structure being fully amorphous at Pcontents ≥13.1 wt.%. Although there is no full con-sensus many authors agree that the phase transitionof Ni-P happens over a wide range of wt.% of Pand not abruptly at a specific phosphorus content[63]. Lin et al. [4] found that not only phosphoruscontent influences the crystallographic structureof Ni-P electrodeposits. At the same phosphoruspercentage substituting direct current depositionby pulse current plating can result in preservingcrystalline structure even at phosphorus contentshigher than a critical value. Contrary, phospho-rus content at which transition from crystalline toamorphous structure occurs has been reported todecrease in the presence of certain additives, suchas saccharin for example [17].

In general, discrepancies which exist in the lit-

13

erature related to the structural transitions of Ni-P electrodeposits can be attributed to the lack ofappropriate techniques that would allow to makea proper differentiation between different crystal-lographic structures. Additionally, Ni-P alloysare frequently not straightforward amorphous orcrystalline but they represent a mixture of severalphases. This can be owing to the low solubility ofphosphorus into the nickel lattice. Namely, oftenNi-P deposits contain more of the alloying element(P) than what the f.c.c. Ni matrix can dissolve, thusthe surplus must separate out resulting in regionsthat have a different concentration of phosphorus,different lattice parameters and different crystal-lite sizes. Vafaei-Makhsoos et al. [90] found alloyswith 3.8 and 6.7 wt.% of P to be solid solutionscomposed of micrometer-sized grains. However,amorphous alloys (11.7 and 13 wt.% P) were not ho-mogeneous on a microscale and films contained mi-crocrystalline regions. Ni-P electrodeposits display-ing a structure that represents a mixture of solidNi(P) solutions and amorphous or nanocrystallinephases have been reported [79].

Microstructure of Ni-P electrodeposits under-goes a transformation if they are exposed to a sub-sequent thermal treatment. This aspect is furtherelaborated in Section 5.

Suitable structure of the electroplated Ni-P al-loy depends on the desired features of the fabri-cated deposit. Crystalline materials are tradition-ally interesting on account of their exceptional me-chanical properties. Amorphous materials are veryappealing owing to the isotropy of their proper-ties and them lacking disadvantages characteristicfor crystalline ones, namely the presence of crystalboundaries, lattice defects, segregation, etc. [91].Amorphous Ni-P electrodeposits are reported topossess better corrosion resistance when comparedto crystalline deposits. However, transition to amor-phous structure and consequent decrease of thegrain size causes deterioration in mechanical prop-erties which is termed an inverse Hall-Petch ef-fect [92]. Its onset is related to the increase of thevolume fraction of the triple junctions relative tothe volume fraction of the grain boundaries whichexerts a detrimental effect on coating’s hardness[93]. Jeong et al. [94] produced Ni-P electrode-posits with grain size of less than 10 nm, therebyachieving hardness values that fall into the range ofthe inverse Hall–Petch behaviour. Recently exten-sively reported are the results testifying even bettermechanical properties of nanocrystalline materials

(Figure 9) when compared to both coarse-grainedand amorphous ones [10, 64, 95, 96]. The mainfeature, which makes nanocrystalline materials dis-tinct from the other two mentioned groups, is theexistence of a larger number of atoms disposed atinterfaces, as grain boundaries and triple junctions.These interfaces are considered to be involved inthe deformation mechanism occurring in the mate-rial. It is postulated that for nanocrystalline materi-als dislocations inside the grains hardly occur andthat other plastic deformation mechanisms, suchas grain boundary diffusion and sliding, grain ro-tation and a viscous flow of interfaces take place[88].

3.5 Variables influencing phosphoruscontent

Phosphorus content of Ni-P electrodeposits report-edly depends on many factors, dominantly on theconcentration of the phosphorus donor in the bath,temperature, pH, current density, waveform in thecase of pulse current deposition, agitation rate andthe presence of additives. Thus, phosphorus quan-tity can be controlled by altering the electroplat-ing bath composition and the employed depositionconditions.

In the 1950s, Brenner [28] found that the amountof phosphorus incorporated in the Ni-P alloy in-creases with the increase of phosphorus acid con-tent in the bath and with the decrease of the currentdensity. However, too high H3PO3 concentrationleads to deterioration of the current efficiency ow-ing to more cathodic charges being spent on thereduction of protons. Additionally, proton reduc-tion results in the atomic and molecular forms ofhydrogen, both of which can be incorporated inthe deposit imposing inauspicious effects on itsproperties and increasing its internal stress [97].Chang et al. [78] found that for the same content ofH3PO3, phosphorous content can be increased bydecreasing electrolytic bath pH value on accountof the consequent increase of the concentration ofnon-dissociated phosphorous acid molecules in theelectrolyte. This effect is however weakened at verylow phosphorus acid concentrations on account ofdiffusion becoming the rate-controlling process.

Yuan and coworkers [5] conducted a study onthe preparation of amorphous-nanocrystalline Ni-Pelectrodeposits from a Brenner type plating bath.Their goal was to interrogate the key electroplatingfactors and their influence on the deposit’s phos-

14

Figure 5: SEM photographs of Ni-P deposits with P contents of (a) 0, (b) 4, (c) 8, (d) 12, (e) 17, (f) 20, (g) 24 and (h) 28 at.%. Reprintedwith permission from [74]. Copyright (2003) Elsevier.

phorus content. Effects of: temperature, currentdensity, pH, H3PO3 concentration and agitationrate were investigated in the orthogonal experimen-tal design study. Findings indicated that only pHvalue and H3PO3 concentration and their interac-tion are the key variables affecting phosphorus con-tent in the deposit. Figure 6 shows contour plotsfor the constant phosphorus content versus Ni-Pelectroplating pH and H3PO3 concentration.

Hu and Bai [42] employed modified Watts Nibath and similarly investigated the influence of themain electroplating variables, namely: temperature,current density, pH, NaH2PO2·H2O concentrationand agitation rate on the phosphorus content in thefabricated Ni-P deposits. By using experimentalstrategies such as: fractional factorial design, pathof steepest ascent and central composite design cou-pled with response surface methodology they cameto the conclusion that the predominant factors af-fecting phosphorus content are temperature and

current density of the electroplating with stronginteractive effect between current density and pH.Figure 7 shows contour plots for the constant phos-phorus content versus Ni-P electroplating tempera-ture and current density.

Figure 6: Contour plots for the constant phosphorus content (Pwt.%) of Ni-P deposits against the electroplating pH (XC) andH3PO3 concentration (XD). Reprinted with permission from [5].Copyright (2007) Elsevier.

15

Figure 7: Contour plots for the constant phosphorus content (Pat.%) of Ni-P deposits against the electroplating temperature(xA) and current density (xB). Reprinted with permission from[42]. Copyright (2001) Elsevier.

Figure 8: Literature results on phosphorus content vs currentdensity (up to 40 A dm−2). Reprinted with permission from [40].Copyright (1993) Elsevier.

Recurring trend of gradual decrease of phos-phorous content with the increase of the appliedcurrent density is observed in the work of manyauthors, however a large scatter between the datapoints of different authors can be perceived (Figure8) [40].

Bredael and coworkers [40] observed, in theirstudy of Ni-P electrodeposition on a rotating-discelectrode, a steep transition from high to low phos-phorus content with increasing local current den-sity, while the literature data according to them donot show such a steep transition owing to the gen-eral approach where average current densities areused. In the bath where phosphorous acid is theonly electrochemically active phosphorus species,dependence of the phosphorus content on the cur-

rent density can be explained in terms of slow dif-fusivity of the large phosphorus acid ions com-pared with Ni+2 ions [40]. Pillai et al.[63] foundthat at higher H3PO3 concentration in the bath (≥15 g L−1) phosphorus content in the coating is in-dependent of current density. However, at lowphosphorous acid amounts in the bath, the decreas-ing trend of phosphorus content in the coating withincreasing current density was evident. Thus, theyconcluded that the observed scatter in the resultspresented by different authors, when it comes to theinfluence of current density on phosphorus content,can be mainly due to the different bath composi-tions used. In their study, an increase in the con-centration of H3PO4 resulted in no significant effecton the phosphorus content in the coating althoughthe rate of deposition decreased continuously. Anincrease in the plating temperature (from 50 ◦C to80 ◦C) resulted in the decrease of the amount ofphosphorous incorporated in the coating and inthe increase of the deposition rate. Naryan andMungole [73] observed contrary to the previouslymentioned study a slight increase of the phospho-rus amount in the coating up to H3PO4 concentra-tions of 125 g L−1 and a slight decrease with furtherincrease in H3PO4 concentration, all at the bath tem-perature of 80 ◦C. At 90 ◦C an increase in the H3PO4concentration generally produced a slight decreasein the phosphorus content in the coating. Naryanand Mungole additionally found that at low H3PO3concentrations, more phosphorus was depositedat 90 ◦C than at 80 ◦C, but at H3PO3 contents in ex-cess of 25 g L−1 more phosphorus was depositedat lower temperature. Sadeghi [21] employed hy-pophosphite as a phosphorous source and foundthat increasing its concentration and decreasing thecurrent density (1-4 A dm−2) both caused higherphosphorous contents in the fabricated Ni-P de-posits. However, the content of phosphorus in thedeposits electroplated from the baths containingvery low phosphorus source amounts did not varyappreciably with the current density. Deposits ob-tained from the baths containing up to 10 g L−1 ofNaH2PO2 exhibited a decrease in the content ofphosphorus as the current density was raised.

The increase in temperature has been observedto cause a decrease in phosphorus content in gen-eral. However, too low temperature leads to unsat-isfactory current efficiency and plating rate, thus acompromise is needed. Temperatures from around50 ◦C to 70 ◦C are generally adopted as acceptablefor optimal feasibility of the Ni-P electroplating

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process.Phosphorus content in the deposit has been re-

ported to be influenced by the presence of certainadditives in the electroplating bath. Increase of thesaccharin concentration was found to induce thedecrease of phosphorus content [17, 67, 68].

Influence of the pulse plating on Ni-P alloystructure and composition will be further elabo-rated in Section 6.

4 Properties of the electroplatedNi-P alloys

4.1 Mechanical, tribological and corro-sion properties

Alloying nickel with phosphorus via electroplatingeffectuates many improvements of its properties:increased hardness (&500 HV as-plated Ni-P), wearand corrosion resistance, decreased coefficient offriction (0.4-0.7 as-plated Ni-P), but also transitiontowards paramagnetic features. Overall, proper-ties and functional behaviour of Ni-P electrode-posits depend on their composition and microstruc-ture. Ni-P coatings with low phosphorus contentdemonstrate high hardness and good wear resis-tance, while coatings with higher phosphorus con-tent exhibit good corrosion resistance but poor me-chanical properties owing to the transition towardsamorphous structure. Amorphous Ni-P depositsare generally brittle and possess low ductility. Atthe same phosphorus content, microstructure ofthe deposit can be modified by altering the depo-sition conditions. Applying pulse plating insteadof direct current plating can help to preserve Ni-Pcrystalline structure even at higher incorporatedphosphorus quantities, thus good mechanical prop-erties characteristic for low-phosphorus depositscan be maintained [4].

Initially, microhardness of the alloy increaseswith phosphorus content augmentation at very lowphosphorous amounts, while further enhancementof phosphorous quantity leads to hardness deteri-oration [18]. Figure 9 shows the evolution of thematerial’s hardness with the change in its grainsize. Nava et al. [20] found that the wear volumeof Ni-P electrodeposits is inversely proportionalto the microhardness of the deposits. They per-ceived also that nanocrystalline Ni-P coatings, withphosphorus content of 2-8 %, exhibit significantlyhigher wear resistance compared to coatings with

higher phosphorus content. Pillai et al. [63] ob-served that Ni-P electrodeposits with phosphoruscontent in the range of 4-7 wt.% exhibit good micro-hardness (7.74–8.57 GPa) and the microhardness ofthese alloys increases up to 12 GPa by annealingat 400 ◦C during 1 h. Yuan and colleagues [5] re-ported that nanocrystalline composites with phos-phorus content 5-9 wt.% demonstrate better corro-sion resistance than that of microcrystalline Ni-Pdeposits, and better abrasion resistance than that ofamorphous Ni-P deposits. Therefore, under certainwork conditions, when a compromise is needed interms of obtaining both corrosion resistance andfavourable mechanical properties, nanocrystallinedeposits could present an optimal choice.

Another way to ensure good mechanical andelectrochemical properties is to design multilayeredor graded coatings. For example, a duplex coatingthat consists of an inner layer having high phospho-rus content and an outer layer with low phosphoruscontent presents a structure that combines advan-tageous features characteristic for each of the twocompositions [14, 15, 98]. Fabricating graded struc-tures can also help to improve coating‘s adhesionon the substrate owing to the gradual structure evo-lution across layer thickness and the lack of abruptinterfaces (Section 7).

Adhesion of the coating on the substrate’s sur-face is critical for all possible functional applica-tions. Ni-P electrodeposits exhibit good adhesionon a range of widely differing substrates (brass, softsteel, copper, etc.). However, stainless steel for ex-ample can be problematic owing to the formationof a passive oxide layer which prevents Ni-P coat-ing from sticking to its surface. Conventional wayof addressing this issue is to apply on the substratesurface a Wood’s or another nickel strike beforeNi-P electroplating [99]. Being highly acidic it dis-solves the oxide and concurrently forms a thin layerof nickel on the stainless steel surface.

Published data offer diverse information regard-ing corrosion characteristics of Ni-P alloy, particu-larly about the nature of its anodic dissolution, abil-ity to passivate and pitting susceptibility. Corrosionresistance of Ni-P coatings is much better than inthe case of pure Ni [18]. In chloride containing andslightly acidic environments Ni-P electrodepositsdemonstrate lower corrosion resistance when com-pared to neutral or slightly alkaline settings [100].

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Figure 9: Hardness of a material as a function of its grain size[64].

Amorphous and crystalline deposits have beenfound to exhibit different corrosion behaviour [17].Generally the weight loss of Ni-P deposits in corro-sive solutions decreases with the increase of theirphosphorus content [101]. Splinter and cowork-ers [102] reported the preferential dissolution of Nion the surface of the Ni-P coating and the surfacewith the enriched P content after corrosion. Theyasserted that the formation of Ni3(PO4)2 film onthe surface impedes the speed of corrosion. Addi-tionally, their study demonstrated that the corro-sion behaviour of nanocrystalline Ni-P alloys withlower phosphorus contents (1.4 wt.% and 1.9 wt.%)in 0.1M H2SO4 approaches that of amorphous Ni-Pelectrodeposits (6.2 wt.% P). X-ray photoelectronspectroscopy revealed that neither the nanocrys-talline (1.9 wt.% P, grain size 8.4 nm) nor the amor-phous (6.2 wt.% P) Ni-P alloys formed a passivelayer in this environment. Diegle et al. [103] re-ported that the addition of phosphorus improvesthe corrosion resistance of nickel owing to the re-action with water in which hypophosphite ionsare formed. These ions prevent further dissolu-tion of nickel through chemical passivation. Kro-likovski and Butkiewicz [104] determined that thebehaviour of Ni-P alloys is similar in neutral solu-tions under open circuit potential. However, underconditions of anodic polarization amorphous al-loys exhibit dissolution suppression while in caseof crystalline alloys intensive dissolution occurs.

4.2 Internal stress

Well controlled internal stress is a very importantcriteria for successful fabrication and use of protec-tive Ni-P coatings. Compressive stresses are advan-tageous in mechanical structures under load, sincethey tend to inhibit crack formation and growth,they rise strength and hardness of the coating [21].Factors influencing stress are many [47]. Among

them, the choice of the electroplating bath com-position can very much aid in obtaining depositswith low values of this quantity. Baudrand [47]fabricated electrodeposits from a sulfamate bathwithout the presence of additives with stress valueof approximately 30 MPa, while the minimal valueof stress for deposits obtained from a Watts bathwas 180 MPa.

The amount of phosphorus, hence crystallo-graphic structure, of the Ni-P electrodeposit in-fluences its stress value. Lin et al. [97] studiedNi-P electrodeposition from a sulfamate bath con-taining no additives. They observed that amor-phous Ni-P deposits exhibit lower internal stressescompared to crystalline ones. The internal stressand the amount of adsorbed hydrogen exhibiteda maximum at intermediate phosphorus contentssuggesting that hydrogen incorporation and subse-quent escape play an important role when it comesto the internal stress of the Ni-P electrodeposits.Nonetheless, high phosphorus content character-istic for amorphous Ni-P alloys comes with lowercathode current efficiency, thus conditions must becarefully optimized in order to secure optimal fea-sibility of the process along with desired propertiesof the obtained deposits.

Certain organic additives are known to induce areduction of the internal stress of the Ni-P deposits.A grain refiner saccharin is typically employed forthis purpose, inducing compressive internal stress[10, 17, 46, 105]. Nevertheless, additives contain-ing sulfur can cause deterioration in the corrosionresistance of the fabricated coatings hence their ap-plication must be carefully optimized [66].

Another possibility to reduce stress of a Ni-Pelectrodeposit is to apply a low temperature heattreatment that would help to desorb hydrogenpresent in the coating [17].

Employing pulse current instead of direct cur-rent deposition is reported to be beneficial for stressreduction. Chen et al. [65] fabricated Ni-P depositswith high phosphorus contents and low internalstresses, ranging from tensile to compressive, withhigh current efficiency by using pulse current in anickel sulfamate bath without the addition of anystress reducers.

4.3 Magnetic properties

Nickel is a typical ferromagnetic material, whilemagnetic properties of the Ni-P alloy present a func-tion of its composition and preparation technique[106]. Magnetic moment and Curie temperature are

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Figure 10: SEM images of wear track of Ni-P coatings, heat treated at different temperatures after sliding against AISI 8620 ball inair: a) without heat treatment, b) 200, c) 300, d) 400, e) 500, f) 600 ◦C. Reprinted with permission from [20]. Copyright (2013) ESG.

found to decrease with the introduction of phospho-rus into Ni matrix [107]. According to Weiss theory,nickel exhibits ferromagnetic properties owing toquantum-mechanical exchange forces which causethe spins of vicinal Ni atoms to be parallel [108].The introduction of phosphorus into nickel matrixenlarges the separation between Ni atoms and withthe increase of interatomic distance exchange forcesdecrease rapidly. Stated effect renders the transi-tion from ferromagnetic to paramagnetic properties[109].

Bakonyi et al. [106] performed a study of themagnetic properties of Ni100-xPx alloys fabricatedby employing various techniques. For electrode-posited alloys the composition range studied was11&x.23. Throughout the whole concentrationrange magnetic inhomogeneities were observed.Alloys were found to exhibit paramagnetism atx&17. Hu and Bai [74] observed that the ferromag-netic property of Ni was transformed into param-agnetism at phosphorus content of 17 at.% . Theycorroborated that the magnetic properties of Ni-Palloys are a function of their phosphorus content.In a subsequent study [110], they demonstratedthat the paramagnetic Ni-P deposits become fer-romagnetic after thermal treatment at 400 ◦C ow-ing to Ni and Ni3P phases separation. Knyazevand colleagues [6] performed magnetization mea-

surements and differential scanning calorimetryanalysis of the Ni-P alloys obtained via electrode-position. Their results indicated that the untreatedalloys with phosphorus contents exceeding 12 at.%were paramagnetic, owing to the lack of exchangeinteractions due to fluctuations in chemical com-position and the formation of a network of phos-phorus rich paramagnetic domains. AmorphousNi-P alloys that were originally paramagnetic wererendered ferromagnetic through thermal treatmentthat also led to their devitrification. Dhanapal etal. [109] studied the influence of phosphorus con-tent, employed duty cycle and current density onthe magnetic properties of Ni-P alloy fabricated bypulse current deposition. They observed satura-tion magnetization dependence on the phosphoruscontent. At low phosphorus amounts saturationmagnetization value of the obtained deposits washigh. Increasing the duty cycle resulted in the in-crease of soft ferromagnetic nature of the Ni-P alloyand decrease of the coercivity and retentivity val-ues.

4.4 Catalytic activity

Ni-P alloy is known to exhibit catalytic activity fore-most in water splitting reaction. This presents avery important feature especially in the light of to-

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day’s rapidly increasing energy demands and pro-gressive exhaustion of fossil fuels resources. Cat-alysts (platinum and ruthenium) conventionallyemployed to reduce the large overpotentials for hy-drogen and oxygen evolution reactions (HER andOER respectively) are fairly scarce and their use notfeasible when it comes to incurred costs, thus suit-able alternatives are in high demand. Nickel is avery popular electrode material owing to its reason-ably high hydrogen generation activity, availabilityas well as its low cost [111]. Although Ni as a cata-lyst does not perform as well as steel from the view-point of its electro-catalytic activity, it possess anexcellent resistance to corrosion in hot concentratedalkaline solutions [112]. Issues encountered whenemploying this metal as cathode material are its lowcatalytic activity or low resistance to intermittentelectrolysis, however alloying nickel with other ele-ments (P, Mo, etc.) can aid in mitigating these issues[112, 113]. According to Paseka [114], the reason forthe improvement of nickel activity in HER achievedthrough alloying it with phosphorus is the augmen-tation of the amount of amorphous phase surround-ing the Ni crystals. Alloys containing amorphousphase which is able to dissolve large amounts ofhydrogen possess high internal stress which con-tributes to their good electro-catalytic activity.

Hu and Bai [115] conducted a study in orderto interrogate the effects of key electroplating vari-ables on the hydrogen evolution activity of Ni-Pdeposits. By employing fractional factorial design,path of steepest ascent and central composite de-sign, they found that the key factors influencingcatalytic activity of Ni-P electrodeposits include:temperature, pH and NaH2PO2·H2O concentra-tion and their interactions. The models they de-veloped indicated that the alloy containing 7 at.%of P should exhibit maximal electro-catalytic activ-ity. In the subsequent study [74], they experimen-tally found deposit with 8 at.% of P to be the bestelectrode material for HER. They attributed thisdeposit’s highest specific activity to its largest truesurface area and hence its maximum roughness.Wei et al. [116] investigated catalytic activity ofNi-P in HER both experimentally and theoretically.Alloys that contain 10,8 at.% P were found to per-form the best. In order to elaborate the influenceof phosphorus content in Ni-P amorphous alloyson their catalytic activity they employed density-functional theory and front molecular orbital theory.Obtained results indicated that alloys with phos-phorus content anywhere from 9.1 at.% to 14.3 at.%

should exhibit optimal activity for the whole HER.Paseka [114] found that the alloy deposited at 65 ◦Cdemonstrates worse catalytic activity than alloysdeposited at temperatures ≤ 53 ◦C. He assertedthat this is owing to the presence of larger amountsof adsorbed hydrogen in alloys fabricated at lowertemperatures, which contributes to their higher in-ternal stresses. According to Paseka, catalytic ac-tivity also depends on the deposit’s thickness andexhibits an increase as deposit grows. Thick de-posits possess great catalytic activity owing to theirhigh internal stress, however their mechanical in-stability presents a problem and thickness increasewill be beneficial only up to a critical width of coat-ing at which it will suffer a final failure. In the studyby Li et al. [76] it was determined that at low bathtemperatures high NH4Cl content in combinationwith low NaH2PO2 concentration can contribute tocreating thick films (6-8 at.% of phosphorus) withoptimum catalytic activity even at high current den-sities. However, ammonia concentration must bechosen carefully because exceeding the ammoniaconcentration above the certain limit can negativelyaffect the cathode current efficiency, hence decreasethe plating rate [44]. Bai and Hu [110] observed thatfor Ni-P electrodeposits with P content from 0 to 28at.% the ability of catalysing hydrogen evolutiondecreases with increasing annealing temperature,concluding that annealed deposits are not suitablefor HER.

Elias et al. [117] studied the efficiency of elec-trodeposited Ni-P for both HER and OER in alka-line media. They found that the alloy thin filmswith 9.0 wt.% of P and 4.2 wt.% of P are thebest electrode materials for HER and OER. Re-cently Tang and coworkers [118] reported a room-temperature electrodeposition of Ni-P nanoparticlefilm on Ni foam which acts as a bifunctonal water-splitting catalyst in strongly alkaline media withvery small overpotentials for HER and OER: 80 mVand 309 mV, respectively.

As an alternative to alkaline media in which wa-ter electrolysis systems are simple but demonstratelow efficiency and high energy consumption, Luet al. [119] interrogated electrocatalytic activity ofNi based alloys in acidic media. They observed adecrease of catalytic activity with the increase ofphosphorus content which they attributed to mov-ing away from the optimal electronic configurationwith phosphorus incorporation but additionally tothe decrease of the number of grain boundarieswhich present active sights for HER. Contrary, au-

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thors of [120] claimed that increasing the phospho-rus content, even past contents that can be achievedthrough conventional electrodeposition would im-part exceptional catalytic properties to these sys-tems for applications in acidic media and hencevery high phosphorus contents ought to be benefi-cial.

5 Thermal treatment of electro-plated Ni-P alloy

With subsequent heat treatment the hardness ofNi-P electrodeposits increases substantially. Thisphenomenon is caused by the precipitation of thecrystalline phases. Most studies report detecting amixture of Ni3P and f.c.c. nickel as a final productof the thermal treatment [20].

The annealing temperature under which crys-tallization occurs depends on the phosphorous con-tent in the deposit as well as on the heating rate[121]. Phase transformation temperatures of thedeposits increase with increasing heating rate anddecreasing phosphorus content [110, 121]. Contin-ued heating at temperatures higher than the tran-sition temperature leads to a decrease of hardnessowing to the subsequent recrystallization and graincoarsening [18, 122, 123]. The degree of loss of hard-ness is higher for deposits having lower phospho-rus content on account of more pronounced graingrowth and coarsening of Ni phase compared toNi3P phase [122]. Transition from amorphous tocrystalline state is followed by a thermal contrac-tion phenomena due to higher density of crystallinestructure when compared to amorphous one, theamplitude of contraction being higher at higherphosphorus contents [105].

Nava et al. [20] fabricated Ni-P electrodepositscontaining 10.6 at.% of P which exhibited maxi-mum hardness (990 HV) after thermal treatment at500 ◦C (comparable to the hardness of hard Cr coat-ings ∼1000 HV). Obtained deposits demonstratedalso the lowest wear rate as indicated by the SEMimages of their wear track patterns that exhibitedthe narrowest width and shallowest plough lines(Figure 10). A linear relationship was detected be-tween the hardness and the wear resistance of theheat treated Ni-P alloy coatings. Corrosion resis-tance of the deposits deteriorated upon annealingowing to the formation of cracked structure in thethermally treated coatings, which promoted local-ized corrosion. Bai and Hu [110] found crystalliza-

tion for Ni-P deposits containing ≤ 24 at.% of P tooccur at 400 ◦C, while deposits with 28 at.% demon-strated a phase transformation at 200 ◦C. Addition-ally, phosphorus content in the deposits decreasedwith increasing the annealing temperature owingto the replacement of phosphorus by the oxygenfrom the air.

Habazaki et al. [124] conducted a study in or-der to interrogate the effect of annealing on themicrostructure and the corrosion behaviour of theelectrodeposited amorphous Ni-P alloys. Obtainedresults indicated that Ni-P alloys with 19.2 at.%phosphorus crystallize directly to f.c.c. Ni and Ni3Pphases. High phosphorus alloys (≥ 24.6 at.% P)were first crystallized to a metastable single phaseand then decomposed to Ni3P. Crystallization ofalloys with the intermediate P content (19.2 at.%< P < 24.6 at.%) resulted first in a mixture of f.c.c.Ni, Ni3P and the metastable phase. The precipita-tion of f.c.c. Ni in the amorphous phase occurredfor Ni-16.7P alloy before complete crystallization.Annealing decreased corrosion resistance for alloyscontaining ≤22.7 at.% P owing to the formationof phosphorus deficient f.c.c. Ni phase. Keong etal. [121] similarly reported that amorphous coat-ings with high phosphorus content follow a se-quence of transformations during annealing andform metastable phases, such as Ni2P and Ni12P5before forming stable Ni3P and f.c.c. Ni phases.Zoikis et al. [125] observed crystallization of theamorphous Ni-P electrodeposit into Ni and Ni3Pphases after annealing at ∼ 400 ◦C. The presenceof Ni2P phase after thermal treatment above 330 ◦Cwas detected. Ni2P phase formation was also re-ported in [43].

Jeong et al. [77] reported the increase of hard-ness and abrasive wear resistance after heat treat-ment for nanocrystalline Ni-P coatings, with Taberabrasive wear resistance being linearly propor-tional to the hardness of the coatings. Heat treat-ment causes also the elastic modulus of the depositto increase significantly according to the authors of[63]. When maximum hardness is achieved uponannealing fracture toughness exhibits the lowestvalue [126].

Chang et al. [127] argued that significantstrengthening by annealing for electrodepositedNi-P alloys with low phosphorus content andnanocrystalline grains is not induced by the pre-cipitation of Ni3P phase. The increase in hardnessupon annealing is according to them a result ofgrain boundaries relaxation, phosphorus segrega-

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tion, reduction of interior defects with the possiblecontribution from the increase of density owing todegassing of hydrogen.

Opting for subsequent Ni-P alloy thermal treat-ment and accordingly choosing the suitable heattreatment temperature, finally depend very muchon the employed substrate nature and the intendedprocess complexity.

6 Application of pulse platingfor Ni-P alloy fabrication

Constant direct current is the most commonly ap-plied regime in which metallic coatings are elec-trodeposited. However, in recent years the useof pulsed, alternating currents is exhibiting an in-crease. This is owing to number of findings thatdemonstrate that pulse current (PC) exerts a benefi-cial influence on the structure and properties of thefabricated deposits. Among others, comprehensivereviews on pulse and pulse reversed plating arecomposed by Devaraj and Seshadri [128] and byChandrasekar and Pushpavanam [129].

Development of modern electronics has granteda great flexibility in programming of the appliedmodulated current waveforms, with trains ofpulses that can be programmed to have very com-plex sequences and forms.

Figure 11: Typical pulse-current waveform.

Rectanglular waves are the easiest waveformsto produce [130]. They have been demonstrated toproduce a higher nucleation rate of the grains com-pared to triangular waveform [131]. Typical wave-forms employed in pulse plating include: cathodicpulse followed by a period without current (or ananodic pulse), direct current (DC) with superim-posed modulations, duplex pulse, pulse-on-pulse,cathodic pulses followed by anodic pulses (pulsereverse current-PRC), superimposing periodic re-

verse on high frequency pulse, modified sine-wavepulses and square-wave pulses [129].

During electroplating, a negatively chargedlayer is formed around the cathode as the processadvances. When performing DC deposition, thislayer charges to a definite thickness and preventsions from reaching the substrate. In a simple formof PC electrodeposition, where cathodic pulse isalternatively switched on and off, when the outputis turned off this layer discharges which allows eas-ier transport of ions from the bulk of the solution[129]. Additionally, during plating high currentdensity areas in the bath become more depleted ofions compared to low current density areas. Dur-ing time when the current is off ions migrate to thedepleted areas and when subsequent current pulseoccurs more evenly distributed ions are availablefor deposition onto the part [129]. In the absenceof current, small grains are recrystallized owing totheir higher surface energy which makes them lessthermodynamically stable than large grains, hydro-gen is also desorbed decreasing the internal stressof the obtained deposits. In general, PC platingresults in finer grain deposits exhibiting improvedproperties, including hardness, roughness, poros-ity, wear resistance, etc. Pulse plating can reduceadditive requirements substantially [128, 129].

Pulsed current results in metal deposition at thesame rate as direct current provided that the aver-age pulse current is equal to the mean direct currentof DC electrodeposition [64, 129].

In PC plating, choice of the applied waveformis critical and care must be taken to appropriatelyoptimize all parameters (peak current, duty cycle,frequency, pulse shape, etc.). In his work Pearson[130] explores the benefits and the limits of the PCelectrodeposition technique. He asserts that verylow duty cycles are not feasible, because in orderto produce the same average deposition rate as forDC, as duty cycle is reduced the pulse peak cur-rent needs to be increased. In practical applications,too high peak current densities are seldom viabledue to limitations of rectifiers capacities. On theother hand, as duty cycle is increased the processbegins to approach direct current deposition, thusa compromise must be made. When it comes tothe frequency, practical maximum frequency whichcan be applied is limited by the capacitance of thedouble layer at the interface between the platingelectrolyte and the article being plated. If the fre-quency is very high, the double layer does not haveenough time to fully charge during the pulse and

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the process begins to resemble DC deposition [130].Maximum useful frequency is around 500 Hz formost applications. However, higher frequenciescan be used where very high peak current densitiesare employed because the double layer charge anddischarge times become shorter as the peak currentdensity is increased.

In pulse reversed current technique (PRC) an-odic pulse is introduced into the plating cycle. PRChas the same effect of replenishing the diffusionlayer as PC does. It results in dissolution of theprotrusions on the metal surface ensuring a moreuniform deposition through elimination of the dis-crepancies between high and low current densityareas and increases coating thickness uniformity[129]. PRC plated amorphous Ni-P deposits arereported to exhibit better ductility owing to the ab-sence of voids and to consist of layers with differentamorphous structures [132].

It has been reported that the application of PCdeposition results in the increase in limiting currentdensity [129]. However according to Pearson [130]total thickness of the diffusion layer is equivalentto that obtained when plating in DC regime, owingto this the use of PC has very little effect on thelimiting current density.

In Ni electrodeposition, pulse plating is exten-sively employed and studied [133–135]. Pavlatouet al. [69] investigated the use of pulse current in or-der to interrupt the columnar growth of the nickelgrains and to produce more compact and thus morecorrosion resistant coatings.

In Ni-P electroplating, pulse plating also pos-sesses several advantages over DC plating. Lin andcoworkers [4] studied Ni-P alloy deposition in aPC regime from a sulfamate bath. They establishedthat compared with DC plating, current efficiencyassociated with high phosphorus content depositscan be improved by applying pulse current havinglow duty cycle, high frequency, and proper peakcurrent density. It was demonstrated that after ap-plying the same total charge associated with DCand PC waveforms of different duty cycles, a moreuniform concentration profile is maintained for PCthan for DC deposition, particularly with the PChaving small duty cycles. By employing 0.1 dutycycle with frequencies exceeding 100 Hz depositswith 14 wt.% of P were plated with an efficiency ofaround 80%. Low duty cycles and high frequenciesare beneficial in terms of maintaining more stablesurface proton concentration distribution inhibit-ing alloy composition modulations induced by pH

value variations. Additionally, unlike DC-platedNi-P deposits that might become amorphous whentheir phosphorus content exceeds a critical value,pulse-plated deposit with 14 wt.% phosphorus stillconsisted of equiaxed crystalline grains. This is inagreement with the study done by Chen et al. [65]who concluded that a reduction in duty cycle from0.5 to 0.1 simultaneously increases phosphorus con-tent and grain size of the deposits. They found thatNi-P deposits with high P content can be achievedwith high current efficiencies by employing pulseplating and that the obtained deposits exhibit lowerinternal stress than the DC-plated coatings. Pulse-plated deposits were also consistently harder thanthe DC-plated ones.

Figure 12: Deposit phosphorus content as a function of the peakcurrent density, pulse frequency, and duty cycle. Reprinted withpermission from [4]. Copyright (2006) Electrochemical Society,Inc.

Figure 13: Dependence of current efficiency on the deposit phos-phorus content. Reprinted with permission from [4]. Copyright(2006) Electrochemical Society, Inc.

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7 Compositionally modulatedNi-P electrodeposits

In DC plating ways of improving deposit propertiesinclude microstructure manipulation by varying de-position conditions or alloying in which case a two-component deposit is formulated by employing anappropriate bath formulation. However, techno-logical development imposes the demand for coat-ings that combine favourable properties of differentmetals, a task unachievable solely by employing in-dividual components neither their resultant alloys.In this case compositionally modulated (function-ally graded or multi-layered) coatings present themselves as a remarkable opportunity owing to thepossibility of engineering coatings fitting a widerange of requirements in terms of functional prop-erties.

In nickel electroplating, production of multi-layer coatings consisting of films of bright and semibright nickel (60-75% of the total nickel thickness)is traditionally employed to improve deposits’ cor-rosion resistance. Preferential corrosion in the elec-trochemically more active upper bright nickel layerand propagation of the corrosion in the lamellardirection protects the columnar lower layer of semi-bright nickel through retarded pitting attack pene-tration [57].

In the number of applications including the useof coatings for thermal, wear or corrosion protec-tion and microelectronics, the mismatch in prop-erties at the interface between the coating and thesubstrate can cause stress concentration that couldresult in the failure of the interface. Employingfunctionally graded materials (FGMs) [136–138]which are characterized by a position-dependentchemical composition, microstructure or atomic or-der [15] and by a subtle gradient of their proper-ties along the coating thickness is reported to helpin reducing the stress usually incurred due to theabrupt composition change when going from sub-strate toward deposit and decreasing the danger ofits delamination.

A non-uniform, graded [14] or layered distribu-tion [139, 140] relative to the phosphorus contentalong the coating thickness or a grain size gradi-ent [141, 142] are deemed to be beneficial also forimparting favourable properties to Ni-P deposits.

Phosphorus content in Ni-P electrodeposits in-herently varies with layer thickness. pH at theelectrode-solution interface rises concomitantlywith the discharge of hydrogen. After escape of

a cohort of hydrogen bubbles from the cathoderesultant enhanced convection increases the interfa-cial proton concentration. Interfacial pH variationscause the variations in the deposit phosphorus con-tent as electroplating continues [97]. Crousier et al.[29, 143] confirmed that Ni-P electrodeposits con-sist alternately of layers having varying phospho-rus contents. Sadeghi [21] observed that the forma-tion of layers with different phosphorus amountsoccurs more readily for alloys with low or mediumphosphorus content and at higher current densi-ties. A homogeneous deposit can be fabricated andstratification avoided by enhancing mass transportto the cathode surface or by employing pulse plat-ing [97]. Additionally, oscillations in pH value aredamped out as coating grows in thickness and elec-troplating continues [17]. However variations inphosphorus content can be sometimes intentionallyinduced in order to produce composition modula-tion in fabricated Ni-P electrodeposits. With carefuloptimization this approach can result in a signifi-cant improvement of features when compared toconventional homogeneous Ni-P electrodeposits.Layered crystalline/amorphous Ni-P alloys are forexample reported to exhibit higher tensile strength,moduli of elasticity and improved corrosion resis-tance [143].

It is known that Ni-P deposits with low phos-phorus content exhibit high hardness and goodwear resistance but poor corrosion resistance, con-versely high phosphorus coatings exhibit good cor-rosion resistance but poor mechanical properties.Developing multilayer coatings can be an effectiveway to obtain deposits characterized by both op-timal mechanical and electrochemical properties.For example, a duplex coating with an outer layerhaving a low phosphorus content and an innerlayer with high phosphorus content is a good wayto ensure corrosion stability in the contact withsurrounding environment and in the same timefavourable mechanical properties [98].

Techniques in order to produce layered Ni-Pelectrodeposits include either using two baths withdifferent phosphorus source contents or a singleelectroplating bath in which case electrochemicalmethods are employed in order to achieve compo-sition modulation (changing cathodic current orpotential).

In the dual-bath electrodeposition technique,the item to be electroplated is moved between twoplating baths of arbitrary composition and a layeris plated from each electrolyte in cycles. First multi-

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Figure 14: Diagram showing the mechanism of increased corrosion protection in the case of multilayer Ni-P coating (left), comparedto monolayer Ni-P coating (right). It demonstrates that the time required for the corroding medium to reach the substrate bypenetrating through the multilayer coating (T1) is much greater than that for the monolayer coating (T2). Reprinted with permissionfrom [147]. Copyright (2016) Royal Society of Chemistry.

layer deposition by employing dual bath techniquewas reported by Blum for Ni(24 µm)/Cu(24 µm)as early as 1921 [144]. This approach is mechani-cally complex compared to the single-bath methodand carries a risk of contamination during the sub-strate transfer [83]. Historically, first realizationof single-bath electrodeposition was reported byBrenner and Pommer in 1948 who produced mul-tilayered coatings CuBi(∼µm)/BiCu(∼µm) by al-ternately switching the deposition current betweenlow and high value [145]. Single bath method isefficient, versatile and technologically simple. Areview of both dual-bath and single-bath electrode-position methods is compiled by Ross [146].

When employing single bath depositionmethod pulsed current is a useful tool to achievethe desired composition modulation. Early attemptwith two-pulse plating was reported by Girard[148] for the electrodeposition of permalloy filmswith a composition as close as possible to the zero-magnetostriction (Ni81Fe19) ensuring the smallestmagnetic coercive force.

In the realm of compositionally modulated Ni-P electrodeposits, Goldman and coworkers [86]demonstrated a method to fabricate Ni/Ni-P films,with wavelengths between 2.1 and 4.0 nm and anaverage phosphorus content around 12 at.%, byalternating electrodeposition in two baths of dif-ferent composition. Ross et al. [83] revealed adual bath electrodeposition technique for the pro-duction of thin-film metal multilayers in whichsubstrate was suspended above nozzles of elec-trolyte and rotated by a motor. Specific steps were

taken, including washing and drying the substratewith N2, in order to mitigate problems related tocross-contamination between electrolytes. Multi-layered films of Ni/NiPx, NiPx/NiPy, Cu/Ni, andCo/NiPx were fabricated with a range of repeatedlengths. Ni/NiPx and NiPx/NiPy multilayers ex-hibited the highest quality, with repeat lengths aslow as 19 Å and up to three orders of reflectionin low angle X-ray scans. NiPx/NiPy multilayerswere fabricated over a wide range of compositionsand with crystalline or amorphous structure. Prob-lems were encountered with other compositions,in case of Cu/Ni and Cu/Co systems due to gal-vanic coupling as well as contamination and in caseof Co/Ni-P owing to non-uniformity of Co nucle-ation.

Wang et al. [14] described electrodeposition andinvestigated properties of Ni-P deposits having avarying phosphorus content in the direction of thecoating thickness. Single bath method was appliedand composition gradient was achieved throughvarying current density (5-30 A dm−2). The wear re-sistance of the fabricated Ni-P electrodeposits wasapproximately two times grater than that of theungraded Ni-P deposits. Beneficial wear proper-ties of the obtained deposits were attributed to theinhibition of formation and propagation of through-thickness cracks during the wear process owing totheir graded structure. Heat treated coatings ex-hibited low friction coefficient and hardness thatwas close to the one of hard Cr coatings. In an-other study, Wang and colleagues [15] examinedcorrosion resistance of the developed Ni-P gradi-

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ent deposits and their tribological behaviour un-der the oil-lubricated conditions. Deposits heattreated at 400 ◦C exhibited two orders of magni-tude better corrosion resistance than hard Cr coat-ings (Figure 15). The best corrosion resistance wasfound for deposits heat-treated at 200 ◦C, whichwas attributed to the preserved amorphous struc-ture and the stress relaxation at this temperature.Heat-treated coatings exhibited relatively higherwear rate and friction coefficients than hard Cr de-posits under oil-lubricated wear conditions.

Figure 15: Potentiodynamic polarization curves obtained forNi-P graded deposits and hard Cr coatings, measured in 10 wt.%HCl solution. Reprinted with permission from [15]. Copyright(2006) Elsevier.

Elias et al. [147] fabricated multilayer Ni-P al-loy coatings for better corrosion protection of mildsteel by employing cyclic modulation of the cath-ode current density. Achieved improvement in thecorrosion protection of fabricated multilayer Ni-Palloy coatings was attributed to the large number ofinterfaces between layers of alloys having differentcomposition and phase structures, at which corro-sion propagates laterally until the interface breaksdown (Figure 14). This mechanism systematicallyslows down corrosion and improves the coatingsstability. Corrosion protection efficiency of multi-layer coatings was found to increase with the num-ber of layers, however only up to a certain point(300 layers). At very high layer numbers corrosionresistance deteriorated owing to the lack of distinctinterfaces between individual films (approaching amonolayer structure). However, not only the num-ber of layers but also their composition determinedthe performance of fabricated deposits.

Multilayers of Ni-P with other metals or alloyscan be engineered also in order to ensure opti-mal functional performance in demanding envi-ronments.

Improved corrosion properties of multilayersof Ni-P/Zn-Ni, when compared to pure Ni-P andZn-Ni alloys, was detected by Liu and colleagues[149]. According to them, this was due to the cor-rosion of the sacrificial sublayers of Zn-Ni whichextends in the direction parallel to the substratesurface. Continued corrosion generation throughthe subsequent sublayers overall slows down thecorrosion reaching the substrate and an eventualmaterial brake down. Bahadormanesh and Ghor-bani [150] recently devised a single bath deposi-tion method for electrodeposition of Ni-P/Zn-Nicompositionally modulated multilayer coatings. Atlow current densities Ni–P was deposited, whileat higher current densities Zn–Ni alloy containing3.2 wt.% P was obtained. It was observed thatthe Ni–P/Zn–Ni compositionally modulated multi-layer coatings were sacrificial to the steel substrate.

Bozzini and colleagues [100] fabricated Ni-Pand Sn multilayer amorphous deposits (layer thick-ness 0.1 µm to 0.5 µm) by employing a dual bathelectrodeposition technique in order to explore thepossibility of improving the Ni-P deposits passi-vation behaviour. They investigated anodic be-haviour of obtained coatings in acidic chloride so-lution. Current densities at the passivation plateau,were in the range of 2 µA cm−2 to 5 µA cm−2 for Ni-P/Sn multi layers, in comparison to a passivationcurrent density of 45 µA cm−2 for Ni-P coatingswith similar phosphorus content. They found thatthe present interfaces improve considerably the pas-sivation behaviour provided that the incorporatedSn layers are thin enough.

8 Conclusion

Ni-P protective coatings can be electrodepositedwith a wide range of crystallographic structures.Deposits extending from fully crystalline to amor-phous ones can be easily fabricated. Properties ofthe obtained coatings very much depend on thedeposition conditions and their phosphorous con-tent. Applied post treatment, such as alloy heating,can bring significant improvement of its overall me-chanical, tribological and electrochemical proper-ties. Designing unconventional structures, such asgraded or multilayered ones, can also with properoptimization give rise to substantial deposits’ char-acteristics amelioration.

However, even though technology of Ni-P elec-troplating is quite mature, there are still many un-knowns and numerous issues still remain to be

26

addressed. Phosphorus incorporation mechanismis still a matter of great disagreement. Factors influ-encing phosphorus content of the deposit are manyand with the plethora of variables characterizingNi-P electroplating process and poor definition ofcertain process parameters it is not easy to estab-lish clearly the key influencers, manner and theextent of their impact. Up-scaling of the electroplat-ing procedure induces new process variables andmore attention in research needs to be bestowedon feasibility study of method transferral to largerscales and its robustness. Current density distri-bution is a parameter that is often poorly defined.Ageing of the electrolytic baths is not addressed inmany of the research works. Additionally, distin-guishing between different Ni-P microstructuresand establishing a point of transition from one stateto another is still a matter of some difficulty.

Acknowledgements

This work was supported by the European Union’sHorizon2020 research and innovation programmeSOLUTION, under grant agreement No. 721642.

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