fretting corrosion of electrolytic nip coatings

ISSN 1068�3666, Journal of Friction and Wear, 2011, Vol. 32, No. 6, pp. 419–423. © Allerton Press, Inc., 2011.Original Russian Text © I.R. Aslanyan, J.�P. Celis, L.Sh. Schuster, 2011, published in Trenie i Iznos, 2011, Vol. 32, No. 6, pp. 556–561.

419

INTRODUCTION

Fretting corrosion emerges on the contacting sur�faces of machine parts which mutually vibrate withsmall amplitude under a given pressure and in corro�sive environment [1–3]. Corrosion upsets the requiredfit and the necessary precision of the mated machineparts. It alters the friction conditions; the spots dam�aged by corrosion become loci of fatigue scuffing andpitting. Fretting corrosion is a complex process inwhich mechanical and chemical wear componentsdominate; i.e., oxide films form and vanish on the rub�bing surfaces [4]. The two components act simulta�neously and influence one another. Fretting corrosionappears frequently in bolted, riveted, splined, and piv�oted joints, predominantly in aircraft designs. In oper�ation, these joints repeatedly perform relative dis�placements resulting in mechanical destruction of sur�face oxide films. The surfaces contacting duringfretting generally stay in contact for a long time andtherefore the debris have no exit from the contactzone. This intensifies the corrosion and wear ofmachine parts; usually corrosion of this type is accom�panied by pitting of the contacting surfaces [5]. Metaloxides and products of abrasion fill up the pits and theybecome evident only after these products are removed.

One of the way to alleviate fretting corrosion is toapply a protective coating. Earlier studies [6, 7] of thefretting wear of electrolytic NiP coatings in air haveestablished the dependence of their wear rate on thepresence of silicon carbides and the crystalline Ni3Pphase liberated when specimens are annealed. The sil�icon carbides harden the composite NiP–SiC coat�ings, but the higher inhomogeneity of the subsurface

layers intensifies the fretting wear. Liberation of thecrystalline Ni3P phase during heat treatment hardensthe coatings still more and reduces their wear rate. TheNiP coating without any additive undergoes the leastfretting wear after heat treatment.

The aim is to study how submicron additives of sil�icon carbides affect the fretting corrosion of compositeNiP–SiC coatings in NaCl solution.

EXPERIMENTAL METHODS

NiP coatings were deposited on the steel substratein an electrolytic bath. The Watts electrolyte was used,containing 20 g/l of phosphorous acid H3PO3 and asuspension of silicon carbides in the amount of 0.80and 200 g/l with the average particle size 600 nm.Some specimens were annealed at 420°C for one hour.

Ball�on�disk couples were subjected to tribotestsunder the normal load 1, 5, and 10 N, frequency ofvibration 2 Hz, and tangential displacement (ampli�tude) 100 and 500 μm. All the specimens underwent20000 test cycles. Corundum balls (Ceratech Co.,Netherlands) acted as the counterbody; the balls were10 mm in diameter and had surface roughness Ra =0.2 μm. The tests were carried out in NaCl solution(pH 5.5) at 22°C. The friction coefficient wasrecorded continuously during the tests. The findings ofthe tribotests were presented as the mean value of threereadings for each coating type. The material volumeloss in wear and the friction coefficient were tribologi�cal wear characteristics .

The morphology of the coating surface and tribo�surfaces were checked with an SEM�Philips 515 scan�

Fretting Corrosion of Electrolytic NiP Coatings I. R. Aslanyana, *, J.�P. Celisb, and L. Sh. Schustera

aUfa State Aviation Engineering University, ul. K. Marksa 12, 450000, Ufa Russia*e�mail: as�[email protected]

bCatholic University Leven, 14 Castelpark Arenberg, B�3001, Leven, BelgiumReceived May 6, 2011

Abstract—The work considers the effect of additives of hardening submicron silicon carbides and heat treat�ment on the fretting corrosion in NaCl solution of nickel–phosphorus (NiP) coatings produced by electro�lytic deposition. Tests are performed at amplitudes 100 and 500 µm. Pitting reveals the abrasive nature of thewear of all of the studied coatings. Introduction of silicon carbides into NiP coatings increases the number ofpits in fretting corrosion. The pitting of NiP coatings is distributed regularly over the subsurface and pits arenot so numerous. As the loading grows, fretting corrosion intensifies, though the friction coefficient declinesin the majority of cases.

Keywords: coatings, friction, wear, fretting corrosion, oxides.

DOI: 10.3103/S106836661106002X

420

JOURNAL OF FRICTION AND WEAR Vol. 32 No. 6 2011

ASLANYAN et al.

ning electron microscope. The chemical analysis wasperformed on an EDAX instrument with the acceler�ating voltage rise from 10 to 20 kV.

The microhardness was measured on the tribosur�faces of the specimens under load 0.005 kg. The wearrate was determined as the crater Δ μm on the speci�men surface using the Wyo NT Series optical 3D pro�filing system.

RESULTS AND DISCUSSION

The NiP and composite NiP–SiC coating struc�ture is represented in [6]. The NiP coating has a

smoothed surface with a typical metallic shine. Themicrostructure of the composite NiP–SiC coating isgrey�colored globular with embedded light inclusionsof silicon carbides SiC.

The coating tribosurface during fretting corrosion inthe NaCl solution has grooves typical of all coatings,parallel to the motion direction (Fig. 1). Such groovesare typical of abrasive wear. This wear was observed dur�ing fretting corrosion on all the studied coatings. Theedges of the tribosurface have in this case smoothboundaries, while the tribosurface boundaries arecoated with oxide films during fretting wear in the air[7]. The debris during fretting corrosion are washedaway from the friction zone, deposit in solutions, and donot influence the wear process in any noticeable way.

Table 1 shows the results of chemical analysis of thetribosurface of the studied coating during fretting cor�rosion in the NaCl solution. The chemical composi�tions of the tribosurfaces of the composite NiP–SiCcoatings were approximately identical, except for the≈2 wt % oxygen registered on the tribosurface of theunannealed coatings. The composite coatings con�taining silicon carbides have light and dark particles onthe tribosurface. These particles are debris: theirchemical analysis shows that the light�colored parti�cles consist predominantly of silicon carbides or sili�con carbides with corundum ball debris; the dark�col�ored particles belong to the oxidized NiP coating basewith silicon carbide particles.

The annealed composite NiP–SiC coatings havedark spots on the tribosurface under load 10 N and atthe displacement amplitude 500 μm (Fig. 2). Detailed

20 µm

Fig. 1. Tribosurface microstructure of annealed compositeNiP–SiC coatings (80 g/l).

Table 1. Chemical composition of the subsurface NiP and composite NiP–SiC coatings (load P = 10 N, amplitudeA = 500 μm)

Coating

Chemical composition

Element, wt %

Ni P Si C O Al

NiPOriginal 90.11 9.89 – – – –

Annealed 89.85 10.15 – – – –

NiP–SiC (80 g/l SiC)

Annealed 80.68 9.53 4.55 3.35 1.90 –

Dark zone 64.22 11.82 6.41 4.46 10.30 2.80

Debris 44.85 11.26 27.36 14.83 1.07 0.62

Annealed 79.46 10.71 3.30 6.52 – –

Light�colored particles – – 6.95 56.28 36.43 0.54

Dark�colored particles 27.80 6.43 24.73 4.54 36.51 –

NiP–SiC (200 g/l SiC)

Original 81.31 8.30 5.16 3.24 1.99 –

Dark zone 33.23 8.90 33.41 17.95 6.50 –

Annealed 75.88 9.48 5.80 8.85 – –

Light�colored particles 2.61 4.41 42.59 50.39 – –

Dark�colored particles 29.00 14.43 13.53 6.47 28.72 –


FRETTING CORROSION OF ELECTROLYTIC NiP COATINGS 421

examination of the dark spots (Fig. 3) shows that somesilicon carbides are present in the friction zone,despite the presence of abrasive grooves, while thereare spots of torn�out silicon carbides and a ramifiedsystem of fine cracks in the friction zone during fret�ting wear [7]. In addition, Figure 3 shows clearly howthe corrosion propagates like pits around silicon car�bides embedded into the NiP matrix. The chemicalanalysis (Table 1) shows oxygen in the dark spots inthe amount of ≈10 wt % in the original state of NiP–SiC coatings when they contain silicon carbides in theconcentration 80 g/l and ≈6.5 wt % oxygen in the samecoatings when the concentration of silicon carbides is200 g/l. After the composite NiP–SiC coating isannealed, only the debris show oxygen�like dark� andlight�colored particles.

Figure 4 shows the NiP coating tribosurface. Thechemical analysis (Table 1) did not reveal any signifi�cant difference between the annealed and unannealedstates. In both cases, grooves are evident on the NiPcoating tribosurface parallel to the direction of frettingcorrosion, while the dark spots show where the corro�sion or pitting develop typically for this wear type.Despite the large number of such points, no crackingor further fracturing are observed. This appearance ofthe tribosurface is typical for all of the studied NiPcoatings irrespective of the load or displacementamplitude.

Table 2 shows the values of the friction coefficientand wear of the studied coatings. Compared with thefriction coefficient of the coatings during fretting wearin air, the friction coefficient during fretting corrosionin the NaCl solution is the smallest of all the studiedcoatings. With the presence of silicon carbides in largerand lager concentrations, the friction coefficientgrows, while it declines after heat treatment, resultingin the liberation of the Ni3P crystalline phase andhardening of the coating. The same is observed duringfretting wear of the coating in air [7].

Table 2 shows that the fretting corrosion of thestudied coatings in the NaCl solution is influenced bytheir composition, heat treatment, loading and dis�placement amplitude during fretting. Let us considerthe mechanical and chemical factors of wear in fret�ting corrosion to analyze the obtained findings.

It is shown in [6] that annealing of the coating at420°C liberates the solid Ni3P crystalline phase, sig�nificantly boosting (1.5–2 times) the microhardness.Table 2 shows additionally that introduction of hard�ening submicron silicon carbides increases the coatingmicrohardness as well, though not as much as heattreatment. The higher the concentration of SC inclu�sions, the higher the microhardness of the coating.These factors favor the reduction of the wear rate dur�ing fretting corrosion. Meanwhile, it is known [8] thatheat treatment of NiP coatings boosts their structuralinhomogeneity due to liberation of the Ni3P crystal�line phase. Introduction of the submicron silicon car�

20 µm

Fig. 2. Tribosurface microstructure of unannealed com�posite NiP—SiC coatings (80 g/l).

20 µm

Fig. 3. Dark zone in Fig. 2 strongly magnified.

5 µm

Fig. 4. Tribosurface microstructure of annealed NiPcoatings.

422


ASLANYAN et al.

bide additives also increases the structural inhomoge�neity of the coating. These factors intensify wear dur�ing fretting corrosion.

Loading is known [9] to intensify the abrasive wearof the stationary friction interface. Meanwhile, aheavier load makes the interface more intimate andincreases the predisplacement [10] during fretting; itdiminishes the probability of penetration of chemi�cally active media into the contact zone, reducing thechemical component of fretting corrosion.

The amplitude of displacement in the tribocontactevidently affects the fretting corrosion in multipleways. If the amplitude is comparable with the predis�placement, then it is more probable that chemicallyactive media will penetrate into the friction zone. Thiswould reduce the chemical component of the frettingcorrosion, while the solid debris entering the frictionzone boost the mechanical component.

If the amplitude exceeds the predisplacement, theconditions favor penetration of the chemically activemedia into the friction zone, boosting the chemical

component of wear. At the same time, the conditionsimprove for elimination (washing away by the solu�tion) of the debris from the friction zone, reducing themechanical component of fretting corrosion.

The combined effect of the above factors can influ�ence the fretting corrosion of NiP coatings in variousways.

CONCLUSIONS

Introduction of silicon carbides into NiP coatingsincreases the number of pits during fretting corrosion.The silicon carbides in composite NiP–SiC coatingsare sites where corrosion develops. Multiple pits aredistributed regularly over the tribosurface in the NiPcoatings.

The conducted study has revealed the following.Irrespective of the composition of the NiP coating

and heat treatment and displacement amplitude, theloading intensifies fretting corrosion, though the fric�tion coefficient declines in most cases.

Table 2. Results of testing of NiP and composite NiP–SiC coatings for fretting corrosion at the displacement amplitudes100 and 500 μm

Composition of coatings Load P, N

Amplitude A = 100 μm

Original state After annealing

Micro� hardness Hµ

Friction coefficient f

Worn layer height Δ,

μm




μm

NiP 1591 0.19

1.121076

0.23 1.09

5 1.94 0.16 1.78

NiP–SiC (80 g/l SiC) 1699 0.20

1.931123

0.30 2.50

5 2.10 0.17 3.40

NiP–SiC (200 g/l SiC) 1727 0.23

2.141454

0.22 2.16

5 2.60 0.19 2.90

Composition of coatings Load P, N

Amplitude A = 500 μm

Original state After annealing




μm




μm

NiP 1 1.07

1076

0.12 0.92

5 591 0.20 2.60 0.18 2.20

10 4.08 0.16 2.81

NiP–SiC (80 g/l SiC) 1

0.21

1.50

1123

0.21 1.47

5 699 2.70 0.19 2.27

10 3.08 0.18 3.76

NiP–SiC (200 g/l SiC) 1 1.50

1454

0.28 1.48

5 727 0.23 2.36 0.22 2.85

10 3.67 0.19 3.80


FRETTING CORROSION OF ELECTROLYTIC NiP COATINGS 423

The various compositions of NiP coatings and dis�placement amplitudes during fretting corrosion (undercomparable loading conditions) are virtually indepen�dent of heat treatment (except the NiP–SiC coatingcontaining 80 g/l SiC at the amplitude A = 100 μm).

SiC additives to NiP coatings (irrespective of theirconcentration within the range in question) intensifyfretting corrosion by approximately 1.5 times only inthe case of small loading; under heavier loading, thepresence of SiC and its concentration do not affect thefretting corrosion noticeably.

The extent of the influence of displacement ampli�tude on the fretting corrosion of the coatings dependsmainly on the loading: the fretting corrosion intensitydeclines under light loading as the amplitude grows(irrespective of the composition and heat treatment);it is virtually independent of the displacement ampli�tude under heavier loading.

In the NiP–SiC coatings containing 80 g/l SiC atamplitude A = 100 μm, the significant influence of theSiC additives in boosting the fretting corrosion of theheat�treated coatings is explained by the unfavorablecombination of the coating composition and frictionconditions, which increase the mechanical compo�nent of wear during fretting corrosion.

DESIGNATIONS

A—displacement amplitude; f—friction coeffi�cient; Hµ—microhardness; P—loading; Δ—wornlayer height.

REFERENCES

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7. Aslanyan, I.R., Celis, J.�P., and Shuster, L.Sh., TheEffect of SiC Additives on Fretting Wear of Electro�plated NiP Coatings, J. Friction Wear, 2011, vol. 32,no. 2, pp. 74–78.

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9. Myshkin, N.K. and Petrokovets, M.I., Trenie, smazka,iznos. Fizicheskie osnovy i tekhnicheskie prilozheniya tri�bologii (Friction, Lubrication, Wear. Physical Funda�mentals and Technical Applications of Tribology),Moscow: Fizmatlit, 2007.

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fretting corrosion of electrolytic nip coatings

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