International

International Journal of Fatigue 28 (2006) 1854–1866

www.elsevier.com/locate/ijfatigue

JournalofFatigue

Fatigue behavior of a 7075-T6 aluminum alloy coatedwith an electroless Ni–P deposit

E.S. Puchi-Cabrera a,*, C. Villalobos-Gutierrez b, I. Irausquın c,J. La Barbera-Sosa a, G. Mesmacque d

a School of Metallurgical Engineering and Materials Science, Faculty of Engineering, Universidad Central de Venezuela,

Apartado Postal 47885, Los Chaguaramos, Caracas 1045, Venezuelab School of Mechanical Engineering, Faculty of Engineering, Universidad Central de Venezuela, Apartado Postal 47885,

Los Chaguaramos, Caracas 1041, Venezuelac Department of Industrial Technology, Universidad Simon Bolıvar, Sede del Litoral, Postal Address 89000, Caracas 1080, Venezuela

d Laboratoire de Mecanique de Lille, Universite de Lille 1, Mecanique des Materiaux, UMR CNRS 8107, Villeneuve D’Ascq, 59650, France

Received 8 February 2005; received in revised form 22 August 2005; accepted 2 December 2005Available online 20 March 2006

Abstract

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloycoated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 lm in thickness and a high P content,of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue andcorrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it isobserved that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the orderof 0.4 r0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 r0.2%, noimprovement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Undercorrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stressapplied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The del-eterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate ofthe yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreementwith the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the ENdeposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength.Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrateand the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Electroless Ni–P coating; 7075-T6 aluminum alloy; Fatigue and corrosion–fatigue properties; Absolute hardness; Fractographic analysis

1. Introduction

7075 Aluminum alloy constitutes a very important engi-neering material widely employed in the aircraft and aero-space industry for the manufacturing of different parts andcomponents due to its high strength-to-density ratio. In the

0142-1123/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijfatigue.2005.12.005

* Corresponding author. Tel.: +58 2 662 8927; fax: +58 2 753 9017.E-mail address: epuchi@reacciun.ve (E.S. Puchi-Cabrera).

aircraft industry, replacement and repair costs representvery significant issues and therefore it is of utmost impor-tance to attempt an extension of the aircraft vessels on asafely basis in order to avoid catastrophic failures. For thispurpose, knowledge about the nucleation and propagationof fatigue cracks is a fundamental matter, which wouldcontribute and allow the inspection and repair of the air-craft structures prior to failure. As pointed out by DeBar-tolo and Hillberry [1], in many aluminum alloys employed

Nomenclature

A, fatigue strength coefficient (MPa)AEN, cross-sectional area of a coated tensile sample

(mm2)AS, cross-sectional area of the uncoated tensile sam-

ples (mm2)d, indent diagonal (mm)dS, diameter of the uncoated tensile sample (mm)fEN, area fraction of the EN deposit (mm2)fS, area fraction of the substrate (mm2)H, apparent hardness of the coating (GPa)H0, absolute hardness of the coating (GPa)n, Meyer’s exponent

Nf, number of cycles to failureNEN

f , number of cycles to failure of the coated samples

NSubst.f , number of cycles to failure of the uncoated sam-

plesm, fatigue exponentS, maximum alternating stress (MPa)t, coating thickness (mm)r0.2%, yield strength (MPa)�r, yield strength of the substrate-coating system

(MPa)rEN, yield strength of the EN deposit (GPa)rS, yield strength of the substrate alloy (MPa)

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1855

in the aircraft industry, fatigue cracks could be nucleatednot only at macroscopic defects, such as machining marksand corrosion damage, but also at cracked constituent par-ticles inherent to the alloy, formed during the solidificationprocess. These observations led the above researchers toconduct a study in order to model the distribution of suchparticles as initial crack sizes to predict fatigue lives in dif-ferent aluminum alloys which included 2024-T3, 2524-T3and 7075-T6.

In general, the heterogeneous microstructure of thesestructural aluminum alloys makes them highly susceptibleto corrosion attack, which can affect seriously the aircraftstructural integrity due to the nucleation and acceleratedgrowth of fatigue cracks from corrosion pits, under theaction of alternating stresses applied in a corrosive environ-ment. In this context, Sankaran et al. [2] conducted aninvestigation in order to study the effects of pitting corro-sion on the fatigue behavior of an uncoated 7075-T6 alumi-num alloy, which allowed the conclusion that pittingcorrosion could decrease the fatigue life of this alloy by afactor ranging from 6 to 8. These researchers also measuredthe fatigue lives of the material and compared such mea-surements with the predictions computed from averagepit depths considered as initial crack sizes. The agreementbetween the measured and predicted fatigue lives obtainedby these authors allowed the conclusion that standardizedspray tests could be potentially used for obtaining quanti-tative measures of corrosion that could be employed asinputs in analytical models for fatigue life prediction, inorder to evaluate the integrity of aircraft structures.

Fonte et al. [3], have also pointed out that fatigue crackgrowth resistance of aluminum alloys is significantlydependent on microstructure and environment, a relation-ship that has been investigated extensively during the pastfew years. These authors conducted a study in order toinvestigate the role of microstructure and environmentalinfluence on fatigue crack growth employing a 7049 alumi-num alloy in different microstructural conditions, includingartificially aged to underaged and overaged microstruc-

tures, leading to similar yield strengths but with differentslip deformation modes. Their results showed clearly thatthe same alloy, with different microstructures, tested inhumid air and vacuum, presented distinct fatigue behavior.Such results were interpreted in terms of a complex rela-tionship between the effects of environment with micro-structure and loading through mechanisms whosecharacterization and understanding require careful system-atic measurements of fatigue data both in vacuum and inselected environments.

In a recent investigation, Liu et al. [4] conducted a studyin order to measure the fatigue crack growth rates of a7075-T76 aluminum alloy in a 3.5% NaCl solution, withthe presence and absence of corrosion inhibiting pigments,including chromate, phosphate and other nontoxic pigmentbased on different chemical components. The investigationalso involved the analysis of the fracture surfaces of thetested specimens by means of SEM techniques and thedetermination of electrochemical polarization curves inthe NaCl solution, with and without the inhibiting pig-ments. Their results showed that the corrosion inhibitorsstudied, gave rise to a decrease in the fatigue crack growthrate of the alloy, particularly at the early stage of crackpropagation.

DuQuesnay et al. [5] have recently carried out a researchstudy in order to investigate the growth of fatigue cracksinitiated at corrosion pits, employing samples of a 7075-T6511 aluminum alloy subjected to a transport aircraftloading spectrum. The material was exposed to a corrosivesolution in order to generate corrosion damage of differentdegree, in the form of pits. The study showed that fatiguelives of components could be severely reduced by corrosiondamage and that the depth of the corrosion pits could be asuitable parameter for characterizing the corrosion damageand predicting the fatigue life of the specimens by means ofa commercial fatigue crack growth software.

Aluminum alloys, after steel and other ferrous alloys,constitute the largest group of substrates suitable for elec-troless nickel (EN) plating, due to the improvement in

Fig. 1. Sketches of the tensile (a) and fatigue (b) specimens employed inthis study. All the dimensions are in mm.

1856 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

hardness, wear, abrasion resistance, solderability and cor-rosion resistance that such a coating imparts to these mate-rials [6]. Particularly in the aerospace industry, EN platinghas been used extensively for coating aluminum alloyscomponents in order to improve its inherent characteris-tics. A good example of a successful combination of alumi-num and EN in this kind of applications is that of pistonheads, since the light weight of aluminum allows the pistonto work more efficiently, while the EN provides wear resis-tance to extend the useful life of such a component [7]. Aninteresting feature of EN coatings deposited on aluminumsubstrates is that residual stresses could be of a compressivenature depending on the P content of the deposit [8], whichwould have a beneficial effect on the fatigue performance ofthe coated system. According to the data published by Par-ker and Shah [8], for aluminum substrates coated with elec-troless Ni–P in the as-deposited condition, the residualstresses within the deposit decrease linearly with the P con-tent of the plating, from approximately 10 MPa for coat-ings with 2 wt% P to about �60 MPa for coatings withapproximately 12 wt% P.

Thus, the present investigation has been conducted inorder to study the effect of an EN deposit, in the as-platedcondition, of about 37 lm in thickness and approximately18 wt% P, on the fatigue and corrosion–fatigue behavior ofa 7075-T6 aluminum alloy and to compare the behavior ofthe coated system with that of the uncoated substrate.According to the data published by Parker and Shah [8]this coating would have a compressive residual stress inthe range of 102 MPa. Also, a fractographic analysis ofsome selected samples, tested both in air and in a NaClsolution, has been carried out in order to assess the roleof the EN deposit on the nucleation of fatigue cracks andits general behavior during fatigue testing conducted underdifferent conditions.

2. Experimental techniques

The present investigation was carried out with samplesof a 7075-T6 aluminum alloy of the following composition(wt%): 6.10 Zn, 2.90 Mg, 2.00 Cu, 0.50 Fe, 0.40 Si, 0.30Mn, 0.28 Cr, and Al bal. The material was supplied as barsof approximately 12.7 mm in diameter and 3.6 m in length,from which a number of tensile and fatigue specimens weremachined following the ASTM standards B557 and E606,respectively. For tensile testing, six specimens were pre-pared, whereas for fatigue testing 96 samples wererequired. Small cylindrical samples of approximately12 mm in height were also machined in order to character-ize the chemical composition and hardness of the coating.Fig. 1 illustrates the sketches of both the tensile and fatiguespecimens. All the dimensions shown are given in mm. Inorder to minimize the introduction of residual stressesthroughout the machining operation of the specimens,the depth of cut of the material during the turning proce-dure was reduced continuously employing a turret lath atlow speed. A ‘‘mirror-like’’ finish was finally achieved by

grounding the samples with successive SiC papers grit600–1200 and subsequently polished with alumina of0.3 lm. Such a procedure allowed the elimination of theremaining circumferential notches that could act as stressconcentrators during the fatigue tests.

The deposition of the EN coating was carried out indus-trially. For conducting the present investigation, it wasrequested the deposition of a coating in the range ofapproximately 35–40 lm and a high phosphorous content,in the range of 18 wt%. Both the chemical composition andthickness of the plating were evaluated by means of second-ary neutron mass spectroscopy (SNMS). However, boththe thickness and state of the substrate-coating interfacewere also evaluated by means of SEM techniques on sam-ples of the cross-section of the coated specimens carefullyprepared metallographically. The structural condition ofthe coating was also analyzed by means of X-ray diffrac-tion methods. In order to assess the hardness of the coat-ing, Vickers hardness measurements were carried outboth on coated and uncoated specimens employing loadsbetween 10 and 300 g. Such measurements were conductedboth on the surface of the coated samples and in cross-section.

The static mechanical properties of both the coated anduncoated specimens were determined by means of tensiletesting, employing a computer-controlled servohydraulicmachine (Instron 8502) at a cross-head speed of 3 mm/min. For this purpose, three samples of each material con-dition were tested. The performance of the uncoated andcoated samples under dynamic loading was evaluated out

Fig. 2. Cross-section view of the substrate alloy illustrating the presenceof an elongated grain structure and second phase particles.

Fig. 3. Cross-section view of a coated sample. The thickness of the depositand the substrate-coating interface can be clearly observed.

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1857

under rotating bending conditions (R = �1), employing aFatigue Dynamics RBF-200 equipment, at a frequency of50 Hz (3000 revolutions per minute). Half of the tests wereconducted in air and half in a 3 wt% NaCl solution, all atapproximately 23 �C.

The fatigue tests carried out in air with the uncoated andcoated samples were conducted at maximum alternatingstresses in the range of 269–430 MPa, whereas all the cor-rosion–fatigue tests carried out in the NaCl solution wereperformed at stresses in the range of 219–377 MPa. Inevery case, for the evaluation of the fatigue life, not lessthan 24 samples were tested, which allows the fulfillmentof the number of specimens required in S–N testing for reli-ability data according to the ASTM standard 739 (12–24samples) and a replication greater than 80%. The conduc-tion of a meaningful comparison of the fatigue resultsbetween the uncoated and coated conditions required thatall the samples were mechanically prepared in a similarmanner, such that all of them had a similar ‘‘mirror-like’’polished surface before testing.

The fracture surfaces of some selected coated samples,tested at the lowest and highest alternating stresses wereanalyzed by means of SEM techniques (Hitachi S-2400).Emphasis was placed on the identification of the initiationsites of fatigue cracks and the different stages undergone bythese during their subsequent propagation. The observa-tions were conducted (at a potential of 20 kV) both onthe plane of fracture and along cross-sections normal toit. An attempt was made to analyze the sequence followedduring the fracture process of the coating-substrate systemunder dynamic loading.

3. Experimental results

3.1. Characteristics of the substrate-deposit system

Fig. 2 illustrates a typical cross-section of the substratealloy taken parallel to the axis of the bar, observed bymeans optical microscopy, indicating that the structure isconstituted of elongated grains and second phase particlesaligned along the extrusion direction of the bar. The SEMand EDS analysis that were conducted on such sectionsshowed that the second phase particles are rich in Fe, Cuand Al, which could be of the type Al23CuFe4, typical ofthis alloy in the T6 condition [9]. Regarding the EN coat-ing, Fig. 3 illustrates a cross-section view of the coatedsamples, which indicates the deposition of a uniform filmwith a mean thickness of approximately 38–40 lm and acoating-substrate interface apparently free of pores andcracks. Fig. 4 shows the results of the SNMS analysis. Inthis figure, it can be clearly observed that the thickness ofthe EN coating is in the range indicated above and thatits composition in P remains constant throughout thedeposit at a value of approximately 30 at% (18.4 wt%).The analysis of the structural condition of the EN coatingby means of XRD allowed the corroboration of its amor-phous state, as expected if no post heat treatment has been

conducted. The XRD spectrum is presented in Fig. 5, inwhich a wide single peak can be seen, located at values of2h of the order of 40�–50�. Such a result is also consistentwith the phosphorous content of the deposit, in the sensethat for values of such an element greater than about7 wt%, the formation of intermetallic compounds, such asNi3P, Ni5P2 and Ni12P5, and Ni rich microcrystallinephases is severely hindered.

The hardness of both substrate and substrate-coatingsystem were evaluated by applying loads in the range of10–300 g. Due to the thickness of the deposit, the substrateshowed a negligible influence on values of the compositehardness of the system. From such measurements as wellas from hardness measurements conducted on the cross-section of the coating employing a load of 100 g, the hard-ness of the latter was found to be somewhat less thanapproximately 5 GPa. This analysis also showed that atapplied loads of less than 300 g, the hardness of the depositincreases slightly with the indent diagonal, as shown inFig. 6. The change in coating hardness as a function of

0,01

0,1

1

10

100

0 10 20 30 40 50

Depth (Microns)

oC

necn

rtn

oita%.t

A( )

(a) Ni(b) P(c) Al

(a)

(b)

(c)

Fig. 4. Composition profile of the coated samples, determined by meansof SNMS analysis.

0

100

200

300

400

500

600

0 20 40 60 80 100 2θ

)s/c( ytisnet

nI

Fig. 5. XRD analysis of the EN deposit.

4.5

4.6

4.7

4.8

4.9

5

5.1

5.2

5.3

0 5 10 15 20 25 30 35

INDENT DIAGONAL (microns)

AH

SE

ND

RS

,G P

a

Fig. 6. Change in hardness with indent diagonal for the coated samples.

Fig. 7. Photomacrographs of different tensile samples of the coatedsubstrate showing the delamination of the EN deposit during testing.

1858 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

the indent diagonal was interpreted in terms of Meyer’s law[10]

H ¼ H 0dn�2 ð1Þwhere H0 represents the absolute hardness of the coatingand n is the Meyer’s exponent. H0 was found to be approx-imately 4.1 GPa, whereas n was determined to be approx-imately 2.1.

3.2. Evaluation of mechanical properties

The tensile tests carried out in the AA 7075-T6 alumi-num alloy substrate indicated a yield strength for this mate-rial of 547 ± 6 MPa and a tensile strength of 594 ± 8 MPa.After coating, the yield strength of the system increased to624 ± 13 MPa whereas the tensile strength also increasedup to 641 ± 19 MPa. It is important to mention that asplastic deformation takes place near the maximum load,during the tension test, extensive delamination and fractureof the coating occurs, as shown in Fig. 7, which is presum-ably associated with the lack of ductility of the EN deposit,as shown later.

Regarding the fatigue and corrosion–fatigue experi-ments that were conducted on both the substrate and sub-strate-coating system, Tables 1–4 summarize the dataconcerning the number of cycles to failure as a function

Table 1Mean number of cycles to failure (Nf) versus stress amplitude (S) for the uncoated specimens tested in air

Stress (MPa) Cycles to failure Mean SD

269 93,000 182,900 72,100 128,400 144,200 88,600 118,200 41,479323 55,500 25,900 69,600 41,200 107,200 59,900 59,883 27,772377 17,400 21,500 25,300 23,800 21,800 12,900 20,450 4560430 9000 6400 8000 4400 9200 8700 7617 1874

Table 2Mean number of cycles to failure (Nf) versus stress amplitude (S) for the coated specimens tested in air

Stress (MPa) Cycles to failure Mean SD

269 186,000 115,200 1565,500 620,700 111,100 157,100 459,267 575,395323 51,900 45,800 60,500 38,500 66,500 41,300 50,750 11,030377 30,100 25,300 21,400 19,500 15,700 18,000 21,667 5256430 13,600 8700 12,400 13,100 15,800 12,800 12,733 2310

Table 3Mean number of cycles to failure (Nf) versus stress amplitude (S) for the uncoated specimens tested in a 3 wt% NaCl solution

Stress (MPa) Cycles to failure Mean SD

219 77,200 72,500 67,400 70,700 80,900 54,000 70,450 9369269 38,500 52,200 34,500 33,000 31,300 35,100 37,433 7623323 16,200 16,300 16,900 19,300 14,800 16,100 16,600 1491377 11,600 8400 9800 9900 8400 9000 9517 1211

Table 4Mean number of cycles to failure (Nf) versus stress amplitude (S) for the coated specimens tested in a 3 wt% NaCl solution

Stress (MPa) Cycles to failure Mean SD

219 84,600 57,500 133,100 74,900 165,100 89,000 100,700 40,312269 52,900 61,300 58,800 51,100 50,400 32,200 51,117 10,240323 22,300 48,600 53,800 23,400 29,600 48,600 37,717 14,171377 20,100 19,200 7800 15,500 16,100 29,300 18,000 7035

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1859

of the maximum alternating stress applied to the material.Such data include the standard deviation of the number ofcycles to failure and are represented graphically in Figs. 8aand b, from which it can be observed that after testing bothin air and in a NaCl solution, the EN coating seems to giverise to a significant increase in fatigue properties. Duringtesting in air, it is observed (Fig. 8a) that the separationbetween the curves corresponding to the uncoated andcoated substrate increases as the alternating stressdecreases, indicating an increase in fatigue life for thecoated alloy. During testing in NaCl solution (Fig. 8b),the separation between the curves is approximately con-stant within the range of alternating stresses explored, indi-cating a better corrosion–fatigue performance for thecoated system. The effect of the corrosive environment onthe fatigue behavior of the uncoated alloy can be analyzedby comparing curves a (Fig. 8a) and a 0 (Fig. 8b), whichshow, as expected, a more pronounced detrimental effectof the corrosive solution as the alternating stress decreasesand failure is delayed to a larger number of cycles.

The quantitative description of the change in the num-ber of cycles to failure with the stress applied was carried

out by means of a simple parametric relationship of theform

S ¼ AN�mf ð2Þ

which involves two constants, A and m, characteristic ofthe materials under investigation and testing conditions,that are determined from the experimental data. The aboverelationship is similar to that advanced earlier by Basquin[11] for the description of this kind of data. A representsthe fatigue strength coefficient, that is, the stress requiredto produced failure of the material in a single load applica-tion and m, the fatigue exponent, which characterizes thesensitivity of the change in the number of cycles to failurewith the stress applied. The values of the parameters A andm that were obtained from the analysis conducted with thedata given in Tables 1–4 and shown in Fig. 8, are presentedin Table 5 for both the uncoated and coated specimenstested in air and under corrosive conditions.

The values of the parameters A and m are very impor-tant from the standpoint of the evaluation of the fatiguelife and design purposes of any component made of thisaluminum alloy that could be coated with this kind of

0

100

200

300

400

500

600

1000 10000 100000 1000000 10000000

NUMa

b

BER OF CYCLES TO FAILURE

MA

MIX

UM

A L

TE

RT

AN

INT

S G

M ,S

SE

RP

a

(a) 7075-AIR(b) 7075+NiP-AIR

(a)

(b)

AIR

0

100

200

300

400

500

600

1000 10000 100000 1000000

NUMBER OF CYCLES TO FAILURE

MA

XI

UM

A M

ET

LR

NA

TNI

S G

TR

ES

S,M

Pa (a') 7075-3% NaCl

(b') 7075+NiP-3% NaCl

(a')

(b')

NaCl Solution

Fig. 8. Mean number of cycles prior to fracture (Nf) as function of thealternating stress applied to the material (S) for the uncoated and coatedspecimens tested both in air (a) and in a 3 wt% NaCl solution (b).

Table 5Parameters involved in Eq. (2) for the different conditions tested

Condition A (MPa) m Standard error r2

Uncoated substratetested in air

1683.4 0.15 0.026 0.949

Coated substratetested in air

1291.4 0.12 0.035 0.918

Uncoated substratetested in NaCl

4005.4 0.26 0.016 0.986

Coated substratetested in NaCl

4032.8 0.25 0.046 0.857

0

20

40

60

80

100

120

200 250 300 350 400 450

MAXIMUM ALTERNATING STRESS, MPa

%NI

CR

EA

ES

UGI

TA

F NI

EL

IFE

(a) FAT IGUE IN AIR

(b) CORROSION-FATIGUE IN NaCl

(b)

(a)

Fig. 9. Change in the percentage of increase in fatigue life for theuncoated and coated specimens tested both in air and in a 3 wt% NaClsolution.

1860 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

EN deposit for imparting corrosion, hardness, wear andabrasion resistance. Once such parameters have been deter-mined, it is possible to evaluate the increase in fatigue lifebrought about by the presence of the coating, by meansof a simple relationship of the form:

%increase ¼ NENf � N Subst.

f

N Subst.f

� 100 ð3Þ

In which the number of cycles to failure are computed fromEq. (2), employing the constants reported in Table 5. Thechange in the % of increase in fatigue life with the stress

applied, for the coated specimens in comparison with theuncoated ones, during testing in air and in the NaCl solu-tion, is shown in Fig. 9. Here, it is clearly observed thatwhen the coated substrate is tested in air, the fatigue lifecan be increased up to more than 100%, particularly if itis tested at low alternating stresses, in the range of approx-imately 0.4 r0.2% of the coated system (0.5 r0.2% of the un-coated substrate). However, as the stress applied increases,the gain in fatigue life decreases and at values in the rangeof approximately 0.7 r0.2% (0.79 r0.2% of the uncoated sub-strate), the coated alloy behaves like the uncoated one, andno improvement in fatigue life is obtained.

Similarly, when the coating-substrate system is testedunder corrosive conditions at alternating stresses in therange of approximately 0.35–0.60 r0.2% of the coated sys-tem (0.4–0.69 r0.2% of the uncoated alloy), an increase infatigue life is also observed, which ranges between about55% and 70%, greater increments in fatigue lives beingassociated with lower alternating stresses. It is noticeablethat under these testing conditions the increase in fatiguelife is less sensitive to any change in the alternating stress,in comparison to the more pronounced change observedduring testing in air.

Fig. 11. General fracture surface of a sample tested in air at 269 MPa. Thefracture process has been dominated by the propagation of a single crack.The convergence of the fracture markings to the crack initiation site isclearly seen. The white arrows indicate the direction of crack propagation.

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1861

3.3. Evaluation of the fracture surfaces of the samples

Fig. 10 shows a close view of a typical coated sampletested in tension, in which both the fracture surface andthe lateral wall are clearly visible. As can be observed, afterfracture of the specimen the coating appears to be severelycracked and has delaminated from the substrate in severalareas, particularly near the fracture surface of the sample.Therefore, the EN deposit can accompany the substrateduring its deformation only to a limited extent, due to itslack of ductility in comparison with the capability of thesubstrate to deform plastically. Besides the tensile samples,a number of fatigue specimens were also analyzed afterfracture. For this purpose, two samples were selected ineach condition (testing in air and in the NaCl solution),corresponding to the lowest and highest alternating stressesthat were applied. The selected specimens were those thatfailed at a number of cycles closest to the mean, given byEq. (2), determined at each stress level.

Fig. 11 illustrates a general view of the fracture surfaceof a sample tested at 269 MPa in air. The fracture surfacewas observed to be somewhat irregular and the fractureprocess dominated by a single crack whose location, P1,is indicated by the arrow. The origin of the crack on thephotomicrograph is clearly revealed by the convergenceof the fracture lines that emanate from such a point. Figs.12a–c illustrate a close view of the crack origin, in which itis possible to observe the EN deposit with a uniform thick-ness and the absence of secondary cracks both in thedeposit and along the interface. The radial marks withinthe coating in Fig. 12c would indicate that the fatigue crackwas nucleated at the outer surface of the specimen and notat the coating-substrate interface. The observations con-ducted on the lateral wall of the specimen near the crackorigin showed the integrity of the coating and the absenceof circumferential cracks.

Fig. 10. View of the cracking and delamination of the coating near thefracture surface of a tensile sample.

Fig. 13, on the other hand, illustrates the general frac-ture surface of a sample also tested in air at 430 MPa. Inthis case, the fracture of the specimen was observed tooccur as a consequence of the propagation of two differentcracks, located close to each other, whose origins are iden-tified as P1 and P2 in the figure, separated by a cleavagestep. One of the fatigue cracks was observed to be associ-ated with a nodular-type of defect located on the outer sur-face of the sample, as shown in detail in Figs. 14a and b. Itis presumed that such defects were formed during the syn-thesis of the coating and could be associated with the highstandard deviation of the number of cycles to failure atconstant maximum alternating stress. The analysis con-ducted on the lateral wall of the specimen, close to the areawhere the fatigue cracks were nucleated, indicated theintegrity of the coating and the absence of parallel circum-ferential cracks, a fact that was corroborated by the obser-vations conducted on the sections normal to the fracturesurface of the sample, as shown in Fig. 15.

Regarding the fatigue samples that were tested in theNaCl solution, Fig. 16 illustrates the general fracture sur-face of a typical specimen tested at 219 MPa. Again, itcan be observed that the fracture process has been domi-nated by the nucleation and propagation of a single crack,whose origin is indicated as P1. The directions of crackpropagation are also shown in the figure. The analysis con-ducted on the lateral area near the crack origin revealedthat the dominant crack was also associated with a nodulardefect located at the outer surface of the specimen, asshown in Figs. 17 a and b. These latter figures also illus-trate that, in spite of the aggressive conditions in whichthe sample was tested, the coating has preserved its integ-rity, being apparent the absence of secondary cracks both

Fig. 12. Closer view of the crack initiation site of the previous sample. (a) Fracture surface of the specimen showing the EN coating and the crack origin,P1. (b) Side view of the sample showing the lateral wall and the crack initiation site, P1. (c) Magnified view showing the EN deposit and the substrate-coating interface.

Fig. 13. General fracture surface of a sample tested in air at 430 MPa.Two different crack initiation sites, P1 and P2 can be observed. The whitearrows indicate the direction of crack propagation.

1862 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

in the coating and along the substrate-coating interface.This aspect can also be seen more clearly in Figs. 18aand b, which show a cross-section normal to the fracturesurface, close to the crack site initiation.

During testing at 377 MPa, almost the same featureswere observed on the typical fracture surfaces, as shownin Figs. 19a and b, where the nucleation and propagationof a single crack is apparent. As for the samples tested atlow alternating stresses, under the present conditions thedominant cracks were also observed to be associated withthe same nodular defects pointed out before, produced dur-ing the coating process. As in the previous cases, the ENdeposit was observed to preserve its integrity and adhesionto the substrate during fatigue testing, as illustrated inFig. 20.

4. Discussion

The fact that both the yield and tensile strength of thesubstrate-coating system are greater than that of the sub-strate alone indicates that the EN deposit can contributein a limited manner to increase the mechanical propertiesof the substrate, while it remains adhered to it, which could

Fig. 14. (a) Side view of one of the crack initiation sites. The crack originis associated with a nodular defect located at the outer surface of thespecimen. The two cracks that gave rise to the fracture of the specimencoalesced into a single crack, which apparently led to the formation of acleavage step (CS). (b) Closer view of the nodular defect associated withthe crack origin.

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1863

be of importance in explaining the fatigue behavior justdescribed in the previous sections. The results also showthat the coating is able to sustain a limited plastic deforma-tion and accompany the substrate up to maximum load intension. However, cracking and delamination of thedeposit from the substrate at stresses above the yieldstrength, as shown in Figs. 7 and 10, limits the contributionof the coating to the increase in the tensile strength of thesystem. Therefore, to a first approximation, it is possibleto compute a crude estimate of the yield strength of theEN deposit by assuming the validity of a linear law of mix-tures, such that

�r ¼ fENrEN þ fSrS ð4Þ

In the above equation �r represents the yield strength of thesubstrate-coating system, fEN and fS the area fractions ofthe EN deposit and substrate that contribute to withstandthe load applied and rEN and rS the yield strength of theEN deposit and substrate, respectively. The cross-sectionalareas of the coated specimen and the uncoated substrateare given by

AEN ¼p4ðdS þ 2tÞ2 ð5Þ

and

AS ¼p4

d2S ð6Þ

respectively, where dS represents the diameter of the tensilespecimen (6.1 mm) and t the mean thickness of the EN de-posit (38–40 lm). Thus,

fEN ¼AEN � AS

AEN

and f S ¼ 1� fEN ð7Þ

Assuming that �r ¼ 624MPa and rS = 547 MPa, a value ofapproximately 3.76 GPa is determined for the yieldstrength of the coating, which is very close to the absolutehardness that was computed by means of the Meyer rela-tionship. Therefore, it would be expected that the fatiguelife of this aluminum alloy were improved after coatingwith such an EN deposit, provided that the coating remainswell adhered to the substrate during fatigue testing andthat the deposit does not become a source of fatigue cracks,giving rise to the early fracture of the coated system.

The results depicted in Figs. 8 and 9 indicate that thefatigue performance of the substrate-coating system is sig-nificantly better than that of the substrate alone, althoughit is clearly observed that during testing in air, there is acontinuous drop in fatigue life gain as the stress appliedincreases, until values in the range of approximately 0.7r0.2% are attained, where no gain at all is achieved. Also,in agreement with the previous statement, the stress-lifedata show that for the substrate-coating system, the magni-tude of the rate of change of the number of cycles to failurewith the stress applied (dNf/dS) is greater than that for theuncoated substrate, both during testing in air and in theNaCl solution.

On the other hand, the fractographic analysis that wasconducted indicated that under most testing conditions,the fatigue mechanism of the coated samples is controlledby the nucleation and propagation of a single crack, exceptduring testing in air at elevated alternating stresses, condi-tions in which more than one crack could be nucleated. Inall the cases analyzed, it was observed that such fatiguecracks were associated with some nodular defects locatedat the surface of the coated samples and that the EN plat-ing preserved its integrity and remained well adhered to thesubstrate during testing.

The above observations could be rationalized as follows:the increase in fatigue life observed during testing both inair and in the NaCl solution, particularly at low alternatingstresses, indicates that fatigue cracks are nucleated at the

Fig. 15. Cross-section views of the previous sample showing the integrity of the EN deposit and the substrate-coating interface. Both the coating and theinterface are observed to be free of cracks. SF refers to the fracture surface of the specimen, near the crack initiation site pointed out in Fig. 13.

Fig. 16. General fracture surface of a sample tested in the NaCl solutionat 219 MPa. The fracture process is observed to be dominated by thenucleation and propagation of a single crack, whose origin is depicted asP1. The arrows show the direction of crack propagation.

Fig. 17. Side views of the above specimen showing the association of thecrack origin with a nodular defect located on the surface. Bothmicrographs illustrate the integrity of the coating on the lateral wall ofthe sample.

1864 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

surface of the coated alloy and have to propagate firstthrough the EN plating before reaching the substrate. Iffatigue cracks were nucleated at the coating-substrate inter-face or at the sub-surface layers of the substrate, the subse-quent propagation would occur towards the substrate andnot towards the EN plating, which not only has a mechan-ical strength higher than that of the 7075-T6 aluminumalloy, as shown by the tensile and hardness results previ-ously presented and analyzed, but also it is presumably ina compressive state of residual stresses, in the range ofapproximately 102 MPa, according to the results providedby Parker and Shah [8].

Given the association of the fatigue cracks initiationsites with the nodular defects observed on the surface ofthe specimens, it could be argued that as the alternating

Fig. 18. Cross-section views of the same specimen described in Fig. 16 showing the integrity of the EN deposit and the substrate-coating interface. Boththe coating and the interface are observed to be free of cracks. The crack origin is located near the area depicted as A.

Fig. 19. (a) General fracture surface of a sample tested in the NaClsolution at 377 MPa. The fracture of the specimen occurred as aconsequence of the nucleation and propagation of a single crack. Thearrows illustrate the direction of crack propagation. (b) Closer view of thecrack site initiation showing its association with a nodular defect locatedon the surface. The integrity of both the coating and substrate-depositinterface are clearly observed.

E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866 1865

stress increases, the stress concentration at such defectsbecomes more pronounced and gives rise to an accelerationof the fatigue mechanism of the coated specimens. A possi-

bility is that such acceleration occurs through the nucle-ation of a larger number of fatigue cracks, in which caseas the alternating stress applied to the material increases,the EN coating would become a source of fatigue cracks,giving rise to a reduction in the number of cycles to failure.Another possibility is that as the alternating stress appliedto the material increases, the fatigue crack growth ratethrough the EN plating also increases, achieving the sub-strate at a significantly less number of cycles. Both effectswould be particularly pronounced during testing in airdue to relatively high stresses applied in comparison withthe tests conducted in the NaCl solution.

Under corrosive conditions, the amorphous state of theEN deposit in the as-plated state, as indicated in Fig. 5,provides an effective barrier against pitting and thereforeagainst multiple nucleation of fatigue cracks, giving riseto a significant increase in the fatigue performance of thecoated system. However, the deleterious effect of the corro-sive medium can be clearly appreciated by comparing thefatigue curves for the different materials tested in air andin the NaCl solution. Accordingly, for the coated alloy,the reduction in fatigue life brought about by the NaClsolution can vary between 31% and 82%, when testing isconducted at alternating stresses in the range of 377–269 MPa, whereas for the uncoated alloy, in the samerange of stresses, the reduction in fatigue life observed var-ies between approximately 46% and 78%.

5. Conclusions

Coating of a 7075-T6 aluminum alloy with an electrolessNi–P deposit of approximately 38–40 lm in thickness anda high P content of about 18 wt%, in the as-deposited con-dition, gives rise to an improvement in the fatigue and cor-rosion–fatigue performance of the substrate. However,when the coated system is tested in air, as the maximumalternating stress increases the gain in fatigue life decreases,until stresses in the range of 0.7 r0.2% of the coated alloy are

Fig. 20. Cross-section view of the previous sample showing the integrity of both the coating and the substrate-coating interface. FS represents the fracturesurface of the specimen.

1866 E.S. Puchi-Cabrera et al. / International Journal of Fatigue 28 (2006) 1854–1866

achieved, at which no improvement in the fatigue perfor-mance of the system is observed and the behavior is similarto that of the uncoated substrate. Fatigue cracks have beenobserved to be associated with nodular defects present onthe surface of the coated samples, whose deleterious effectseems to be more pronounced as the alternating stressincreases. A crude estimate of the yield strength of theEN coating from tensile measurements indicate that sucha parameter is in the range of 3.8 GPa, in agreement withthe computation of the absolute hardness of the deposit,of about 4 GPa, by means of Meyer’s law. In general, ithas been shown that the EN deposit has a very good adhe-sion to the substrate even when the system is subjected totensile stresses greater than the yield strength. Such charac-teristics as well as its higher mechanical properties in com-parison with the aluminum alloy substrate, the presumablycompressive nature of its residual stresses, in the range ofapproximately 102 MPa, and the preservation of its integ-rity during fatigue testing contribute to the better fatigueperformance of the coated system.

Acknowledgments

This investigation has been conducted with the financialsupport of the Venezuelan National Fund for Science,Technology and Innovation (FONACIT) through the pro-

ject UCV G-2000001591, and the Scientific and HumanisticDevelopment Council of the Universidad Central de Vene-zuela through the project PG-08-17-4595-2000. Theauthors are deeply grateful to Ms. Aramaris Prieto andMr. Anıbal Rodrıguez for conducting a large part of theexperimental work and to Reliable Plating Inc. (Chicago,Illinois, USA), for carrying out the coating of the samples.

References

[1] DeBartolo EA, Hillberry BM. Int J Fatigue 2001;23:S79–86.[2] Sankaran KK, Perez R, Jata KV. Mater Sci Eng A 2001;297:223–9.[3] Fonte MA, Stanzl-Tschegg SE, Holper B, Tschegg EK, Vasudevan

AK. Int J Fatigue 2001;23:S311–7.[4] Liu XF, Huang SJ, Gu HC. Int J Fatigue 2002;24:803–9.[5] DuQuesnay DL, Underhill PR, Britt HJ. Int J Fatigue 2003;25:371–7.[6] Hajdu J. Surface preparation for electroless nickel plating. In:

Mallory GO, Hajdu JB, editors. Electroless plating: fundamentalsand applications. FL, USA: American Electroplaters and SurfaceFinishers Society; 1990. p. 193.

[7] Colaruotolo J, Tramontana D. Engineering applications of electrolessnickel. In: Mallory GO, Hajdu JB, editors. Electroless plating:fundamentals and applications. FL, USA: American Electroplatersand Surface Finishers Society; 1990. p. 207.

[8] Parker K, Shah H. Plating 1971;58:319.[9] Gao M, Feng CR, Wei R. Met Mater Trans 1998;29A:1145–51.

[10] Tabor D. The hardness of metals. Oxford, UK: Oxford UniversityPress; 1951.

[11] Basquin OH. Proc ASTM 1910;10(2):625.