Materials Science and Engineering A 507 (2009) 29–36
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Materials Science and Engineering A
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ffect of WC–10%Co–4%Cr coating on the Ti–6Al–4V alloy fatigue strength
.Y.P. Costa ∗, M.L.R. Venditti, H.J.C. Voorwald, M.O.H. Cioffi, T.G. Cruzatigue and Aeronautical Materials Research Group, DMT/FEG/UNESP, Av. Ariberto Pereira da Cunha, 333, Guaratinguetá, Cep: 12516 410, S.P., Brazil
r t i c l e i n f o
rticle history:eceived 25 September 2008eceived in revised form2 November 2008ccepted 24 November 2008
eywords:i–6Al–4VatigueVOFhot peening
a b s t r a c t
High strength/weight ratio and effective corrosion resistance are primary reasons to use titanium alloysreplacing steel and aluminum in some aeronautical components. However, titanium alloys have poortribological properties, which reduce devices performance under friction; making surface treatmentsa requirement to improve wear. Thermal spray coatings have attractive characteristics as high hardnessand strong coating/substrate adhesion. Compared with thermal spray processes, the High Velocity OxygenFuel (HVOF) presents less porosity and oxide contents due to the lower flame temperature used in theprocess operation.
Electroplated coatings used to improved abrasive wear and corrosion properties, affects nega-tively the fatigue strength, providing lower results than those for uncoated parts. To increase fatiguestrength of coated materials, techniques as compressive residual stresses induced by shot peening are
used.
In this study the influence of WC–10%Co–4%Cr coating deposited by HVOF on the fatigue strength ofTi–6Al–4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base materialshows also a decrease when parts are coated. It was observed that the influence is more significant in highcycle fatigue tests. The shot peening prior to the thermal spray coating is an efficient surface treatmentto improve fatigue resistance of coated Ti–6Al–4V. Scanning electron microscopy technique (SEM) wasused to observe crack origin sites and thickness in all the coatings.
. Introduction
In the aeronautical industry, materials selection is mostlyocused on optimized performance and cost reduction. Base
aterial performance is frequently considered as well as other char-cteristics are also taken into account, such as weight reduction,aintenance and manufacturing costs. For landing gears, titanium
lloys are responsible for high strength/weight ratio; in addition,igh corrosion resistance increases the durability of titanium com-onents in aggressive environment [1–3].
Ti–6Al–4V is the most manufactured titanium alloy and repre-ents 60% of all production. Related to the Ti–6Al–4V alloy intrinsicow wear resistance, coatings are used to improve properties of
echanical components in sliding wear [4–6].Shibata et al. [7] investigated the effect of a TiN surface layer
btained by gas nitriding on rotating bending fatigue behav-
or of the Ti–6Al–4V alloy. Results showed that an increase initriding time is followed by a fatigue strength decrease fromaximum stress of 720 MPa for the base material to 540 MPa
nd 480 MPa, for 4 h and 15 h of gas nitriding, respectively. This
behavior was attributed to premature fatigue cracks in the nitridelayer [7].
Nan et al. [8] evaluated three-point bending fatigue tests inTi–6Al–4V alloy coated with TiN. The presence of TiN applied by ionbeam enhanced deposition (IBED) increased the Ti–6Al–4V fatiguestrength from 850 MPa to 950 MPa. The IBED layer presented higherdislocation density than substrate and blocked surface dislocationmovement, increasing sample fatigue life.
Chromium base coatings are found in applications where com-binations of adhesion, hardness, wear and corrosion resistanceare required. However, electroplated baths produce hexavalentchromium, restricted by environmental legislation [9]. Thereforethe development of alternative processes to manufacture wearresistant coatings is required [9–11].
Thermal spraying process is a feasible alternative coatingto replace chromium electroplating, among other ones such asPVD (physical vapor deposition). The High Velocity Oxygen Fuel(HVOF) technology permits the production of cermets coatingswith superior properties by spraying particles at a higher aver-
age velocity and a lower average temperature than other thermalspray processes [10]. This fact produces a lesser significant par-ticle oxidation of semi-melting material during the recoveringprocess, which generates a higher coating corrosion resistance andallows smaller amount of phase transformation. As a consequence,
arbides decomposition is limited improving wear coating resis-ance [9–11].
Recent studies investigated the influence of HVOF coatingsn the mechanical performance of high strength steel. Voor-ald and co-authors [12] showed that AISI 4340 steel fatigue
trength for 105 cycles was 950 MPa and for AISI 4340 steelC–10%Co–4%Cr thermal spray coated, it decreased to 700 MPa.ard chromium electroplated specimens present a fatigue strengthqual to 525 MPa, at 105 cycles. From HVOF coated specimensxperimental results, wear and corrosion properties improvementn comparison with hard chrome coatings was obtained [12–16].
As surface treatments in general reduce fatigue life due to theensile residual stress induced, the shot peening was indicateds a treatment useful to improve this property. This process pre-ents fatigue crack initiation and delay fatigue crack propagationy inducing a compressive residual stress field in the upper lay-rs of the substrate. The fatigue strength obtained for shot peenedISI 4340 steel specimens WC–10%Co–4%Cr coated was 1100 MPat 105 cycles in comparison to the 700 MPa for base materialC–10%Co–4%Cr coated [12,17–20].This research evaluates the influence of WC–10%Co–4%Cr HVOF
hermal spray coated on the axial fatigue strength of Ti–6Al–4V.hree groups of specimens were prepared to obtain S–N curves:ase material, base material WC–10%Co–4%Cr HVOF coated andase material shot peened and WC–10%Co–4%Cr HVOF coated.
. Experimental procedures
The base material used in this research was the Ti–6Al–4V alloyith the following chemical composition: 69.86% Ti, 6.03% Al, 4.58%, 0.61% Fe wt% obtained by atomic absorption spectrophotometer.
t is characterized by a metallurgical duplex structure with a 30%olume fraction of equiaxied primary � and 70% correspond to aamellar � + � structure.
Tensile tests were conducted according to ASTM E-8M [21]tandard procedure. Mechanical properties of the alloy are: elas-ic modulus 107.5 ± 0.8 GPa, elongation 13.3 ± 1.4%, yield tensiletrength 935.1 ± 11.5 MPa (0.2% offset), ultimate tensile strength
001.0 ± 7.7 MPa and hardness 410.7 ± 2.3 HV0.3Kg.f, in the annealedondition.
Titanium alloy specimens were obtained by grind machining,hich represents surface roughness Ra = 0.76 ± 0.05 �m, cut-off
.8 mm.
Fig. 3. Fracture surface of Ti–6Al–4V alloy
Fig. 2. S–N curves for Ti–6Al–4V alloy, Ti–6Al–4V alloy WC–10%Co–4%Cr thermalspray coated Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr thermal spraycoated.
2.1. Tungsten carbide coating
The tungsten carbide thermal spray coating applied by Prax-air with a HVOF spray system used WC powder with 10%Co–4%Cr,resulting in average thickness equal to 150 �m with surface rough-
ness Ra = 2.77 ± 0.19 �m. Prior to the tungsten carbide thermalspray coating process, specimens were blasted with aluminumoxide mesh 90 to enhance adhesion. Praxair performed the bondstrength tests, as designated in ASTM C633 [22]. Minimum adhesionstrength of 68.9 MPa was observed.
—�max = 975 MPa: (a)15×; (b) 150×.
M.Y.P. Costa et al. / Materials Science and Engineering A 507 (2009) 29–36 31
Fig. 4. Fracture surface of Ti–6Al–4V alloy WC–10%Co–4%Cr therma
Fig. 5. Fracture surface of Ti–6Al–4V alloy WC–10%Co–4%Cr th
A polished specimen WC–10%Co–4%Cr thermal spray coated wasvaluated by SEM analyses to investigate the coating morphol-gy. The blasted specimen with aluminum oxide was observed asell.
.2. Shot peening
Shot peening parameters were: intensity of 0.008 A, out flow ofkg, speed of 250 mm/min, distance 200 mm and rotation equal to0 rpm. The steel shot used was S230 (∅ 0.7 mm) for the process
hat was carried out on an air-blast machine according to stan-ard SAE-AMS-S-13165 [23]. Shot peened specimens presented aurface roughness Ra = 1.08 ± 0.14 �m. For the HVOF coated speci-ens this process was evaluated before the aluminum oxide blas-
Fig. 7. WC–10%Co–4%Cr morphology (a), (b), (c) and (d): regions a, b, c and d in Fig. 6.
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.3. Fatigue tests
Axial fatigue tests according to ASTM E 466 [24] were conductedsing a sinusoidal constant amplitude load of frequency 20 Hz andtress ratio R = 0.1 at room temperature considering, as fatiguetrength, specimens fractured or 107 load cycles. Three groups ofatigue specimens were prepared, according to Fig. 1, to obtain S–Nurves for axial fatigue tests:
Specimens of base material;Specimens of base material WC–10%Co–4%Cr HVOF thermal spraycoated;Specimens of base material, shot peened and WC–10%Co–4%CrHVOF thermal spray coated.
Fracture planes of fatigue specimens were examined using acanning electron microscope model JEOL JSM 5310 in order todentify the crack origin sites.
. Results and discussion
Fig. 2 shows axial fatigue S–N curves for the Ti–6Al–4V alloyn the following conditions: base material and base material
ig. 8. Ti–6Al–4V specimen blasted with aluminum oxide: (a) 1500×; (b) EDS from (a); (c
Engineering A 507 (2009) 29–36 33
WC–10%Co–4%Cr thermal spray coated. Experimental data indicatethat the WC–10%Co–4%Cr coating had a negative effect on the axialfatigue strength of the Ti–6Al–4V alloy. This tendency is observedfrom 104 to 105 load cycles and also for 107 cycles. For maximumapplied stress of 965 MPa, which represents 96% of the ultimate ten-sile strength, a decrease of 96.7% in the fatigue life for the titaniumalloy coated with WC–10%Co–4%Cr was observed. From the litera-ture it was informed that tungsten carbide thermal spray coatingapplied by HP/HVOF process decreased the AISI 4340 steel rotatingbending fatigue strength [13]. In the case of aluminum 7050 T7451alloy the same behavior was reported [25].
According to the experimental data represented in Fig. 2,the uncoated Ti–6Al–4V alloy presents elevated fatigue strength,approximately 900 MPa for 107 load cycles. For the tungsten car-bide thermal spray coated specimens, 400 MPa was associated to107 cycles.
Fig. 3a shows the uncoated fatigue specimen fracture surfacetested at maximum stress 975 MPa, which supported 32,000 cycles
to failure. Several crack fronts at surface, as well as fatigue crackpropagation throughout base material, were observed in Fig. 3b.
The axial fatigue fracture surface of the Ti–6Al–4V alloyWC–10%Co–4%Cr thermal spray coated tested at maximum stress965 MPa with 4000 cycles to failure, is represented in Fig. 4. The
) transversal view of Ti–6Al–4V specimen blasted; (d) EDS from region A in (c).
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racture surface appearance in Fig. 4a is comparable to a typical cup-one fracture of a ductile material tested in tension. From Fig. 4bne sees cracks starting at the coating/substrate interface and inig. 4c equiaxied dimples relatively uniform in size are associatedo a tensile mode of fracture due to the relatively high maximumtress used.
It is clear from Fig. 5a and b that the tungsten carbide thermalpray coating has a detrimental effect on the axial fatigue strengthf the Ti–6Al–4V alloy.
Several crack origin sites in Fig. 5a are visible. In Fig. 5b, fatigueracks nucleation at coating, which not coalesced into the substrateirection are represented. Fatigue cracks nucleation and propaga-ion from coating/substrate interface before penetrate the substrateas also observed. The specimen was axial fatigue tested at maxi-um stress 450 MPa and presented 24,500 cycles to failure.Ogawa and co-authors [26] observed that the coating fatigue
esistance determines sprayed materials fatigue strength when theoating is harder than the substrate and presents high bonding
trength. Cracks in the coating increase stress concentration andonduct the propagation process; as a consequence, the fatiguetrength of the material is reduced. In this work, for maximumtress 965 MPa, cracks developed inside the coating increased the
local stress to a value higher than the substrate mechanical resis-tance.
A thermal spray coated specimen was polished and observedaccording to Fig. 6, which represents regions a, b, c, and d, detailedin Fig. 7. Recent papers showed a dense morphology and lowerporosity in HVOF coatings than other spray processes [12,27]. Thismorphology was confirmed in Fig. 7a that presents a uniformcarbide distribution. The presence of porous reduces fatigue life[12,13,15].
Fig. 7b indicates that the aluminum oxide blasting is responsi-ble for the increase of roughness increases at the substrate surface,improving coating/substrate adhesion. Fig. 7c and d show a crack inthe coating with approximately 60 �m of extension. In the sprayingprocedure, cermet powder is semi-molted and accelerated towardsthe substrate, assuming a characteristic lamellar shape (splats).Cracks result from voids in the splats interface [14].
The microstructure of HVOF coatings is particularly multiphasewith pores, oxide inclusions and splats interfaces, which acted as
stress concentrators and became crack initiation sites (Fig. 5b),reducing fatigue life [13,14,25].
Recent studies [13,26,28] showed that when the coating isharder than the substrate, the fatigue crack propagates across the
max = 650 MPa: (a) 350×; (b) 2000×; (c) 2000×, backscattered electron image.
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M.Y.P. Costa et al. / Materials Scien
nterface into base material. The WC–10%Co–4%Cr thermal sprayoating hardness was 1900 HV0.3Kg.f and the substrate was 410V0.3Kg.f.
A Ti–6Al–4V grid-blasted specimen with aluminum oxide isresented in Fig. 8a. Energy Dispersive Spectroscopy analysisEDS), represented in Fig. 8b, showed an increase in base mate-ial aluminum weight percent when compared with the unblastedpecimen: 9.84% Al, 4.16% V and 86.01% Ti (wt%). Fig. 8c shows aransversal image of the blasted Ti–6Al–4V alloy, in which a sur-ace roughness increase was observed. The chemical compositionn region A was: 6.29% Al, 4.95% V and 88.76% Ti (wt%), showed inig. 8d. From the literature it is well know that aluminum oxide par-icles in most cases are incorporated in the titanium and in this casects as stress concentrator, reducing the fatigue strength [29,30].
Aluminum oxide particles partly embedded in Ti–6Al–4V alloyre showed in Fig. 9a and b obtained from secondary electronmages. The bright area is associated to the aluminum oxide par-icles. The backscattered electron image confirms the presence ofluminum oxide particles by contrast based on the atomic num-er, as indicated in Fig. 9c. The distinct microscopic compositionariations are Tungsten (74—atomic number), Titanium (22) andluminum (13), which are described from the brighter to the dark-st, respectively.
In Fig. 2, it is possible to observe a significant reductionn the fatigue strength of the Ti–6Al–4V alloy associated to
C–10%Co–4%Cr coating and a slightly better fatigue performanceor specimens shot peened prior to the thermal spray coating pro-ess. For maximum stress equal to 450 MPa, the ratio between theverage number of cycles to failure of shot peened and thermalpray coated and just WC–10%Co–4%Cr thermal spray coated spec-
ig. 10. Fracture surface of Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr thermal sp
Engineering A 507 (2009) 29–36 35
imens is 2.2. The same comparison for maximum stress 425 MPaindicates ratio equal to 2.3.
From previous work [13], it is known that the HVOF ther-mal spray process produces compressive residual internal stresseswithin the substrate, due to the mechanical deformation on the sur-face during particle impact. Fatigue crack propagation is delayedwhen this compressive residual stress site is achieved. As men-tioned earlier, despite of the compressive residual stress induced bythe HVOF thermal spray process, the decrease in fatigue strengthis associated to the pores intrinsic to the coating and particle oxideinclusions in the base material.
Analysis of Fig. 10a, which represents a fracture surface froman axial fatigue specimen shot peened and WC–10%Co–4%Cr ther-mal spray coated 160 �m thick, tested at maximum stress 375 MPawith 1,150,000 cycles to failure. From Fig. 10b, one sees that theshot peening process increased the substrate strength and delayedfatigue crack propagation through base metal. In Fig. 10c, fatiguecrack propagation inside base metal was deflected due to the effectof the compressive residual stress field induced by the shot peeningprocess. This behavior explains the increase in the fatigue strengthfor Ti–6Al–4V alloy specimens after the shot peening process.
Fig. 11 presents a fatigue fracture surface of a Ti–6Al–4V alloyspecimen shot peened and WC–10%Co–4%Cr thermal spray coated,tested at 350 MPa. It is possible to observe cracks growth at andparallel to the coating/substrate interface, associated to the influ-
ence of compressive residual stresses, which avoid or even retardthe crack propagation into the substrate.
According to Nascimento et al. [13], when the coating is harderthan base material, fatigue crack may propagate through interfaceinside base material. In this work, crack propagation at interface
ray coated—�max = 375 MPa: (a) 200×; (b) 500×, region 1; (c) 500×, region 2.
ig. 11. Fracture surface of Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr ther-al spray coated—�max = 350 MPa.
oating/substrate is related to the compressive residual stressnduced by HVOF process [31,32].
The shot peening process was effective to increase theC–10%Co–4%Cr thermal spray coated Ti–6Al–4V alloy high cycle
atigue strength in about twice. Despite this benefit, the originalase material fatigue life was not restored.
Nalla et al. [33] investigated the peening effect on Ti–6Al–4V, at00 MPa maximum stress level, fatigue life increased from 104 to06 cycles for the treated specimens. Studies from Gao [34] showedwo shot peened titanium alloys, Ti–5Al–5Mo–5V–1Cr–1Fe andi–10V–2Fe–3Al, in which the fatigue limit increased 27% and 29%y shot peening, respectively. An increase in specimen roughnessfter shot peening treatment promotes a fatigue strength reductions well as insufficient shot peening and over peening will not resultn good fatigue performance [34].
In this work, the influence of the thermal spray coating was toecrease the base material fatigue strength.
The fact that compressive residual stresses were not effective toncrease low cycle fatigue strength is related to reduction or relief
hen the material is subjected to high stress levels [35].
. Conclusions
The effect of WC–10%Co–4%Cr thermal spray coating applied byHVOF process was to decrease the fatigue strength of Ti–6Al–4Valloy. Several crack fronts all around the specimen were observed.
Aluminum oxides used to increase roughness and small cracksinside the coating were the main factors associated to the reduc-tion in the fatigue strength.The shot peening pre-treatment improved the fatigue strengthof coated Ti–6Al–4V alloy, which means that the fatigue life may
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Engineering A 507 (2009) 29–36
increase if crack nucleation is retarded or avoided and propaga-tion is delayed or even arrested.
Acknowledgements
The authors are grateful for the research support by CAPES, byFAPESP through the processes numbers 2006/03570-9 and 2006/02121-6 and by CNPq through the processes numbers 304155/2006-4, 470074/2006-0, 427570/2006-4 and 300233/2006-0. Wewould like to extend our thanks to Mrs. Maria Lucia Brison de Mat-tos (INPE) for the scanning electron microscope images.
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