Materials Science and Engineering A Effect of WCâ10

Materials Science and Engineering A 507 (2009) 29–36

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Effect of WC–10%Co–4%Cr coating on the Ti–6Al–4V alloy fatigue strength M.Y.P. Costa ∗ , M.L.R. Venditti, H.J.C. Voorwald, M.O.H. Cioffi, T.G. Cruz Fatigue and Aeronautical Materials Research Group, DMT/FEG/UNESP, Av. Ariberto Pereira da Cunha, 333, Guaratinguetá, Cep: 12516 410, S.P., Brazil

a r t i c l e

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Article history: Received 25 September 2008 Received in revised form 22 November 2008 Accepted 24 November 2008 Keywords: Ti–6Al–4V Fatigue HVOF Shot peening

a b s t r a c t High strength/weight ratio and effective corrosion resistance are primary reasons to use titanium alloys replacing steel and aluminum in some aeronautical components. However, titanium alloys have poor tribological properties, which reduce devices performance under friction; making surface treatments a requirement to improve wear. Thermal spray coatings have attractive characteristics as high hardness and strong coating/substrate adhesion. Compared with thermal spray processes, the High Velocity Oxygen Fuel (HVOF) presents less porosity and oxide contents due to the lower flame temperature used in the process operation. Electroplated coatings used to improved abrasive wear and corrosion properties, affects negatively the fatigue strength, providing lower results than those for uncoated parts. To increase fatigue strength 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 of Ti–6Al–4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base material shows also a decrease when parts are coated. It was observed that the influence is more significant in high cycle fatigue tests. The shot peening prior to the thermal spray coating is an efficient surface treatment to improve fatigue resistance of coated Ti–6Al–4V. Scanning electron microscopy technique (SEM) was used to observe crack origin sites and thickness in all the coatings. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In the aeronautical industry, materials selection is mostly focused on optimized performance and cost reduction. Base material performance is frequently considered as well as other characteristics are also taken into account, such as weight reduction, maintenance and manufacturing costs. For landing gears, titanium alloys are responsible for high strength/weight ratio; in addition, high corrosion resistance increases the durability of titanium components in aggressive environment [1–3]. Ti–6Al–4V is the most manufactured titanium alloy and represents 60% of all production. Related to the Ti–6Al–4V alloy intrinsic low wear resistance, coatings are used to improve properties of mechanical components in sliding wear [4–6]. Shibata et al. [7] investigated the effect of a TiN surface layer obtained by gas nitriding on rotating bending fatigue behavior of the Ti–6Al–4V alloy. Results showed that an increase in nitriding time is followed by a fatigue strength decrease from maximum stress of 720 MPa for the base material to 540 MPa and 480 MPa, for 4 h and 15 h of gas nitriding, respectively. This

∗ Corresponding author. Tel.: +55 1231 232865; fax: +55 1231 232852. E-mail address: [email protected] (M.Y.P. Costa). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.11.068

behavior was attributed to premature fatigue cracks in the nitride layer [7]. Nan et al. [8] evaluated three-point bending fatigue tests in Ti–6Al–4V alloy coated with TiN. The presence of TiN applied by ion beam enhanced deposition (IBED) increased the Ti–6Al–4V fatigue strength from 850 MPa to 950 MPa. The IBED layer presented higher dislocation density than substrate and blocked surface dislocation movement, increasing sample fatigue life. Chromium base coatings are found in applications where combinations of adhesion, hardness, wear and corrosion resistance are required. However, electroplated baths produce hexavalent chromium, restricted by environmental legislation [9]. Therefore the development of alternative processes to manufacture wear resistant coatings is required [9–11]. Thermal spraying process is a feasible alternative coating to replace chromium electroplating, among other ones such as PVD (physical vapor deposition). The High Velocity Oxygen Fuel (HVOF) technology permits the production of cermets coatings with superior properties by spraying particles at a higher average velocity and a lower average temperature than other thermal spray processes [10]. This fact produces a lesser significant particle oxidation of semi-melting material during the recovering process, which generates a higher coating corrosion resistance and allows smaller amount of phase transformation. As a consequence,

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M.Y.P. Costa et al. / Materials Science and Engineering A 507 (2009) 29–36

carbides decomposition is limited improving wear coating resistance [9–11]. Recent studies investigated the influence of HVOF coatings on the mechanical performance of high strength steel. Voorwald and co-authors [12] showed that AISI 4340 steel fatigue strength for 105 cycles was 950 MPa and for AISI 4340 steel WC–10%Co–4%Cr thermal spray coated, it decreased to 700 MPa. Hard chromium electroplated specimens present a fatigue strength equal to 525 MPa, at 105 cycles. From HVOF coated specimens experimental results, wear and corrosion properties improvement in comparison with hard chrome coatings was obtained [12–16]. As surface treatments in general reduce fatigue life due to the tensile residual stress induced, the shot peening was indicated as a treatment useful to improve this property. This process prevents fatigue crack initiation and delay fatigue crack propagation by inducing a compressive residual stress field in the upper layers of the substrate. The fatigue strength obtained for shot peened AISI 4340 steel specimens WC–10%Co–4%Cr coated was 1100 MPa at 105 cycles in comparison to the 700 MPa for base material WC–10%Co–4%Cr coated [12,17–20]. This research evaluates the influence of WC–10%Co–4%Cr HVOF thermal spray coated on the axial fatigue strength of Ti–6Al–4V. Three groups of specimens were prepared to obtain S–N curves: base material, base material WC–10%Co–4%Cr HVOF coated and base material shot peened and WC–10%Co–4%Cr HVOF coated.

Fig. 1. Axial fatigue testing specimens.

2. Experimental procedures The base material used in this research was the Ti–6Al–4V alloy with the following chemical composition: 69.86% Ti, 6.03% Al, 4.58% V, 0.61% Fe wt% obtained by atomic absorption spectrophotometer. It is characterized by a metallurgical duplex structure with a 30% volume fraction of equiaxied primary ␣ and 70% correspond to a lamellar ␣ + ␤ structure. Tensile tests were conducted according to ASTM E-8M [21] standard procedure. Mechanical properties of the alloy are: elastic modulus 107.5 ± 0.8 GPa, elongation 13.3 ± 1.4%, yield tensile strength 935.1 ± 11.5 MPa (0.2% offset), ultimate tensile strength 1001.0 ± 7.7 MPa and hardness 410.7 ± 2.3 HV0.3Kg.f , in the annealed condition. Titanium alloy specimens were obtained by grind machining, which represents surface roughness Ra = 0.76 ± 0.05 ␮m, cut-off 0.8 mm.

Fig. 2. S–N curves for Ti–6Al–4V alloy, Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr thermal spray coated.

2.1. Tungsten carbide coating The tungsten carbide thermal spray coating applied by Praxair with a HVOF spray system used WC powder with 10%Co–4%Cr, resulting in average thickness equal to 150 ␮m with surface roughness Ra = 2.77 ± 0.19 ␮m. Prior to the tungsten carbide thermal spray coating process, specimens were blasted with aluminum oxide mesh 90 to enhance adhesion. Praxair performed the bond strength tests, as designated in ASTM C633 [22]. Minimum adhesion strength of 68.9 MPa was observed.

Fig. 3. Fracture surface of Ti–6Al–4V alloy— max = 975 MPa: (a)15×; (b) 150×.


Fig. 4. Fracture surface of Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated— max = 965 MPa: (a) 15×; (b) 150×; (c) 2000×.

Fig. 5. Fracture surface of Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated— max = 450 MPa: (a) 15×; (b) 150×.

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The following process parameters were employed: • • • • • • •

Process: TAFA JP 5000; Oxygen pressure: 136–146 psi; Fuel: kerosene; Fuel pressure: 114–124 psi; Powder supply pressure: 3–6 psi; Spray distance: 300 mm; Maximum substrate temperature during spraying: 170 ◦ C.

A polished specimen WC–10%Co–4%Cr thermal spray coated was evaluated by SEM analyses to investigate the coating morphology. The blasted specimen with aluminum oxide was observed as well. 2.2. Shot peening Shot peening parameters were: intensity of 0.008 A, out flow of 3 kg, speed of 250 mm/min, distance 200 mm and rotation equal to 30 rpm. The steel shot used was S230 (∅ 0.7 mm) for the process that was carried out on an air-blast machine according to standard SAE-AMS-S-13165 [23]. Shot peened specimens presented a surface roughness Ra = 1.08 ± 0.14 ␮m. For the HVOF coated specimens this process was evaluated before the aluminum oxide blasting.

Fig. 6. Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated polished surface.

Fig. 7. WC–10%Co–4%Cr morphology (a), (b), (c) and (d): regions a, b, c and d in Fig. 6.


2.3. Fatigue tests Axial fatigue tests according to ASTM E 466 [24] were conducted using a sinusoidal constant amplitude load of frequency 20 Hz and stress ratio R = 0.1 at room temperature considering, as fatigue strength, specimens fractured or 107 load cycles. Three groups of fatigue specimens were prepared, according to Fig. 1, to obtain S–N curves for axial fatigue tests: • Specimens of base material; • Specimens of base material WC–10%Co–4%Cr HVOF thermal spray coated; • Specimens of base material, shot peened and WC–10%Co–4%Cr HVOF thermal spray coated. Fracture planes of fatigue specimens were examined using a scanning electron microscope model JEOL JSM 5310 in order to identify the crack origin sites. 3. Results and discussion Fig. 2 shows axial fatigue S–N curves for the Ti–6Al–4V alloy in the following conditions: base material and base material

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WC–10%Co–4%Cr thermal spray coated. Experimental data indicate that the WC–10%Co–4%Cr coating had a negative effect on the axial fatigue strength of the Ti–6Al–4V alloy. This tendency is observed from 104 to 105 load cycles and also for 107 cycles. For maximum applied stress of 965 MPa, which represents 96% of the ultimate tensile strength, a decrease of 96.7% in the fatigue life for the titanium alloy coated with WC–10%Co–4%Cr was observed. From the literature it was informed that tungsten carbide thermal spray coating applied by HP/HVOF process decreased the AISI 4340 steel rotating bending fatigue strength [13]. In the case of aluminum 7050 T7451 alloy 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 carbide thermal spray coated specimens, 400 MPa was associated to 107 cycles. Fig. 3a shows the uncoated fatigue specimen fracture surface tested at maximum stress 975 MPa, which supported 32,000 cycles to failure. Several crack fronts at surface, as well as fatigue crack propagation throughout base material, were observed in Fig. 3b. The axial fatigue fracture surface of the Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated tested at maximum stress 965 MPa with 4000 cycles to failure, is represented in Fig. 4. The

Fig. 8. Ti–6Al–4V specimen blasted with aluminum oxide: (a) 1500×; (b) EDS from (a); (c) transversal view of Ti–6Al–4V specimen blasted; (d) EDS from region A in (c).

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fracture surface appearance in Fig. 4a is comparable to a typical cupcone fracture of a ductile material tested in tension. From Fig. 4b one sees cracks starting at the coating/substrate interface and in Fig. 4c equiaxied dimples relatively uniform in size are associated to a tensile mode of fracture due to the relatively high maximum stress used. It is clear from Fig. 5a and b that the tungsten carbide thermal spray coating has a detrimental effect on the axial fatigue strength of the Ti–6Al–4V alloy. Several crack origin sites in Fig. 5a are visible. In Fig. 5b, fatigue cracks nucleation at coating, which not coalesced into the substrate direction are represented. Fatigue cracks nucleation and propagation from coating/substrate interface before penetrate the substrate was also observed. The specimen was axial fatigue tested at maximum stress 450 MPa and presented 24,500 cycles to failure. Ogawa and co-authors [26] observed that the coating fatigue resistance determines sprayed materials fatigue strength when the coating is harder than the substrate and presents high bonding strength. Cracks in the coating increase stress concentration and conduct the propagation process; as a consequence, the fatigue strength of the material is reduced. In this work, for maximum stress 965 MPa, cracks developed inside the coating increased the

local stress to a value higher than the substrate mechanical resistance. A thermal spray coated specimen was polished and observed according to Fig. 6, which represents regions a, b, c, and d, detailed in Fig. 7. Recent papers showed a dense morphology and lower porosity in HVOF coatings than other spray processes [12,27]. This morphology was confirmed in Fig. 7a that presents a uniform carbide distribution. The presence of porous reduces fatigue life [12,13,15]. Fig. 7b indicates that the aluminum oxide blasting is responsible for the increase of roughness increases at the substrate surface, improving coating/substrate adhesion. Fig. 7c and d show a crack in the coating with approximately 60 ␮m of extension. In the spraying procedure, cermet powder is semi-molted and accelerated towards the substrate, assuming a characteristic lamellar shape (splats). Cracks result from voids in the splats interface [14]. The microstructure of HVOF coatings is particularly multiphase with 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 is harder than the substrate, the fatigue crack propagates across the

Fig. 9. Fracture surface of Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated— max = 650 MPa: (a) 350×; (b) 2000×; (c) 2000×, backscattered electron image.


interface into base material. The WC–10%Co–4%Cr thermal spray coating hardness was 1900 HV0.3Kg.f and the substrate was 410 HV0.3Kg.f . A Ti–6Al–4V grid-blasted specimen with aluminum oxide is presented in Fig. 8a. Energy Dispersive Spectroscopy analysis (EDS), represented in Fig. 8b, showed an increase in base material aluminum weight percent when compared with the unblasted specimen: 9.84% Al, 4.16% V and 86.01% Ti (wt%). Fig. 8c shows a transversal image of the blasted Ti–6Al–4V alloy, in which a surface roughness increase was observed. The chemical composition in region A was: 6.29% Al, 4.95% V and 88.76% Ti (wt%), showed in Fig. 8d. From the literature it is well know that aluminum oxide particles in most cases are incorporated in the titanium and in this case acts as stress concentrator, reducing the fatigue strength [29,30]. Aluminum oxide particles partly embedded in Ti–6Al–4V alloy are showed in Fig. 9a and b obtained from secondary electron images. The bright area is associated to the aluminum oxide particles. The backscattered electron image confirms the presence of aluminum oxide particles by contrast based on the atomic number, as indicated in Fig. 9c. The distinct microscopic composition variations are Tungsten (74—atomic number), Titanium (22) and Aluminum (13), which are described from the brighter to the darkest, respectively. In Fig. 2, it is possible to observe a significant reduction in the fatigue strength of the Ti–6Al–4V alloy associated to WC–10%Co–4%Cr coating and a slightly better fatigue performance for specimens shot peened prior to the thermal spray coating process. For maximum stress equal to 450 MPa, the ratio between the average number of cycles to failure of shot peened and thermal spray coated and just WC–10%Co–4%Cr thermal spray coated spec-

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imens is 2.2. The same comparison for maximum stress 425 MPa indicates ratio equal to 2.3. From previous work [13], it is known that the HVOF thermal spray process produces compressive residual internal stresses within the substrate, due to the mechanical deformation on the surface during particle impact. Fatigue crack propagation is delayed when this compressive residual stress site is achieved. As mentioned earlier, despite of the compressive residual stress induced by the HVOF thermal spray process, the decrease in fatigue strength is associated to the pores intrinsic to the coating and particle oxide inclusions in the base material. Analysis of Fig. 10a, which represents a fracture surface from an axial fatigue specimen shot peened and WC–10%Co–4%Cr thermal spray coated 160 ␮m thick, tested at maximum stress 375 MPa with 1,150,000 cycles to failure. From Fig. 10b, one sees that the shot peening process increased the substrate strength and delayed fatigue crack propagation through base metal. In Fig. 10c, fatigue crack propagation inside base metal was deflected due to the effect of the compressive residual stress field induced by the shot peening process. This behavior explains the increase in the fatigue strength for Ti–6Al–4V alloy specimens after the shot peening process. Fig. 11 presents a fatigue fracture surface of a Ti–6Al–4V alloy specimen shot peened and WC–10%Co–4%Cr thermal spray coated, tested at 350 MPa. It is possible to observe cracks growth at and parallel to the coating/substrate interface, associated to the influence of compressive residual stresses, which avoid or even retard the crack propagation into the substrate. According to Nascimento et al. [13], when the coating is harder than base material, fatigue crack may propagate through interface inside base material. In this work, crack propagation at interface

Fig. 10. Fracture surface of Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr thermal spray coated— max = 375 MPa: (a) 200×; (b) 500×, region 1; (c) 500×, region 2.

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increase if crack nucleation is retarded or avoided and propagation is delayed or even arrested. Acknowledgements The authors are grateful for the research support by CAPES, by FAPESP 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. We would like to extend our thanks to Mrs. Maria Lucia Brison de Mattos (INPE) for the scanning electron microscope images. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 11. Fracture surface of Ti–6Al–4V alloy shot peened and WC–10%Co–4%Cr thermal spray coated— max = 350 MPa.

coating/substrate is related to the compressive residual stress induced by HVOF process [31,32]. The shot peening process was effective to increase the WC–10%Co–4%Cr thermal spray coated Ti–6Al–4V alloy high cycle fatigue strength in about twice. Despite this benefit, the original base material fatigue life was not restored. Nalla et al. [33] investigated the peening effect on Ti–6Al–4V, at 500 MPa maximum stress level, fatigue life increased from 104 to 106 cycles for the treated specimens. Studies from Gao [34] showed two shot peened titanium alloys, Ti–5Al–5Mo–5V–1Cr–1Fe and Ti–10V–2Fe–3Al, in which the fatigue limit increased 27% and 29% by shot peening, respectively. An increase in specimen roughness after shot peening treatment promotes a fatigue strength reduction as well as insufficient shot peening and over peening will not result in good fatigue performance [34]. In this work, the influence of the thermal spray coating was to decrease the base material fatigue strength. The fact that compressive residual stresses were not effective to increase low cycle fatigue strength is related to reduction or relief when the material is subjected to high stress levels [35].

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

[25] [26] [27]

4. Conclusions • The effect of WC–10%Co–4%Cr thermal spray coating applied by HVOF process was to decrease the fatigue strength of Ti–6Al–4V alloy. Several crack fronts all around the specimen were observed. • Aluminum oxides used to increase roughness and small cracks inside the coating were the main factors associated to the reduction in the fatigue strength. • The shot peening pre-treatment improved the fatigue strength of coated Ti–6Al–4V alloy, which means that the fatigue life may

[28] [29] [30] [31] [32] [33] [34] [35]

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Materials Science and Engineering A Effect of WCâ10

Materials Science and Engineering A Effect of WCâ10

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