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Fatigue strength of HVOF sprayed Cr3C2–25NiCr and WC-10Ni on AISI 4340 steel
Surface & Coatings Technology 203 (2008) 191–198
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Fatigue strength of HVOF sprayed Cr3C2–25NiCr and WC-10Ni on AISI 4340 steel R.C. Souza a, H.J.C. Voorwald b, M.O.H. Cioffi b,⁎ a b
EEL/DEMAR – Department of Materials Engineering, Polo Urbo-Industrial, Gleba AI-6, CP 116, CEP 12600-000 – Lorena/SP, Brazil Fatigue and Aeronautical Materials Research Group-Department of Materials and Technology, UNESP, Av. Ariberto Pereira da Cunha, 333 – CEP 12516 410, Guaratinguetá/SP, Brazil
a r t i c l e
i n f o
Article history: Received 3 January 2008 Accepted in revised form 26 July 2008 Available online 1 September 2008 Keywords: Thermal spray coatings Fatigue Wear Corrosion AISI 4340 steel
a b s t r a c t The fatigue strength of coated material is significantly influenced by internal residual stresses. Chromium coatings are used in applications to guarantee protection against wear and corrosion, combined with chemical resistance and good lubricity. The reduction in the fatigue strength of base material and since this technology presents detrimental environmental and health effects, resulted in the search on coatings viewed as being capable of replacing hard chrome plating. Thermally sprayed HVOF coatings are being considered to replace galvanic chromium deposits in industrial applications with, at least, comparable performance with respect to wear and corrosion resistance. The aim of the present study is to compare the influence of Cr3C2–25NiCr and WC–10Ni coatings applied by HVOF process and hard chromium electroplating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel. S–N curves were obtained in axial fatigue tests for base material, chromium plated and HVOF coated specimens. Experimental data showed higher axial fatigue resistance for HVOF coated specimens in comparison to electroplated chromium. The wear weight loss tests indicated better results for the HVOF thermal spray processing in comparison to the chromium electroplating. An increase in the corrosion resistance of steel protected with WC–10Ni HVOF coatings occurred with increased coating thickness. For Cr3C2–25NiCr HVOF coating, results indicate clearly the higher salt spray resistance. © 2008 Published by Elsevier B.V.
1. Introduction Fatigue failures are mostly the result of initiation and growth of cracks caused by the application of cyclic loading [1]. Because high strength materials are used in structures that should be able to carry a predetermined load, the critical crack size to fracture is normally very small. Hard chromium coating electroplated are used to guarantee combinations of adhesion, hardness, corrosion and wear resistance [2]. However chromium plating has negative environmental and health effects [3]. With respect to mechanical properties, the hard chromium electroplating decreases the fatigue resistance of a component, attributed to high tensile residual stresses and microcracks density contained in the coating [4]. Considering that the fracture process from fatigue usually arises at the surface, failure of components hard chromium electroplated results from the initiation and propagation of these microcracks [5]. As a consequence of the problems already mentioned, the aeronautical industries have been working in view of chrome coatings replacement to alternatives with comparable or superior properties, in accordance with environmental requirements.
⁎ Corresponding author. Tel.: +55 1231232865; fax: +55 1231232852. E-mail address: cioffi@feg.unesp.br (M.O.H. Cioffi). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.07.038
Thermal spray technologies, in particular high velocity oxygen-fuel (HVOF) thermal spraying, are types of coatings capable of replacing hard chrome plating [6]. In HVOF spraying, a fuel such as hydrogen, oxygen, ethylene or kerosene is burned with oxygen into a combustion chamber of a gun, in which the coating material in powder form is fed. Supersonic gases expel as a spray the heated powders which, in contact with the component substrate, produces coatings with low porosity and high adherence. The evaluation of WC-17Co and WC10Co–4Cr thermal spray coatings by HVOF on the fatigue strength of AISI 4340 steel indicated better performance in comparison to chromium electroplated [5]. With respect to abrasive wear resistance, better results of samples coated with WC and lower wear weight loss than hard chromium electroplated specimens was observed. Experimental results from tests under high temperature (≈310 °C) erosion indicated for HVOF thermal spray coated WC-12Co, Cr3C2–NiCr and WC-Cr3C2–Ni, better erosive wear resistance, respectively 18, 13 and 9 times in comparison to bare steel. The erosive material used was coal fly ash [7]. Tribological performance of Cr3C2–25%NiCr reactive plasma sprayed coatings indicate the influence of process parameters like pressure, spraying distance and substrate temperature during deposition on the sliding wear results [8]. Analysis of thickness effects on anticorrosive properties of thermal sprayed Cr3C2–25NiCr coatings indicate the importance of stress generation during process on behavior against corrosion [9].
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In salt spray tests results, no visual corrosion was observed on samples WC-10Co–4Cr coated. S–N curves for axial fatigue tests of specimens WC-12Co coated 100 μm thickness by the HVOF process showed a lower decrease in fatigue strength in comparison to hard chromium electroplated, 100 μm and 160 μm thicknesses [6]. The application of HVOF Cr3C2–25NiCr coatings resulted in superior mechanical and tribological properties in comparison to hard chromium electroplated [3]. The recognition for the high velocity oxygen-fuel process coating quality is therefore associated to the reason why HVOF has a great potential to replace chromium plating. The effects of Cr3C2–25NiCr and WC-10Ni coatings applied by HVOF and hard chromium electroplating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel is evaluated in the present paper. Scanning electron microscopy and optical metallograph were used to investigate the fatigue source appearance in the specimen groups, thickness and adhesion in all coatings. 2. Experimental procedures 2.1. Material and mechanical properties The coatings studied in this work were Cr3C2–25NiCr and WC10Ni, deposited on a steel substrate using a high-velocity oxy-fuel system (HVOF). The chemical composition of AISI 4340 steel was 0.41C–0.73Mn–0.8Cr–1.74Ni–0.25Mo and 0.25Si wt.%. The fatigue experimental program was performed on axial fatigue test specimens machined from hot-rolled, quenched and tempered bars, according to Fig. 1. Mechanical properties obtained by means of quenching from 815 °C to 845 °C in oil (20 °C) followed by tempering in the range 520 ± 5 °C for 2 h, are: 39 HRc–42 HRc, yield tensile strength of 1118 MPa and ultimate tensile strength 1210 MPa. After final preparation samples were subjected to a stress relieve heat treatment at 190 °C for 4 h to reduce residual stress induced by machining. Average superficial roughness in the reduced section of the specimens was Ra ≈ 2.75 µm and standard deviation of 0.89 μm.
2.3. Hard chromium electroplating Hard chromium electroplating was carried out from a chrome acid solution with 250 g/l of CrO3 and 2.5 g/l of H2SO4 at 50–55 °C, with a current density from 31 A/dm2 and a deposition rate of 25 μm/h. A bath with a single catalyst base on sulfate was used. After deposition samples were subjected to a hydrogen embrittlement relief treatment at 190 °C for 8 h. The average surface roughness of the hard chromium electroplating was Ra ≈ 3.13 µm in the reduced section and standard deviation of 0.79 µm in the plated condition. 2.4. HVOF thermal spray processing Coatings were deposited using a high velocity oxy-fuel torch, model JP-5000, HOBART-TAFA Technologies. The spraying parameters used for Cr3C2–25NiCr and WC-10Ni are: 1. Cr3C2–25NiCr Spray distance: Flows:
2. WC-10Ni Distance: Flows:
355 mm Kerosene: Oxygen: Nitrogen: Powder:
22.7 l/h; 52.4 m3/h; 9.9 l/min; 5.9 kg/h.
380 mm Kerosene: Oxygen: Nitrogen: Powder:
19.3 l/h; 53.8 m3/h; 10.8 l/min; 5.9 kg/h.
2.5. Abrasive wear test For abrasive wear tests, samples were prepared from annealed AISI 4340 steel 4 mm thick and 100 mm square, according to FED-STD141C. For both powders, specimens were coated with 200 μm thicknesses and tested with a Taber Abraser, at room temperature, using a 10-N load. Results were analyzed by wear index (mg/1000 cycles) and total wear (mg/10,000 cycles) data.
2.2. Axial fatigue tests 2.6. Salt spray test For axial fatigue tests, a sinusoidal load of 10 Hz frequency and load ratio of R = 0.1 was applied throughout this study. Experimental tests consider as fatigue strength the complete fracture of the specimen or 107 load cycles. Four groups of fatigue specimens were prepared to obtain S–N curves for axial fatigue tests: – 12 smooth specimens of base material; – 12 smooth specimens of base material hard chromium electroplated, 160 μm thick; – 13 smooth specimens of base material with Cr3C2–25NiCr thermal spray coated by HVOF process, 200 μm thick; – 13 smooth specimens of base material with WC-10Ni thermal spray coated by HVOF process, 200 μm thick. The thermal spray coated specimens were blasted with aluminum oxide mesh 90 to enhance adhesion.
The corrosion resistance of the coatings generated was evaluated in specific environment in accordance to ASTM B117, in 5 wt.%NaCl, pH of 6.5–7.2 at 35 °C. Samples were prepared from normalized AISI 4340 steel with 1 mm thickness, 76 mm width and 254 mm length, surface roughness Ra ≈ 0.2 μm and in the following conditions: – Cr3C2–25 Ni Cr-HVOF/TAFA – 150 μm and 200 μm; – WC-10Ni–HVOF/TAFA – 150 μm, 200 μm and 250 μm. Results were analyzed by Image Pro Plus software. 2.7. Residual stress measurement The X-ray diffraction method was used to determine the residual stress field induced by the thermal spray coatings. The equipment characteristics are [10]: Ψ goniometer geometer, Cr-kα radiation and registration of {221} diffraction lines. The accuracy of the stress measurement was Δσ = ± 20 MPa. In order to obtain the stress distribution by depth, the layers of specimens were removed by electrolytic polishing with a nonacid solution. 3. Results and discussion
Fig. 1. Axial fatigue testing specimen.
The coating hardness has been determined with a microhardness testing system using a Vickers diamond indenter on the top surface of polished cross sections and represented in Figs. 2 and 3 for Cr3C2–
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Fig. 4. Optical microscopy cross section. Cr3C2–25NiCr–100X. Fig. 2. Vickers microhardness. 100 g. Cr3C2–25NiCr.
25NiCr and WC-10Ni, respectively. To perform the indentation, a load of 100 g was used and maintained for 15 s. The through-thickness Vickers microhardness variation of chromium coating electroplated was: surface: 897; core: 906; interface: 912. Figs. 2 and 3 show, for Cr3C2–25NiCr and WC-10Ni, higher coatings microhardness than for chromium electroplated. For both coatings the same tendency was observed, with lower values near the coating surface, increasing slowly until a maximum close to the interface, decreasing again at interface coating-substrate. The increase in hardness near the interface coating-substrate may be associated to the fact that the thermal spray coated specimens were blasted to enhance adhesion, resulting in work-hardening effects [11]. The impact of coating droplets during sample preparation was also associated to the same mechanical behavior [12]. Also from Figs. 2 and 3, the thickness of the coatings was kept around 400 μm. The standardization coating thickness is approximately 300–500 μm for cermets coatings, instead of 2 mm for metallic alloys such as stainless steel [9]. Small cracks between different deposited layers were observed in specimens with the highest number of layers and higher thickness [9], which could be associated to residual stresses during spraying and coating solidification. Therefore thick coatings may result in coating depletion and/or crack propagation through the layer. Figs. 4 and 5 represent the Cr3C2–
25NiCr and WC-10Ni coated 4340 steel cross-section, respectively. It is possible to observe, in both cases, coatings homogeneity, increase in roughness at interface coating/substrate due to aluminum oxide blasting increasing adhesion and that the deposition process did not affect the microstructure. The average thickness for the Cr3C2–25NiCr coating was 465 µm and 490 µm for the WC-10Ni coating. Porosity in both cases was low, around 1–2%. Results of corrosion testing in accordance to ASTM B117 for AISI 4340 steel Cr3C2–25NiCr thermal spray coated with 150 µm and 200 µm thicknesses are represented in Figs. 6 and 7, in which no visual corrosion was observed. For AISI 4340 steel, WC-10Ni HVOF thermal spray coated, salt spray tests results for thickness 150 µm, and 200 µm and 250 µm are indicated in Figs. 8, 9 and 10, respectively. The surface aspect after salt spray testing indicated that the visual corrosion decreases with increase in the coating thickness. Experimental results may be explained by the lack of effectiveness of thinner coatings to correctly protect the base metal. The better corrosion resistance of AISI 4340 steel protected with WC-10Ni thermally sprayed coating with the increase in thickness is associated to the ability to close remaining porosity. On the other hand, it is important to consider that the residual stresses also influence the electrochemical behavior of the thermally sprayed HVOF coating. Corrosion products on the sample surface were also observed for WC-17% Co HP/HVOF I, 250 µm in thickness [5]. The salt spray test
Fig. 3. Vickers microhardness. 100 g. WC-10Ni.
Fig. 5. Optical microscopy cross section. WC-10Ni. 100X.
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results for specimens WC-10Co–4Cr coated by HP/HVOF I, for thickness equal to 150 µm and 200 µm indicated no visual corrosion [5]. Bodger and co-workers [13] observed no corrosion products in tungsten carbide samples of 200 µm thickness over 30 days in salt spray test. With respect to hard chromium plated specimens, 100% visually corrosion occurred with 49 µm thickness and subjected to 48 h in salt spray environment [6]. This behavior is related to the microcracks density in the deposit. The increase in thickness enhanced the electroplated chromium protection against corrosion [6]. The experimental results from chromium coatings applied on specimens of AISI 4340 steel, indicated higher microcracks density for the high velocity of deposition and fluoride-free chromium electroplating in comparison to the conventional one [6]. It was observed that the greater microcracks density resulted in the better protection against corrosion [14]. The corrosion resistance is also associated to the surface roughness, in a way in which the higher surface roughness, the higher the corrosion attack due to higher surface area [15]. The abrasive wear resistance of Cr3C2–25NiCr and WC-10Ni HVOF thermal spray coated was evaluated and the results in terms of wear weight loss are represented in Fig. 11. Comparing the abrasive wear resistance, one sees the better performance of both coatings with lower wear weight loss than the chromium electroplated specimen. The behavior may be associated to the higher hardness and oxide content into the coating. Coatings of high oxide content are usually harder and more wear resistant [16–18]. The lower hardness on the chromium electroplated surface and its increase through-thickness may explain the reduction in the wear weight loss after some number
Fig. 7. Salt spray test results for Cr3C2 –25NiCr. 200 µm thickness.
Fig. 6. Salt spray tests results for Cr3C2–25NiCr. 150 µm thickness.
of cycles. The mass loss results for Cr3C2–25NiCr and WC-10Ni coatings, may also be associated to the coating hardness. Theoretical calculations of the respectively wear depth caused by abrasive wheels after 10,000 cycles were 38.0 µm and 40.8 µm for the accelerated and conventional chromium electroplating, respectively and 9.50 µm for the tungsten carbide thermal spray coating [6]. The factors related to this tendency are: higher microcrack density and hardness in the accelerated chromium electroplating. Studies on coatings of WC-12Co and WC-10Ni produced on steel substrates using as thermal spraying the high velocity oxy-fuel (HVOF) and high power plasma spraying (HPPS), indicated that both techniques results in the formation of the W2C phase in the coating [19]. This phase plays an important role in wear applications, associated to the influence of hardness and brittleness on the wear resistance of the coating. For the HVOF spraying technique, the mass loss measured for rotors and stators is higher for the WC-10 Ni than for WC-12Co [19]. Sliding wear tests results of plasma sprayed coatings showed that pressure, spraying distance and substrate temperature during deposition are parameters of fundamental importance [8]. The wear behavior of HVOF thermal sprayed WC-10 Ni coatings was improved when solid lubricants were added to the WC-10Ni cermets powder [20]. The S–N curves for the axial fatigue tests for the base metal and coated specimens, are represented in Fig. 12. Comparison of curves shown in Fig. 12 for hardness 39 HRc–42 HRc, indicates the effect of chromium electroplated coating on the fatigue strength reduction of AISI 4340 steel, for thickness equal to 160 µm. The presence of tensile residual stresses which are relieved by chromium coating microcracking during electroplating and crack growth through interface inside substrate, are factors associated to the decrease in fatigue strength.
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The presence of plastic deformation, for high maximum stresses, reduces the influence of hard chromium electroplating on the number of cycles to failure, as observed in Fig. 12. The residual stresses profile for AISI4340 steel hard chromium electroplated 100 µm thickness, for hardness 39HRc–42HRc, is represented in Fig. 13. The through thickness residual stresses change from tensile near the coating surface (300 MPa) to compressive near the interface coating/substrate (−100 MPa). Inside base metal, stress changes from compressive to tensile at the same distance from the coating surface. For base metal, residual stress is tensile near surface (400 MPa) and decreases inside substrate thickness (0 MPa for 0.05 mm). Figs. 14 and 15 show the residual internal stresses profile for AISI4340 steel HVOF thermal spray coated with Cr3C2–25NiCr and WC-10Ni, respectively. According to Figs. 14 and 15, the through-thickness residual stresses change from tensile near the coating surface to compressive inside coating thickness, with maximum compressive stresses near to the interface coating/substrate, around −50 MPa for Cr3C2–25NiCr and WC-10Ni. Inside base metal, stresses change from compressive to tensile at 0.08 mm and 0.04 mm for the Cr3C2–25NiCr and WC-10 Ni thermal spray coatings, respectively. As indicated in Figs. 14 and 15, the HVOF thermal spray process produces compressive residual internal stresses within the substrate, formed from mechanical deformation on the surface during particle impact. The fast cooling and solidification as particles strike the surface are responsible for the tensile shrinkage stresses of the coating. The reduction in the axial fatigue strength of AISI 4340 steel WC17Co thermal spray coated by HVOF was associated to high density of
Fig. 9. Salt spray tests results for WC-10Ni. 200 µm thickness.
Fig. 8. Salt spray tests results for WC-10Ni. 150 µm thickness.
pores and oxide inclusions into the coating [5]. Tensile residual stresses at coating surface and compressive inside coating thickness to a maximum value near to the interface coating/substrate were obtained. The fatigue life of AISI 4340 steel was less reduced by WC-17Co and WC-10Co–4Cr HVOF thermal spray coated than by chromium electroplating [5]. The shot peening process was effective to restore the coated steel fatigue strength. An important role was played by the superposition of the compressive residual stresses caused by the HVOF thermal spray and shot peening processes. According to Fig. 12, AISI 4340 steel Cr3C2–25NiCr thermal spray coated showed for maximum applied stresses σmax = 1207 MPa (108%σys), σmáx = 1103 MPa (99%σys), and σmáx = 1000 MPa (89%σys) fatigue strength similar to base material. On the other hand, for high cycle fatigue with σmáx = 793 MPa (71%σys), reduction in fatigue life in comparison to bare material is observed. As a matter of fact the fatigue limit for coated material may be considered as σmáx = 690 MPa (62%σys). Thermally sprayed Cr3C2–25NiCr results in higher fatigue strength when compared to chromium electroplated. Analysis of the experimental data indicated in Fig. 12 revealed the insignificant reduction in the fatigue strength of AISI 4340 steel, associated to the WC-10Ni thermally sprayed coating. For high cycle fatigue, tests with maximum stress σmáx = 896 MPa (80%σys), were interrupted at 107 cycles. It is important to note that for AISI 4340 steel Cr3C2−25NiCr and WC-10Ni thermal sprayed coated, delamination occurred for low and high cycle fatigue. It is well known that fatigue crack growth through the interface between different solid materials is associated to the direction in which the crack approaches the interface and to the
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Fig. 12. S–N curves for axial fatigue tests.
Fig. 10. Salt spray tests results for WC-10Ni. 250 µm thickness.
strength of materials involved [21]. In case of ductile layer and brittle substrate, crack propagation may change direction at interface and cause delamination. If the layer is brittle and the substrate ductile, fatigue crack may propagate through interface inside base metal. In the case a ductile interlayer is present, fatigue crack nucleated at superficial layer will propagate through interface and intermediate layer without any difficulty. The moment the crack tip plastic zone reaches the interface between intermediate layer and substrate, the crack will be arrested, deflected and delamination may occur [22].
Fig. 11. Abrasive wear weight loss versus number of cycles.
Fig. 13. Residual stresses. 39 HRc–42 HRc. 100 µm [23].
The influence of residual stresses in the fatigue crack propagation process need to be considered and is not necessarily associated to the nature ductile/brittle of the materials involved. Analysis of Figs. 14 and 15 indicate low values for the compressive residual stresses at interface coating/substrate and inside base metal. The residual stresses profile change from compressive to tensile at
Fig. 14. Residual stresses. Cr3C2–25NiCr HVOF thermal spray coated.
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Fig. 15. Residual stresses. WC-10Ni HVOF thermal spray coated.
0.08 mm and 0.04 mm for Cr3C2–25NiCr and WC-10Ni thermal spray coatings, respectively. Higher compressive residual stresses were identified around interface coating substrate for WC-17Co and WC10Co–4Cr thermal spray coatings by HVFO [5]. The intense delamination which occurred at the interface between Cr3C2–25NiCr and WC-
Fig. 17. a. Fracture surface of axial fatigue specimen. WC-10Ni. b. Fracture surface of axial fatigue specimen. WC-10Ni.
10Ni HVOF thermal spray coated and substrate is clearly shown in Figs. 16a/b and 17a/b, respectively. The superficial cracking indicates the substrate strength to crack penetration. But it was not the case for both HVOF thermal spray coatings studied. Figs. 16b and 17b show fatigue cracks initiation and propagation at interface coating /substrate. Compressive residual stresses at interface coating/ substrate and inside base metal were not effective to avoid or delay fatigue crack nucleation and growth. Intense fatigue crack initiation and propagation all around the specimen section and inside base metal was responsible for the delamination observed. The residual stresses profile was not in order to explain differences in the fatigue behavior associated to both thermal spray coatings. The delamination observed is associated to a weak interface coating/substrate. 4. Conclusions
Fig. 16. a. Fracture surface of axial fatigue specimen. Cr3C2–25NiCr. b. Fracture surface of axial fatigue specimen. Cr3C2–25NiCr.
1. Experimental results indicated for Cr3C2–25NiCr and WC-10Ni HVOF thermal spray coated higher microhardness than for chromium electroplated. 2. For AISI 4340 steel protected with WC-10Ni HVOF thermal sprayed coated, the surface aspect after salt spray testing indicated that the visual corrosion decreases with increase in coating thickness (150 µm, 200 µm, 250 µm). 3. The better corrosion resistance of AISI 4340 steel Cr3C2–25NiCr HVOF thermal spray coated in comparison to WC-10Ni was observed from salt spray tests.
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4. HVOF thermal sprayed coatings (Cr3C2–25NiCr and WC-10Ni) presented better abrasive wear resistance with lower wear weight loss than chromium electroplated. An interesting performance of specimens coated with WC-10Ni was observed. 5. The effect of chromium electroplating was to decrease the axial fatigue strength of AISI 4340 steel. Thermally sprayed Cr3C2–25NiCr results in higher fatigue strength when compared to chromium electroplated. In comparison to bare AISI4340 steel, reduction in fatigue life, for high cycle fatigue, occurred. With respect to WC10Ni thermal spray coated, insignificant influence on the fatigue strength was detected. 6. Intense delamination occurred at interface Cr3C2–25NiCr, WC-10Ni HVOF thermal spray coated and substrate. Acknowledgements Authors are grateful to the support provided by FAPESP through the processes numbers 2006/03570-9 and 2006/02121-6 and to CNPq through the processes numbers 479534/2007-1, 479534/2007-1, 482816/2007-4 and 300233/2006-0. References [1] V.A. Pastoukhov, H.J.C. Voorwald, Introdução à Mecânica da Integridade Estrutural, Editora UNESP, São Paulo/ SP, junho, 1995. [2] H.J.C. Voorwald, W.L. Pigatin, Scientific American Brasil, Brasil, vol. 01, 2003, p. 60. [3] J.A. Picas, A. Forn, G. Matthäus, Wear 261 (2006) 477. [4] H.J.C. Voorwald, R. Padilha, M.Y.P. Costa, W.L. Pigatin, M.O.H. Cioffi, Int. J. Fatigue 29 (2007) 695. [5] H.J.C. Voorwald, R.C. Souza, W.L. Pigatin, M.O.H. Cioffi, Surf. Coat. Technol. 190 (2005) 155.
[6] M.P. Nascimento, R.C. Souza, I.M. Miguel, W.L. Pigatin, H.J.C. Voorwald, Surf. Coat. Technol. 138 (2001) 113. [7] J. Vicenzi, D.L. Villanova, M.D. Lime, A.S. Takimi, C.M. Marques, C.P. Bergmann, Mat. Design 27 (2006) 236. [8] M.H. Staia, T. Valente, C. Bartuli, D.B. Lewis, C.P. Constable, A. Roman, J. Lesage, D. Chicot, G. Mesmacque, Surf. Coat. Technol. 146–147 (2001) 563. [9] J.M. Guilemany, J. Fernandez, J. Delgado, A.V. Benedetti, F. Climent, Surf. Coat. Technol. 153 (2002) 107–113. [10] T. Gurova, J.R. Teodósio, J.M.A. Rebello, V. Monin, Scr. Mater. 36 (9) (1997) 1031. [11] T. Sundararajan, S. Kuroda, F. Abe, Mater. Trans. 45 (4) (2004) 1299. [12] B. Sidher, S. Prakash, Surf. Coat. Technol. 166 (2003) 89. [13] B.E. Bodger, R.T.R. Mcgram, D.A. Somerville, Plat. Surf. Finish. (1997) 28. [14] A.R. Jones, Plat. Surf. Finish. (1989) 62. [15] E. Budke, J. Krempel-Hesse, H. Maidhof, H. Schüssler, Surf. Coat. Technol. 112 (1999) 108. [16] M. Schroeder, R. Unger, Advanc. Mat. Proc. 8 (1997) 19. [17] H. Wang, W. Xia, Y. Jin, Wear 195 (1996) 47. [18] E. Budke, J.K. Pel-Hesse, H. Maiidhof, H. Schüssler, Surf. Coat. Technol. 112 (1999) 108. [19] J. Voyer, B.R. Marple, Wear 225–229 (1999) 135. [20] J. Voyer, B.R. Marple, Surf. Coat. Technol. 127 (2000) 155. [21] Y. Sugimura, S. Suresh, Ceramic Coatings, vol. 44, ASME, 1993, p. 9. [22] M.P. Nascimento, R.C. Souza, W.L. Pigatin, H.J.C. Voorwald, Int. J. Fatigue 23 (2001) 607. [23] M.A.S. Torres. Uma avaliação do efeito do shot peening na vida em fadiga do aço ABNT 4340 com e sem revestimento de cromo duro. Doctor theses. Guaratinguetá, UNESP, (2002) 223p.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Fatigue strength of HVOF sprayed Cr3C2–25NiCr and WC-10Ni on AISI 4340 steel R.C. Souza a, H.J.C. Voorwald b, M.O.H. Cioffi b,⁎ a b
EEL/DEMAR – Department of Materials Engineering, Polo Urbo-Industrial, Gleba AI-6, CP 116, CEP 12600-000 – Lorena/SP, Brazil Fatigue and Aeronautical Materials Research Group-Department of Materials and Technology, UNESP, Av. Ariberto Pereira da Cunha, 333 – CEP 12516 410, Guaratinguetá/SP, Brazil
a r t i c l e
i n f o
Article history: Received 3 January 2008 Accepted in revised form 26 July 2008 Available online 1 September 2008 Keywords: Thermal spray coatings Fatigue Wear Corrosion AISI 4340 steel
a b s t r a c t The fatigue strength of coated material is significantly influenced by internal residual stresses. Chromium coatings are used in applications to guarantee protection against wear and corrosion, combined with chemical resistance and good lubricity. The reduction in the fatigue strength of base material and since this technology presents detrimental environmental and health effects, resulted in the search on coatings viewed as being capable of replacing hard chrome plating. Thermally sprayed HVOF coatings are being considered to replace galvanic chromium deposits in industrial applications with, at least, comparable performance with respect to wear and corrosion resistance. The aim of the present study is to compare the influence of Cr3C2–25NiCr and WC–10Ni coatings applied by HVOF process and hard chromium electroplating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel. S–N curves were obtained in axial fatigue tests for base material, chromium plated and HVOF coated specimens. Experimental data showed higher axial fatigue resistance for HVOF coated specimens in comparison to electroplated chromium. The wear weight loss tests indicated better results for the HVOF thermal spray processing in comparison to the chromium electroplating. An increase in the corrosion resistance of steel protected with WC–10Ni HVOF coatings occurred with increased coating thickness. For Cr3C2–25NiCr HVOF coating, results indicate clearly the higher salt spray resistance. © 2008 Published by Elsevier B.V.
1. Introduction Fatigue failures are mostly the result of initiation and growth of cracks caused by the application of cyclic loading [1]. Because high strength materials are used in structures that should be able to carry a predetermined load, the critical crack size to fracture is normally very small. Hard chromium coating electroplated are used to guarantee combinations of adhesion, hardness, corrosion and wear resistance [2]. However chromium plating has negative environmental and health effects [3]. With respect to mechanical properties, the hard chromium electroplating decreases the fatigue resistance of a component, attributed to high tensile residual stresses and microcracks density contained in the coating [4]. Considering that the fracture process from fatigue usually arises at the surface, failure of components hard chromium electroplated results from the initiation and propagation of these microcracks [5]. As a consequence of the problems already mentioned, the aeronautical industries have been working in view of chrome coatings replacement to alternatives with comparable or superior properties, in accordance with environmental requirements.
⁎ Corresponding author. Tel.: +55 1231232865; fax: +55 1231232852. E-mail address: cioffi@feg.unesp.br (M.O.H. Cioffi). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.07.038
Thermal spray technologies, in particular high velocity oxygen-fuel (HVOF) thermal spraying, are types of coatings capable of replacing hard chrome plating [6]. In HVOF spraying, a fuel such as hydrogen, oxygen, ethylene or kerosene is burned with oxygen into a combustion chamber of a gun, in which the coating material in powder form is fed. Supersonic gases expel as a spray the heated powders which, in contact with the component substrate, produces coatings with low porosity and high adherence. The evaluation of WC-17Co and WC10Co–4Cr thermal spray coatings by HVOF on the fatigue strength of AISI 4340 steel indicated better performance in comparison to chromium electroplated [5]. With respect to abrasive wear resistance, better results of samples coated with WC and lower wear weight loss than hard chromium electroplated specimens was observed. Experimental results from tests under high temperature (≈310 °C) erosion indicated for HVOF thermal spray coated WC-12Co, Cr3C2–NiCr and WC-Cr3C2–Ni, better erosive wear resistance, respectively 18, 13 and 9 times in comparison to bare steel. The erosive material used was coal fly ash [7]. Tribological performance of Cr3C2–25%NiCr reactive plasma sprayed coatings indicate the influence of process parameters like pressure, spraying distance and substrate temperature during deposition on the sliding wear results [8]. Analysis of thickness effects on anticorrosive properties of thermal sprayed Cr3C2–25NiCr coatings indicate the importance of stress generation during process on behavior against corrosion [9].
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In salt spray tests results, no visual corrosion was observed on samples WC-10Co–4Cr coated. S–N curves for axial fatigue tests of specimens WC-12Co coated 100 μm thickness by the HVOF process showed a lower decrease in fatigue strength in comparison to hard chromium electroplated, 100 μm and 160 μm thicknesses [6]. The application of HVOF Cr3C2–25NiCr coatings resulted in superior mechanical and tribological properties in comparison to hard chromium electroplated [3]. The recognition for the high velocity oxygen-fuel process coating quality is therefore associated to the reason why HVOF has a great potential to replace chromium plating. The effects of Cr3C2–25NiCr and WC-10Ni coatings applied by HVOF and hard chromium electroplating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel is evaluated in the present paper. Scanning electron microscopy and optical metallograph were used to investigate the fatigue source appearance in the specimen groups, thickness and adhesion in all coatings. 2. Experimental procedures 2.1. Material and mechanical properties The coatings studied in this work were Cr3C2–25NiCr and WC10Ni, deposited on a steel substrate using a high-velocity oxy-fuel system (HVOF). The chemical composition of AISI 4340 steel was 0.41C–0.73Mn–0.8Cr–1.74Ni–0.25Mo and 0.25Si wt.%. The fatigue experimental program was performed on axial fatigue test specimens machined from hot-rolled, quenched and tempered bars, according to Fig. 1. Mechanical properties obtained by means of quenching from 815 °C to 845 °C in oil (20 °C) followed by tempering in the range 520 ± 5 °C for 2 h, are: 39 HRc–42 HRc, yield tensile strength of 1118 MPa and ultimate tensile strength 1210 MPa. After final preparation samples were subjected to a stress relieve heat treatment at 190 °C for 4 h to reduce residual stress induced by machining. Average superficial roughness in the reduced section of the specimens was Ra ≈ 2.75 µm and standard deviation of 0.89 μm.
2.3. Hard chromium electroplating Hard chromium electroplating was carried out from a chrome acid solution with 250 g/l of CrO3 and 2.5 g/l of H2SO4 at 50–55 °C, with a current density from 31 A/dm2 and a deposition rate of 25 μm/h. A bath with a single catalyst base on sulfate was used. After deposition samples were subjected to a hydrogen embrittlement relief treatment at 190 °C for 8 h. The average surface roughness of the hard chromium electroplating was Ra ≈ 3.13 µm in the reduced section and standard deviation of 0.79 µm in the plated condition. 2.4. HVOF thermal spray processing Coatings were deposited using a high velocity oxy-fuel torch, model JP-5000, HOBART-TAFA Technologies. The spraying parameters used for Cr3C2–25NiCr and WC-10Ni are: 1. Cr3C2–25NiCr Spray distance: Flows:
2. WC-10Ni Distance: Flows:
355 mm Kerosene: Oxygen: Nitrogen: Powder:
22.7 l/h; 52.4 m3/h; 9.9 l/min; 5.9 kg/h.
380 mm Kerosene: Oxygen: Nitrogen: Powder:
19.3 l/h; 53.8 m3/h; 10.8 l/min; 5.9 kg/h.
2.5. Abrasive wear test For abrasive wear tests, samples were prepared from annealed AISI 4340 steel 4 mm thick and 100 mm square, according to FED-STD141C. For both powders, specimens were coated with 200 μm thicknesses and tested with a Taber Abraser, at room temperature, using a 10-N load. Results were analyzed by wear index (mg/1000 cycles) and total wear (mg/10,000 cycles) data.
2.2. Axial fatigue tests 2.6. Salt spray test For axial fatigue tests, a sinusoidal load of 10 Hz frequency and load ratio of R = 0.1 was applied throughout this study. Experimental tests consider as fatigue strength the complete fracture of the specimen or 107 load cycles. Four groups of fatigue specimens were prepared to obtain S–N curves for axial fatigue tests: – 12 smooth specimens of base material; – 12 smooth specimens of base material hard chromium electroplated, 160 μm thick; – 13 smooth specimens of base material with Cr3C2–25NiCr thermal spray coated by HVOF process, 200 μm thick; – 13 smooth specimens of base material with WC-10Ni thermal spray coated by HVOF process, 200 μm thick. The thermal spray coated specimens were blasted with aluminum oxide mesh 90 to enhance adhesion.
The corrosion resistance of the coatings generated was evaluated in specific environment in accordance to ASTM B117, in 5 wt.%NaCl, pH of 6.5–7.2 at 35 °C. Samples were prepared from normalized AISI 4340 steel with 1 mm thickness, 76 mm width and 254 mm length, surface roughness Ra ≈ 0.2 μm and in the following conditions: – Cr3C2–25 Ni Cr-HVOF/TAFA – 150 μm and 200 μm; – WC-10Ni–HVOF/TAFA – 150 μm, 200 μm and 250 μm. Results were analyzed by Image Pro Plus software. 2.7. Residual stress measurement The X-ray diffraction method was used to determine the residual stress field induced by the thermal spray coatings. The equipment characteristics are [10]: Ψ goniometer geometer, Cr-kα radiation and registration of {221} diffraction lines. The accuracy of the stress measurement was Δσ = ± 20 MPa. In order to obtain the stress distribution by depth, the layers of specimens were removed by electrolytic polishing with a nonacid solution. 3. Results and discussion
Fig. 1. Axial fatigue testing specimen.
The coating hardness has been determined with a microhardness testing system using a Vickers diamond indenter on the top surface of polished cross sections and represented in Figs. 2 and 3 for Cr3C2–
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Fig. 4. Optical microscopy cross section. Cr3C2–25NiCr–100X. Fig. 2. Vickers microhardness. 100 g. Cr3C2–25NiCr.
25NiCr and WC-10Ni, respectively. To perform the indentation, a load of 100 g was used and maintained for 15 s. The through-thickness Vickers microhardness variation of chromium coating electroplated was: surface: 897; core: 906; interface: 912. Figs. 2 and 3 show, for Cr3C2–25NiCr and WC-10Ni, higher coatings microhardness than for chromium electroplated. For both coatings the same tendency was observed, with lower values near the coating surface, increasing slowly until a maximum close to the interface, decreasing again at interface coating-substrate. The increase in hardness near the interface coating-substrate may be associated to the fact that the thermal spray coated specimens were blasted to enhance adhesion, resulting in work-hardening effects [11]. The impact of coating droplets during sample preparation was also associated to the same mechanical behavior [12]. Also from Figs. 2 and 3, the thickness of the coatings was kept around 400 μm. The standardization coating thickness is approximately 300–500 μm for cermets coatings, instead of 2 mm for metallic alloys such as stainless steel [9]. Small cracks between different deposited layers were observed in specimens with the highest number of layers and higher thickness [9], which could be associated to residual stresses during spraying and coating solidification. Therefore thick coatings may result in coating depletion and/or crack propagation through the layer. Figs. 4 and 5 represent the Cr3C2–
25NiCr and WC-10Ni coated 4340 steel cross-section, respectively. It is possible to observe, in both cases, coatings homogeneity, increase in roughness at interface coating/substrate due to aluminum oxide blasting increasing adhesion and that the deposition process did not affect the microstructure. The average thickness for the Cr3C2–25NiCr coating was 465 µm and 490 µm for the WC-10Ni coating. Porosity in both cases was low, around 1–2%. Results of corrosion testing in accordance to ASTM B117 for AISI 4340 steel Cr3C2–25NiCr thermal spray coated with 150 µm and 200 µm thicknesses are represented in Figs. 6 and 7, in which no visual corrosion was observed. For AISI 4340 steel, WC-10Ni HVOF thermal spray coated, salt spray tests results for thickness 150 µm, and 200 µm and 250 µm are indicated in Figs. 8, 9 and 10, respectively. The surface aspect after salt spray testing indicated that the visual corrosion decreases with increase in the coating thickness. Experimental results may be explained by the lack of effectiveness of thinner coatings to correctly protect the base metal. The better corrosion resistance of AISI 4340 steel protected with WC-10Ni thermally sprayed coating with the increase in thickness is associated to the ability to close remaining porosity. On the other hand, it is important to consider that the residual stresses also influence the electrochemical behavior of the thermally sprayed HVOF coating. Corrosion products on the sample surface were also observed for WC-17% Co HP/HVOF I, 250 µm in thickness [5]. The salt spray test
Fig. 3. Vickers microhardness. 100 g. WC-10Ni.
Fig. 5. Optical microscopy cross section. WC-10Ni. 100X.
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results for specimens WC-10Co–4Cr coated by HP/HVOF I, for thickness equal to 150 µm and 200 µm indicated no visual corrosion [5]. Bodger and co-workers [13] observed no corrosion products in tungsten carbide samples of 200 µm thickness over 30 days in salt spray test. With respect to hard chromium plated specimens, 100% visually corrosion occurred with 49 µm thickness and subjected to 48 h in salt spray environment [6]. This behavior is related to the microcracks density in the deposit. The increase in thickness enhanced the electroplated chromium protection against corrosion [6]. The experimental results from chromium coatings applied on specimens of AISI 4340 steel, indicated higher microcracks density for the high velocity of deposition and fluoride-free chromium electroplating in comparison to the conventional one [6]. It was observed that the greater microcracks density resulted in the better protection against corrosion [14]. The corrosion resistance is also associated to the surface roughness, in a way in which the higher surface roughness, the higher the corrosion attack due to higher surface area [15]. The abrasive wear resistance of Cr3C2–25NiCr and WC-10Ni HVOF thermal spray coated was evaluated and the results in terms of wear weight loss are represented in Fig. 11. Comparing the abrasive wear resistance, one sees the better performance of both coatings with lower wear weight loss than the chromium electroplated specimen. The behavior may be associated to the higher hardness and oxide content into the coating. Coatings of high oxide content are usually harder and more wear resistant [16–18]. The lower hardness on the chromium electroplated surface and its increase through-thickness may explain the reduction in the wear weight loss after some number
Fig. 7. Salt spray test results for Cr3C2 –25NiCr. 200 µm thickness.
Fig. 6. Salt spray tests results for Cr3C2–25NiCr. 150 µm thickness.
of cycles. The mass loss results for Cr3C2–25NiCr and WC-10Ni coatings, may also be associated to the coating hardness. Theoretical calculations of the respectively wear depth caused by abrasive wheels after 10,000 cycles were 38.0 µm and 40.8 µm for the accelerated and conventional chromium electroplating, respectively and 9.50 µm for the tungsten carbide thermal spray coating [6]. The factors related to this tendency are: higher microcrack density and hardness in the accelerated chromium electroplating. Studies on coatings of WC-12Co and WC-10Ni produced on steel substrates using as thermal spraying the high velocity oxy-fuel (HVOF) and high power plasma spraying (HPPS), indicated that both techniques results in the formation of the W2C phase in the coating [19]. This phase plays an important role in wear applications, associated to the influence of hardness and brittleness on the wear resistance of the coating. For the HVOF spraying technique, the mass loss measured for rotors and stators is higher for the WC-10 Ni than for WC-12Co [19]. Sliding wear tests results of plasma sprayed coatings showed that pressure, spraying distance and substrate temperature during deposition are parameters of fundamental importance [8]. The wear behavior of HVOF thermal sprayed WC-10 Ni coatings was improved when solid lubricants were added to the WC-10Ni cermets powder [20]. The S–N curves for the axial fatigue tests for the base metal and coated specimens, are represented in Fig. 12. Comparison of curves shown in Fig. 12 for hardness 39 HRc–42 HRc, indicates the effect of chromium electroplated coating on the fatigue strength reduction of AISI 4340 steel, for thickness equal to 160 µm. The presence of tensile residual stresses which are relieved by chromium coating microcracking during electroplating and crack growth through interface inside substrate, are factors associated to the decrease in fatigue strength.
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The presence of plastic deformation, for high maximum stresses, reduces the influence of hard chromium electroplating on the number of cycles to failure, as observed in Fig. 12. The residual stresses profile for AISI4340 steel hard chromium electroplated 100 µm thickness, for hardness 39HRc–42HRc, is represented in Fig. 13. The through thickness residual stresses change from tensile near the coating surface (300 MPa) to compressive near the interface coating/substrate (−100 MPa). Inside base metal, stress changes from compressive to tensile at the same distance from the coating surface. For base metal, residual stress is tensile near surface (400 MPa) and decreases inside substrate thickness (0 MPa for 0.05 mm). Figs. 14 and 15 show the residual internal stresses profile for AISI4340 steel HVOF thermal spray coated with Cr3C2–25NiCr and WC-10Ni, respectively. According to Figs. 14 and 15, the through-thickness residual stresses change from tensile near the coating surface to compressive inside coating thickness, with maximum compressive stresses near to the interface coating/substrate, around −50 MPa for Cr3C2–25NiCr and WC-10Ni. Inside base metal, stresses change from compressive to tensile at 0.08 mm and 0.04 mm for the Cr3C2–25NiCr and WC-10 Ni thermal spray coatings, respectively. As indicated in Figs. 14 and 15, the HVOF thermal spray process produces compressive residual internal stresses within the substrate, formed from mechanical deformation on the surface during particle impact. The fast cooling and solidification as particles strike the surface are responsible for the tensile shrinkage stresses of the coating. The reduction in the axial fatigue strength of AISI 4340 steel WC17Co thermal spray coated by HVOF was associated to high density of
Fig. 9. Salt spray tests results for WC-10Ni. 200 µm thickness.
Fig. 8. Salt spray tests results for WC-10Ni. 150 µm thickness.
pores and oxide inclusions into the coating [5]. Tensile residual stresses at coating surface and compressive inside coating thickness to a maximum value near to the interface coating/substrate were obtained. The fatigue life of AISI 4340 steel was less reduced by WC-17Co and WC-10Co–4Cr HVOF thermal spray coated than by chromium electroplating [5]. The shot peening process was effective to restore the coated steel fatigue strength. An important role was played by the superposition of the compressive residual stresses caused by the HVOF thermal spray and shot peening processes. According to Fig. 12, AISI 4340 steel Cr3C2–25NiCr thermal spray coated showed for maximum applied stresses σmax = 1207 MPa (108%σys), σmáx = 1103 MPa (99%σys), and σmáx = 1000 MPa (89%σys) fatigue strength similar to base material. On the other hand, for high cycle fatigue with σmáx = 793 MPa (71%σys), reduction in fatigue life in comparison to bare material is observed. As a matter of fact the fatigue limit for coated material may be considered as σmáx = 690 MPa (62%σys). Thermally sprayed Cr3C2–25NiCr results in higher fatigue strength when compared to chromium electroplated. Analysis of the experimental data indicated in Fig. 12 revealed the insignificant reduction in the fatigue strength of AISI 4340 steel, associated to the WC-10Ni thermally sprayed coating. For high cycle fatigue, tests with maximum stress σmáx = 896 MPa (80%σys), were interrupted at 107 cycles. It is important to note that for AISI 4340 steel Cr3C2−25NiCr and WC-10Ni thermal sprayed coated, delamination occurred for low and high cycle fatigue. It is well known that fatigue crack growth through the interface between different solid materials is associated to the direction in which the crack approaches the interface and to the
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Fig. 12. S–N curves for axial fatigue tests.
Fig. 10. Salt spray tests results for WC-10Ni. 250 µm thickness.
strength of materials involved [21]. In case of ductile layer and brittle substrate, crack propagation may change direction at interface and cause delamination. If the layer is brittle and the substrate ductile, fatigue crack may propagate through interface inside base metal. In the case a ductile interlayer is present, fatigue crack nucleated at superficial layer will propagate through interface and intermediate layer without any difficulty. The moment the crack tip plastic zone reaches the interface between intermediate layer and substrate, the crack will be arrested, deflected and delamination may occur [22].
Fig. 11. Abrasive wear weight loss versus number of cycles.
Fig. 13. Residual stresses. 39 HRc–42 HRc. 100 µm [23].
The influence of residual stresses in the fatigue crack propagation process need to be considered and is not necessarily associated to the nature ductile/brittle of the materials involved. Analysis of Figs. 14 and 15 indicate low values for the compressive residual stresses at interface coating/substrate and inside base metal. The residual stresses profile change from compressive to tensile at
Fig. 14. Residual stresses. Cr3C2–25NiCr HVOF thermal spray coated.
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Fig. 15. Residual stresses. WC-10Ni HVOF thermal spray coated.
0.08 mm and 0.04 mm for Cr3C2–25NiCr and WC-10Ni thermal spray coatings, respectively. Higher compressive residual stresses were identified around interface coating substrate for WC-17Co and WC10Co–4Cr thermal spray coatings by HVFO [5]. The intense delamination which occurred at the interface between Cr3C2–25NiCr and WC-
Fig. 17. a. Fracture surface of axial fatigue specimen. WC-10Ni. b. Fracture surface of axial fatigue specimen. WC-10Ni.
10Ni HVOF thermal spray coated and substrate is clearly shown in Figs. 16a/b and 17a/b, respectively. The superficial cracking indicates the substrate strength to crack penetration. But it was not the case for both HVOF thermal spray coatings studied. Figs. 16b and 17b show fatigue cracks initiation and propagation at interface coating /substrate. Compressive residual stresses at interface coating/ substrate and inside base metal were not effective to avoid or delay fatigue crack nucleation and growth. Intense fatigue crack initiation and propagation all around the specimen section and inside base metal was responsible for the delamination observed. The residual stresses profile was not in order to explain differences in the fatigue behavior associated to both thermal spray coatings. The delamination observed is associated to a weak interface coating/substrate. 4. Conclusions
Fig. 16. a. Fracture surface of axial fatigue specimen. Cr3C2–25NiCr. b. Fracture surface of axial fatigue specimen. Cr3C2–25NiCr.
1. Experimental results indicated for Cr3C2–25NiCr and WC-10Ni HVOF thermal spray coated higher microhardness than for chromium electroplated. 2. For AISI 4340 steel protected with WC-10Ni HVOF thermal sprayed coated, the surface aspect after salt spray testing indicated that the visual corrosion decreases with increase in coating thickness (150 µm, 200 µm, 250 µm). 3. The better corrosion resistance of AISI 4340 steel Cr3C2–25NiCr HVOF thermal spray coated in comparison to WC-10Ni was observed from salt spray tests.
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4. HVOF thermal sprayed coatings (Cr3C2–25NiCr and WC-10Ni) presented better abrasive wear resistance with lower wear weight loss than chromium electroplated. An interesting performance of specimens coated with WC-10Ni was observed. 5. The effect of chromium electroplating was to decrease the axial fatigue strength of AISI 4340 steel. Thermally sprayed Cr3C2–25NiCr results in higher fatigue strength when compared to chromium electroplated. In comparison to bare AISI4340 steel, reduction in fatigue life, for high cycle fatigue, occurred. With respect to WC10Ni thermal spray coated, insignificant influence on the fatigue strength was detected. 6. Intense delamination occurred at interface Cr3C2–25NiCr, WC-10Ni HVOF thermal spray coated and substrate. Acknowledgements Authors are grateful to the support provided by FAPESP through the processes numbers 2006/03570-9 and 2006/02121-6 and to CNPq through the processes numbers 479534/2007-1, 479534/2007-1, 482816/2007-4 and 300233/2006-0. References [1] V.A. Pastoukhov, H.J.C. Voorwald, Introdução à Mecânica da Integridade Estrutural, Editora UNESP, São Paulo/ SP, junho, 1995. [2] H.J.C. Voorwald, W.L. Pigatin, Scientific American Brasil, Brasil, vol. 01, 2003, p. 60. [3] J.A. Picas, A. Forn, G. Matthäus, Wear 261 (2006) 477. [4] H.J.C. Voorwald, R. Padilha, M.Y.P. Costa, W.L. Pigatin, M.O.H. Cioffi, Int. J. Fatigue 29 (2007) 695. [5] H.J.C. Voorwald, R.C. Souza, W.L. Pigatin, M.O.H. Cioffi, Surf. Coat. Technol. 190 (2005) 155.
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