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HVOF and hard chromium electroplating on AISI 4340 high strength steel
Surface and Coatings Technology 138 Ž2001. 113᎐124
Effects of tungsten carbide thermal spray coating by HPrHVOF and hard chromium electroplating on AISI 4340 high strength steel Marcelino P. Nascimento a,U , Renato C. Souzab, Ivancy M. Miguel a , Walter L. Pigatin c , Herman J.C. Voorwalda a
State Uni¨ ersity of Sao ˜ Paulo- DMT-UNESPr FEG, A¨ . Ariberto Pereira da Cunha, 333-Guaratinguetar ´ SP r BR-CEP: 12500-000, Brazil b FAENQUILr DEMAR, Lorenar SP r BR-CEP:12600-000, Brazil c EMBRAER-LIEBHERRr EDE, Sao ˜ Jose´ dos Camposr SP r BR-CEP: 12237-540, Brazil Received 26 August 1999; received in revised form 5 October 2000; accepted 7 November 2000
Abstract In cases of decorative and functional applications, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. However, pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics has increased in recent years, related to the reduction in the fatigue strength of the base material and to environmental requirements. The high efficiency and fluoride-free hard chromium electroplating is an improvement to the conventional process, considering chemical and physical final properties. One of the most interesting, environmentally safer and cleaner alternatives for the replacement of hard chrome plating is tungsten carbide thermal spray coating, applied by the high velocity oxy-fuel ŽHVOF. process. The aim of this study was to analyse the effects of the tungsten carbide thermal spray coating applied by the HPrHVOF process and of the high efficiency and fluoride-free hard chromium electroplating Žin the present paper called ‘accelerated’., in comparison to the conventional hard chromium electroplating on the AISI 4340 high strength steel behaviour in fatigue, corrosion, and abrasive wear tests. The results showed that the coatings were damaging to the AISI 4340 steel behaviour when submitted to fatigue testing, with the tungsten carbide thermal spray coatings showing the better performance. Experimental data from abrasive wear tests were conclusive, indicating better results from the WC coating. Regarding corrosion by salt spray test, both coatings were completely corroded after 72 h exposure. Scanning electron microscopy technique ŽSEM. and optical microscopy were used to observe crack origin sites, thickness and adhesion in all the coatings and microcrack density in hard chromium electroplatings, to aid in the results analysis. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten carbide thermal spray coating; Hard chromium electroplating; Abrasive wear; Corrosion; Fatigue; HPrHVOF
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Corresponding author. Tel.: q55-12-525-2800; fax: q55-12-515-2466. E-mail address: [email protected] ŽM.P. Nascimento..
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 4 8 - 8
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1. Introduction Chromium plating is the most used electrodeposited coating to obtain high levels of hardness, resistance to wear and corrosion and a low coefficient of friction for applications in the aerospace, automotive and petrochemical fields w1,2x. Chromium plating properties, such as hardness and microcrack density, change with the bath composition, current density, bath agitation, temperature, etc. w3,4x. Among other things, a significant characteristic of chromium electroplating is the high tensile residual internal stresses originating from the decomposition of chromium hydrides during the electrodeposition process w3᎐5x. These high tensile stresses in electroplated chromium coatings increase as thickness increases and are relieved by local microcracking during electroplating. Therefore, basically, microcrack density is related to the high tensile residual internal stresses, hardness, and corrosion resistance w2,3,6x. It was observed that the residual stresses through-thickness decrease with the depth of the coating and increase again at the coating᎐substrate interface w7x. Bending fatigue tests on samples with different coatings and coating conditions indicate that the fatigue strength is dependent on the fracture behaviour of the substrates and on the hardness and residual stresses at the substrate surface. It was also observed that the hard chromium electroplating reduces the fatigue strength of a component w8x. Due to this fact, the design of hard chromium plated components, which are subjected to dynamic loads, may consider this negative influence to guarantee safety during operation. Therefore, the use of effective methods to improve the fatigue strength shall be considered. Shot peening is a well-known process to increase fatigue life of structures subjected to constant and variable amplitude loading. The compressive residual stress obtained by surface plastic deformation is responsible for the increase in fatigue strength in shot peened mechanical components w9x. Compressive residual stresses induced by machining processes are also responsible for the improvement in fatigue resistance of AISI 4340 steel w10x. Increase in the fatigue crack propagation resistance in AISI 4340 steel with electron beam surface hardening was associated to residual stress distribution and microstructural characteristics w11x. However, problems concerning chrome plating, such as health and environmental hazards, increasing costs and a performance not in accordance with the specifications, have resulted in a search to identify possible alternatives w12x. Aircraft landing gear manufacturers are considering tungsten carbide ŽWC. thermal spray coatings applied by the high velocity oxy-fuel ŽHVOF. process as an alternative to hard chrome plating. The question to be answered is if the performance of the alternative candidate is at
least comparable to results obtained for hard chrome plating. Comparisons of experimental data showed better corrosion resistance for several HVOF coatings with respect to chrome plating. In the case of fatigue and friction tests, the results were acceptable, indicating interesting perspectives on the use of tungsten carbide coating to replace chrome-plating w1x. Analysis of the wear performance of tungsten carbide coated samples in the presence of air, aqueous and aqueous abrasive media indicated better results in terms of volume loss and change in surface roughness than for the mild steel substrate w13x. The objective of this research is to compare the influence of the tungsten carbide thermal spray coating applied by HPrHVOF and hard-chromium plating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel. S᎐N curves were obtained in rotating bending and axial fatigue tests for the base material, chromium plated, and tungsten carbide coated specimens.
2. Experimental procedures AISI 4340 steel is widely used in aircraft components where strength and toughness are fundamental design requirements. The chemical analysis of the material used in this research indicates accordance with specifications. The fatigue experimental program was performed on rotating bending and axial fatigue test specimens machined from hot rolled, quenched and tempered bars according to Figs. 1 and 2, respectively. The specimens were polished in the reduced section with 600 grit papers, inspected dimensionally and by magnetic particle inspection. Fatigue tests specimens were quenched from 815᎐845⬚C in oil Ž20⬚C. and tempered in the range of 520 " 5⬚C for 2 h. The mechanical properties of the material after the heat treatment are: hardness of 39 HRC; yield tensile strength of 1118 MPa, and ultimate tensile strength of 1210 MPa. After final preparation, samples were subjected to a stress relieve heat treatment at 190⬚C for 4 h to reduce residual stresses induced by machining. Average superficial roughness in the reduced section of the samples was R a f 2.75 m and a standard deviation ŽS.D.. of 0.89 m. Rotating bending fatigue tests were conducted using a sinusoidal load of frequency 50 Hz, and load ratio R s y1, at room temperature. For axial fatigue tests, a sinusoidal load of frequency 50 Hz and load ratio R s 0.1 was applied throughout this study. Both tests consider as fatigue strength the complete fracture of the specimens or 10 7 load cycles. Four groups of fatigue specimens were prepared to obtain S᎐N curves for rotating bending fatigue and axial fatigue tests.
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124
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Fig. 1. Rotating bending fatigue testing specimen.
2.1. For rotating bending fatigue tests
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12 smooth samples of base material; 13 samples with 160 m thickness of conventional hard chromium electroplated; 13 samples with 100 m thickness of accelerated hard chromium electroplated; and 13 samples with 100 m thickness of tungsten carbide thermal spray coated by HPrHVOF process.
2.2. For axial fatigue tests
rotating bending fatigue specimens were tested without blasting. 2.3. Salt spray test The performance of the coatings was evaluated with respect to chemical corrosion in specific environment. The samples were prepared from normalised AISI 4340 steel with 1 mm thickness, and 76 mm width and 254 mm length, surface roughness R a f 0.2 m, and in the following conditions: 䢇
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10 smooth samples of base material; 15 samples with 160 m thickness of conventional hard chromium electroplated; 7 samples with 100 m thickness of accelerated hard chromium electroplated; and 13 samples with 100 m thickness of tungsten carbide thermal spray coated by HPrHVOF process.
The tungsten carbide thermal spray coated specimens were blasted with aluminium oxide mesh 90 to enhance adhesion. To compare experimental data, six
Fig. 2. Axial fatigue testing specimen.
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accelerated hard chromium electroplated ª 16, 36 and 49 m thickness; conventional hard chromium electroplated ª 16, 36 and 49 m thickness; and tungsten carbide thermal spray coated ª 100 m thickness.
Experimental tests were conducted in accordance with ASTM B 117, in 5 wt.% NaCl, pH of 6.5᎐7.2, at 35⬚C. The samples were supported at 20⬚ from the vertical. The results were analysed by Image Pro Plus software. 2.4. Abrasi¨ e wear test The performance of the coating was also evaluated with respect to abrasive wear. For abrasive wear tests, samples were prepared from annealed AISI 4340 steel with 4 mm thickness and 100 mm square, according to FED-STD-141C. The samples were divided in three groups; two coated with 100-m thickness of accelerated and conventional hard chromium electroplating, respectively, and one group coated with 100-m thickness of tungsten carbide coating. The wear tests were conducted with a Taber abraser, at room temperature, using a 10-N load and CS-17 abrading wheel for hard chromium electroplating and diamond wheel for tungsten carbide coating. The results were analysed by wear index Žmgr1000 cycles. and total wear Žmgr10 000 cycles. data.
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2.5. Tungsten carbide coating The tungsten carbide thermal spray coating applied by HPrHVOF system, used WC powder with 12% Co, resulting in thickness equal to 100 m. The average superficial roughness in the reduced section of the samples was R a f 4 m and a S.D. of 0.39 m, in the as-deposited condition. 2.6. Hard chromium electroplating The conventional hard chromium electroplating was carried out from a chromic acid solution with 250 grl of CrO 3 and 2.5 grl of H 2 SO4 , at 50᎐55⬚C, with a current density from 31 to 46 Ardm2 , and a speed of deposition equal to 25 mrh. A bath with a single catalyst based on sulfate was used. The accelerated hard chromium electroplating was carried out from a chromic acid solution with 250 grl of CrO 3 and 2.7 grl of H 2 SO4 , at 55᎐60⬚C, with a current density from 55 to 65 Ardm2 , and a speed of deposition equal to 80 mrh. A bath with a double catalyst, one based on sulfate and the other without fluoride, was used. After the coating deposition, the 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 R a f 3.13 m in the reduced section and a S.D. of 0.79 m in the as-electroplated condition. For the microcrack determination in both hard chromium electroplating, samples were prepared from normalised AISI 4340 steel Ž R a f 0.2 m., 1 mm thickness, 25 mm width and length, and with accelerated and conventional hard chromium electroplating both
with 100 m thickness, which resulted in a surface roughness of R a f 0.74 m for the former and R a f 1.6 m for the later, in the as-electroplated condition. The surface microcracks were enhanced through anodic etching for 30 s with a current density equal to 25 Ardm2 in the same chromium bath and later analysed using an optical microscope model NikonrApophot. All surface roughness data measured in this research was obtained by Mitutoyo 301 equipment using a cut-off of 0.8 mm. The analysis of fracture surface was carried out on rotating bending fatigue specimens by scanning electron microscope, model LEO 435 vpi and Zeiss DSM 950. The metallographic analysis was carried out on optical microscope model Neophot 21.
3. Results and discussion 3.1. Fatigue test The S᎐N curves for the rotating bending and axial fatigue tests for the base metal and coated specimens are presented in Figs. 3 and 4, respectively. Fig. 3 shows that the effect of coating in the rotating bending fatigue test is to decrease the fatigue strength of AISI 4340 steel. The tendency is observed for low number of cycles Ž10 4 ., high number of cycles Ž10 5 . and for the fatigue limit, 10 7 cycles, and is represented in Table 1. One sees that the specimens coated with tungsten carbide applied by the HVOF process show a lower decrease in fatigue strength. This may be attributed to the process itself. It is well known that HVOF thermal spray process produces compressive
Fig. 3. S᎐N curves for rotating bending fatigue tests.
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Fig. 4. S᎐N curves for axial fatigue tests.
residual internal stresses within the substrate, which are formed from mechanical deformation on the surface during particle impact. This is confirmed by the through-thickness residual stress behaviour shown in Fig. 5. These surface deformations counteract the ten-
sile shrinkage stresses of the coating caused by fast cooling and solidification as particles strike the surface. These tensile stresses in the coating also generate compressive stresses within the surface of the substrate. However, there was a reduction in the fatigue
Table 1 Rotating bending fatigue strength. Rotating bending fatigue strength Group
Low cycles Ž104 .
High cycles Ž105 .
limit Ž107 .
Base material Treat. T. carb Ž100 m. Tungsten carb. Ž100 m. Conv. chrome Ž160 m. Accel. chrome Ž100 m.
s 950 MPa Ž85%ys . s 900 MPa Ž80%ys . ( 900 MPa Ž80%ys . ( 840 MPa Ž75%ys . ( 730 MPa Ž65%ys .
( 700 MPa Ž63%ys . ( 610 MPa Ž54%ys . ( 570 MPa Ž51%ys . ( 500 MPa Ž45%ys . ( 340 MPa Ž30%ys .
( 615 MPa Ž59%ys . ( 531 MPa Ž47.5%ys . ( 531 MPa Ž47.5%ys . ( 321 MPa Ž29%ys . ( 280 MPa Ž25%ys .
Fig. 5. Through-thickness residual stress distribution for WC HPrHVOF thermal spray coating.
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Fig. 6. Microcracks surface network in conventional Ža. and accelerated Žb. hard chromium electroplating. Anodic etching from 25 Ardm2 for 30 s Ž200 = ..
strength of AISI 4340, despite the compressive residual stresses induced by the process. This can be due to the high density of pores and oxide inclusions in the coating that commonly form during the process. Thermal spray is generally conducted in air so chemical interactions occur, notably oxidation, which can be evident in the coating microstructure as oxide inclusions, mainly in grain boundaries w14x. These inclusions in coating subsurfaces are possible cracks or nucleationrinitiation sites. From Fig. 3, it is possible to observe that the aluminium oxide blasting is responsible for a small improvement in the fatigue strength. Considering both hard chromium electroplated rotating bending fatigue results, one sees the negative influence of coating on the fatigue strength of the steel. From the analysis of these two coatings, it is possible to observe the better performance of the conventional hard chromium in relation to the accelerated hard chromium electroplating, despite the higher thickness of the former. This may be attributed to the lower microcrack density of the conventional hard chromium electroplating in comparison with the accelerated hard chromium electroplating as showed in Fig. 6. The microcracks density quantitative analysis indicated median values of 1512
microcracksrcm and a S.D. of 190.6 microcracksrcm for the accelerated hard chromium electroplating, and 223 microcracksrcm with standard deviation of 57.5 microcracksrcm for the conventional hard chromium electroplating. Microcracks form when the high tensile residual internal stresses exceed the cohesive strength of the chromium deposits and affect the fatigue behaviour of a plated part. Therefore, microcrack density arises as a relief of the tensile residual internal stresses, which increase when the chromium thickness increases. Pina et al. w7x showed that the microcrack density changes along the thickness, being higher at the core and lower at the surface of the coating and in the substratercoating interface due to the balance between the residual stresses. On the surface of the coating, the microcracks arise in a network shape, without preferential direction and characterising an equi-biaxial residual stress state. With respect to residual stresses, an inverse behaviour from that observed for the microcrack occurred. Therefore, in general, the higher the microcrack density, the higher the tensile residual internal stresses andror their relief. This means that the accelerated hard chromium electroplating is responsible for higher tensile residual internal stresses andror present the highest crack initiationrpropagation front amount. However, the different microcrack densities between both hard chromium electroplatings practically produced the same effect in low cycle fatigue, since crack growth occurs after few cycles of fatigue testing. In general, the fatigue strength for all conditions studied was reduced due to the high tensile residual internal stresses, microcrack density, and high adhesion at the coatingrsubstrate interface, which allows the crack to grow from the coating through the interface into the base metal. Fig. 5 shows the residual internal stress profile for tungsten carbide thermal spray coating. From experimental points, one sees that the residual internal stresses change throughout coating thickness, from 100 MPa tensile stress near to the surface, reaching 350 MPa maximum tensile stress at a 0.025-mm depth and decreasing to the maximum compressive stress of 680 MPa at a 0.07-mm depth, increasing again into the base metal, becoming tensile stress at a 0.20-mm depth. The curve of through-thickness residual stress was plotted based on three specimens and obtained by the modified layer-removal method for thermal spray coating and substrates w18,19x. The through-thickness residual stresses change from approximately 300 MPa tensile at 0.025-mm depth to approximately 680 MPa, compressive at 0.06 mm from the surface. This means that the crack initiation may occur easily on the coating surface, but its propagation throughout the thickness may be delayed when the compressive residual stress site is reached.
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Table 2 Axial fatigue strength Axial fatigue strength Group
Low cycles Ž104 .a
High cycles Ž105 .
Limit Ž107 .
Base material Tungsten carb. Ž100 m. Conv. chrome Ž160 m. Accel. chrome Ž100 m.
( 1330 MPa Ž119%ys . ( 1350 MPa Ž121%ys . ( 1200 MPa Ž107%ys . ( 1150 MPa Ž103%ys .
( 1125 MPa Ž101%ys . ( 850 MPa Ž76%ys . ( 650 MPa Ž58%ys . ( 650 MPa Ž58%ys .
( 850 MPa Ž76%ys . ( 750 MPa Ž67%ys . ( 400 MPa Ž36%ys . ( 400 MPa Ž36%ys .
a
ª Projection of the curves until the respective number of cycles Ž10 4 ..
For hard chromium electroplating residual stresses, Pina et al. w7x showed that, despite the fact that microcracks result in residual stress relief in the coating, the stresses still remain high at the surface Žapprox. 800 MPa., decreasing in direction to the core Žapprox. 200 to 300 MPa., and increasing again at the interface to values which depend on the substrate material. Fig. 4 shows the axial fatigue testing results, indicating the decrease in fatigue strength for all specimens coated with tungsten carbide thermal spray and hard chromium electroplating, in comparison to the base material. Comparing the curves, one sees the negative influence of coatings on the fatigue strength of the steel, with the same tendency observed previously in the rotating bending fatigue tests. This behaviour can also be explained by high tensile residual internal stresses, oxide inclusions, pores and microcracks inherent from each process. Table 2 indicates the axial fatigue strength tendency of all specimens groups, based on Fig. 4. The better performance of tungsten carbide coated specimens in comparison to the hard chromium plating may also be attributed to the lower tensile residual internal stresses in the coating of the former, compressive residual stresses on the substrate surface and in the subsurface due to the particles impact effect, as well as by interactions between surfacersubstrate residual stresses. Note in the axial fatigue test that the different microcrack density between both hard chromium electroplatings did not play an important role in the specimens performance, as occurred in
rotating bending fatigue tests. A comparison of Tables 1 and 2, as shown in Table 3, indicates the higher fatigue strength shown in axial fatigue test in relation to the rotating bending fatigue tests. In addition to the lower specimen dimensions, this is in accordance with the fact that the rotating bending fatigue tests are more severe as a result of the effect of the bending moment which increases the tensile stresses on surface from where, in general, the fatigue cracks grow. However, Table 3 shows a higher decrease in the fatigue strength as a function of the number of cycles in comparison to the rotating bending fatigue test results, for each level of stress. This may also be due to radial throughout thickness crack propagation to the base metal, in a direction normal to the maximum tensile stress and resulting in lower fatigue life of the specimen w15x. Fig. 7 shows a typical fracture surface from the base metal, indicating that the fatigue crack nucleation started at the surface. In Fig. 8, several crack fronts that may be associated to the microcracks density originated from the plating process are represented. From Fig. 9, which represents fracture surface from a rotating bending fatigue specimen electroplated with accelerated hard chromium and tested at 871 MPa, one sees the coating homogeneity, strong adhesion substratercoating, and microcracks distributed along thickness in a radial shape. Fig. 10 shows a micrograph of a tungsten carbide thermal spray coated specimen blasted with aluminium
Table 3 Fatigue strength in number of cycles in the rotating bending and axial fatigue tests Fatigue testing Stress
Rotating. bending fatigue data Ž%.a
Axial fatigue data Ž%.a
ŽMPa.
Base mat. Cycles
W.C. Cycles
C.H.C. Cycles
A.H.C. Cycles
Base mat. Cycles
W.C. Cycles
C.H.C. Cycles
A.H.C. Cycles
850 750 650
22 000 60 000 299 000
12 000 Ž55. 24 000 Ž40. 56 000 Ž19.
9500 Ž43. 18 000 Ž36. 36 500 Ž12.
6000 Ž27. 9000 Ž18. 14 200 Ž5.
107 107 107
105 Ž1. 106 Ž10. 107 Ž100.
30 000Ž0.30. 42 000Ž0.42. 60 000Ž0.60.
30 000Ž0.30. 42 000Ž0.42. 60 000Ž0.60.
a ª Values contained in parenthesis are the rate between the number of cycles of the coated material and the number of cycles of base metal, in percentage.
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Fig. 7. Typical fracture surface from base metal. Rotating bending fatigue test.
oxide. To compare, Fig. 11 indicates a coating profile without the blasting treatment. It is possible to observe that the aluminium oxide blasting is responsible for the increase in roughness at the substratercoating interface, with a consequent improvement in adhesion. The important characteristics of the hard chromium electroplating are the homogeneity of the coatings and
Fig. 8. Fracture surface from 100 m of accelerated hard chromium electroplating.
the excellent adhesion with the base metal, represented in Fig. 12. From Figs. 10᎐12, it is also possible to observe that, in both cases, the base metal microstructure was not affected by the deposition process.
Fig. 9. Fracture surface of samples accelerated hard chromium electroplated and tested at 871 MPa Ž500 = ..
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124
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Fig. 11. Tungsten carbide coating specimen Ž100 = .. Fig. 10. Tungsten carbide coating aluminium blasted specimen Ž200 = ..
3.2. Abrasi¨ e wears tests The abrasive wear resistance of HPrHVOF WC coating and hard chromium plating was evaluated, and the results in terms of wear weight loss are represented in Table 4 and Fig. 13. Comparing the abrasive wear resistance, one sees the better performance of samples coated with WC, with lower wear weight loss than the hard chromium electroplated specimens. This may be attributed to the higher hardness and oxide content into the tungsten carbide coating. Coatings of high oxide content are usually harder and more wear resistant w14x. However, initially, in the first 1000 cycles, an almost equivalent value of wear weight loss was observed for the tungsten carbide coating and conventional hard chromium electroplating, and a higher wear weight loss for the accelerated hard chromium. This higher hardness of tungsten carbide coating, which should result in lower wear weight loss in comparison with the conventional hard chromium electroplating, was damaged due to its higher surface roughness. With respect to both hard chromium electroplating, in the subsequent cycles, the wear weight loss of the accelerated hard chromium electroplating decreases with the increase in number of cycles in a parabolic way, resulting after 10 000 cycles in lower wear weight loss than the conventional hard chromium plating. This may be explained by the through-thickness hardness variation, in accordance with Table 5, in which the microhardness data of tungsten carbide coating and conventional hard chromium electroplating is also indicated. The lower
hardness on the accelerated hard chromium electroplating surface and its increase through-thickness may explain the decrease in the wear weight loss after a number of cycles. Theoretical calculations of the respective wear depth caused by abrasive wheels after 10 000 cycles were 38.0 and 40.8 m for the accelerated and conventional hard chromium electroplating, respectively, and 9.50 m for the tungsten carbide thermal spray coating. Therefore, for both hard chromium electroplatings the wear depth values are associated to the hardness change site through-thickness. This may also be associated to the higher microcrack density and higher hardness in the accelerated hard chromium electroplating,
Fig. 12. Hard chromium electroplated specimen Ž100 = ..
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124
122 Table 4 Abrasive wears weight loss
Abrasive wear ŽTaber abraser . Cycles ŽN.
Tungsten carbide Total mg
1000 2000 3000 4000 5000 6000 7000 8000 9000 10 000
2.20 3.90 4.70 7.97 10.80 13.03 14.20 15.80 17.40 18.87
Median Standard deviation
1.89 mgr1000 cycles 0.75 mgr1000 cycles
mgr1000 2.20 1.70 0.80 3.27 2.83 2.23 1.17 1.60 1.60 1.47
Accelerated hard chromium Depth Žm.
Total mg
1.57 2.78 3.56 5.69 7.72 9.31 10.15 11.29 12.43 13.48
8.30 13.40 16.50 19.10 21.00 22.60 24.10 25.30 26.30 27.10
mgr1000 8.30 5.10 3.10 2.60 1.90 1.60 1.50 1.20 1.00 0.80
Conventional hard chromium Depth Žm. 11.67 18.81 23.16 26.81 29.47 31.72 33.82 35.51 36.91 38.80
2.71 mgr1,000 cycles 2.34 mgr1000 cycles
and so in the higher amount of edges, resulting in lower fracture toughness and higher brittleness. In addition, the higher the crack density, the higher the amount of previously detached solid particles, which are suppressed in the microcracks and decrease the wear strength. This may result in micro-cutting, which is considered to be the predominant wear weight loss mechanism w16x. Hard chromium electroplates with hardness of approximately 750᎐800 Vickers were found to have the best frictional wear resistance, if the hardness was obtained as-deposited or by moderate heat treatment of harder deposit w4x. 3.3. Salt spray tests The results of the corrosion testing, performed in a qualitative way, were obtained by visual inspection and
Total mg 2.83 5.57 8.33 11.33 14.83 17.63 20.60 22.93 25.83 29.13
mgr1000 2.83 2.74 2.76 3.00 3.50 2.80 2.97 2.33 2.90 3.30
Depth Žm. 3.97 7.82 11.70 15.90 20.81 24.74 28.91 32.20 36.25 40.80
2.91 mgr1000 cycles 0.88 mgr1000 cycles
by image analyser software of the specimen surface after exposure to salt spray test. Both coatings completed the tests fully corroded. Table 6, Figs. 14 and 15 show the results of the salt spray test after 24, 48 and 72 h, for all the cases. For the HPrHVOF tungsten carbide coating, a better corrosion resistance was observed after sealing the application before testing. However, HPrHVOF tungsten carbide coating did not quite protect the substrate against the aggressive action of salt spray environment after 72 h in the test chamber. The HPrHVOF thermal spray tungsten carbide process has a high content of pores and oxides, which can be detrimental towards corrosion strength w14x. In addition, it is well known that the HPrHVOF tungsten carbide coating does not yield consistent thickness, which also made easy the salt spray action on sites of low thickness. On the other
Fig. 13. Abrasive wear weight loss vs. number of cycles.
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124 Table 5 Through-thickness HV microhardness with 1 N load Microhardness-HV Coatings
Surface
Core
Interface
Accel. hard chromium Conv. hard chromium Tungsten carbide
864 897 1070
920 906 1159
913 912 1354
hand, HPrHVOF tungsten carbide coating shows the better performance in comparison to the hard chromium electroplating. Bodger and co-workers w1x observed no corrosion products in tungsten carbide samples of 200 m thickness over 30 days in salt spray test. Higher nominal thicknesses of tungsten carbide lead to discontinuous micropore distributions along the thickness, which may delay the corrosive process. With respect to both hard chromium plated samples, Fig. 14 clearly shows the higher salt spray resistance of the accelerated hard chromium electroplated specimen during all tests. In relation to the accelerated hard chromium plated specimen with 49 m thickness and subjected to 48 h in salt spray environment, it can be observed that its surface showed approximately 5% corrosion products. On the other hand, in the same condition, the corrosion of the conventional hard chromium electroplated specimen was full, i.e. visually, 100% was corrosion. This experimental behaviour is related to the number of microcracks in the deposit in a way that the greater the microcrack density, the more corrosion along the sample surface and, so, better protection against corrosion w3x. In spite of the higher microcrack density of the accelerated hard chromium electroplating, the surface roughness measurements indicates lower values than the conventional hard chromium electroplating, as mentioned before. In general, the corrosion resistance is related to the surface roughness of a part; i.e. the higher surface roughness, the higher the corrosion attack due to higher surface area w17x. Therefore, the conventional hard chromium electroplating process yielded lower density and, consequently, deeper microcracks. It is also clear that the increase in the thickness enhanced the hard chromium
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protection to the salt spray corrosion. However, here also, there was no protection of the substrate against the aggressive action of salt spray environment. This corrosion is due to the high content of pores and microcracks inherent to the process itself, that act as canals, leading the corrosive process to the substratercoating interface, and getting intensity.
4. Conclusions
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The effect of tungsten carbide thermal spray coating applied by HPrHVOF process and hard chromium electroplating for the rotating bending and axial fatigue tests was to decrease the fatigue strength of AISI 4340 steel. The influence is more significant in high cycle fatigue tests than in low cycle fatigue tests. The decrease of the fatigue strength was higher in chromium electroplated specimens than in tungsten carbide coated specimens. The higher rotating bending fatigue strength of the conventional hard chromium in comparison to the accelerated hard chromium electroplating, despite the higher thickness of the former, is associated with the lower microcrack density of the conventional hard chromium electroplating. A small increase in rotating fatigue strength was obtained for tungsten carbide thermal spray coated specimens blasted with aluminium oxide in comparison to samples without superficial treatment. No change in the microstructure of the base metal due to deposition process was observed for tungsten carbide thermal spray coating applied by HPrHVOF process and for chromium electroplating. For axial fatigue tests, the negative influence of coatings on the fatigue strength of the steel followed the same tendency observed in rotating bending fatigue tests. Analysis of the hard chromium electroplated results revealed that the different microcracks density did not play an important role in their performance.
Table 6 Results of the salt spray test in 24, 48 and 72 h Salt spray test Coating Time
Tungs. carbide 100 m
16 m
Accel. hard chrome 36 m
49 m
16 m
Conv. hard chrome 36 m
49 m
24 h 48 h 72 h
30% 50% 100%
80% 100% 100%
10% 30% 100%
OK 5% 100%
90% 100% 100%
70% 100% 100%
50% 100% 100%
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spect to both hard chromium electroplated samples, the results indicate clearly the higher salt spray resistance of the accelerated hard chromium electroplated specimens.
Acknowledgements The authors are grateful for the support of this research by CAPES, FAPESP, EMBRAER-LIEBHERRrEDE and CTArAMR. Fig. 14. Salt spray test results for hard chromium electroplated samples after 48 h. 䢇
䢇
The wear weight loss tests showed better results for the HPrHVOF tungsten carbide coating in comparison to the chromium electroplating. An initially higher wear weight loss for the accelerated hard chromium electroplating occurred, decreasing continuously with the increase in test cycles. Both coatings completed the test of 72 h with full corrosion. For the HPrHVOF tungsten carbide coating, a better corrosion resistance was observed after sealing application before testing. With re-
Fig. 15. Salt spray test results for tungsten carbide coated samples after 72 h.
References w1x B.E. Bodger, R.T.R. McGrann, D.A. Somerville, Plating and Surface Finishing ŽSeptember 1997. 28. w2x J.M. Tyler, Metal Finishing ŽOctober 1995. 11. w3x A.R. Jones, Plating and Surface Finishing ŽApril 1989. 62. w4x G. Dubpernell, F.A. Lowenheim, Modern Electroplating Ž1968. 80. w5x H.S. Kuo, J.K. Wu, J. Mater. Sci. 31 Ž1996. 6095. w6x K.L. Lin, C.-J. Hsu, J.-T. Chang, J. Mater. Eng. Perform. 1 Ž1992. 359. w7x J. Pina, A. Dias, M. Franc¸ois, J.L. Lebrun, Surf. Coat. Technol. 96 Ž1997. 148. w8x S. Hotta, Y. Itou, K. Saruki, T. Arai, Surf. Coat. Technol. 73 Ž1995. 5. w9x S. Wang, Y. Li, M. Yao, R. Wang, J. Mats. Process. Technol. 73 Ž1998. 64. w10x Y. Matsumoto, D. Magda, D.W. Hoeppner, T.Y. Kim, J. Eng. Ind. 113 Ž1991. 154. w11x J.R. Hwang, C.-P. Fung, Surf. Coat. Technol. 80 Ž1996. 271. w12x D.C. Bolles, Welding J., October Ž1995. 31 w13x W. Coulson, E.R. Leheup, M.G. Marsh, Trans. IMF 73 Ž1. Ž1995. 7. w14x Available from World Wide Web: http:rrmember.aol. comrenglandgrtsc.htm w15x H.E. Boyer, Am. Soc. Metals. Metals Park, Ohio 44073 Ž1986. 1. w16x H. Wang, W. Xia, Y. Jin, Wear 195 Ž1996. 47. w17x E. Budke, J. Krempel-Hesse, H. Maidhof, H. Schussler, Surf. ¨ Coat. Technol. 112 Ž1999. 108. w18x D.J. Greving, E.F. Rybicki, J.R. Shadley, J. Thermal Spray Technol. 3 Ž4. ŽDecember 1994. 379. w19x L. Pejryd, J. Wigren, D.J. Greving, J.R. Shadley, E.F. Rybicki, J. Thermal Spray Technol. 4 Ž3. Ž1995. 268.
Effects of tungsten carbide thermal spray coating by HPrHVOF and hard chromium electroplating on AISI 4340 high strength steel Marcelino P. Nascimento a,U , Renato C. Souzab, Ivancy M. Miguel a , Walter L. Pigatin c , Herman J.C. Voorwalda a
State Uni¨ ersity of Sao ˜ Paulo- DMT-UNESPr FEG, A¨ . Ariberto Pereira da Cunha, 333-Guaratinguetar ´ SP r BR-CEP: 12500-000, Brazil b FAENQUILr DEMAR, Lorenar SP r BR-CEP:12600-000, Brazil c EMBRAER-LIEBHERRr EDE, Sao ˜ Jose´ dos Camposr SP r BR-CEP: 12237-540, Brazil Received 26 August 1999; received in revised form 5 October 2000; accepted 7 November 2000
Abstract In cases of decorative and functional applications, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. However, pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics has increased in recent years, related to the reduction in the fatigue strength of the base material and to environmental requirements. The high efficiency and fluoride-free hard chromium electroplating is an improvement to the conventional process, considering chemical and physical final properties. One of the most interesting, environmentally safer and cleaner alternatives for the replacement of hard chrome plating is tungsten carbide thermal spray coating, applied by the high velocity oxy-fuel ŽHVOF. process. The aim of this study was to analyse the effects of the tungsten carbide thermal spray coating applied by the HPrHVOF process and of the high efficiency and fluoride-free hard chromium electroplating Žin the present paper called ‘accelerated’., in comparison to the conventional hard chromium electroplating on the AISI 4340 high strength steel behaviour in fatigue, corrosion, and abrasive wear tests. The results showed that the coatings were damaging to the AISI 4340 steel behaviour when submitted to fatigue testing, with the tungsten carbide thermal spray coatings showing the better performance. Experimental data from abrasive wear tests were conclusive, indicating better results from the WC coating. Regarding corrosion by salt spray test, both coatings were completely corroded after 72 h exposure. Scanning electron microscopy technique ŽSEM. and optical microscopy were used to observe crack origin sites, thickness and adhesion in all the coatings and microcrack density in hard chromium electroplatings, to aid in the results analysis. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten carbide thermal spray coating; Hard chromium electroplating; Abrasive wear; Corrosion; Fatigue; HPrHVOF
U
Corresponding author. Tel.: q55-12-525-2800; fax: q55-12-515-2466. E-mail address: [email protected] ŽM.P. Nascimento..
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 4 8 - 8
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1. Introduction Chromium plating is the most used electrodeposited coating to obtain high levels of hardness, resistance to wear and corrosion and a low coefficient of friction for applications in the aerospace, automotive and petrochemical fields w1,2x. Chromium plating properties, such as hardness and microcrack density, change with the bath composition, current density, bath agitation, temperature, etc. w3,4x. Among other things, a significant characteristic of chromium electroplating is the high tensile residual internal stresses originating from the decomposition of chromium hydrides during the electrodeposition process w3᎐5x. These high tensile stresses in electroplated chromium coatings increase as thickness increases and are relieved by local microcracking during electroplating. Therefore, basically, microcrack density is related to the high tensile residual internal stresses, hardness, and corrosion resistance w2,3,6x. It was observed that the residual stresses through-thickness decrease with the depth of the coating and increase again at the coating᎐substrate interface w7x. Bending fatigue tests on samples with different coatings and coating conditions indicate that the fatigue strength is dependent on the fracture behaviour of the substrates and on the hardness and residual stresses at the substrate surface. It was also observed that the hard chromium electroplating reduces the fatigue strength of a component w8x. Due to this fact, the design of hard chromium plated components, which are subjected to dynamic loads, may consider this negative influence to guarantee safety during operation. Therefore, the use of effective methods to improve the fatigue strength shall be considered. Shot peening is a well-known process to increase fatigue life of structures subjected to constant and variable amplitude loading. The compressive residual stress obtained by surface plastic deformation is responsible for the increase in fatigue strength in shot peened mechanical components w9x. Compressive residual stresses induced by machining processes are also responsible for the improvement in fatigue resistance of AISI 4340 steel w10x. Increase in the fatigue crack propagation resistance in AISI 4340 steel with electron beam surface hardening was associated to residual stress distribution and microstructural characteristics w11x. However, problems concerning chrome plating, such as health and environmental hazards, increasing costs and a performance not in accordance with the specifications, have resulted in a search to identify possible alternatives w12x. Aircraft landing gear manufacturers are considering tungsten carbide ŽWC. thermal spray coatings applied by the high velocity oxy-fuel ŽHVOF. process as an alternative to hard chrome plating. The question to be answered is if the performance of the alternative candidate is at
least comparable to results obtained for hard chrome plating. Comparisons of experimental data showed better corrosion resistance for several HVOF coatings with respect to chrome plating. In the case of fatigue and friction tests, the results were acceptable, indicating interesting perspectives on the use of tungsten carbide coating to replace chrome-plating w1x. Analysis of the wear performance of tungsten carbide coated samples in the presence of air, aqueous and aqueous abrasive media indicated better results in terms of volume loss and change in surface roughness than for the mild steel substrate w13x. The objective of this research is to compare the influence of the tungsten carbide thermal spray coating applied by HPrHVOF and hard-chromium plating on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel. S᎐N curves were obtained in rotating bending and axial fatigue tests for the base material, chromium plated, and tungsten carbide coated specimens.
2. Experimental procedures AISI 4340 steel is widely used in aircraft components where strength and toughness are fundamental design requirements. The chemical analysis of the material used in this research indicates accordance with specifications. The fatigue experimental program was performed on rotating bending and axial fatigue test specimens machined from hot rolled, quenched and tempered bars according to Figs. 1 and 2, respectively. The specimens were polished in the reduced section with 600 grit papers, inspected dimensionally and by magnetic particle inspection. Fatigue tests specimens were quenched from 815᎐845⬚C in oil Ž20⬚C. and tempered in the range of 520 " 5⬚C for 2 h. The mechanical properties of the material after the heat treatment are: hardness of 39 HRC; yield tensile strength of 1118 MPa, and ultimate tensile strength of 1210 MPa. After final preparation, samples were subjected to a stress relieve heat treatment at 190⬚C for 4 h to reduce residual stresses induced by machining. Average superficial roughness in the reduced section of the samples was R a f 2.75 m and a standard deviation ŽS.D.. of 0.89 m. Rotating bending fatigue tests were conducted using a sinusoidal load of frequency 50 Hz, and load ratio R s y1, at room temperature. For axial fatigue tests, a sinusoidal load of frequency 50 Hz and load ratio R s 0.1 was applied throughout this study. Both tests consider as fatigue strength the complete fracture of the specimens or 10 7 load cycles. Four groups of fatigue specimens were prepared to obtain S᎐N curves for rotating bending fatigue and axial fatigue tests.
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Fig. 1. Rotating bending fatigue testing specimen.
2.1. For rotating bending fatigue tests
䢇 䢇
䢇
䢇
12 smooth samples of base material; 13 samples with 160 m thickness of conventional hard chromium electroplated; 13 samples with 100 m thickness of accelerated hard chromium electroplated; and 13 samples with 100 m thickness of tungsten carbide thermal spray coated by HPrHVOF process.
2.2. For axial fatigue tests
rotating bending fatigue specimens were tested without blasting. 2.3. Salt spray test The performance of the coatings was evaluated with respect to chemical corrosion in specific environment. The samples were prepared from normalised AISI 4340 steel with 1 mm thickness, and 76 mm width and 254 mm length, surface roughness R a f 0.2 m, and in the following conditions: 䢇
䢇
䢇 䢇
䢇
䢇
10 smooth samples of base material; 15 samples with 160 m thickness of conventional hard chromium electroplated; 7 samples with 100 m thickness of accelerated hard chromium electroplated; and 13 samples with 100 m thickness of tungsten carbide thermal spray coated by HPrHVOF process.
The tungsten carbide thermal spray coated specimens were blasted with aluminium oxide mesh 90 to enhance adhesion. To compare experimental data, six
Fig. 2. Axial fatigue testing specimen.
䢇
accelerated hard chromium electroplated ª 16, 36 and 49 m thickness; conventional hard chromium electroplated ª 16, 36 and 49 m thickness; and tungsten carbide thermal spray coated ª 100 m thickness.
Experimental tests were conducted in accordance with ASTM B 117, in 5 wt.% NaCl, pH of 6.5᎐7.2, at 35⬚C. The samples were supported at 20⬚ from the vertical. The results were analysed by Image Pro Plus software. 2.4. Abrasi¨ e wear test The performance of the coating was also evaluated with respect to abrasive wear. For abrasive wear tests, samples were prepared from annealed AISI 4340 steel with 4 mm thickness and 100 mm square, according to FED-STD-141C. The samples were divided in three groups; two coated with 100-m thickness of accelerated and conventional hard chromium electroplating, respectively, and one group coated with 100-m thickness of tungsten carbide coating. The wear tests were conducted with a Taber abraser, at room temperature, using a 10-N load and CS-17 abrading wheel for hard chromium electroplating and diamond wheel for tungsten carbide coating. The results were analysed by wear index Žmgr1000 cycles. and total wear Žmgr10 000 cycles. data.
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2.5. Tungsten carbide coating The tungsten carbide thermal spray coating applied by HPrHVOF system, used WC powder with 12% Co, resulting in thickness equal to 100 m. The average superficial roughness in the reduced section of the samples was R a f 4 m and a S.D. of 0.39 m, in the as-deposited condition. 2.6. Hard chromium electroplating The conventional hard chromium electroplating was carried out from a chromic acid solution with 250 grl of CrO 3 and 2.5 grl of H 2 SO4 , at 50᎐55⬚C, with a current density from 31 to 46 Ardm2 , and a speed of deposition equal to 25 mrh. A bath with a single catalyst based on sulfate was used. The accelerated hard chromium electroplating was carried out from a chromic acid solution with 250 grl of CrO 3 and 2.7 grl of H 2 SO4 , at 55᎐60⬚C, with a current density from 55 to 65 Ardm2 , and a speed of deposition equal to 80 mrh. A bath with a double catalyst, one based on sulfate and the other without fluoride, was used. After the coating deposition, the 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 R a f 3.13 m in the reduced section and a S.D. of 0.79 m in the as-electroplated condition. For the microcrack determination in both hard chromium electroplating, samples were prepared from normalised AISI 4340 steel Ž R a f 0.2 m., 1 mm thickness, 25 mm width and length, and with accelerated and conventional hard chromium electroplating both
with 100 m thickness, which resulted in a surface roughness of R a f 0.74 m for the former and R a f 1.6 m for the later, in the as-electroplated condition. The surface microcracks were enhanced through anodic etching for 30 s with a current density equal to 25 Ardm2 in the same chromium bath and later analysed using an optical microscope model NikonrApophot. All surface roughness data measured in this research was obtained by Mitutoyo 301 equipment using a cut-off of 0.8 mm. The analysis of fracture surface was carried out on rotating bending fatigue specimens by scanning electron microscope, model LEO 435 vpi and Zeiss DSM 950. The metallographic analysis was carried out on optical microscope model Neophot 21.
3. Results and discussion 3.1. Fatigue test The S᎐N curves for the rotating bending and axial fatigue tests for the base metal and coated specimens are presented in Figs. 3 and 4, respectively. Fig. 3 shows that the effect of coating in the rotating bending fatigue test is to decrease the fatigue strength of AISI 4340 steel. The tendency is observed for low number of cycles Ž10 4 ., high number of cycles Ž10 5 . and for the fatigue limit, 10 7 cycles, and is represented in Table 1. One sees that the specimens coated with tungsten carbide applied by the HVOF process show a lower decrease in fatigue strength. This may be attributed to the process itself. It is well known that HVOF thermal spray process produces compressive
Fig. 3. S᎐N curves for rotating bending fatigue tests.
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Fig. 4. S᎐N curves for axial fatigue tests.
residual internal stresses within the substrate, which are formed from mechanical deformation on the surface during particle impact. This is confirmed by the through-thickness residual stress behaviour shown in Fig. 5. These surface deformations counteract the ten-
sile shrinkage stresses of the coating caused by fast cooling and solidification as particles strike the surface. These tensile stresses in the coating also generate compressive stresses within the surface of the substrate. However, there was a reduction in the fatigue
Table 1 Rotating bending fatigue strength. Rotating bending fatigue strength Group
Low cycles Ž104 .
High cycles Ž105 .
limit Ž107 .
Base material Treat. T. carb Ž100 m. Tungsten carb. Ž100 m. Conv. chrome Ž160 m. Accel. chrome Ž100 m.
s 950 MPa Ž85%ys . s 900 MPa Ž80%ys . ( 900 MPa Ž80%ys . ( 840 MPa Ž75%ys . ( 730 MPa Ž65%ys .
( 700 MPa Ž63%ys . ( 610 MPa Ž54%ys . ( 570 MPa Ž51%ys . ( 500 MPa Ž45%ys . ( 340 MPa Ž30%ys .
( 615 MPa Ž59%ys . ( 531 MPa Ž47.5%ys . ( 531 MPa Ž47.5%ys . ( 321 MPa Ž29%ys . ( 280 MPa Ž25%ys .
Fig. 5. Through-thickness residual stress distribution for WC HPrHVOF thermal spray coating.
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Fig. 6. Microcracks surface network in conventional Ža. and accelerated Žb. hard chromium electroplating. Anodic etching from 25 Ardm2 for 30 s Ž200 = ..
strength of AISI 4340, despite the compressive residual stresses induced by the process. This can be due to the high density of pores and oxide inclusions in the coating that commonly form during the process. Thermal spray is generally conducted in air so chemical interactions occur, notably oxidation, which can be evident in the coating microstructure as oxide inclusions, mainly in grain boundaries w14x. These inclusions in coating subsurfaces are possible cracks or nucleationrinitiation sites. From Fig. 3, it is possible to observe that the aluminium oxide blasting is responsible for a small improvement in the fatigue strength. Considering both hard chromium electroplated rotating bending fatigue results, one sees the negative influence of coating on the fatigue strength of the steel. From the analysis of these two coatings, it is possible to observe the better performance of the conventional hard chromium in relation to the accelerated hard chromium electroplating, despite the higher thickness of the former. This may be attributed to the lower microcrack density of the conventional hard chromium electroplating in comparison with the accelerated hard chromium electroplating as showed in Fig. 6. The microcracks density quantitative analysis indicated median values of 1512
microcracksrcm and a S.D. of 190.6 microcracksrcm for the accelerated hard chromium electroplating, and 223 microcracksrcm with standard deviation of 57.5 microcracksrcm for the conventional hard chromium electroplating. Microcracks form when the high tensile residual internal stresses exceed the cohesive strength of the chromium deposits and affect the fatigue behaviour of a plated part. Therefore, microcrack density arises as a relief of the tensile residual internal stresses, which increase when the chromium thickness increases. Pina et al. w7x showed that the microcrack density changes along the thickness, being higher at the core and lower at the surface of the coating and in the substratercoating interface due to the balance between the residual stresses. On the surface of the coating, the microcracks arise in a network shape, without preferential direction and characterising an equi-biaxial residual stress state. With respect to residual stresses, an inverse behaviour from that observed for the microcrack occurred. Therefore, in general, the higher the microcrack density, the higher the tensile residual internal stresses andror their relief. This means that the accelerated hard chromium electroplating is responsible for higher tensile residual internal stresses andror present the highest crack initiationrpropagation front amount. However, the different microcrack densities between both hard chromium electroplatings practically produced the same effect in low cycle fatigue, since crack growth occurs after few cycles of fatigue testing. In general, the fatigue strength for all conditions studied was reduced due to the high tensile residual internal stresses, microcrack density, and high adhesion at the coatingrsubstrate interface, which allows the crack to grow from the coating through the interface into the base metal. Fig. 5 shows the residual internal stress profile for tungsten carbide thermal spray coating. From experimental points, one sees that the residual internal stresses change throughout coating thickness, from 100 MPa tensile stress near to the surface, reaching 350 MPa maximum tensile stress at a 0.025-mm depth and decreasing to the maximum compressive stress of 680 MPa at a 0.07-mm depth, increasing again into the base metal, becoming tensile stress at a 0.20-mm depth. The curve of through-thickness residual stress was plotted based on three specimens and obtained by the modified layer-removal method for thermal spray coating and substrates w18,19x. The through-thickness residual stresses change from approximately 300 MPa tensile at 0.025-mm depth to approximately 680 MPa, compressive at 0.06 mm from the surface. This means that the crack initiation may occur easily on the coating surface, but its propagation throughout the thickness may be delayed when the compressive residual stress site is reached.
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Table 2 Axial fatigue strength Axial fatigue strength Group
Low cycles Ž104 .a
High cycles Ž105 .
Limit Ž107 .
Base material Tungsten carb. Ž100 m. Conv. chrome Ž160 m. Accel. chrome Ž100 m.
( 1330 MPa Ž119%ys . ( 1350 MPa Ž121%ys . ( 1200 MPa Ž107%ys . ( 1150 MPa Ž103%ys .
( 1125 MPa Ž101%ys . ( 850 MPa Ž76%ys . ( 650 MPa Ž58%ys . ( 650 MPa Ž58%ys .
( 850 MPa Ž76%ys . ( 750 MPa Ž67%ys . ( 400 MPa Ž36%ys . ( 400 MPa Ž36%ys .
a
ª Projection of the curves until the respective number of cycles Ž10 4 ..
For hard chromium electroplating residual stresses, Pina et al. w7x showed that, despite the fact that microcracks result in residual stress relief in the coating, the stresses still remain high at the surface Žapprox. 800 MPa., decreasing in direction to the core Žapprox. 200 to 300 MPa., and increasing again at the interface to values which depend on the substrate material. Fig. 4 shows the axial fatigue testing results, indicating the decrease in fatigue strength for all specimens coated with tungsten carbide thermal spray and hard chromium electroplating, in comparison to the base material. Comparing the curves, one sees the negative influence of coatings on the fatigue strength of the steel, with the same tendency observed previously in the rotating bending fatigue tests. This behaviour can also be explained by high tensile residual internal stresses, oxide inclusions, pores and microcracks inherent from each process. Table 2 indicates the axial fatigue strength tendency of all specimens groups, based on Fig. 4. The better performance of tungsten carbide coated specimens in comparison to the hard chromium plating may also be attributed to the lower tensile residual internal stresses in the coating of the former, compressive residual stresses on the substrate surface and in the subsurface due to the particles impact effect, as well as by interactions between surfacersubstrate residual stresses. Note in the axial fatigue test that the different microcrack density between both hard chromium electroplatings did not play an important role in the specimens performance, as occurred in
rotating bending fatigue tests. A comparison of Tables 1 and 2, as shown in Table 3, indicates the higher fatigue strength shown in axial fatigue test in relation to the rotating bending fatigue tests. In addition to the lower specimen dimensions, this is in accordance with the fact that the rotating bending fatigue tests are more severe as a result of the effect of the bending moment which increases the tensile stresses on surface from where, in general, the fatigue cracks grow. However, Table 3 shows a higher decrease in the fatigue strength as a function of the number of cycles in comparison to the rotating bending fatigue test results, for each level of stress. This may also be due to radial throughout thickness crack propagation to the base metal, in a direction normal to the maximum tensile stress and resulting in lower fatigue life of the specimen w15x. Fig. 7 shows a typical fracture surface from the base metal, indicating that the fatigue crack nucleation started at the surface. In Fig. 8, several crack fronts that may be associated to the microcracks density originated from the plating process are represented. From Fig. 9, which represents fracture surface from a rotating bending fatigue specimen electroplated with accelerated hard chromium and tested at 871 MPa, one sees the coating homogeneity, strong adhesion substratercoating, and microcracks distributed along thickness in a radial shape. Fig. 10 shows a micrograph of a tungsten carbide thermal spray coated specimen blasted with aluminium
Table 3 Fatigue strength in number of cycles in the rotating bending and axial fatigue tests Fatigue testing Stress
Rotating. bending fatigue data Ž%.a
Axial fatigue data Ž%.a
ŽMPa.
Base mat. Cycles
W.C. Cycles
C.H.C. Cycles
A.H.C. Cycles
Base mat. Cycles
W.C. Cycles
C.H.C. Cycles
A.H.C. Cycles
850 750 650
22 000 60 000 299 000
12 000 Ž55. 24 000 Ž40. 56 000 Ž19.
9500 Ž43. 18 000 Ž36. 36 500 Ž12.
6000 Ž27. 9000 Ž18. 14 200 Ž5.
107 107 107
105 Ž1. 106 Ž10. 107 Ž100.
30 000Ž0.30. 42 000Ž0.42. 60 000Ž0.60.
30 000Ž0.30. 42 000Ž0.42. 60 000Ž0.60.
a ª Values contained in parenthesis are the rate between the number of cycles of the coated material and the number of cycles of base metal, in percentage.
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Fig. 7. Typical fracture surface from base metal. Rotating bending fatigue test.
oxide. To compare, Fig. 11 indicates a coating profile without the blasting treatment. It is possible to observe that the aluminium oxide blasting is responsible for the increase in roughness at the substratercoating interface, with a consequent improvement in adhesion. The important characteristics of the hard chromium electroplating are the homogeneity of the coatings and
Fig. 8. Fracture surface from 100 m of accelerated hard chromium electroplating.
the excellent adhesion with the base metal, represented in Fig. 12. From Figs. 10᎐12, it is also possible to observe that, in both cases, the base metal microstructure was not affected by the deposition process.
Fig. 9. Fracture surface of samples accelerated hard chromium electroplated and tested at 871 MPa Ž500 = ..
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Fig. 11. Tungsten carbide coating specimen Ž100 = .. Fig. 10. Tungsten carbide coating aluminium blasted specimen Ž200 = ..
3.2. Abrasi¨ e wears tests The abrasive wear resistance of HPrHVOF WC coating and hard chromium plating was evaluated, and the results in terms of wear weight loss are represented in Table 4 and Fig. 13. Comparing the abrasive wear resistance, one sees the better performance of samples coated with WC, with lower wear weight loss than the hard chromium electroplated specimens. This may be attributed to the higher hardness and oxide content into the tungsten carbide coating. Coatings of high oxide content are usually harder and more wear resistant w14x. However, initially, in the first 1000 cycles, an almost equivalent value of wear weight loss was observed for the tungsten carbide coating and conventional hard chromium electroplating, and a higher wear weight loss for the accelerated hard chromium. This higher hardness of tungsten carbide coating, which should result in lower wear weight loss in comparison with the conventional hard chromium electroplating, was damaged due to its higher surface roughness. With respect to both hard chromium electroplating, in the subsequent cycles, the wear weight loss of the accelerated hard chromium electroplating decreases with the increase in number of cycles in a parabolic way, resulting after 10 000 cycles in lower wear weight loss than the conventional hard chromium plating. This may be explained by the through-thickness hardness variation, in accordance with Table 5, in which the microhardness data of tungsten carbide coating and conventional hard chromium electroplating is also indicated. The lower
hardness on the accelerated hard chromium electroplating surface and its increase through-thickness may explain the decrease in the wear weight loss after a number of cycles. Theoretical calculations of the respective wear depth caused by abrasive wheels after 10 000 cycles were 38.0 and 40.8 m for the accelerated and conventional hard chromium electroplating, respectively, and 9.50 m for the tungsten carbide thermal spray coating. Therefore, for both hard chromium electroplatings the wear depth values are associated to the hardness change site through-thickness. This may also be associated to the higher microcrack density and higher hardness in the accelerated hard chromium electroplating,
Fig. 12. Hard chromium electroplated specimen Ž100 = ..
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122 Table 4 Abrasive wears weight loss
Abrasive wear ŽTaber abraser . Cycles ŽN.
Tungsten carbide Total mg
1000 2000 3000 4000 5000 6000 7000 8000 9000 10 000
2.20 3.90 4.70 7.97 10.80 13.03 14.20 15.80 17.40 18.87
Median Standard deviation
1.89 mgr1000 cycles 0.75 mgr1000 cycles
mgr1000 2.20 1.70 0.80 3.27 2.83 2.23 1.17 1.60 1.60 1.47
Accelerated hard chromium Depth Žm.
Total mg
1.57 2.78 3.56 5.69 7.72 9.31 10.15 11.29 12.43 13.48
8.30 13.40 16.50 19.10 21.00 22.60 24.10 25.30 26.30 27.10
mgr1000 8.30 5.10 3.10 2.60 1.90 1.60 1.50 1.20 1.00 0.80
Conventional hard chromium Depth Žm. 11.67 18.81 23.16 26.81 29.47 31.72 33.82 35.51 36.91 38.80
2.71 mgr1,000 cycles 2.34 mgr1000 cycles
and so in the higher amount of edges, resulting in lower fracture toughness and higher brittleness. In addition, the higher the crack density, the higher the amount of previously detached solid particles, which are suppressed in the microcracks and decrease the wear strength. This may result in micro-cutting, which is considered to be the predominant wear weight loss mechanism w16x. Hard chromium electroplates with hardness of approximately 750᎐800 Vickers were found to have the best frictional wear resistance, if the hardness was obtained as-deposited or by moderate heat treatment of harder deposit w4x. 3.3. Salt spray tests The results of the corrosion testing, performed in a qualitative way, were obtained by visual inspection and
Total mg 2.83 5.57 8.33 11.33 14.83 17.63 20.60 22.93 25.83 29.13
mgr1000 2.83 2.74 2.76 3.00 3.50 2.80 2.97 2.33 2.90 3.30
Depth Žm. 3.97 7.82 11.70 15.90 20.81 24.74 28.91 32.20 36.25 40.80
2.91 mgr1000 cycles 0.88 mgr1000 cycles
by image analyser software of the specimen surface after exposure to salt spray test. Both coatings completed the tests fully corroded. Table 6, Figs. 14 and 15 show the results of the salt spray test after 24, 48 and 72 h, for all the cases. For the HPrHVOF tungsten carbide coating, a better corrosion resistance was observed after sealing the application before testing. However, HPrHVOF tungsten carbide coating did not quite protect the substrate against the aggressive action of salt spray environment after 72 h in the test chamber. The HPrHVOF thermal spray tungsten carbide process has a high content of pores and oxides, which can be detrimental towards corrosion strength w14x. In addition, it is well known that the HPrHVOF tungsten carbide coating does not yield consistent thickness, which also made easy the salt spray action on sites of low thickness. On the other
Fig. 13. Abrasive wear weight loss vs. number of cycles.
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124 Table 5 Through-thickness HV microhardness with 1 N load Microhardness-HV Coatings
Surface
Core
Interface
Accel. hard chromium Conv. hard chromium Tungsten carbide
864 897 1070
920 906 1159
913 912 1354
hand, HPrHVOF tungsten carbide coating shows the better performance in comparison to the hard chromium electroplating. Bodger and co-workers w1x observed no corrosion products in tungsten carbide samples of 200 m thickness over 30 days in salt spray test. Higher nominal thicknesses of tungsten carbide lead to discontinuous micropore distributions along the thickness, which may delay the corrosive process. With respect to both hard chromium plated samples, Fig. 14 clearly shows the higher salt spray resistance of the accelerated hard chromium electroplated specimen during all tests. In relation to the accelerated hard chromium plated specimen with 49 m thickness and subjected to 48 h in salt spray environment, it can be observed that its surface showed approximately 5% corrosion products. On the other hand, in the same condition, the corrosion of the conventional hard chromium electroplated specimen was full, i.e. visually, 100% was corrosion. This experimental behaviour is related to the number of microcracks in the deposit in a way that the greater the microcrack density, the more corrosion along the sample surface and, so, better protection against corrosion w3x. In spite of the higher microcrack density of the accelerated hard chromium electroplating, the surface roughness measurements indicates lower values than the conventional hard chromium electroplating, as mentioned before. In general, the corrosion resistance is related to the surface roughness of a part; i.e. the higher surface roughness, the higher the corrosion attack due to higher surface area w17x. Therefore, the conventional hard chromium electroplating process yielded lower density and, consequently, deeper microcracks. It is also clear that the increase in the thickness enhanced the hard chromium
123
protection to the salt spray corrosion. However, here also, there was no protection of the substrate against the aggressive action of salt spray environment. This corrosion is due to the high content of pores and microcracks inherent to the process itself, that act as canals, leading the corrosive process to the substratercoating interface, and getting intensity.
4. Conclusions
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The effect of tungsten carbide thermal spray coating applied by HPrHVOF process and hard chromium electroplating for the rotating bending and axial fatigue tests was to decrease the fatigue strength of AISI 4340 steel. The influence is more significant in high cycle fatigue tests than in low cycle fatigue tests. The decrease of the fatigue strength was higher in chromium electroplated specimens than in tungsten carbide coated specimens. The higher rotating bending fatigue strength of the conventional hard chromium in comparison to the accelerated hard chromium electroplating, despite the higher thickness of the former, is associated with the lower microcrack density of the conventional hard chromium electroplating. A small increase in rotating fatigue strength was obtained for tungsten carbide thermal spray coated specimens blasted with aluminium oxide in comparison to samples without superficial treatment. No change in the microstructure of the base metal due to deposition process was observed for tungsten carbide thermal spray coating applied by HPrHVOF process and for chromium electroplating. For axial fatigue tests, the negative influence of coatings on the fatigue strength of the steel followed the same tendency observed in rotating bending fatigue tests. Analysis of the hard chromium electroplated results revealed that the different microcracks density did not play an important role in their performance.
Table 6 Results of the salt spray test in 24, 48 and 72 h Salt spray test Coating Time
Tungs. carbide 100 m
16 m
Accel. hard chrome 36 m
49 m
16 m
Conv. hard chrome 36 m
49 m
24 h 48 h 72 h
30% 50% 100%
80% 100% 100%
10% 30% 100%
OK 5% 100%
90% 100% 100%
70% 100% 100%
50% 100% 100%
124
M.P. Nascimento et al. r Surface and Coatings Technology 138 (2001) 113᎐124
spect to both hard chromium electroplated samples, the results indicate clearly the higher salt spray resistance of the accelerated hard chromium electroplated specimens.
Acknowledgements The authors are grateful for the support of this research by CAPES, FAPESP, EMBRAER-LIEBHERRrEDE and CTArAMR. Fig. 14. Salt spray test results for hard chromium electroplated samples after 48 h. 䢇
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The wear weight loss tests showed better results for the HPrHVOF tungsten carbide coating in comparison to the chromium electroplating. An initially higher wear weight loss for the accelerated hard chromium electroplating occurred, decreasing continuously with the increase in test cycles. Both coatings completed the test of 72 h with full corrosion. For the HPrHVOF tungsten carbide coating, a better corrosion resistance was observed after sealing application before testing. With re-
Fig. 15. Salt spray test results for tungsten carbide coated samples after 72 h.
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