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Effect of detonation gun sprayed Cu–Ni–In coating on plain fatigue and fretting fatigue behaviour of Al–Mg–Si alloy
Surface & Coatings Technology 201 (2006) 1548 – 1558 www.elsevier.com/locate/surfcoat
Effect of detonation gun sprayed Cu–Ni–In coating on plain fatigue and fretting fatigue behaviour of Al–Mg–Si alloy B. Rajasekaran a , S. Ganesh Sundara Raman a,⁎, S.V. Joshi b , G. Sundararajan b a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai — 600 036, India b International Advanced Research Centre for New Materials and Powder Metallurgy, Hyderabad — 500 005, India Received 6 December 2005; accepted in revised form 15 February 2006 Available online 6 March 2006
Abstract Detonation gun spray technique was employed to coat Al–Mg–Si alloy (AA 6063) specimens with Cu–Ni–In powder. Coated samples were characterized with reference to the microstructure, porosity, residual stresses, microhardness and surface roughness. Plain fatigue (without fretting) and fretting fatigue tests were carried out at room temperature on uncoated and coated specimens. The detonation gun spray process resulted in a dense coating of almost uniform deposition with low porosity (0.3%) and good adhesion between the substrate and the coating. Under plain fatigue loading 40 μm thick coated samples exhibited superior lives compared with uncoated and 100 μm thick coated specimens due to the presence of higher surface compressive residual stress in the former. Delamination-induced failure resulted in inferior lives of 100 μm thick coated specimens. Under fretting fatigue deformation 40 μm thick coated specimens exhibited superior lives compared with 100 μm thick coated samples owing to higher compressive residual stress at the surface and better interfacial adhesion. At 120 MPa stress level 40 μm thick coated specimens exhibited superior fretting fatigue life compared with uncoated sample and at stress levels above 120 MPa the converse was true. This was attributed to interface cracking at higher stress levels. © 2006 Elsevier B.V. All rights reserved. Keywords: Fretting fatigue; Cu–Ni–In; Detonation gun; Residual stress; Al–Mg–Si alloy
1. Introduction Fretting, a small amplitude relative motion between two contacting bodies, greatly accelerates fatigue crack initiation process. The joint action of fretting and fatigue may produce strength reduction factor to 2–5 or even greater [1,2]. Surface modifications employing different processes like physical vapour deposition (PVD), chemical vapour deposition (CVD), ion implantation, thermal spraying and laser treatments are widely used to mitigate fretting damages [3–6]. Hard coatings deposited by spraying can provide high wear resistance but there is limited information on their application in fretting fatigue. Most of these types of coatings have been employed to inhibit fretting wear rather than fretting fatigue.
⁎ Corresponding author. Tel.: +91 44 2257 4768; fax: +91 44 2257 0509. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.023
Generally, the fatigue properties of thermal sprayed materials are controlled by many factors such as coating thickness, adhesion, residual stresses, and so on. There are few reports available in open literature dealing with the influence of coating thickness on the fatigue properties of spray-coated materials (e.g. [7–9]). Watanabe et al. reported that the rotating bending fatigue limit of 50 μm thick coated specimens was higher than that of 100 μm thick coated samples in case of a medium carbon steel coated with high velocity oxy fuel thermally sprayed WC– Co powder coating [7]. This was attributed to good cohesion between particles and good adhesion at the interface between the coating and substrate of 50 μm thick coated samples. Kubota et al. reported that the axial fatigue strength of AISI 1038 steel samples sprayed with molybdenum coating of 0.5 mm thickness was higher than that of specimens sprayed with 0.25 mm thick coating [8]. In a study on a medium carbon steel coated with gas flame thermally sprayed Co-based alloy, Oh et al. reported that the rotating bending fatigue strength of the specimens with 1 mm
B. Rajasekaran et al. / Surface & Coatings Technology 201 (2006) 1548–1558
Proving Ring
Load Adjusting Screw
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Test Specimen
Fretting Pad
Fig. 3. SEM picture showing the coating surface.
plain fatigue (without fretting) behaviour was investigated. The effect of coating thickness was also studied. 2. Experimental details Fig. 1. Flat-on-flat contact bridge type uniaxial fretting fatigue test setup used in the present study.
thick coating was remarkably higher compared with that of samples with 0.3 and 0.5 mm thick coating. This was explained in terms of differences in the failure mechanisms. While fatigue cracks initiated from the porosity located in the coating of 1 mm thickness, they initiated inside the substrate of samples having a coating thickness of 0.3 and 0.5 mm. Detonation gun (D-gun) spray coating process is a thermal spray process, which gives an extremely good adhesive strength, low porosity and coating surfaces with compressive residual stresses. In general, any surface coating will alter the nature of the contact surface by changing its surface microstructure, surface hardness, surface chemistry, coefficient of friction, surface roughness and surface residual stress. D-gun coating also imparts these changes in the surface properties on the coated surfaces. The present work was undertaken to study the influence of the D-gun sprayed Cu–Ni–In coating on the fretting fatigue behaviour of Al–Mg–Si alloy (AA6063). For reference
Substrate material AA 6063 of chemical composition (wt.%) 0.44 Mg, 0.41 Si, 0.35 Fe, 0.1 Cu, 0.1 Mn, 0.1 Zn, 0.1 Ti, 0.1 Cr and rest Al was used to prepare test specimens and fretting pads. The gauge dimensions of the specimens were: 8 mm thickness, 10 mm width and 65 mm length. The specimens were solution treated at 540 °C for 1 h and water quenched to room temperature. Subsequently they were aged at 160 °C for 12 h. The room temperature mechanical properties of the substrate material were: 0.2% yield strength = 208 MPa, ultimate tensile strength = 247 MPa, elongation = 21% and hardness = 80 HV0.2. Cu–36Ni–5In powder having spherical particles of size in the range of 15–40 μm was used as a coating material. Prior to coating process, the test specimens were cleaned, degreased and shot blasted using alumina grit of 60-mesh size and then ultrasonically cleaned using acetone. The specimens were fixed in the specimen holder of AWAAZ detonation spray coating system (horizontal type) and then sprayed with Cu–Ni–In powder. The gun barrel length was 1.3 m and diameter was 22 mm. Dgun coating was carried out using a combustible mixture of O2 and C2H2 with a frequency of 3 Hz and a spraying distance of 180 mm. Multipass coating of Cu–Ni–In was formulated on all four sides of the gauge portions of the fatigue specimens to achieve the desired coating thickness. Two sets of specimens were prepared — one with a coating thickness of 40 μm and the other with 100 μm. Porosity of the coating was determined as 0.3% using an image analyzer system on the basis of measurements carried out in 10 different regions. Microhardness of coating was found out Table 1 Characteristics of D-gun sprayed Cu–Ni–In coating and substrate AA 6063 Thickness
Fig. 2. Optical micrograph showing the cross section. Indentation made during hardness testing can be seen.
Porosity
Hardness
Roughness (μm)
(μm)
(%)
(HV0.2)
Ra
Rmax
40 100 Substrate
0.30 0.30 –
350 350 80
4.80 6.33 0.60
33.69 34.15 3.83
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Fig. 4. XRD analysis done on: (a) Cu–Ni–In powder; (b) coating.
Schenck servo-hydraulic testing machine. Normal (contact) load was applied using a proving ring with the help of load adjusting screws. Plain fatigue and fretting fatigue tests were conducted on uncoated and coated specimens at room temperature with a stress ratio of 0.1 at different maximum cyclic stresses. For plain fatigue tests the cycle frequency was 30 Hz and for fretting fatigue tests it was 10 Hz. The fretting fatigue tests were carried out at a constant average contact pressure of 100 MPa. The 210 190
Max. Stress (MPa)
as 350 HV0.2 (average of 20 measurements) using Leitz microhardness tester at a load of 200 g. The cross section and surface morphology of the coating were observed using a scanning electron microscope. The X-ray diffraction (XRD) analysis on the coating powder and coated surface was done to study the phases present. The surface roughness parameters Ra and Rmax were determined using a surface roughness tester. Residual stress measurements (at the surface level) were carried out on Rigaku X-ray diffractometer employing Cr–Kα target. Sin2 Ψ diffraction method was used. An experimental facility, with a ring type load cell and bridge type fretting pads, which can simulate flat-on-flat contact fretting fatigue conditions was used in fretting fatigue tests. Full details of the test setup are given elsewhere [10]. The contact surfaces of the uncoated pads were polished with four grades (1/ 0, 2/0, 3/0 and 4/0) of silicon carbide paper and cleaned with acetone prior to each test. Fig. 1 shows a schematic view of the uniaxial fretting fatigue test setup employed in the present study. The specimen was gripped and loaded cyclically in
170 150 130 110 90 70
Table 2 Surface residual stress values Condition Uncoated (as machined) Coated (40 μm thickness) Coated (100 μm thickness)
FF uncoated FF coated (40 microns) FF coated (100 microns) PF uncoated PF coated (40 microns) PF coated (100 microns)
50
Surface residual stress (MPa) −57 to − 66 −68 to − 167 +16 to − 85
1.0E+04
1.0E+05
1.0E+06
1.0E+07
No. of Cycles to Failure
Fig. 5. Plain fatigue (PF) and fretting fatigue (FF) test results of uncoated and coated specimens. Arrows indicate non-failure.
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(a) 3000
Friction Force (N)
2500 2000 1500
Uncoated
1000
185 MPa 160 MPa 140 MPa 120 MPa
500 0
1.0E+00
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
(b) 3500
Friction Force (N)
3000 2500 2000 1500 Coated (40 μm)
1000
185 MPa 160 MPa 130 MPa 120 MPa
500 0 1.0E+00
1.0E+02
1.0E+04
1.0E+06
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specimen and fretting pads were recorded and the contact load applied by the proving ring was monitored and maintained constant throughout the test with the help of a data acquisition system (HBM — Spider8 — 600 Hz and catman Express 4.0 software). To measure the displacement of specimen (in the gauge portion) and pad the following method was adopted. Two steel strips were fixed — one on the top of the pad and the other on the specimen surface using an adhesive. Non-contact inductive displacement sensors (Micro-Epsilon make) were positioned above the strips in order to measure the displacement of pad and the specimen during tests. The difference between the two displacement ranges was calculated to obtain the relative slip values. In literature relative slip has been measured by extensometers and/or strain gauges of different configurations (for example, see Refs. [11–13]). The common principle in these procedures is the measurement of relative displacement between two points, one on the fretting pad and other on specimen. Ideally, the two measurement points should be as close as possible to the contact surface. However, this condition cannot be satisfied due to practical experimental difficulties. So the measurements give only the apparent and macroscopic slip value, which are considerably larger than the actual relative slip at the contact surface. It should be noted that the relative slip values reported in the present study are macroscopic or global relative slip values. Observations on fracture surfaces and fretted area were made using a scanning electron microscope (SEM). 3. Results and discussion
No. of Cycles
The detonation spray process resulted in a dense coating of almost uniform deposition with low porosity content and good adhesion between the substrate and the coating without any subsurface cracks (see Fig. 2). SEM observations made on the coating surface indicated partially melted zone with irregular distribution of unmelted particles (Fig. 3). The coated surface was very rough (see Table 1). XRD analysis revealed that same peaks (Cu) were observed both in coating and powder (Fig. 4).
(c) 3500
Friction Force (N)
3000 2500 2000 1500 Coated (100 μm)
1000
185 MPa 160 MPa 130 MPa 120 MPa
500
3500 3000
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
Fig. 6. Variation of friction force with number of fretting cycles at different stress levels for specimens in different conditions: (a) uncoated; (b) coated (40 μm); (c) coated (100 μm).
average contact pressure was calculated by dividing the contact (normal) load by the apparent contact area (= pad foot size × specimen thickness = 2 × 8 = 16 mm 2 ). Friction force between the fretting pads and the specimen was measured by bonding strain gauges to the underside of the fretting pads, with the strain gauge grid centered between the pad feet. During fretting fatigue testing the values of friction force between the
Friction Force (N)
0 1.0E+00
2500 2000 1500 Uncoated Coated (40 microns) Coated (100 microns)
1000 500 100
120
140
160
180
200
Max. Stress (MPa)
Fig. 7. Influence of applied cyclic stress on friction force (at half life) of specimens in different conditions.
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(a) 100
170
Relative Slip Range (μm)
90
Relative Slip Range (μm)
Uncoated 160 MPa 140 MPa
80
120 MPa
70 60
Uncoated Coated (40 microns)
150
Coated (100 microns)
130 110 90 70
50
50 40 1.0E+00
100 1.0E+02
1.0E+04
1.0E+06
160
180
200
Max. Stress (MPa)
Fig. 9. Effect of applied cyclic stress on the relative slip range (at half life) of specimens in different conditions.
220
Relative Slip Range (μm)
140
No. of Cycles
(b) 190
Coated (40 μm)
160
160 MPa 130 MPa
130
120 MPa
stresses arise from two sources — (a) cooling of spray particles from their solidification temperature to substrate temperature and (b) differences between the thermal expansion coefficient values of coating and substrate materials. To develop 100 μm
100
(a)
70
σmax= 120 MPa
10 1.0E+00
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
(c) 140 120
Friction Force (N)
40
Relative Slip Range (μm)
120
Uncoated Coated (40 microns) Coated (100 microns)
100
400 N 20 μm
80 60 Coated (100 μm)
40
185 MPa 160 MPa 130 MPa 120 MPa
20
Relative Slip (μm)
(b)
σmax= 185 MPa
0 1.0E+02
1.0E+04
1.0E+06
No. of Cycles
Fig. 8. Variation of relative slip range with number of fretting cycles for specimens in different conditions: (a) uncoated; (b) coated (40 μm); (c) coated (100 μm).
However, peak broadening was noticed in case of coating indicating heavy strain hardening. The coating material exhibited higher hardness (350 HV0.2) compared with the substrate material (80 HV0.2). A hard coating will improve fretting resistance provided it has good adhesion or bond strength with the substrate. Residual stress at interface plays an important role in affecting the adhesion of the coating with the substrate. In a thermal spraying process, residual
Friction Force (N)
1.0E+00
Uncoated Coated (40 microns) Coated (100 microns)
800 N
50 μm
Relative Slip (μm)
Fig. 10. Fretting hysteresis loops for different specimens tested at two different maximum stress (σmax) levels: (a) 120 MPa; (b) 185 MPa.
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thick coating on the substrate, more number of passes was used compared with 40 μm thick coating. This resulted in different residual stresses in both coatings. This had an influence on the fatigue behaviour. Table 2 lists the values of surface residual stresses for the uncoated and coated specimens. The surface residual stresses in 40 μm thick coating were mainly compressive. On the other hand, 100 μm thick coating had lower values of compressive residual stresses and even tensile residual stresses at the surface in some places. The differences in the residual stresses and the wide range of values in the coating of two different thicknesses may be attributed to difference in the heat dissipation, cooling rates, quenching stresses, relative movement between the work piece and gun and the coefficient of thermal expansion of the substrate and the physiothermal properties of coating material Cu–Ni–In [14]. In the present study, the residual stress measurements were carried out only at the surface of coatings. A thorough analysis of residual stresses beneath the surface and
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more importantly at the interface is necessary to get a clear understanding. Fig. 5 shows the results of plain fatigue and fretting fatigue tests conducted on the uncoated and coated specimens. Fretting fatigue lives of the uncoated and coated specimens were shorter than plain fatigue lives. Fretting reduced the fatigue lives of the uncoated and coated specimens to a large extent. For example, at a maximum stress of 160 MPa both coated and uncoated samples did not fail even after 2 × 106 cycles under plain fatigue loading. On the other hand, under fretting fatigue loading the samples failed even before 105 cycles. At a maximum cyclic stress of 185 MPa the influence of coating thickness on the plain fatigue life may be clearly observed. When the coating thickness was 40 μm, the coated specimen exhibited superior life compared with the uncoated sample. On the other hand, when the coating thickness was 100 μm the coated specimen exhibited inferior life compared with the uncoated sample (the reason for the observed behaviour
Fig. 11. Appearance of untested surface and fretting scar regions in different specimens tested at a maximum stress of 185 MPa: (a) uncoated, untested; (b) uncoated, fretted; (c) coated (40 μm), untested; (d) coated (40 μm), fretted; (e) coated (100 μm), untested; (f) coated (100 μm), fretted.
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Fig. 12. EDAX pattern obtained on the fretting scar region of a 40 μm thick coated specimen tested under fretting fatigue loading.
condition. However, in the present study, coated samples exhibited higher friction force. In a study on Ti–6Al–4V, Fu et al. [15] reported that ion-beam-enhanced deposition of Cu–Ni– In film on Ti–6Al–4V improved fretting fatigue and fretting wear resistance by decreasing friction force. In the present study, Cu–Ni–In coating on AA6063 gave rise to higher friction forces that should have a deleterious effect on fretting fatigue life.
(a) 6 5
Ra (μm)
4 Uncoated
3
Coated (40 microns) Coated (100 microns)
2 1 0 100
120
140
160
180
200
Max. Stress (MPa)
(b) 35 30 25
Rmax (μm)
is given in the latter part of the section). At lower stress levels, all samples, whether uncoated or coated and whether the coating thickness was 40 or 100 μm, did not fail even after 2 × 106 cycles. In case of specimens tested under fretting fatigue loading the coating deteriorated the lives of samples at cyclic stress levels above 120 MPa. Uncoated samples exhibited superior fretting fatigue lives compared with coated specimens. The influence of coating thickness on the fretting fatigue lives may be clearly observed. 100 μm thick coated specimens exhibited inferior fretting fatigue lives compared with 40 μm thick coated samples. At a stress level of 120 MPa, 40 μm thick coated specimen exhibited superior life compared with uncoated and 100 μm thick coated samples. This indicates that if the coating thickness is properly chosen, the fretting fatigue life can be improved. The life of 100 μm thick coated specimen was lower than that of uncoated sample. The reasons for the observed behaviour are given in the latter part of the section. It may be noted that only one sample was tested at each stress level in each condition. With the available limited data, a clear trend has emerged. However, it is a qualitative assessment. D-gun coatings have very low porosity levels (less than 1%) compared with plasma spray coatings having very high porosity levels of 8–15% [14]. In the present study, the porosity of D-gun coatings was 0.3% and the repeatability of process was very good. So it was assumed that the experimental scatter due to the coating process was very small. The variation of friction force with number of fretting fatigue cycles at different stress levels is shown in Fig. 6. At a given stress level friction force was nearly a constant for a major portion of the life except at 185 MPa stress level. In case of 40 μm thick coated specimen the friction force continuously decreased after reaching a nearly constant value at 185 MPa. In the case of 100 μm thick coated sample the friction force continuously increased. The reason for this behaviour is not known. In all cases, the friction force decreased towards the end of the test, which may be related to the growth of macroscopic cracks. The friction force (half life values) increased with applied stress level (see Fig. 7). In general it may be said that the uncoated samples experienced the lowest friction force followed by 40 μm thick coated specimens and then 100 μm thick coated samples. Usually a coating should be chosen in such a way that it would provide lower friction force compared with the uncoated
20
Uncoated Coated (40 microns)
15
Coated (100 microns)
10 5 0
100
120
140
160
180
200
Max. Stress (MPa)
Fig. 13. Relation between surface roughness parameters measured across the fretting scar region and applied cyclic stress: (a) Ra; (b) Rmax.
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Fig. 8 shows the variation of relative slip range with the number of fretting fatigue cycles at different stress levels. As has been already mentioned in Experimental details section, the relative slip values were macroscopic values and the actual relative slip values between the particles in contact would be much lower than these values. Large fluctuations in relative slip values were observed. The relation between maximum cyclic stress and relative slip (corresponding to half life) is shown in Fig. 9. The best fit lines are also given. Lower relative slip values were observed in uncoated samples compared with those in the coated specimens. 40 μm thick coated specimens experienced larger relative slip compared with 100 μm thick coated samples. Fig. 10 shows fretting hysteresis loops. The values of friction force and relative slip corresponding to half life were considered. From the shape of the loops it may be said that partial slip regime was operating in all tests. Due to this Coulomb law was not employed to determine the coefficient of friction. The discussion is made in terms of friction force only. Fig. 11 shows the appearance of fretting scar regions of tested specimens in different conditions. For reference, the appearance of the surface of an untested sample is also shown. EDAX done on the fretting scar of the coating surface revealed the presence of Al and Si peaks in addition to peaks corresponding to Cu, Ni and In (see Fig. 12). This clearly indicated the material transfer from the uncoated Al–Mg–Si alloy pad to the coated specimen surface due to fretting fatigue. The presence of oxygen peak indicated the formation of oxide due to fretting.
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Surface roughness measurements were made across the fretting scar region of the tested specimens. The relation between the surface roughness parameters (Ra and Rmax) and applied cyclic stress is shown in Fig. 13. The values of Ra increased with increasing cyclic stress in all conditions. The fretting damage increased the surface roughness (Ra) of the specimens. In case of the uncoated samples Rmax increased with increasing cyclic stress. However, Rmax remained almost the same for 40 μm thick coated specimens and it decreased for 100 μm thick coated samples with increasing cyclic stress. It may be noted that the coated specimens had rough surfaces with larger undulations prior to fretting fatigue deformation. Due to the deformation, the surface undulations were reduced. As the applied cyclic stress increased, this effect was increased. The Rmax value of 100 μm thick coated specimen surface was higher than that of 40 μm thick coated sample surface before deformation (see Table 1). This could explain the observed variation. Beneficial effect of surface roughness in enhancing fretting fatigue life has been reported in literature [2,16,17]. During fretting, hard oxide debris formed at the contact surface may cause severe abrasion. However, on a rough surface, the wear debris may escape from the contact area into the adjacent depressions instead of causing abrasion damage. Based on this hypothesis, in the present study, the coated samples that had very rough surfaces should exhibit longer fretting fatigue lives. This was not the case. As there were other factors affecting fretting fatigue life, the beneficial influence of surface roughness could not be realized in the present study.
Fig. 14. Appearance of fracture surface of coated samples tested at a maximum stress of 185 MPa under plain fatigue loading: (a) and (b) 40 μm thick coating; (c) and (d) 100 μm thick coating.
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Fig. 14 shows the appearances of fracture surfaces of coated samples tested under plain fatigue loading. Fatigue cracks initiated at the corner and propagated. In case of 40 μm thick coated specimen the adhesion between the coating and substrate
was very good. Crack initiated at the surface of the coating and propagated through the interface into the substrate. As compressive residual stress was present at the surface of the coating and hardness of the coating was higher than that of the
Fig. 15. SEM observations made on the fracture surfaces (a–g) and side surface (h) of coated samples tested under fretting fatigue loading at two stress levels: (a) and (b) 40 μm, 120 MPa; (c) and (d) 40 μm, 185 MPa; (e) 100 μm, 120 MPa; (f)–(h) 100 μm, 185 MPa.
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substrate, the crack initiation resistance was higher compared with the uncoated substrate material. It may be noted that surface roughness has an adverse effect on plain fatigue resistance. 40 μm thick coated samples had very rough surfaces compared with uncoated specimens. However, the adverse effect of surface roughness was overcome by the beneficial effects of higher compressive residual stress and higher surface hardness in 40 μm thick coated samples. So the 40 μm thick coated specimens exhibited superior plain fatigue life compared with the uncoated sample (see Fig. 5). In case of 100 μm thick coated specimen the adhesion between the coating and the substrate was very poor and cracking was observed at the interface (Fig. 14d). Delamination-induced failure was noticed. Interface cracked and the crack propagated through the substrate leading to a reduction in life compared with the life of uncoated specimen. Crack initiation at the interface and the subsequent growth in the substrate are shown in Fig. 14(d). Godoy et al. [18] reported in case of plasma spray coating of NiCrAl on AISI 304 steel that coating adhesion decreased with increasing coating thickness indicating a correlation between adhesion and average residual stress at the interface. Fig. 15 shows SEM observations made on the fracture surfaces and side surface of the tested samples in different conditions under fretting fatigue loading. Fatigue cracks initiated from the contact region of the specimens due to stress concentration effect introduced by fretting. (It may be noted that under plain fatigue loading fatigue cracks initiated from the corners.) In fretting fatigue loading multiple crack initiation sites were observed in the contact region. Severe delaminationinduced fracture was noticed in 100 μm thick coated specimens (see Fig. 15e and h). As the interfacial adhesion was relatively stronger in 40 μm thick coated samples, the interface did not crack easily and no severe delamination was observed. In the coated samples, the cracks initiated at the contact region, propagated into the coating thickness, interface and then penetrated into the substrate. As higher compressive residual stress was present at the surface and interfacial adhesion was relatively stronger in 40 μm thick coated samples, they exhibited superior fretting fatigue lives compared with 100 μm thick coated specimens (see Fig. 5). At 120 MPa stress level 40 μm thick coated specimen exhibited superior fretting fatigue life compared with uncoated sample and above 120 MPa, the converse was true. At higher stress levels cracking was observed along the interface and some amount of delamination was noticed (see Fig. 15c). At 120 MPa this was not observed. This may explain the result that at 120 MPa the 40 μm thick coated specimen exhibited superior fretting fatigue life compared with the uncoated sample. Ahmed and Hadfield have reported the failure modes of thermal spray coatings on a steel substrate under rolling/sliding contact fatigue [19]. They have observed that below a certain contact stress delamination of coating was absent. Also, they have identified the limiting coating thickness below which the delamination was not observed. The results obtained in the present study are in accordance with the results reported by Ahmed and Hadfield [19]. Fu et al. suggested four different mechanisms for increasing fretting resistance of coated components [2]: presence of com-
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pressive residual stress, reduced friction force (coefficient of friction), increased surface hardness and increased surface roughness. The role of compressive residual stress is to close up fretting fatigue cracks at the surface and to prevent their propagation. A low value of friction force is desirable. A higher value of friction force produces higher shear stress on the surface and at the interface between coating and substrate, which can intensify fatigue failure or generate delamination cracks. An increase in surface hardness will prevent adhesion and abrasive wear during fretting. A hard coating will improve fretting resistance provided it has good adhesion or bond strength with the substrate. A rough surface will provide escape routes for oxide debris from the contact area and reduces the possibility of abrasive wear damage. In the present study, the 40 μm thick coated samples exhibited higher compressive residual stress, increased surface hardness, increased surface roughness and higher friction force compared with the uncoated specimens. While the first three parameters are expected to play a beneficial role, the last one is detrimental to fretting fatigue life. It was not possible to identify the relative importance of each factor and their influence on the fretting fatigue life. It may be said that the overall behaviour was decided by the relative importance of each factor and was dependent on the applied cyclic stress level. Also, in case of the coated specimens, there could be a reduction in the coating thickness due to wear during fretting fatigue tests. This was not measured. 4. Conclusions Based on the results obtained in the present study on the effect of detonation gun sprayed Cu–Ni–In coating on Al–Mg– Si alloy the following conclusions were drawn. 1. The surface residual stress was compressive in 40 μm thick coating. On the other hand, tensile residual stress was also observed on the surface of 100 μm thick coating. 2. Fretting reduced the fatigue lives of the uncoated and coated samples to a large extent. 3. The 40 μm thick coated specimens exhibited superior plain fatigue lives compared with 100 μm thick coated samples and uncoated specimens owing to the presence of higher compressive residual stress at the surface of the former. The 100 μm thick coated samples showed inferior performance due to delamination-induced failure. 4. The 40 μm thick coated samples exhibited superior fretting fatigue lives at all stress levels compared with 100 μm coated specimens due to higher surface compressive residual stress and stronger interfacial adhesion. At a stress level of 120 MPa, 40 μm thick coated specimen showed longer fretting fatigue life compared with uncoated sample and at high stress levels above 120 MPa the converse was true. This was attributed to interface cracking at higher stress levels. 5. The 40 μm thick coated samples exhibited higher compressive residual stress, increased surface hardness, increased surface roughness and higher friction force compared with the uncoated specimens. While the first three parameters are
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expected to play a beneficial role, the last one is detrimental to fretting fatigue life. The overall behaviour was decided by the relative importance of each factor and was dependent on the applied cyclic stress level.
[9]
[10]
Acknowledgements [11]
One of the authors (Rajasekaran) is grateful to International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad for the award of a Doctoral Fellowship. References [1] R.B. Waterhouse, Int. Mater. Rev. 37 (1992) 77. [2] Y. Fu, J. Wei, A.W. Batchelor, J. Mater. Process. Technol. 99 (2000) 231. [3] J. Beard, in: R.B. Waterhouse, T.C. Lindley (Eds.), Fretting Fatigue, ESIS, vol. 18, Mechanical Engineering Publications, London, UK, 1994, p. 419. [4] S.C. Gordelier, T.C. Chivers, Wear 56 (1979) 177. [5] T.C. Chivers, S.C. Gordelier, Wear 96 (1984) 153. [6] M.B.K. Gabel, J.J. Bathke, Wear 46 (1978) 81. [7] S. Watanabe, T. Tajiri, N. Sakoda, J. Amano, J. Therm. Spray Technol. 7 (1998) 93. [8] M. Kubota, K. Tsutsni, T. Makino, K. Hirakawa, in: D.W. Hoeppner, V. Chandrasekaran, C.B. Elliott (Eds.), Fretting Fatigue: Current
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[14] [15] [16]
[17] [18] [19]
Technology and Practices, ASTM STP, vol. 1367, ASTM, West Conshohocken, PA, USA, 2000, p. 477. J. Oh, J. Komotori, M. Shimizu, in: C.C. Bendt, K.A. Khor, E.F. Lugscheider (Eds.), Thermal Spray 2001: New Surfaces for a New Millennium, ASM Int., Materials Park, Ohio, USA, 2001, p. 1023. N.K. Ramakrishna Naidu, S. Ganesh Sundara Raman, Int. J. Fatigue 27 (2005) 323. B.U. Wittkowsky, P.R. Birch, J. Dominguez, S. Suresh, Fatigue Fract. Eng. Mater. Struct. 22 (1999) 307. Y. Ochi, Y. Kido, T. Akiyama, T. Matsumura, in: Y. Mutoh, S. Kinyon, D.W. Hoeppner (Eds.), Fretting Fatigue: Advances in Basic Understanding and Applications, ASTM STP, vol. 1425, ASTM, West Conshohocken, PA, USA, 2003, p. 220. A. Puglia, F. Pratesi, G. Zonfrillo, in: R.B. Waterhouse, T.C. Lindley (Eds.), Fretting Fatigue, ESIS, vol. 18, Mechanical Engineering Publications, London, UK, 1994, p. 219. Y.A. Kharlamov, Mater. Sci. Eng. 93 (1987) 1. Y. Fu, N.L. Loh, A.W. Batchelor, D. Liu, X. Zhu, J. He, K. Xu, Surf. Coat. Technol. 106 (1998) 193. R.B. Waterhouse, in: R.B. Waterhouse, T.C. Lindley (Eds.), Fretting Fatigue, ESIS, vol. 18, Mechanical Engineering Publications, London, UK, 1994, p. 339. C.X. Li, Y. Sun, T. Bell, Mater. Sci. Technol. 16 (2000) 1067. C. Godoy, E.A. Souza, M.M. Lima, J.C.A. Batista, Thin Solid Films 420–421 (2002) 438. R. Ahmed, M. Hadfield, J. Therm. Spray Technol. 11 (2002) 333.
Effect of detonation gun sprayed Cu–Ni–In coating on plain fatigue and fretting fatigue behaviour of Al–Mg–Si alloy B. Rajasekaran a , S. Ganesh Sundara Raman a,⁎, S.V. Joshi b , G. Sundararajan b a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai — 600 036, India b International Advanced Research Centre for New Materials and Powder Metallurgy, Hyderabad — 500 005, India Received 6 December 2005; accepted in revised form 15 February 2006 Available online 6 March 2006
Abstract Detonation gun spray technique was employed to coat Al–Mg–Si alloy (AA 6063) specimens with Cu–Ni–In powder. Coated samples were characterized with reference to the microstructure, porosity, residual stresses, microhardness and surface roughness. Plain fatigue (without fretting) and fretting fatigue tests were carried out at room temperature on uncoated and coated specimens. The detonation gun spray process resulted in a dense coating of almost uniform deposition with low porosity (0.3%) and good adhesion between the substrate and the coating. Under plain fatigue loading 40 μm thick coated samples exhibited superior lives compared with uncoated and 100 μm thick coated specimens due to the presence of higher surface compressive residual stress in the former. Delamination-induced failure resulted in inferior lives of 100 μm thick coated specimens. Under fretting fatigue deformation 40 μm thick coated specimens exhibited superior lives compared with 100 μm thick coated samples owing to higher compressive residual stress at the surface and better interfacial adhesion. At 120 MPa stress level 40 μm thick coated specimens exhibited superior fretting fatigue life compared with uncoated sample and at stress levels above 120 MPa the converse was true. This was attributed to interface cracking at higher stress levels. © 2006 Elsevier B.V. All rights reserved. Keywords: Fretting fatigue; Cu–Ni–In; Detonation gun; Residual stress; Al–Mg–Si alloy
1. Introduction Fretting, a small amplitude relative motion between two contacting bodies, greatly accelerates fatigue crack initiation process. The joint action of fretting and fatigue may produce strength reduction factor to 2–5 or even greater [1,2]. Surface modifications employing different processes like physical vapour deposition (PVD), chemical vapour deposition (CVD), ion implantation, thermal spraying and laser treatments are widely used to mitigate fretting damages [3–6]. Hard coatings deposited by spraying can provide high wear resistance but there is limited information on their application in fretting fatigue. Most of these types of coatings have been employed to inhibit fretting wear rather than fretting fatigue.
⁎ Corresponding author. Tel.: +91 44 2257 4768; fax: +91 44 2257 0509. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.023
Generally, the fatigue properties of thermal sprayed materials are controlled by many factors such as coating thickness, adhesion, residual stresses, and so on. There are few reports available in open literature dealing with the influence of coating thickness on the fatigue properties of spray-coated materials (e.g. [7–9]). Watanabe et al. reported that the rotating bending fatigue limit of 50 μm thick coated specimens was higher than that of 100 μm thick coated samples in case of a medium carbon steel coated with high velocity oxy fuel thermally sprayed WC– Co powder coating [7]. This was attributed to good cohesion between particles and good adhesion at the interface between the coating and substrate of 50 μm thick coated samples. Kubota et al. reported that the axial fatigue strength of AISI 1038 steel samples sprayed with molybdenum coating of 0.5 mm thickness was higher than that of specimens sprayed with 0.25 mm thick coating [8]. In a study on a medium carbon steel coated with gas flame thermally sprayed Co-based alloy, Oh et al. reported that the rotating bending fatigue strength of the specimens with 1 mm
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Proving Ring
Load Adjusting Screw
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Test Specimen
Fretting Pad
Fig. 3. SEM picture showing the coating surface.
plain fatigue (without fretting) behaviour was investigated. The effect of coating thickness was also studied. 2. Experimental details Fig. 1. Flat-on-flat contact bridge type uniaxial fretting fatigue test setup used in the present study.
thick coating was remarkably higher compared with that of samples with 0.3 and 0.5 mm thick coating. This was explained in terms of differences in the failure mechanisms. While fatigue cracks initiated from the porosity located in the coating of 1 mm thickness, they initiated inside the substrate of samples having a coating thickness of 0.3 and 0.5 mm. Detonation gun (D-gun) spray coating process is a thermal spray process, which gives an extremely good adhesive strength, low porosity and coating surfaces with compressive residual stresses. In general, any surface coating will alter the nature of the contact surface by changing its surface microstructure, surface hardness, surface chemistry, coefficient of friction, surface roughness and surface residual stress. D-gun coating also imparts these changes in the surface properties on the coated surfaces. The present work was undertaken to study the influence of the D-gun sprayed Cu–Ni–In coating on the fretting fatigue behaviour of Al–Mg–Si alloy (AA6063). For reference
Substrate material AA 6063 of chemical composition (wt.%) 0.44 Mg, 0.41 Si, 0.35 Fe, 0.1 Cu, 0.1 Mn, 0.1 Zn, 0.1 Ti, 0.1 Cr and rest Al was used to prepare test specimens and fretting pads. The gauge dimensions of the specimens were: 8 mm thickness, 10 mm width and 65 mm length. The specimens were solution treated at 540 °C for 1 h and water quenched to room temperature. Subsequently they were aged at 160 °C for 12 h. The room temperature mechanical properties of the substrate material were: 0.2% yield strength = 208 MPa, ultimate tensile strength = 247 MPa, elongation = 21% and hardness = 80 HV0.2. Cu–36Ni–5In powder having spherical particles of size in the range of 15–40 μm was used as a coating material. Prior to coating process, the test specimens were cleaned, degreased and shot blasted using alumina grit of 60-mesh size and then ultrasonically cleaned using acetone. The specimens were fixed in the specimen holder of AWAAZ detonation spray coating system (horizontal type) and then sprayed with Cu–Ni–In powder. The gun barrel length was 1.3 m and diameter was 22 mm. Dgun coating was carried out using a combustible mixture of O2 and C2H2 with a frequency of 3 Hz and a spraying distance of 180 mm. Multipass coating of Cu–Ni–In was formulated on all four sides of the gauge portions of the fatigue specimens to achieve the desired coating thickness. Two sets of specimens were prepared — one with a coating thickness of 40 μm and the other with 100 μm. Porosity of the coating was determined as 0.3% using an image analyzer system on the basis of measurements carried out in 10 different regions. Microhardness of coating was found out Table 1 Characteristics of D-gun sprayed Cu–Ni–In coating and substrate AA 6063 Thickness
Fig. 2. Optical micrograph showing the cross section. Indentation made during hardness testing can be seen.
Porosity
Hardness
Roughness (μm)
(μm)
(%)
(HV0.2)
Ra
Rmax
40 100 Substrate
0.30 0.30 –
350 350 80
4.80 6.33 0.60
33.69 34.15 3.83
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Fig. 4. XRD analysis done on: (a) Cu–Ni–In powder; (b) coating.
Schenck servo-hydraulic testing machine. Normal (contact) load was applied using a proving ring with the help of load adjusting screws. Plain fatigue and fretting fatigue tests were conducted on uncoated and coated specimens at room temperature with a stress ratio of 0.1 at different maximum cyclic stresses. For plain fatigue tests the cycle frequency was 30 Hz and for fretting fatigue tests it was 10 Hz. The fretting fatigue tests were carried out at a constant average contact pressure of 100 MPa. The 210 190
Max. Stress (MPa)
as 350 HV0.2 (average of 20 measurements) using Leitz microhardness tester at a load of 200 g. The cross section and surface morphology of the coating were observed using a scanning electron microscope. The X-ray diffraction (XRD) analysis on the coating powder and coated surface was done to study the phases present. The surface roughness parameters Ra and Rmax were determined using a surface roughness tester. Residual stress measurements (at the surface level) were carried out on Rigaku X-ray diffractometer employing Cr–Kα target. Sin2 Ψ diffraction method was used. An experimental facility, with a ring type load cell and bridge type fretting pads, which can simulate flat-on-flat contact fretting fatigue conditions was used in fretting fatigue tests. Full details of the test setup are given elsewhere [10]. The contact surfaces of the uncoated pads were polished with four grades (1/ 0, 2/0, 3/0 and 4/0) of silicon carbide paper and cleaned with acetone prior to each test. Fig. 1 shows a schematic view of the uniaxial fretting fatigue test setup employed in the present study. The specimen was gripped and loaded cyclically in
170 150 130 110 90 70
Table 2 Surface residual stress values Condition Uncoated (as machined) Coated (40 μm thickness) Coated (100 μm thickness)
FF uncoated FF coated (40 microns) FF coated (100 microns) PF uncoated PF coated (40 microns) PF coated (100 microns)
50
Surface residual stress (MPa) −57 to − 66 −68 to − 167 +16 to − 85
1.0E+04
1.0E+05
1.0E+06
1.0E+07
No. of Cycles to Failure
Fig. 5. Plain fatigue (PF) and fretting fatigue (FF) test results of uncoated and coated specimens. Arrows indicate non-failure.
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(a) 3000
Friction Force (N)
2500 2000 1500
Uncoated
1000
185 MPa 160 MPa 140 MPa 120 MPa
500 0
1.0E+00
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
(b) 3500
Friction Force (N)
3000 2500 2000 1500 Coated (40 μm)
1000
185 MPa 160 MPa 130 MPa 120 MPa
500 0 1.0E+00
1.0E+02
1.0E+04
1.0E+06
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specimen and fretting pads were recorded and the contact load applied by the proving ring was monitored and maintained constant throughout the test with the help of a data acquisition system (HBM — Spider8 — 600 Hz and catman Express 4.0 software). To measure the displacement of specimen (in the gauge portion) and pad the following method was adopted. Two steel strips were fixed — one on the top of the pad and the other on the specimen surface using an adhesive. Non-contact inductive displacement sensors (Micro-Epsilon make) were positioned above the strips in order to measure the displacement of pad and the specimen during tests. The difference between the two displacement ranges was calculated to obtain the relative slip values. In literature relative slip has been measured by extensometers and/or strain gauges of different configurations (for example, see Refs. [11–13]). The common principle in these procedures is the measurement of relative displacement between two points, one on the fretting pad and other on specimen. Ideally, the two measurement points should be as close as possible to the contact surface. However, this condition cannot be satisfied due to practical experimental difficulties. So the measurements give only the apparent and macroscopic slip value, which are considerably larger than the actual relative slip at the contact surface. It should be noted that the relative slip values reported in the present study are macroscopic or global relative slip values. Observations on fracture surfaces and fretted area were made using a scanning electron microscope (SEM). 3. Results and discussion
No. of Cycles
The detonation spray process resulted in a dense coating of almost uniform deposition with low porosity content and good adhesion between the substrate and the coating without any subsurface cracks (see Fig. 2). SEM observations made on the coating surface indicated partially melted zone with irregular distribution of unmelted particles (Fig. 3). The coated surface was very rough (see Table 1). XRD analysis revealed that same peaks (Cu) were observed both in coating and powder (Fig. 4).
(c) 3500
Friction Force (N)
3000 2500 2000 1500 Coated (100 μm)
1000
185 MPa 160 MPa 130 MPa 120 MPa
500
3500 3000
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
Fig. 6. Variation of friction force with number of fretting cycles at different stress levels for specimens in different conditions: (a) uncoated; (b) coated (40 μm); (c) coated (100 μm).
average contact pressure was calculated by dividing the contact (normal) load by the apparent contact area (= pad foot size × specimen thickness = 2 × 8 = 16 mm 2 ). Friction force between the fretting pads and the specimen was measured by bonding strain gauges to the underside of the fretting pads, with the strain gauge grid centered between the pad feet. During fretting fatigue testing the values of friction force between the
Friction Force (N)
0 1.0E+00
2500 2000 1500 Uncoated Coated (40 microns) Coated (100 microns)
1000 500 100
120
140
160
180
200
Max. Stress (MPa)
Fig. 7. Influence of applied cyclic stress on friction force (at half life) of specimens in different conditions.
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(a) 100
170
Relative Slip Range (μm)
90
Relative Slip Range (μm)
Uncoated 160 MPa 140 MPa
80
120 MPa
70 60
Uncoated Coated (40 microns)
150
Coated (100 microns)
130 110 90 70
50
50 40 1.0E+00
100 1.0E+02
1.0E+04
1.0E+06
160
180
200
Max. Stress (MPa)
Fig. 9. Effect of applied cyclic stress on the relative slip range (at half life) of specimens in different conditions.
220
Relative Slip Range (μm)
140
No. of Cycles
(b) 190
Coated (40 μm)
160
160 MPa 130 MPa
130
120 MPa
stresses arise from two sources — (a) cooling of spray particles from their solidification temperature to substrate temperature and (b) differences between the thermal expansion coefficient values of coating and substrate materials. To develop 100 μm
100
(a)
70
σmax= 120 MPa
10 1.0E+00
1.0E+02
1.0E+04
1.0E+06
No. of Cycles
(c) 140 120
Friction Force (N)
40
Relative Slip Range (μm)
120
Uncoated Coated (40 microns) Coated (100 microns)
100
400 N 20 μm
80 60 Coated (100 μm)
40
185 MPa 160 MPa 130 MPa 120 MPa
20
Relative Slip (μm)
(b)
σmax= 185 MPa
0 1.0E+02
1.0E+04
1.0E+06
No. of Cycles
Fig. 8. Variation of relative slip range with number of fretting cycles for specimens in different conditions: (a) uncoated; (b) coated (40 μm); (c) coated (100 μm).
However, peak broadening was noticed in case of coating indicating heavy strain hardening. The coating material exhibited higher hardness (350 HV0.2) compared with the substrate material (80 HV0.2). A hard coating will improve fretting resistance provided it has good adhesion or bond strength with the substrate. Residual stress at interface plays an important role in affecting the adhesion of the coating with the substrate. In a thermal spraying process, residual
Friction Force (N)
1.0E+00
Uncoated Coated (40 microns) Coated (100 microns)
800 N
50 μm
Relative Slip (μm)
Fig. 10. Fretting hysteresis loops for different specimens tested at two different maximum stress (σmax) levels: (a) 120 MPa; (b) 185 MPa.
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thick coating on the substrate, more number of passes was used compared with 40 μm thick coating. This resulted in different residual stresses in both coatings. This had an influence on the fatigue behaviour. Table 2 lists the values of surface residual stresses for the uncoated and coated specimens. The surface residual stresses in 40 μm thick coating were mainly compressive. On the other hand, 100 μm thick coating had lower values of compressive residual stresses and even tensile residual stresses at the surface in some places. The differences in the residual stresses and the wide range of values in the coating of two different thicknesses may be attributed to difference in the heat dissipation, cooling rates, quenching stresses, relative movement between the work piece and gun and the coefficient of thermal expansion of the substrate and the physiothermal properties of coating material Cu–Ni–In [14]. In the present study, the residual stress measurements were carried out only at the surface of coatings. A thorough analysis of residual stresses beneath the surface and
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more importantly at the interface is necessary to get a clear understanding. Fig. 5 shows the results of plain fatigue and fretting fatigue tests conducted on the uncoated and coated specimens. Fretting fatigue lives of the uncoated and coated specimens were shorter than plain fatigue lives. Fretting reduced the fatigue lives of the uncoated and coated specimens to a large extent. For example, at a maximum stress of 160 MPa both coated and uncoated samples did not fail even after 2 × 106 cycles under plain fatigue loading. On the other hand, under fretting fatigue loading the samples failed even before 105 cycles. At a maximum cyclic stress of 185 MPa the influence of coating thickness on the plain fatigue life may be clearly observed. When the coating thickness was 40 μm, the coated specimen exhibited superior life compared with the uncoated sample. On the other hand, when the coating thickness was 100 μm the coated specimen exhibited inferior life compared with the uncoated sample (the reason for the observed behaviour
Fig. 11. Appearance of untested surface and fretting scar regions in different specimens tested at a maximum stress of 185 MPa: (a) uncoated, untested; (b) uncoated, fretted; (c) coated (40 μm), untested; (d) coated (40 μm), fretted; (e) coated (100 μm), untested; (f) coated (100 μm), fretted.
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Fig. 12. EDAX pattern obtained on the fretting scar region of a 40 μm thick coated specimen tested under fretting fatigue loading.
condition. However, in the present study, coated samples exhibited higher friction force. In a study on Ti–6Al–4V, Fu et al. [15] reported that ion-beam-enhanced deposition of Cu–Ni– In film on Ti–6Al–4V improved fretting fatigue and fretting wear resistance by decreasing friction force. In the present study, Cu–Ni–In coating on AA6063 gave rise to higher friction forces that should have a deleterious effect on fretting fatigue life.
(a) 6 5
Ra (μm)
4 Uncoated
3
Coated (40 microns) Coated (100 microns)
2 1 0 100
120
140
160
180
200
Max. Stress (MPa)
(b) 35 30 25
Rmax (μm)
is given in the latter part of the section). At lower stress levels, all samples, whether uncoated or coated and whether the coating thickness was 40 or 100 μm, did not fail even after 2 × 106 cycles. In case of specimens tested under fretting fatigue loading the coating deteriorated the lives of samples at cyclic stress levels above 120 MPa. Uncoated samples exhibited superior fretting fatigue lives compared with coated specimens. The influence of coating thickness on the fretting fatigue lives may be clearly observed. 100 μm thick coated specimens exhibited inferior fretting fatigue lives compared with 40 μm thick coated samples. At a stress level of 120 MPa, 40 μm thick coated specimen exhibited superior life compared with uncoated and 100 μm thick coated samples. This indicates that if the coating thickness is properly chosen, the fretting fatigue life can be improved. The life of 100 μm thick coated specimen was lower than that of uncoated sample. The reasons for the observed behaviour are given in the latter part of the section. It may be noted that only one sample was tested at each stress level in each condition. With the available limited data, a clear trend has emerged. However, it is a qualitative assessment. D-gun coatings have very low porosity levels (less than 1%) compared with plasma spray coatings having very high porosity levels of 8–15% [14]. In the present study, the porosity of D-gun coatings was 0.3% and the repeatability of process was very good. So it was assumed that the experimental scatter due to the coating process was very small. The variation of friction force with number of fretting fatigue cycles at different stress levels is shown in Fig. 6. At a given stress level friction force was nearly a constant for a major portion of the life except at 185 MPa stress level. In case of 40 μm thick coated specimen the friction force continuously decreased after reaching a nearly constant value at 185 MPa. In the case of 100 μm thick coated sample the friction force continuously increased. The reason for this behaviour is not known. In all cases, the friction force decreased towards the end of the test, which may be related to the growth of macroscopic cracks. The friction force (half life values) increased with applied stress level (see Fig. 7). In general it may be said that the uncoated samples experienced the lowest friction force followed by 40 μm thick coated specimens and then 100 μm thick coated samples. Usually a coating should be chosen in such a way that it would provide lower friction force compared with the uncoated
20
Uncoated Coated (40 microns)
15
Coated (100 microns)
10 5 0
100
120
140
160
180
200
Max. Stress (MPa)
Fig. 13. Relation between surface roughness parameters measured across the fretting scar region and applied cyclic stress: (a) Ra; (b) Rmax.
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Fig. 8 shows the variation of relative slip range with the number of fretting fatigue cycles at different stress levels. As has been already mentioned in Experimental details section, the relative slip values were macroscopic values and the actual relative slip values between the particles in contact would be much lower than these values. Large fluctuations in relative slip values were observed. The relation between maximum cyclic stress and relative slip (corresponding to half life) is shown in Fig. 9. The best fit lines are also given. Lower relative slip values were observed in uncoated samples compared with those in the coated specimens. 40 μm thick coated specimens experienced larger relative slip compared with 100 μm thick coated samples. Fig. 10 shows fretting hysteresis loops. The values of friction force and relative slip corresponding to half life were considered. From the shape of the loops it may be said that partial slip regime was operating in all tests. Due to this Coulomb law was not employed to determine the coefficient of friction. The discussion is made in terms of friction force only. Fig. 11 shows the appearance of fretting scar regions of tested specimens in different conditions. For reference, the appearance of the surface of an untested sample is also shown. EDAX done on the fretting scar of the coating surface revealed the presence of Al and Si peaks in addition to peaks corresponding to Cu, Ni and In (see Fig. 12). This clearly indicated the material transfer from the uncoated Al–Mg–Si alloy pad to the coated specimen surface due to fretting fatigue. The presence of oxygen peak indicated the formation of oxide due to fretting.
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Surface roughness measurements were made across the fretting scar region of the tested specimens. The relation between the surface roughness parameters (Ra and Rmax) and applied cyclic stress is shown in Fig. 13. The values of Ra increased with increasing cyclic stress in all conditions. The fretting damage increased the surface roughness (Ra) of the specimens. In case of the uncoated samples Rmax increased with increasing cyclic stress. However, Rmax remained almost the same for 40 μm thick coated specimens and it decreased for 100 μm thick coated samples with increasing cyclic stress. It may be noted that the coated specimens had rough surfaces with larger undulations prior to fretting fatigue deformation. Due to the deformation, the surface undulations were reduced. As the applied cyclic stress increased, this effect was increased. The Rmax value of 100 μm thick coated specimen surface was higher than that of 40 μm thick coated sample surface before deformation (see Table 1). This could explain the observed variation. Beneficial effect of surface roughness in enhancing fretting fatigue life has been reported in literature [2,16,17]. During fretting, hard oxide debris formed at the contact surface may cause severe abrasion. However, on a rough surface, the wear debris may escape from the contact area into the adjacent depressions instead of causing abrasion damage. Based on this hypothesis, in the present study, the coated samples that had very rough surfaces should exhibit longer fretting fatigue lives. This was not the case. As there were other factors affecting fretting fatigue life, the beneficial influence of surface roughness could not be realized in the present study.
Fig. 14. Appearance of fracture surface of coated samples tested at a maximum stress of 185 MPa under plain fatigue loading: (a) and (b) 40 μm thick coating; (c) and (d) 100 μm thick coating.
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Fig. 14 shows the appearances of fracture surfaces of coated samples tested under plain fatigue loading. Fatigue cracks initiated at the corner and propagated. In case of 40 μm thick coated specimen the adhesion between the coating and substrate
was very good. Crack initiated at the surface of the coating and propagated through the interface into the substrate. As compressive residual stress was present at the surface of the coating and hardness of the coating was higher than that of the
Fig. 15. SEM observations made on the fracture surfaces (a–g) and side surface (h) of coated samples tested under fretting fatigue loading at two stress levels: (a) and (b) 40 μm, 120 MPa; (c) and (d) 40 μm, 185 MPa; (e) 100 μm, 120 MPa; (f)–(h) 100 μm, 185 MPa.
B. Rajasekaran et al. / Surface & Coatings Technology 201 (2006) 1548–1558
substrate, the crack initiation resistance was higher compared with the uncoated substrate material. It may be noted that surface roughness has an adverse effect on plain fatigue resistance. 40 μm thick coated samples had very rough surfaces compared with uncoated specimens. However, the adverse effect of surface roughness was overcome by the beneficial effects of higher compressive residual stress and higher surface hardness in 40 μm thick coated samples. So the 40 μm thick coated specimens exhibited superior plain fatigue life compared with the uncoated sample (see Fig. 5). In case of 100 μm thick coated specimen the adhesion between the coating and the substrate was very poor and cracking was observed at the interface (Fig. 14d). Delamination-induced failure was noticed. Interface cracked and the crack propagated through the substrate leading to a reduction in life compared with the life of uncoated specimen. Crack initiation at the interface and the subsequent growth in the substrate are shown in Fig. 14(d). Godoy et al. [18] reported in case of plasma spray coating of NiCrAl on AISI 304 steel that coating adhesion decreased with increasing coating thickness indicating a correlation between adhesion and average residual stress at the interface. Fig. 15 shows SEM observations made on the fracture surfaces and side surface of the tested samples in different conditions under fretting fatigue loading. Fatigue cracks initiated from the contact region of the specimens due to stress concentration effect introduced by fretting. (It may be noted that under plain fatigue loading fatigue cracks initiated from the corners.) In fretting fatigue loading multiple crack initiation sites were observed in the contact region. Severe delaminationinduced fracture was noticed in 100 μm thick coated specimens (see Fig. 15e and h). As the interfacial adhesion was relatively stronger in 40 μm thick coated samples, the interface did not crack easily and no severe delamination was observed. In the coated samples, the cracks initiated at the contact region, propagated into the coating thickness, interface and then penetrated into the substrate. As higher compressive residual stress was present at the surface and interfacial adhesion was relatively stronger in 40 μm thick coated samples, they exhibited superior fretting fatigue lives compared with 100 μm thick coated specimens (see Fig. 5). At 120 MPa stress level 40 μm thick coated specimen exhibited superior fretting fatigue life compared with uncoated sample and above 120 MPa, the converse was true. At higher stress levels cracking was observed along the interface and some amount of delamination was noticed (see Fig. 15c). At 120 MPa this was not observed. This may explain the result that at 120 MPa the 40 μm thick coated specimen exhibited superior fretting fatigue life compared with the uncoated sample. Ahmed and Hadfield have reported the failure modes of thermal spray coatings on a steel substrate under rolling/sliding contact fatigue [19]. They have observed that below a certain contact stress delamination of coating was absent. Also, they have identified the limiting coating thickness below which the delamination was not observed. The results obtained in the present study are in accordance with the results reported by Ahmed and Hadfield [19]. Fu et al. suggested four different mechanisms for increasing fretting resistance of coated components [2]: presence of com-
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pressive residual stress, reduced friction force (coefficient of friction), increased surface hardness and increased surface roughness. The role of compressive residual stress is to close up fretting fatigue cracks at the surface and to prevent their propagation. A low value of friction force is desirable. A higher value of friction force produces higher shear stress on the surface and at the interface between coating and substrate, which can intensify fatigue failure or generate delamination cracks. An increase in surface hardness will prevent adhesion and abrasive wear during fretting. A hard coating will improve fretting resistance provided it has good adhesion or bond strength with the substrate. A rough surface will provide escape routes for oxide debris from the contact area and reduces the possibility of abrasive wear damage. In the present study, the 40 μm thick coated samples exhibited higher compressive residual stress, increased surface hardness, increased surface roughness and higher friction force compared with the uncoated specimens. While the first three parameters are expected to play a beneficial role, the last one is detrimental to fretting fatigue life. It was not possible to identify the relative importance of each factor and their influence on the fretting fatigue life. It may be said that the overall behaviour was decided by the relative importance of each factor and was dependent on the applied cyclic stress level. Also, in case of the coated specimens, there could be a reduction in the coating thickness due to wear during fretting fatigue tests. This was not measured. 4. Conclusions Based on the results obtained in the present study on the effect of detonation gun sprayed Cu–Ni–In coating on Al–Mg– Si alloy the following conclusions were drawn. 1. The surface residual stress was compressive in 40 μm thick coating. On the other hand, tensile residual stress was also observed on the surface of 100 μm thick coating. 2. Fretting reduced the fatigue lives of the uncoated and coated samples to a large extent. 3. The 40 μm thick coated specimens exhibited superior plain fatigue lives compared with 100 μm thick coated samples and uncoated specimens owing to the presence of higher compressive residual stress at the surface of the former. The 100 μm thick coated samples showed inferior performance due to delamination-induced failure. 4. The 40 μm thick coated samples exhibited superior fretting fatigue lives at all stress levels compared with 100 μm coated specimens due to higher surface compressive residual stress and stronger interfacial adhesion. At a stress level of 120 MPa, 40 μm thick coated specimen showed longer fretting fatigue life compared with uncoated sample and at high stress levels above 120 MPa the converse was true. This was attributed to interface cracking at higher stress levels. 5. The 40 μm thick coated samples exhibited higher compressive residual stress, increased surface hardness, increased surface roughness and higher friction force compared with the uncoated specimens. While the first three parameters are
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expected to play a beneficial role, the last one is detrimental to fretting fatigue life. The overall behaviour was decided by the relative importance of each factor and was dependent on the applied cyclic stress level.
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