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TiAl on AISI 4140 steel
Surface & Coatings Technology 201 (2006) 1511 – 1518 www.elsevier.com/locate/surfcoat
Fretting wear of multilayered PVD TiAlCN/TiAlN/TiAl on AISI 4140 steel A. Ben Cheikh Larbi ⁎, B. Tlili Laboratoire de Mécanique Matériaux et Procédés Ecole Supérieure des Sciences et Techniques de Tunis, 1008 Tunisia Received 21 July 2005; accepted in revised form 11 February 2006 Available online 6 March 2006
Abstract Physical vapor deposition (PVD) coatings are recommended for metallic components to mitigate the damage induced by fretting wear. Several experimental investigations have led to the development of a TiAlCN/TiAlCN/TiAl coating in preference to the traditional TiN coating. The present work enabled quantification of the resistance to fretting wear of a TiAlCN/TiAlCN/TiAl multilayer coating deposited by reactive DC magnetron sputtering of Ti–Al alloys on AISI 4140 steel. Experimental simulations of the fretting wear induced by 20,000 cycles at a frequency of 5 Hz made it possible to establish a fretting map and delimit the partial-slip and gross-slip regions. The influence of normal force and displacement amplitude on the coefficients of instantaneous and stabilized friction is deferred for coated and uncoated steels. The PVD coating reduces the friction. The relation between worn volume and dissipated cumulated energy was established for coated and uncoated steels, and the energetic wear coefficients of fretting wear were deduced. The multilayer TiAlCN/TiAlCN/TiAl coating reduces the energetic wear coefficient, which is synonymous with improving the resistance to fretting wear. © 2006 Elsevier B.V. All rights reserved. Keywords: Fretting wear; PVD; TiAlCN; Fretting map; Friction coefficient; Dissipated energy
1. Introduction The performances of metallic components employed in mechanical applications depend mainly on the type and level of solicitation imposed, as well as on factors such as the structure and the surface and mass mechanical characteristics of the component [1]. Among the solicitations frequently encountered in mechanical structures, fretting remains one of the modern plagues of the mechanical engineering industry [2]. Waterhouse described it as an established phenomenon when two surfaces in contact undergo a reciprocal displacement of amplitude, limited in general to 150 μm [3]. According to the intensity of the imposed parameters (normal force, amplitude of displacement and frequency) and the local environment of the surfaces in contact, fretting wear, fretting fatigue or fretting corrosion are caused, resulting in wear and cracking. These degradations strongly limit the lifespan of the components.
⁎ Corresponding author. E-mail address: [email protected] (A. Ben Cheikh Larbi). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.025
Various investigations have been carried out to determine the behavior in fretting wear of many materials. Three types of study are commonly employed. 1. Establishment of a fretting map: Vingsbo and Sodeberg [4] developed running condition fretting maps (RCFMs) and materials response fretting maps (MRFMs). RCFMs delimit the sliding regime. According to normal force and displacement amplitude, one distinguishes the stick situation, the mixed-slip and gross-slip regimes. It is established that the mixed-slip regime is associated with cracking, whereas the gross-slip regime is associated with wear [5]. 2. Determination of the tribological characteristics and the specific wear rate: the wear coefficient is provided by the relation of Archard, V = KFL / H, where V is the worn volume, F the normal force, L the kinematics distance, H the hardness and K the wear coefficient. A new energy approach developed by Celis et al. [6] and Fouvry et al. [7] allows determination of an energy coefficient of wear by establishing the relation between worn volume and cumulated energy dissipated in friction during fretting. These various coefficients represent a useful measurement of resistance to fretting.
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Table 1 Chemical composition of AISI 4140 steel
2. Experimental details
C Mn Si S Cr P Mo
0.410 0.770 0.280 0.026 1.020 0.019 0.160
3. Identification of the dominant mechanisms of degradation: according to the nature of the bodies in contact and the operating parameters, the modes of degradation most often seen are adhesive or abrasive or oxidative wear, detachment of articles and cracking. The methods used to mitigate the effects of fretting are multiple and varied. Among them, PVD coatings offer an efficient and economically viable solution [2,8]. There are currently several different processes of PVD coating, providing monolayers, multilayers or nanostructured layers. The reference coating remains titanium nitride. A TiN coating on mechanical components improves their lifespan owing to a structured composition, offering good tribological properties, and particularly a low friction coefficient in fretting (about 0.15 to 0.2 [9]). However, TiN coatings show some disadvantages such as weak resistance to oxidation and poor adherence [10]. In order to remedy these imperfections, additional constituents such as aluminum and/or carbon are recommended. The addition of aluminum to TiN coating provides a ternary coating, TiAlN, which has better resistance to oxidation and improved mechanical properties [11] but weakened tribolic capacities [12]. The addition of carbon to TiAlN coating yields a quaternary coating, TiAlCN, which has good adherence [13]. Although some studies are available on TiAlCN layers and their behavior in terms of wear and fatigue [14,15], data on resistance to fretting wear are not available. In this article, we present the results of our studies on the behavior in fretting wear of AISI 4140 steel and the multilayer TiAlCN/TiAlN/TiAl deposited by DC magnetron sputtering. The main purpose of our study was to measure the effect of this coating on steel against fretting wear over a wide range of normal forces and imposed slip amplitudes.
Table 2 Deposition conditions
Unit
Target Bias power (kW) (V)
2.1. Substrate The substrate material used in the present study was commercial AISI 4140 steel with the composition presented in Table 1. The substrates were hardened and tempered to a hardness of 420 HV0.05. They were ground, polished and cleaned with trichloroethylene, acetone and alcohol in an ultrasonic cleaner. 2.2. Deposition TiAlCN/TiAlN/TiAl coatings were deposited on AISI 4140 steel by reactive DC magnetron sputtering from high purity Ti– Al targets (50% at Ti, 50% at Al). The substrates were mounted on a continuously rotating planetary holder inside the vacuum chamber. The atmosphere was chosen in order to produce successively a TiAl underlayer, a TiAlN buffer layer and then a TiAlCN layer. The TiAl and TiAlN films were deposited onto the substrate to improve adhesion of the TiAlCN [13]. Optimum deposition conditions such as target power, bias voltage, temperature, rotational velocity, deposition time and gas flow were determined in preliminary studies. Table 2 shows the detailed deposition conditions. Argon, nitrogen and acetylene gas of very high purity (99.999%) was introduced into the vacuum chamber. The base pressure in the deposition chamber was 5 · 10−5 Pa, which rose to 0.1 Pa during deposition. The distance between the target and surface substrate was 35 mm. Prior to deposition, the surface substrates were further cleaned by argon ion bombardment. 2.3. Characterization of the coating The surface morphology of the TiAlCN coating was examined by atomic force microscopy (AFM). Microstructural characterization of the deposited film was investigated by Xray diffraction (Philips X'pert system). Scans were carried out in the grazing angle mode with an incident beam angle of 3° and the normal θ − 2θ method classically used in the same situation [16,17]. Young's modulus and hardness were measured by nanoindentation tests with a nanoindenter MTS-XP. The indentation was performed using a triangular Berkovich diamond pyramid.
TiAlCN (1000 nm) Temper Rotation Time Gas flow velocity (°C) (rpm) (mn) Ar N2 (sccm)
TiAl 6 TiAlN 7.5
− 300 250 − 200 150
0.7 0.6
22 17
TiAlCN 7.5
− 200 100
0.6
42
400 770– 800 950
TiAlN (200 nm) C2H2
– – 90–150 – 150–135 20–135
TiAl (400 nm) Substrate
Fig. 1. Distribution and thickness of the layers in the coating.
A. Ben Cheikh Larbi, B. Tlili / Surface & Coatings Technology 201 (2006) 1511–1518
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Fig. 2. AFM morphologies of surface layer TiAlCN.
2.4. Fretting tests The fretting tests were carried out on an MTS tension compression hydraulic machine. A sphere-onplane configuration was employed. The counter-body was a polycrystalline alumina ball with of diameter 24 mm, Young's modulus 310 GPa and hardness 2300 HV0.05. The coating steel flat specimen was 10 mm thick. During the test, the instantaneous displacement, the normal force and the tangential force were monitored and recorded for every cycle. The experimental parameters were selected as displacement amplitudes of 25, 50 and 100 μm, and normal forces of 50, 100, 200, 500 and 750 N. The tests were of 20,000 cycles and the frequency was set at 5 Hz. The fretting tests were conducted in dry conditions at an ambient temperature 25 °C and relative humidity 60%. Prior to the fretting tests, the specimen and counter-body were cleaned with acetone and alcohol. After each test, the mor-
phology of the wear scar was observed by scanning electron microscopy (SEM). The profile of the fretting scar was assessed using surface roughness profilometry and the wear volumes deduced. 3. Results and discussion 3.1. Coating SEM and AFM observations allowed determination of the global coating thickness and the morphology of the surface respectively. The coating had a mean thickness of 1.6 μm distributed in three layers as shown in Fig. 1. The surface was globally uniform with some domes and tiny craters spread all over the area (Fig. 2). Dimensional measurements showed that the
Intensity
(200)
(220) (111)
30
40
50
60
70
80
2 θ CuKα [°] Fig. 3. XRD diffractogram of TiAlCN/TiAlN/TiAl multilayer coating deposited onto AISI 4140 steel.
Fig. 4. Scanning electron micrograph of a cross-section of multilayered TiAlCN/ TiAlN/TiAl coating as made by fractography.
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Load (mN)
1600
Ft
Ft
1200
δ
800
δ
400
Ft
Ft
0 0
1000
2000
3000
4000
Displacement (nm)
δ
δ
Fig. 5. Typical loading–unloading curves for TiAlCN/TiAlN/TiAl multilayer coating.
3.2. Fretting map The evolution of the hysteresis buckles made up by the tangential force and movement during fretting cycles allows the sliding rates of each surface layer to be distinguished. For applied operating parameters in the range FN = 20 N to 750 N and δ = 20 μm to 100 μm, two sliding regimes are seen: these are the partial-slip and the gross-slip regimes. At a certain number of cycles, the partial-slip regime presents a transition shown by a change in hysteresis loop form, whereas the gross-slip regime maintains the buckle form with a variation of tangential force (Fig. 6). The specification report of each sliding rate on a bidimensional map, made up along the abscissa by the imposed slip amplitude and along the ordinate by applied normal force, allows RCFMs [4] to be determined. Fig. 7 shows the delimitation of both sliding rates for coated and non-coated steel. It is shown that the gross-slip regime region of the coated AISI 4140 steel is extended by the presence of the TiAlCN/
(a)
(b)
Fig. 6. Hysteresis loops “tangential load (Ft)-slip amplitude (δ)”: (a) transition partial-slip/gross-slip condition and (b) gross-slip condition.
TiAlN/TiAl layer. From a phenomenological consideration, the gross-slip regime corresponds to wear and, in the partial-slip regime, the wear is associated with cracking. The contribution of the multilayer to reducing the partial-slip regime in favor of the gross-slip regime should be interpreted positively. Indeed, in such a situation, wear is favored with the cracking of the covered part, which makes it possible to sacrifice the surface in order to protect the volume of the part. The applied PVD coating thus reduces the partial-slip regime field, which is the most detrimental for fretting. 3.3. Friction coefficient Measurement of the tangential force during the fretting tests allowed us to study the evolution of the friction coefficient. At the beginning of the test, the TiAlCN/TiAlN/TiAl coating presents a friction coefficient of 0.15, whereas the non-coated steel presents a coefficient of 0.3. During the test, the friction coefficient increases progressively towards a level known as
Gross slip /partial slip limit for TiAlCN/TiAlN/TiAl
Gross slip /partial slip limit for Uncoated steel
800 700
Normal load (N)
domes had a mean diameter of about 180 nm and the craters a maximum depth of 110 nm. The crystallographic structure and orientation of the coating were determined by X-ray diffraction. Phase identification showed the presence of reflection peaks corresponding to stripes (111), (200) and (220) as represented in Fig. 3. The presence of crystallographic structure is linked to the columnar morphology of the layers observed by SEM on a cross-section (Fig. 4). The growth of columnar structure has occurred perpendicularly at the surface and is comparable to that obtained in zone T of the Thornton model [18]. The measurements of nanoindentation carried out on a depth exceeding the thickness of the coating (Fig. 5) made it possible to determine the average hardness and average Young modulus in this multilayer TiAlCN/TiAlN/TiAl. The results are presented in Table 3.
Partial slip
600 500 400
Gross slip
300 200 100 0
Table 3 Mechanical proprieties of coating layer and substrate
TiAlCN/TiAlN/TiAl AISI 4140 steel (substrate)
0
Hardness (GPa)
Young's modulus (GPa)
15 0.42
260 210
50
100
150
Slip amplitude (μm) Fig. 7. Effect of TiAlCN/TiAlN/TiAl multilayer coating on the running condition fretting maps.
A. Ben Cheikh Larbi, B. Tlili / Surface & Coatings Technology 201 (2006) 1511–1518
Friction coefficient
0.6
1515
Uncoated steel
0.5 0.4
TiAlCN/TiAlN/TiAl
0.3 0.2
Transition period
0.1 0 0
5000
10000
15000
20000
Number of cycles
the stabilized friction coefficient. The transition period is systematically longer in the presence of the PVD coating (Fig. 8). de Wit [19] showed that the transition period corresponds, in the case of the PVD layer, to the formation of debris made up of amorphous rutiles and nanocrystallines. Beyond this transition, the amorphous phase is transformed into a crystalline phase and contributes to further wear. SEM observations showed that the debris appeared on this coating during the first cycles, in the form of particles less than 1 μm in size (Fig. 9). A flow of such particles was observed by pull-off during various interrupted tests. Some of the particles are ejected outside the contact zone, whereas the rest remain trapped (Fig. 10). The trapped particles constitute a third body, which contributes to kinematic adapting of the contact. When the pull-off flow of particles is steady, the transition period is reached. For non-coated steel, the transition period corresponds instead to the formation and growth of oxides that appear during the first cycles. The impact of normal loading and sliding amplitude on the stabilized friction coefficient of coated steel is reported in Fig. 11a. It is seen that there is a slight incidence of the sliding amplitude on the friction coefficient level, whereas the normal force is directly correlated with the friction coefficient. The steady friction coefficient of non-coated steel is relatively inde-
Fig. 10. Fretting wear scar morphology of TiAlCN/TiAlN/TiAl coating (FN = 200 N, δ = 100 μm).
pendent of the loading parameters and is between 0.55 and 0.65 (Fig. 11b). Thus, the TiAlCN/TiAlN/TiAl layer reduces the stabilized friction coefficient at all loading levels. This advantage should be attributed both to differences in hardness between two states of the surface as well as to the results of tribochemical reactions occurring between the coating and the substrate.
(a) Stabilized friction coefficient
Fig. 8. Evolution of the friction coefficient for the TiAlCN/TiAlN/TiAl multilayer coating and uncoated steel (FN = 100 N, δ = 20 μm).
0,6
0,4
0,2
0 50
100
200
500
Normal load (N)
750
100 50 20 Slip amplitude (μm)
Stabilized friction coefficient
(b) 0,6
0,4 0,2 0
100 50
100
50 200
500
Normal load (N)
Fig. 9. Detached debris from worn surface.
750
20 Slip amplitude (μm)
Fig. 11. Stabilized friction coefficient as a function of normal load and slip amplitude of (a) TiAlCN/TiAlN/TiAl multilayer coating and (b) non-coated steel.
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The resistance to fretting wear, quantified by the volume loss, is related to the slip amplitude, the normal force and the energy dissipated during sliding. The used volume is determined further by SEM observations of the fretting trace and by profilometric measurement of the trace depth. 3.4.1. Variation of wear with slip amplitude The used volume of coated and non-coated steel is shown in Fig. 12 as a function of slip amplitude and normal force. For non-coated steel, the used volume increases with slip amplitude for all the applied normal force levels. This increase becomes more important at higher normal forces. With TiAlCN/TiAlN/ TiAl coating, the used volume shows two different behaviors. It remains insignificant up to 50 μm whatever the normal force imposed and then it increases with slip amplitude. The threshold of 50 μm corresponds to the critical slip amplitude of the TiAlCN/TiAlN/TiAl layer. This amplitude is slightly lower than that reported by Sung et al. [20], 70 μm, for TiN coating of hardness 2300 HV0.05. Application of a PVD layer on steel widens its use field in fretting wear without prejudice to the layer resistance. No critical threshold is seen for non-coated steel.
(a) 20
Wear volume (106 μm3)
Normal load (N)
16
50
100
200
500
750
12 8 4 0 20
50
Lost volume *106 μm3
3.4. Wear properties
AISI 4140 Steel
14 12 10 8 6 4 2 0 0
50
150
200
Fig. 13. Wear volume as a function of cumulated dissipated energy.
3.4.2. Friction energy The volume loss is related to the energy dissipated during a fretting cycle, as determined by integrating the hysteresis loop. This assumes that wear is associated with work of the interfacial shear constraint. Fig. 13 represents the relation between the lost volume and dissipated energy obtained after 20,000 cycles. This relation is linear for both studied surfaces states. The slope associated with each surface is its energetic wear coefficient and represents its resistance capacity to wear in fretting. The energetic wear coefficients of uncoated steel with martensite structure and TiAlCN/TiAlN/TiAl coating are reported in Fig. 14. This result shows that the applied PVD coating provides an energetic wear coefficient of 23,000 μm3/J, that of AISI 4140 steel is 67,000 μm3/J. The coating improves the resistance to fretting wear of AISI 4140 steel. The principal factors favoring this tendency are generally related to the presence of compression residual stress, the decrease of friction coefficient, the increase of superficial hardness and the roughness effect of the surface [8]. Without measuring the residual stress, the contribution of the reduction of friction coefficient and the increase in hardness are confirmed in this study. The TiAlCN/TiAlN/TiAl coating reduces the instantaneous and stabilized friction coefficients of uncoated steel, reported to
100 80
78
(b)
67
Energy wear rate 103 μm3/J
20
Normal load (N)
Wear volume (106 μm3)
100
Energy (J)
Slip amplitude (μm)
16
TiAlCN/TiAlN/TiAl
50
100
200
500
750
12 8 4 0 20
50
100
Slip amplitude (μm) Fig. 12. Wear volume as a function of slip amplitude at various normal loads (a) TiAlCN/TiAlN/TiAl multilayer coating and (b) uncoated steel.
60
40 33 23
22 20
0 AISI 4140 [22]
AISI 4140 This steady
TiN [23]
TiAlN [23]
TiAlCN This steady
Fig. 14. Energy wear rates of different substrates and hard coatings in fretting wear tests.
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4. Conclusions TiAlCN/TiAlN/TiAl multilayers deposited by DC magnetron sputtering on AISI 4140 steel present a crystalline structure dominated by the orientations (111), (200) and (220). The hardness and Young's modulus are respectively 15 GPa and 260 GPa. The behavior of this coating in fretting wear is compared to that of non-coated steel. The results are summarized below:
Fig. 15. Vickers pyramid indentation on TiAlCN/TiAlN/TiAl multilayer coating.
vary by up to 50% as a function of the normal force. This diminishes the interfacial shear stress causing the damage measured in that work in terms of material removal during fretting cycles. At the same time, this coating provides a hardness equal to 1500 HV0.05, a value that is significantly higher than that of the substrate (420 HV0.05). It should also be noted that Vickers indentations with different loads indicated that the coating adhered well to the substrate. Fig. 15 shows a Vickers indentation trace in which no delaminations or cracking appear, indicating no adherence defects [21]. The edge circling the indentation trace corresponds to a localized deformation and does not show any damage. This adherence quality is mainly due to the good adherence properties of the TiAlN balancing layer as shown by Han et al. [14]. Thus, increase in the hardness due to the coating, associated with its good adherence, reduces the wear during fretting. Fig. 14 compares the energetic wear coefficients with those reported in the literature [22,23]. The energetic wear coefficient of non-coated AISI 4140 steel in contact with an alumina ball is 15% lower than that obtained by Varenberg et al. [22] for the same steel in contact with a steel ball. The level of this coefficient is high, showing significant wear. The energetic wear coefficient obtained for the TiAlCN/TiAlN/TiAl layer is almost the same as that of TiN and is inferior to that of TiAlN with an equivalent level of relative humidity. The performance coefficient of the coating, defined as the ratio between the energetic coefficients of the substrate and the coating, provides a measure of the coating efficiency. According to energetic considerations, TiN monolayer and TiAlCN/TiAlN/TiAl multilayer coatings improve the resistance to fretting wear by a factor of 3 and 2.8, respectively, compared with non-coated steel. The TiAlN layer has a lower performance as it improves the resistance only by a factor of 2. This result seems to be coherent, as the addition of aluminum to titanium nitrate essentially improves the adherence of this layer, but provides less good tribolic qualities [24]. These properties are improved by the addition of carbon to TiAlN. Similar results were found in the works of Lackner et al. [25].
• The local solicitation maps for coated and non-coated steel show that a TiAlCN/TiAlN/TiAl coating reduces the region of partial-slip in favor of the gross-slip regime. • Application of TiAlCN/TiAlN/TiAl coating reduces the instantaneous and stabilized friction coefficients. The variation between the two surface states depends more on the normal force than on the slip amplitude. • The volume used in fretting wear depends on the loading parameters. The coating reduces the used volume, which becomes insignificant when the slip amplitude is less than the critical slip amplitude. • The resistance to fretting wear can be described by the energetic wear coefficient deduced from the linear relation between the used volume and dissipated cumulated energy. TiAlCN/TiAlN/TiAl coating improves the resistance to fretting wear of AlSI 4140 steel by a factor of about three after the application of 20,000 cycles. Acknowledgments The authors thank T. Gendre and Co. (Waterman S.A France) for providing the TiAlCN/TiAlN/TiAl coating. References [1] C. Subramanian, K.N. Strafford, T.P. Wilks, L.P. Ward, J. Mater. Process. Technol. 56 (1996) 385. [2] Y. Fu, J. Wei, A.W. Batchelor, J. Mater. Process. Technol. 99 (2000) 231. [3] R.B. Waterhouse, Fretting Corrosion, Pergamon, Oxford, 1972. [4] O. Vingsbo, S. Sodeberg, Wear 126 (1988) 131. [5] S. Fouvry, Ph. Kapsa, L. Vincent, Wear 247 (2001) 41. [6] J.-P. Celis, L. Stals, E. Vancoille, H. Mohrbacher, Surf. Eng. 14 (3) (1998) 205. [7] S. Fouvry, T. Liskiewicz, Ph. Kapsa, S. Hannel, E. Sauger, Wear 255 (2003) 287. [8] K. Holmberg, A. Matthewst, H. Ronkainen, Tribol. Int. 31 (1–3) (1998) 107. [9] S-Y. Yoon, K.O. Lee, .S. Kang, K.H. Kim, J. Mater. Process. Technol. 130–131 (2002) 260. [10] S. PalDey, S.C. Deevi, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 342 (2003) 58. [11] F.-R. Weber, F. Fontaine, M. Scheib, W. Bock, Surf. Coat. Technol. 177–178 (2004) 227. [12] S.-Y. Yoon, J.-K. Kim, K.H. Kim, Surf. Coat. Technol. 161 (2002) 237. [13] D.-F. Lii, J.-L. Huang, M.-H. Lin, Surf. Coat. Technol. 99 (1998) 197. [14] J.G. Han, K.H. Nam, I.S. Choi, Wear 214 (1998) 91. [15] K-D. Bouzakis, N. Vidakis, T. Leyendecker, O.H-G. Fuss, G. Erkens, Surf. Coat. Technol. 86–87 (1996) 549. [16] M. Nordin, M. Larsson, S. Hogmark, Surf. Coat. Technol 106 (1998) 234. [17] N.J.M. Carvalho, E. Zoestbergen, B.J. Kooi, J.T.M. De Hosson, Thin Solid Films 249 (2003) 179. [18] J.A. Thornton, J. Vac. Sci. Technol. 11 (1974) 666.
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A. Ben Cheikh Larbi, B. Tlili / Surface & Coatings Technology 201 (2006) 1511–1518 E. de Wit, B. Blanpain, L. Froyen, J.-P. Celis, Wear 217 (1998) 215. J.H. Sung, T.H. Kim, S.S. Kim, Wear 250 (2001) 658. J. Richter, Surf. Coat. Technol. 162 (2003) 119. M. Varenberg, G. Halperin, I. Etsion, Wear 252 (2002) 902.
[23] B. Prakash, C. Ftikos, J.-P. Celis, Surf. Coat. Technol. 154 (2002) 182. [24] S-Y. Yoon, J-K. Kim, K.H. Kim, Surf. Coat. Technol. 161 (2002) 237. [25] J.M. Lakner, W. Waldhauser, R. Ebner, J. Keskés, T. Schoberl, Surf. Coat. Technol. 177- 178 (2004) 447.
Fretting wear of multilayered PVD TiAlCN/TiAlN/TiAl on AISI 4140 steel A. Ben Cheikh Larbi ⁎, B. Tlili Laboratoire de Mécanique Matériaux et Procédés Ecole Supérieure des Sciences et Techniques de Tunis, 1008 Tunisia Received 21 July 2005; accepted in revised form 11 February 2006 Available online 6 March 2006
Abstract Physical vapor deposition (PVD) coatings are recommended for metallic components to mitigate the damage induced by fretting wear. Several experimental investigations have led to the development of a TiAlCN/TiAlCN/TiAl coating in preference to the traditional TiN coating. The present work enabled quantification of the resistance to fretting wear of a TiAlCN/TiAlCN/TiAl multilayer coating deposited by reactive DC magnetron sputtering of Ti–Al alloys on AISI 4140 steel. Experimental simulations of the fretting wear induced by 20,000 cycles at a frequency of 5 Hz made it possible to establish a fretting map and delimit the partial-slip and gross-slip regions. The influence of normal force and displacement amplitude on the coefficients of instantaneous and stabilized friction is deferred for coated and uncoated steels. The PVD coating reduces the friction. The relation between worn volume and dissipated cumulated energy was established for coated and uncoated steels, and the energetic wear coefficients of fretting wear were deduced. The multilayer TiAlCN/TiAlCN/TiAl coating reduces the energetic wear coefficient, which is synonymous with improving the resistance to fretting wear. © 2006 Elsevier B.V. All rights reserved. Keywords: Fretting wear; PVD; TiAlCN; Fretting map; Friction coefficient; Dissipated energy
1. Introduction The performances of metallic components employed in mechanical applications depend mainly on the type and level of solicitation imposed, as well as on factors such as the structure and the surface and mass mechanical characteristics of the component [1]. Among the solicitations frequently encountered in mechanical structures, fretting remains one of the modern plagues of the mechanical engineering industry [2]. Waterhouse described it as an established phenomenon when two surfaces in contact undergo a reciprocal displacement of amplitude, limited in general to 150 μm [3]. According to the intensity of the imposed parameters (normal force, amplitude of displacement and frequency) and the local environment of the surfaces in contact, fretting wear, fretting fatigue or fretting corrosion are caused, resulting in wear and cracking. These degradations strongly limit the lifespan of the components.
⁎ Corresponding author. E-mail address: [email protected] (A. Ben Cheikh Larbi). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.025
Various investigations have been carried out to determine the behavior in fretting wear of many materials. Three types of study are commonly employed. 1. Establishment of a fretting map: Vingsbo and Sodeberg [4] developed running condition fretting maps (RCFMs) and materials response fretting maps (MRFMs). RCFMs delimit the sliding regime. According to normal force and displacement amplitude, one distinguishes the stick situation, the mixed-slip and gross-slip regimes. It is established that the mixed-slip regime is associated with cracking, whereas the gross-slip regime is associated with wear [5]. 2. Determination of the tribological characteristics and the specific wear rate: the wear coefficient is provided by the relation of Archard, V = KFL / H, where V is the worn volume, F the normal force, L the kinematics distance, H the hardness and K the wear coefficient. A new energy approach developed by Celis et al. [6] and Fouvry et al. [7] allows determination of an energy coefficient of wear by establishing the relation between worn volume and cumulated energy dissipated in friction during fretting. These various coefficients represent a useful measurement of resistance to fretting.
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Table 1 Chemical composition of AISI 4140 steel
2. Experimental details
C Mn Si S Cr P Mo
0.410 0.770 0.280 0.026 1.020 0.019 0.160
3. Identification of the dominant mechanisms of degradation: according to the nature of the bodies in contact and the operating parameters, the modes of degradation most often seen are adhesive or abrasive or oxidative wear, detachment of articles and cracking. The methods used to mitigate the effects of fretting are multiple and varied. Among them, PVD coatings offer an efficient and economically viable solution [2,8]. There are currently several different processes of PVD coating, providing monolayers, multilayers or nanostructured layers. The reference coating remains titanium nitride. A TiN coating on mechanical components improves their lifespan owing to a structured composition, offering good tribological properties, and particularly a low friction coefficient in fretting (about 0.15 to 0.2 [9]). However, TiN coatings show some disadvantages such as weak resistance to oxidation and poor adherence [10]. In order to remedy these imperfections, additional constituents such as aluminum and/or carbon are recommended. The addition of aluminum to TiN coating provides a ternary coating, TiAlN, which has better resistance to oxidation and improved mechanical properties [11] but weakened tribolic capacities [12]. The addition of carbon to TiAlN coating yields a quaternary coating, TiAlCN, which has good adherence [13]. Although some studies are available on TiAlCN layers and their behavior in terms of wear and fatigue [14,15], data on resistance to fretting wear are not available. In this article, we present the results of our studies on the behavior in fretting wear of AISI 4140 steel and the multilayer TiAlCN/TiAlN/TiAl deposited by DC magnetron sputtering. The main purpose of our study was to measure the effect of this coating on steel against fretting wear over a wide range of normal forces and imposed slip amplitudes.
Table 2 Deposition conditions
Unit
Target Bias power (kW) (V)
2.1. Substrate The substrate material used in the present study was commercial AISI 4140 steel with the composition presented in Table 1. The substrates were hardened and tempered to a hardness of 420 HV0.05. They were ground, polished and cleaned with trichloroethylene, acetone and alcohol in an ultrasonic cleaner. 2.2. Deposition TiAlCN/TiAlN/TiAl coatings were deposited on AISI 4140 steel by reactive DC magnetron sputtering from high purity Ti– Al targets (50% at Ti, 50% at Al). The substrates were mounted on a continuously rotating planetary holder inside the vacuum chamber. The atmosphere was chosen in order to produce successively a TiAl underlayer, a TiAlN buffer layer and then a TiAlCN layer. The TiAl and TiAlN films were deposited onto the substrate to improve adhesion of the TiAlCN [13]. Optimum deposition conditions such as target power, bias voltage, temperature, rotational velocity, deposition time and gas flow were determined in preliminary studies. Table 2 shows the detailed deposition conditions. Argon, nitrogen and acetylene gas of very high purity (99.999%) was introduced into the vacuum chamber. The base pressure in the deposition chamber was 5 · 10−5 Pa, which rose to 0.1 Pa during deposition. The distance between the target and surface substrate was 35 mm. Prior to deposition, the surface substrates were further cleaned by argon ion bombardment. 2.3. Characterization of the coating The surface morphology of the TiAlCN coating was examined by atomic force microscopy (AFM). Microstructural characterization of the deposited film was investigated by Xray diffraction (Philips X'pert system). Scans were carried out in the grazing angle mode with an incident beam angle of 3° and the normal θ − 2θ method classically used in the same situation [16,17]. Young's modulus and hardness were measured by nanoindentation tests with a nanoindenter MTS-XP. The indentation was performed using a triangular Berkovich diamond pyramid.
TiAlCN (1000 nm) Temper Rotation Time Gas flow velocity (°C) (rpm) (mn) Ar N2 (sccm)
TiAl 6 TiAlN 7.5
− 300 250 − 200 150
0.7 0.6
22 17
TiAlCN 7.5
− 200 100
0.6
42
400 770– 800 950
TiAlN (200 nm) C2H2
– – 90–150 – 150–135 20–135
TiAl (400 nm) Substrate
Fig. 1. Distribution and thickness of the layers in the coating.
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Fig. 2. AFM morphologies of surface layer TiAlCN.
2.4. Fretting tests The fretting tests were carried out on an MTS tension compression hydraulic machine. A sphere-onplane configuration was employed. The counter-body was a polycrystalline alumina ball with of diameter 24 mm, Young's modulus 310 GPa and hardness 2300 HV0.05. The coating steel flat specimen was 10 mm thick. During the test, the instantaneous displacement, the normal force and the tangential force were monitored and recorded for every cycle. The experimental parameters were selected as displacement amplitudes of 25, 50 and 100 μm, and normal forces of 50, 100, 200, 500 and 750 N. The tests were of 20,000 cycles and the frequency was set at 5 Hz. The fretting tests were conducted in dry conditions at an ambient temperature 25 °C and relative humidity 60%. Prior to the fretting tests, the specimen and counter-body were cleaned with acetone and alcohol. After each test, the mor-
phology of the wear scar was observed by scanning electron microscopy (SEM). The profile of the fretting scar was assessed using surface roughness profilometry and the wear volumes deduced. 3. Results and discussion 3.1. Coating SEM and AFM observations allowed determination of the global coating thickness and the morphology of the surface respectively. The coating had a mean thickness of 1.6 μm distributed in three layers as shown in Fig. 1. The surface was globally uniform with some domes and tiny craters spread all over the area (Fig. 2). Dimensional measurements showed that the
Intensity
(200)
(220) (111)
30
40
50
60
70
80
2 θ CuKα [°] Fig. 3. XRD diffractogram of TiAlCN/TiAlN/TiAl multilayer coating deposited onto AISI 4140 steel.
Fig. 4. Scanning electron micrograph of a cross-section of multilayered TiAlCN/ TiAlN/TiAl coating as made by fractography.
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Load (mN)
1600
Ft
Ft
1200
δ
800
δ
400
Ft
Ft
0 0
1000
2000
3000
4000
Displacement (nm)
δ
δ
Fig. 5. Typical loading–unloading curves for TiAlCN/TiAlN/TiAl multilayer coating.
3.2. Fretting map The evolution of the hysteresis buckles made up by the tangential force and movement during fretting cycles allows the sliding rates of each surface layer to be distinguished. For applied operating parameters in the range FN = 20 N to 750 N and δ = 20 μm to 100 μm, two sliding regimes are seen: these are the partial-slip and the gross-slip regimes. At a certain number of cycles, the partial-slip regime presents a transition shown by a change in hysteresis loop form, whereas the gross-slip regime maintains the buckle form with a variation of tangential force (Fig. 6). The specification report of each sliding rate on a bidimensional map, made up along the abscissa by the imposed slip amplitude and along the ordinate by applied normal force, allows RCFMs [4] to be determined. Fig. 7 shows the delimitation of both sliding rates for coated and non-coated steel. It is shown that the gross-slip regime region of the coated AISI 4140 steel is extended by the presence of the TiAlCN/
(a)
(b)
Fig. 6. Hysteresis loops “tangential load (Ft)-slip amplitude (δ)”: (a) transition partial-slip/gross-slip condition and (b) gross-slip condition.
TiAlN/TiAl layer. From a phenomenological consideration, the gross-slip regime corresponds to wear and, in the partial-slip regime, the wear is associated with cracking. The contribution of the multilayer to reducing the partial-slip regime in favor of the gross-slip regime should be interpreted positively. Indeed, in such a situation, wear is favored with the cracking of the covered part, which makes it possible to sacrifice the surface in order to protect the volume of the part. The applied PVD coating thus reduces the partial-slip regime field, which is the most detrimental for fretting. 3.3. Friction coefficient Measurement of the tangential force during the fretting tests allowed us to study the evolution of the friction coefficient. At the beginning of the test, the TiAlCN/TiAlN/TiAl coating presents a friction coefficient of 0.15, whereas the non-coated steel presents a coefficient of 0.3. During the test, the friction coefficient increases progressively towards a level known as
Gross slip /partial slip limit for TiAlCN/TiAlN/TiAl
Gross slip /partial slip limit for Uncoated steel
800 700
Normal load (N)
domes had a mean diameter of about 180 nm and the craters a maximum depth of 110 nm. The crystallographic structure and orientation of the coating were determined by X-ray diffraction. Phase identification showed the presence of reflection peaks corresponding to stripes (111), (200) and (220) as represented in Fig. 3. The presence of crystallographic structure is linked to the columnar morphology of the layers observed by SEM on a cross-section (Fig. 4). The growth of columnar structure has occurred perpendicularly at the surface and is comparable to that obtained in zone T of the Thornton model [18]. The measurements of nanoindentation carried out on a depth exceeding the thickness of the coating (Fig. 5) made it possible to determine the average hardness and average Young modulus in this multilayer TiAlCN/TiAlN/TiAl. The results are presented in Table 3.
Partial slip
600 500 400
Gross slip
300 200 100 0
Table 3 Mechanical proprieties of coating layer and substrate
TiAlCN/TiAlN/TiAl AISI 4140 steel (substrate)
0
Hardness (GPa)
Young's modulus (GPa)
15 0.42
260 210
50
100
150
Slip amplitude (μm) Fig. 7. Effect of TiAlCN/TiAlN/TiAl multilayer coating on the running condition fretting maps.
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Friction coefficient
0.6
1515
Uncoated steel
0.5 0.4
TiAlCN/TiAlN/TiAl
0.3 0.2
Transition period
0.1 0 0
5000
10000
15000
20000
Number of cycles
the stabilized friction coefficient. The transition period is systematically longer in the presence of the PVD coating (Fig. 8). de Wit [19] showed that the transition period corresponds, in the case of the PVD layer, to the formation of debris made up of amorphous rutiles and nanocrystallines. Beyond this transition, the amorphous phase is transformed into a crystalline phase and contributes to further wear. SEM observations showed that the debris appeared on this coating during the first cycles, in the form of particles less than 1 μm in size (Fig. 9). A flow of such particles was observed by pull-off during various interrupted tests. Some of the particles are ejected outside the contact zone, whereas the rest remain trapped (Fig. 10). The trapped particles constitute a third body, which contributes to kinematic adapting of the contact. When the pull-off flow of particles is steady, the transition period is reached. For non-coated steel, the transition period corresponds instead to the formation and growth of oxides that appear during the first cycles. The impact of normal loading and sliding amplitude on the stabilized friction coefficient of coated steel is reported in Fig. 11a. It is seen that there is a slight incidence of the sliding amplitude on the friction coefficient level, whereas the normal force is directly correlated with the friction coefficient. The steady friction coefficient of non-coated steel is relatively inde-
Fig. 10. Fretting wear scar morphology of TiAlCN/TiAlN/TiAl coating (FN = 200 N, δ = 100 μm).
pendent of the loading parameters and is between 0.55 and 0.65 (Fig. 11b). Thus, the TiAlCN/TiAlN/TiAl layer reduces the stabilized friction coefficient at all loading levels. This advantage should be attributed both to differences in hardness between two states of the surface as well as to the results of tribochemical reactions occurring between the coating and the substrate.
(a) Stabilized friction coefficient
Fig. 8. Evolution of the friction coefficient for the TiAlCN/TiAlN/TiAl multilayer coating and uncoated steel (FN = 100 N, δ = 20 μm).
0,6
0,4
0,2
0 50
100
200
500
Normal load (N)
750
100 50 20 Slip amplitude (μm)
Stabilized friction coefficient
(b) 0,6
0,4 0,2 0
100 50
100
50 200
500
Normal load (N)
Fig. 9. Detached debris from worn surface.
750
20 Slip amplitude (μm)
Fig. 11. Stabilized friction coefficient as a function of normal load and slip amplitude of (a) TiAlCN/TiAlN/TiAl multilayer coating and (b) non-coated steel.
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The resistance to fretting wear, quantified by the volume loss, is related to the slip amplitude, the normal force and the energy dissipated during sliding. The used volume is determined further by SEM observations of the fretting trace and by profilometric measurement of the trace depth. 3.4.1. Variation of wear with slip amplitude The used volume of coated and non-coated steel is shown in Fig. 12 as a function of slip amplitude and normal force. For non-coated steel, the used volume increases with slip amplitude for all the applied normal force levels. This increase becomes more important at higher normal forces. With TiAlCN/TiAlN/ TiAl coating, the used volume shows two different behaviors. It remains insignificant up to 50 μm whatever the normal force imposed and then it increases with slip amplitude. The threshold of 50 μm corresponds to the critical slip amplitude of the TiAlCN/TiAlN/TiAl layer. This amplitude is slightly lower than that reported by Sung et al. [20], 70 μm, for TiN coating of hardness 2300 HV0.05. Application of a PVD layer on steel widens its use field in fretting wear without prejudice to the layer resistance. No critical threshold is seen for non-coated steel.
(a) 20
Wear volume (106 μm3)
Normal load (N)
16
50
100
200
500
750
12 8 4 0 20
50
Lost volume *106 μm3
3.4. Wear properties
AISI 4140 Steel
14 12 10 8 6 4 2 0 0
50
150
200
Fig. 13. Wear volume as a function of cumulated dissipated energy.
3.4.2. Friction energy The volume loss is related to the energy dissipated during a fretting cycle, as determined by integrating the hysteresis loop. This assumes that wear is associated with work of the interfacial shear constraint. Fig. 13 represents the relation between the lost volume and dissipated energy obtained after 20,000 cycles. This relation is linear for both studied surfaces states. The slope associated with each surface is its energetic wear coefficient and represents its resistance capacity to wear in fretting. The energetic wear coefficients of uncoated steel with martensite structure and TiAlCN/TiAlN/TiAl coating are reported in Fig. 14. This result shows that the applied PVD coating provides an energetic wear coefficient of 23,000 μm3/J, that of AISI 4140 steel is 67,000 μm3/J. The coating improves the resistance to fretting wear of AISI 4140 steel. The principal factors favoring this tendency are generally related to the presence of compression residual stress, the decrease of friction coefficient, the increase of superficial hardness and the roughness effect of the surface [8]. Without measuring the residual stress, the contribution of the reduction of friction coefficient and the increase in hardness are confirmed in this study. The TiAlCN/TiAlN/TiAl coating reduces the instantaneous and stabilized friction coefficients of uncoated steel, reported to
100 80
78
(b)
67
Energy wear rate 103 μm3/J
20
Normal load (N)
Wear volume (106 μm3)
100
Energy (J)
Slip amplitude (μm)
16
TiAlCN/TiAlN/TiAl
50
100
200
500
750
12 8 4 0 20
50
100
Slip amplitude (μm) Fig. 12. Wear volume as a function of slip amplitude at various normal loads (a) TiAlCN/TiAlN/TiAl multilayer coating and (b) uncoated steel.
60
40 33 23
22 20
0 AISI 4140 [22]
AISI 4140 This steady
TiN [23]
TiAlN [23]
TiAlCN This steady
Fig. 14. Energy wear rates of different substrates and hard coatings in fretting wear tests.
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4. Conclusions TiAlCN/TiAlN/TiAl multilayers deposited by DC magnetron sputtering on AISI 4140 steel present a crystalline structure dominated by the orientations (111), (200) and (220). The hardness and Young's modulus are respectively 15 GPa and 260 GPa. The behavior of this coating in fretting wear is compared to that of non-coated steel. The results are summarized below:
Fig. 15. Vickers pyramid indentation on TiAlCN/TiAlN/TiAl multilayer coating.
vary by up to 50% as a function of the normal force. This diminishes the interfacial shear stress causing the damage measured in that work in terms of material removal during fretting cycles. At the same time, this coating provides a hardness equal to 1500 HV0.05, a value that is significantly higher than that of the substrate (420 HV0.05). It should also be noted that Vickers indentations with different loads indicated that the coating adhered well to the substrate. Fig. 15 shows a Vickers indentation trace in which no delaminations or cracking appear, indicating no adherence defects [21]. The edge circling the indentation trace corresponds to a localized deformation and does not show any damage. This adherence quality is mainly due to the good adherence properties of the TiAlN balancing layer as shown by Han et al. [14]. Thus, increase in the hardness due to the coating, associated with its good adherence, reduces the wear during fretting. Fig. 14 compares the energetic wear coefficients with those reported in the literature [22,23]. The energetic wear coefficient of non-coated AISI 4140 steel in contact with an alumina ball is 15% lower than that obtained by Varenberg et al. [22] for the same steel in contact with a steel ball. The level of this coefficient is high, showing significant wear. The energetic wear coefficient obtained for the TiAlCN/TiAlN/TiAl layer is almost the same as that of TiN and is inferior to that of TiAlN with an equivalent level of relative humidity. The performance coefficient of the coating, defined as the ratio between the energetic coefficients of the substrate and the coating, provides a measure of the coating efficiency. According to energetic considerations, TiN monolayer and TiAlCN/TiAlN/TiAl multilayer coatings improve the resistance to fretting wear by a factor of 3 and 2.8, respectively, compared with non-coated steel. The TiAlN layer has a lower performance as it improves the resistance only by a factor of 2. This result seems to be coherent, as the addition of aluminum to titanium nitrate essentially improves the adherence of this layer, but provides less good tribolic qualities [24]. These properties are improved by the addition of carbon to TiAlN. Similar results were found in the works of Lackner et al. [25].
• The local solicitation maps for coated and non-coated steel show that a TiAlCN/TiAlN/TiAl coating reduces the region of partial-slip in favor of the gross-slip regime. • Application of TiAlCN/TiAlN/TiAl coating reduces the instantaneous and stabilized friction coefficients. The variation between the two surface states depends more on the normal force than on the slip amplitude. • The volume used in fretting wear depends on the loading parameters. The coating reduces the used volume, which becomes insignificant when the slip amplitude is less than the critical slip amplitude. • The resistance to fretting wear can be described by the energetic wear coefficient deduced from the linear relation between the used volume and dissipated cumulated energy. TiAlCN/TiAlN/TiAl coating improves the resistance to fretting wear of AlSI 4140 steel by a factor of about three after the application of 20,000 cycles. Acknowledgments The authors thank T. Gendre and Co. (Waterman S.A France) for providing the TiAlCN/TiAlN/TiAl coating. References [1] C. Subramanian, K.N. Strafford, T.P. Wilks, L.P. Ward, J. Mater. Process. Technol. 56 (1996) 385. [2] Y. Fu, J. Wei, A.W. Batchelor, J. Mater. Process. Technol. 99 (2000) 231. [3] R.B. Waterhouse, Fretting Corrosion, Pergamon, Oxford, 1972. [4] O. Vingsbo, S. Sodeberg, Wear 126 (1988) 131. [5] S. Fouvry, Ph. Kapsa, L. Vincent, Wear 247 (2001) 41. [6] J.-P. Celis, L. Stals, E. Vancoille, H. Mohrbacher, Surf. Eng. 14 (3) (1998) 205. [7] S. Fouvry, T. Liskiewicz, Ph. Kapsa, S. Hannel, E. Sauger, Wear 255 (2003) 287. [8] K. Holmberg, A. Matthewst, H. Ronkainen, Tribol. Int. 31 (1–3) (1998) 107. [9] S-Y. Yoon, K.O. Lee, .S. Kang, K.H. Kim, J. Mater. Process. Technol. 130–131 (2002) 260. [10] S. PalDey, S.C. Deevi, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 342 (2003) 58. [11] F.-R. Weber, F. Fontaine, M. Scheib, W. Bock, Surf. Coat. Technol. 177–178 (2004) 227. [12] S.-Y. Yoon, J.-K. Kim, K.H. Kim, Surf. Coat. Technol. 161 (2002) 237. [13] D.-F. Lii, J.-L. Huang, M.-H. Lin, Surf. Coat. Technol. 99 (1998) 197. [14] J.G. Han, K.H. Nam, I.S. Choi, Wear 214 (1998) 91. [15] K-D. Bouzakis, N. Vidakis, T. Leyendecker, O.H-G. Fuss, G. Erkens, Surf. Coat. Technol. 86–87 (1996) 549. [16] M. Nordin, M. Larsson, S. Hogmark, Surf. Coat. Technol 106 (1998) 234. [17] N.J.M. Carvalho, E. Zoestbergen, B.J. Kooi, J.T.M. De Hosson, Thin Solid Films 249 (2003) 179. [18] J.A. Thornton, J. Vac. Sci. Technol. 11 (1974) 666.
1518 [19] [20] [21] [22]
A. Ben Cheikh Larbi, B. Tlili / Surface & Coatings Technology 201 (2006) 1511–1518 E. de Wit, B. Blanpain, L. Froyen, J.-P. Celis, Wear 217 (1998) 215. J.H. Sung, T.H. Kim, S.S. Kim, Wear 250 (2001) 658. J. Richter, Surf. Coat. Technol. 162 (2003) 119. M. Varenberg, G. Halperin, I. Etsion, Wear 252 (2002) 902.
[23] B. Prakash, C. Ftikos, J.-P. Celis, Surf. Coat. Technol. 154 (2002) 182. [24] S-Y. Yoon, J-K. Kim, K.H. Kim, Surf. Coat. Technol. 161 (2002) 237. [25] J.M. Lakner, W. Waldhauser, R. Ebner, J. Keskés, T. Schoberl, Surf. Coat. Technol. 177- 178 (2004) 447.