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HVOF-sprayed WC-Co as hard interlayer for DLC films
Surface & Coatings Technology 203 (2008) 699–703
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
HVOF-sprayed WC-Co as hard interlayer for DLC films G. Bolelli a,⁎, L. Lusvarghi a, M. Montecchi a, F. Pighetti Mantini a, F. Pitacco b, H. Volz b, M. Barletta c a b c
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, Via Vignolese 905, 41100 Modena (MO), Italy Protec Surface Technologies S.r.l., Via Benaco 88, 25081 Bedizzole (BS), Italy Department of Mechanical Engineering, University of Roma - Tor Vergata, Via del Politecnico 1, 00133 Roma, Italy
a r t i c l e
i n f o
Available online 13 August 2008 Keywords: High Velocity Oxygen-Fuel Spraying (HVOF) Electron Cyclotron Resonance— Chemical Vapour Deposition (ECR-CVD) Nano-indentation Scratch test Ball-on-disk test
a b s t r a c t This study evaluates the technical improvement achieved by applying a thick HVOF-sprayed WC-17Co interlayer between a DLC thin film and an AISI1040 steel substrate. The chemical composition (from XPS analysis) and hardness (from nano-indentation test) of the DLC films deposited on both bare and HVOFcoated AISI1040 steel are analogous. Nevertheless, the better adhesion of the DLC film on WC-17Co interlayer is demonstrated by higher critical loads recorded in scratch testing. During ball-on-disk dry sliding tribological tests, the low hardness of the AISI1040 substrate causes delamination phenomena in the DLC film. When contact pressure is low, delamination is limited, both at room temperature and at 400 °C; instead, delamination under high contact pressure is severe at room temperature and complete at 300 °C. The hard WC-17Co interlayer significantly improves the tribological performance, although delamination eventually occurs, after long sliding distances, under high contact pressure at 300 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction DLC (“Diamond-Like Carbon”) films, deposited either by PVD or CVD technique, combine good wear resistance and low friction behaviour. Moreover, the low substrate temperature during coating deposition (b200 °C) confers these films a large range of applicability [1]. Various factors (including quite low deposition rates and large residual stresses) limit the thickness of DLC films to b5 µm [1]. When a DLC-coated component comes into contact with its counterbody, the resulting stress distribution cannot be entirely carried by such thin film; thus, the substrate is also involved. Consequently, the film performance is affected by the mechanical properties of the substrate. Specifically, if the substrate deforms plastically, stresses increase dramatically in the coating [2], so that cracking and/or delamination from the substrate interface can occur [3,4]. This problem affects all substrates having moderate or low hardness (low and medium carbon steels, light alloys [5], etc…). A thick interlayer, capable of bearing the contact stresses, can relieve the problem. Although multi-layer systems comprising a thick interlayer and a DLC-based thin film have shown promising characteristics [5–7], they have been seldom dealt with in literature. More research is therefore needed in order to provide a better assessment of their technical benefits and limits. Thermal spraying
⁎ Corresponding author. Via Vignolese, 905; I-41100 Modena (MO), Italy. Tel.: +39 059 2056281; fax: +39 059 2056243. E-mail address: [email protected] (G. Bolelli). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.014
appears a particularly suitable technique for the deposition of these thick interlayers, because it is highly flexible and it finds numerous industrial applications on a wide range of simple- or complex-shaped components [8–12]. This versatility is well-matched to the versatility of PVD or CVD deposition processes. Specifically, HVOF-sprayed WCCo seems highly promising as an interlayer, on account of its high hardness, good toughness and high modulus [13,14]. The aim of the research is therefore assessing the improvement in the tribological behaviour of DLC-based thin films by using a HVOFsprayed WC-Co interlayer. 2. Experimental WC-17%Co layers (200 μm thick) were deposited onto grit-blasted (100 × 100 × 5) mm3 AISI1040 steel plates by HVOF-spraying, using a Praxair-Tafa JP5000 torch operated with parameters previously described in [15]. DLC-based thin films were deposited both on AISI1040 steel plates and on WC-Co layers. Both surfaces were manually polished to Ra ≈ 0.02 µm using diamond slurry (up to 0.5 µm), degreased by acid-alkaline washing and finally sputtercleaned inside the film deposition chamber, using a high energy ion bombardment (Ar) with a −800 V bias for 10 min. The films were deposited in a single-batch operation, using a hybrid PVD-CVD coating system equipped with magnetron sputtering (MS) sources and electron cyclotron resonance (ECR) plasma sources (for ECR-CVD). These films consist of two sputtered adhesion layers (a thin Cr layer and a WC/C layer), and a DLC top layer deposited by ECR-CVD (gaseous precursor: C2H2). Process parameters are proprietary. The deposition temperature was below 200 °C.
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G. Bolelli et al. / Surface & Coatings Technology 203 (2008) 699–703
Fig. 1. FEG-SEM images (secondary electrons, tilt angle = 52°) of FIB sections produced on the thin film/AISI1040 system (A) and on the thin film/WC-Co interlayer system (B).
The films were sectioned using a Focused Ion Beam (FIB) device (5 nA ion beam current for cutting, 300 pA ion beam current for polishing) and observed using a high-resolution FEG-SEM column under a tilt angle of 52° (FEI StrataTM-DB235 dual-beam machine equipped with Ga Liquid Metal Ion Source and FEG-SEM electron source). Their chemical composition was analysed by X-ray Photoelectron Spectroscopy (XPS): the Mg-Kα1.2 line from a Vacuum Generators XR3 dual anode X-ray tube, operated at 240 W (15 kV, 16 mA), was used as the source. Overview scans were taken with pass energy of 100 eV; higher resolution spectra of carbon-1s (C1s) and oxygen-1s (O1s) core levels were acquired with a 50 eV pass energy. The surface hardness of polished WC-Co and AISI1040 was measured by depth-sensing Berkovich microindentation (Micro-Combi tester, CSM Instruments, Switzerland: 15 indentations with 6 μm penetration depth). Depth-sensing Berkovich nano-indentation (Nano-Indentation Tester, CSM Instruments) was performed on the surface of the DLC films, under increasingly high penetration depth (15 indentations for each penetration depth value); hardness was determined according to the Oliver–Pharr procedure [16] (Poisson's ratio: 0.15). Scratch testing (Micro-Combi tester) was performed on the DLC-coated samples using a conical diamond indenter with spherical tip (200 μm radius). The load was linearly increased from 0.1 N to 30 N (15 N/min loading rate, 4 mm scratch length). The critical loads Lc1 (first crack), Lc2 (edge spallation) and Lc3 (spallation inside the groove)
were determined by optical microscopy after testing [17]. SEM investigation on the scratch grooves was also performed. Rotating unidirectional ball-on-disk tribological tests were performed on DLC films (High-Temperature Tribometer, CSM Instruments), using sintered alumina balls (nominal hardness HV = 19 GPa). Two different test settings were adopted: a less severe setting (load: 5 N; ball diameter: 6 mm; sliding distance: 500 m; sliding speed: 0.20 m/s), hereafter labelled as “Test I”, and a more severe setting (load: 10 N; ball diameter: 3 mm; sliding distance: 5000 m; sliding speed: 0.30 m/s), hereafter labelled as “Test II”. “Test I” was performed both at room temperature (RT) and at 400 °C; “Test II” was performed at RT and at 300 °C. The coating wear rate was assessed by optical profilometry (Conscan Profilometer, CSM Instruments), the friction coefficient was monitored during the tests. Wear scars were observed by SEM and sectioned in-situ by the Focused Ion Beam (FIB) technique, as described above. 3. Results and discussion The three-layered thin film deposited onto the polished AISI1040 steel surface does not exhibit any major defect (Fig. 1A). Imperfections on the polished WC-Co surface, by contrast, induce some small defects in the sputtered WC/C and Cr layers (Fig. 1B, circle); however, these defects do not propagate to the DLC top layer. DLC top layers on both surfaces have the same chemical composition, namely C: 91%, O: 9%, and they possess the same sp3/ sp2 ratio, equal to (70 ± 4)%. This was evaluated from the C (KVV) Auger structure in XPS spectra [18]. The first derivatives of the electron distribution curves were calculated and the sp3/sp2 ratio was evaluated from the length difference between the minimum and maximum positions of the first derivatives, with respect to the values obtained for pure sp3 (diamond) and pure sp2 (graphite). Accordingly, the deposition parameters were the same for all films, and the interposition of the sputtered layers might have further prevented possible influences of the substrate composition on the nucleation and growth processes of DLC during ECR-CVD. The similarity between the chemical compositions of the DLC top layers deposited on the AISI1040 and WC-Co surfaces is reflected by their analogous hardness
Table 1 Critical loads of DLC-based films
Fig. 2. Hardness vs. indentation depth for DLC-based films on WC-Co and AISI1040.
Sample
Lc1 (N)
Lc2 (N)
Lc3 (N)
DLC/AISI1040 DLC/WC-Co
4.1 ± 0.5 13.5 ± 0.6
8.2 ± 0.8 21.8 ± 2.6
10.4 ± 0.3 N30 (not detectable)
G. Bolelli et al. / Surface & Coatings Technology 203 (2008) 699–703
Fig. 3. Wear rates of DLC-based films after ball-on-disk “Test I” (at RT and 400 °C) and “Test II” (at RT).
values, measured by nano-indentation tests (Fig. 2) performed at 200 nm penetration depth (i.e. less than 10% of the thickness of the DLC top layer, which is around 2.5 μm [19,20]). Hardness measurements performed at higher indentation depth return a composite hardness, bearing the influence of both the coating and the underlying surface [19,20]: the latter influence grows larger as the indentation depth increases, lowering the composite hardness value (Fig. 2), until, at 6 μm depth, it equals the hardness of the underlying surface. Among the numerous equations describing such dependence of composite hardness on indentation depth, the present data is well-fitted using the simple model by Korsunsky et al. [21] (1), whose greatest merit is the use of a limited number of fitting parameters (Fig. 2): HC ¼ HS þ
HF −HS 2
d 1 þ tTα
ð1Þ
Where: HC d t Hs HF α
composite hardness (dependent variable); indentation depth (independent variable); film thickness (constant); substrate hardness (constant); film hardness (fitting parameter); parameter related to the coating fracture energy (fitting parameter).
The substrate hardness to be used as input in Eq. (1) was measured by depth-sensing Berkovich microindentation, as specified in the Experimental section: the values are HS = (10.3 ± 1.8) GPa for WC-Co, HS = (3.2 ± 0.8) GPa for AISI1040.
701
Fig. 5. Friction coefficient evolution of DLC-based thin films under “Test II” configuration at 300 °C.
As this model is strictly valid only for single-layer thin films, its application to the present three-layer films involves an approximation; nonetheless, the satisfactory fitting quality suggests the validity of this approximation. The adjusted values of the fitting parameters are: HF= (20.9 ± 0.8)GPa; α = (0.48 ± 0.13) μmforfilmsonWC-Counderlayer; HF= (20.2 ± 1.4) GPa; α = (0.36 ± 0.11) μm for films on AISI1040 substrate. The similarity between the fitted H F values confirms the substantial independence of film hardness on the nature of the underlying surface. The parameter α is related to the resistance of the thin film-coated system against cracking, dependent on the coating's fracture toughness, on the difference between the hardness of the coating and of the underlying surface, and on the overall characteristics of the film-underlying surface system [21]. Although the error bars on α are quite large, the thin film/WC-Co system seems to exhibit a somewhat larger α value, suggesting that this system could be more resistant to cracking than the thin film/ AISI1040 steel one. This observation is confirmed by scratch test results (Table 1): whereas delamination occurs quite early on the DLC/AISI1040 system, no complete film delamination could be produced on the DLC/WC-Co system. Indeed, the softer steel substrate yields much earlier than the hard WC-Co thick layer; the brittle DLC-based film is thus subjected to a more severe stress concentration in the former case [2,22]. It can be noted that, in a previous literature paper, Lc1 values lower than the one presently measured on the DLC/WC-Co system were reported for nitrided medium carbon steel substrates [3].
Fig. 4. Wear scars after ball-on-disk “Test II” at RT: (A) DLC-based thin film on AISI1040; (B) DLC-based thin film on HVOF-sprayed WC-Co interlayer.
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Ball-on-disk tests performed under the less severe “Test I” setting, both at RT and at 400 °C, and under the more severe “Test II” setting at RT enabled the determination of the wear rate of the DLC-based films (Fig. 3). By contrast, the films were always delaminated before the end of the tests run under the “Test II” setting at 300 °C, so that the substrate started to be worn, too. The wear rates (Fig. 3) clearly indicate that the hard WC-Co underlayer is definitely beneficial in improving the tribological performance of the DLC-based thin film, consistently with all the previous results. Indeed, when the DLC-based thin film is directly deposited on the bare steel surface, it undergoes cracking and delamination during the wear tests performed at room temperature, because of yielding and plastic deformation of steel under the contact stress distribution (Fig. 4A) [23], in accordance with scratch test results. The hard WC-Co interlayer, by contrast, does not plastically deform under load; therefore, a mild abrasion groove, free of any delamination phenomena, appears in the wear scar (Fig. 4B). At 400 °C, the wear rate of the thin film/steel system becomes significantly larger, whereas the wear rate of the thin film/WC-Co system even seems to decrease slightly. Two concurrent phenomena can influence the wear behaviour of the coatings at 400 °C. On the one hand, the mechanical properties of the substrates decrease [24,25]. A further loss in load-carrying capability of the steel substrate clearly enhances the previouslydiscussed delamination phenomena, thus contributing to the observed increase in wear loss. The WC-Co interlayer also experiences a decrease in hardness [25]; nonetheless, its initially superior hardness enables it to withstand the stress distribution produced by the less severe “Test I” contact conditions, so that a smooth wear scar (similar to the room temperature one in Fig. 4B) appears again. On the other hand, the DLC top layer itself may undergo some chemical and structural alteration (release of hydrogen and decrease of the sp3/sp2 bonding ratio), resulting in a loss of hardness and wear resistance [26]. Even at 400 °C, however, the wear rate of the DLC film deposited on the WC-Co interlayer, remains close to the lowest values reported in
literature for these coatings (≈1.2⁎10− 7 mm3/(Nm)) [26–28], suggesting that the loss in the film's wear resistance does not play a major role under the present test conditions. In all the above-mentioned tests, the friction coefficient remains around or slightly below 0.1 over the whole test duration: as the DLCbased film never delaminates completely, even on the steel substrate (Fig. 4A), the film can still exert its tribological characteristics. Contrarily, complete delamination occurs during sliding when adopting the “Test II” configuration at 300 °C (Fig. 5). Under these conditions, the decrease in substrate hardness at high temperature [24,25] is combined to the extreme severity of the contact stress distribution, resulting in a complete failure of the film. This failure obviously occurs much earlier on the soft steel substrate, but it also affects the film on WC-Co. In order to investigate more deeply the reasons underlying the failure of the DLC/WC-Co system under these conditions, one ball-on-disk test was interrupted after 2000 m (i.e. before complete film failure, Fig. 5) and analysed by the FIB technique. FIB sections show a microstructural alteration of the WC-Co layer under the wear groove (compare Fig. 6A,B: the altered area is pointed by a circle): the near-surface material seems to have collapsed under the action of the contact stress and of the test temperature. Probably, the Co matrix could not support the carbide grains, which were overloaded and fractured. As plastic flow in the near-surface region of the WC-Co layer went on during the test, the fractured carbides were dispersed in the matrix itself. 4. Conclusions This research investigated the effectiveness of an HVOF-sprayed WC-Co interlayer in enhancing adhesion and wear resistance of DLCbased thin films on an AISI1040 steel substrate. Micromechanical investigation (nano-indentation, scratch testing) and ball-on-disk tribological tests indicated that the interlayer definitely reduces the film tendency to cracking and delamination, especially under localized contact. Indeed, the stress distribution produced by such contact can easily yield the steel substrate, thus overloading the brittle DLC-based thin film up to failure. Instead, the hard and thick WC-Co interlayer bears the stress distribution with reduced or no plastic deformation, keeping stresses in the film to a moderate level. Accordingly, no complete delamination could be produced during scratch testing; in ball-on-disk tests, only extremely severe conditions (high hertzian contact pressure and high temperature) could overcome the mechanical strength of the WC-Co interlayer and produce film failure. Acknowledgements Thanks to Dr. Enrico Gualtieri and Dr. Giancarlo Gazzadi for FIB imaging and to Ing. Andrea Bassani and Mr. Binit Kumar for the help with the sample preparation. The production of HVOF-sprayed WC-Co layers by Ing. Fabrizio Casadei, Mr. Edoardo Severini, Mr. Francesco Barulli and Mr. Carlo Costa (Centro Sviluppo Materiali S.p.A., Roma, Italy) is acknowledged. Partially supported by PRRIITT (Regione Emilia Romagna), Net-Lab “Surface and Coatings for Advanced Mechanics and Nanomechanics” (SUP&RMAN). References
Fig. 6. FIB sections of the DLC/WC-Co system subjected to ball-on-disk testing (“Test II” configuration at 300 °C) interrupted after 2000 m. (A) region outside the wear track; (B) region inside the wear track. Circle indicates microstructurally altered WC-Co.
[1] D.W. Wheeler, in: B.G. Mellor (Ed.), Surface Coatings for Protection against Wear, Woodhead PublishingLimited, AbingtonHall,Cambridge,England, 2006,pp.129–132. [2] K. Holmberg, A. Laukkanen, H. Ronkainen, K. Wallin, S. Varjus, J. Koskinen, Surf. Coat. Technol. 200 (2006) 3810. [3] B. Podgornik, J. Vižintin, Diamond Relat. Mater. 10 (2001) 2232. [4] Y. Liu, E.I. Meletis, Surf. Coat. Technol. 153 (2002) 178. [5] R. Gadow, D. Scherer, Surf. Coat. Technol. 151–152 (2002) 471. [6] F. Casadei, R. Pileggi, R. Valle, A. Matthews, Surf. Coat. Technol. 201 (2006) 1200. [7] E. Bemporad, M. Sebastiani, D. De Felicis, F. Carassiti, R. Valle, F. Casadei, Thin Solid Films 515 (2006) 186. [8] X. Liu, P.K. Chu, C. Ding, Mater. Sci. Eng. R 47 (2004) 49.
G. Bolelli et al. / Surface & Coatings Technology 203 (2008) 699–703 [9] J. Vetter, G. Barbezat, J. Crummenauer, J. Avissar, Surf. Coat. Technol. 200 (2005) 1962. [10] R. Henne, J. Therm. Spray Technol. 16 (2007) 381. [11] J.T. DeMasi-Marcin, D.K. Gupta, Surf. Coat. Technol. 68–69 (1994) 1. [12] R.B. Heimann, Key Eng. Mater. 122–124 (1996) 399. [13] F.N. Longo, in: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, Materials Park, OH, USA, 2004, pp. 105–107. [14] P. Chivavibul, M. Watanabe, S. Kuroda, K. Shinoda, Surf. Coat. Technol. 202 (2007) 509. [15] G. Bolelli, V. Cannillo, L. Lusvarghi, S. Riccò, Surf. Coat. Technol. 200 (2006) 2995. [16] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [17] S.J. Bull, Surf. Coat. Technol. 50 (1991) 25. [18] J.C. Lascovich, R. Giorgi, S. Scaglione, Appl. Surf. Sci. 47 (1991) 17.
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
HVOF-sprayed WC-Co as hard interlayer for DLC films G. Bolelli a,⁎, L. Lusvarghi a, M. Montecchi a, F. Pighetti Mantini a, F. Pitacco b, H. Volz b, M. Barletta c a b c
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, Via Vignolese 905, 41100 Modena (MO), Italy Protec Surface Technologies S.r.l., Via Benaco 88, 25081 Bedizzole (BS), Italy Department of Mechanical Engineering, University of Roma - Tor Vergata, Via del Politecnico 1, 00133 Roma, Italy
a r t i c l e
i n f o
Available online 13 August 2008 Keywords: High Velocity Oxygen-Fuel Spraying (HVOF) Electron Cyclotron Resonance— Chemical Vapour Deposition (ECR-CVD) Nano-indentation Scratch test Ball-on-disk test
a b s t r a c t This study evaluates the technical improvement achieved by applying a thick HVOF-sprayed WC-17Co interlayer between a DLC thin film and an AISI1040 steel substrate. The chemical composition (from XPS analysis) and hardness (from nano-indentation test) of the DLC films deposited on both bare and HVOFcoated AISI1040 steel are analogous. Nevertheless, the better adhesion of the DLC film on WC-17Co interlayer is demonstrated by higher critical loads recorded in scratch testing. During ball-on-disk dry sliding tribological tests, the low hardness of the AISI1040 substrate causes delamination phenomena in the DLC film. When contact pressure is low, delamination is limited, both at room temperature and at 400 °C; instead, delamination under high contact pressure is severe at room temperature and complete at 300 °C. The hard WC-17Co interlayer significantly improves the tribological performance, although delamination eventually occurs, after long sliding distances, under high contact pressure at 300 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction DLC (“Diamond-Like Carbon”) films, deposited either by PVD or CVD technique, combine good wear resistance and low friction behaviour. Moreover, the low substrate temperature during coating deposition (b200 °C) confers these films a large range of applicability [1]. Various factors (including quite low deposition rates and large residual stresses) limit the thickness of DLC films to b5 µm [1]. When a DLC-coated component comes into contact with its counterbody, the resulting stress distribution cannot be entirely carried by such thin film; thus, the substrate is also involved. Consequently, the film performance is affected by the mechanical properties of the substrate. Specifically, if the substrate deforms plastically, stresses increase dramatically in the coating [2], so that cracking and/or delamination from the substrate interface can occur [3,4]. This problem affects all substrates having moderate or low hardness (low and medium carbon steels, light alloys [5], etc…). A thick interlayer, capable of bearing the contact stresses, can relieve the problem. Although multi-layer systems comprising a thick interlayer and a DLC-based thin film have shown promising characteristics [5–7], they have been seldom dealt with in literature. More research is therefore needed in order to provide a better assessment of their technical benefits and limits. Thermal spraying
⁎ Corresponding author. Via Vignolese, 905; I-41100 Modena (MO), Italy. Tel.: +39 059 2056281; fax: +39 059 2056243. E-mail address: [email protected] (G. Bolelli). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.014
appears a particularly suitable technique for the deposition of these thick interlayers, because it is highly flexible and it finds numerous industrial applications on a wide range of simple- or complex-shaped components [8–12]. This versatility is well-matched to the versatility of PVD or CVD deposition processes. Specifically, HVOF-sprayed WCCo seems highly promising as an interlayer, on account of its high hardness, good toughness and high modulus [13,14]. The aim of the research is therefore assessing the improvement in the tribological behaviour of DLC-based thin films by using a HVOFsprayed WC-Co interlayer. 2. Experimental WC-17%Co layers (200 μm thick) were deposited onto grit-blasted (100 × 100 × 5) mm3 AISI1040 steel plates by HVOF-spraying, using a Praxair-Tafa JP5000 torch operated with parameters previously described in [15]. DLC-based thin films were deposited both on AISI1040 steel plates and on WC-Co layers. Both surfaces were manually polished to Ra ≈ 0.02 µm using diamond slurry (up to 0.5 µm), degreased by acid-alkaline washing and finally sputtercleaned inside the film deposition chamber, using a high energy ion bombardment (Ar) with a −800 V bias for 10 min. The films were deposited in a single-batch operation, using a hybrid PVD-CVD coating system equipped with magnetron sputtering (MS) sources and electron cyclotron resonance (ECR) plasma sources (for ECR-CVD). These films consist of two sputtered adhesion layers (a thin Cr layer and a WC/C layer), and a DLC top layer deposited by ECR-CVD (gaseous precursor: C2H2). Process parameters are proprietary. The deposition temperature was below 200 °C.
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G. Bolelli et al. / Surface & Coatings Technology 203 (2008) 699–703
Fig. 1. FEG-SEM images (secondary electrons, tilt angle = 52°) of FIB sections produced on the thin film/AISI1040 system (A) and on the thin film/WC-Co interlayer system (B).
The films were sectioned using a Focused Ion Beam (FIB) device (5 nA ion beam current for cutting, 300 pA ion beam current for polishing) and observed using a high-resolution FEG-SEM column under a tilt angle of 52° (FEI StrataTM-DB235 dual-beam machine equipped with Ga Liquid Metal Ion Source and FEG-SEM electron source). Their chemical composition was analysed by X-ray Photoelectron Spectroscopy (XPS): the Mg-Kα1.2 line from a Vacuum Generators XR3 dual anode X-ray tube, operated at 240 W (15 kV, 16 mA), was used as the source. Overview scans were taken with pass energy of 100 eV; higher resolution spectra of carbon-1s (C1s) and oxygen-1s (O1s) core levels were acquired with a 50 eV pass energy. The surface hardness of polished WC-Co and AISI1040 was measured by depth-sensing Berkovich microindentation (Micro-Combi tester, CSM Instruments, Switzerland: 15 indentations with 6 μm penetration depth). Depth-sensing Berkovich nano-indentation (Nano-Indentation Tester, CSM Instruments) was performed on the surface of the DLC films, under increasingly high penetration depth (15 indentations for each penetration depth value); hardness was determined according to the Oliver–Pharr procedure [16] (Poisson's ratio: 0.15). Scratch testing (Micro-Combi tester) was performed on the DLC-coated samples using a conical diamond indenter with spherical tip (200 μm radius). The load was linearly increased from 0.1 N to 30 N (15 N/min loading rate, 4 mm scratch length). The critical loads Lc1 (first crack), Lc2 (edge spallation) and Lc3 (spallation inside the groove)
were determined by optical microscopy after testing [17]. SEM investigation on the scratch grooves was also performed. Rotating unidirectional ball-on-disk tribological tests were performed on DLC films (High-Temperature Tribometer, CSM Instruments), using sintered alumina balls (nominal hardness HV = 19 GPa). Two different test settings were adopted: a less severe setting (load: 5 N; ball diameter: 6 mm; sliding distance: 500 m; sliding speed: 0.20 m/s), hereafter labelled as “Test I”, and a more severe setting (load: 10 N; ball diameter: 3 mm; sliding distance: 5000 m; sliding speed: 0.30 m/s), hereafter labelled as “Test II”. “Test I” was performed both at room temperature (RT) and at 400 °C; “Test II” was performed at RT and at 300 °C. The coating wear rate was assessed by optical profilometry (Conscan Profilometer, CSM Instruments), the friction coefficient was monitored during the tests. Wear scars were observed by SEM and sectioned in-situ by the Focused Ion Beam (FIB) technique, as described above. 3. Results and discussion The three-layered thin film deposited onto the polished AISI1040 steel surface does not exhibit any major defect (Fig. 1A). Imperfections on the polished WC-Co surface, by contrast, induce some small defects in the sputtered WC/C and Cr layers (Fig. 1B, circle); however, these defects do not propagate to the DLC top layer. DLC top layers on both surfaces have the same chemical composition, namely C: 91%, O: 9%, and they possess the same sp3/ sp2 ratio, equal to (70 ± 4)%. This was evaluated from the C (KVV) Auger structure in XPS spectra [18]. The first derivatives of the electron distribution curves were calculated and the sp3/sp2 ratio was evaluated from the length difference between the minimum and maximum positions of the first derivatives, with respect to the values obtained for pure sp3 (diamond) and pure sp2 (graphite). Accordingly, the deposition parameters were the same for all films, and the interposition of the sputtered layers might have further prevented possible influences of the substrate composition on the nucleation and growth processes of DLC during ECR-CVD. The similarity between the chemical compositions of the DLC top layers deposited on the AISI1040 and WC-Co surfaces is reflected by their analogous hardness
Table 1 Critical loads of DLC-based films
Fig. 2. Hardness vs. indentation depth for DLC-based films on WC-Co and AISI1040.
Sample
Lc1 (N)
Lc2 (N)
Lc3 (N)
DLC/AISI1040 DLC/WC-Co
4.1 ± 0.5 13.5 ± 0.6
8.2 ± 0.8 21.8 ± 2.6
10.4 ± 0.3 N30 (not detectable)
G. Bolelli et al. / Surface & Coatings Technology 203 (2008) 699–703
Fig. 3. Wear rates of DLC-based films after ball-on-disk “Test I” (at RT and 400 °C) and “Test II” (at RT).
values, measured by nano-indentation tests (Fig. 2) performed at 200 nm penetration depth (i.e. less than 10% of the thickness of the DLC top layer, which is around 2.5 μm [19,20]). Hardness measurements performed at higher indentation depth return a composite hardness, bearing the influence of both the coating and the underlying surface [19,20]: the latter influence grows larger as the indentation depth increases, lowering the composite hardness value (Fig. 2), until, at 6 μm depth, it equals the hardness of the underlying surface. Among the numerous equations describing such dependence of composite hardness on indentation depth, the present data is well-fitted using the simple model by Korsunsky et al. [21] (1), whose greatest merit is the use of a limited number of fitting parameters (Fig. 2): HC ¼ HS þ
HF −HS 2
d 1 þ tTα
ð1Þ
Where: HC d t Hs HF α
composite hardness (dependent variable); indentation depth (independent variable); film thickness (constant); substrate hardness (constant); film hardness (fitting parameter); parameter related to the coating fracture energy (fitting parameter).
The substrate hardness to be used as input in Eq. (1) was measured by depth-sensing Berkovich microindentation, as specified in the Experimental section: the values are HS = (10.3 ± 1.8) GPa for WC-Co, HS = (3.2 ± 0.8) GPa for AISI1040.
701
Fig. 5. Friction coefficient evolution of DLC-based thin films under “Test II” configuration at 300 °C.
As this model is strictly valid only for single-layer thin films, its application to the present three-layer films involves an approximation; nonetheless, the satisfactory fitting quality suggests the validity of this approximation. The adjusted values of the fitting parameters are: HF= (20.9 ± 0.8)GPa; α = (0.48 ± 0.13) μmforfilmsonWC-Counderlayer; HF= (20.2 ± 1.4) GPa; α = (0.36 ± 0.11) μm for films on AISI1040 substrate. The similarity between the fitted H F values confirms the substantial independence of film hardness on the nature of the underlying surface. The parameter α is related to the resistance of the thin film-coated system against cracking, dependent on the coating's fracture toughness, on the difference between the hardness of the coating and of the underlying surface, and on the overall characteristics of the film-underlying surface system [21]. Although the error bars on α are quite large, the thin film/WC-Co system seems to exhibit a somewhat larger α value, suggesting that this system could be more resistant to cracking than the thin film/ AISI1040 steel one. This observation is confirmed by scratch test results (Table 1): whereas delamination occurs quite early on the DLC/AISI1040 system, no complete film delamination could be produced on the DLC/WC-Co system. Indeed, the softer steel substrate yields much earlier than the hard WC-Co thick layer; the brittle DLC-based film is thus subjected to a more severe stress concentration in the former case [2,22]. It can be noted that, in a previous literature paper, Lc1 values lower than the one presently measured on the DLC/WC-Co system were reported for nitrided medium carbon steel substrates [3].
Fig. 4. Wear scars after ball-on-disk “Test II” at RT: (A) DLC-based thin film on AISI1040; (B) DLC-based thin film on HVOF-sprayed WC-Co interlayer.
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Ball-on-disk tests performed under the less severe “Test I” setting, both at RT and at 400 °C, and under the more severe “Test II” setting at RT enabled the determination of the wear rate of the DLC-based films (Fig. 3). By contrast, the films were always delaminated before the end of the tests run under the “Test II” setting at 300 °C, so that the substrate started to be worn, too. The wear rates (Fig. 3) clearly indicate that the hard WC-Co underlayer is definitely beneficial in improving the tribological performance of the DLC-based thin film, consistently with all the previous results. Indeed, when the DLC-based thin film is directly deposited on the bare steel surface, it undergoes cracking and delamination during the wear tests performed at room temperature, because of yielding and plastic deformation of steel under the contact stress distribution (Fig. 4A) [23], in accordance with scratch test results. The hard WC-Co interlayer, by contrast, does not plastically deform under load; therefore, a mild abrasion groove, free of any delamination phenomena, appears in the wear scar (Fig. 4B). At 400 °C, the wear rate of the thin film/steel system becomes significantly larger, whereas the wear rate of the thin film/WC-Co system even seems to decrease slightly. Two concurrent phenomena can influence the wear behaviour of the coatings at 400 °C. On the one hand, the mechanical properties of the substrates decrease [24,25]. A further loss in load-carrying capability of the steel substrate clearly enhances the previouslydiscussed delamination phenomena, thus contributing to the observed increase in wear loss. The WC-Co interlayer also experiences a decrease in hardness [25]; nonetheless, its initially superior hardness enables it to withstand the stress distribution produced by the less severe “Test I” contact conditions, so that a smooth wear scar (similar to the room temperature one in Fig. 4B) appears again. On the other hand, the DLC top layer itself may undergo some chemical and structural alteration (release of hydrogen and decrease of the sp3/sp2 bonding ratio), resulting in a loss of hardness and wear resistance [26]. Even at 400 °C, however, the wear rate of the DLC film deposited on the WC-Co interlayer, remains close to the lowest values reported in
literature for these coatings (≈1.2⁎10− 7 mm3/(Nm)) [26–28], suggesting that the loss in the film's wear resistance does not play a major role under the present test conditions. In all the above-mentioned tests, the friction coefficient remains around or slightly below 0.1 over the whole test duration: as the DLCbased film never delaminates completely, even on the steel substrate (Fig. 4A), the film can still exert its tribological characteristics. Contrarily, complete delamination occurs during sliding when adopting the “Test II” configuration at 300 °C (Fig. 5). Under these conditions, the decrease in substrate hardness at high temperature [24,25] is combined to the extreme severity of the contact stress distribution, resulting in a complete failure of the film. This failure obviously occurs much earlier on the soft steel substrate, but it also affects the film on WC-Co. In order to investigate more deeply the reasons underlying the failure of the DLC/WC-Co system under these conditions, one ball-on-disk test was interrupted after 2000 m (i.e. before complete film failure, Fig. 5) and analysed by the FIB technique. FIB sections show a microstructural alteration of the WC-Co layer under the wear groove (compare Fig. 6A,B: the altered area is pointed by a circle): the near-surface material seems to have collapsed under the action of the contact stress and of the test temperature. Probably, the Co matrix could not support the carbide grains, which were overloaded and fractured. As plastic flow in the near-surface region of the WC-Co layer went on during the test, the fractured carbides were dispersed in the matrix itself. 4. Conclusions This research investigated the effectiveness of an HVOF-sprayed WC-Co interlayer in enhancing adhesion and wear resistance of DLCbased thin films on an AISI1040 steel substrate. Micromechanical investigation (nano-indentation, scratch testing) and ball-on-disk tribological tests indicated that the interlayer definitely reduces the film tendency to cracking and delamination, especially under localized contact. Indeed, the stress distribution produced by such contact can easily yield the steel substrate, thus overloading the brittle DLC-based thin film up to failure. Instead, the hard and thick WC-Co interlayer bears the stress distribution with reduced or no plastic deformation, keeping stresses in the film to a moderate level. Accordingly, no complete delamination could be produced during scratch testing; in ball-on-disk tests, only extremely severe conditions (high hertzian contact pressure and high temperature) could overcome the mechanical strength of the WC-Co interlayer and produce film failure. Acknowledgements Thanks to Dr. Enrico Gualtieri and Dr. Giancarlo Gazzadi for FIB imaging and to Ing. Andrea Bassani and Mr. Binit Kumar for the help with the sample preparation. The production of HVOF-sprayed WC-Co layers by Ing. Fabrizio Casadei, Mr. Edoardo Severini, Mr. Francesco Barulli and Mr. Carlo Costa (Centro Sviluppo Materiali S.p.A., Roma, Italy) is acknowledged. Partially supported by PRRIITT (Regione Emilia Romagna), Net-Lab “Surface and Coatings for Advanced Mechanics and Nanomechanics” (SUP&RMAN). References
Fig. 6. FIB sections of the DLC/WC-Co system subjected to ball-on-disk testing (“Test II” configuration at 300 °C) interrupted after 2000 m. (A) region outside the wear track; (B) region inside the wear track. Circle indicates microstructurally altered WC-Co.
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