- Email: [email protected]
Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering
Surface & Coatings Technology 201 (2006) 4099 – 4104 www.elsevier.com/locate/surfcoat
Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering M. Fenker a,⁎, M. Balzer a , H. Kappl a , A. Savan b a
b
FEM, Department POT, Katharinenstr. 17, D-73525 Schwäbisch Gmünd, Germany Combinatorial Materials Science, Forschungszentrum caesar, Ludwig-Erhard-Strasse 2, D-53175 Bonn, Germany Available online 15 September 2006
Abstract MoSx-based films were deposited using magnetron sputtering from a pure MoS2 target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr+-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoSx and Al- and Ti-alloyed MoSx coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations. The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoSx-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoSx shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoSx coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential Ecorr in potentiodynamic corrosion tests was the highest for non-alloyed MoSx coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoSx–Al (Cr+) coating on HSS. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid lubricant; MoSx; PVD; Magnetron sputtering; High speed steel; Corrosion
1. Introduction MoS2- or diamond-like-carbon-based coatings are well known materials for use as solid lubricants in tribological applications [1–3]. Generally, alloying elements have to be used to adapt the properties of these materials to specific applications. This can be accomplished in physical vapour deposition (PVD) by adding the alloying element via co-sputtering. For example, metals like Au, Ni, Pb, Cr and Ti or compounds like TiN, TiB2 were added to MoS2 coatings to increase their stability in humid air and their hardness in low-friction applications [4–8]. The tribological behaviour of MoS2-based coatings has been reported in many articles, but nothing has been published about their corrosion performance. Hence, it will be of scientific interest to fill this knowledge gap. ⁎ Corresponding author. Tel.: +49 7171 1006 49; fax: +49 7171 1006 54. E-mail address: [email protected] (M. Fenker). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.028
The high corrosion resistance of hard coatings used for decorative and/or wear protection applications is well known. However, when deposited on less noble materials like steel, brass, Al or Mg alloys, the coated parts suffer from severe corrosive attack due to inherent coating defects or inhomogeneities (cracks, pores, transient grain boundaries) [9–12]. They open possible paths for the corrosive media to reach the substrate. In the case of a less noble substrate material, galvanic corrosion at the substrate will occur. This kind of corrosion is localised to the defect area and is characterised by the anodic dissolution of the substrate material with a high anodic current density at the defect site. It is also called contact corrosion and, in the case of pores and pinholes as defects, it is called pitting corrosion. The intensity of the corrosive attack depends strongly on the potential difference of the coating and the substrate material in the respective electrolyte. If this potential difference is large enough, contact corrosion will occur.
4100
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
In this study we report on the corrosion behaviour of PVD MoSx-based coatings deposited onto high speed steel and glass substrates using magnetron sputtering. The influence of alloying elements like Al and Ti and a special interface modification (Cr+-ion etching step + Cr interlayer) on the corrosion behaviour of MoSx in NaCl-containing media is investigated. 2. Experimental details The films used in this study were grown in an industrial PVD machine (HTC-1000/4, Hauzer Techno Coatings, Venlo, NL) having the targets and samples in a vertical (sputter sideways) orientation. In this 4-target machine, two pure MoS2 targets (99%, Cerac Inc. Milwaukee, USA) were placed opposing each other, with Al or Ti in between. The Cr target was opposite to the Al. The MoS2 and Al targets were on cathodes configured for unbalanced magnetron sputtering, while the Cr target was on a cathodic arc evaporation source. The growth rates for the pure films were measured separately using stylus profilometry, and the results were used to calculate the power required for each target in order to achieve a specific alloying element concentration. The samples underwent a two-fold rotation on a continuously moving substrate table. More details about the process parameters for the depositions can be found in Table 1. Using a cathodic arc evaporation cathode, a high-power Crion etch is sometimes added to the deposition process for removing the native oxide from metal surfaces prior to thin film coating. By subsequently reducing the arc power, a Cr adhesion layer can also be deposited. However, because the arc is unfiltered, a substantial density of droplets, generally consisting of pure Cr and ranging in size from sub-micron to a few tens of microns, will also be deposited on the substrate surface. Some of these droplets may detach during later processing or handling, or even when covered by the MoS2 or MoS2–metal film can still have shadowed areas or cracks that would present a direct route for corrosion to the substrate. Therefore one sample type was prepared using the Cr-ion etch and adhesion layer method, while it was omitted for the others. High speed steel (standard no. 1.3207, S10-4-3-10) and glass substrates have been used. The depositions on glass were additionally carried out on microscope slides which where roughened on one half of the slide to improve adhesion.
Table 1 Process parameters for magnetron sputtering of MoSx-based coatings Sputtering device Target size [mm2] Cathode power [kW] MoS2 Ti Al Substrate temperature [°C] Distance target–substrate [mm] pbase [Pa] ptotal [Pa]
HTC-1000/4 (Hauzer Techno Coatings, NL) 820 × 160 2 × 2.0 1.5 0.8 150 (pumping to base vacuum; target preclean) ∼ 90 (deposition) ∼ 200 (closest approach in two-fold planetary rotation) b2 × 10− 5 (base vacuum) 1.2 × 10− 1 (sputtering, Ar)
The surface morphology of the coatings was observed by high resolution scanning electron microscopy (SEM, Leo Supra 55 VP) equipped with an energy dispersive X-ray spectroscopy (EDX) system for measuring the chemical composition. Only the chemical concentration of the alloying elements Ti and Al will be presented because of peak overlapping of the Mo and S signal in EDX measurements. The crystallographic structure was investigated by X-ray diffraction (XRD, Siemens D5000) using monochromatised Cu Kα radiation (λ = 0.15406 nm) in Bragg-Brentano configuration. The instrumented indentation test (Fischerscope H100, DIN EN ISO 14577-1) delivered the indentation hardness of the MoSx-based coatings. The maximum applied load during indentation was 30 mN. Additionally, information on the adhesion was obtained from the Rockwell C indentation test (according to the European prestandard CEN/ TS 1071-8:2004 [13]). The coating thickness was determined using a calotest device (CSEM instruments). The corrosion behaviour was electrochemically studied using open-circuit-potential (OCP) measurements and potentiodynamic corrosion (polarisation) tests. For the potentiodynamic corrosion tests a scan rate of 1 mV/s at a temperature of 25 °C was adjusted. Furthermore, a three electrode configuration and an EG&G potentiostat was used. For buffering the 0.8 M NaCl solution at pH 7 acetic acid (Fixanal®, Riedel-de Haën) was used. The deaeration was performed by bubbling nitrogen gas through the solution. The reference electrode for the measurement of the sample potential was a saturated calomel electrode (SCE). The salt spray tests were performed according to DIN 50021 (ISO 9227). The test duration was 24 h. The measuring area was about 1–2 cm2 for the OCP and the salt spray tests and about 0.2 cm2 for the potentiodynamic corrosion tests. 3. Results and discussion 3.1. Chemical composition The chemical composition of MoSx-based coatings was measured by energy-dispersive X-ray spectroscopy (EDX). The measurements are problematical due to peak overlapping of the Mo and the S signal. Similar difficulties in quantification due to peak overlaps were seen with Auger electron spectroscopy, compounded by peak position shifts of around 20 eV due to the relatively low conductivity of the films. Therefore, only the values of the alloying elements are listed in Table 2. The alloying element concentration for Al and Ti was measured to be about 20 at.%. 3.2. Crystallographic structure X-ray diffraction pattern of the MoSx-based coatings are shown in Fig. 1. As a reference also the high speed steel substrate material is given. For the non-alloyed MoSx coating several X-ray reflections can be found at the following 2θ positions: 13°, 33.5° and 59.2°. The reflections at 13° and 33.2° can belong to three different phases: hexagonal Mo15S19, rhombohedral Mo7S8 and hexagonal or rhombohedral MoS2. The reflection at 59.2° can have its origin in a Mo7S8 or a MoS2
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104 Table 2 Element analysis of Al and Ti, hardness, coating thickness, adhesion and results of the salt spray test of MoSx-based coatings on high speed steel Coating
Element analysis Al Ti [at.%] [at.%]
MoSx MoSx–Ti MoSx–Al 22 MoSx–Al + Cr+ 20
23
Hardness Coating Rockwell Salt HIT spray thickness class test [GPa] [μm] (r/a)a [pits/cm2] 4.5 ± 0.3 8.8 ± 0.6 5.7 ± 0.1 6.5 ± 0.6
2.8 3.5 5.2 5.3
3 (1.6) 3 (1.7) 3 (1.5) ???c
57–76 85–93 ???b 15–18
a Ratio of the radius r of adhesive coating failure and the radius a of the indent. b In some areas a large area corrosive attack was observable, therefore the pits could not be counted. c Classification could not be performed, due to a non-classified coating failure.
phase. The XRD patterns of all other coatings possess only a diffuse peak at 2θ ∼ 40°, resulting from overlapping of broad peaks from different MoSx phases [14]. Renevier et al. reported in [15] on MoS2/Ti composite coatings (MoST™). They found in their studies that these coatings consist of MoS2 nanocrystals or of an amorphous material with a low-order crystal symmetry. It was assumed that the titanium atoms surround the MoS2 nanocrystals. The authors of Ref. [15] stated further that the addition of other metals would lead to similar effects. Hence, it can be assumed that the addition of alloying elements like Ti and Al to MoSx inhibits the formation of crystalline MoSx phases, resulting in a diffuse peak at about 40° in the XRD patterns. 3.3. Morphology The surface morphology of the MoSx-based coatings has been studied by SEM. The morphology of the non-alloyed MoSx coating resembles a granular, cauliflower appearance. Addition of Ti and Al to MoSx seems to have no significant influence on the coating surface structure. As an example an SEM micrograph of the surface morphology of the Al-alloyed MoSx coating is shown in Fig. 2a. A dramatic change in the surface morphology can be observed if the chromium interface modification is added to the MoSx–Al deposition process (Fig. 2b). This interface modification leads to much larger grains with lateral dimensions of up to 1–2 μm. The two-fold rotation of the substrates in front of the targets could lead in principle to a multilayer structure. The study of wear scars obtained by the Calotest device for measuring the coating thickness did not show any multilayer structure in an optical microscope nor in a high resolution SEM.
to a higher hardness in the range of 5.7–8.8 GPa, with the Tialloying having the highest hardness. Fox et al. assumed for Tialloyed MoS2 (MoST™) that the strain produced within the lattice, due to the Ti addition, is responsible for the hardness increase [16]. The adhesion of the coatings to the HSS substrate was measured by Rockwell C indentation test and determined according to the European prestandard CEN/TS 1071-8:2004 (see Table 2). The Rockwell classification indicates that the adhesion is very low for three of the coatings (class 3). However, the classification could not be performed for the MoSx–Al coating with chromium interface modification, because the indent of the Rockwell diamond looked different from the classification in the European prestandard. EDX analysis of the indent showed that only at a few sites around the indent a Cr interlayer flaking occurred. The main failure mechanism was the cohesive failure of the MoSx–Al coating and adhesive failure between MoSx–Al coating and Cr interlayer. The coating thickness is in the range of 2.8–5.3 μm. It is lowest for the MoSx and MoSx–Ti coating (around 3 μm) and highest for the two MoSx–Al-based coatings (around 5 μm). This has to be taken into account when discussing the corrosion behaviour of the coatings, because the higher the coating thickness the higher usually is the corrosion resistance of the coating/substrate system [17,18]. 3.5. Open-circuit-potential measurements OCP measurements in 0.8 M NaCl solution (pH 7) of MoSxbased coatings on glass substrates are displayed in Fig. 3. The measurements reveal that the non-alloyed MoSx coating is the most noble coating. The OCP is relatively stable for 20 h and has a value of + 10 mV at the end of the measurement. All other coatings have nearly the same OCP in the range of − 70 to − 100 mV for durations N3 h. The chromium interface modification of the Al-alloyed MoSx coating has only a minor influence and makes it only a little bit more noble (about 15 mV) as compared to the non-modified coating. OCP measurements in 0.8 M NaCl solution (pH 7) of MoSxbased coatings on HSS substrates are plotted in Fig. 4. As a
3.4. Mechanical properties Hardness of the MoSx-based coatings was measured as the indentation hardness HIT and is listed in Table 2. The lowest hardness of about 4.5 GPa is found for the pure MoSx coating. This is in accordance with the reported literature values in the range of 0.4–5 GPa, but also higher values of up to 10 GPa have been published [5]. The alloying of MoSx with Ti and Al leads
4101
Fig. 1. XRD spectra of MoSx-based coatings.
4102
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
Fig. 4. Open-circuit-potential measurements of MoSx-based coatings on HSS in 0.8 M NaCl solution (pH 7).
higher thickness of the MoSx–Al coatings can be a reason for the better corrosion behaviour of these coatings in OCP measurements compared to the other two coatings. 3.6. Potentiodynamic corrosion tests Fig. 2. Two SEM micrographs of the surface morphology of the Al-alloyed MoSx coatings with and without Cr interface modification.
reference also the curve of the HSS is displayed. Only the two MoSx–Al coatings do not show any influence of the steel substrate. The OCP curves are very smooth and they are nearly identical to the MoSx–Al coatings on glass. No corrosion spot could be detected after the tests for these coatings. For the MoSx and the MoSx–Ti coating the influence of the steel substrate can be observed at test durations N 4 h. After OCP measurements one corrosion spot was visible for each of these samples. The
Fig. 3. Open-circuit-potential measurements of MoSx-based coatings on glass in 0.8 M NaCl solution (pH 7).
Potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7) of MoSx-based coatings on HSS are plotted in Fig. 5. Again the curve for the uncoated HSS is shown as a reference with a typical passivation range as it is observable for passivating steels at about − 200 mV. For the coated HSS samples a kind of “passivation range” at about +680 mV is measured only for the MoSx–Al coating with chromium interface modification. A similar curve behaviour was obtained for this coating deposited on glass substrates (not shown here). A possible reason for this behaviour could be that the MoSx–Al coating is removed in a large area due to the corrosive attack (e.g. MoSx–Al dissolution or coating spallation) and the chromium interlayer is
Fig. 5. Potentiodynamic corrosion tests of MoSx-based coatings on HSS in 0.8 M NaCl solution (pH 7).
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
coming into contact with the electrolyte. The chromium at the interface starts to passivate and forms chromium oxides and for a small potential range the coated steel obtains a certain amount of protection. But at higher potentials the chromium oxide is also dissolved as already reported by Schönjahn et al. and Lee et al. [19,20]. All other samples do not show this passivating behaviour. The corrosion behaviour of the MoSx coating is also remarkable. The corrosion potential Ecorr is at about − 50 mV, whereas the other coatings have an Ecorr in the range of about − 160 to − 200 mV. This is again an indication that the MoSx coating is more noble compared to the other coatings [21]. Additionally, in the potential range − 100 mV up to + 750 mV, the corrosion current density of the MoSx coating is lower (up to about two orders of magnitude) than that of the other coated steel samples, despite the fact that this coating has the lowest coating thickness of all the studied samples. After potentiodynamic corrosion tests a strong attack of all samples in the measured area could be observed, due to the high anodic polarisation. 3.7. Salt spray tests Salt spray tests have been performed on MoSx-based coatings on HSS. The test duration was 24 h. The tests revealed that the lowest number of corrosion pits (15–18 pits/cm2) was found for the MoSx–Al (Cr+) coating on HSS (see Table 2). This coating has the same thickness as the MoSx–Al coating, where a large area corrosive attack occurred. Observations with an optical light microscope lead to the assumption that the large area corrosive attack of the MoSx–Al coating is caused by pitting corrosion which causes coating spallation on a large area due to the low adhesion of the MoSx–Al coating to the HSS. In OCP measurements no difference between these two coatings could be identified. As already discussed, a difference was found only in potentiodynamic corrosion tests, in that a passive range at high anodic potential occurred for the MoSx–Al (Cr+) coating. For the MoSx–Al sample, we assume that the big difference in the corrosion behaviour between OCP and salt spray tests is based on the different presence of oxygen in the corrosive atmosphere. In OCP the electrolyte is deaerated, but not in salt spray tests and also air comes into contact with the samples. That means oxygen seems to play a significant role in the detrimental failure of the MoSx–Al sample. The exact failure mechanism is not clear at the moment. The corrosion pit density for the MoSx and the MoSx–Ti coating was much higher than for the MoSx–Al (Cr+) coating. The difference between these three coatings was the higher coating thickness and the coarser granular surface morphology for the MoSx–Al (Cr+) coating. It can be expected that the corrosion pit density of the MoSx and the MoSx–Ti coatings would be lower if they would have the same thickness as the MoSx–Al-based coatings. Salt spray tests on approximately 3 μm thick coatings like CrN, NbN, NbON, TiN, TiBN and TiMgN deposited by magnetron sputtering on the same HSS showed a much lower corrosion pit density (b 10 pits/cm2) [22]. We assume that the reason for this is the lower defect density (pores, cracks, grain boundaries) in the coatings compared to
4103
the MoSx-based coatings. Hence, it can be stated that the corrosion behaviour of MoSx-based coatings can be improved by e.g. a Cr interface modification and/or a higher coating thickness, but the corrosion resistance is much lower than for standard (and other) PVD coatings. The granular appearance of the MoSx-based coatings could be one reason for this much lower corrosion resistance. 4. Conclusions MoSx-based films, like MoSx, MoSx–Ti, MoSx–Al and MoSx–Al (Cr+) with a chromium interface modification were deposited onto glass and high speed steel substrates using magnetron sputtering. Properties like hardness, adhesion, chemical composition, morphology, crystallographic structure and corrosion resistance have been studied. From the investigations the following conclusion can be drawn: • The hardness of MoSx coatings is improved up to a factor of 2 by alloying the coating with Al or Ti (alloying element concentration about 20 at.%) and is in the range of 4.5– 8.8 GPa for the MoSx-based coatings. The chromium interface modification has improved the adhesion to some extent. • Alloying of the MoSx coating with Al and Ti led to the vanishing of crystalline phase and resulted in an X-ray amorphous microstructure. A granular surface morphology was found for the MoSx-based coatings. The chromium interface modification of the MoSx–Al coating led to larger grains. • In OCP measurements the non-alloyed MoSx coating on a glass substrate is the most noble coating. Alloying of the MoSx coating with Al or Ti shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoSx coating a little bit less noble. • In potentiodynamic corrosion tests a kind of “passivation range” was observed only for the MoSx–Al coating with chromium interface modification. • In salt spray tests the lowest number of corrosion pits was found for the MoSx–Al (Cr+) coating on HSS. The higher coating thickness and maybe an improved adhesion through the chromium interface modification can be responsible for this result. Hence, it can be stated that the corrosion behaviour of MoSxbased coatings can be improved by a Cr interface modification and/or a higher coating thickness, but the corrosion resistance is much lower than for standard PVD coatings. The granular appearance of the MoSx-based coatings could be one reason for this much lower corrosion resistance. Acknowledgement The authors wish to commemorate the death of Dr. Henry Haefke (CSEM, Switzerland), who initiated this collaboration between CSEM and FEM and who died in 2005 due to cancer. M. Fenker would like to thank R. Bretzler, L. Schmalz and K.
4104
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
Petrikowski (FEM) for performing different characterisation tests. We also thank Dr. Jürgen Feydt (Forschunszentrum caesar) for AES measurements. References [1] M. Berger, E. Bergmann, D. Stouder, Metall 43 (7) (1989) 631. [2] R. Gadow, D. Scherer, Ceram. Trans. 129 (2002) 125. [3] G.J. Van Der Kolk, W. Fleischer, M. Eerden, T. Hurkmans, Galvanotechnik 92 (11) (2001) 3058. [4] A. Savan, M.C. Simmonds, E. Pflüger, H. van Swygenhoven, Surf. Coat. Technol. 126 (2000) 159. [5] M.C. Simmonds, A. Savan, E. Pflüger, H. van Swygenhoven, Surf. Coat. Technol. 126 (2000) 15. [6] S. Zhang, J.H. Hsieh, B.H. Tan, P. Hing, D.G. Teer, Proc. Int. Conf. 10th, IOM, London, 1997, S. 50. [7] R. Gilmore, M.A. Baker, P.N. Gibson, W. Gissler, Surf. Coat. Technol. 105 (1998) 45. [8] J. Patscheider, R. Hauert, Nachhaltige Material-und Systemtechnik, ISSN :3-905594-21-8, vol. 113-121, 2001. [9] M.J. Park, A. Leyland, A. Matthews, Surf. Coat. Technol. 43/44 (1990) 481. [10] H.A. Jehn, M.E. Baumgärtner, Surf. Coat. Technol. 54/55 (1992) 108.
[11] H.A. Jehn, I. Pfeiffer-Schäller, Galvanotechnik 84 (10) (1993) 3283. [12] J. Creus, H. Mazille, H. Idrissi, Surf. Coat. Technol. 130 (2000) 224. [13] European prestandard: Advanced technical ceramics – Methods of test for ceramic coatings – Part 8: Rockwell indentation test for evaluation of adhesion; German version CEN/TS 1071-8:2004 [14] A. Savan, M.C. Simmonds, Y. Huang, C.P. Constable, S. Creasy, Y. Gerbig, H. Haefke, D.B. Lewis, Thin Solid Films 489 (2005) 137. [15] N.M. Renevier, J. Hamphire, V.C. Fox, J. Witts, T. Allen, D.G. Teer, Surf. Coat. Technol. 142–144 (2001) 67. [16] V. Fox, A. Jones, N.M. Renevier, D.G. Teer, Surf. Coat. Technol. 125 (2000) 347. [17] H.A. Jehn, Surf. Coat. Technol. 125 (2000) 212. [18] J. Aromaa, H. Ronkainen, A. Mahiout, S.-P. Hannula, Surf. Coat. Technol. 49 (1991) 353. [19] C. Schönjahn, H. Paritong, W.-D. Münz, R.D. Twesten, I. Petrov, J. Vac. Sci. Technol., A, Vac. Surf. Films 19 (4) (2001) 1392. [20] K.H. Lee, C.H. Park, Y.S. Yoon, H.A. Jehn, J.J. Lee, Surf. Coat. Technol. 142–144 (2001) 971. [21] I. Milosev, B. Navinsek, H.-H. Strehblow, Scientific Series of the Int. Bureau, vol. 37, Forschungszentrum Jülich GmbH, Zentralbibliothek, 1995. [22] M. Fenker, M. Balzer, H. Kappl, Thin Solid Films 515 (2006) 27.
Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering M. Fenker a,⁎, M. Balzer a , H. Kappl a , A. Savan b a
b
FEM, Department POT, Katharinenstr. 17, D-73525 Schwäbisch Gmünd, Germany Combinatorial Materials Science, Forschungszentrum caesar, Ludwig-Erhard-Strasse 2, D-53175 Bonn, Germany Available online 15 September 2006
Abstract MoSx-based films were deposited using magnetron sputtering from a pure MoS2 target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr+-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoSx and Al- and Ti-alloyed MoSx coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations. The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoSx-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoSx shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoSx coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential Ecorr in potentiodynamic corrosion tests was the highest for non-alloyed MoSx coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoSx–Al (Cr+) coating on HSS. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid lubricant; MoSx; PVD; Magnetron sputtering; High speed steel; Corrosion
1. Introduction MoS2- or diamond-like-carbon-based coatings are well known materials for use as solid lubricants in tribological applications [1–3]. Generally, alloying elements have to be used to adapt the properties of these materials to specific applications. This can be accomplished in physical vapour deposition (PVD) by adding the alloying element via co-sputtering. For example, metals like Au, Ni, Pb, Cr and Ti or compounds like TiN, TiB2 were added to MoS2 coatings to increase their stability in humid air and their hardness in low-friction applications [4–8]. The tribological behaviour of MoS2-based coatings has been reported in many articles, but nothing has been published about their corrosion performance. Hence, it will be of scientific interest to fill this knowledge gap. ⁎ Corresponding author. Tel.: +49 7171 1006 49; fax: +49 7171 1006 54. E-mail address: [email protected] (M. Fenker). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.028
The high corrosion resistance of hard coatings used for decorative and/or wear protection applications is well known. However, when deposited on less noble materials like steel, brass, Al or Mg alloys, the coated parts suffer from severe corrosive attack due to inherent coating defects or inhomogeneities (cracks, pores, transient grain boundaries) [9–12]. They open possible paths for the corrosive media to reach the substrate. In the case of a less noble substrate material, galvanic corrosion at the substrate will occur. This kind of corrosion is localised to the defect area and is characterised by the anodic dissolution of the substrate material with a high anodic current density at the defect site. It is also called contact corrosion and, in the case of pores and pinholes as defects, it is called pitting corrosion. The intensity of the corrosive attack depends strongly on the potential difference of the coating and the substrate material in the respective electrolyte. If this potential difference is large enough, contact corrosion will occur.
4100
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
In this study we report on the corrosion behaviour of PVD MoSx-based coatings deposited onto high speed steel and glass substrates using magnetron sputtering. The influence of alloying elements like Al and Ti and a special interface modification (Cr+-ion etching step + Cr interlayer) on the corrosion behaviour of MoSx in NaCl-containing media is investigated. 2. Experimental details The films used in this study were grown in an industrial PVD machine (HTC-1000/4, Hauzer Techno Coatings, Venlo, NL) having the targets and samples in a vertical (sputter sideways) orientation. In this 4-target machine, two pure MoS2 targets (99%, Cerac Inc. Milwaukee, USA) were placed opposing each other, with Al or Ti in between. The Cr target was opposite to the Al. The MoS2 and Al targets were on cathodes configured for unbalanced magnetron sputtering, while the Cr target was on a cathodic arc evaporation source. The growth rates for the pure films were measured separately using stylus profilometry, and the results were used to calculate the power required for each target in order to achieve a specific alloying element concentration. The samples underwent a two-fold rotation on a continuously moving substrate table. More details about the process parameters for the depositions can be found in Table 1. Using a cathodic arc evaporation cathode, a high-power Crion etch is sometimes added to the deposition process for removing the native oxide from metal surfaces prior to thin film coating. By subsequently reducing the arc power, a Cr adhesion layer can also be deposited. However, because the arc is unfiltered, a substantial density of droplets, generally consisting of pure Cr and ranging in size from sub-micron to a few tens of microns, will also be deposited on the substrate surface. Some of these droplets may detach during later processing or handling, or even when covered by the MoS2 or MoS2–metal film can still have shadowed areas or cracks that would present a direct route for corrosion to the substrate. Therefore one sample type was prepared using the Cr-ion etch and adhesion layer method, while it was omitted for the others. High speed steel (standard no. 1.3207, S10-4-3-10) and glass substrates have been used. The depositions on glass were additionally carried out on microscope slides which where roughened on one half of the slide to improve adhesion.
Table 1 Process parameters for magnetron sputtering of MoSx-based coatings Sputtering device Target size [mm2] Cathode power [kW] MoS2 Ti Al Substrate temperature [°C] Distance target–substrate [mm] pbase [Pa] ptotal [Pa]
HTC-1000/4 (Hauzer Techno Coatings, NL) 820 × 160 2 × 2.0 1.5 0.8 150 (pumping to base vacuum; target preclean) ∼ 90 (deposition) ∼ 200 (closest approach in two-fold planetary rotation) b2 × 10− 5 (base vacuum) 1.2 × 10− 1 (sputtering, Ar)
The surface morphology of the coatings was observed by high resolution scanning electron microscopy (SEM, Leo Supra 55 VP) equipped with an energy dispersive X-ray spectroscopy (EDX) system for measuring the chemical composition. Only the chemical concentration of the alloying elements Ti and Al will be presented because of peak overlapping of the Mo and S signal in EDX measurements. The crystallographic structure was investigated by X-ray diffraction (XRD, Siemens D5000) using monochromatised Cu Kα radiation (λ = 0.15406 nm) in Bragg-Brentano configuration. The instrumented indentation test (Fischerscope H100, DIN EN ISO 14577-1) delivered the indentation hardness of the MoSx-based coatings. The maximum applied load during indentation was 30 mN. Additionally, information on the adhesion was obtained from the Rockwell C indentation test (according to the European prestandard CEN/ TS 1071-8:2004 [13]). The coating thickness was determined using a calotest device (CSEM instruments). The corrosion behaviour was electrochemically studied using open-circuit-potential (OCP) measurements and potentiodynamic corrosion (polarisation) tests. For the potentiodynamic corrosion tests a scan rate of 1 mV/s at a temperature of 25 °C was adjusted. Furthermore, a three electrode configuration and an EG&G potentiostat was used. For buffering the 0.8 M NaCl solution at pH 7 acetic acid (Fixanal®, Riedel-de Haën) was used. The deaeration was performed by bubbling nitrogen gas through the solution. The reference electrode for the measurement of the sample potential was a saturated calomel electrode (SCE). The salt spray tests were performed according to DIN 50021 (ISO 9227). The test duration was 24 h. The measuring area was about 1–2 cm2 for the OCP and the salt spray tests and about 0.2 cm2 for the potentiodynamic corrosion tests. 3. Results and discussion 3.1. Chemical composition The chemical composition of MoSx-based coatings was measured by energy-dispersive X-ray spectroscopy (EDX). The measurements are problematical due to peak overlapping of the Mo and the S signal. Similar difficulties in quantification due to peak overlaps were seen with Auger electron spectroscopy, compounded by peak position shifts of around 20 eV due to the relatively low conductivity of the films. Therefore, only the values of the alloying elements are listed in Table 2. The alloying element concentration for Al and Ti was measured to be about 20 at.%. 3.2. Crystallographic structure X-ray diffraction pattern of the MoSx-based coatings are shown in Fig. 1. As a reference also the high speed steel substrate material is given. For the non-alloyed MoSx coating several X-ray reflections can be found at the following 2θ positions: 13°, 33.5° and 59.2°. The reflections at 13° and 33.2° can belong to three different phases: hexagonal Mo15S19, rhombohedral Mo7S8 and hexagonal or rhombohedral MoS2. The reflection at 59.2° can have its origin in a Mo7S8 or a MoS2
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104 Table 2 Element analysis of Al and Ti, hardness, coating thickness, adhesion and results of the salt spray test of MoSx-based coatings on high speed steel Coating
Element analysis Al Ti [at.%] [at.%]
MoSx MoSx–Ti MoSx–Al 22 MoSx–Al + Cr+ 20
23
Hardness Coating Rockwell Salt HIT spray thickness class test [GPa] [μm] (r/a)a [pits/cm2] 4.5 ± 0.3 8.8 ± 0.6 5.7 ± 0.1 6.5 ± 0.6
2.8 3.5 5.2 5.3
3 (1.6) 3 (1.7) 3 (1.5) ???c
57–76 85–93 ???b 15–18
a Ratio of the radius r of adhesive coating failure and the radius a of the indent. b In some areas a large area corrosive attack was observable, therefore the pits could not be counted. c Classification could not be performed, due to a non-classified coating failure.
phase. The XRD patterns of all other coatings possess only a diffuse peak at 2θ ∼ 40°, resulting from overlapping of broad peaks from different MoSx phases [14]. Renevier et al. reported in [15] on MoS2/Ti composite coatings (MoST™). They found in their studies that these coatings consist of MoS2 nanocrystals or of an amorphous material with a low-order crystal symmetry. It was assumed that the titanium atoms surround the MoS2 nanocrystals. The authors of Ref. [15] stated further that the addition of other metals would lead to similar effects. Hence, it can be assumed that the addition of alloying elements like Ti and Al to MoSx inhibits the formation of crystalline MoSx phases, resulting in a diffuse peak at about 40° in the XRD patterns. 3.3. Morphology The surface morphology of the MoSx-based coatings has been studied by SEM. The morphology of the non-alloyed MoSx coating resembles a granular, cauliflower appearance. Addition of Ti and Al to MoSx seems to have no significant influence on the coating surface structure. As an example an SEM micrograph of the surface morphology of the Al-alloyed MoSx coating is shown in Fig. 2a. A dramatic change in the surface morphology can be observed if the chromium interface modification is added to the MoSx–Al deposition process (Fig. 2b). This interface modification leads to much larger grains with lateral dimensions of up to 1–2 μm. The two-fold rotation of the substrates in front of the targets could lead in principle to a multilayer structure. The study of wear scars obtained by the Calotest device for measuring the coating thickness did not show any multilayer structure in an optical microscope nor in a high resolution SEM.
to a higher hardness in the range of 5.7–8.8 GPa, with the Tialloying having the highest hardness. Fox et al. assumed for Tialloyed MoS2 (MoST™) that the strain produced within the lattice, due to the Ti addition, is responsible for the hardness increase [16]. The adhesion of the coatings to the HSS substrate was measured by Rockwell C indentation test and determined according to the European prestandard CEN/TS 1071-8:2004 (see Table 2). The Rockwell classification indicates that the adhesion is very low for three of the coatings (class 3). However, the classification could not be performed for the MoSx–Al coating with chromium interface modification, because the indent of the Rockwell diamond looked different from the classification in the European prestandard. EDX analysis of the indent showed that only at a few sites around the indent a Cr interlayer flaking occurred. The main failure mechanism was the cohesive failure of the MoSx–Al coating and adhesive failure between MoSx–Al coating and Cr interlayer. The coating thickness is in the range of 2.8–5.3 μm. It is lowest for the MoSx and MoSx–Ti coating (around 3 μm) and highest for the two MoSx–Al-based coatings (around 5 μm). This has to be taken into account when discussing the corrosion behaviour of the coatings, because the higher the coating thickness the higher usually is the corrosion resistance of the coating/substrate system [17,18]. 3.5. Open-circuit-potential measurements OCP measurements in 0.8 M NaCl solution (pH 7) of MoSxbased coatings on glass substrates are displayed in Fig. 3. The measurements reveal that the non-alloyed MoSx coating is the most noble coating. The OCP is relatively stable for 20 h and has a value of + 10 mV at the end of the measurement. All other coatings have nearly the same OCP in the range of − 70 to − 100 mV for durations N3 h. The chromium interface modification of the Al-alloyed MoSx coating has only a minor influence and makes it only a little bit more noble (about 15 mV) as compared to the non-modified coating. OCP measurements in 0.8 M NaCl solution (pH 7) of MoSxbased coatings on HSS substrates are plotted in Fig. 4. As a
3.4. Mechanical properties Hardness of the MoSx-based coatings was measured as the indentation hardness HIT and is listed in Table 2. The lowest hardness of about 4.5 GPa is found for the pure MoSx coating. This is in accordance with the reported literature values in the range of 0.4–5 GPa, but also higher values of up to 10 GPa have been published [5]. The alloying of MoSx with Ti and Al leads
4101
Fig. 1. XRD spectra of MoSx-based coatings.
4102
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
Fig. 4. Open-circuit-potential measurements of MoSx-based coatings on HSS in 0.8 M NaCl solution (pH 7).
higher thickness of the MoSx–Al coatings can be a reason for the better corrosion behaviour of these coatings in OCP measurements compared to the other two coatings. 3.6. Potentiodynamic corrosion tests Fig. 2. Two SEM micrographs of the surface morphology of the Al-alloyed MoSx coatings with and without Cr interface modification.
reference also the curve of the HSS is displayed. Only the two MoSx–Al coatings do not show any influence of the steel substrate. The OCP curves are very smooth and they are nearly identical to the MoSx–Al coatings on glass. No corrosion spot could be detected after the tests for these coatings. For the MoSx and the MoSx–Ti coating the influence of the steel substrate can be observed at test durations N 4 h. After OCP measurements one corrosion spot was visible for each of these samples. The
Fig. 3. Open-circuit-potential measurements of MoSx-based coatings on glass in 0.8 M NaCl solution (pH 7).
Potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7) of MoSx-based coatings on HSS are plotted in Fig. 5. Again the curve for the uncoated HSS is shown as a reference with a typical passivation range as it is observable for passivating steels at about − 200 mV. For the coated HSS samples a kind of “passivation range” at about +680 mV is measured only for the MoSx–Al coating with chromium interface modification. A similar curve behaviour was obtained for this coating deposited on glass substrates (not shown here). A possible reason for this behaviour could be that the MoSx–Al coating is removed in a large area due to the corrosive attack (e.g. MoSx–Al dissolution or coating spallation) and the chromium interlayer is
Fig. 5. Potentiodynamic corrosion tests of MoSx-based coatings on HSS in 0.8 M NaCl solution (pH 7).
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
coming into contact with the electrolyte. The chromium at the interface starts to passivate and forms chromium oxides and for a small potential range the coated steel obtains a certain amount of protection. But at higher potentials the chromium oxide is also dissolved as already reported by Schönjahn et al. and Lee et al. [19,20]. All other samples do not show this passivating behaviour. The corrosion behaviour of the MoSx coating is also remarkable. The corrosion potential Ecorr is at about − 50 mV, whereas the other coatings have an Ecorr in the range of about − 160 to − 200 mV. This is again an indication that the MoSx coating is more noble compared to the other coatings [21]. Additionally, in the potential range − 100 mV up to + 750 mV, the corrosion current density of the MoSx coating is lower (up to about two orders of magnitude) than that of the other coated steel samples, despite the fact that this coating has the lowest coating thickness of all the studied samples. After potentiodynamic corrosion tests a strong attack of all samples in the measured area could be observed, due to the high anodic polarisation. 3.7. Salt spray tests Salt spray tests have been performed on MoSx-based coatings on HSS. The test duration was 24 h. The tests revealed that the lowest number of corrosion pits (15–18 pits/cm2) was found for the MoSx–Al (Cr+) coating on HSS (see Table 2). This coating has the same thickness as the MoSx–Al coating, where a large area corrosive attack occurred. Observations with an optical light microscope lead to the assumption that the large area corrosive attack of the MoSx–Al coating is caused by pitting corrosion which causes coating spallation on a large area due to the low adhesion of the MoSx–Al coating to the HSS. In OCP measurements no difference between these two coatings could be identified. As already discussed, a difference was found only in potentiodynamic corrosion tests, in that a passive range at high anodic potential occurred for the MoSx–Al (Cr+) coating. For the MoSx–Al sample, we assume that the big difference in the corrosion behaviour between OCP and salt spray tests is based on the different presence of oxygen in the corrosive atmosphere. In OCP the electrolyte is deaerated, but not in salt spray tests and also air comes into contact with the samples. That means oxygen seems to play a significant role in the detrimental failure of the MoSx–Al sample. The exact failure mechanism is not clear at the moment. The corrosion pit density for the MoSx and the MoSx–Ti coating was much higher than for the MoSx–Al (Cr+) coating. The difference between these three coatings was the higher coating thickness and the coarser granular surface morphology for the MoSx–Al (Cr+) coating. It can be expected that the corrosion pit density of the MoSx and the MoSx–Ti coatings would be lower if they would have the same thickness as the MoSx–Al-based coatings. Salt spray tests on approximately 3 μm thick coatings like CrN, NbN, NbON, TiN, TiBN and TiMgN deposited by magnetron sputtering on the same HSS showed a much lower corrosion pit density (b 10 pits/cm2) [22]. We assume that the reason for this is the lower defect density (pores, cracks, grain boundaries) in the coatings compared to
4103
the MoSx-based coatings. Hence, it can be stated that the corrosion behaviour of MoSx-based coatings can be improved by e.g. a Cr interface modification and/or a higher coating thickness, but the corrosion resistance is much lower than for standard (and other) PVD coatings. The granular appearance of the MoSx-based coatings could be one reason for this much lower corrosion resistance. 4. Conclusions MoSx-based films, like MoSx, MoSx–Ti, MoSx–Al and MoSx–Al (Cr+) with a chromium interface modification were deposited onto glass and high speed steel substrates using magnetron sputtering. Properties like hardness, adhesion, chemical composition, morphology, crystallographic structure and corrosion resistance have been studied. From the investigations the following conclusion can be drawn: • The hardness of MoSx coatings is improved up to a factor of 2 by alloying the coating with Al or Ti (alloying element concentration about 20 at.%) and is in the range of 4.5– 8.8 GPa for the MoSx-based coatings. The chromium interface modification has improved the adhesion to some extent. • Alloying of the MoSx coating with Al and Ti led to the vanishing of crystalline phase and resulted in an X-ray amorphous microstructure. A granular surface morphology was found for the MoSx-based coatings. The chromium interface modification of the MoSx–Al coating led to larger grains. • In OCP measurements the non-alloyed MoSx coating on a glass substrate is the most noble coating. Alloying of the MoSx coating with Al or Ti shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoSx coating a little bit less noble. • In potentiodynamic corrosion tests a kind of “passivation range” was observed only for the MoSx–Al coating with chromium interface modification. • In salt spray tests the lowest number of corrosion pits was found for the MoSx–Al (Cr+) coating on HSS. The higher coating thickness and maybe an improved adhesion through the chromium interface modification can be responsible for this result. Hence, it can be stated that the corrosion behaviour of MoSxbased coatings can be improved by a Cr interface modification and/or a higher coating thickness, but the corrosion resistance is much lower than for standard PVD coatings. The granular appearance of the MoSx-based coatings could be one reason for this much lower corrosion resistance. Acknowledgement The authors wish to commemorate the death of Dr. Henry Haefke (CSEM, Switzerland), who initiated this collaboration between CSEM and FEM and who died in 2005 due to cancer. M. Fenker would like to thank R. Bretzler, L. Schmalz and K.
4104
M. Fenker et al. / Surface & Coatings Technology 201 (2006) 4099–4104
Petrikowski (FEM) for performing different characterisation tests. We also thank Dr. Jürgen Feydt (Forschunszentrum caesar) for AES measurements. References [1] M. Berger, E. Bergmann, D. Stouder, Metall 43 (7) (1989) 631. [2] R. Gadow, D. Scherer, Ceram. Trans. 129 (2002) 125. [3] G.J. Van Der Kolk, W. Fleischer, M. Eerden, T. Hurkmans, Galvanotechnik 92 (11) (2001) 3058. [4] A. Savan, M.C. Simmonds, E. Pflüger, H. van Swygenhoven, Surf. Coat. Technol. 126 (2000) 159. [5] M.C. Simmonds, A. Savan, E. Pflüger, H. van Swygenhoven, Surf. Coat. Technol. 126 (2000) 15. [6] S. Zhang, J.H. Hsieh, B.H. Tan, P. Hing, D.G. Teer, Proc. Int. Conf. 10th, IOM, London, 1997, S. 50. [7] R. Gilmore, M.A. Baker, P.N. Gibson, W. Gissler, Surf. Coat. Technol. 105 (1998) 45. [8] J. Patscheider, R. Hauert, Nachhaltige Material-und Systemtechnik, ISSN :3-905594-21-8, vol. 113-121, 2001. [9] M.J. Park, A. Leyland, A. Matthews, Surf. Coat. Technol. 43/44 (1990) 481. [10] H.A. Jehn, M.E. Baumgärtner, Surf. Coat. Technol. 54/55 (1992) 108.
[11] H.A. Jehn, I. Pfeiffer-Schäller, Galvanotechnik 84 (10) (1993) 3283. [12] J. Creus, H. Mazille, H. Idrissi, Surf. Coat. Technol. 130 (2000) 224. [13] European prestandard: Advanced technical ceramics – Methods of test for ceramic coatings – Part 8: Rockwell indentation test for evaluation of adhesion; German version CEN/TS 1071-8:2004 [14] A. Savan, M.C. Simmonds, Y. Huang, C.P. Constable, S. Creasy, Y. Gerbig, H. Haefke, D.B. Lewis, Thin Solid Films 489 (2005) 137. [15] N.M. Renevier, J. Hamphire, V.C. Fox, J. Witts, T. Allen, D.G. Teer, Surf. Coat. Technol. 142–144 (2001) 67. [16] V. Fox, A. Jones, N.M. Renevier, D.G. Teer, Surf. Coat. Technol. 125 (2000) 347. [17] H.A. Jehn, Surf. Coat. Technol. 125 (2000) 212. [18] J. Aromaa, H. Ronkainen, A. Mahiout, S.-P. Hannula, Surf. Coat. Technol. 49 (1991) 353. [19] C. Schönjahn, H. Paritong, W.-D. Münz, R.D. Twesten, I. Petrov, J. Vac. Sci. Technol., A, Vac. Surf. Films 19 (4) (2001) 1392. [20] K.H. Lee, C.H. Park, Y.S. Yoon, H.A. Jehn, J.J. Lee, Surf. Coat. Technol. 142–144 (2001) 971. [21] I. Milosev, B. Navinsek, H.-H. Strehblow, Scientific Series of the Int. Bureau, vol. 37, Forschungszentrum Jülich GmbH, Zentralbibliothek, 1995. [22] M. Fenker, M. Balzer, H. Kappl, Thin Solid Films 515 (2006) 27.