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Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater
Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater Vladimir Totolin, Vladimir Pejakovi´c, Thomas Csanyi, Oliver Hekele, Martin Huber, Manel Rodr´ıguez Ripoll PII: DOI: Reference:
S0264-1275(16)30563-9 doi: 10.1016/j.matdes.2016.04.080 JMADE 1724
To appear in: Received date: Revised date: Accepted date:
25 January 2016 22 March 2016 25 April 2016
Please cite this article as: Vladimir Totolin, Vladimir Pejakovi´c, Thomas Csanyi, Oliver Hekele, Martin Huber, Manel Rodr´ıguez Ripoll, Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater, (2016), doi: 10.1016/j.matdes.2016.04.080
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ACCEPTED MANUSCRIPT Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater
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AC2T research GmbH, Wiener Neustadt, Austria IMI Critical Engineering / IMI CCI / CCI Valve Technology GmbH, Vienna, Austria
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Vladimir Totolin a,*, Vladimir Pejaković a, Thomas Csanyi b, Oliver Hekele b, Martin Huber b and Manel Rodríguez Ripoll a
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* Corresponding author: Dr. Vladimir Totolin AC2T research GmbH Viktor-Kaplan-Strasse 2/C 2700 Wiener Neustadt, Austria Email: [email protected]
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Abstract
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Titanium and its alloys are materials with excellent corrosion resistant properties, but under sliding contact exhibit poor wear and friction performance. Therefore, the aim of this study was to find suitable surface treatments for titanium alloy substrates to be used under saline environmental conditions. Various surface engineering techniques have been applied to Ti6Al4V substrates, and the static corrosion performance of these surface treatments in presence of artificial seawater was evaluated using potentiodynamic investigations. Furthermore, the combined sliding-corrosion performance was investigated using a microtribometer coupled to an electrochemical cell. Using this set-up, the selected samples were tested in order to check the synergistic effect of wear and corrosion, also known as tribocorrosion. The results clearly revealed a poor tribological performance of the untreated Ti6Al4V substrate, thus highlighting the need for surface treatments. Furthermore, it was highlighted that some of the compared surface engineering treatments applied to Ti6Al4V although exhibited improved corrosion resistance under static conditions, they showed a radically different behaviour under tribocorrosion. The underlying reasons leading to a different tribocorrosion performance are discussed and the main mechanisms associated with surface degradation under combined wear and corrosion are revealed.
Keywords: Ti6Al4V, DLC, HVOF, ion implantation, tribocorrosion, seawater
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ACCEPTED MANUSCRIPT 1.
Introduction Tribocorrosion is a surface degradation process resulting from simultaneous
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tribological and electrochemical actions in a corrosive environment [1-4]. The synergism
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between wear and corrosion may lead to an accelerated degradation of passive metals. For
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instance, the mechanical removal of a passivation layer usually leads to wear-accelerated corrosion due to chemical reactivity of the nascent metal surface exposed to the corrosive environment. Therefore, it is important to identify the contribution of corrosion and wear to
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material removal during a tribocorrosion process in order to minimize material degradation [1,
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4]. Over the past two decades, significant work has been done regarding to tribocorrosion as previously reported in several reviews [1, 2]. Various triboelectrochemical techniques have
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been applied on a variety of sliding contact conditions such as unidirectional or reciprocating
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motion, fretting or spinning contacts. In an attempt to identify the fundamental tribocorrosion mechanisms, the most investigated materials were model alloys [3, 5, 6], followed by
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biomedical alloys [7, 8, 9], sealing materials [10, 11], materials for nuclear reactors [12] and materials used in chemical mechanical polishing process [13, 14].
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It is widely known that the synergistic action of wear and corrosion can also lead to premature failure of engineering components in seawater environments. Nevertheless, some crucial components used in marine equipment, such as pumps, valves, gears and propellers have to be directly lubricated by seawater [15, 16]. Their safety, reliability and service life greatly depend on their combined tribological and corrosive performance in harsh marine environments [17]. Common corrosion resistant materials used in seawater environments, such as titanium alloys, offer a very poor tribological performance and failed to meet the mechanical requirements of highly-demanding applications [18]. One of the most costeffective approaches for improving the tribological properties of titanium alloys is to include advanced surface engineering technologies in the component design. By doing so, surface
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ACCEPTED MANUSCRIPT engineered titanium components could be able to withstand seawater environment while simultaneously offering an improved wear protection.
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The work regarding tribocorrosion of engineered surfaces is rather limited and only a
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few studies have been reported in literature [17-26]. Moreover, understanding the
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tribocorrosion of surface engineered surfaces is often more difficult in comparison to bulk metals due to the porosity and oxide pockets that are usually formed during the coating process [2]. In this case subsurface corrosion can occur at the interface with the substrate or
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interlayers.
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In recent years, diamond-like carbon (DLC) films have been the subject of extensive investigations due to their potential of attaining a combination of highly desirable properties such as high hardness, low friction and wear, electrical insulation and chemical inertness [21,
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22]. Azzi et al. investigated the tribocorrosion behavior of DLC-coated 316L stainless steel exposed to the Ringer’s solution in the context of biomedical applications [21]. Two different
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bond layers were studied, namely amorphous hydrogenated silicon nitride (a-SiNx:H) and plasma nitrided layers. It was found that the nitrided bond layer showed a significant
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improvement in wear resistance of the DLC coatings in dry conditions. However, under sliding wear in simulated body fluid conditions, this bond layer proved to be insufficient, mainly due to the infiltration of the liquid through the pores and the weakening of the interface due to corrosion processes. On the other hand, the use of a-SiNx:H as an interface layer significantly improved the wear resistance of DLC films in Ringer’s solution. Costa et al. studied the friction and wear rate of DLC films under seawater and saline solutions using two different tribological systems, a stainless steel vessel and a PTFE one [23]. Although the 20% hydrogenated DLC film confirmed to be effective in terms of corrosion and wear resistance under saline solutions for both vessels, better results were achieved using the stainless steel vessel.
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ACCEPTED MANUSCRIPT Another effective method to reduce wear and corrosion of metallic components is using ceramic-metallic (cermet) or hard oxide coatings. High Velocity Oxygen Fuel (HVOF)
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coatings with compositions of 86WC-10Co-4Cr or corrosion resistant alloys such as nickel
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aluminum bronze have been considered for wear-corrosion applications [27-29]. In general, a
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particular concern for these coatings is their level of porosity (>2%) that can accelerate crack propagation and coating removal under erosive conditions. Moreover, the permeation of the electrolyte through the interconnected pores into the coating/substrate interface could lead to
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corrosion-driven coating-substrate disbondment [2].
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Surface engineering of metals by plasma nitriding techniques can be used to improve tribocorrosion [18, 25]. Galliano et al. investigated the tribocorrosion performance of a plasma nitrided Ti6Al4V alloy coating in 0.9 wt% NaCl solutions under reciprocating
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alumina ball-on-flat sliding wear [18]. It was found that samples nitrided at higher temperatures (1173K) showed better wear resistance without crack formation compared to
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samples nitrided at lower temperatures (973K) which micro-cracked causing failure. The improvement in anti-wear properties was attributed to a surface hardened layer of TiN-Ti2N.
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It is noteworthy that, in general, the various tribocorrosion studies previously reported in literature mostly focused on the assessment of a single surface engineering technology. Furthermore, the investigated surface technology was often evaluated in terms of the role of process parameters or was simply compared with the untreated substrate material. A systematic approach that investigates the wear and corrosion protection of different surface engineering techniques in artificial seawater seems to be lacking in the existing literature. Such a study is essential for providing design rules for components able to withstand operations under offshore environments. For this reason, the aim of the present work is to investigate and compare the tribocorrosion behavior of DLC coatings, HVOF coatings and Ion implantation treatments applied to titanium substrates exposed to artificial seawater in the context of offshore applications. 4
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ACCEPTED MANUSCRIPT 2.
Experimental
2.1.
Surface technologies
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The substrate investigated was a titanium alloy (Ti6Al4V), commercially available
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were applied to the substrate and are summarized in Table 1.
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from Alba Industrial AS (Bekkestua, Norway). Three different surface engineering treatments
Sample pre-treatments and their corresponding hardness and roughness
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Table 1.
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Sample Treatment Hardness (GPa) Roughness, Ra (µm) Ti6Al4V None 3.3 0.3 W-DLC CrN + a-C:H:W Topcoat 15.9 0.3 Cr3C2 embedded in Ni/Cr matrix HVOF 14.0 0.3 Ion implantation Low pressure nitriding 16.0 0.4
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The first surface treatment (referred as W-DLC) comprised of a multifunctional coating applied in a single-pass Physical-Vapor Deposition process at temperatures between
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180 and 350 °C and was commercially available from Oerlikon Balzers Coating Austria GmbH (Stainz, Austria). The multifunctional coating consisted of a tribologically effective
resistance.
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tungsten-doped hydrogenated DLC top layer and a CrN intermediate layer for corrosion
The second surface treatment (referred as HVOF), commercially available from TeroLab Surface GmbH (Vienna, Austria) consisted of a hard metallic structure made of chromium carbides with a content of 75% weight. The carbides were embedded in a metallic matrix made of nickel and chromium. The coating was deposited using a HVOF thermal spray process to obtain a dense, homogenous and ultra-hard coating. The third surface treatment (referred as Ion implantation), commercially available from Bodycote SAS (Pusignan, France) was based on nitrogen diffusion into titanium performed under vacuum at low pressure, without the use of a plasma. The diffusion of 6
ACCEPTED MANUSCRIPT nitrogen created a compound layer of TiN+Ti2N and a diffusion layer of Ti (N) underneath. The chemical composition of the samples is given in Table 2.
Table 2.
Al 8.2 1.6
Si 1.0
N 55.9
Chemical composition (at. %) of the samples
Tribocorrosion investigations
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2.2.
Chemical composition (atomic %) Cr Ni W 4.3 2.6 11.5 43.6 9.1 0.1 -
Ti 91.0 1.2 41.5
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O 15.8 11.2 -
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Ti6Al4V W-DLC HVOF Ion implantation
C 64.6 36.0 -
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Sample
Tribocorrosion experiments were performed at room temperature using a using a
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linear reciprocating ball-on-flat tribometer TETRA BASALT®-N from Falex Tribology N.V. (Rotselaar, Belgium) described in a previous publication [30]. The detailed experimental
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parameters are displayed in Table 3. The average contact pressures for all the tested materials
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have been estimated from the Hertzian contact stress theory. The calculated average contact
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pressures for Ti6Al4V, W-DLC, HVOF and Ion implantation are 200 MPa, 230 MPa, 160
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MPa and 240 MPa, respectively.
Test rig parameters
Ball Specimens Disc
Applied load (mN) Speed (mm/s) Stroke (mm) Temperature (°C) Number of cycles Repetitions/ sample
100 2 1 22 5000 3
Material Diameter (mm) Young's modulus (GPa)
Al2 O3 Ceramic 5 300
Poisson's ratio Bulk Material Size (mm²) Young's modulus (GPa) Poisson's ratio
0.21 Ti6Al4V 10 113.8 0.34
Table 3.
Test rig parameters
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ACCEPTED MANUSCRIPT During a typical experiment an alumina ball (5 mm diameter) slides against the sample specimen fully immersed in artificial seawater (2.5% NaCl, pH = 7.6) made according to
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ASTM Standard D1141-13. The testing sample serves as the working electrode (WE) and its
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potential is monitored using a potentiostat VERSASTAT 3F from AMETEK GmbH
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(Meerbusch, Germany). The reference electrode (REF) was an Ag/AgCl in 3 M saturated
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NaCl solution and the counter electrode was a platinum plate (Fig. 1).
Figure 1.
Experimental setup for tribocorrosion experiments
The tribocorrosion method used consisted of four sequences: cathodic cleaning of the samples at -1.2 V (vs. Ag/AgCl) for 60 s for dissolution of air-formed oxides [1]; stabilization of the system under Open Circuit Potential (OCP) for 8000 s (in absence of rubbing) to achieve a stable passive surface; rubbing under OCP for 6500 s, and finally, re-stabilization of the system under OCP (after rubbing was stopped) for 1800 s. The tribocorrosion experiments 8
ACCEPTED MANUSCRIPT were repeated three times for the same surface condition in order to validate the results obtained.
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Prior to the tribocorrosion experiments, potentiodynamic polarization scans were
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performed in order to monitor the materials behavior under controlled electrochemical
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conditions. The samples were fully immersed in the artificial seawater and the measurements were carried out using the same three electrode setup described earlier. In this case, the potentiodynamic polarization method consisted of the following sequences: initial cathodic
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cleaning of the samples at - 1.2 V (vs. Ag/AgCl) for 60 s; stabilization of the system under
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OCP for 3600 s, and finally, potentiodynamic polarization measurements were carried out between -1.2 V and +1.2 V (vs. Ag/AgCl) at a scan rate of 1 mV/s. The measured currentvoltage data were plotted as Tafel plot in the form of potential versus log (i) (the current
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density). The corrosion potential (Ecorr) and corrosion current (Icorr) were derived from the Tafel plot. The corrosion current was obtained using the Stern-Geary model for a corroding
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system [31]. Two potentiodynamic experiments for each sample have been conducted, and due to the similarity of the curves, only one representative curve for each sample is illustrated.
Surface characterization
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2.3.
The surface topography before and after the tribocorrosion experiments was analyzed with a Leica DCM 3D confocal microscope Ernst Leiz GmbH (Wetzlar, Germany). The removed material volumes (µm³) from all the tested samples were calculated from the topographical data. An ideal flat surface was defined as the reference surface for the discs and the missing volume between this surface and the measured (worn) surface was calculated with a Matlab based program. Hardness measurements were performed on cross-sections of the samples using Vickers hardness test (HV 0.05 kgf) performed with a Microhardness tester FM 700 (FutureTech Corp., Kawasaki-City, Japan) for the thick coatings (HVOF) and nanoindentation 9
ACCEPTED MANUSCRIPT hardness (10 mN applied pressure) performed with a Hysitron TriboIndenter TI900 (Hysitron, Inc., Minneapolis, USA) for the surface treatments leading to thinner coatings (W-DLC and
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Ion implantation).
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The wear scar morphology and cross-section analyses of the coated substrates were
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investigated with a scanning electron microscope (SEM) ZEISS 1540XB crossbeam Carl Zeiss Microscopy GmbH (Jena, Germany) operated at an accelerated voltage of 20 kV. The chemical surface composition of the tested samples was characterized by X-ray Photoelectron
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Spectroscopy (XPS). The analyses were conducted using a Thetaprobe Thermo Fisher
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Scientific (East Grinstead, United Kingdom) equipped with a monochromatic Al Kα X-ray source (hv = 1486.6 eV) and a hemispherical analyzer.
Results and discussion
3.1.
Characterization of the engineered surfaces
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SEM images of the polished cross-section from W-DLC (top), HVOF (middle) and Ion implantation (bottom) coated Ti6Al4V substrates are shown in Fig. 2. As described in section 2.1., the W-DLC coated Ti6Al4V substrate has a double layer coating with the
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approximate overall thickness of 2.2 µm (1.4 µm for the a-C:H:W top layer and 800 nm for the CrN inter layer). For the HVOF coated Ti6Al4V substrate, the thickness is approximately 300 µm and the coating exhibits a relatively dense and coherent microstructure, consisting of a Ni-Cr metallic binder and a dispersed Cr3C2 phase (Fig. 2, middle). Some debris from sand blasting can be observed at the interface between the substrate and coating. In case of the Ion implantation treated substrate, the presence of the nitride layer is confirmed by the SEM micrograph, and the layer thickness is approximately 7 µm (Fig. 2, bottom).
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Figure 2.
SEM cross-sectional micrographs of W-DLC (top), HVOF (middle) and Ion implantation treated Ti6Al4V substrate (bottom)
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ACCEPTED MANUSCRIPT Hardness measurements (Table 1) reveal a dramatic increase in the surface hardness (between 14 and 16 GPa), regardless of the applied surface treatment, and the results are in
Potentiodynamic polarization measurements
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3.2.
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good agreement with the literature values [18, 21].
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The potentidynamic polarization curves are presented in Fig. 3. The untreated Ti6Al4V substrate exhibited a corrosion potential of – 0.2 V (vs. Ag/AgCl). A very wide
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passive region with a passive current plateau of approximately 10-5 A/cm² was reached above 0.15 V (vs. Ag/AgCl), indicating a high stability of the titanium oxide film and therefore,
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good corrosion resistance under static conditions. This behavior is typical for passive metals like Ti6Al4V, as previously reported [18].
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The W-DLC coated Ti6Al4V substrate showed a slightly lower corrosion potential (~
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0.3 V vs. Ag/AgCl) and a slightly higher passive current plateau when compared to the
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untreated Ti6Al4V. The passive region was followed by an increase in the anodic current density and a transpassivation region occurred at higher potentials (> 1 V vs. Ag/AgCl). Although DLC coatings are usually electrochemically nobler than the substrates, it has been
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reported that the presence of nanopores in the coatings can lead to the electrochemical dissolution of the substrates due to permeation of water, environmental oxygen and ions [32]. In case of the HVOF coated Ti6Al4V substrate the corrosion potential was shifted even more towards the cathodic region (approximately -0.6 V vs. Ag/AgCl). The coating showed a passive potential range between -0.5 and +0.1 V vs. Ag/AgCl, and a breakdown potential occurred at approximately +0.12 V vs. Ag/AgCl. At this potential, a passivity breakdown took place, coinciding with the start of the transpassive region that corresponds to the oxidative dissolution of the passivation layer. Due to the very complex structure of the coating, it is difficult to point out which of the constituting phases are active in a corrosive media. Nevertheless, it has been reported that the electrochemical behavior of this type of 12
ACCEPTED MANUSCRIPT coatings is more controlled by pores and defects present in the coating (see Fig. 2, middle) than by expected active-passive behavior of the NiCr matrix [33].
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In case of the Ion implantation treated Ti6Al4V substrate, the nitriding treatment
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induced a shift in the corrosion potential towards the noble direction with respect to the
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untreated Ti6Al4V substrate. Moreover, the current density in the passive region was about 2 orders of magnitude lower (~ 10-7 A/ cm²), indicating excellent corrosion resistant capability
Figure 3. 3.3.
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under static conditions.
Potentiodynamic polarization curves of all samples tested in artificial seawater
Tribocorrosion performance The tribocorrosion tests were performed at open circuit potential (OCP) conditions and
the evolution of the potential as a function of time are shown in Fig. 4 (bottom) along with their corresponding friction curves in Fig. 4 (top). The change in OCP of the untreated Ti6Al4V substrate, W-DLC coated Ti6Al4V substrate, HVOF coated Ti6Al4V substrate and Ion implantation treated Ti6Al4V substrate was measured before the onset of sliding, during sliding and after the sliding motion was stopped. 13
ACCEPTED MANUSCRIPT Before the onset of sliding, the OCP of the untreated Ti6Al4V substrate was -0.25 V, while for the W-DLC, HVOF and Ion implantation, the OCP were -0.09 V, -0.05 V and +0.47
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V, respectively. This indicates that the applied surface treatments imparted a more noble
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potential in case of the coated samples with respect to the untreated Ti6Al4V substrate,
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pointing out a higher protection of the surface against corrosion [34-36]. When sliding started, it can be clearly seen that the W-DLC and HVOF coatings did not exhibit any fluctuations in the OCP. This indicates that the coatings were not destroyed
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during sliding under these testing conditions, hence no changes in the potential. In fact, the
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cross-section scans performed across each wear showed that the maximum wear depth average for W-DLC and HVOF were 0.9 µm and 1.1 µm, respectively (Fig. 5). This clearly indicates that due to the low loading conditions used, these coatings were not fully penetrated.
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Therefore, the underlying substrate material was neither exposed to the corrosive environment nor affected by plastic deformation.
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On the other hand, in case of the untreated Ti6Al4V, when sliding started, a sudden drop in the potential of ~ 0.12 V can be observed. Such a drop appears to be lower than other
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values previously reported in the literature [33, 34] and it could be attributed to the low normal load applied in the present work (100 mN). The voltage drop indicates a partial removal of the passive layer by rubbing [17, 18, 35, 36] suggesting an increase in susceptibility of the nascent active Ti6Al4V surface for corrosion, and therefore a need for surface protection. In this case, the maximum wear depth average of the investigated wear scars was found to be approximately 1.5 µm (Fig. 5). It should be noted that the OCP stayed low during the sliding experiment, constant at ~ -0.3 V, with some oscillations that are usually attributed to depassivation/ repassivation events induced by rubbing inside the wear scar. [5, 18]. Regarding the Ion implantation treated Ti6Al4V substrate, the OCP did not drop suddenly once sliding started, but rather decayed as the number of cycles increased. This 14
ACCEPTED MANUSCRIPT progressive drop in the potential with increasing time indicates that the TiN layer was not immediately destroyed when the sliding started, but rather gradually removed during rubbing,
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as previously observed by other authors [18, 36]. Furthermore, the OCP during the wear test
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for the Ion implantation treated sample was higher than the OCP of the untreated Ti6Al4V
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sample, indicating that the TiN was effective in protecting the Ti6Al4V substrate from corrosion during sliding.
When the sliding motion was stopped, the potential of both untreated and Ion
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implantation treated Ti6Al4V substrate exhibited a shift in the noble direction with respect to
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the OCP, suggesting the repassivation of the active area in the rubbed zone. It should be noted that after the sliding motion was stopped, the untreated Ti6Al4V substrate reached the steady state potential, whereas the Ion implantation treated substrate failed to reach the initial steady
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state value in the given duration of time, suggesting a possible surface damage, as discussed in more detail in section 3.4. Furthermore, the repassivation time is higher for the Ion
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implantation treated Ti6Al4V substrate (~ 700 s) when compared to the untreated one (~ 100 s) and this could be attributed to a relatively higher damaged area of the rubbed zone on the
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Ion implantation treated Ti6Al4V substrate that increases the time required for repassivation. The corresponding high friction coefficient fluctuations (Fig. 4, top) for the untreated and Ion implantation treated Ti6Al4V substrate provide supplemental evidence for the OCP variations obtained in this study. The untreated Ti6Al4V substrate exhibited the highest and most unstable friction, confirming its already known poor tribological characteristics, and thus the need for surface protective treatments. Such fluctuations in friction have been attributed to the formation and ejection of wear particles from the contact area [37, 38]. On the other hand, it can be clearly seen that the W-DLC coated Ti6Al4V substrate had the lowest and most stable friction among all the tested samples. The tribological behavior of carbon-based coatings is strongly affected by their chemical composition, polycrystalline structure and surface morphology [39]. Nevertheless, it has been reported that 15
ACCEPTED MANUSCRIPT in all environments, the tribological behavior of these coatings is controlled by a low shear strength interfacial transfer layer formed during friction [40].
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The HVOF sample also shows a lower and more stable friction in comparison to the
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untreated and Ion implantation treated Ti6Al4V substrate. This could be attributed due to the
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presence of oxides (discussed in section 3.5) formed due to the hydration of HVOF coating in aqueous environment. It has been reported that such an oxide layer could decrease the shear strength and in turn the frictional forces, reducing adhesive/ delamination wear and resulting
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in a lower and more stable friction coefficient [41].
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The Ion implantation treated Ti6Al4V substrate showed less fluctuations in coefficient of friction in comparison to the untreated Ti6Al4V, but the friction level was similar. It is believed that the hardness increase resulted from the nitriding process was responsible for a
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to the untreated Ti6Al4V.
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more stable friction in case of the Ion implantation treated Ti6Al4V substrate when compared
Figure 4.
Evolution of friction and OCP recorded in-situ before, during and after sliding in artificial seawater 16
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Surface morphology and wear mechanisms
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3.4.
Maximum wear scar depth of the investigated surface treatments. The results are taken from the 3D topography images using cross-section profiles of the wear scars
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Figure 5.
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To further examine the wear and corrosion behavior of the samples, SEM images of the worn surfaces were taken (Figs. 6 and 7). Using a backscattered electrons detector, some
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changes in chemical surface composition can be noticed, especially in case of the untreated Ti6Al4V substrate (Fig. 6). A wide wear scar covered with smeared material (the black spots) is indicative for the wear accelerated corrosion occurring during the sliding process. A large number of agglomerated corrosion debris inside the wear scar and the wear marks present in the secondary electron image reveal that the untreated Ti6Al4V was worn severely by adhesive wear mechanisms (Fig. 7, top-left). Conversely, the W-DLC coating experienced only mild wear caused by the abrasion of the counter body. The resultant wear track on the W-DLC coated Ti6Al4V surface was narrow with a smooth and polished appearance (Figs. 6 and 7, top-right). No spallation of the coating was observed in the wear scar after the sliding under these testing conditions. The
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ACCEPTED MANUSCRIPT superior wear resistance of W-DLC is derived from its good adhesion with the substrate, high hardness and much improved frictional behavior during sliding.
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In case of HVOF coating, the surface inside the wear scar was characterized by
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flattening of the asperities during rubbing and crack initiation at the interface (Figs. 6 and 7,
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bottom-left). Permeation of electrolyte into these cracks could induce localized environments that will lead to crevice corrosion, as previously reported [2].
Regarding the morphology of the Ion implantation treated Ti6Al4V substrate, several
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cracks were detected inside the wear scar, and abrasive wear combined with spalling due to
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localized brittle fracture was the dominant mechanism under these conditions (Figs. 6 and 7, bottom-right). The average wear volume losses for each tested surface treatment is included in Fig. 8. As detected by the SEM analyses, the W-DLC showed the lowest wear volume when
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superior wear resistance.
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compared to the other samples, thus confirming the aforementioned remarks regarding its
Figure 6.
Backscattered SEM images of the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-right), HVOF (bottom-left) and Ion implantation (bottom-right) taken at 500 X magnification
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Secondary electron SEM images of the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-right), HVOF (bottom-left) and Ion implantation (bottom-right) taken at 1000 X magnification
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Figure 7.
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Figure 8.
Average wear volume loss for the investigated surface treatments
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ACCEPTED MANUSCRIPT 3.5.
Chemical surface composition The relative surface atomic concentrations of all elements detected by the XPS
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analyses performed inside the wear scars of all the samples tested under tribocorrosion
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conditions in artificial seawater are given in Table 4. The presence of Si 2p in form of SiO2 at
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a binding energy of 102.4 eV on all the surfaces can be related to the sand paper used in the polishing process of the samples. Moreover, the presence of Ca 2p in form of Ca(CO) 3 at
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347.9 eV as well as N 1s in form of organic nitrogen at a binding energy of 399.9 eV on all the samples could be attributed to the chemical composition of the artificial sea water, as
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previously reported [42].
The high resolution XPS spectra of the relevant elements are displayed in Fig. 9.
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Regarding the untreated Ti6Al4V substrate, the high resolution XPS spectrum of Ti 2p shows
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two spin orbit components (Ti 2p1/2 and Ti 2p3/2) that are centered at binding energies of 464.8 eV and 459.1 eV, respectively. According to previous findings, these correspond to TiO2. The
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results indicate that inside the wear scar, the metal particles resulted from the passive layer removal by mechanical action mainly consist of TiO2 [42].
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C1s spectrum of W-DLC has been fitted into three different bands. The first component located at 285 eV, is related to saturated and unsaturated hydrocarbon groups. A band at 286.4 eV can be related to C-O bonds, and the last peak was obtained at 288.9 eV, which can be related to the O-C=O bonds, according to prior findings [43]. The O1s core level spectrum from W-DLC coated substrate shows two main sub peaks around 531.1 and 532.6 eV, which are attributed to C=O and C-O bonds in the film structure, respectively. It should be noted that the chemical surface composition of W-DLC coated Ti6Al4V substrate is relatively similar to the as-deposited DLC films, thus confirming that the coating was not chemically affected during the rubbing process.
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ACCEPTED MANUSCRIPT The XPS results of HVOF coating show a broad Cr 2p peak with a spin orbit splitting for Cr 2p1/2 and Cr 2p3/2 between 573 eV and 592.2 eV, respectively. Each spectrum line
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consists of two peaks, the metallic form and the oxide form of chromium (Cr2O3). For the Cr
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2p3/2 line, these peaks were found at binding energies of 574 eV and 576.5 eV for metallic
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chromium and for the third valence state of chromium, respectively [44]. In case of the high resolution C 1s spectrum from HVOF coating, besides the main peak of C-C (at 285 eV), other peaks around 283 eV and 288.7 eV are observed and can be attributed to the carbide
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phase and O-C=O bonds, respectively. It is suggested that the formation of the low shear
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strength Cr2O3 layer contributed to the superior tribological characteristics exhibited by the HVOF coating [44]. Moreover, the presence of chlorine (Cl 2p) on the HVOF surface could suggest a permeation of the electrolyte through the interconnected pores of the coating.
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Although noisy, the high resolution XPS spectrum of N 1s from Ion implantation treated substrate may be interpreted as the convolution of three components: TiN at about
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397.4 eV, TiOxNy at about 396 eV and organic surface contaminants above 398 eV [45]. The presence of an oxynitride film (TiOxNy) on the surface of TiN in aqueous environments has
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already been confirmed in literature, and corresponds to the TiN enhanced oxidation process [18]. According to previous works, it has been pointed out that the high friction of TiN coating is attributed to the formation of a high shear strength TiOx layer [46, 47].
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3.6.
Ti 2p 7.5 3.3
Element concentration (atomic %) W 4f Cr 2p N 1s Si 2p 5.1 3.5 1.1 2.6 3.9 2.3 4.3 6.1 -
Ca 2p 1.2 1.1 1.6 2.3
Cl 2p 1.4 -
Relative surface atomic concentrations of elements inside the wear scar as detected by the XPS analysis
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Table 4.
O 1s 33.2 24.9 19.7 17.1
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Ti6Al4V W-DLC HVOF Ion implantation
C 1s 49.5 65.8 70.8 71.2
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Sample
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XPS high resolution spectra taken from inside the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-middle-right), HVOF (bottomleft-middle) and Ion implantation (bottom-right)
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Figure 9.
Limitations and scope Although this current study represents a systematic investigation of tribocorrosion
performance of various surface engineering technologies in artificial seawater under OCP conditions, additional investigations are needed to clarify and better understand the tribocorrosion behavior of these surface treatments. The current findings can be extended to analyze the tribocorrosion behavior of these materials under potentiostatic conditions (cathodic and/or anodic fixed potentials). Such analyses will facilitate the understanding of the individual contributions from wear and corrosion and their synergistic effects on the degradation of the surface treatments. 22
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Conclusions Several surface engineering treatments have been applied to Ti6Al4V substrates in
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order to enhance their wear and corrosion characteristics in artificial seawater. The following
In terms of static corrosion, the Ion implantation treated Ti6Al4V substrate exhibited
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conclusions can be derived from this study:
the best corrosion resistance among all the samples, as shown by the potentiodynamic polarization curves.
Regarding the friction behavior, the W-DLC coated Ti6Al4V substrate exhibited the
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lowest and most stable friction which was attributed by the formation of a low shear strength transfer layer at the sliding interface. On the other hand, the untreated Ti6Al4V substrate had the highest and most unstable friction due to the formation and
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ejection of wear particles from the contact area. The XPS analyses revealed that these wear particles were composed of TiO2. Under OCP conditions, the W-DLC and HVOF coatings exhibited better
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tribocorrosion performance when compared to the Ion implantation and the untreated
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Ti6Al4V substrate. This was attributed to their improved frictional characteristics, and low wear scars depth that coincided with the absence of fluctuations in the OCP.
The surface morphology after the sliding revealed a smooth and polished appearance of the W-DLC coating, highlighting the outstanding wear and corrosion performance of this coating in artificial seawater under OCP conditions.
The crack initiation at the interface was characteristic for the HVOF coating and Ion implantation treated Ti6Al4V substrate, which can provide the direct diffusion paths for the corrosion medium and thereby enable localized corrosion.
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ACCEPTED MANUSCRIPT Acknowledgements This work was funded by the Austrian COMET-Program (Project K2 XTribology,
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Grant No. 849109) and has been carried out within the Excellence Centre of Tribology. The
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authors would like to thank Christoph Gabler for performing the XPS analyses and Fjorda
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Xhiku for the topography measurements.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
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Enhancement of tribocorrosion performance in artificial seawater of titanium alloy by various surface engineering technologies Outstanding tribocorrosion performance of diamond-like carbon coating by formation of a low shear strength transfer layer at the sliding interface Hydration of the thermal spray coating led to formation of chromium oxides that improved its frictional characteristics in artificial seawater Nitriding the titanium alloy enhanced its tribocorrosion properties in artificial sea water
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S0264-1275(16)30563-9 doi: 10.1016/j.matdes.2016.04.080 JMADE 1724
To appear in: Received date: Revised date: Accepted date:
25 January 2016 22 March 2016 25 April 2016
Please cite this article as: Vladimir Totolin, Vladimir Pejakovi´c, Thomas Csanyi, Oliver Hekele, Martin Huber, Manel Rodr´ıguez Ripoll, Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater, (2016), doi: 10.1016/j.matdes.2016.04.080
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Surface engineering of Ti6Al4V surfaces for enhanced tribocorrosion performance in artificial seawater
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AC2T research GmbH, Wiener Neustadt, Austria IMI Critical Engineering / IMI CCI / CCI Valve Technology GmbH, Vienna, Austria
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Vladimir Totolin a,*, Vladimir Pejaković a, Thomas Csanyi b, Oliver Hekele b, Martin Huber b and Manel Rodríguez Ripoll a
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* Corresponding author: Dr. Vladimir Totolin AC2T research GmbH Viktor-Kaplan-Strasse 2/C 2700 Wiener Neustadt, Austria Email: [email protected]
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Abstract
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Titanium and its alloys are materials with excellent corrosion resistant properties, but under sliding contact exhibit poor wear and friction performance. Therefore, the aim of this study was to find suitable surface treatments for titanium alloy substrates to be used under saline environmental conditions. Various surface engineering techniques have been applied to Ti6Al4V substrates, and the static corrosion performance of these surface treatments in presence of artificial seawater was evaluated using potentiodynamic investigations. Furthermore, the combined sliding-corrosion performance was investigated using a microtribometer coupled to an electrochemical cell. Using this set-up, the selected samples were tested in order to check the synergistic effect of wear and corrosion, also known as tribocorrosion. The results clearly revealed a poor tribological performance of the untreated Ti6Al4V substrate, thus highlighting the need for surface treatments. Furthermore, it was highlighted that some of the compared surface engineering treatments applied to Ti6Al4V although exhibited improved corrosion resistance under static conditions, they showed a radically different behaviour under tribocorrosion. The underlying reasons leading to a different tribocorrosion performance are discussed and the main mechanisms associated with surface degradation under combined wear and corrosion are revealed.
Keywords: Ti6Al4V, DLC, HVOF, ion implantation, tribocorrosion, seawater
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Introduction Tribocorrosion is a surface degradation process resulting from simultaneous
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tribological and electrochemical actions in a corrosive environment [1-4]. The synergism
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between wear and corrosion may lead to an accelerated degradation of passive metals. For
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instance, the mechanical removal of a passivation layer usually leads to wear-accelerated corrosion due to chemical reactivity of the nascent metal surface exposed to the corrosive environment. Therefore, it is important to identify the contribution of corrosion and wear to
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material removal during a tribocorrosion process in order to minimize material degradation [1,
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4]. Over the past two decades, significant work has been done regarding to tribocorrosion as previously reported in several reviews [1, 2]. Various triboelectrochemical techniques have
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been applied on a variety of sliding contact conditions such as unidirectional or reciprocating
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motion, fretting or spinning contacts. In an attempt to identify the fundamental tribocorrosion mechanisms, the most investigated materials were model alloys [3, 5, 6], followed by
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biomedical alloys [7, 8, 9], sealing materials [10, 11], materials for nuclear reactors [12] and materials used in chemical mechanical polishing process [13, 14].
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It is widely known that the synergistic action of wear and corrosion can also lead to premature failure of engineering components in seawater environments. Nevertheless, some crucial components used in marine equipment, such as pumps, valves, gears and propellers have to be directly lubricated by seawater [15, 16]. Their safety, reliability and service life greatly depend on their combined tribological and corrosive performance in harsh marine environments [17]. Common corrosion resistant materials used in seawater environments, such as titanium alloys, offer a very poor tribological performance and failed to meet the mechanical requirements of highly-demanding applications [18]. One of the most costeffective approaches for improving the tribological properties of titanium alloys is to include advanced surface engineering technologies in the component design. By doing so, surface
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The work regarding tribocorrosion of engineered surfaces is rather limited and only a
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few studies have been reported in literature [17-26]. Moreover, understanding the
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tribocorrosion of surface engineered surfaces is often more difficult in comparison to bulk metals due to the porosity and oxide pockets that are usually formed during the coating process [2]. In this case subsurface corrosion can occur at the interface with the substrate or
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interlayers.
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In recent years, diamond-like carbon (DLC) films have been the subject of extensive investigations due to their potential of attaining a combination of highly desirable properties such as high hardness, low friction and wear, electrical insulation and chemical inertness [21,
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22]. Azzi et al. investigated the tribocorrosion behavior of DLC-coated 316L stainless steel exposed to the Ringer’s solution in the context of biomedical applications [21]. Two different
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bond layers were studied, namely amorphous hydrogenated silicon nitride (a-SiNx:H) and plasma nitrided layers. It was found that the nitrided bond layer showed a significant
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improvement in wear resistance of the DLC coatings in dry conditions. However, under sliding wear in simulated body fluid conditions, this bond layer proved to be insufficient, mainly due to the infiltration of the liquid through the pores and the weakening of the interface due to corrosion processes. On the other hand, the use of a-SiNx:H as an interface layer significantly improved the wear resistance of DLC films in Ringer’s solution. Costa et al. studied the friction and wear rate of DLC films under seawater and saline solutions using two different tribological systems, a stainless steel vessel and a PTFE one [23]. Although the 20% hydrogenated DLC film confirmed to be effective in terms of corrosion and wear resistance under saline solutions for both vessels, better results were achieved using the stainless steel vessel.
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coatings with compositions of 86WC-10Co-4Cr or corrosion resistant alloys such as nickel
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aluminum bronze have been considered for wear-corrosion applications [27-29]. In general, a
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particular concern for these coatings is their level of porosity (>2%) that can accelerate crack propagation and coating removal under erosive conditions. Moreover, the permeation of the electrolyte through the interconnected pores into the coating/substrate interface could lead to
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corrosion-driven coating-substrate disbondment [2].
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Surface engineering of metals by plasma nitriding techniques can be used to improve tribocorrosion [18, 25]. Galliano et al. investigated the tribocorrosion performance of a plasma nitrided Ti6Al4V alloy coating in 0.9 wt% NaCl solutions under reciprocating
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alumina ball-on-flat sliding wear [18]. It was found that samples nitrided at higher temperatures (1173K) showed better wear resistance without crack formation compared to
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samples nitrided at lower temperatures (973K) which micro-cracked causing failure. The improvement in anti-wear properties was attributed to a surface hardened layer of TiN-Ti2N.
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It is noteworthy that, in general, the various tribocorrosion studies previously reported in literature mostly focused on the assessment of a single surface engineering technology. Furthermore, the investigated surface technology was often evaluated in terms of the role of process parameters or was simply compared with the untreated substrate material. A systematic approach that investigates the wear and corrosion protection of different surface engineering techniques in artificial seawater seems to be lacking in the existing literature. Such a study is essential for providing design rules for components able to withstand operations under offshore environments. For this reason, the aim of the present work is to investigate and compare the tribocorrosion behavior of DLC coatings, HVOF coatings and Ion implantation treatments applied to titanium substrates exposed to artificial seawater in the context of offshore applications. 4
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Experimental
2.1.
Surface technologies
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The substrate investigated was a titanium alloy (Ti6Al4V), commercially available
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were applied to the substrate and are summarized in Table 1.
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from Alba Industrial AS (Bekkestua, Norway). Three different surface engineering treatments
Sample pre-treatments and their corresponding hardness and roughness
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Table 1.
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Sample Treatment Hardness (GPa) Roughness, Ra (µm) Ti6Al4V None 3.3 0.3 W-DLC CrN + a-C:H:W Topcoat 15.9 0.3 Cr3C2 embedded in Ni/Cr matrix HVOF 14.0 0.3 Ion implantation Low pressure nitriding 16.0 0.4
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The first surface treatment (referred as W-DLC) comprised of a multifunctional coating applied in a single-pass Physical-Vapor Deposition process at temperatures between
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180 and 350 °C and was commercially available from Oerlikon Balzers Coating Austria GmbH (Stainz, Austria). The multifunctional coating consisted of a tribologically effective
resistance.
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tungsten-doped hydrogenated DLC top layer and a CrN intermediate layer for corrosion
The second surface treatment (referred as HVOF), commercially available from TeroLab Surface GmbH (Vienna, Austria) consisted of a hard metallic structure made of chromium carbides with a content of 75% weight. The carbides were embedded in a metallic matrix made of nickel and chromium. The coating was deposited using a HVOF thermal spray process to obtain a dense, homogenous and ultra-hard coating. The third surface treatment (referred as Ion implantation), commercially available from Bodycote SAS (Pusignan, France) was based on nitrogen diffusion into titanium performed under vacuum at low pressure, without the use of a plasma. The diffusion of 6
ACCEPTED MANUSCRIPT nitrogen created a compound layer of TiN+Ti2N and a diffusion layer of Ti (N) underneath. The chemical composition of the samples is given in Table 2.
Table 2.
Al 8.2 1.6
Si 1.0
N 55.9
Chemical composition (at. %) of the samples
Tribocorrosion investigations
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2.2.
Chemical composition (atomic %) Cr Ni W 4.3 2.6 11.5 43.6 9.1 0.1 -
Ti 91.0 1.2 41.5
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O 15.8 11.2 -
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Ti6Al4V W-DLC HVOF Ion implantation
C 64.6 36.0 -
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Sample
Tribocorrosion experiments were performed at room temperature using a using a
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linear reciprocating ball-on-flat tribometer TETRA BASALT®-N from Falex Tribology N.V. (Rotselaar, Belgium) described in a previous publication [30]. The detailed experimental
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parameters are displayed in Table 3. The average contact pressures for all the tested materials
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have been estimated from the Hertzian contact stress theory. The calculated average contact
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pressures for Ti6Al4V, W-DLC, HVOF and Ion implantation are 200 MPa, 230 MPa, 160
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MPa and 240 MPa, respectively.
Test rig parameters
Ball Specimens Disc
Applied load (mN) Speed (mm/s) Stroke (mm) Temperature (°C) Number of cycles Repetitions/ sample
100 2 1 22 5000 3
Material Diameter (mm) Young's modulus (GPa)
Al2 O3 Ceramic 5 300
Poisson's ratio Bulk Material Size (mm²) Young's modulus (GPa) Poisson's ratio
0.21 Ti6Al4V 10 113.8 0.34
Table 3.
Test rig parameters
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ASTM Standard D1141-13. The testing sample serves as the working electrode (WE) and its
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potential is monitored using a potentiostat VERSASTAT 3F from AMETEK GmbH
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(Meerbusch, Germany). The reference electrode (REF) was an Ag/AgCl in 3 M saturated
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NaCl solution and the counter electrode was a platinum plate (Fig. 1).
Figure 1.
Experimental setup for tribocorrosion experiments
The tribocorrosion method used consisted of four sequences: cathodic cleaning of the samples at -1.2 V (vs. Ag/AgCl) for 60 s for dissolution of air-formed oxides [1]; stabilization of the system under Open Circuit Potential (OCP) for 8000 s (in absence of rubbing) to achieve a stable passive surface; rubbing under OCP for 6500 s, and finally, re-stabilization of the system under OCP (after rubbing was stopped) for 1800 s. The tribocorrosion experiments 8
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Prior to the tribocorrosion experiments, potentiodynamic polarization scans were
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performed in order to monitor the materials behavior under controlled electrochemical
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conditions. The samples were fully immersed in the artificial seawater and the measurements were carried out using the same three electrode setup described earlier. In this case, the potentiodynamic polarization method consisted of the following sequences: initial cathodic
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cleaning of the samples at - 1.2 V (vs. Ag/AgCl) for 60 s; stabilization of the system under
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OCP for 3600 s, and finally, potentiodynamic polarization measurements were carried out between -1.2 V and +1.2 V (vs. Ag/AgCl) at a scan rate of 1 mV/s. The measured currentvoltage data were plotted as Tafel plot in the form of potential versus log (i) (the current
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density). The corrosion potential (Ecorr) and corrosion current (Icorr) were derived from the Tafel plot. The corrosion current was obtained using the Stern-Geary model for a corroding
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system [31]. Two potentiodynamic experiments for each sample have been conducted, and due to the similarity of the curves, only one representative curve for each sample is illustrated.
Surface characterization
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2.3.
The surface topography before and after the tribocorrosion experiments was analyzed with a Leica DCM 3D confocal microscope Ernst Leiz GmbH (Wetzlar, Germany). The removed material volumes (µm³) from all the tested samples were calculated from the topographical data. An ideal flat surface was defined as the reference surface for the discs and the missing volume between this surface and the measured (worn) surface was calculated with a Matlab based program. Hardness measurements were performed on cross-sections of the samples using Vickers hardness test (HV 0.05 kgf) performed with a Microhardness tester FM 700 (FutureTech Corp., Kawasaki-City, Japan) for the thick coatings (HVOF) and nanoindentation 9
ACCEPTED MANUSCRIPT hardness (10 mN applied pressure) performed with a Hysitron TriboIndenter TI900 (Hysitron, Inc., Minneapolis, USA) for the surface treatments leading to thinner coatings (W-DLC and
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Ion implantation).
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The wear scar morphology and cross-section analyses of the coated substrates were
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investigated with a scanning electron microscope (SEM) ZEISS 1540XB crossbeam Carl Zeiss Microscopy GmbH (Jena, Germany) operated at an accelerated voltage of 20 kV. The chemical surface composition of the tested samples was characterized by X-ray Photoelectron
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Spectroscopy (XPS). The analyses were conducted using a Thetaprobe Thermo Fisher
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Scientific (East Grinstead, United Kingdom) equipped with a monochromatic Al Kα X-ray source (hv = 1486.6 eV) and a hemispherical analyzer.
Results and discussion
3.1.
Characterization of the engineered surfaces
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SEM images of the polished cross-section from W-DLC (top), HVOF (middle) and Ion implantation (bottom) coated Ti6Al4V substrates are shown in Fig. 2. As described in section 2.1., the W-DLC coated Ti6Al4V substrate has a double layer coating with the
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approximate overall thickness of 2.2 µm (1.4 µm for the a-C:H:W top layer and 800 nm for the CrN inter layer). For the HVOF coated Ti6Al4V substrate, the thickness is approximately 300 µm and the coating exhibits a relatively dense and coherent microstructure, consisting of a Ni-Cr metallic binder and a dispersed Cr3C2 phase (Fig. 2, middle). Some debris from sand blasting can be observed at the interface between the substrate and coating. In case of the Ion implantation treated substrate, the presence of the nitride layer is confirmed by the SEM micrograph, and the layer thickness is approximately 7 µm (Fig. 2, bottom).
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Figure 2.
SEM cross-sectional micrographs of W-DLC (top), HVOF (middle) and Ion implantation treated Ti6Al4V substrate (bottom)
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ACCEPTED MANUSCRIPT Hardness measurements (Table 1) reveal a dramatic increase in the surface hardness (between 14 and 16 GPa), regardless of the applied surface treatment, and the results are in
Potentiodynamic polarization measurements
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3.2.
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good agreement with the literature values [18, 21].
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The potentidynamic polarization curves are presented in Fig. 3. The untreated Ti6Al4V substrate exhibited a corrosion potential of – 0.2 V (vs. Ag/AgCl). A very wide
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passive region with a passive current plateau of approximately 10-5 A/cm² was reached above 0.15 V (vs. Ag/AgCl), indicating a high stability of the titanium oxide film and therefore,
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good corrosion resistance under static conditions. This behavior is typical for passive metals like Ti6Al4V, as previously reported [18].
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The W-DLC coated Ti6Al4V substrate showed a slightly lower corrosion potential (~
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0.3 V vs. Ag/AgCl) and a slightly higher passive current plateau when compared to the
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untreated Ti6Al4V. The passive region was followed by an increase in the anodic current density and a transpassivation region occurred at higher potentials (> 1 V vs. Ag/AgCl). Although DLC coatings are usually electrochemically nobler than the substrates, it has been
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reported that the presence of nanopores in the coatings can lead to the electrochemical dissolution of the substrates due to permeation of water, environmental oxygen and ions [32]. In case of the HVOF coated Ti6Al4V substrate the corrosion potential was shifted even more towards the cathodic region (approximately -0.6 V vs. Ag/AgCl). The coating showed a passive potential range between -0.5 and +0.1 V vs. Ag/AgCl, and a breakdown potential occurred at approximately +0.12 V vs. Ag/AgCl. At this potential, a passivity breakdown took place, coinciding with the start of the transpassive region that corresponds to the oxidative dissolution of the passivation layer. Due to the very complex structure of the coating, it is difficult to point out which of the constituting phases are active in a corrosive media. Nevertheless, it has been reported that the electrochemical behavior of this type of 12
ACCEPTED MANUSCRIPT coatings is more controlled by pores and defects present in the coating (see Fig. 2, middle) than by expected active-passive behavior of the NiCr matrix [33].
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In case of the Ion implantation treated Ti6Al4V substrate, the nitriding treatment
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induced a shift in the corrosion potential towards the noble direction with respect to the
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untreated Ti6Al4V substrate. Moreover, the current density in the passive region was about 2 orders of magnitude lower (~ 10-7 A/ cm²), indicating excellent corrosion resistant capability
Figure 3. 3.3.
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under static conditions.
Potentiodynamic polarization curves of all samples tested in artificial seawater
Tribocorrosion performance The tribocorrosion tests were performed at open circuit potential (OCP) conditions and
the evolution of the potential as a function of time are shown in Fig. 4 (bottom) along with their corresponding friction curves in Fig. 4 (top). The change in OCP of the untreated Ti6Al4V substrate, W-DLC coated Ti6Al4V substrate, HVOF coated Ti6Al4V substrate and Ion implantation treated Ti6Al4V substrate was measured before the onset of sliding, during sliding and after the sliding motion was stopped. 13
ACCEPTED MANUSCRIPT Before the onset of sliding, the OCP of the untreated Ti6Al4V substrate was -0.25 V, while for the W-DLC, HVOF and Ion implantation, the OCP were -0.09 V, -0.05 V and +0.47
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V, respectively. This indicates that the applied surface treatments imparted a more noble
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potential in case of the coated samples with respect to the untreated Ti6Al4V substrate,
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pointing out a higher protection of the surface against corrosion [34-36]. When sliding started, it can be clearly seen that the W-DLC and HVOF coatings did not exhibit any fluctuations in the OCP. This indicates that the coatings were not destroyed
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during sliding under these testing conditions, hence no changes in the potential. In fact, the
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cross-section scans performed across each wear showed that the maximum wear depth average for W-DLC and HVOF were 0.9 µm and 1.1 µm, respectively (Fig. 5). This clearly indicates that due to the low loading conditions used, these coatings were not fully penetrated.
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Therefore, the underlying substrate material was neither exposed to the corrosive environment nor affected by plastic deformation.
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On the other hand, in case of the untreated Ti6Al4V, when sliding started, a sudden drop in the potential of ~ 0.12 V can be observed. Such a drop appears to be lower than other
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values previously reported in the literature [33, 34] and it could be attributed to the low normal load applied in the present work (100 mN). The voltage drop indicates a partial removal of the passive layer by rubbing [17, 18, 35, 36] suggesting an increase in susceptibility of the nascent active Ti6Al4V surface for corrosion, and therefore a need for surface protection. In this case, the maximum wear depth average of the investigated wear scars was found to be approximately 1.5 µm (Fig. 5). It should be noted that the OCP stayed low during the sliding experiment, constant at ~ -0.3 V, with some oscillations that are usually attributed to depassivation/ repassivation events induced by rubbing inside the wear scar. [5, 18]. Regarding the Ion implantation treated Ti6Al4V substrate, the OCP did not drop suddenly once sliding started, but rather decayed as the number of cycles increased. This 14
ACCEPTED MANUSCRIPT progressive drop in the potential with increasing time indicates that the TiN layer was not immediately destroyed when the sliding started, but rather gradually removed during rubbing,
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as previously observed by other authors [18, 36]. Furthermore, the OCP during the wear test
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for the Ion implantation treated sample was higher than the OCP of the untreated Ti6Al4V
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sample, indicating that the TiN was effective in protecting the Ti6Al4V substrate from corrosion during sliding.
When the sliding motion was stopped, the potential of both untreated and Ion
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implantation treated Ti6Al4V substrate exhibited a shift in the noble direction with respect to
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the OCP, suggesting the repassivation of the active area in the rubbed zone. It should be noted that after the sliding motion was stopped, the untreated Ti6Al4V substrate reached the steady state potential, whereas the Ion implantation treated substrate failed to reach the initial steady
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state value in the given duration of time, suggesting a possible surface damage, as discussed in more detail in section 3.4. Furthermore, the repassivation time is higher for the Ion
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implantation treated Ti6Al4V substrate (~ 700 s) when compared to the untreated one (~ 100 s) and this could be attributed to a relatively higher damaged area of the rubbed zone on the
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Ion implantation treated Ti6Al4V substrate that increases the time required for repassivation. The corresponding high friction coefficient fluctuations (Fig. 4, top) for the untreated and Ion implantation treated Ti6Al4V substrate provide supplemental evidence for the OCP variations obtained in this study. The untreated Ti6Al4V substrate exhibited the highest and most unstable friction, confirming its already known poor tribological characteristics, and thus the need for surface protective treatments. Such fluctuations in friction have been attributed to the formation and ejection of wear particles from the contact area [37, 38]. On the other hand, it can be clearly seen that the W-DLC coated Ti6Al4V substrate had the lowest and most stable friction among all the tested samples. The tribological behavior of carbon-based coatings is strongly affected by their chemical composition, polycrystalline structure and surface morphology [39]. Nevertheless, it has been reported that 15
ACCEPTED MANUSCRIPT in all environments, the tribological behavior of these coatings is controlled by a low shear strength interfacial transfer layer formed during friction [40].
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The HVOF sample also shows a lower and more stable friction in comparison to the
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untreated and Ion implantation treated Ti6Al4V substrate. This could be attributed due to the
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presence of oxides (discussed in section 3.5) formed due to the hydration of HVOF coating in aqueous environment. It has been reported that such an oxide layer could decrease the shear strength and in turn the frictional forces, reducing adhesive/ delamination wear and resulting
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in a lower and more stable friction coefficient [41].
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The Ion implantation treated Ti6Al4V substrate showed less fluctuations in coefficient of friction in comparison to the untreated Ti6Al4V, but the friction level was similar. It is believed that the hardness increase resulted from the nitriding process was responsible for a
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to the untreated Ti6Al4V.
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more stable friction in case of the Ion implantation treated Ti6Al4V substrate when compared
Figure 4.
Evolution of friction and OCP recorded in-situ before, during and after sliding in artificial seawater 16
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Surface morphology and wear mechanisms
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3.4.
Maximum wear scar depth of the investigated surface treatments. The results are taken from the 3D topography images using cross-section profiles of the wear scars
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Figure 5.
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To further examine the wear and corrosion behavior of the samples, SEM images of the worn surfaces were taken (Figs. 6 and 7). Using a backscattered electrons detector, some
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changes in chemical surface composition can be noticed, especially in case of the untreated Ti6Al4V substrate (Fig. 6). A wide wear scar covered with smeared material (the black spots) is indicative for the wear accelerated corrosion occurring during the sliding process. A large number of agglomerated corrosion debris inside the wear scar and the wear marks present in the secondary electron image reveal that the untreated Ti6Al4V was worn severely by adhesive wear mechanisms (Fig. 7, top-left). Conversely, the W-DLC coating experienced only mild wear caused by the abrasion of the counter body. The resultant wear track on the W-DLC coated Ti6Al4V surface was narrow with a smooth and polished appearance (Figs. 6 and 7, top-right). No spallation of the coating was observed in the wear scar after the sliding under these testing conditions. The
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ACCEPTED MANUSCRIPT superior wear resistance of W-DLC is derived from its good adhesion with the substrate, high hardness and much improved frictional behavior during sliding.
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In case of HVOF coating, the surface inside the wear scar was characterized by
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flattening of the asperities during rubbing and crack initiation at the interface (Figs. 6 and 7,
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bottom-left). Permeation of electrolyte into these cracks could induce localized environments that will lead to crevice corrosion, as previously reported [2].
Regarding the morphology of the Ion implantation treated Ti6Al4V substrate, several
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cracks were detected inside the wear scar, and abrasive wear combined with spalling due to
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localized brittle fracture was the dominant mechanism under these conditions (Figs. 6 and 7, bottom-right). The average wear volume losses for each tested surface treatment is included in Fig. 8. As detected by the SEM analyses, the W-DLC showed the lowest wear volume when
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superior wear resistance.
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compared to the other samples, thus confirming the aforementioned remarks regarding its
Figure 6.
Backscattered SEM images of the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-right), HVOF (bottom-left) and Ion implantation (bottom-right) taken at 500 X magnification
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Secondary electron SEM images of the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-right), HVOF (bottom-left) and Ion implantation (bottom-right) taken at 1000 X magnification
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Figure 7.
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Figure 8.
Average wear volume loss for the investigated surface treatments
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ACCEPTED MANUSCRIPT 3.5.
Chemical surface composition The relative surface atomic concentrations of all elements detected by the XPS
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analyses performed inside the wear scars of all the samples tested under tribocorrosion
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conditions in artificial seawater are given in Table 4. The presence of Si 2p in form of SiO2 at
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a binding energy of 102.4 eV on all the surfaces can be related to the sand paper used in the polishing process of the samples. Moreover, the presence of Ca 2p in form of Ca(CO) 3 at
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347.9 eV as well as N 1s in form of organic nitrogen at a binding energy of 399.9 eV on all the samples could be attributed to the chemical composition of the artificial sea water, as
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previously reported [42].
The high resolution XPS spectra of the relevant elements are displayed in Fig. 9.
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Regarding the untreated Ti6Al4V substrate, the high resolution XPS spectrum of Ti 2p shows
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two spin orbit components (Ti 2p1/2 and Ti 2p3/2) that are centered at binding energies of 464.8 eV and 459.1 eV, respectively. According to previous findings, these correspond to TiO2. The
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results indicate that inside the wear scar, the metal particles resulted from the passive layer removal by mechanical action mainly consist of TiO2 [42].
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C1s spectrum of W-DLC has been fitted into three different bands. The first component located at 285 eV, is related to saturated and unsaturated hydrocarbon groups. A band at 286.4 eV can be related to C-O bonds, and the last peak was obtained at 288.9 eV, which can be related to the O-C=O bonds, according to prior findings [43]. The O1s core level spectrum from W-DLC coated substrate shows two main sub peaks around 531.1 and 532.6 eV, which are attributed to C=O and C-O bonds in the film structure, respectively. It should be noted that the chemical surface composition of W-DLC coated Ti6Al4V substrate is relatively similar to the as-deposited DLC films, thus confirming that the coating was not chemically affected during the rubbing process.
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ACCEPTED MANUSCRIPT The XPS results of HVOF coating show a broad Cr 2p peak with a spin orbit splitting for Cr 2p1/2 and Cr 2p3/2 between 573 eV and 592.2 eV, respectively. Each spectrum line
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consists of two peaks, the metallic form and the oxide form of chromium (Cr2O3). For the Cr
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2p3/2 line, these peaks were found at binding energies of 574 eV and 576.5 eV for metallic
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chromium and for the third valence state of chromium, respectively [44]. In case of the high resolution C 1s spectrum from HVOF coating, besides the main peak of C-C (at 285 eV), other peaks around 283 eV and 288.7 eV are observed and can be attributed to the carbide
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phase and O-C=O bonds, respectively. It is suggested that the formation of the low shear
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strength Cr2O3 layer contributed to the superior tribological characteristics exhibited by the HVOF coating [44]. Moreover, the presence of chlorine (Cl 2p) on the HVOF surface could suggest a permeation of the electrolyte through the interconnected pores of the coating.
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Although noisy, the high resolution XPS spectrum of N 1s from Ion implantation treated substrate may be interpreted as the convolution of three components: TiN at about
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397.4 eV, TiOxNy at about 396 eV and organic surface contaminants above 398 eV [45]. The presence of an oxynitride film (TiOxNy) on the surface of TiN in aqueous environments has
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already been confirmed in literature, and corresponds to the TiN enhanced oxidation process [18]. According to previous works, it has been pointed out that the high friction of TiN coating is attributed to the formation of a high shear strength TiOx layer [46, 47].
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3.6.
Ti 2p 7.5 3.3
Element concentration (atomic %) W 4f Cr 2p N 1s Si 2p 5.1 3.5 1.1 2.6 3.9 2.3 4.3 6.1 -
Ca 2p 1.2 1.1 1.6 2.3
Cl 2p 1.4 -
Relative surface atomic concentrations of elements inside the wear scar as detected by the XPS analysis
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Table 4.
O 1s 33.2 24.9 19.7 17.1
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Ti6Al4V W-DLC HVOF Ion implantation
C 1s 49.5 65.8 70.8 71.2
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Sample
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XPS high resolution spectra taken from inside the wear tracks after 5000 cycles of sliding on Ti6Al4V (top-left), W-DLC (top-middle-right), HVOF (bottomleft-middle) and Ion implantation (bottom-right)
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Figure 9.
Limitations and scope Although this current study represents a systematic investigation of tribocorrosion
performance of various surface engineering technologies in artificial seawater under OCP conditions, additional investigations are needed to clarify and better understand the tribocorrosion behavior of these surface treatments. The current findings can be extended to analyze the tribocorrosion behavior of these materials under potentiostatic conditions (cathodic and/or anodic fixed potentials). Such analyses will facilitate the understanding of the individual contributions from wear and corrosion and their synergistic effects on the degradation of the surface treatments. 22
ACCEPTED MANUSCRIPT 4.
Conclusions Several surface engineering treatments have been applied to Ti6Al4V substrates in
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order to enhance their wear and corrosion characteristics in artificial seawater. The following
In terms of static corrosion, the Ion implantation treated Ti6Al4V substrate exhibited
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conclusions can be derived from this study:
the best corrosion resistance among all the samples, as shown by the potentiodynamic polarization curves.
Regarding the friction behavior, the W-DLC coated Ti6Al4V substrate exhibited the
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lowest and most stable friction which was attributed by the formation of a low shear strength transfer layer at the sliding interface. On the other hand, the untreated Ti6Al4V substrate had the highest and most unstable friction due to the formation and
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ejection of wear particles from the contact area. The XPS analyses revealed that these wear particles were composed of TiO2. Under OCP conditions, the W-DLC and HVOF coatings exhibited better
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tribocorrosion performance when compared to the Ion implantation and the untreated
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Ti6Al4V substrate. This was attributed to their improved frictional characteristics, and low wear scars depth that coincided with the absence of fluctuations in the OCP.
The surface morphology after the sliding revealed a smooth and polished appearance of the W-DLC coating, highlighting the outstanding wear and corrosion performance of this coating in artificial seawater under OCP conditions.
The crack initiation at the interface was characteristic for the HVOF coating and Ion implantation treated Ti6Al4V substrate, which can provide the direct diffusion paths for the corrosion medium and thereby enable localized corrosion.
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ACCEPTED MANUSCRIPT Acknowledgements This work was funded by the Austrian COMET-Program (Project K2 XTribology,
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Grant No. 849109) and has been carried out within the Excellence Centre of Tribology. The
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authors would like to thank Christoph Gabler for performing the XPS analyses and Fjorda
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Xhiku for the topography measurements.
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Enhancement of tribocorrosion performance in artificial seawater of titanium alloy by various surface engineering technologies Outstanding tribocorrosion performance of diamond-like carbon coating by formation of a low shear strength transfer layer at the sliding interface Hydration of the thermal spray coating led to formation of chromium oxides that improved its frictional characteristics in artificial seawater Nitriding the titanium alloy enhanced its tribocorrosion properties in artificial sea water
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