Friction and adhesion of fluorine containing hydrophobic hydrogenated diamond-like carbon (F-H-DLC) coating against magnesium alloy AZ91

Surface & Coatings Technology 267 (2015) 21–31

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Friction and adhesion of fluorine containing hydrophobic hydrogenated diamond-like carbon (F-H-DLC) coating against magnesium alloy AZ91 S. Bhowmick, F.G. Sen, A. Banerji, A.T. Alpas ⁎ Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Ave, Windsor, Ontario, N9B 3P4, Canada

a r t i c l e

i n f o

Available online 26 November 2014 Keywords: Diamond-like carbon Fluorine Hydrophobicity Adhesion Friction

a b s t r a c t Tribological tests were performed on hydrogenated diamond-like carbon (H-DLC) and F containing H-DLC (F-HDLC) coatings sliding against a magnesium alloy (AZ91) in air with relative humidity (RH) ranging between 0% and 75%. Pin-on-disk type friction tests typically exhibited high running-in COF, followed by a steady state regime with a low and constant coefficient of friction (μS). At 32% RH both the F-H-DLC and the H-DLC showed similar friction behavior, but μS of H-DLC increased with an increase in RH while μS of F-H-DLC decreased. Carbonaceous transfer layers were formed on the AZ91 surfaces and incorporated F transferred from F-H-DLC. The transfer layers were passivated by OH molecules, as detected by the Fourier transform infrared and microRaman spectroscopy. A first principles interface model that examined the hydrophobic interactions between F and OH terminated diamond (111) surfaces showed that high repulsive electrostatic forces would contribute to low friction in high humidity atmospheres. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Diamond-like carbon (DLC) coatings have attracted scientific and industrial attention due to their low coefficient of friction (COF) and low wear rates against steel surfaces and against themselves [1]. More recently it has been shown that DLC coatings reduced adhesion of lightweight materials, such as aluminum and magnesium [2–10] to steel and tungsten carbide counterfaces much more effectively than other coatings. However, the tribological properties of the DLC coatings tend to vary widely with the environment. The environmental sensitivity of the DLC coatings depends on whether the coating is hydrogenated (30–40 at.% H) or non-hydrogenated (b 2 at.% H) [11–13]. The hydrogenated DLC (H-DLC) coatings consistently showed low COF of ≤ 0.01 in vacuum (6.5 × 10−3 Pa) [12] or under inert (Ar, N2) atmospheres [13], whereas non-hydrogenated DLC (NH-DLC) coatings exhibited high COF when tested under similar conditions [14]. Hydrogen passivation of the dangling bonds of surface carbon atoms that reduced covalent interactions between the carbon atoms and the counterface was shown to be responsible for the low COF under these conditions [13]. When tested under ambient atmospheres with high relative humidity (RH N 50%), the COF of H-DLC increased, whereas, the NH-DLC showed low COF [1,11]. The surface passivation of carbon atoms by the OH and H radicals, which according to first principles calculations [15] were formed as a result of the dissociation of water molecules into H and OH at the carbon surfaces, was suggested to be responsible for the low COF encountered in the NH-DLC coatings tested ⁎ Corresponding author. Tel.: +1 519 253 3000x2602; fax: +1 519 973 7085. E-mail address: [email protected] (A.T. Alpas).

http://dx.doi.org/10.1016/j.surfcoat.2014.11.047 0257-8972/© 2014 Elsevier B.V. All rights reserved.

under high atmospheric humidity [16]. Another way of reducing the friction of DLC coatings can be provided by incorporating fluorine (F) atoms into the DLC structure. F-DLC coatings were effective in decreasing the surface energy and increasing the water contact angle [17–21]. H-DLC coatings containing between 2 and 35 at.% F (F-HDLC) were reported to possess a lower surface energy than H-DLC coatings, and tended to form thermodynamically more stable surfaces [22–24]. First principles calculations revealed that two F-terminated diamond surfaces sliding against each other would exert higher repulsive forces compared to two H-terminated surfaces [25]. According to observations made on the DLC-coated molds used for nanoimprinting polymers, F-DLC coatings displayed more effective antisticking properties compared to the H-DLC-coated mold surfaces [26,27]. Hakovirta [19] performed tests with a F-H-DLC coating having 19.7 at.% of F and 3.1 at.% of H, and reported a slightly lower COF of 0.10 for F-H-DLC coatings, compared to a teflon coating (67 at.% F, 0 at.% H) with a COF of 0.13 tested against ruby (Al2O3:Cr). In another study, measurements carried out using an atomic force microscope on F-DLC coatings (35 at.% F, 2 at.%H) tested against Si3N4 showed a low COF of 0.15, whereas a COF of 0.21 was obtained for Si3N4 tested against H-DLC (22 at.% H) coating [22]. Other studies have shown that increasing the relative humidity of the test atmosphere would increase the COF values of H-DLC coatings [12,13], while higher humidity levels resulted in a decrease in the F-DLC's COF [28,29]. A recent study [24] has shown that when a F-H-DLC (3 at.% F, 25 at.% H) was placed in sliding contact against Al 1100, F-passivated carbonaceous transfer layers were formed on the Al surface and a low COF of 0.14 was achieved. Fpassivated carbon interfaces at the sliding contact interface (consisting of F-H-DLC and F-enriched transfer layers on the counterface) would

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exert repulsive forces to each other [24] that could be responsible for the observed low COF between Al and the F-H-DLC. The lowest COF of 0.10 obtained for F-H-DLC tested against steel while it was submerged in distilled water [19]. Butter et al. [30] deposited F-H-DLC using mixtures of CF4 with butane and acetylene and proposed that the formation of CF2 and CF3 groups was responsible for the hydrophobic nature of the coating as evidenced form the ~ 90° contact angle of water measured using the sessile drop tests. They suggested that ideal hydrophobicity will be attained when the CF groups were associated with sp3 hybridized carbon atoms. Hatada and Baba [31] showed that increasing the F content of the coating increased the hydrophobicity of the coating, with the highest contact angle being 99° for C6F6 compared to 51° for C6H5CH3 and 60° for C6H5F. In summary, both experimental and computational studies suggested that the F-H-DLC coatings could provide an effective route to reducing adhesion in the systems studied so far, namely ceramic, steel, and aluminum surfaces. Experimental and computational studies that address material transfer and adhesion between F-H-DLC coatings and Mg surfaces have yet to be undertaken. These studies are important for understanding the surface properties of F-H-DLC coatings, as well as assessing the feasibility of applying these coatings on the surfaces of lightweight engineering components subjected to sliding motion for addressing tool/dies adhesion problems encountered during magnesium alloy machining and forming. The use of DLC coatings sliding against magnesium alloys has received limited attention. In most cases, the material transfer to the counterface limits the life of Mg components in sliding applications. Similarly in case of machining of Mg, adhesion to the tool is the primary problem [32]. Only few studies are available on the friction and wear mechanisms of Mg against DLC and other potential tool coatings. Matsumoto and Osakada [33] investigated the friction behavior of ZK60 Mg alloy (6% Zn–0.5% Zr) specimens sliding against H-DLC as well as TiC/TiCN/TiN coated cemented carbide disks. A low COF of 0.08 was observed with the HDLC coatings that compared favorably with TiC/TiCN/TiN (0.25) coating and the uncoated cemented carbide (0.30). Konca et al. [34] studied the dry sliding wear behavior of NH-DLC coatings against pure Mg (99.9 wt.%) at room temperature and in argon to investigate the effect of test atmosphere on the COF of this system. In argon, the NH-DLC coatings showed a very low COF of 0.05 after an initial running-in period. Carbonaceous material transfer from the NH-DLC to the contact surface of the Mg pin occurred. Changing the test atmosphere from argon to ambient air increased the COF to 0.40, which was accompanied by the formation of oxidized Mg debris and higher wear rate. Based on these two studies H-DLC exhibited lower and stable COF compared to NH-DLC against Mg, yet, this behavior cannot be confirmed as the test conditions were different the underlying tribo-chemical mechanisms were not clear. This study examines the tribological behavior of H-DLC and F-H-DLC against a cast magnesium alloy (AZ91) at different humidity levels. The role of transfer layers has been discussed. Atomistic simulations were used to interpret the experimental observations. 2. Experimental and computational methods 2.1. Materials: magnesium pins and substrates Pin-on-disk tests were performed using 15 mm long AZ91 alloy pins with one end machined into a hemispherical shape of 4.05 mm in diameter. The composition of the AZ91 was (in wt.%): 8.90 Al, 0.91 Zn, 0.20 Mn, 0.0025 Fe, 0.0007 Cu, 0.0006 Ni, 0.005 Si and the balance Mg. The hardness of the AZ91 alloy was 67 ± 8.0 kg-mm− 2 on a Brinell hardness scale. An M2 tool steel bar of 25.40 mm diameter was used to machine the substrates for both H- and F-H-DLC coating depositions. The M2 steel bar was first cut into disks of 10 mm in thickness and polished according to standard metallographic procedures. The hardness of the M2 steel disks was 64 ± 3 Rc.

2.2. DLC coatings The DLC coatings (designated as F-H-DLC and H-DLC) were deposited using a plasma-assisted chemical vapor deposition (PACVD) technique on M2 grade tool steel coupons. A vacuum reactor (330 mm × 300 mm) with a base vacuum pressure below 10−6 mbar and a typical working pressure maintained between 10− 4 and 10−3 mbar was used. The plasma was created from a fluid organic precursor (CH4) using electron emission from a W-filament (0.4 mm diameter) heated using direct current (DC). During the deposition of F-H-DLC a fluorine gas with CH4 precursor was injected inside the vacuum chamber. A negative self-bias voltage of 150 V–800 V was used to direct the plasma towards the substrate while the substrate temperature did not exceed 225 °C. The carbon and fluorine atomic percentages of the coatings were determined using Rutherford backscattering spectroscopy (RBS), and their hydrogen content was determined using the elastic recoil detection (ERD) technique. The hydrogenated DLC coating designated as H-DLC incorporated 30 at.% of H. The fluorinated DLC, F-H-DLC, also possessed 30 at.% H and additionally contained 3 at.% F, an apparently small amount, but known to reduce COF significantly [24]. The microhardness and elastic modulus of F-HDLC were determined as 31 ± 4 GPa and 172 ± 12 GPa using a Hysitron TI 900 Nanoindenter. The microhardness of H-DLC was 27 ± 3 GPa with an elastic modulus of 153 ± 8 GPa. The water contact angles (θ) of H-DLC and F-H-DLC measured using the sessile-drop method were 67° and 80° respectively. The higher contact angle of F-H-DLC, closer to 90°, indicates the hydrophobic nature of this coating against water. The r.m.s. roughness (Ra) of the F-H-DLC and H-DLC samples was measured using a non-contact surface optical profilometer (Wyko NT-1100) as 20 ± 3 nm and 16 ± 3 nm respectively. Cross-sections of F-H-DLC were obtained using Carl Zeiss NVision 40 CrossBeam focused ion beam (FIB) milling. The details of FIB technique can be found in [35]. Fig. 1(a) shows the SEM cross-sectional microstructure of the F-H-DLC coating with a thickness of 1.1 ± 0.1 μm. The spectra revealing the uniform distributions of C and F across the thickness of the coating are shown in Figs. 1 (b, c). 2.3. Pin-on-disk tests Pin-on-disk type wear tests were performed using a CSM tribometer to measure coefficient of friction, COF, between AZ91 pin and DLC coatings in ambient air. Preliminary tests were conducted on the uncoated M2 tool steel within in a load range of 0.5–5.0 N. The highest friction was recorded at 5.0 N and the tests on coated M2 substrates were conducted at this load using a linear sliding speed of 0.12 m/s. As indicated in Section 2.1, the counterface was a pin made of AZ91 alloy with a hemispherical contact surface which created a wear track with a radius of 1.5 mm on the coated M2 disk surfaces during sliding. An aqueous KCl solution was placed in the test chamber to create an atmosphere with 75% RH [6]. For dry air tests the test chamber was purged with argon gas to reduce moisture (b1% RH) as confirmed using a hygrometer (designated as 0% RH). The COF was measured continuously during the sliding test. On each coating three tests were conducted at each humidity level, and the typical COF curves plotted as a function of sliding distance (number of revolutions) are reported. The wear rate of the DLC coatings was calculated from the volume of the material removed during sliding by measuring the cross-sectional area at eight different locations along the wear track using a white light optical surface profilometer and the averages are reported. The details of the wear rate calculations for the coatings can be found in [36,37]. Sliding tests were also conducted on a polycrystalline diamond, PCD, with a hardness of 80 GPa, against the same AZ91 pins. The surface roughness PCD was Ra = 13 ± 3 nm. The average grain size of PCD samples was 0.70 ± 23 nm. The PCD was selected for comparison of its tribological properties with the coating as PCD tool inserts are extensively used in the machining of magnesium alloys. In addition to the carbon based coatings, tribological tests were

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Fig. 1. a) FIB/SEM image of the cross-section of F-H-DLC coating and EDS elemental maps for b) C and c) F. The arrow shows the thickness of the coating on the M2 substrate.

also performed on traditional N-based hard coatings namely, TiN, TiAlN and TiCN as well as uncoated M2 steel. The details of deposition techniques and properties of N-based coatings can be found in [35]. 2.4. Surface analysis methods Morphologies of transfer layers that were formed on the AZ91 pin surfaces during the sliding contact experiments were observed using a

FEI Quanta 200 FEG scanning electron microscope equipped with an EDAX SiLi detector spectrometer. EDS spectra (and the maps) were acquired at an accelerating voltage of 10.0 kV using 512 × 400 image resolution and 64 frames. The probe size used for acquiring the EDS maps was 3.4 nm with a working distance of 10 mm and a beam current of 1900 pA. The chemical compositions of the transfer layers were determined using Thermoelectron Nicolet 760 FourierTransformed Infrared (FTIR) spectroscopy. For this purpose the pin

Fig. 2. The interface model formed among 6 bilayers of a (a) F-terminated diamond surface and H-terminated diamond surface; (b) F-terminated diamond surface and OH-terminated diamond surface. dinterface is the distance between the F and H atoms at the interface for (a) and the distance between O and F atoms at the interface for (b). Surface constructions of (c) F-terminated diamond (111) and (d) OH-terminated diamond (111) surface.

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(a) TiN N

TiAlN

H-DLC

F-H-DLC TiCN N

2 Steel M2

H-D DLC

PC CD

Average Coefficient of Friction

(b)0.5

Fig. 5. Summary plot comparing average steady state COF (μS) of H-DLC and F-H-DLC at different RH levels. Each point in the plot represents the average value of three tests performed at a particular RH. The error bars represent the variations about the mean steady state COF values obtained from the three test results.

0.4

0.3

spectroscopy (XPS) using a Kratos Axis Ultra X-ray photoelectron spectrometer. The survey scan analyses were carried out with a pass energy of 160 eV, while the high-resolution analyses were carried out with a pass energy of 40 eV.

0.2

0.1

0.0 PCD

LC H-DL

M2 Stteel

T TiN

TiC CN

TiiAlN

Fig. 3. (a) Variations of the COF with the number of revolutions for uncoated M2 steel, TiN, TiAlN, TiCN, PCD and H-DLC tested against AZ91 in an ambient air atmosphere with 32% RH. (Load = 5.00 N and speed = 0.12 m/s). (b) Bar chart showing the average COF values (and standard deviations) of uncoated M2 steel, TiN, TiAlN, PCD, H-DLC and TiCN against AZ91—as determined from three tests performed for each coating under the tests conditions specified in (a). For PCD and H-DLC the average steady state COF values μS are plotted.

surfaces were analyzed in reflectance mode at two spots using an aperture of 100 μm × 100 μm. Micro-Raman studies of the transfer layers and the coating surfaces were carried out using a 50 mW Nd-YAG solid state laser (532 nm excitation line) through the 50 × objective lens of a Horiba Raman micro-spectrometer. Chemical analyses of the transfer layers formed on AZ91 were conducted by X-ray photoelectron

2.5. First principles computational studies of adhesion between contact surfaces The adhesion between the F, H and OH terminated diamond surfaces was studied by constructing an interface model between H-terminated (diamond:H) or F-terminated (diamond:F) and OH-terminated (diamond:OH) diamond (111) surfaces (Fig. 2 [a, b]). All the stable structures of diamond:H, diamond:F and diamond:OH had 1 × 1 construction [38–40] with 4 terminating groups on the 2 × 2 diamond (111) surface with an edge length of 5.05 Å (Fig. 2 [c, d]) so that interfaces were fully coherent. During simulations, the interface separation distance, dinterface, was decreased from 3.7 to 1.5 Å by allowing the atomic positions to relax gradually while the total energy of the system was computed. The dinterface, is defined as the distance between H and F atoms at the interfaces created between diamond:H and diamond:F as shown in Fig. 2(a). The dinterface also defines the distance between O and F (or H) atoms at the interfaces created between diamond:OH and

F-H-DLC C

H-DLC C

0% RH

75% RH

52% % RH

15% RH 32% RH 32% % RH

52% RH

15% RH

75% RH H 0% RH H

Fig. 4. Variations of the COF with the number of revolutions when H-DLC coating was tested against AZ91 at 0% RH, 15% RH, 32% RH, 52% RH and 75% RH.

Fig. 6. Variations of the COF with the number of revolutions when F-H-DLC coating was tested against AZ91 at 0% RH, 15% RH, 32% RH, 52% RH and 75% RH.

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diamond:F (or diamond:H) as shown in Fig. 2(b). The change in the energy during interface separation can be used to calculate the interfacial normal force, FN, exerted between surfaces, as F N ¼ − ∂d∂Etot where interface Etot is the total energy of the slab (5–10 layers of atoms separated by a 10 Ǻ vacuum gap). The total energy and the ground state structure of each diamond interface were calculated using the first principles methods based on density functional theory (DFT) by solving the single particle Kohn– Sham [41] equation with a plane wave basis set. A projectoraugmented wave (PAW) method with exchange correlation energy approximated in the generalized gradient approximation (GGA) [42] to DFT as implemented in the Vienna Ab initio Simulation Package (VASP) [38,43–44] was used in all computations. Calculations were carried out using PAW-PBE [45] potentials, supplied by VASP. The total energy values were obtained by relaxing the atomic positions and minimizing the Hellman–Feynman forces using a conjugate gradient method. In all calculations, energy convergence to 1–2 meV was obtained using a plane wave cut off energy of 600 eV. The electronic degrees of freedom for surface structures were converged to 10−5 eV/cell and Hellman–Feynman forces were relaxed to less than 10−4 eV/Å. Details of the computational methods are given in [24].

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about 150 revolutions. The COF then decreased again to a low and stable μS of 0.11 ± 0.01 (Fig. 3a). The average values of μS obtained from three friction tests performed on PCD and H-DLC, and the average of the mean COF values obtained from three tests conducted on each of the N-based coatings are reported in Fig. 3(b). According to this ranking H-DLC coating emerges as a suitable candidate for further evaluation of its tribological performance against AZ91 at different humidity levels as described in Section 3.2. It was observed that the COF of F-H-DLC coating (μS = 0.12) was as low as that of the H-DLC coating, with the same H atomic percentage, and hence F-H-DLC coating was also tested at different humidity levels (see Section 3.2). 3.2. Changes in friction behavior of H-DLC and F-H-DLC with humidity Fig. 4 shows the variations of COF of H-DLC with the number of revolutions for the tests conducted at 0% RH, 15% RH, 32% RH, 52% RH and 75% RH. A common trend was observed in all COF curves such that they exhibited a high running-in friction regime followed by a

(a) 0% RH

H-DLC

3. Experimental and computational results and discussion 3.1. Evaluation and ranking of COF of different coatings Initially three types of N-based coatings, namely TiN, TiAlN and TiCN, were tested against AZ91 in order to compare their performance with C-based coatings at a constant humidity level of 32% RH. All N-based coatings exhibited high COF values and large fluctuations similar to the behavior of the M2 steel as shown in Fig. 3(a). The COF values calculated for sliding between 400 and 1000 revolutions were 0.34 ± 0.04 for the M2 steel, 0.36 ± 0.04 for TiN, 0.38 ± 0.06 for TiAlN and 0.33 ± 0.07 for TiCN. In contrast, C-based coatings (PCD, H-DLC) initially exhibited a distinct running- in period with high COF, μR, which was followed by a decrease to a low steady-state COF, μS, characterized by minor fluctuations (± 0.01). The μR of PCD was 0.13 for about the first 400 revolutions followed by steady-state COF regime with a low μS of 0.05 ± 0.01. A similar behavior was observed during sliding of H-DLC coatings for which μR of 0.28 was recorded for the running-in period that lasted

Mg debris

200 µm

(b) 75% RH

H-DLC

H-DLC

Mg debris

F-H--DLC

Adhered Mg

200 µm Fig. 7. Variations of wear rate with change in RH for H-DLC and F-H-DLC coatings after 1000 revolutions. Each point in the plot represents the average value of three tests performed at a particular RH. The error bar represents the variations about the mean wear rate from the three test results.

Fig. 8. Typical secondary electron images of wear tracks formed on H-DLC surface when tested against AZ91 after 1000 revolutions at (a) 0% RH and (b) 75% RH. The Mg debris and adhered Mg were indicated using arrows.

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low steady state friction regime. The running-in regime corresponds to the initial time period during which a transfer layer forms on the AZ91 counterface. Three tests were performed on samples tested under each constant RH atmosphere and the average steady state COF is shown in Fig. 5, where it becomes clear that μS of H-DLC increased with increasing the RH: at 0% RH μS was 0.10 ± 0.003 and a μS of 0.32 ± 0.01 was reached at 75% RH. The values of μR showed no particular trend that depended on: RH and varied between 0.35 and 0.45. As the steady state friction values of H-DLC were found to increase with RH, an alternative DLC coating namely F-H-DLC was also tested under high humidity atmospheres and the results are plotted in Fig. 5. In Fig. 6 the typical curves illustrating the variations of COF of the F-HDLC at different humidity levels are shown as a function of the number of revolutions. At 0% RH, a steady state regime was not observed and a high COF of 0.35 ± 0.02 with large fluctuations persisted throughout the sliding test. A steady state COF, μS of 0.21 ± 0.003 was attained when the tests were conducted at 15% RH following a lengthy running-in period. The duration of running-in period decreased with an increase in the RH (similar to H-DLC). A more salient observation

was that the RH dependence of the μS for F-H-DLC exhibited an opposite behavior to that of H-DLC. For tests conducted at 32% RH the F-H-DLC had a μS of 0.18 ± 0.003, whereas at 75% RH μS was 0.13 ± 0.002. The average μS values obtained from the three tests conducted on F-H-DLC at each constant humidity level were calculated and plotted in Fig. 5, together with the corresponding μS results for H-DLC. 3.3. Wear rates of H-DLC and F-H-DLC with change in relative humidity Fig. 7 shows the opposing trends for wear behavior of the surfaces of H-DLC and F-H-DLC. Similar to the friction (μS) trend, tests in high RH containing atmosphere increased the wear rates of H-DLC, whereas a reduction in wear was observed for F-H-DLC. The normalized wear rate of H-DLC was 2.03 × 10− 5 mm3/Nm at 0% RH, and slightly increased to 2.12 × 10− 5 mm3/Nm at 15% RH. The highest wear of 7.33 × 10−5 mm3/Nm was observed at 75% RH. In contrast, the highest wear for F-H-DLC (4.68 × 10−5 mm3/Nm) was observed at 0% RH. At 15% RH, the wear decreased to 3.67 × 10−5 mm3/Nm and continued to decrease to 1.03 × 10−5 mm3/Nm as the humidity increased to 75% RH.

0% RH R

Mg

(b)

Transfer laye er

O

(c) C

(a)

2 200 µm

75% RH

(d) Mg

(f) O

(g) C

(e)

2 200 µm

(h)

Fig. 9. Secondary electron image of the AZ91 pin surface tested against H-DLC coating after 1000 revolutions at (a) 0% RH. The elemental EDS maps taken from the whole contact area on (a) are shown for (b) Mg, (c) O, and (d) C. (e) Secondary electron image of the AZ91 pin surface tested against H-DLC coating after 1000 revolutions at 75% RH. The elemental EDS maps taken from the whole area on (e) are shown for (f) Mg, (g) O, and (h) C.

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3.4. Changes on sliding surfaces of H-DLC and F-H-DLC with humidity and formation of transfer layers Examination of the wear tracks formed on the H-DLC surfaces by SEM revealed that negligible amount of material transfer occurred from AZ91 during the tests performed at 0% RH as shown by the secondary electron image, SEI, in Fig. 8(a). Traces of AZ91 transfer could be found occasionally along the wear tracks when the tests were done at 32% RH. On the other hand, AZ91 transfer occurred to the H-DLC surfaces both at 52% RH and at 75% RH as shown in Fig. 8(b). The AZ91 pin contact surfaces were examined by SEM and EDS. During the tests conducted at 0% RH, 15% RH and 32% RH the contact surfaces of the AZ91 pins were covered with patches of material transferred from the H-DLC. Figs. 9(a-d) shows the morphology of the AZ91 pin surface tested at 0% RH, which is covered with transfer layer rich in C (Fig. 9 d) and O (Fig. 9 c) as determined by EDS. For tests conducted at 52% RH and 75% RH transfer layers were not formed on the AZ91 pin. According to Fig. 9(e), for example, there was no transfer layer on the contact surface of the AZ91 when the test was conducted at 75% RH. The EDS maps of the relevant elements Mg, O and C shown in Fig. 9 (f–h) revealed that the contact surface of the AZ91 pin tested at 75% RH was oxidized but devoid of any carbonaceous material. SEI image of F-H-DLC, shown in Fig. 10(a), indicated extensive amount of AZ91 material transfer to the coating surface at 0% RH as expected from the high COF. Also for tests at 15% RH and 32% RH, Mg transfer occurred despite the fact that these samples reached low steady-state COF values. The amount of material transferred from AZ91 to the F-H-DLC decreased with an increase in RH. At 75% RH, there was no evidence of magnesium adhesion inside the wear tracks of F-H-DLC (Fig. 10b). The AZ91 pin tested at 0% RH against F-H-DLC showed scuffing marks along the sliding direction but no material transfer occurred from the coating as indicated by Figs. 11 (a–e). Patches of C-rich transfer layers were found on the AZ91 pin surface when tested against F-H-DLC in atmospheres containing 15% RH and 52% RH. At 75% RH, a transfer layer covered the contact surface of the AZ91 pin, as shown in Fig. 11(f). The radius of the contact area was smaller, indicating less wear. It was noted that the composition of this transfer layer was distinctly different from others. The contact area was oxidized as evident from the high O concentration shown in Fig. 11(h). A high carbon concentration was found as shown in Fig. 11(i) typical of the layers formed due to material transfer from DLC coatings. A particularly important feature of the compositional analyses however was the fact that the transfer layer was rich in F (Fig. 11j). The role of F in reducing the COF is discussed in Section 3.7.

27

(a) C F-H-DLC

0% RH

Mg deb bris

ered Mg Adhe

2 200 µm

(b) F-H-DLC C

7 75% RH

Mg debrris

2 200 µm Fig. 10. Typical secondary electron images of wear tracks formed on the F-H-DLC surface when tested against AZ91 for 1000 revolutions at (a) 0% RH and (b) 75% RH.

3.5. Characterization of transfer layers The micro-Raman spectra obtained from the transfer layers that were formed on AZ91 surfaces sliding against H-DLC and F-H-DLC under different testing conditions are shown in Fig. 12a and b respectively. The Raman spectra of the unworn H-DLC coating had a broad peak between 1250 and 1550 cm− 1, which is a common feature of amorphous hydrogenated DLC coatings. The Raman spectra of the transfer layers that were generated on the AZ91 pin surface showed distinct peaks at 1388 cm−1 and 1583 cm−1, which were assigned as the D and G bands of carbon [35], and indicated an increase in the sp2 fraction in the carbon transfer layer (Fig. 12a). This may suggest sliding induced graphitization as previously proposed in the literature [5,35]. For tests conducted on H-DLC at 75% RH as well as on F-H-DLC at 0% RH with, no distinguishable peaks were recorded. Peaks between 1190 cm− 1 and 1250 cm− 1 were observed for cases where transfer layers were formed, namely at 0 − 52% RH for H-DLC and 15%–75% RH for F-HDLC. These peaks were assigned to C\O bonds [46] indicating passivation of the carbonaceous transfer layer. Further evidence for passivation was gathered using FTIR analyses of the transfer layer.

FTIR analyses conducted on the transfer layers formed on the AZ91 tested against H-and F-H-DLC showed that the layers had different chemical compositions depending on the test atmosphere (Fig. 13 [a–f]). The transfer layer formed during tests carried out at 0% RH exhibited a peak at 1500 cm−1 that can be attributed to C\H group which is consistent with the material transferred from the H-DLC coating (Fig. 13a). FTIR spectra obtained from the material transferred to the AZ91 during tests conducted in an atmosphere with 52% RH (Fig. 13(b)) showed peaks between 2000 and 2500 cm−1 which could be attributed to C\O groups. The peaks at 1454 cm−1 and 2925 cm−1 could be ascribed to C\H groups. A broad band observed at 3458 cm−1 was assigned to hydroxyl (OH) group [47,48]. This observation is interesting as it infers the possibility of OH passivation of the carbonaceous transfer layers. The contact surface of AZ91 at 75% RH showed the less intense peaks assigned as C\H and some other weak peaks (Fig. 13c). Microscopic observations in Fig. 9(h) indicated the absence of a carbonaceous transfer layer for tests conducted in an atmosphere with 75% RH.

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RH 0% R

(a)

20 00 µm

75% RH

(f)

Mg

C

(b)

(d)

O

F

(c)

(e)

Mg

C

(g)

(i)

O

F

(h)

(j)

Transferr layer 20 00 µm

Fig. 11. Secondary electron image of the AZ91 pin surface after the tested against F-H-DLC coating after 1000 revolutions at (a) 0% RH. The elemental EDS maps taken from the whole area on (a) are shown for (b) Mg, (c) O, (d) C and (e) F of the AZ91 surface. (f) Secondary electron image of the AZ91 pin surface tested against F-H-DLC coating after 1000 revolutions at 75% RH. The elemental EDS maps taken from the whole area on (f) are shown for (g) Mg, (h) O, i) C and (j) F of the AZ91 surface.

FTIR spectra of F-H-DLC tested against AZ91 in an atmosphere with 0% RH (Fig. 13d) showed no OH peak. Under this condition no carbonaceous transfer layer was formed on the AZ91 (Fig. 11d). The carbonaceous material transfer layers were formed during the tests with 52% and 75% RH, and consequently high intensity peaks that correspond to C\H (1500 cm−1) and OH (3458 cm−1) were formed as shown in Fig. 13 [e, f]. When analyzed together Figs. 9, 11 and 13 indicate that surface passivation occurs only when there is evidence of carbonaceous transfer layer formation. In case of F-H-DLC the increase in atmospheric humidity promoted transfer layer formation while the opposite behavior was observed for H-DLC. 3.6. Forces between contact surfaces First principles calculations were used to compute the forces operating between the contact surfaces when either H-DLC or F-HDLC sliding against the AZ91 surface led to formation of transfer layers (Fig. 14). When two F-terminated diamond surfaces approach each other, the negatively charged F atoms are expected to develop a large repulsive electrostatic interaction. This assumption was tested by performing adhesion simulations of a fully F-terminated surface against another fully F-terminated surface. Fig. 14 shows that when two C (diamond):F surfaces were brought together the F atoms at the diamond surface led to repulsive interactions between F atoms in the transfer layer and the F-H-DLC coating surface. The high magnitude of F\F interaction in Fig. 14 compared to other pairs suggested generation of high repulsive forces that may reduce the

COF. The low steady state COF μs of F-H-DLC running against AZ91 (when F containing transfer layers are formed) is consistent with this hypothesis. In comparison, the normal forces for H\H interaction, during contact between the H in the transfer layer and H-DLC, were lower possibly leading to higher steady state COF. The first principle calculations also showed that the F\OH repulsion is high as principles in Fig. 14. The importance of F\OH interactions and its role in reducing friction is discussed in Section 3.7. 3.7. Role of fluorine on friction reduction The results presented in Sections 3.2–3.4 showed that the formation of transfer layers and passivation of these layers by H and OH would contribute to a low COF. The increase in atmospheric humidity led to the formation of a stable transfer layer on the AZ91 sliding against F-H-DLC with a low μS (Fig. 5). An opposite trend was observed for H-DLC with high friction (Fig. 5) and wear (Fig. 6) at high atmospheric humidity environments and in this case a transfer layer did not form (Fig. 9). To gain insight into the low friction observed in F-H-DLC it is pertinent to refer to Fig. 11 that depicted that transfer layers on AZ91 incorporated F (along with C, and O) when tested against F-H-DLC at 75% RH. The transfer of F from the coating surface to the AZ91 contact surface is particularly important. High resolution XPS spectra of C 1s obtained from the transfer layers (Fig. 15) provided further evidence for the formation of C\F bonds in the transfer layers under the high humidity conditions. In Fig. 15, the binding energies of peaks formed at 284.8, 286.3, and 289.1 eV can be assigned to the \C\C/\C\H, \C\OH/

S. Bhowmick et al. / Surface & Coatings Technology 267 (2015) 21–31

29

(b)

(a) 0% RH

C-O C

15% RH C-O C

32% RH

D

D

G

G

D

G

C-O D

G

C-O C

52% RH

D

G

D

G

D

G

D

G

D

G

15% RH

32% RH

52% RH

75% RH

75% RH

H-DLC

0% RH

F-H-DLC

C C-O

C-O

G D

D

G

C-O

D

G

Fig. 12. Raman spectra of the transfer layers formed on AZ91 surface at 0% RH, 15% RH, 32% RH, 52% RH and 75% RH against (a) H-DLC and (b) F-H-DLC.

\C\O\C, \O\C_O bonding states respectively. The peak generated at 287.8 eV can be interpreted as due to C\F which infers that F in the transfer layer formed bonds with C. Therefore when F-H-DLC is placed in sliding contact with AZ91, following the initial running in period where the transfer layers become established (which requires testing in an atmosphere with N 10 % RH) the sliding interface consists of F and OH bonds at both sides. These bonds are highly repulsive as evident from the results shown in Fig. 14. At this point a discussion of the hydrophobicity of F-H-DLC coatings is relevant in order to examine the contribution of fluorine on low friction behavior at high humidity atmospheres. The water contact angle of the F-H-DLC coating is close to 90° (Section 2.2). A contact angle atomistic simulation [49] that compared fluorocarbon and hydrocarbon surfaces indicated that the hydrophobicity of fluorinated surfaces is due to less dense packing of fluorocarbons at the surfaces leading to weaker van der Waals interactions with water. Another study [50] indicated that the lack of hydrogen bonding ability of the C\F bonds was responsible for the hydrophobic nature of fluorinated surfaces. In this case, hydrophobic interactions could be attributed to the repulsive forces between the F and OH groups at each side of the interface. Atomistic simulation results shown in Fig. 14 indicate that the interfacial force required to bring F and OH molecules together is greater than that of the H and OH interfacial force. Consequently, the low μS of 0.13 measured for F-H-DLC at 75% RH could be attributed to the hydrophobicity induced by strong F and OH repulsive forces. Taking advantage of low friction of F-H-DLC at high humidity levels, a set of drilling tests were carried out successfully on AZ91 using F-HDLC coated drills and by H2O applied as of a minimum quantity

lubricating (MQL) agent. F-H-DLC coated tools maintained a low and stable torque and provided a performance equal to flooded drilling. Although demanding machining applications such as deep hole drilling, thread cutting and other machining operations—usually carried out without use of lubricants—may not particularly benefit from the deposition of F-H-DLC coatings on the cutting tools, the use of F-H-DLC in conjunction with H2O (aqueous)—MQL may be a feasible machining technology for magnesium alloys. 4. Conclusions The main conclusions arising from this work can be summarized as follows: 1. Tribological tests conducted on H-DLC and F-H-DLC against AZ91 alloy showed that the average steady state COF of F-H-DLC decreased with an increase in relative humidity while H-DLC showed an opposite trend. F-H-DLC exhibited lower wear rates at high atmospheric humidity compared to H-DLC. 2. During sliding contact, carbonaceous transfer layers were formed on the counterfaces when the low COF was observed. These layers were observed at high humidity levels for F-H-DLC but not for H-DLC. C-H, C-OH bonds were detected in the transfer layers, which also contained C-F bonds when formed from F-H-DLC. 3. Atomistic simulations showed that the interfacial force required to bring F and OH molecules together was greater than that for H and OH which could contribute to the hydrophobic nature of F-H-DLC coatings and the low COF of F-H-DLC coatings at high humidity atmospheres.

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S. Bhowmick et al. / Surface & Coatings Technology 267 (2015) 21–31

(a)

(d) 0% RH

H-D DLC

0% RH

F-H-DLC

C-H CO2

(b)

C-H C--H

H-DL LC

CO2

O-H

52% RH

(e)

C-H

F-H-DLC

O O-H

5 52% RH

C-H C--H

CO2

O-H CO2

C-H H

(c)

(f) H-D DLC

75% RH R

C-H

F-H-DLC

O O-H

7 75% RH

C-H

CO2 C C-H

CO2

H C-H

C-H

O O-H

O-H

Fig. 13. FTIR spectra of the transfer layers formed on AZ91 surface tested against H-DLC at (a) 0% RH, (b) 52% RH and (c) 75% RH. FTIR spectra of the transfer layer formed on the AZ91 surface against F-H-DLC at (d) 0% RH, (e) 52% RH and (f) 75% RH.

4. F-H-DLC coated surfaces may be used in sliding applications of AZ91 that require low friction under high atmospheric humidity and may find usage as tool coatings in manufacturing processes of magnesium alloys such as aqueous machining. Acknowledgments The authors would like to thank Auto 21 Innovation through Research Excellence, Canada for their financial support. Authors acknowledge the SHARCNET (Shared Hierarchical Academic Research

Computing Network)/Compute Canada for the computations carried out in this work.

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