Tribol Lett (2017)65:76 DOI 10.1007/s11249-017-0856-2
ORIGINAL PAPER
Effect of WS2 Addition on Tribological Behavior of Aluminum at Room and Elevated Temperatures Sara Rengifo1 • Cheng Zhang1 • Sandip Harimkar2 • Benjamin Boesl1 Arvind Agarwal1
•
Received: 8 January 2017 / Accepted: 19 April 2017 Ó Springer Science+Business Media New York 2017
Abstract The objective of this study is to evaluate the role of WS2-based two-dimensional material as solid lubricant additive to aluminum to reduce the friction. Al-2 vol% WS2 was consolidated by spark plasma sintering into a 99% dense composite. Tribological behavior of Al-2 vol% WS2 and pure Al was evaluated at room temperature and 200 °C using the ball-on-disk method in dry sliding wear conditions. Wear mechanism was studied using wear surface and sub-surface via electron microscopy and focused ion beam milling. Al-2 vol% WS2 showed the lowest coefficient of friction value of 0.55 at 200 °C as compared to 0.82 that of pure aluminum. The wear rate of Al-2 vol% WS2 showed 54 and 29% improvement at room temperature and 200 °C, respectively, as compared to pure Al. The improved tribological properties of Al-2 vol% WS2 are attributed to a tribofilm formation due to the breaking of the weak Van der Waals forces that hold the S–W–S structure. Due to shear forces, a portion of the tribofilm is transferred on the counter surface providing lubrication properties resulting in a stable and low COF. Keywords Friction Tribology Self-lubricant WS2 Tribofilm
& Arvind Agarwal
[email protected] 1
Plasma Forming Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA
2
School of Mechanical and Aerospace Engineering, Oklahoma State University, 218 Engineering North, Stillwater, OK 74078, USA
1 Introduction Overcoming friction in engine systems can significantly improve the fuel efficiency of automobiles [1–3]. Passenger cars use one-third of fuel energy to overcome friction, which is mostly detected in the engine transmission, tires, and brakes [1]. Major research efforts to reduce friction and wear in automotive materials include the development of novel aluminum alloys and composites [4–6], advanced surface engineering techniques, anti-friction coatings [7], and solid lubricant additives [8]. With the aim of improving wear and coefficient of friction in aluminum, [9–12], researchers found that solid lubricants such as graphite [11, 13] and metal dichalcogenides such as MoS2 and WS2 can significantly ameliorate the tribological properties of the composite. Liu et al. [11] obtained an order of magnitude reduction in the wear rate of 2014 Al alloy reinforced with 50 vol% flaky graphite synthesized by pressure infiltration. However, graphite/graphene phase tends to react with aluminum to form aluminum carbide which is hygroscopic and can result in lower tensile strength of the composite [14]. Wang [12] developed a porous anodic film of MoS2 on aluminum surface by anodizing to improve lubrication. Both wear life and friction improved due to the filling of the porous anodic layer by MoS2 solid lubricant. Tungsten disulfide, which is another novel 2D layered material, has shown excellent performance by reducing friction and wear in composites with different matrices. Tribological behavior of spark-plasma-sintered Ni3Al matrix reinforced with WS2 ? Ag ? hBN (WAh) was reported from room temperature to 800 °C [15]. The results show that Ni3Al ?15 wt% WAh reduced the coefficient of friction (COF) by 62–72% than a Ni3Al matrix with a wear rate improvement of *40% at 600 °C [15]. In a separate study, sputtered WS2 ? 9 at.% Ag composite film
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presented a very long wear life of 7.6 9 105 cycles in a vacuum and 6.7 9 105 cycles in the humid air [16]. Polymer matrix such PTFE/Kevlar reinforced with 7.5 wt% sub-micron WS2 ? 12.5 wt% nano-Si3N4 also displayed a very low wear rate with 53% improvement and a low COF of 0.045 [17]. Different forms of WS2 such as fullerene-like [18] and nanotube [19] have also been explored as additives to reduce friction. Tribological properties of hot-pressed Cu– MoS2–graphite–WS2 nanotube composites were investigated in both air and vacuum conditions [13]. The COF and wear rate were lower in the air than in vacuum due to lubrication from the mixture of sulfides and graphite [20]. However, the role of WS2 as nanotubes was not discussed. WS2 has also been deposited as a protective coating on different substrates such as Al–SiC composite [21], carbon steel [22], quartz [23]. WS2-based coatings have resulted in excellent tribological performance, as they allowed for the very low coefficient of friction (0.04 and below) [16], and improvement of wear rate. Hence, WS2 is recognized for its excellent lubricating performance under a high load, high pressure, and high temperature. WS2 creates a tribochemical transfer film on the counter surface during wear resulting in the reduced COF [24, 25]. The coefficient of friction of the WSx film decreases gently with an increasing S/W ratio [26]. The biggest concern about WS2 as a lubricant is its high reactivity in a humid environment and formation of amorphous WO3, which affects tribological performance [27]. The objective of the present study is to investigate the tribological behavior of Al-2 vol% nano-WS2 composite synthesized by spark plasma sintering (SPS). The effect of WS2 as solid lubricant additive to aluminum is evaluated by ball-on-disk wear test at room and elevated temperature (200 °C). Wear mechanism at two different temperatures is elucidated in terms of transfer film formation and subsurface analysis by focused ion beam (FIB)-based milling and imaging. Wear debris has been analyzed by high-resolution transmission electron microscopy (HR-TEM) to understand the chemical reactions occurring during wear. To the best knowledge of authors, the addition of tungsten disulfide (WS2) to aluminum to study tribological properties has never been reported in the literature.
2 Materials and Methods
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size of WS2 is 90 nm with hexagonal faceted morphology. Two vol% percent of nano-sized WS2 powder was added to Al powder into acetone followed by ultrasonication for 90 min. Subsequently, the powder was dried in a vacuum oven at 75 °C for 24 h. A lower concentration of 2 vol% WS2 was selected as it has been observed that lower concentration (2–3 vol%) is more effective in improving the properties due to reduced agglomeration of nanomaterials including 2D materials like graphene and WS2 [28]. 2.2 Spark Plasma Sintering (SPS) The mixed Al-2 vol% nano-WS2 powders were packed in a graphite foil and consolidated using a Spark Plasma Sintering 10-3 model (Thermal Technology Corp., CA, USA). The consolidation was carried out in a 20-mm-diameter graphite die at a maximum sintering temperature of 500 °CC using a holding pressure of 50 MPa for a 10-min duration. A heating rate of 50 °C/min was used in the vacuum environment. Pure Al powder was also sintered under similar conditions as a control sample. 2.3 Microstructural and Phase Characterization The density of the sintered pellets was measured using AccuPyc II 1340 Series Pycnometer using Helium gas. The cross section of the samples was metallographically polished up to 0.5 lm alumina powder. Vickers micro-hardness of the polished cross section of the sintered pellets was measured under 100 g-force applied for 15 s. X-ray diffraction (XRD) experiments were performed on the powder and sintered composites using a D5000 Siemens Diffractometer at an operating voltage of 40 kV and a current of 35 mA. The radiation used was Cu Ka which has ˚. a wavelength of 1.542 A Field emission scanning electron microscopy (JEOL JSM-633OF) was used to characterize the powders, sintered pellets, and the fracture surface of each pellet. Raman spectroscopy was carried out using Spectra-Physics model 3900S with Ti-sapphire crystal as the target (514 nm), a laser power of 18 mW, and a detector with 4 cm-1 spectral resolution from Kaiser Optical. JEOL-JIB 4500 MultiBeam Focused Ion Beam (FIB) was used to machine the wear track to observe the sub-surface for a better understanding of the wear mechanisms. Phillips CM-200 200 kV Transmission Electron Microscope (TEM) was used to observe the reaction products in the wear debris.
2.1 Powder Preparation 2.4 Tribological Test Spherical aluminum powder (H3) with an average particle size of 2–10 lm was obtained from Valimet Inc., CA, USA. The nano-sized WS2 powder was obtained from Graphene Supermarket, NY, USA. The average particle
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The tribological tests were conducted in the rotative mode (ASTM G99) using a ball-on-disk Nanovea tribometer. All the samples were ground using 600 grit paper followed by
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cloth polishing. The surface roughness (Ra) was measured to be 0.62–0.69 lm. The top surface of the sintered pellets was subjected to 1 N normal load against 3-mm-diameter Al2O3 ball as the counter surface at a speed of 100 RPM. The test was conducted for 30 min at RT and 200 °C under dry sliding conditions. High-temperature tests were conducted at 200 °C to simulate gasoline operated engines [29, 30]. The coefficient of friction was recorded in real time using an LVDT sensor. Nanovea PS50 Optical Profilometer was used to scan the wear surface to obtain 2D wear profile and wear depths of each track. Scanning Probe Image Processor (SPIP) and Origin 6.0 were used to convert 2D wear profiles to 3D profiles and compute wear volume. Energy-dispersive spectroscopy (EDS) of the wear track was carried out using a JEOL JIB 4500.
3 Results and Discussion 3.1 Microstructural Characterization and Phase Identification Figure 1a shows the starting spherical Al powder with a diameter varying between 2 and 10 lm. The faceted hexagonal structure of nano-WS2 with few layers is shown in TEM micrograph in Fig. 1b. The inset shows a cluster of faceted nano-WS2 powder particles. The overall cross-sectional microstructure of sintered Al and Al-2 vol% WS2 is shown in low-magnification SEM micrographs in Fig. 2. Pure Al (Fig. 2a) shows sintered spherical aluminum particles with a higher degree of porosity. However, Al-2 vol% WS2 shows a very dense structure with fine WS2 particles (white phase) homogeneously dispersed in the matrix (Fig. 2b). The high density of Al-2 vol% WS2 sample is attributed to the nano-sized WS2 powder which fills the voids between spherical Al powders resulting in a better packing density due to bimodal powder size distribution. Pure Al displays poor
(a)
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sintering due to large spherical particles which form small neck during sintering but leaves greater trapped porosity. The densification measured by helium pycnometer showed 96% density for pure Al and greater than 99.8% for Al-2 vol% nano-WS2 which is reflected in Fig. 2. X-ray diffraction of the sintered pellets was carried out to observe if any new phase(s) were formed during sintering. All major peaks representing pure Al are located at 2h equal to 38.74°, 44.96°, 65.29°, 78.29°, and 82.81 in both samples. Pure Al also shows a trace amount of Al2O3 formation (Fig. 3). WS2 peaks at 2h equal to 14.17° (002), 34.79° (100), and 39.87° (103) confirm the hexagonal structure in sintered Al-2 vol% WS2 sample. To have a better understanding of the retention of the 2D structure of the solid lubricants (WS2) after SPS, Raman spectroscopy was performed on the starting powder and sintered pellet. Typical Raman spectra of WS2 show two main peaks located at 355 and 420 cm-1, which correspond to the E2g and A1g modes [31]. The A1g peak represents the S–S vibration between two different WS2 layers. The relative intensity of the A1g peak suggests that the vibration energy is small and that the particles, therefore, contain only a few stacked layers. It has been demonstrated that the ratio I2LA/IA1g is always greater than 2 for monolayer films [32]. In the present study, the ratio I2LA/IA1g for the powder is * 0.86 and for the composite is *1.68 indicating the presence of few layered structure in both WS2 powder and Al-2 vol% WS2 composite (Fig. 4). Figure 5a is a mass contrast TEM image showing black areas corresponding to agglomerated spherical WS2 particles. Each WS2 particle retains it starting size of 50–90 nm and is fully embedded in the Al matrix. No porosity or separation between WS2 particles and Al is observed. Figure 5b shows WS2 particle embedded between Al grains and corresponding EDS line profiles showing the presence of tungsten, sulfur, and aluminum. EDS spectrum from the region indicates the presence of aluminum, tungsten, and sulfur. A trace amount of oxygen is also
(b)
500 nm
10 µm
10 nm
Fig. 1 a SEM images of as-received Al powder b TEM image of nano-WS2 powder showing faceted surfaces with layers within each facet. Inset is a SEM image showing a cluster of faceted nano-WS2 powder
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(a)
(b)
15µm
12µm Fig. 2 Cross-sectional SEM images of spark plasma sintered: a pure Al and b Al-2 vol% WS2
3.2 Tribological Behavior: Wear Rate and Coefficient of Friction
Fig. 3 X-ray diffraction patterns of sintered pure Al and Al-2 vol% WS2
( )
352
418
WS2 Powder 352 404
Al-2 vol.% WS2
Fig. 4 Raman spectra for as-received nano-WS2 powder and sintered Al-2 vol% WS2
detected. Cu peak is observed due to the presence of the sample grid/holder. Based on the microstructural characterization and phase identification, it is confirmed that nano-WS2-based 2D solid lubricant additive is retained in the sintered composite.
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Wear resistance of pure Al and Al-2 vol% WS2 is char V acterized regarding its wear rate SP , which is expressed as volume loss (V) divided by the sliding distance (S) and applied normal load (P). Wear track was scanned using optical profilometer which provides wear depth and crosssectional area of the track to enable estimation of wear volume. Figure 6a shows a representative 2D profile of the wear track. The corresponding wear depths from both samples at RT and 200 °C are plotted in Fig. 6a. Pure Al shows wear depths of 25 and 55 lm at RT and 200 °C, respectively. Al-2 vol% WS2 shows lower wear depths of 20 and 24 lm RT and 200 °C, respectively. Wear volume was obtained by multiplying the cross-sectional area of the wear track with the wear depth for the entire track length. The corresponding wear rates for pure Al and Al-2 vol% WS2 are shown in Fig. 6b. Pure Al shows larger deviation (error bars in Fig. 6b) in the wear rate due to poor densification which leads to reduced homogeneity of the sintered microstructure. It can be observed that Al-2 vol% WS2 shows 54% reduction in the wear rate at room temperature as compared to pure aluminum. At 200 °C, Al-2 vol% WS2 shows 29% reduction in the wear rate as compared to pure aluminum. This can be attributed to a combination of high densification (99.8%) of Al-2 vol% WS2 and lubrication effect provided by WS2. Although WS2 is a softer phase, microhardness is similar for both Al and Al- 2 vol% WS2 (Table 1) due to improved densification as seen in Fig. 2. Figure 6b shows that wear rate increases at high temperature. It is well known that materials become softer at a higher temperature resulting in more wear. In addition to the wear rate, the coefficient of friction is a critical parameter to obtain an indirect measurement of energy efficiency in automotive materials. Figure 7 shows the COF variation during the room temperature and 200 °C for
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Fig. 5 a Mass contrast TEM image showing embedded WS2 particles in Al matrix. b TEM image showing WS2 and aluminum grains. EDS spectrum and line profile of the sample showing the presence of Al, W and S. Cu peak is due to the TEM sample grid
both samples. Aluminum has the highest COF (0.87), whereas Al-2 vol% WS2 has a lower COF (0.66) at room temperature. Aluminum exhibits a COF of 0.82 at 200 °C,
whereas Al-2 vol% WS2 has the lowest COF (0.55) at the same temperature. It is also observed that COF variation is more stable at 200 °C as compared to RT. Moreover, Al-2
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vol% WS2 sample displayed most stable COF without significant oscillations both at RT and 200 °C. The wear rate and coefficient of friction values of both samples at room temperature and 200 °C are summarized in Table 1. 3.3 Wear Mechanism and Sub-Surface Analysis The worn surfaces of the samples were studied using SEM to understand the underlying wear mechanisms. Figure 8a shows the wear track of pure Al after RT test. The surface
Fig. 6 a Wear depth profile for sintered Al and Al- 2 vol% WS2 at room temperature and 200 °C. The inset shows a representative 2D profile of the wear track. b Wear rate of sintered Al and Al- 2 vol% WS2 at room temperature and 200 °C
of the wear track reveals crack formation and is very rough in the appearance. The debris size was bimodal with a size distribution of 20 and 80 lm. These big detachments are due to poor sintering and interparticle bonding in aluminum, whereas finer debris is indicative of mixed nature due to the presence of brittle Al2O3 resulting in a higher COF. Micro-cracks are seen (inset image) which are attributed to the cracking of brittle Al2O3 layer. Figure 8c shows a smoother wear surface of aluminum at 200 °C, which is due to slight softening at the elevated temperature. The addition of nano-sized WS2 particles resulted in homogeneous dispersion and dense microstructure as previously observed in Fig. 2b. It is expected that uniform dispersion of nano-sized WS2 reinforcement will facilitate the formation of a uniform tribofilm, which will promote the reduction of COF and improve the wear resistance. Figure 8b shows wear track of Al-2 vol% WS2 at room temperature which is rough in appearance and looks similar to that obtained from pure Al (Fig. 8a). However, no micro-cracks were observed on the wear surface of Al-2 vol% WS2 at room temperature. The nano-size of WS2 particles allows these particles to act as solid lubricants without altering the matrix’s properties. The wear debris (2–20 lm) was finer and attached to the Al2O3 ball which
Fig. 7 The coefficient of friction variation as a function of time for sintered Al and Al- 2 vol% WS2 at room temperature and 200 °C
Table 1 Wear rate and coefficient of friction of pure Al and Al-2 vol% WS2 Sample
Densification (%)
Micro-hardness (Hv)
Wear rate-RT (mm3/Nmm) (910-6)
Wear rate-200 °C (mm3/Nmm) (910-6)
COF-RT
COF200 °C
Pure Al
96.9
59.2 ± 3.9
5.7 ± 1.6
7.5 ± 1.9
0.87 ± 0.04
0.82 ± 0.03
Al-2 vol% WS2
99.8
59.4 ± 5.1
2.6 ± 0.2
5.3 ± 0.5
0.66 ± 0.09
0.55 ± 0.10
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Fig. 8 SEM images of wear track: a Pure Al at room temperature, b Al-2 vol% WS2 at room temperature, c Pure Al at 200 °C, and d Al-2 vol% WS2 at 200 °C
suggests that sliding occurred between the tribolayer and transfer film on the counter surface, resulting in a stable and lower COF. The worn surface of Al-2 vol% WS2 obtained after 200 °C wear test is observed in Fig. 8d which is smoother in appearance and also suggests the formation of a tribofilm. The sub-surface of the wear track was investigated using optical microscopy and FIB to understand the tribolayer formation and dynamic wear features such as
cracks progression. Figure 9a shows a tribolayer formed on the cross-sectional wear surface of Al- 2 vol% WS2 at RT. The thickness of the tribofilm is *7 lm and is pointed out by the red arrow. Tribolayer is adherent to the underlying substrate. Figure 9b shows the sub-surface of the wear track prepared by FIB machining. A crack is propagating along the tribofilm through WS2 particles indicating the progression of the wear in the sliding mode.
Fig. 9 Sub-surface of the room temperature wear track of Al- 2 vol% WS2: a Optical micrograph shows a tribolayer and b SEM image after FIB milling shows propagating the crack
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(a)
0.304 nm WS2 (004)
The wear debris peeled off the counter surface (Al2O3 ball) was examined using TEM to understand the phases present in the tribofilm. High-resolution TEM image of the wear debris (Fig. 10a) confirms retention of WS2 phase. Small domains of crystalline WO3 (Fig. 10b) are also observed due to oxidation of a portion of WS2 as shown by the equation below. 3 WS2 þ O2 ¼ WO3 þ 2S 2
2 2 nm nm
(b)
Amorphous
0.364n WO3
Crystalline 2 2 nm nm
Fig. 10 HR-TEM images of the wear debris of Al-2 vol% WS2 obtained at room temperature: a Retention of the crystalline structure of WS2 is observed, and b WO3 phase is also formed due to oxidation
Oxidation during wear is thermodynamically feasible due to localized high temperature between the sample and Al2O3 ball. At high temperature (200 °C), it is expected that Al-2 vol% WS2 would have higher wear rate due to accelerated oxidation and formation of WO3. A higher wear rate is observed in Table 1, but this condition also presented the lowest COF at 200 °C. The lowest COF is attributed to continuous tribofilm formation which is dense, crack-free, and adherent as shown by the red arrow in Fig. 11a. The adherent tribofilm has a thickness of 5–10 lm. FIB image (Fig. 11b) shows the sub-surface of the wear track having a crack-free and adherent tribofilm. The absence of crack is attributed to an elevated temperature which provides localized sintering. WS2 nanoparticles can be seen as white spots with intact shape, reflecting the ability of WS2 nanoparticles to transfer and cleave to the Al2O3 counter surface due to the weak bonding forces between layers of sulfur–tungsten–sulfur (S–W–S). The schematic representation of the wear mechanism observed during this study for WS2 is depicted in Fig. 12. A tribofilm is formed on the wear surface due to the breaking of the weak Van der Waals forces that hold the S– W–S structure. Due to shear forces, a portion of the tribofilm is transferred on the counter surface. The wear occurs between transfer film and tribolayer providing lubrication properties and helping to reduce COF.
Fig. 11 The sub-surface of the wear track of Al-2 vol% WS2 at 200 °C: a Optical micrograph shows a dense tribolayer, and b SEM image after FIB milling shows the crack-free layer
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Fig. 12 Schematic representation of wear mechanism of WS2
4 Conclusions Tribological behavior of spark-plasma-sintered Al and Al-2 vol% WS2 is evaluated at room temperature and 200 °C using ball-on-disk in dry sliding conditions. Al-2 vol% WS2 composite displayed excellent densification (99%) due to nano-size WS2 particles filling the void between spherical aluminum particles. At room temperature, the wear rate of Al-2 vol% WS2 composite reduced by 54% as compared to pure aluminum. Also, Al-2 vol% WS2 has a lower coefficient of friction (0.66) as compared to pure aluminum which has a COF of 0.87. At 200 °C, the wear rate of Al-2 vol% WS2 composite reduced by 29% as compared to pure aluminum. Al-2 vol% WS2 has the lowest COF (0.55) at 200 °C, whereas pure aluminum has a high COF (0.82). The reduced COF in Al-2 vol% WS2 involves the formation of a protective transfer tribofilm. Thus, the sliding occurs between the tribolayer and the transfer film located on the counter surface, resulting in a very stable and low COF. Acknowledgement Authors thank Advanced Materials Engineering Research Institute at Florida International University for providing characterization facilities.
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