Self-adaptive and self-healing nanocomposite tribocoatings
Cao, Huatang
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Publication date: 2019
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Cao, H. (2019). Self-adaptive and self-healing nanocomposite tribocoatings. University of Groningen.
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6
C
HAPTER
6
*
In this chapter, we investigated the self-healing capacity of WS2/a-C tribocoating. We
found that a pre-notch up to 45 µm wide in the WS2/a-C coating surface could be
completely healed under the stimulus of sliding operations. In-situ tribotests of 100 laps, 500 laps, 2000 laps and 6000 laps confirm a dynamic filling of tribofilms that patch the voids and pre-notched damages. The stabilized coefficient of friction (CoF) remains at an ultralow value down to 0.02, independent of the pre-notched damage at the top of coating. The notched damages act even as lubricant reservoirs to accumulate the otherwise “wasted” debris, which are restored as lubricant by the sliding operation. HR-TEM unravels that WS2 (002) nano platelets in the healed notch straightly parallel
to the top coating surface but conformal to the coating/notch interface. The patchy tribofilm holds excellent promise for self-repairing of damages in the field of tribology.
* This chapter has been published/submitted in the following journals:
(a) H.T. Cao, J.Th.M. De Hosson, Y.T. Pei. Mater. Res. Lett. 7 (2019) 103–109. DOI:
10.1080/21663831.2018.1561538
6.1 I
NTRODUCTION
During their use in service materials accumulate defects which may lead to critical damage accumulation and catastrophic failure. If advanced materials could counter degradation through the initiation of an intrinsic repair mechanism responding to damages occurring during applications in real practice, the production costs will be reduced drastically, with substantial savings of materials resources and energy [1]. Self-healing materials may also stretch the necessary safety margins for the design of mechanically, thermally and chemically exposed components in various technical areas, which, in turn, will streamline component construction. As a result, self-healing materials are attracting increasingly growing attentions and were significantly developed during the last decade.
As of to date, however, self-healing phenomena are not strictly intrinsic and autonomous but thanks to a subsequent extrinsic influence triggered by external thermo-mechanical, electrical, or optical stimuli. Self-healing was introduced by White [2] in an autonomic healing of polymer composites, where embedded pre-filled microcapsules rupture during mechanical loading, subsequently releasing a liquid healing agent into the crack plane through a capillary action. The contact between healing agent and catalyst triggers polymerization which bridges the crack faces to be closed. Further approaches achieving healing functionality were explored into polymer cross-linking [3], shape memory alloy wires [4], microbial healing [5,6], and high-temperature ceramic oxidations [7,8]. However, in the field of tribology, rather scant information has been reported on the self-healing potential in tribocoatings. It is known that tribo-induced wear is damaging and gradual removal of materials leading to wear debris.
For tribological applications, initial cracks may run along columnar boundaries, branch and trigger catastrophic flaking or spalling of PVD coatings. This may result in sudden failures. Considerable efforts, e.g. applying an interlayer between coating and substrate [9], tuning the PVD deposition distance [10], controlling the microstructure [11–13] and doping with carbon or other metallic elements [14–16] were devoted to enhance the mechanical integrity of the coatings and substrate system under various conditions. In particular, nanocomposite coatings were designed to retain a high toughness to resist the nucleation and propagation of cracks under severe stresses or a high energy absorbance to suppress crack propagation [13,17–19], whereas it turned out rather difficult to produce defect free coatings.
Transition metal dichalcogenides (TMDs) such as WS2 and MoS2 are recognized for
their outstanding solid lubricating behavior [20–22]. Their endowed lubricating property is attributed to the crystallographic anisotropic structure and atomic bonding
On the self-healing performance of WS2/a-C Tribocoatings
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characteristics. TMDs are in fact hexagonal lamellar compounds with their layers of tungsten or molybdenum atoms are sandwiched in between layers of packed sulfur atoms. Each unit is characterized by strong covalent bonds, whereas the sandwiched layers are coupled through weak Van der Waals interactions [23–26]. As such, the ultralow shear strength (τ = 1-2 MPa) in (002) basal orientation renders each WS2 unit
to easy glide along the sliding interfaces leading to an ultralow friction [20]. Therefore, WS2 or MoS2 is introduced into either tribocoatings prepared by vacuum deposition
methods such as magnetron sputtering [27,28] and laser ablation [29], or into oil lubrication used as solid additives [30] and external dry lubricating agent [31,32]. For example, numerous studies aimed to enhance the tribological properties of less lubricating substrates via laser texturing reservoirs which were further burnished with WS2 or MoS2 [33–35]. The fundamental idea of the texturing techniques is to
precisely fabricate micron-scaled tiny dimples that serve as hydrodynamic bearings, reservoirs of lubricant or even as a sink to capture the wear debris. Oksanen et al. [31] claimed that it is inefficient to texture diamond-like carbon (DLC) coated surface, whereas only the indirectly processed samples were reliably associated with good tribological properties; therefore, they instead explored WS2 additions on the laser
surface textured hard ta-C films.
It should be pointed out that, on the one hand, pure WS2 is rather soft, yet porous
enhancing oxidation as a result of passivation of active sites of its edge planes in moisture and thereby diminishing the lubricative properties [36]. On the other hand, HR-TEM in-situ observations of the wear track have revealed subsurface reorientations of WS2 platelets, where their basal planes are realigned, parallel to the
coating surface into the “frictionless” (002) direction [37]. The latter confirms that soft WS2 can self-adapt itself to reorientate during a sliding contact [20,38]. A tribofilm
formed during wear can normally act as a self-lubricating buffer layer to isolate the metal-to-metal direct contact. This further encourages us consider taking advantage of the reorientated WS2 nanoplatelets as ‘patches’ to simultaneously heal microcracks
occurring in the coatings. Our approach sets aside from the standard idea of depositing an “ideal” coating by focusing instead on the exploration of a healing and self-adaptive tribo-system. The hypothesis is that during sliding contact with the counterpart, the wear debris is compacted to a closed continuous tribofilm which in this way ‘self-repairs’ the damage near the sliding surface.
6.2 E
XPERIMENTAL PROCEDURES
Nanocomposite WS2/a-C coatings were deposited on single crystal silicon (100)
wafers via a TEER UDP400/4 closed-field unbalanced magnetron sputtering system (CFUMS). The substrates were ultrasonically cleansed in acetone before Ar plasma
etching for 20 min at a negative bias voltage of 400 V (pulsed direct current, p-DC mode at 250 kHz, 87.5% duty cycle). The nanocomposite coatings were co-sputtered from two WS2 targets (99.9% purity) at an average current of 0.5A (p-DC 150 kHz, 70% duty
cycle) and one graphite target (0.5A, direct current mode). The substrates were placed vertically on a carousel holder that rotated at 3 rpm in front of the targets. No additional substrate heating was applied during the deposition. First a thin Cr interlayer (300 nm thick) was deposited to facilitate good interfacial adhesion between the coating and substrate. The WS2/a-C coatings 1.6-2 µm thick were
prepared at an Ar pressure of around 0.6 Pa and the deposition time was 2h at a target-substrate distance of 220-290 mm.
A CSM scratch tester was used to deliberately induce two types of notches, namely around 2 and 45 µm wide, respectively, onto the coating. The first narrow one (not shown) is for cross-section and high-resolution TEM observations, and the wide one penetrates through the coating until it reaches the Si substrate, which triggers a total coating failure to simulate the damages. To observe the healing efficiency, a tribotest was interrupted at 100, 500, 2000 and 6000 laps, respectively, for microscopic characterization, using a CSM ball-on-disk tribometer against a 100Cr6 ball of 6 mm diameter at a fixed sliding speed of 10 cm s-1. The test was performed under a 5N
normal load generating a Hertzian contact pressure of approximately 0.75 GPa. The wear track diameter was set to 15 mm. The relative humidity (RH) was controlled at
5-7% by a home-made humidity adjustor. After each interrupted test, the wear scar of the ball counterpart and the wear track of the coating sample were examined by optical microscopy (OM, Olympus VANOX-T) and scanning electron microscopy (SEM, Lyra Tescan) equipped with energy dispersive X-ray spectroscopy (EDS, EDAX, performed under an accelerating voltage of 20 kV). A confocal microscope (µsurf, NanoFocus) was used to plot the morphology of the notch before and after sliding. Besides, an atomic force microscope (AFM, Digital Instruments NanoScope 3100) was used to compare the surface morphology of raw WS2/a-C coatings, tribofilm and healed part at
nanoscale. In addition, an MTS Nano indentation XP® equipped with a Berkovich indenter was employed to measure the hardness and modulus of the coatings, formed tribofilm and healed part, respectively, with a depth-controlled mode of 200 nm. The grazing incidence X-ray diffraction (GI-XRD) spectra were conducted at a fixed incident angle of 2° with a PANalytical-X'Pert MRD to character the phases in the coatings. Furthermore, the structure and microstructure were scrutinized using Raman spectroscopy (632.8 nm wavelength, Thorlabs HeNe laser, at around 1-2 mW power) and high-resolution transmission electron microscope (HR-TEM, 2010F-JEOL, operated at 200 kV). Focused ion beam (FIB, Lyra Tescan) was employed to slice the healed part of the narrow notch and also to prepare TEM lamella at the center of the
On the self-healing performance of WS2/a-C Tribocoatings
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wear tracks. Before FIB milling, a protective Pt layer was deposited to protect the specimen from severe Ga ion irradiation damage.
6.3 R
ESULTS
6.3.1 Microstructure characterization
The SEM image in Figure 6.1a displays that the pristine WS2/a-C coating exhibits a
cauliflower-like surface morphology, which is characteristic of magnetron sputtered coatings. The fractured cross section in the inset reveals that the coating has a columnar structure. The column boundaries may be preferential cracking path and potentially induce coating failure during applications. HR-TEM micrograph in Figure 6.1b reveals that short WS2 platelets (< 5 nm in length) are randomly distributed in
an amorphous carbon (a-C) matrix. It is also confirmed by the GI-XRD spectrum shown in Figure 6.1c where a broad shoulder peak around WS2 (100) is observed. The XRD
Figure 6.1 Characterizations of magnetron sputtered WS2/a-C coating: (a) top-view SEM
observation with the inset showing the columnar structure on fractured cross-section where an arrow indicating potential crack along the columnar boundary; (b) HR-TEM micrograph revealing randomly orientated short WS2 platelets embedded in an amorphous
carbon matrix; (c) GI-XRD spectrum and (d) indicative EDS composition.
10 20 30 40 50 60 70 80 Int en sity (a .u. ) 2 (°) c 002 100 110 0 2 4 6 8 10 C O W S W Energy (keV) Int en sity (a .u. ) d Element Wt. % At. % C 2.6 16.2 O 0.5 2.2 S 22.5 51.8 W 74.4 29.8
spectrum also indicates WS2 (002) basal plane located at 2θ = ~ 12°, more or less
compatible with the JCPDS card (No. 008-0237), although the peak with a slight shift towards a lower diffraction angle (e.g. from ~ 14° of the standard diffraction down to 12°). This is reasonable because carbon incorporation from WS2/a-C nanocomposite
increases the lattice parameters of WS2 (002) planes [10,38]. The EDS spectrum in
Figure 6.1d quantifies the chemical composition of the coating roughly as 16C-52S-30W-2O (at.%). The S/W ratio of approximately 1.7 indicates certain sulfur deficiencies as substantially reported in the sputtered WS2-based coatings due to the
preferential resputtering of sulfur [10]. More details about the microstructure of the coating can refer to our previous work [10,36,37].
6.3.2 Self-healing of damages
First we use the scratch machine to induce a smaller crack on the coating. Figures 6.2a and b compare SEM images of the pre-notch before and after healing. Figure 6.2a shows the notch with a variable width 2-5 µm, whereas Figure 6.2b clearly indicates that the notch is filled and flattened. In fact, a thin cap is formed bridging the notch. Figure 6.2c compares the transverse profiles of the notch and the healed notch, where the maximum depth of the pre-notch is 0.34 µm. After healing, the notch is filled up
Figure 6.2 SEM images of a representative pre-notch (a) and after being healed (b) by the “patching effect” of the tribofilm; (c) typical transverse profiles of the pre-notch before and after being healed; (d) SEM micrograph showing gradual crack self-healing behavior from the edge towards the center of the wear track and (e, f) FIB-cut cross-sections of the healed pre-notch with the locations marked in (d). Note that open arrows of the same colors indicate the identical locations.
On the self-healing performance of WS2/a-C Tribocoatings
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with a slight protrusion ~ 0.1 µm high. At higher magnification, SEM observations of the interface (Figure 6.2d), together with two FIB cuts (Figure 6.2e and f) that are made from the side edge towards the center of the wear track, unveil a gradual healing process. The cross-section near the edge of the wear track as displayed in Figure 6.2e shows a couple of cracks formed by scratching that penetrates throughout the entire coating until the Cr interlayer (red arrows). Yet the cracks are still only partially recovered as indicated by the crevasse (blue arrows). Moving 10 µm towards the center of the wear track, the second FIB cut (Figure 6.2f) confirms that the notch is entirely healed without any remaining crevasses.
Figure 6.3 (a, b) SEM observation and overlaid EDS elemental mapping of the pre-notch damaged coating; (c) close-view of the marked area in (a) showing coating fracture and complete delamination to simulate service damages.
Further, Figure 6.3a shows the notched coating by scratching to a large scale. It can be seen that the coating has a dimple-like delamination of a width as wide as ~ 45 µm. The
overlaid EDS elemental mapping is shown in Figure 6.3b, where in the central part of delaminated dimples strong signal of Si substrate (colored in white), together with the Cr interlayer (colored in cyan) exposed along the rim of delaminated dimples, is in contrast to the mixed appearance of S (colored in red), W (colored in blue) and C (colored in purple) of the pristine coating on the two sides of the notched damages. This suggests a localized complete failure of the coating. The close-up in Figure 6.3c further confirms failure of the coating and delamination, which are pre-introduced mimicking severe damage of the coating during the practical use. Such damages may include catastrophic events and break-down due to accumulated fatigue in the subsurface area.
Figure 6.4 SEM micrographs (top) and overlaid EDS elements mapping (bottom) showing the self-healing process of damages after sliding 100, 500, 2000 and 6000 laps, respectively. To check the self-healing capacity of the investigated coating, a set of interrupted tribotests were in-situ performed, and the wear track was checked by SEM and EDS after sliding in dry air for 100, 500, 2000 and 6000 laps, respectively. SEM morphology
On the self-healing performance of WS2/a-C Tribocoatings
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and overlaid EDS mapping of the wear track are presented in Figure 6.4. Further details of each individual element mapping are depicted in Figure 6.5. It can be seen from Figure 6.4a, although a large number of cracks still remain, 100 laps sliding already results in a large part of the delaminated spots being healed as evidenced by the disappearance of the Cr and Si signals from Figure 6.4a. Comparing Figure 6.4a with the overlaid EDS mapping of Figure 6.3b and detailed element mappings in Figures 6.5a1-a6 with Figures 6.5b1-b6, it is seen that a tribofilm is formed and transferred to
Figure 6.5 Detailed EDS mapping of individual elements during the healing process: (a) notched sample before sliding, and after sliding for (b) 100 laps, (c) 500 laps and (d) 2000 laps, respectively. The associated number 1-6 refers to the element of S, W, C, O, Cr and Si, respectively. Note there is a slight shift of the wear track after each interruption.
the voids. In particular, the sulfur and tungsten elements are traced, whereas there is also some oxygen traceable along the wear track. Figure 6.5b5 and Figure 6.5b6 also show the intensities of Cr and Si being weakened due to the shielding of tribofilm filled in the notch if compared to that in Figure 6.5a5 and Figure 6.5a6, respectively, where the element of Cr and Si noticeably cover the whole dimple-like damages.
After 500 laps sliding, the large residue voids are almost repaired, as the Si signal in the EDS of Figures 6.5c6 is almost disappeared. Meanwhile, the oxygen seems to spread further forward along the sliding direction, indicating the occurrence of oxidizing at the sliding interface. Sliding of 2000 laps results in full healing of the damage. Figure 6.4c shows a continuous and dense tribofilm covering the entire voids. It is confirmed by Figures 6.5d1-d6 where the EDS mappings compare the element transport, and in particular the distribution of tungsten and carbon are uniform and nearly indistinguishable across the original coating and the healed notch. This indicates that the notched damage has been successfully patched by the tribofilm that was transferred towards the damage sites triggering the subsequent local repairs. In addition, the intensity of Si completely disappeared, suggesting a gradual overlayer built-up onto the top of the damage, pointing to a full recovery.
Figure 6.6a presents a comparison of the transverse profiles of the notched damage before and after healing, where the initial notch was around 1.8 µm deep and 45 µm wide. After tribo-sliding induced healing, the notch is filled up and presents a flattened top surface morphology. In line with the cross-section profiles, linear EDS scans of the relevant elements across the notch are conducted accordingly. Figure 6.6b shows a rather steep decrease in the content of W and S where the notched damage locates, together with an obvious increase in the content of Cr and Si. In particular, the sharp peak of Si at the center indicates the removal of substrate material after the scratching. In contrast, after healing by 6000 laps sliding, the signals of Cr and Si basically disappear because they are submerged under the upper filled materials. Apart from some minor increase in O and only a partial recovery of S as shown in Figure 6.6c, the signals of tungsten and carbon hardly change over the outside of the pristine coating passing through the healed notch. As a consequence, EDS reconfirms the elements of W, S, C are indeed transported into the notch and trigger the repairing reactions. Note the O is introduced by oxidation during the sliding/healing process.
To unravel the self-healing process, SEM observations at different stages and locations of the wear track are scrutinized in Figure 6.7. Figure 6.7a shows that one dimple-like pit at the edge of wear track after sliding 500 laps has accumulated and therefore could store substantially wear debris. The very tiny particles as seen at high magnification in Figure 6.7c generally originate from the removal of the domed protuberances (Figure 6.1a) of the typical PVD coating during wear. At the same stage toward the inner side
On the self-healing performance of WS2/a-C Tribocoatings
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of wear track, Figures 6.7c and d show that the debris, under long-time sliding stimuli and load pressure, are continuously filled into the notch. Later the debris is connected
Figure 6.6 Optical confocal morphologic profiles (a) and linear EDS compositional profiles across the notched damage before (b) and after (c) healing by sliding 6000 laps, respectively.
Figure 6.7 Close-ups of the self-healing behaviors in the wear track: (a, b) a largely notched damage accumulates wear debris; (c, d) particulate debris are bonded and gradually form a continuous layer; (e, f) the notched damage is nearly fully restored by the filled tribofilm and (g, h) layer-by-layer built-up of continuous and thick tribofilms covering the wear track. and flattened until dense and continuous tribofilms are formed. As a consequence, Figure 6.7e shows that the damaged notch after sliding 6000 laps is restored by the filled tribofilm to various extents. The closer view of Figure 6.7f from the marked area in Figure 6.7e confirms a full repair, at least locally. It is noteworthy that continuous
On the self-healing performance of WS2/a-C Tribocoatings
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tribofilms (as long as tens of microns) can be formed, exhibiting the capability to cover the whole wide wear track and thus heal over wide damage areas. Also the tribofilm may grow thicker and thicker due to the gradual layer-by-layer built-up (see Figures 6.7g and h at the stage of 2000 laps). Note that since the low shear strength inevitably results in a rapid loss of materials from the surface, the active regeneration of tribofilm is necessary for the tribo-system to escape from a quick rise in friction.
AFM was further used to compare the surface morphology of raw WS2/a-C coatings,
tribofilm on the wear track and healed part at nanoscale. Figure 6.8a indicates that the raw WS2/a-C coating has a clear domed-like morphology with a high root mean square
roughness (Rq) of around 6.8 nm. In contrast, after the sliding process, the wear track is smoother in comparison to the raw coating, as confirmed with a lower Rq = 2.2 nm shown in Figure 6.8b. Similar sliding induced smoothing was also reported in TiC/a-C nanocomposite coating [39]. The cross-section profiles of the two line scans (L1 and L2) throughout in Figures 6.8a and b also confirm the variations in roughness where L1 in the raw coating is apparently fluctuating and the latter is rather flattened. This is so because during sliding, the peaks of the surface asperities on the rough coating are truncated, resulting in a rather flat and polished surface [39]. AFM image of Figure 6.8c, corresponding to the view in Figure 6.7f, indicates some pile-up at the interface of tribofilm/healed part, and L3 in Figure 6.8d confirms that on the healed part, there is a 35-nm thick protuberance above the normal tribofilm on the left of the notch. This indicates some compaction of the softened tribofilm occurs during the sliding process.
Figure 6.8 AFM of surface topography: (a) rough raw WS2/a-C coating; (b) smoother
tribofilm on the wear track; (c) pile-up at the interface of tribofilm/healed part and (d) cross-section profile of three lines as indicated in (a-c).
In terms of the tribological performance, the on-going CoF of four tests by sliding 100, 500, 2000 and 6000 laps are presented in Figure 6.9, with the corresponding insets showing the scar morphology of the counterpart ball. Note that after each interrupted test, the ball was checked with optical microscopy and then placed at the same position
for continued tests. At the onset of the first 100 laps of running-in period (Figure 6.9a), the coating starts with CoF of 0.1. The CoF peaks at about 0.14, and thereafter it fluctuates around the value of 0.075. The inset of the ball scar in Figure 6.9a reveals that transfer films were formed. However, they are islands-like and dispersedly distributed, indicating that some local areas of ball scar are contacting with the lubricating tribofilm while other local areas are still partially in contact with the original raw coating (similarly as a boundary lubrication state in the Stribeck curve with many residue micro-asperities of high surface points). This explains the fluctuations of CoF yet at a relatively high value in the running-in period. It should be noted that although the test was performed under dry air condition (5-7% RH), the red
or purple color areas in the insert of Figure 6.9a indicate that oxidations take place in the tribofilm, particularly at the first sliding period. Earlier HR-TEM results [37] reported a clear 5-nm thick WO3 interlayer between the raw bulk coating with the
formed tribofilm at the wear track interface.
Figure 6.9 Coefficient of friction plot together with an inserted OM image showing the corresponding wear scar of ball counterpart: (a) after 100 laps; (b) after 500 laps; (c) after 2000 laps and (d) after 6000 laps. Note that the open arrows pointing at the colorized ripples in the insets indicate the gradual formation of transfer film (multilayered). For the second round of tribotest, from 101 to 500 laps, a gradual decrease of CoF down to 0.05 is observed, with an average CoF value of 0.057 for the whole period. This is confirmed by the formation of almost fully covered transfer film on the wear scar of ball counterpart as indicated in the inset of Figure 6.9b. Similarly, the CoF continues to
On the self-healing performance of WS2/a-C Tribocoatings
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decrease as the sliding laps increase, with an mean value to be 0.035 and 0.027 in the period of 501-2000 laps and 2001- 6000 laps, respectively. It should be noticed that by re-starting each tribotest, there is always a new running-in period; in fact, the initial CoF of a later test cannot exactly continue the ultralow state of the previous one. However, the running-in period is shortened for later tests. For instance, both Figures 6.9c and d indicate that it takes around 500 laps for the CoF to stabilize at an ultralow value of 0.02, while the first run of 500 laps (see Figure 6.9b) sliding end up with a high CoF value of ~ 0.05. This is understandable considering that the pre-accumulated tribofilms covering the ball reduce the friction by increasingly disconnecting the direct contact between the steel and coating.
Figure 6.10 Details of the self-healing behavior on the wear scar of counterpart ball after sliding 6000 laps: (a) overview of the scar; close-ups of the marked areas in (a) in corresponding colors showing debris accumulated in front of the scar densifying the transfer film (b); flattened particles (c); built-up tribofilm layer by layer (d, e); squeezed into voids (f) and continuous tribofilms covering the scar (g).
Another intriguing finding from the apparent changes of the scar morphologies at four stages is that there are many fully colorized ripples around the scars shown in the insets of Figures 6.9b-d. The decreasing diameter of the circular loops from outside to inside indicates multi-layers are accumulated during the sliding process, which is in accordance with the formation of tribofilm on the wear track as shown in Figure 6.7h. This suggests that WS2 tribofilms are transferred and aligned continuously throughout
the whole wear process. Note the building-up of transfer film is a dynamic process, i.e., it can be formed, removed and regenerated simultaneously during sliding. SEM examination on the ball scar after sliding 6000 laps is shown in Figure 6.10, and several close-ups marked in Figure 6.10a are presented in Figures 6.10b-g, respectively. In line
with Figures 6.7a and b, Figure 6.10b shows that some segmented small debris are worn off from the bulk coating, and then transferred and accumulated in front of the ball scar. Note the shear force can slide the basal planes of WS2 over one another by
intra-crystalline slip and transfer to the rubbing counterface [40]. In contrast, at the right side of ball scar (end of contact, see Figure 6.10c), a dense transfer film, with substantial particles being compacted and flattened, can be observed when compared with the fresh steel ball. Comparison of Figure 6.10d and e shows the gradual building-up of multilayered transfer film on the ball scar. The tribofilm/transfer film between the coating and counterpart ball can be squeezed to spread out and interconnect (Figure 6.10e), leading to the self-healing of cracks/damages underneath the tribofilm. Figure 6.10f further reveals some residual voids (see yellow arrows) exist in the almost flattened transfer film. The white arrows clearly indicate that the transfer films experience some plastic deformation to heal the voids, ultimately resulting in a completely flat and smooth transfer film as seen in Figure 6.10g. This is comparable with the tribofilm healing the notched damages in the wear track as discussed in Figure 6.7c-f. Note the squeezed tribofilm and transfer film keep the surfaces of the steel ball and the bulk coating apart, leading to a self-lubrication corresponding to an ultralow friction. It should be noted that the completely flattened transfer film in the center part of the scar can be very thin as confirmed in EDS mapping shown in Figure 6.11, where minor S and W elements are traceable in comparison with that at the side edges. This may resort to Auger electron spectroscopy (AES) mapping due to its surface sensitivity for the elemental mapping of such thin layers. Previous TEM observations have already confirmed tens of nanometer thick well-aligned TMD tribofilms are transferred onto the wear scar of steel counterpart balls [37,40]. The aligned part of the tribofilm hardly grows in thickness, as once a thin layer has formed, the shear forces turn extremely
Figure 6.11 Elemental distribution of indicated elements on the wear scar of ball counterpart.
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small, thereby eliminating the driving force for its further growth. Consequently wear is confined within this thin buffer layer, which could not transfer shear force high enough to induce damages to the stronger underneath substrates (coating/steel ball) and thereby effectively protecting it from further wear [41].
6.4
H
EALING MECHANISM
To reveal the healing quality and mechanisms, a cross-section TEM specimen was sliced at a small healed part located at the center part of the wear track by FIB, more details refer to our earlier work [42]. Figure 6.12a clearly shows a compacted and healed area underneath the Pt protecting layer as indicated. The arc curved line in Figure 6.12a distinguishes the pristine coating (bottom) and the filled tribofilm (upper). This provides a solid evidence for the self-healing mechanism in the investigated tribocoating. Besides, no cracks neither voids remain in the entire healed area, pointing to an effective self-healing process. HR-TEM images of Figures 6.12b-e display some closer views at the locally healed areas as marked in Figure 6.12a. For instance, in the top of the healed part showing in Figure 6.12b, arresting WS2 lamellae
are well aligned with (002) basal parallel to the coating surface (i.e. the same to the ball sliding direction). Figure 6.12c shows that the WS2 platelets are highly densified,
as compared with that in the original coating as shown in Figure 6.1b. However they are rather randomized at the center of the healed the notch damage. At the bottom of the notch, a few WS2 platelets are again observed parallel to the bottom of the notch
surface (Figure 6.12d).
The key-question is what is the real driving force for the reorientation of WS2 inside
the healed notch. It should be emphasized that the so-called TMD (002) basal plane subsurface reorientation was generally reported to be exclusively parallel with the coating surface [28,37,38], namely, the “straight” TMD rearrangement is expected to be parallel to the sliding direction of counterpart ball as shown in in Figure 6.12b. That is also exactly the lubricating mechanism explaining the outstanding triboperformance of TMD-based coatings. Notwithstanding, an intriguing finding in this study is that WS2
platelets are also able to conformally spread over the curved interface. In other words, WS2 platelets can self-adapt themselves under shear when the filled tribofilm
confronts with the hard bulk coating as observed in Figure 6.12e and f. For example, one can easily observe that two directional WS2 platelets (see the arrows) converge at
the upper right corner shown in Figure 6.12e. In particular, the angle (φ) as indicated between the localized reorientation and ball sliding direction is around 40° instead of zero. Note that the angle φ changes along the notch surface as its curvature changes. This implies that short randomly oriented WS2 platelets in close proximity can be
of the shape provided with the stimuli from sliding contact. It is also notable that the thickness of perfectly well-aligned WS2 platelets is more than 20 nm (Figure 6.12b and
f), much thicker than the thickness (~ 5 nm) reported [27,38,43]. To conclude, TMD basal plane reorientation seems not to occur necessarily parallel along with the top coating surface during sliding contact, but also get influenced with local sliding along
Figure 6.12 TEM confirmation of the healed notch: (a) overview; (b) HR-TEM image revealing WS2 (002) platelets in the upper part of the healed notch parallel to the coating
surface; (c) densified yet randomized WS2 platelets at the center part of the healed notch;
(d, e) WS2 platelets flexibly parallel to the notch/coating interface and (f) close HR-TEM
On the self-healing performance of WS2/a-C Tribocoatings
6
the interface of healed notch during tribofilm filling, which leads to a strong densification [28]. Accordingly, suchcompliance of the tribofilm adds flexibility to heal irregular damage.
To further reveal the local chemical phases and mechanical properties, nanoindentation and Raman spectroscopy analysis were conducted on different zones in the wear track, and their results are shown in Figure 6.13a-c. Figure 6.13a illustrates the optical image of a typical healed notch by the tribofilm. The nanohardness and indentation impressions of the tribofilm on wear track, the healed notch damage and the raw WS2/a-C coating (marked by triangles in respective colors in Figure 6.13a),
together with the pure WS2 coating prepared under the same sputtering condition are
presented in Figure 6.13b. It can be seen that the raw WS2/a-C coating has the highest
hardness of 6.7±1.5 GPa, substantially higher than that of the pure sputtered WS2
Figure 6.13 (a) OM of a typical healed notch by tribofilm; (b) hardness and nanoindentation impression of the tribofilm on the wear track and healed notch as indicated in (a) as compared to the raw WS2/a-C coating and pure sputtered WS2 coating prepared under the
same sputtering conditions (all scale bars: 500 nm), respectively; (c) Raman spectra of circled areas in (a) with the inset showing typical deconvoluted D and G peaks of diamond-like carbon matrix from the healed part (area 4); (d) the distance of ten WS2 platelets in the
healed notch with A and B marked in Figure 6.12b and f, while C are from the WS2 target
coating of merely 0.25±0.05 GPa. Interestingly, the hardness of the tribofilms both on the wear track and the healed notch part decreases, among which the first is about 4.3±1.0 GPa, almost twice as high as that of the latter (2.2±0.3 GPa). The hardness differences also shed light on the reason why WS2 platelets tend to flexibly reorientate
along the curved interface of the notch and bulk coating (see Figures 6.12d-f): the healed part mainly consists of soft WS2 whereas the bulk coating is much harder; as a
result, the frictional force F drives the reorientation of WS2 platelets first parallel with
the sliding direction (arrow 1 in Figure 6.12e). However, when WS2 platelets interact
with harder obstacles, i.e., the inclined (θ) frontier of the hard raw coatings, they start to deviate from their original direction and instead orientate themselves along the curved interface (arrow 2 in Figure 6.12e).
F = µL (1)
τ = F/A = F/(πD2/4) = 4 µL/ πD2 (2)
where L is the normal load (5 N), and µ is the measured coefficient of friction (CoF = 0.027 in Figure 6.9d), D is the diameter (~ 120 µm, see the inset in Figure 6.9d) of the wear scar of the ball counterpart at steady-state CoF, and τ is the nominal shear stress. Accordingly τ is about 12 MPa over the healing notch. This is much larger than the interfacial shear strength (1-2 MPa) required to drive the WS2 cleavage to glide within
the hexagonal layers [20]. Even along the inclined frontier of the hard bulk coating, the shear stress component can be resolved as τ × cosφ (see Figure 6.12e), which is still high enough to reorientate the WS2 platelets, i.e. the shear stress becomes ~ 9 MPa at
φ = 40°. It should be pointed out that far away from the direct sliding contact (compare Figure 6.12b and c), WS2 platelets lose the alignment degree of basal planes. This is
confirmed by the randomized distribution of TMD in Figure 6.12c in the middle part of the healed notch and previous results in TMD tribofilms at tens of nanometer away from the sliding interface as reported in [16,27,28,44]. Moser et al. [28] found 'turbulent-like' or 'convective-like' patterns formed by the basal planes in MoS2
lubricating films, suggesting the occurrence of relatively easy sliding displacements between local crystallites in the buffer tribofilm, in accordance with that in the central part of the healed notch (see Figure 6.12c).
The hardness decrease in the tribofilm is plausibly caused by the release of carbon from the original WS2/a-C nanocomposite during the sliding process. It weakens the
hardness as the less C content, the closer the chemical coating composition approaches pure WS2, which is well-known for its extremely low hardness and porous
characteristics [36,45]. In fact, there are some earlier conflicting results concerning the underlying lubricating mechanism for the tribological properties of WS2/a-C coating.
For instance, Voevodin et al.[29,46] attribute the coating self-adaptive tribological behavior in dry and humid atmospheres to the collective contributions of DLC and WS2.
On the self-healing performance of WS2/a-C Tribocoatings
6
However some recent results [47–50] proposed that only TMDs provide lubrication as pure TMD platelets were exclusively observed adjacent to the sliding interface. EDS mapping on the wear scars shows indeed some carbon release from the coating and transfer in the debris (see Figure 6.11c). The carbon content of the debris near the edges of the wear track is relatively high (see Figures 6.5b3 and e3). The EDS mappings on the wear track in Figures 6.5 (a3-d3) and the horizontal C line of EDS scan in Figure 6.6c, however, all point at negligible change in the content of carbon from outside bulk coating across the healed notch.
Raman analysis results shown in Figure 6.13c confirm that apart from clear WS2 peaks
at E12g (355 cm−1) and A1g (421 cm−1), the areas 1-4 circled in the wear track of Figure
6.13a all present the typical amorphous carbon peaks, indicating the existence of carbon in the tribofilm. For instance, the Raman shift from 100 to 1800 cm-1 in area 4
(tribofilm on the healed notch) can be deconvoluted into D peak at 1376 cm-1 and G
peak at 1560 cm-1. It is interesting to note that both the peaks of WS2 and a-C are not
very strong in the raw bulk coating (area 5), indicating that sliding may help the two phases becoming Raman-active. The Raman spectra of areas 3-4 confirm similar chemical phases of the tribofilms either on the healed notch (area 4) or on the wear track (area 3). Besides, one can see a gradual increase in the Raman excitation intensity of DLC from the center of the wear track to the side edge, which is in accordance with the EDS results and previous reports [51]. In addition, we compared the spacing of WS2(002) crystalline planes as shown in Figure 6.13d, it can be seen that the distance
of each WS2 layer in the WS2 target is about 0.63 nm, while those in the top (Figure
6.12b) and side zone (Figure 6.12f) of the healed notch are 0.65-0.66 nm, indicating the distance between the WS2 layers is expanded. Other intercalants can be inserted in
between the layers due to the weak interlayer coupling. For instance, Teer [52] reported the positioning of titanium atoms between the neighboring sulfur planes in the MoST structure. This can also be supported by the shift of (002) peak to the left of a lower diffraction angle in the GI-XRD of WS2/a-C coating as compared with that of the
sputtered pure WS2 coating [37]. Based on the Raman analysis, EDS and HR-TEM
results, it is concluded that carbon atoms are partially released from the WS2/a-C
nanocomposite under sliding shear while some carbon atoms are likely filled in between different WS2 layers. This overall demonstrates a good combination of
ultralow friction and wear resistance.
It should be pointed out that Figures 6.5(a-e) all show certain content of oxygen at tribofilm either on the healed notch or on the wear track. Comparisons of the selected area electron diffractions (SAED, not shown here) of the tribofilm-healed notch with the bulk WS2/a-C coating confirms the additional WO3 (002) plane, unravelling the
oxidation of WS2 into WO3. It is known that the passivation of active sites of
properties. HR-TEM micrographs in Figure 6.14 reveal that WO3 nano-sized particles
may partially block the alignment of WS2 platelets. Nevertheless, the formation of WO3
nano-sized particles seems hardly to influence the healing efficiency as confirmed by Figures 6.4-7 at micron-scale.
Figure 6.14 HR-TEM showing the negative blocking effects of WO3 on the continuous
reordering/reorientation of WS2 platelets.
Due to the healing process, notched damages were not expected to deteriorate tribo-performance as usual, and no catastrophic coating peel-off problem occurred. The formation of regeneration of new-aligned WS2 is induced for self-lubricating & healing
once the local friction escalates. Figure 6.15 shows the schematic illustration of the self-healing process where tribofilms heal damages. Note that the damage self-healing behavior is a more or less spontaneous process, where the damage itself initiates the healing process without any other external interventions except the stimuli of sliding. More
On the self-healing performance of WS2/a-C Tribocoatings
6
importantly, the healing agent (e.g. tribofilm) originates directly from the bulk coating. Such intrinsic mechanism is potentially superior to the extrinsic approaches (which are restricted due to the exhaust of the extrinsic healing agents or their degrading reactions with the matrix) as reported in previous studies, e.g. depositing lubricating materials into the textured reservoirs of substrates so as to enhance their tribological performance [32–34,53] or alternatively making use of the conformal characteristics of atomic layer deposition to deposit extrinsic upper DLC layers to heal pinholes of some porous coatings[54].
Figure 6.15 Schematic illustration (not in scale) of the in-situ self-healing mechanism (“tribofilm reverses damage to work”): (a) defective raw coating with aligned voids and cracks along the columnar boundaries and potential damages in service; (b) contact sliding ploughs the protuberance of the coating into debris that penetrates into voids/grooves and fills into damage; (c) under continuous sliding stimuli the tribofilms start to form and become flattened, completely patching the damages that reversely act as microreservoirs to replenish super-lubricant; (d) inside the tribofilm the WS2 platelets are reordered,
connected and reoriented with the (002) basal planes parallel with the top coating/notch surface. The mate of transfer film and tribofilm yields an interfacial “basal on basal” frictionless sliding.
Figure 6.15a displays the defective raw coating with cracks in columnar boundaries and potential damages in services. Figure 6.15b shows that contact sliding ploughs the asperities of the coating into debris that later penetrate into the intrinsic coating voids and or fill into the service damages. Figure 6.15c shows under continuous sliding stimuli that the tribofilms form and become flattened and continuous. Meanwhile, the
WS2 flakes are transferred to cover the ball counterpart, resulting in predominantly
self-mated “basal on basal” interfacial sliding as illustrated in Figure 6.15d. These jointly yield a simultaneous self-healing by recycling of the wear debris (otherwise wasted) between the mating surfaces and self-lubricating behavior corresponding to a low friction and wear between the coating and counterpart, as supported by the horizontally aligned WS2 with frictionless orientation shown in Figure 6.12b. In fact,
Figure 6.15d reveals that the notched damages are acting as microreservoirs for storing re-aligned WS2 lubricant ready for replenishing frictionless responses. Wang et
al. [32] controlled the texture of a CrN interlayer to support WS2 film and found that
nanocone array textured WS2 film exhibited an enhanced lifetime in wear because the
regular CrN nanocone arrays serve as favorable reservoirs of WS2 lubricants.
Therefore, the presence of cracks - common adverse products in tribology - could be restored with beneficial lubricating effects by in-situ filling WS2 tribofilms. As a
consequence, we may lift the constraints of preparing ideal coating (e.g. defect-free) and rather focus on the self-adaptive and self-repairing system upon the idea to allow the flexible reorientation of hexagonal basal planes towards the frictionless direction, whereby the “WS2 tribofilm patches” heal damages and prolong the lifetime of coated
inferior parts.
6.5 C
ONCLUSIONS
This study explored the potential self-healing capability and mechanisms of WS2/a-C
tribocoatings. In-situ formation of flexible WS2 tribofilms provide self-healing
functionality ‘patching’ voids and repair cracks/damages in the bulk coating while maintaining or even enhancing the tribological properties simultaneously. Through intrinsically repairing damage, coatings could promote tribo-efficiency over time and prevent costs incurred by coating failure. The self-healing capacity of tribofilms also lifts the constraints of producing ideal coatings for practical tribo-applications.
R
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