Microstructure and lubrication mechanism of multilayered MoS2/Sb2O3

thin films

J.J. Hu*, J.E. Bultman and J.S. Zabinski

Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/MLBT), Bldg. 654, 2941 Hobson Way, Wright–Patterson Air

Force Base, Dayton, Ohio 45433-7750, USA

Received 9 December 2005; accepted 23 January 2006; published online 1 March 2006

Multilayered MoS2/Sb2O3 thin films were prepared by pulsed laser deposition on steel substrates. A rotary multi-target holder

was used to switch the laser targets for alternative growth of MoS2 and Sb2O3 layers providing nanometers thickness. The

tribological properties of the films were measured in dry and wet environments and the wear scars were observed using a scanning

electron microscope. The multilayer films showed a much longer wear life than pure MoS2 films in wet air tribotests. Focused ion

beam and transmission electron microscopies were used to investigate the cross-sectional microstructures of wear scars. Lubricious

MoS2/Sb2O3 tribofilms were built up on wear scar surfaces, and produced low friction. Micro-cracks occurred along the interface

between the tribofilm and the neighboring/topmost Sb2O3 underlayer, where the Sb2O3 layer effectively inhibited the crack

propagation perpendicular to the interface. The orientation of MoS2 crystals in as-deposited films was mostly random and friction-

induced stress oriented the MoS2 basal planes parallel to the surface. The reorientation was confined to the topmost MoS2 layer and

was not observed below the first intact Sb2O3 layer.

KEY WORDS: pulsed laser deposition, multilayer thin film, lubrication, molybdenum disulfide, antimony trioxide, transmission

electron microscopy, focused ion beam

1. Introduction

MoS2 is a well-known solid lubricant and is refer-enced as long ago as the early 17th century. Lubrica-tion with MoS2-based materials is excellent in vacuum/dry environments, but degrades in wet environments,where an increase in friction coefficient and a decreasein wear life occur [1]. By incorporating additives intoMoS2, for example, inorganic sulfides/oxides [2–12] andmetals [13–16], increased load-carrying capacity,endurance, and environmental adaptability can beachieved. Sb2O3 is an effective additive, while not alubricant itself, it can act synergistically with MoS2 toimprove friction and wear properties [6,7,11]. MoS2and Sb2O3 have been combined in composite com-pacts/coatings made by mixing the component materi-als [6], and by co-sputtering [11].

Multilayer architectures have been shown to improvethe tribological and mechanical properties, for example,MoS2 multilayers have been created using metals[17–19], PbO [20] and WS2 [21]. The films were grown bymulti-target r.f. magnetron sputtering. Laser ablationoffers an alternative to the sputter growth of MoS2[22,23]. High-purity films can be produced by pulsedlaser deposition (PLD) in an ultrahigh vacuum envi-

ronment. The contamination effect of residual contam-inants (e.g., H2O+O2) and argon (typically used inmagnetron sputtering) can therefore be minimized.

In our experiments, we used a multi-target holderPLD system to grow multilayered MoS2/Sb2O3 thinfilms for tribological investigations. In contrast to theMoS2–Sb2O3 composites in bulk, the films consisted ofperiodic MoS2 and Sb2O3 layers produced by cyclingtargets in the laser beam. The friction coefficients of thefilms were measured in both dry and wet conditions. Thesamples were characterized using X-ray diffraction(XRD), scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). Particularly,the cross-sectional microstructure of wear scars wasstudied using a focused ion beam (FIB) microscope.Lubrication mechanism of the films was discussed fromthe observed results.

2. Experimental

MoS2/Sb2O3 multilayer films were grown on 440Cstainless steel substrates using pulses from a LambdaPhysik COMPex 205 KrF excimer laser, which provideda pulsed beam of UV radiation of 248 nm wavelength,20 ns duration, 1–50 Hz rate, and 200–600 mJ energy.The laser targets made of pure MoS2 and Sb2O3,respectively, were mounted on a multi-target holder for

*To whom correspondence should be addressed.

E-mail: Jianjun.Hu@WPAFB.AF.MIL

1023-8883/06/0200–0169/0 � 2006 Springer Science+Business Media, Inc.

Tribology Letters, Vol. 21, No. 2, February 2006 (� 2006) 169

DOI: 10.1007/s11249-006-9035-6

PLD in a UHV compatible chamber. Layers of MoS2and Sb2O3 were alternatively grown by direct laserablation of each target that was switched back-and-forthvia a rotary motion feedthrough. For comparisonstudies, pure MoS2 films were also prepared at the samedeposition conditions. The films were grown at a sub-strate temperature of 100 �C, and the growth was ter-minated at a film thickness of about 1 lm.

Crystallographic data of the films were collectedusing a Rigaku thin film XRD system with a mono-chromator in front of the Cu Ka X-ray source, whichwas operated in Q-2Q mode. Standard powder XRDpatterns of hexagonal MoS2 and body-centered cubic Fewere used for data interpretation. Friction coefficientswere collected using a ball-on-disc tribometer run withthe specimens in horizontal position. The measurementswere made at room temperature in dry nitrogen and inwet air environment at 40% relative humidity (RH).6.35 mm diameter 440C steel balls were used as thecounterface. A sliding speed of about 0.2 m/s and nor-mal load of 100 g were used. According to Hertziancontact mechanics, the maximum value of normal con-tact stress was calculated as about 600 MPa, and thehighest shear stress in the static condition occurred atthe depth of about 13 lm. Therefore, the stress distri-bution extends across the entire film and into the sub-

strate. A Leica 360 field-emission-gun (FEG) SEM,operated at 25 keV, was used for observing wear scarson the films after they ran for thousands of cycles.

To study microstructural changes from the frictionsurface to the interface between film and substrate,cross-sectional TEM specimens were prepared by lift-out in a FIB microscope, FEI-DB235. It was operated at5 keV of electron beams and 30 keV of Ga+ ion beams.To protect friction surfaces, a 2 lm-thick Pt cap wasdeposited on the top of wear scars by using a gasinjection system, which first gently deposited with elec-tron beams and then with ion beams. Microstructureswere observed at high-resolution using a Philips CM200-FEG TEM operated at 200 keV. The probe size ofincident electron beams was adjustable from 25 nmdown to 1 nm for chemical microanalyses using a NO-RAN X-ray energy dispersive spectrometer (EDS)installed on the TEM. Cross-sectional TEM resultsshowed the microstructure of wear scars in depth andwere used to elucidate lubrication mechanisms.

3. Results and discussion

Figure 1(a) presents the XRD spectrum of MoS2/Sb2O3 multilayer films grown on a steel substrate, whichwas indexed according to the 2H-MoS2 phase and iron.

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Figure 1. (a) XRD spectrum of the MoS2/Sb2O3 multilayer film grown on steel substrates, which was indexed to 2H-MoS2. (b) Selected area

electron diffraction pattern from the multilayer film, which was indexed to 2H-MoS2. (c) Cross-sectional TEM image from the multilayer film,

where bright and dark stripes represented alternative MoS2 and Sb2O3 layers, respectively. The insert color image shows the EDS elemental

mapping, where Mo is blue and Sb is red.

170 J.J. Hu et al./Multilayered MoS2/Sb2O3 thin films

Broad peaks with less intensity resulted from tiny/smallcrystallites in the films. The polycrystallization was alsoproven by electron diffraction of the films, as shown infigure 1(b), where the ring-like patterns resulted fromdiffractions on randomly oriented (002), (004), (100) and(110) planes. The overall microstructures of the as-deposited films were observed by cross-sectional TEM,as shown in figure 1(c), where the bright and darkstripes respectively represented MoS2 and Sb2O3 layers.The insert color image shows the EDS elemental map-ping for the multilayer film, where Mo and Sb arerespectively shown in blue and red. The films wereconsistently grown by alternative PLD of MoS2/Sb2O3

layers at a periodicity of 65 nm. The total thickness ofthe film is measured as 1.1 lm.

Ball-on-disc tribotest results are shown in figure 2along with SEM images taken from the wear scars.Figure 2(a) shows the friction trace recorded frommultilayer films under a normal load of 100 g in air of40% RH. Lubrication by the films approached to near100,000 cycles at an average friction coefficient of 0.15–0.20. Friction coefficients measured in dry nitrogen werearound 0.01–0.02, which is about the same as pure MoS2(not shown). Friction and wear data were determinedfrom a minimum of three repetitions at each conditionfrom three films; and all tests showed a significantimprovement to the wear life. By precisely controlling

the layer structures, the wear life had been increased tobeyond 150,000 cycles in our tests. To distinguish mul-tilayer mechanisms, the same tests were performed onpure MoS2 films grown on steel substrates. They startedto fail after running for 25,000 cycles, as shown infigure 2(c), which is only one fourth of the endurance ofmultilayer films. In the corresponding SEM observa-tions of wear scars, the multilayer film shows manyscratch-like tracks with some slight cracks along thesliding direction (figure 2(b)). The pure MoS2 film showspartial delamination features with branched fracturesand detachments appearing beyond the delaminationregion (figure 2(d)). Therefore, the multilayer film musthave a higher resistance to wear-off than the pure MoS2film.

FIB-based lift-out preparation of TEM specimensprovides an effective method to directly visualize thecross-sectional microstructures of wear scars, which iscritical to measure the friction and wear properties ofthe films. Figure 3(a) shows the cross-sectional TEMimage taken from the wear scar on the multilayer film.The MoS2-like tribofilm formed on the wear scar surfaceshows a bright image contrast as the same as the MoS2layers. It was built up and thickened above the surface.The present results allow direct visualization of tribofilmformations that had been previously studied by using anin situ Raman tribometry [24]. A dashed green line was

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Figure 2. (a) Friction trace of the MoS2/Sb2O3 multilayer film recorded in 40% RH air. (b) SEM image taken from the wear scar surface on the

multilayer film. (c) Friction trace of the pure MoS2 film recorded in 40% RH air. (d) SEM image taken from the wear scar surface on the pure

film.

J.J. Hu et al./Multilayered MoS2/Sb2O3 thin films 171

drawn across the multilayer film surface indicating theas-deposited thickness. Here the tribofilms showed akind of movability and were unevenly re-distributedamong the surfaces with a wavy-like morphology. Sometiny voids were formed inside the tribofilms accompa-nying their built-up, as indicated by the white arrows infigure 3(a). More micro-cracks occurred along theinterface between the tribofilm and the neighboringSb2O3 underlayer, as indicated by the black arrows infigure 3(a). However, it is very important to notice thatthere is no void/crack occurring beneath the topmostremaining Sb2O3 layers. It implies that the Sb2O3 layercan provide a successful barrier to inhibit crack propa-gation perpendicular to the interface, which decreasesthe wear rate. Friction and wear properties were thenprovided by the tribofilm formed above the Sb2O3 bar-rier. The film wears slowly until an Sb2O3 layer isexposed, whereupon rapid wear occurs followed byreformation of an oriented MoS2/Sb2O3 mixed layer.

The color images inserted in figure 3(a) show theEDS elemental mapping of the wear scar cross-section,where Mo and Sb compositions are displayed in blueand red images, respectively. The Mo and Sb composi-tions are alternatively distributed inside the multilayerfilm, while both of them presented in the tribofilm layer.Characteristic peaks of individual Sb2O3 crystals fromthe wear scars were not observed by using a micro-Raman spectroscope. The Sb2O3 crystals could be toosmall to provide Raman peaks. Lubricant transfer filmswere also observed on the balls by micro-Raman. Thespectra from the balls closely matched those on thedisc. The debris contained both MoS2 and Sb2O3

components. It appears that some parts of upper Sb2O3

layers were broken under the friction-induced stress asindicated by a circle in figure 3(a). Therefore, the MoS2tribofilm consisted of intimately mixed fine particles ofSb2O3 additives and MoS2 with the topmost layersclearly being basal oriented MoS2.

In contrast to the multilayer results, figure 3(b) showsthe cross-sectional TEM image taken from the wear scaron the pure MoS2 film. A vertical crack propagatedthrough the whole MoS2 film thickness and approachedto the roughly horizontal cracks along the MoS2/sub-strate interface, as indicated by an arrow. Furtherdevelopment of those cracks would result in filmdelamination/wear-off, as observed in figure 2(d).Comparison of the multilayer (see figure 2(b)) and pureMoS2 SEM micrographs clearly reveals severe cracks onthe pure MoS2 surface. Sb2O3 interlayers in the multi-layer film inhibited crack propagation perpendicular tothe interface. Therefore, the multilayer film has a muchhigher endurance than the pure MoS2 film.

The crystalline structures inside the wear scar ofthe multilayer film were further observed using a high-resolution TEM, as shown in figure 4(a). The incidentelectron beam was in parallel to the rubbing surface.Above the top most layer of Sb2O3, the tribofilm wasformed with 2H-MoS2 (002) basal planes in a goodorientation parallel to the rubbing surface. Beneath theSb2O3, the basal planes within the as-deposited MoS2layer exhibited more random orientations, as shown infigure 4(a). To further verify the reorientation, the fastFourier transformation (FFT) was separately performedon the tribofilm area and the as-deposited MoS2 layer.

Figure 3. Cross-sectional TEM images show the microstructure details of wear scars on the multilayer film (a) and on the pure MoS2 film (b).

The color image inserted in (a) shows the EDS elemental mapping for the wear scar, where Mo and Sb are in blue and red, respectively. Also in

(a), a dashed green line on the top of the film shows the as-deposited thickness. Some micro-cracks are indicated by arrows, and broken layers of

Sb2O3 are indicated by a circle.

172 J.J. Hu et al./Multilayered MoS2/Sb2O3 thin films

Two dominant (002) diffraction spots aligned in per-pendicular to the rubbing surface were revealed in thedigital Fourier spectrum of the tribofilm (figure 4(b)),while a ring-like (002) pattern resulted from the as-deposited MoS2 layer (figure 4(c)). Reorientation ofbasal planes was also measured by selected area electrondiffractions similar to figure 4(b). The crystal realign-ment occurred at a depth of about 0.1–0.5 lm suggest-ing that the friction energy, film transport and plasticflow at the surface drove the observed phenomena. Itwas not caused by the compressive/tensile stress distri-bution at the contact point alone. Both Raman andTEM analyses demonstrated that it was MoS2 on thewear scar surface. In addition, the friction coefficientsmatched with pure MoS2. Therefore, the MoS2 com-ponent in the multilayer films was the active lubricant.

Based on the above observations, the lubricationmechanism of multilayer films are summarized as fol-lows: (a) MoS2 layers were alternatively deposited withSb2O3 interlayers, which effectively blocked/slowed thepropagation of cracks perpendicular to the MoS2/Sb2O3

interface; (b) on the surface, friction-induced stressresulted in parallel orientation of 2H-MoS2 basal planes,which formed a lubricious tribofilm of layer structureswith low shear stress; (c) the composition was a mixtureof MoS2 and very small Sb2O3 particles; and (d) wearcaused slow removal of oriented MoS2 top layers, rapidremoval/mixing of Sb2O3, and reformation of an ori-ented friction surface. Various multilayer films of MoS2such as in the present studies and those studied in pre-

vious reports have shown that significant increases infilm endurance and load-carrying capacity can beachieved by nanostructure tailoring [17–21]. Here, thecross-sectional characterization of wear scars andmultilayer structures provided important evidence toexplain how and why the multilayer films improveperformance.

4. Conclusions

Multilayered MoS2/Sb2O3 thin films were success-fully grown on steel substrates by using a multi-targetholder PLD system. The tribological properties of themultilayer films were measured and analyzed in com-parison to pure MoS2 films. The endurance in wet airtribotests was significantly improved by the multilayerarchitecture. The cross-sectional microstructures ofwear scars were intensely studied by employing FIBand high-resolution TEM. The lubricious tribofilmwas revealed on wear scar surfaces, which supplied thelow friction. Some micro-cracks were revealed at theinterface between the MoS2 tribofilm and its Sb2O3

underlayer. They were developing parallel to theinterface. Therefore, the Sb2O3 interlayers worked asan effective barrier to inhibit crack propagation per-pendicular to the interface, as well as providing alubricant reservoir to gradually discharge fresh MoS2materials to rubbing contacts. Friction-induced stresscaused the parallel orientation of 2H-MoS2 basal

Figure 4. (a) High-resolution TEM image shows the crystal lattice orientation of the 2H-MoS2 basal planes from inside the film to the surface.

Fourier transformation spectra of the tribofilm (b) and of the as-deposited MoS2 layer (c).

J.J. Hu et al./Multilayered MoS2/Sb2O3 thin films 173

planes in the tribofilm, while the as-deposited MoS2had a random orientation.

Acknowledgments

The Air Force Office of Scientific Research (AFOSR)is gratefully acknowledged for financial support.

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