Tribol Lett (2013) 49:95–102
`DOI 10.1007/s11249-012-0048-z
`
`O R I G I N A L P A P E R
`
`Plasmonic Diagnostics for Tribology: In Situ Observations
`Using Surface Plasmon Resonance in Combination
`with Surface-Enhanced Raman Spectroscopy
`
`Brandon A. Krick • David W. Hahn •
`W. Gregory Sawyer
`
`Received: 12 March 2012 / Accepted: 15 September 2012 / Published online: 30 September 2012
`Ó Springer Science+Business Media New York 2012
`
`Abstract The generation of transfer films is a common
`wear and lubrication mechanism of solid lubricants, such as
`polymers and lamellar solids. Material can transfer from a
`solid lubricant to a counter surface as early as the first cycle
`of sliding, initiating the formation of a transfer film, which
`can persist for the duration of sliding. Surface plasmon
`resonance (SPR) was used to monitor incipient molecular
`wear of a solid lubricant as performed here using a gold
`coated (50 nm) quartz prism in situ during sliding experi-
`ments. Surface-enhanced Raman spectroscopy (SERS) is a
`complementary technique enabling the analysis of ultra-
`thin transfer films. SPR and SERS experiments confirm
`that polytetrafluoroethylene and graphite transfer readily,
`with observed changes in SPR signal after one cycle of
`sliding, while ultra-high-molecular-weight polyethylene
`shows little transfer to the gold during sliding in the in situ
`SPR experiment. This shows the feasibility of SPR and
`SERS as important diagnostic tools for tribological studies.
`Keywords Surface plasmon resonance (SPR)
`Surface-enhanced Raman spectroscopy (SERS)
`Polytetrafluoroethylene (PTFE) Molecular wear
`Transfer film Plasmonics In situ
`
`1 Introduction and Background
`
`Friction and wear involve many complex interactions within
`interfaces buried between contacting surfaces; fundamental
`studies of friction and wear can benefit significantly from
`
`B. A. Krick (&) D. W. Hahn W. G. Sawyer
`Department of Mechanical and Aerospace Engineering,
`University of Florida, Gainesville, FL 32611, USA
`e-mail: bakrick@ufl.edu
`
`complementary analytical tools specifically aimed to inter-
`rogate interactions at the buried interface of two solids [1–3].
`Tribology has a rich history and even more promising
`opportunity for applications of analytical and in situ tech-
`niques [1–30].
`One complexity of tribological system is the concept of
`the transfer film (i.e., removal of material from one mating
`surface and subsequent deposition on the opposing surface).
`This may entail material movement from the molecular scale
`upward, while temporal scales span from chemical kinetic
`rates to hours and days [9, 13–16, 20, 23, 24, 31–38]. Ana-
`lytical
`techniques,
`including atomic force microscopy,
`optical profilometry and microscopy for monitoring and
`quantifying transfer film and third body formation have their
`respective advantages and disadvantages; the considerable
`footprint, relatively long analysis times, resolution limita-
`tions on optical techniques, and lack of in situ capabilities of
`these instruments remain limiting factors [2]. We, therefore,
`focus on complementary analytical tools specifically aimed
`to interrogate the buried interfaces with goals of elucidating
`the temporal and spatial scales of surface kinetics and
`material transfer by developing an in situ surface plasmon
`resonance (SPR) tribometer.
`Until
`recently, spectroscopy techniques with single
`molecule sensitivity have been precluded from in situ tri-
`bological efforts [2]. SPR appears to be uniquely suited to
`provide such high fidelity measurements within a tribo-
`logical apparatus. In this paper, SPR was used for in situ
`tribological studies of the dynamics of transfer films of
`polytetrafluoroethylene
`(PTFE),
`ultra-high-molecular-
`weight polyethylene (UHMWPE), and graphite. This
`approach offers the ability for in situ investigation of the
`development of incipient
`transfer films (ideally at
`the
`molecular or near-molecular level), providing experimental
`capabilities unavailable
`to date
`in the
`tribological
`
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`Tribol Lett (2013) 49:95–102
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`community. For example, the transfer of discrete funda-
`mental wear units (e.g., molecules, macromolecules, and
`nano-particulates) is of key importance in understanding
`the underlying physics and origins of wear processes.
`Additionally,
`surface-enhanced Raman spectroscopy
`(SERS) is ideally suited to analyze the chemical identity of
`transfer films generated during in situ SPR experiments.
`This can confirm the presence and chemical identity of a
`transfer film and complement the results of the SPR stud-
`ies. Furthermore, it shows promise to be used for in situ
`tribology experiments with similar light pathways used for
`prior in situ optical and Raman experiments [2–4, 6, 10,
`13–15, 18–20, 23, 26, 27] and the present in situ SPR
`experiment.
`
`1.1 Surface Plasmon Resonance
`
`A surface plasmon can be described as a fluctuating electron
`density wave that propagates parallel and along the
`boundary of a metal and a dielectric medium [39–42]. The
`surface plasmon phenomenon was first described to explain
`unexpected attenuation in diffraction gratings over a cen-
`tury ago [43]. A surface plasmon can be excited by light, in
`which the energy of a photon is transferred into the elec-
`trons of the metal, thereby propagating itself as a surface
`
`plasmon. In order for this to occur, the metal must possess
`conduction band electrons that resonate with the exciting
`photons. Many types of metals contain the properties to
`produce surface plasmons (e.g., copper, titanium, chro-
`mium, silver, and others), however, gold is often the most
`suitable. Since this is a surface phenomenon in which both
`mediums play a role, any slight perturbation to the boundary
`can greatly alter the induction and propagation of a surface
`plasmon. For this reason, surface plasmon resonance is a
`technique which is very sensitive in detecting adsorbed
`materials on the metal surface, including the detection of
`mono-layers and single nanoparticles [39, 40, 44].
`A simple schematic of the Kretschmann configuration
`[45] can be seen in Fig. 1. In such configurations, a thin metal
`film is deposited on one surface of a prism. The prism’s high
`refractive index acts to condition the incoming light ray,
`changing its wave function so that coupling into a surface
`plasmon is possible. The angle dependence in the wave
`function through a prism is such that the plasmon will only be
`induced at a specific incidence angle, a, characterized by a
`near total attenuation of the reflected ray.
`When the appropriate incident angle is reached and a
`surface plasmon is generated, reflectance of the incident
`light is greatly reduced as the majority of the light energy is
`coupled into the surface plasmon propagating along the
`
`(a)
`
`Fig. 1 In situ surface plasmon
`resonance tribometry.
`a Schematic for in situ surface
`plasmon resonance tribometer
`that slides a sample across the
`metallic film sensing layer on
`a quartz prism utilizing the
`Kretschmann configuration.
`b Schematic of the
`Kretschmann configuration for
`generation of a surface plasmon.
`c Schematic of the modification
`of the surface plasmon due to
`molecular adsorbates or transfer
`on the metallic sensing layer
`
`(b)
`
`(c)
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`97
`
`metal and dielectric interface. As material becomes
`adsorbed (e.g., a monolayer) onto the surface of the metal
`opposite the plasmon, the angle, a, at which resonance
`occurs is altered. This allows for a measurement of
`reflectance intensity versus incident angle to be made
`indicating changes due to adsorbed material at the surface
`of the metal. Alternatively, one may expand the incident
`beam to cover a large surface area and then tune the
`incident angle to plasmon resonance, and an image may be
`recorded of the reflected beam. Any changes to the sensing
`surface are detected by changes in intensity of the reflected
`image as the surface plasmon is disrupted by adsorbed
`analyte [44]. As described above, since the surface plas-
`mon is so highly dependent on the interface, measureable
`differences in reflectance will occur due to the smallest
`perturbations to the boundary, allowing for very sensitive
`surface measurements.
`We note that the SPR technique generally responds to
`refractive index changes on the sensing layer to a depth of a
`few hundred nanometers. Jung et al. explored the inter-
`pretation of the SPR surface response in the context of
`quantifying the thickness or surface concentration of an
`adsorbed layer, including calculations of the SPR angle as
`a function of adsorbed layer thickness for various refractive
`index pairs, and assessing the effect of refractive index
`change on the SPR wavelength. For a bulk refractive index
`mismatch of 0.1, detection limits for an adsorbed layer
`thickness are estimated to be sub-angstrom [46]. Kooyman
`et al.[47] also examine the sensitivity of SPR sensors in the
`context of sensing layer thickness.
`Here, the SPR method was implemented into an in situ
`tribometer to monitor transfer from solid lubricant mate-
`rials to a gold coated quartz prism. Increases in reflected
`intensity of the incident beam at a fixed incident angle
`correspond to molecular transfer from the solid lubricant
`sample to the gold sensing surface.
`
`1.2 Surface-Enhanced Raman Spectroscopy
`
`Raman spectroscopy is an inelastic-scattering molecular
`probe (i.e., vibrational
`spectroscopy)
`that
`is
`readily
`implemented with a single excitation laser source. The
`Raman effect arises from coupling of the EM wave-
`induced dipole moment with the vibrational quanta of the
`targeted analyte. While a very versatile spectroscopic tool,
`Raman scattering is characterized by very low scattering
`cross sections that limit applicability. These limitations
`may be overcome with surface-enhanced Raman spec-
`troscopy,
`in which surface plasmons, primarily on the
`surfaces of gold and silver, interact with an adjacent or
`adsorbed analyte to boost the Raman signal by orders of
`magnitude in many cases, including factors greater than
`106 for single molecules [48]. The physics of the SERS
`
`effect is rooted in the localized surface plasmons created by
`the incident laser beam on the SERS substrate, which we
`consider here in terms of the classical electromagnetic field
`effect [48–50]. Silver and gold are the most common
`choices for the SERS substrate because visible excitation
`wavelengths (e.g., 488, 514, and 632 nm laser lines)
`effectively couple to the resonance plasmon frequencies.
`The localized electric field is then greatly increased by the
`surface plasmon, which subsequently increases the field
`that interacts with a targeted analyte molecule adsorbed to
`the substrate surface, thereby enhancing the Raman scat-
`tering through the induced-dipole interactions. Surface
`roughness and curvature can influence the surface plasmon
`and resulting SERS enhancement, as edge effects can
`greatly alter the EM field enhancement.
`
`2 Materials
`
`2.1 Gold Coated Quartz Prisms
`
`Counterfaces for in situ SPR experiments were gold coated
`quartz prisms. The prisms were right angle triangular
`prisms defined by a right
`triangle with a 35.36 mm
`hypotenuse and 25 mm legs. A 5 nm chromium bond layer
`and a 50 nm gold layer were coated on the hypotenuse face
`of the prism.
`
`2.2 Polytetrafluoroethylene
`
`Polytetrafluoroethylene (PTFE) is an excellent solid lubri-
`cant because of
`its low friction coefficient
`(reported
`between 0.04 and 0.2),
`its chemical
`inertness, and its
`resistance to a wide variety of environments [34, 51–56].
`Disadvantages of PTFE include its high wear
`rate
`(*1 9 10-4 to 1 9 10-3 mm3/Nm [57]) and the large
`amount of wear debris produced [56–58]. PTFE is known
`to develop a transfer film; although an incubation period
`for wear has been observed and discussed [34, 56–59],
`little is understood about this mild-wear incubation process
`Ò
`for unfilled material. DuPont Teflon
`7C PTFE was used
`for in situ SPR experiments. PTFE samples were machined
`to a tip radius of 1 mm. Samples were washed with soap
`and water then sonicated in methanol for 30 min and then
`dried prior to testing.
`
`2.3 Ultra-High-Molecular-Weight Polyethylene
`(UHMWPE)
`
`another
`is
`polyethylene
`Ultra-high-molecular-weight
`excellent polymeric candidate for use as a solid lubricant
`[31, 60–62]; it differs in that its performance is often
`thought not to require transfer films, but rather smooth
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`Tribol Lett (2013) 49:95–102
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`counter surfaces [60]. Pooley and Tabor [61] found that
`high density polyethylene and ‘‘extended chain polyethyl-
`ene’’ had noticeably less transfer than PTFE. They also
`noted that transfer films were observed for PTFE when slid
`against steel, while no detectable films were observed in
`the case of polyethylene against steel [61]. This makes
`UHMWPE an ideal candidate as a negative control for SPR
`experiments; where there should be minimal increase in
`reflectivity during SPR experiments. UHMWPE physical
`properties and friction coefficient are similar to PTFE so
`the main difference in the experiments should be the
`amount of transfer measured by SPR. UHMWPE samples
`were also machined to a 1 mm tip radius. Samples were
`washed with soap and water, sonicated in methanol for
`30 min, and dried prior to testing.
`
`sensing layer of the prism, which in this case would be
`material transfer.
`The tribometer was designed around the gold coated
`prism (the counterface) and mounted on a rotational stage.
`The rotational stage is used to adjust the angle of incidence
`to tune the initial plasmon effect in the system. The sample
`(i.e., PTFE, UHMWPE, or graphite) was mounted to a
`bi-axial cantilever force transducer [63]. The sample was
`loaded into contact with the gold surface of the prism with
`a manual micrometer. The beam was 1.6 mm in diameter at
`the point of contact and was centered on the transfer film at
`the midpoint of the sliding cycle.
`
`3.2 Surface-Enhanced Raman Spectroscopy (SERS)
`and Raman Spectroscopy
`
`2.4 Graphite
`
`Graphite is commonly used for its ability to transfer and
`make low friction films. One of its most common uses is in
`composites used for writing in pencils. The most prevalent
`composition in pencils is the HB (number 2) pencil, which
`consists of clay and graphite. This composition is ideal for
`transfer film experiments in that it is designed to transfer
`readily and is not too hard to damage the counterface. HB
`graphite composites were extracted from standard #2
`pencils and finished to a tip radius of 1 mm. Samples were
`rinsed in methanol and then wiped with a clean laboratory
`wipe.
`
`3 Methods
`
`3.1 In Situ Surface Plasmon Resonance (SPR)
`Tribometer
`
`SPR was used to monitor transfer film formation as a
`function of sliding cycle. A linear reciprocating pin on flat
`tribometer was used to perform friction and SPR mea-
`surements. A 632 nm laser with power of 540 mW is
`transmitted through one leg face of the prism, directed at
`the hypotenuse (gold coated) face. The reflected intensity
`of this light is measured through the other leg face of the
`prism with an optical power meter as shown in Fig. 1. The
`angle of incidence is adjusted such that the surface plasmon
`resonance effect is a maximum and the reflected power is a
`minimum (*0.005 %). After this,
`the sample (PTFE,
`UHMWPE, or graphite) is loaded against the gold surface
`of the prism and sliding is initiated. After each sliding
`cycle (one forward and reverse motion), a value of
`reflected intensity is recorded. An increase in this intensity
`corresponds to a change in the adsorbates on the gold
`
`123
`
`For all Raman spectroscopy, a fully automated commercial
`micro-Raman system was used (LabRam Infinity). This
`system used a 15 mW continuous helium:neon laser (k =
`632.8 nm). A 1009 magnification (N.A. = 0.9) objective
`was used to focus the laser to a diameter of 5 lm. An
`internal camera was used to find the region of interest
`(wear tracks) and neutral density filters were used to adjust
`the excitation energy of the laser at the sample surface. The
`scattered light was collected via backscatter by the same
`microscope objective with elastic scattered light rejected
`by a sharp-edge filter; it was dispersed by an 1,800 grove/
`mm grating and imaged by a CCD detector (1,024 9
`256 pixels).
`Surface-enhanced Raman was achieved utilizing the
`micro-Raman system on the transfer films that were
`deposited on the gold surface of the prism. The gold
`coating required for the SPR experiments also produced the
`surface-enhanced scattering effect for spectral analysis.
`Spectra on transfer films on the gold coating were com-
`pared to transfer films on uncoated quartz to validate the
`SERS effect.
`
`3.3 Optical In Situ Micro-tribometer
`
`A micro-tribometer equipped with an in situ microscope
`and camera [27] was used to analyze the wear of PTFE
`when slid against glass. The objective is mounted beneath
`the contact and focuses through the glass on the contact
`spot between the glass and PTFE sample. Real contact
`area, transfer film formation, wear debris formation, and
`wear debris motion can be observed with the optics as the
`PTFE is worn by sliding against the glass. The contact spot
`is a dark spot in the image, while higher order fringes
`surround the contact and map the near contact separation
`distance. Furthermore, larger third bodies under the contact
`are distinguished from the sample contact, as they are
`outlined with higher order interference fringes.
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`4 Results
`
`Sliding experiments were performed with a linear velocity
`of 1 mm/s for PTFE, UHMWPE, and graphite. In an effort
`to increase material transfer for one experiment, PTFE was
`heated to phase I (*370 K); heating of PTFE has been
`shown to increase transfer [59]. An externally applied
`normal load of 1 N was used for UHMWPE, PTFE (room
`temperature), and heated PTFE (phase I) experiments while
`a load of 0.2 N was used for the graphite composite. Fig-
`ure 2 shows the reflected laser power normalized to the
`incident laser power as a function of sliding cycle. The
`arbitrary intensity of the SPR measurement, I, is equal to
`the measured intensity, I offset by the initial intensity
`before sliding, I0, all normalized by the incident laser
`intensity, Iin, as shown in Eq. 1.
`I ¼ ðI I0Þ=Iin
`
`ð1Þ
`
`(a)
`
`(b)
`
`99
`
`For both PTFE samples, a strong increase in reflectance
`is observed, followed by a slow reduction. The graphite
`sample shows a steady increase in SPR signal with each
`additional sliding cycle. Minimal
`increases in surface
`reflectance were observed in the UHMWPE experiments.
`After each experiment, surface-enhanced Raman was
`performed on each transfer film. For the PTFE and heated
`PTFE transfer film, a spectrum was observed that is consistent
`with a bulk PTFE reference (Fig. 3). This can be contrasted
`with the non-surface-enhanced Raman spectrum of a PTFE
`transfer film generated by sliding PTFE on quartz (without a
`gold layer). The graphite track also produced a surface-
`enhanced Raman spectrum consistent with the graphite ref-
`erence. No signal outside of the noise was observed for the
`UHMWPE transfer film. This is consistent with the minimal
`change in signal observed by the SPR, implying that there was
`minimal (if any) transfer of UHMWPE.
`PTFE was slid against glass in an optical in situ micro-
`tribometer. Figure 4 shows a sequence of images collected
`during sliding. From the images it was observed that a
`shearing event can produce a transfer filament (Fig. 4c) of
`PTFE to the glass. The PTFE sample will slide over the
`PTFE filament for some time without moving the filament
`(Fig. 4d–g). After several cycles of sliding, the filament
`may be picked up and moved from its initial
`transfer
`location to another location as shown in Fig. 4h.
`
`5 Discussion
`
`5.1 UHMWPE
`
`Minimal changes in surface reflectance were observed in
`the SPR signal when UHMWPE was slid against gold. The
`
`Fig. 2 In situ surface plasmon resonance reflectivity versus sliding
`cycle for sliding experiments of phase I PTFE, room temperature
`PTFE, UHMWPE, and graphite. SPR reflectivity intensity measured
`and calculated by Eq. 1 is shown in a log–linear scale and b linear–
`linear for visualization convenience
`
`Fig. 3 Surface-enhanced Raman scattering for PTFE transfer film
`formed during in situ SPR experiments. A PTFE reference spectrum
`and a spectrum for PTFE transferred to glass by sliding are shown for
`comparison
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`(a)
`
`(b)
`
`(c)
`
`(d)
`
`(e)
`
`(f)
`
`(g)
`
`(h)
`
`Fig. 4 In situ optical microscopy of PTFE, sliding on glass, and
`generating a wear filament. Sequential images show: a static contact;
`b sliding; c static with wear filament in contact; d; e sliding over wear
`filament, f; g wear filament moved during sliding; h wear filament
`moved to outside of static contact. For all images, the dark area is the
`contact between PTFE and glass. It is surrounded by interference
`fringes caused by the interference of reflections from the surface of
`the glass and from the surface of the PTFE in the near contact region
`
`lack of change in intensity implies that the surface plasmon
`was unaltered during sliding and should be interpreted as
`UHMWPE sliding without transferring material to the gold
`layer. Mechanically UHMWPE is very similar to PTFE,
`with the important difference in the ability to form transfer
`films at low sliding cycles under these sliding conditions.
`The nearly unaltered plasmon in the UHMWPE experi-
`ments confirms that
`the mechanical
`interaction of the
`polymer and gold did not cause false SPR signal due to
`
`123
`
`mechanical deformation of the gold layer. This further
`validates that the response of SPR during the sliding of
`PTFE and similar materials is not simply caused by the
`wearing or deformation of the gold layer. Additionally, the
`SERS results confirm that there is no detectable UHMWPE
`transfer. These tests provide an excellent negative control
`for in situ SPR tribological experiments.
`
`5.2 Graphite
`
`A monotonic increase in SPR signal is observed when the
`graphite sample was slid against the gold sensing layer.
`This increase is consistent with observations of transfer
`from the graphite sample to the gold in surface-enhanced
`Raman measurements. This is no surprise, as the graphite
`composite used in pencils is designed to transfer continu-
`ous deposition of graphite with each sliding cycle. Anyone
`who has ever sketched with a pencil knows that one can
`darken a line by either pressing harder or by increasing the
`number of strokes over the line. We also note that pre-
`liminary experiments of higher loads (*1 N) were found
`to remove the gold layer from the prism.
`
`5.3 PTFE
`
`Large increases in the reflected intensities came in the
`PTFE systems, with the phase I PTFE (heated) experiment
`producing the largest change. This large increase in
`reflectance corresponds to material transfer from the PTFE
`to the gold layer on the prism and as expected, the phase I
`PTFE transferred material more readily than room tem-
`perature PTFE during sliding. After approximately 10
`sliding cycles,
`the SPR signal decreased,
`implying a
`decrease in transferred material on the gold layer in the
`analysis zone. It has been shown by the results from the
`optical in situ tribometer (Fig. 4) that PTFE will transfer
`and the transferred material may be moved around. The
`transferred material will most likely be moved to and
`accumulate at the ends of the sliding cycle as shown in the
`results from sliding PTFE on glass in the optical in situ
`micro-tribometer. Since the SPR measurements are taking
`place halfway along the sliding cycle, this accumulation
`would not be measured; instead there would be some bal-
`ance of transfer and removal.
`The in situ SPR results suggest a new understanding of
`the first transfer of PTFE and the transition from the mild
`wear of PTFE. As previously discussed, PTFE wears
`through a delamination wear process in which subsurface
`cracks propagate through the polymer and flakey wear
`debris is ejected. The SPR results suggest that there is also
`a gradual wear process in which PTFE is transferred in as
`early as one sliding cycle. This mild transfer mechanism is
`different than the delamination wear process of PTFE and
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`101
`
`is possibly dominated by attractive forces between the
`PTFE and the metal counterface. Generally, the first sliding
`cycle of PTFE has higher friction than the sliding cycles
`immediately following it. This could be a result of the
`sliding interface changing from PTFE on metal to PTFE on
`a molecularly thin transfer film of PTFE. As sliding is
`continued, crack propagations in the subsurface allow
`debris to be ejected and the system transitions to the poor
`wear behavior commonly observed in PTFE.
`Surface-enhanced Raman confirms the transfer of PTFE
`to the gold sensing layer. A transfer film of PTFE on quartz
`is unobservable by traditional Raman spectroscopy. The
`surface-enhanced effect provided by the gold sensing layer
`makes it possible to confirm that the increase in surface
`plasmon resonance is caused by transfer of PTFE. Future
`incorporation of in situ SERS appears feasible based on the
`current findings.
`
`6 Concluding Remarks
`
`In situ surface plasmon resonance measurements were used
`to detect material transfer from PTFE, UHMWPE, and
`graphite when slid against a gold coated quartz prism.
`Incipient transfer film formation was observed from PTFE
`and graphite, while minimal transfer was observed from
`UHMWPE. Surface-enhanced Raman spectroscopy was
`used to confirm the chemical identity of transferred species.
`Furthermore, an in situ optical tribometer was used to
`explain the increase and subsequent decrease in SPR
`intensity during sliding of PTFE on gold.
`Surface plasmonics such as SPR and SERS are appli-
`cable to the field of tribology and show promise for in situ
`studies of friction and wear. As the use of surface plasmon
`resonance progresses in tribological studies, future work
`will be to quantify material transfer as a function of SPR
`signal. Furthermore, there is a clear opportunity to apply
`surface-enhanced Raman spectroscopy for in situ tribo-
`logical studies using the in situ strategies discussed here
`and throughout the literature.
`
`Acknowledgments The authors thank Kathryn Harris and Jeff Ewin
`for their assistance in sample preparation, and Bret Windom for the
`preliminary work on the feasibility of SPR. The authors also
`acknowledge the University of Florida Microfabritech group for
`depositing gold coatings for SPR and SERS experiments.
`
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