1-nanocomposite-coatings-aerospace-applications by mrbenugwe


									                                              Surface and Coatings Technology 116–119 (1999) 36–45

      Nanocomposite tribological coatings for aerospace applications
                                      A.A. Voevodin *, J.P. O’Neill, J.S. Zabinski
 Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLBT, Wright–Patterson Air Force Base, OH 45433-7750, USA


    Challenges in aerospace tribology and composite coatings for aerospace applications are briefly reviewed. Attention is given to
nanocomposite coatings made of carbide, diamond-like carbon (DLC ) and transition-metal dichalcogenide phases. The preparation
of such coatings within the W–C–S material system using a hybrid of magnetron sputtering and pulsed laser deposition is
described. Coatings consist of 1–2 nm WC and 5–10 nm WS grains embedded in an amorphous DLC matrix. These
WC/DLC/WS nanocomposites demonstrate low friction and wear in tests performed in high vacuum, dry nitrogen and humid
air. Coatings are found to adapt to the test conditions, which results in: (1) crystallization and reorientation of initially
nanocrystalline and randomly oriented WS grains; (2) graphitization of the initially amorphous DLC matrix; (3) reversible
regulation of the composition of the transfer film between WS and graphite with environmental cycling from dry to humid; and
(4) possible DLC/WS synergistic effects, providing friction reduction in oxidizing environments. These adaptive mechanisms
achieve low friction coefficients of 0.02–0.05 and an endurance above two million cycles in space simulation tests. This also
provides stable coating performance and recovery of low friction in tests simulating ambient/space environmental cycling.
Correlations among WC/DLC/WS chemistry, structure, hardness, friction and wear are discussed. The tremendous potential of
such composites for aerospace tribology is demonstrated. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Aerospace tribology; Carbide; Diamond-like carbon; Dichalcogenide; Nanocomposite coating

1. Introduction                                                               assembly or launch, and contact atomic oxygen in low
                                                                              Earth orbit. Another environmental factor is the broad
1.1. Challenges of aerospace tribology                                        temperature range of operation; e.g., −100 to +100°C
                                                                              for space-borne devices and −40 to +300°C for aircraft
   One life-limiting problem of aerospace systems is                          components. Shuttle-type operations cause environmen-
friction and wear in various movable devices, such as                         tal cycling, and planetary explorations add the problem
reaction wheels, gyroscopes, solar arrays, antenna                            of abrasive wear by electrostatically attracted dust, such
drives, sensor-pointing mechanisms, gears, pumps, actu-                       as fine dust in the dry CO atmosphere of Mars experi-
ators, latches and releases. Together they cover a broad                      enced during the recent 1997 Pathfinder mission [1].
range of contact stresses from 107 to 1010 Pa and sliding                        The expected life of aerospace devices rapidly pro-
speeds from near zero in restraining mechanisms to                            gresses, which generates new challenges in mechanism
above 20 m s−1 in control-moment gyros. They experi-                          endurance. Examples are the more than two decades
ence low-frequency launch vibrations, and some of them                        service of high-altitude SR-71 ‘Blackbird’ reconnais-
operate with dithering motion at frequencies of up to                         sance planes [2] and the increase of space station service
500 Hz and high peak loads; e.g., gimbal bearings on                          from the several weeks of early Apollo and Salyut
space platforms capable of rapid re-targeting.                                programs to the more than 10 years of the MIR station
   Another challenge is extreme operation conditions.                         and forthcoming International Space Station [3].
Aircraft are repeatedly subjected to high and low humid-                      Estimates for 15–30 years of service give about
ity with altitude changes. Satellites designed for the high                   109–1010 cycles in gimbal bearings of momentum-control
vacuum of space can be exposed to moisture during                             devices or 106–108 oscillations in solar-array joints.
                                                                                 The mechanical, environmental and endurance
  * Corresponding author. Tel.: +1-937-255-9001;                              requirements of the aerospace applications exceed the
fax: +1-937-255-2176.                                                         available lubrication and wear-reduction technologies,
  E-mail address: andrey.voevodin@afrl.af.mil (A.A. Voevodin)                 demanding novel materials and advanced technologies.

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 22 8 - 5
                             A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45                    37

Presently, low outgassing liquids (e.g., mineral oils,               posites for the use in tool industry [42,43]. Similar
synthetic hydrocarbons, perfluoropolyalkylethers) and                 nanocomposites can be developed for aerospace needs.
solid lubricants [e.g., polytetrafluoroethylene (PTFE)                   One relatively new tribological material for aerospace
polymers, soft metals, dichalcogenides] are the predomi-             applications is diamond-like carbon (DLC ), which has
nant materials used for aerospace lubrication [4–6 ].                high hardness, low friction and low wear [44,45]. An
Liquids are used when the tribo-assembly can be sealed               effort was made to combine superhard unhydrogenated
from the outside environment. In other cases, solid                  DLC with transition-metal carbides in nanocrystalline/
lubrication is preferred. Aerospace solid lubrication and            amorphous composites. TiC or WC nanograins have
wear reduction with advanced composite coatings is the               been successfully embedded in a DLC matrix [46–49].
focus of this paper.                                                 Both TiC/DLC and WC/DLC composites demonstrated
                                                                     the desirable combination of hardness, toughness, low
                                                                     friction and wear in tests in ambient environments
1.2. Composite coatings for aerospace applications                   [47,49].
                                                                        The use of two-phase carbide/DLC nanocomposites
   Transition-metal dichalcogenides (MoS , WS ,                      in aerospace applications may create a problem due to
                                                  2     2
NbSe , etc.) are the most common solid lubricants for                DLC graphitization in friction contacts and the associ-
space applications. They are applied as powders mixed                ated increase of friction coefficient in the high-vacuum
with various organic and inorganic binders, burnished                environment [50]. A hydrogenated DLC phase provides
to the surface from powders, or deposited by spray and               a remedy by hydrogen termination of active carbon
vacuum deposition methods [7]. These lubricants are                  bonds, but not for long owing to hydrogen depletion
soft, not abrasion-resistant and oxidize in air. To comply           after about 104 cycles [51]. An alternative approach may
with endurance requirements of aerospace applications,               be incorporation of dichalcogenide space lubricants,
more advanced approaches were explored. One of them                  such as MoS or WS , into a carbide/DLC/
                                                                                       2           2
was self-lubricating composites, where solid lubricant               dichalcogenide composite. One design explored included
was pressed into a supporting matrix, such as in                     formation of MoS reservoirs within a TiC/DLC nano-
bronze/lead composites and glass-fiber- and polyamade-                composite by laser milling and MoS sputtering [52].
reinforced PTFE composites coated with MoS [8–11].                   The coating maintained more than a million cycles of
The composite approach was also used in high-temper-                 operation in space simulation tests and with humid/dry
ature space lubrication coatings, such as the NASA                   cycling. The deposition process was, however, quite
PS-200 series coatings produced by plasma spraying                   sophisticated.
which consisted of BaF /CaF lubricant, silver lubricant                 The nanocomposite coating approach using hard and
                         2     2
and binder, and Cr C support [12].                                   lubricating phases for both humid and dry environments
                    3 2
   The development of vacuum deposition methods                      can be more technologically robust. A synergetic effect
added new capabilities for controlling the chemistry,                of graphite/WS powder lubricants was reported to be
structure, morphology and thickness of solid lubricants.             beneficial for air/vacuum environmental cycling [53].
This improved their friction, endurance and environmen-              Taking into account DLC graphitization in friction
tal adaptation, and allowed precision components to be               contacts, a WC/DLC/WS composite coating was
coated. Examples are textured MoS films [13–18],                      designed, where hard nanocrystalline WC provides wear
metal-doped MoS            and WS          films [19–23],             resistance and mechanical stability, hard amorphous
                      2               2
CF -doped WS films [24], metal–MoS multilayers                        DLC provides low friction and wear in ambient condi-
    x           2                             2
[25–28], temperature-adaptive PbO/MoS , ZnO/MoS ,                    tions, and WS provides lubrication in vacuum.
                                             2          2                               2
ZnO/WS        and      moisture-resistant      PTFE/MoS ,            Structural and tribological characterization of
          2                                             2
LaF /MoS composites [29–33], etc. Vacuum deposition                  WC/DLC/WS coatings are discussed in this paper.
      3     2                                                                       2
also provided a variety of advanced hard coatings for
wear protection [34–37], which are currently expanding
toward aerospace applications due to the endurance                   2. Experimental details
requirement. For example, surface-treated diamond
coatings were developed for space friction and wear                     WC/DLC/WS coatings approximately 0.5 mm thick
reduction [38,39], while TiC and TiN coatings were                   were grown on 440C stainless steel disks, 25.4 mm in
used to delay fatigue wear in ball bearings lubricated               diameter, and polished to a surface roughness of
with perfluoropolyalkylether fluids [40]. The possibility              0.02 mm R . Coatings were produced by either magnet-
to mix hard and lubricious phases in thin nanocomposite              ron-assisted pulsed laser deposition (MSPLD) [54] or
coatings is another benefit of vacuum deposition. There               by laser ablation of a composite target made of graphite
were reports of producing TiN/MoS composites by                      and WS sectors. The deposition chamber was evacuated
                                          2                                   2
chemical vapor deposition [41] and, most recently, on                to less than 10−6 Pa. A magnetron with a WS target
magnetron-sputtered TiB /MoS and TiN/MoS com-                        was operated at 8 W cm−2 in 0.2 Pa argon to create a
                            2     2                  2
38                                A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45

                                                                          40 000 cycles. Coating endurance in space simulating
                                                                          conditions was estimated by long-duration tests of up
                                                                          to 2 000 000 cycles, which were run continuously in
                                                                          vacuum. The cross-sectional area and diameter of the
                                                                          wear tracks were determined with a Dektak profilometer
                                                                          and an optical microscope, respectively. Normalized
                                                                          wear volumes (mm3 N−1 m−1) were then calculated
                                                                          based on these measurements.
                                                                             Raman spectroscopy was used to analyze the micro-
                                                                          structure of tribofilms formed on the ball wear scar and
                                                                          disk wear track. The spectra were obtained with a
                                                                          Renishaw Ramascope 2000 equipped with a 514.5 nm
                                                                          laser and an air-cooled charge coupled device (CCD)
                                                                          detector. Spectrum identification was based on a Raman
Fig. 1. The W–C–S ternary composition diagram with dots represent-        database of tribological materials [55].
ing the chemistry of investigated coatings. The line of WS stoichiome-
try is also plotted.

                                                                          3. Results and discussion
plasma flux directed at 45° to the substrate. The KrF
excimer laser beam (200 mJ, 20 ns, 1–20 Hz) ablated a                     3.1. Chemistry and structure
graphite target at a 45° incidence angle. The coating
composition was controlled by varying the laser pulse                         The goal of this study was to investigate nanocom-
rate when using MSPLD, and by varying the WS sector                       posites, consisting of WC, DLC and WS . Thus, the
                                                   2                                                                  2
area on the target when using only laser ablation.                        elemental composition of the coatings was adjusted to
   Coating elemental composition was investigated using                   satisfy two criteria: (1) enough carbon should be avail-
a Surface Science Instruments M-Probe X-ray photo-                        able to form both nanocrystalline b-WC and a free
electron spectroscope. Relative peak areas were used for                  carbon phase with DLC properties; and (2) enough
compositional quantification, and corrected for a spec-                    sulfur should be available to form WS . The composi-
trometer factor and X-ray cross-section. The coatings                     tions investigated are indicated with dots in the ternary
were not sputtered prior to scanning because sulfur is                    W–C–S diagram in Fig. 1, neglecting 2–4 at% oxygen
preferentially removed. There was some oxygen contam-                     contamination.
ination due to the environment but it was mainly on                           All coatings had carbon contents within 60 to 90 at%
the surface; upon sputtering 5 nm deep, the oxygen                        ( Fig. 1). From previous studies of WC/DLC nanocom-
content was less than 4 at%. Coating structural investi-                  posites, the region of 65–75 at% was found to provide
gations were performed with transmission electron                         the formation of b-WC nanograins embedded in an
microscopy ( TEM ), using a Philips CM200 instrument                      amorphous DLC matrix [48]. This region produced an
operated at 200 keV.                                                      optimum combination of hardness and low friction in
   Hardness and Young’s moduli were determined with                       ambient environments [49]. Fig. 1 indicates also the
a Nanoindenter IIs microprobe. A Berkovich indenter                       deviation of the W/S ratio from the WS stoichiometry
was loaded in the range of 1 mN. Hardness was found                       line due to the consumption of tungsten in both WC
for the maximum penetration, and moduli were calcu-                       and WS phases.
lated from unloading portions of the load–displace-                           TEM images and electron diffraction patterns of W–
ment curves.                                                              C–S coatings with different sulfur contents are compared
   Coating friction was measured using a ball-on-disk                     in Fig. 2. The coating without sulfur had WC nanoclus-
tribometer. A 100 g load was applied to a 440C stainless                  ters (more dense dark spots) surrounded by amorphous
steel ball, which was about 6 mm in diameter. A lower                     DLC material (gray areas) [Fig. 2(a)]. The formation
limit estimate of Hertzian contact stress was 0.5 GPa,                    of fine, 1–2 nm sized WC clusters was confirmed by
assuming steel-on-steel contact. Tests were conducted in                  electron diffraction patterns, containing largely diffuse
air with relative humidity (RH ) controlled at 50±1%,                     rings for the main b-WC planes. The DLC phase was
dry nitrogen (<0.1% RH ) and 10−6 Pa vacuum. The                          detected by characteristic Raman scattering in experi-
tests were run over 50 000 cycles (approximately 2500 m)                  ments described previously for room-temperature depos-
for sliding friction investigations, and over 10 000 cycles               ition of b-WC/DLC nanocomposites [48].
(approximately 500 m) for wear-rate estimations. Sliding                      When the coating was doped with 15 at% sulfur, the
speeds were close to 10 m min−1. Coating friction was                     microstructure changed slightly [Fig. 2(b)]. There were
also investigated by cycling between dry nitrogen and                     also some changes in the diffraction pattern, where new
humid air every 5000 cycles for total test duration of                    rings appeared, indicating the beginning of WS forma-
                                 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45                               39

tion. For 29 at% sulfur, the microstructure and diffrac-                   spacing of about 0.3 nm, corresponding to the
tion pattern changed completely [Fig. 2(c)]. They clearly                 (10Z)           group of planes of a WS standard [56 ].
                                                                                Z=0,1,2,3                        2
demonstrated the presence of hexagonal WS . Two types                     Some curvature of widely spaced basal planes is visible,
of WS crystal plane were identified: (1) basal planes                      but few of them extend more than 10 nm in length.
perpendicular to the c-axis of WS with a spacing of                       Most of the WS grains had sizes of about 5–10 nm and
                                   2                                                       2
about 0.6 nm, corresponding to (002) planes of a WS                       were randomly oriented in the surrounding material,
standard; and (2) planes parallel to the c-axis with a                    consisting of the same dark and gray areas that were




Fig. 2. Transmission electron microscope images and selected-area diffraction patterns for the W–C–S coatings with various sulfur contents: (a)
0 at%; (b) 15 at%; (c) 29 at%.
40                               A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45

identified as b-WC and DLC in composites without                          3.3. Friction
   Grazing-angle X-ray diffraction studies did not reveal                    Variation of the friction coefficient of the composite
well-defined diffraction patterns, confirming that the                      coatings with sulfur content is shown in Fig. 4 for dry
grain sizes of WC and WS were on average less than                       nitrogen, vacuum and moist air environments. The data
10 nm. Also, Raman scattering analyses could not                         are averaged over 50 000 sliding cycles and represent
resolve characteristic peaks of free carbon or WS from                   steady-state wear conditions. The dependence of friction
the background noise. This added to the conclusion that                  coefficients on sulfur content for sliding in dry nitrogen
all phases in the WC/DLC/WS composites are either                        [Fig. 4(a)] and vacuum [Fig. 4(b)] was very similar.
nearly amorphous or extremely nanocrystalline with 1–
10 nm dimensions.

3.2. Hardness

   Fig. 3 plots the variation of the composite hardness
as a function of sulfur content. The coating hardness is
maintained at the level of about 15 GPa up to 15 at%
sulfur. This level of hardness corresponds to the
WC/DLC nanocomposite deposited at room temper-
ature without sulfur [48]. At 15–20 at% sulfur the
hardness decreased, reaching the level of about 7 GPa
above 20 at% sulfur ( Fig. 3). The elastic modulus also
decreased with sulfur content, from 230–250 GPa for
the coatings with less than 15 at% sulfur to 100–130 GPa
for the coatings with above 20 at% sulfur. This drop of
mechanical chacteristics was associated with the segre-
gation of soft WS within the composite.
   The hardness of WC and other carbide coatings is as
high as 24–28 GPa [57]. At the same time, the hardness
of WS and other solid lubricants (MoS , graphite, lead)
       2                                 2
is generally below the 3 GPa level [7,58]. Thus, the
7 GPa hardness of WC/DLC/WS composites is not
comparable with the hardness of carbides, but is signifi-
cantly higher than that of typical solid lubricants. This
creates the potential for low wear rates and longer
endurance, providing they maintain low friction coeffi-
cients similar to solid lubricants.



Fig. 3. Hardness variation of the coatings as a function of sulfur       Fig. 4. Variation of the friction coefficient with sulfur content for tests
content. Dotted line provides guidance for the eye.                      in: (a) dry nitrogen; (b) vacuum; and (c) moist air.
                                  A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45                               41

There was a sharp reduction of friction coefficient in
these environments at 15–20 at% sulfur. For coatings
with more than 20 at% sulfur, there was formation of
thick transfer films after tests in vacuum (Fig. 5). This
was observed on ball counterparts [Fig. 5(a)] and within
wear tracks [Fig. 5(b)], appearing as gray patches along
the sliding direction. A reduction of the friction coeffi-
cient to below 0.1 and third-body formation within
friction contacts were connected to the presence of
WS .
   The low friction of WS originates from weak Van
der Waals’ forces between sandwiches of SMWMS hex-
agonal basal planes in its crystal lattice [59], providing                Fig. 6. Micro-Raman spectra recorded for friction surfaces of a
easy intra- and intercrystalline slip in the friction contact             WC/DLC/WS composite with about 20 at% sulfur after tests in
[60]. These slip mechanisms require the presence of                       vacuum.
developed crystals with basal planes oriented parallel to
                                                                          the substrate surface. However, TEM studies indicated
                                                                          extremely nanocrystalline and randomly oriented WS
                                                                          grains in composites with above 20 at% sulfur. There
                                                                          were several reports about friction-induced crystalliza-
                                                                          tion and basal plane reorientation for vacuum-deposited
                                                                          MoS [15,61–63] and WS films [64,65]. Reorientation
                                                                                2                    2
                                                                          and conglomeration of initially randomly oriented
                                                                          WS nanograins was verified in this research by micro-
                                                                          Raman analyses of the contact areas indicated in Fig. 5.
                                                                          The spectra clearly indicated the presence of hexagonal
                                                                          WS planes identified by peaks at about 400 cm−1
                                                                          ( Fig. 6). The contribution of carbon to friction in
                                                                          vacuum was negligible, which was supported by negligi-
                                                                          ble Raman activity in the 1300–1600 cm−1 region, corre-
                                                                          sponding to the scattering from amorphous and/or
                                                                          graphite carbon [66 ].
                                                                             The composites with sulfur content below 15 at% had
                                                                          higher friction coefficients of 0.5–0.7 in vacuum
                                                                          [Fig. 4(b)]. Raman studies did not reveal the presence
                                                                          of hexagonal WS in friction contacts, but rather indi-
                                                                          cated the formation of amorphous carbon with partial
                                                                          graphitization detected by broad overlapped peaks, cen-
                                                                          tered at about 1380 and 1530 cm−1 ( Fig. 7, upper curve).
                                                                          Graphitization of the DLC phase was suggested as the


Fig. 5. Microphotographs of ball wear spot (a) and wear track (b) in
the WC/DLC/WS coating with about 20 at% sulfur formed after fric-
tion tests in vacuum. Arrows indicate regions used for micro-Raman        Fig. 7. Micro-Raman spectra recorded for wear track surfaces of coat-
studies.                                                                  ings with about 15 at% sulfur after tests in vacuum and moist air.
42                                 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45

main process influencing the tribological characteristics.
Friction coefficients of single-phase unhydrogenated
DLC in vacuum can be as high as 0.5–0.6 [50].
   The dependence of the friction coefficients on sulfur
content in humid air was more complicated [Fig. 4(c)].
For composites with lower sulfur content the formation
of amorphous carbon in wear tracks was detected
(Fig. 7, lower curve). This helped to achieve lower
friction coefficients of about 0.1, approaching the friction
of unhydrogenated DLC in humid air [50]. For films
with about 15–20 at% sulfur, phases of WS , WO and
                                             2      3
amorphous carbon were detected inside wear tracks and
debris along the sides of wear tracks ( Fig. 8). There
were also peaks of Fe O at about 220 cm−1 and some                         Fig. 9. Coating wear rates as a function of sulfur content for three
                       2 3
iron and/or chrome oxides peaks at about 900 cm−1                          environments: moist air (&), dry nitrogen (#) and high vacuum ($).
[55]. These oxide peaks appeared to be due to oxidation                    Lines represent trend fits.
and transfer of the ball material. These processes lead
to friction coefficients of 0.40–0.45 for composites with                    rates increased to about 6×10−6 mm3 N−1 m−1. This
about 15–20 at% sulfur with hardness comparable to                         level of wear rate was comparable to the wear rates of
that of the 440C steel balls (10–12 GPa). For softer                       various ceramic and metal-doped DLC materials tested
coatings with more than 20 at% sulfur, friction coeffi-                      in ambient conditions [67–69]. The wear rates in vacuum
cients reduced to the level of 0.2, which is typical for                   for the coatings with 20–30 at% sulfur were as low as
WS in moist air [65].                                                      10−6 mm3 N−1 m−1.
   Friction tests demonstrated the need for about 20 at%                      Raman spectra were taken from wear debris formed
sulfur in the WC/DLC/WS composite coatings to gen-                         in vacuum tests of the coating with about 20 at% sulfur
erate lubrication. The friction of such composites is                      ( Fig. 10). The spectra indicated the domination of
mainly governed by the WS phase reorientation and/or                       graphite-like carbon in the wear debris (Fig. 5). There
DLC phase graphitization. The resulting friction coeffi-                     was also WO in debris along the wear tracks (Fig. 10,
cients depend on both sulfur content and humidity.                                       3
                                                                           lower curve). The formation of WO in vacuum tests
                                                                           can be caused by oxygen contamination during coating
3.4. Wear and endurance                                                    deposition or by residual water pressure during wear
                                                                           tests. There were no peaks indicating the presence of
   Fig. 9 shows the variation of coating wear rates as a                   oxides due to counterpart ball wear. It was concluded
function of sulfur content for tests with 10 000 sliding                   that the wear debris formed in vacuum consisted pre-
cycles in three environments. Similar to the friction                      dominantly of graphitic carbon and some amount of
coefficients, the wear rates in dry environments (nitrogen                   WO ( Fig. 10), while the wear debris formed in moist
and vacuum) decreased considerably above 20 at%                                 3
                                                                           air consisted predominantly of oxides ( WO and ball
sulfur, with initiation of WS lubrication. In moist air,                                                                3
                              2                                            metal oxides) and some amount of graphite carbon
a reverse dependence was found (Fig. 9), where wear                        ( Fig. 8).

Fig. 8. Micro-Raman spectra recorded inside wear tracks and alongside      Fig. 10. Micro-Raman spectra recorded for wear debris of a
wear debris for coatings with about 20 at% sulfur after tests in moist     WC/DLC/WS composite with about 20 at% sulfur after tests in
air.                                                                       vacuum.
                              A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45                                43

   The tests of WC/DLC/WS coating endurance in
high-vacuum environment were particularly important.
Fig. 11 shows an example of friction coefficient variation
over two million sliding cycles for a coating with 22 at%
sulfur. The friction coefficient stayed in the range of
0.03 to 0.06. Some friction reduction with the number
of cycles indicated the ongoing process of WS phase
reorientation and accumulation in the friction contact.
The wear-rate estimate after this long run was about
10−7 mm3 N−1 m−1, which was an order of magnitude
lower than the wear rate of similar composites estimated
after 10 000 cycles ( Fig. 9). The wear rate decrease with
the increase of test duration is typical for solid lubri-
cants, since most of the wear occurs initially and is
followed by a long period of a very low wear [15,70].
   The WS/DLC/WS composites demonstrated low
wear rates in different environments and excellent endu-
                                                                      Fig. 12. Friction coefficient variation in tests with environment cycled
rance in long runs simulating space applications. Their
                                                                      from dry nitrogen to moist air every 5000 cycles.
low wear and friction fulfil the criteria of self-lubricated
wear-resistant coatings for space applications, which
was proposed to be 10−6 mm3 N−1 m−1 for wear rates                    An interesting observation was a slightly lower friction
and <0.1 for friction coefficients [38].                                coefficient recorded for moist air cycles, in comparison
                                                                      to the tests when the humidity was kept constant
3.5. Friction stability in environmental cycling                      [Fig. 4(c)]. One explanation can be that 5000 cycles in
                                                                      moist air are not sufficient to develop a strongly oxidized
   A critical requirement of aerospace applications is                film such as that formed after 50 000 cycles of fixed-
stable performance in environments with varied humid-                 humidity tests. Another can be a synergistic effect of
ity. Fig. 12 provides an example of friction coefficient                DLC and WS , similar to the one observed for
variation for the environment varied every 5000 sliding               graphite/MoS mixtures [71–73].
cycles between dry nitrogen (<0.05% RH ) and moist                        Based on the observed friction variation and the
air (50% RH ) for a WC/DLC/WS coating. This test                      analyses of wear debries in Section 3.3, the following
procedure was selected to simulate friction in                        hypothesis was made about environmental adaptation
ambient/space cycling, since friction and wear results                of the WC/DLC/WS coatings. In dry conditions, which
for dry nitrogen and vacuum were similar, and moist                   are favorable for WS but severe for graphite, there is
air/high vacuum cycling was difficult to implement.                     a predominant formation of hexagonal WS planes in
   For all coatings with above 15 at% sulfur, a low                   the friction contact. Once the humidity is cycled high,
friction coefficient of 0.02–0.05 was recorded in low-                  the WS planes in friction surfaces are destroyed by
humidity cycles, corresponding to lubrication by WS .                 oxidation and replaced with graphite planes formed by
In high-humidity cycles, the friction coefficients were                 the friction-induced transformation of DLC. For graph-
about 0.10–0.20, corresponding to lubrication by DLC.                 ite, the intercalation of basal planes with water molecules
                                                                      is essential for low friction and wear rates [74,75]. In
                                                                      the next dry cycle, the desorbtion of water molecules
                                                                      causes a high wear of graphite sheets and reveals fresh
                                                                      WS grains. The friction-induced reorientation of WS
                                                                           2                                                    2
                                                                      basal planes promotes the low friction and wear in dry
                                                                      conditions, and so on.
                                                                          To verify this hypothesis, the tests were stopped in
                                                                      the middle of a dry cycle, and in the middle of a moist
                                                                      cycle, to investigate predominant phases in the friction
                                                                      contacts by micro-Raman spectroscopy (Fig. 13). For
                                                                      the dry cycle, the results found Raman activity in the
                                                                      WS region and almost the absence of carbon-related
                                                                      scattering ( lower curve). For the moist cycle, the signal
                                                                      from graphite-like carbon was clearly identified in the
Fig. 11. Friction variation for the WC/DLC/WS composite with          spectra (Fig. 13, upper curve). This supported the adap-
about 22 at% sulfur in a long-duration test in vacuum.                tation of the coatings to these environments.
44                               A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45

                                                                         tion of an initially amorphous DLC matrix; (3)
                                                                         reversible regulation of the composition of the transfer
                                                                         film between WS and graphite with environmental
                                                                         cycling from dry to humid; and (4) possible DLC/WS
                                                                         synergistic effects, providing friction reduction in oxidiz-
                                                                         ing environments. The activation of these adaptation
                                                                         mechanisms in the WC/DLC/WS nanocomposites
                                                                         helped to achieve low friction coefficients of 0.02–0.05
                                                                         and an endurance of two million cycles in space simula-
                                                                         tion tests. This also provided stable coating performance
                                                                         and recovery of low friction in tests simulating
                                                                         ambient/space environmental cycling.
                                                                            The low friction and wear as well as unique environ-
Fig. 13. Micro-Raman spectra recorded for friction surfaces of a         mental adaptation of WC/DLC/WS coatings open new
WC/DLC/WS composite with about 20 at% sulfur in tests with cycled                                               2
              2                                                          perspectives for aerospace applications of these or sim-
humidity: (a) last cycle was in moist air; (b) last cycle was in dry
nitrogen.                                                                ilar carbide/DLC/dichalcogenide nanocomposites.

   The effect on friction regulation with environmental                   References
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