Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Nanocomposite Coatings for Severe Applications 701
demonstrate adequate stability up to 1000
◦
C, and above, but are characterized by strong
covalent bonding, which suggests that the surfaces would be difficult to shear. In fact, ceramic
materials typically yield high friction coefficients (0.8 to 1.0) that generate high tensile stresses
and produce surface cracks and debris particles [95]. However, substoichiometric oxides of
several transition metals, known as Magneli phases, form a homologous series of compounds
with the formula Me
n
O
2n−1
,Me
n
O
3n−1
,orMe
n
O
3n−2
with planar lattice faults that result in
crystallographic shear planes with reduced binding strength [96]. In general, the friction
coefficients for single transition metal oxides forming Magneli phases range from 0.2 to 0.5 at
homologous temperatures above 0.7 [97].
Several transition metal oxides (MO
x
,WO
x
,VO
x
, etc.) and double oxides, such as molybdates
and tungstates, deform by plastic flow at high temperature rather than brittle fracture at
elevated temperatures. For example, MoO
3
has a friction coefficient of 0.2 at 700
◦
C [98]. One
possible mechanism for such shear at elevated temperatures was linked to formation of an
oxygen vacancy, as it was explored on rutile [99]. Erdemir [100] also suggested that the ionic
potential of metal oxides and, for double oxides, the difference in relative ionic potentials of
the oxides is related to the reduced shear strength required for friction reduction. Oxidation of
some metals results in a reduction of the melting point, thereby forming a lubricious glaze
with low shear strength at high homologous surface temperatures. Alkaline halides, such as
CaF, can provide low-shear surface glazes at high temperatures (500–900
◦
C), close to their
melting points, with friction coefficients around 0.2 [101]. At temperatures < 500
◦
C,
coatings of CaF and CeF improve wear resistance with friction coefficients < 0.3 [102, 103],
but have been reported to become brittle at low temperatures after one thermal cycle in air
[104].
In general, the brittleness of oxides significantly limits their tribological applicability as
monolithic coatings, especially when applied to less rigid substrates, such as steels and
superalloys. Therefore, recent efforts with nanocomposite coating architectures have focused
on a realization of thin oxide layers at the coating surface for friction and oxidation control,
while the bulk of the coating remains in a non-oxidized state to ensure load support and
fracture toughness. Both multilayer and nanocomposite concepts are being explored for this
purpose, with surface adaptation by high temperature tribo-oxidation being one of the most
common. For example, TiAlN and many other cutting tool coatings are based on hard phases
with metal additions that form thermally stable protective oxide layers [105] that can be
somewhat lubricous, with friction coefficients of 0.3–0.6 and low wear rates. Self-hardening
and microstructure evolution in TiAlN, TiCrN, and TiBN coatings can occur over extended
periods at high temperature to further increase wear resistance [9, 20, 106, 107]. The properties
of these thin film compounds are sometimes modified further with one or more elements, such
as vanadium [69], known to form Magneli phases at very high temperatures (> 600
◦
C). Recent
studies have included exploration of Mo–N and W–N based coatings, designed to oxidize in
dry machining operations where high contact temperatures are common, and to create
wear-resistant and somewhat lubricious oxide phases upon heating [108, 109].
702 Chapter 14
Explorations of more complex nanocomposite architectures, such as yttria-stabilized zirconia
(YSZ)–Ag–MoS
2
, have shown that the chemistry of the oxide layer formation can be
controlled catalytically to guide formation of different lubricious oxides over a broad
temperature range. For example, the addition of MoS
2
was found effective as both lubricant
and catalyst of silver molybdate formation, as the friction coefficient dropped to below 0.2
over the temperature range 25–700
◦
C for a series of coatings with different compositions. The
low friction coefficients were attributed to the presence of MoS
2
and silver at < 300
◦
C, silver
molybdate compounds at 300–600
◦
C, and MoO
3
at higher temperatures [110]. Similar
examples provide coatings consisting of a Mo
2
N matrix with nanoscopic MoS
2
and Ag
lubricant phases [111].
An alternative mechanism to oxide lubrication at moderate temperatures (300–600
◦
C) is the
use of noble metals in the hard nanocomposite coating. This approach relies on the activation
of metal diffusion from coating bulk to the surface to provide lubrication in contact areas at
elevated temperatures. For example, inclusions of nanoscopic grains of gold in YSZ matrices
were used to provide a temperature-actuated metal lubrication with friction coefficients to
about 0.2 [112]. Other examples include TiC/Ag [113], YSZ–Ag [114], and CrN–Ag [115,
116] nanocomposite coatings. Typically up to 20 at.% of noble metal can be dispersed in
ceramic matrices before significant softening occurs [117]. Such coatings are produced in a
metastable condition by PVD methods, using forced co-deposition of metal and
nanocrystalline ceramics at substrate temperatures of 150–200
◦
C. Heating during coating use
activates metal diffusion and segregation at the contact surface, providing a low shear
strength interface with typical lubrication up to 500
◦
C. Possible shortcomings include
excessive contact deformations at elevated temperatures, reduction in mechanical stability of
coating matrix, and non-reversibility of the diffusion process important for temperature
cycling.
Ongoing research shows promise in combining oxide chemistry adaptation and controlled
metal diffusion for lubrication during temperature cycles [118–120]. Insertion of diffusion
control layers into a nanocomposite coating provides the means to restrict oxygen diffusion
into the coating bulk as well as restrain noble metal diffusion to the surface. The coating
immediately below the next intact diffusion barrier layer would essentially be in the
as-deposited condition and ready to adapt to any ambient temperature upon exposure by the
wear process. A three-layer coating made of the two adaptive YSZ–Ag–Mo lubricant layers
with a TiN diffusion barrier between them is shown in Figure 14.9 after being thermally
‘actuated’ by postdeposition heating to 500
◦
C. While the top layer has the composition
modification due to the silver diffusion to the surface, the adaptive layer below TiN was intact
and ready to go for the next temperature cycle as soon as the wear process breached it. A
similar coating with four adaptive layers provided stable friction coefficients over several
temperature cycles, as illustrated in Figure 14.10.
Nanocomposite Coatings for Severe Applications 703
Figure 14.9: Cross-section TEM image of a multilayer coating made of two adaptive YSZ–Mo–Ag
nanocomposite layers separated with a TiN diffusion barrier and applied to an Inconel 718
superalloy substrate with a Ti/YSZ bonding layer. Image was taken after coating heating to
500
C and subsequent cooling in air, demonstrating Ag diffusion to the coating surface and
preservation of the coating uniformity under the TiN barrier layer. Top Pt layer was added during
TEM cross-section preparation [120].
These multilayer coatings lend themselves to incorporation of sensor layers to allow in situ
measurement of the extent of wear, and can be designed to provide a warning prior to failure,
since diffusion barrier materials doped with rare earth elements that fluoresce with
characteristic spectra can be placed at known thicknesses. If the area being worn is
illuminated, the spectrum of scattered light gives off distinctive and intense spectra, allowing
an easy reporting of the in situ wear state condition, which is critical for some aerospace and
transportation applications [121].
Figure 14.10: Friction coefficients measured during pin-on-disk sliding experiments with repeated
heating and cooling of a multilayered coating with four YSZ–Ag–Mo adaptive nanocomposite
layers separated by TiN diffusion barrier layers and applied to an Inconel 718 superalloy
substrate. Data were obtained in laboratory air with a Si
3
N
4
ball as a sliding counterpart [119].
704 Chapter 14
14.4.4 Nanostructured Diamond and Diamond-Like Carbon Films
Diamond and DLC films have attracted tremendous interest during the past three decades
mainly because of their unusual mechanical, tribological, thermal, optical, and electrical
properties. Both coatings are effective in reducing sliding friction and wear of machine
elements and, hence, are used as solid lubricants in many industrial applications. Diamond is
crystalline and represents the hardest known material, while DLC is amorphous but combines
some of the many attractive properties of diamond. Besides diamond and DLC, there exist
several other kinds of carbon-based materials (graphite, graphite fluoride, glassy carbon, and
carbon–carbon or carbon–graphite composites) that also provide very impressive tribological
properties in a wide range of applications, including seal materials for the rotating-equipment
industry. The high-friction carbon–carbon composites are used in the making of
high-performance brakes for racing cars and various aircraft.
14.4.4.1 Nanocrystalline Diamond Films
Diamond is the hardest natural material known and hence offers huge possibilities for
combating friction and wear under some of the harshest application conditions. Reported
friction coefficients for diamond are generally below 0.1, and its wear resistance against many
engineering materials is superb. At elevated temperature, it may oxidize and undergo oxidative
wear, especially during sliding against ferrous-based materials. The high chemical inertness of
diamond ensures extreme resistance to corrosive attacks in highly acidic media. Because bulk
diamond is very expensive, it cannot be used as a tribomaterial in many engineering
applications. However, for the past 30 years, synthetic diamond films have been produced on
appropriate tribological surfaces by CVD methods [122, 123]. These films are highly
crystalline and possess many of the attractive properties of natural diamond. One major
drawback in applications involving tribology has been their relatively rough surface finish,
which can cause high friction and wear. During the last two decades, great strides have been
made in controlling surface roughness of diamond films by effective polishing or by reducing
the grain size to the nanoscale. Polishing of superhard diamond has proved to be difficult and
expensive, while producing diamond films with nanoscale grains is much easier and is now a
common practice for applications involving friction and wear. In one of the methods,
appropriate mixtures of Ar–C
60
and Ar–CH
4
were used as the main precursors in a microwave
CVD process, and high-quality nanocrystalline diamond films with a smooth surface finish
were produced by Gruen [46]. The grain size of these films is in the range of 2–5 nm (Figure
14.11), and their surface finish is 10–20 nm (Roughness Measurement System (RMS)). The
friction coefficients of such films are in the range of 0.02–0.05 in open air, mainly because of
their smooth surface finish. A rough or microcrystalline diamond film would have provided
friction coefficients of more than 0.2 under the same test conditions [124–126].
Nanocrystalline diamond films are currently used in machining, sliding or rotating-bearing
applications (such as mechanical seals), microelectromechanical systems, atomic force
microscopy tips, and other engineering applications.
Nanocomposite Coatings for Severe Applications 705
Figure 14.11: TEM image of nanocrystalline diamond film (left inset shows details of nanograins
and grain boundaries; right inset shows grain size distribution) [72] (Courtesy of CRC
Publications).
14.4.4.2 Nanostructured Carbide-Derived Carbon Coatings
Another interesting carbon film that emerged in recent years is carbide-derived carbon (CDC).
It is produced at atmospheric pressures in a tube furnace by simply passing chlorine or a
mixture of chlorine plus hydrogen gases over the carbide-based materials at elevated
temperatures (600–1100
◦
C). Chlorine leaches out the metallic part and leaves the carbon
behind as a coating. For example, in the case of SiC, Si reacts with chlorine and leaves the
reaction chamber as SiCl
4
gas, and the carbon left behind consolidates and becomes a solid
film [127–130]. For the production of CDC, one does not need a high vacuum or plasma
deposition system. As shown in Figure 14.12, the CDC film is truly nanostructured and
composed of nanocrystalline diamond and graphite phases, carbon nano-onions, and
amorphous carbon [127, 128].
Like diamond and DLC, CDC is an excellent tribological material. It provides low friction and
wear coefficients under a wide range of sliding conditions. Figure 14.13 compares the friction
coefficient of CDC with that of SiC. As evident, CDC provides three to four times lower
706 Chapter 14
Figure 14.12: High-resolution TEM photomicrograph of nanostructured CDC film on SiC
substrate. The sample was treated for 24 hours at 1000
C in Ar/3.5% Cl
2
[127] (Courtesy of
Nature).
friction coefficient than SiC. By subjecting CDC to hydrogen plasma treatment, much lower
friction coefficients (in the range of 0.02–0.05) have been achieved, and such dramatic
reductions in friction have been attributed to the hydrogen termination of the dangling -bonds
of surface carbon atoms.
Figure 14.13: Friction performance of CDC against Si
3
N
4
ball in humid air and dry nitrogen. For
comparison, the friction coefficient of SiC is also provided [72] (Courtesy of CRC Publications).
Nanocomposite Coatings for Severe Applications 707
14.4.4.3 Nanocomposite DLC Films
Structurally, DLC films are amorphous and made of sp
2
- and sp
3
-hybridized carbon atoms.
They can be alloyed with additional elements without changing their amorphous structures.
For example, hydrogen and nitrogen atoms can be introduced in large quantities without
causing any structural transformations. Those with large amounts of hydrogen are called
‘hydrogenated’ DLCs, while those containing nitrogen are known as carbon nitride films. The
DLC films with a high proportion of sp
3
-bonding are superhard and hence resistant to wear. In
contrast, DLC films with high amounts of sp
2
-bonded carbon are soft and hence may have
relatively poor frictional behavior. Almost all of the DLC films available today provide very
low friction and wear coefficients in open air. Test environments and conditions can have
significant effects on the friction and wear of DLC films. Some DLC films work best in inert or
vacuum-like environments, while others operate best in the presence of humidity or other
gaseous species. To further improve the tribological functionality, researchers have been
introducing additional crystalline phases into these films. Such nanocomposite films are
thought to provide even lower friction and wear and significantly higher load-carrying
capacity. These DLC films are used in tribological applications that range from magnetic hard
disks to various automotive parts and components.
Multilayer DLC coatings have been studied extensively by several investigators for their
mechanical and tribological properties. In particular, films of C/W and C/Cr multilayers have
attracted the most attention [50, 66, 131]. These coatings have been produced on steel
substrates and evaluated for their tribological properties under dry- and lubricated–sliding
conditions [26, 132, 133]. The results indicated that these coatings indeed possess superior
load-carrying capacity and wear resistance not only in laboratory tests but also in actual
applications.
Some of the nanostructured carbon films consisted of nanoscale carbide and other crystalline
phases that are evenly dispersed or embedded into the amorphous carbon matrix. Figure 14.3
shows nanolayered and nanocomposite carbon films that are currently used for tribological
applications. As another example, a nanocomposite TiC/DLC system is shown in Figure
14.14(a) [14]. Owing to a large network of grain boundaries between the nanocrystalline TiC
phase and the amorphous DLC matrix, such a film is able to effectively limit crack initiation.
Even if a crack is present or has initiated, grain boundaries effectively deflect or terminate
further crack growth and film delamination. Figure 14.14(b) shows how such a composite film
behaves under severe deformation conditions. Apparently, encapsulation of 3–10 nm sized
grains of hard crystalline TiC within the amorphous DLC matrix restricts dislocation activity,
diverts and arrests macrocrack development, and maintains a high level of hardness (32 GPa)
similar to that of other superhard coatings. Overall, the combination of the nanocrystalline TiC
and amorphous DLC phases with a functionally graded interface provides excellent toughness
and high resistance to adhesive failure under severe tribological situations. There are numerous
708 Chapter 14
Figure 14.14: (a) TEM image and electron diffr action of TiC/DLC nanocomposite coating, and
(b) TEM image of plastic deformation obtained by scratching a diamond needle of 0.2 mm radius
under 50 N. Note that the coating remains intact on the surface and does not show any evidence
of delamination [14].
other tribological coatings with nanolayered or composite architectures. Tribological test
results from them further confirm that these films are better able to resist wear and provide
much lower friction coefficients in dry, humid, and vacuum environments, as well as in oils,
compared with the monolithic or single-phase DLC coatings.
14.5 Summary
In this chapter, we attempted to summarize key recent developments in the production and use
of nanostructured and composite coatings and their tribological properties. Structurally, these
coatings are unique and may consist of two or more phases in a nanolayered or evenly
distributed nanocomposite fashion. At present, several PVD and CVD techniques are available
for the deposition of nanocomposite coatings on all kinds of substrates. In particular, with the
cathodic arc and HIPIMS methods, formation of multiphase and hence multifunctional
coatings has become much easier. Reported laboratory and field test results show that unlike
conventional coatings, nanostructured and composite coatings provide far superior friction and
wear properties even under very harsh tribological conditions, including high contact loads
and elevated temperatures in oxidizing environments. Some of the nanocomposite coatings are
superhard and able to extend the wear life of metal-cutting and metal-forming tools
significantly. The selection of grain-boundary phases and their thickness seems to control the
hardness, toughness, and other mechanical properties of these films. In the case of multilayer
coatings, the thickness of the layers and their chemical compositions seems to play important
roles in their mechanical and tribological properties. These coatings can be designed to provide
self-lubricating properties regardless of the test environments and conditions with a surface
Nanocomposite Coatings for Severe Applications 709
self-adaptation capability to cope with high mechanical loads and contact speeds, as well as
variable humidity and elevated temperatures in oxidizing environments. With the selection of
optimal coating ingredients and/or phases, the effects of test environments and/or conditions
on friction and wear may be reduced. Nanocrystalline diamond and nanocomposite CDC and
DLC films are some of the prime examples that can provide self-lubricating capability with
friction coefficients below 0.1, even under very harsh sliding conditions. Overall,
nanostructured and nanocomposite coatings hold great promise for applications involving
severe operating conditions. In particular, these coatings can be ideal for transportation,
aerospace, and manufacturing applications. Because of their superior mechanical properties
and high chemical and structural stability, nanocomposite coatings can significantly lower
friction and wear losses under rolling, rotating, sliding, cutting, grinding, chopping, drilling,
milling, and forming operations that exist in numerous industrial sectors. They can also
provide significant energy, environmental, and economic benefits in such operations.
Acknowledgments
This work was supported by the US Department of Energy, Office of Energy Efficiency and
Renewable Energy, Freedom Car and Vehicle Technologies Program, under Contract No.
DE-AC02-06CH11357. Additional support is provided in part by the Air Force Office of
Scientific Research (Grant No. FA9550-08-1-0010).
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