Table Of ContentChapter 1
SMART NANOCOMPOSITE COATINGS WITH
CHAMELEON SURFACE ADAPTATION IN
TRIBOLOGICAL APPLICATIONS
A. A. Voevodin and J. S. Zabinski
Air Force Research Laboratory, Materials and Manufacturing Directorate
Wright Patterson Air Force Base, OH 45433-7750, U.S.A.
Smart nanocomposite tribological coatings were designed to respond to changing
environmental conditions by self-adjustment of their surface properties to maintain
good tribological performance in any environment. These coatings have been
dubbed “chameleon” because of their ability to change their surface chemistry and
structure to avoid wear. The first “chameleon” coatings were made of WC, WS2,
and DLC; these coatings provided superior mechanical toughness and performance
in dry/humid environmental cycling. In order to address temperature variation, the
second generation of “chameleon” coatings were made of yttria stabilized zirconia
(YSZ) in a gold matrix with encapsulated nano-sized reservoirs of MoS2 and DLC.
High temperature lubrication with low melting point glassy ceramic phases was also
explored. All coatings were produced using a combination of laser ablation and
magnetron sputtering. They were thoroughly characterized by various analytical,
mechanical, and tribological methods. Coating toughness was remarkably enhanced
by activation of a grain boundary sliding mechanism. Friction and wear endurance
measurements were performed in controlled humidity air, dry nitrogen, and vacuum
environments, as well as at 500-600 ºC in air. Unique friction and wear
performance in environmental cycling was demonstrated.
Keywords: nanostructured materials, composite, coating, hard, tough, low friction,
wear reduction, environment cycling.
1. INTRODUCTION
Nanocomposite coatings offer a unique opportunity to design and
produce adaptive or smart tribological coatings, which were termed
“chameleon” for their ability to resist friction and wear by changing surface
chemistry and microstructure in response to environment and loading
changes [1,2], much like a chameleon changing its skin color to avoid
predators.
Although quite challenging, practical realization of smart coatings is
extremely rewarding for tribological pairs subjected to multiple
environmental changes, as for example in aerospace applications. The first
1
A. A. Voevodin et al. (eds.),
Nanostructured Thin Films and Nanodispersion Strengthened Coatings, 1–8.
© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
2
realization of such coating growth was made using a mixture of oxides and
dichalcogenides (PbO/MoS2, CFx/WS2, ZnO/WS2), which could operate in a
broad range of temperatures [3-5]. Advanced multilayer structures were then
designed to combine these composites with buried diffusion barrier layers
and achieve surface self-adaptation during repeated temperature cycling.
Recently, novel wear resistant materials were developed, which combine
nanocrystalline carbides (TiC, WC), oxide based ceramics (YSZ and AlON),
dichalcogenides (MoS2, WS2), and amorphous diamond-like carbon (DLC)
into nanocomposite structures [1,2,6].
For example, “chameleon” coatings made of amorphous diamond-like
carbon (DLC) matrix with incorporation of nanocrystalline TiC [7,8], WC
[9,10], WS2 [1,11] and laser processed MoS2 reservoirs [12] had
demonstrated an order of magnitude improvement in toughness above that of
single phase carbides while maintaining the same level of hardness, a low
friction coefficient in cycling from dry to humid environments, and an
extremely long life in both ambient and space environments. The surface
chemistry, structure, and mechanical behavior of these nanocomposite
materials were shown to reversibly change in the tribological contact,
depending on applied loads and operational environment to maintain low
friction and prevent wear.
While keeping the low friction in any environment is important, the
coating wear resistance requires an additional blend of both hardness and
fracture toughness. Nanocomposite coating designs provide a unique
opportunity to achieve the best blend of the toughness, wear resistance, and a
low friction performance across multiple environments.
This paper reviews the most recent developments in smart nanocomposite
tribological coatings, starting with design criteria and examples of tough
tribological nanocomposites and progressing to the design and examples of
“chameleon” coatings.
2. TOUGH NANOCOMPOSITE TRIBOLOGICAL COATINGS
An approach with embedding grains of a hard, high yield strength phase
into a ductile matrix has been widely explored in macro-composites made of
ceramics and metals which are known as cermets [34]. However, if grain
sizes and matrix separations are reduced to a nanometer level, dislocation
activity as a source of ductility is eliminated and new mechanisms are
required to provide toughness enhancement. Uniquely, these
nanocomposites contain a high volume of grain boundaries with a
crystalline/amorphous transition across grain-matrix interfaces, limiting
initial crack sizes and helping to deflect and terminate growing cracks.
These mechanisms helped to explain the fracture resistance of novel super-
hard composites, where both amorphous ceramic [13-15] and metal [16-18]
3
matrices were used to encapsulate nanocrystalline ceramic grains. More on
the super-hard nanocomposite coating design and discussion of the involved
mechanisms can be found in available review articles [14,15,17,19,20].
Grain boundary diffusion
[21] and grain boundary sliding
[21-25] were suggested as
mechanisms for improving
ductility and providing super-
plasticity of single phase
ceramic nanocrystaline
systems. The most recent
research indicates also that high
ductility can be more easily
achieved in multiphase
structures [26] and that grain
Figure 1. Schematic of a tough nanocomposite
boundary sliding is a primary
coating design, combining a nanocrystalline /
mechanism of super-plasticity amorphous structure with a functionally
[27-30]. It was also found that gradient interface.
equiaxial grain shapes, high
angle grain boundaries, low surface energy, and the presence of an
amorphous boundary phase facilitate grain boundary sliding [21,22]. These
findings can be expanded into the field of hard wear resistant coatings to
introduce ductility and prevent fracture under a high contact load.
In the course of the development of tough tribological nanocomposite
coatings the following design concepts were used:
(1) a graded interface layer is applied between the substrate and
crystalline/amorphous composite coating to enhance adhesion strength and
relieve interface stresses (combination of functional gradient and
nanocomposite designs) [31-33];
(2) encapsulation of 3-10 nm sized hard crystalline grains in an
amorphous matrix restricts dislocation activity, diverts and arrests macro-
crack development, and maintains a high level of hardness similar to super-
hard coating designs [14,17];
(3) a large volume fraction of grain boundaries provides ductility through
grain boundary sliding and nano-cracking along grain/matrix interfaces
[8,34,35].
These design concepts are targeting toughness enhancement through
stress minimization, crack deflection, and ductility introduction. They are
close but still differ from the design concepts of superhard nanocomposite
coatings. The primary differences are viewed in the selection of a matrix
phase with a lower elastic modulus, relaxation of the requirement for strong
binding between matrix and grains, and selection from a greater range of
acceptable grain sizes of nanocrystals phase embedded in an amorphous
matrix phase.
4
Combination of the
nanocrystalline/amorphous
designs with a functionally
graded interface is shown in
Figure 1. This design
provides both high cohesive
toughness and high interface
(adhesive) toughness in a
single coating. Several
examples of tough wear
resistant composite coatings
have been produced. Two of
them combined nano-
crystalline carbides with an Figure 2. TEM image of an YSZ/Au
nanocomposite coating with improved toughness
amorphous DLC matrix
characteristics
designated as TiC/DLC and
WC/DLC composites. In another example, nanocrystalline YSZ grains were
encapsulated in a mixed YSZ-Au amorphous matrix as shown in Figure 2.
In all cases, the large fraction of grain boundary phase provided ductility by
activating grain boundary slip and crack termination by nanocrack splitting.
This provided a combination of high hardness and toughness in these
coatings. Figure 3, compares Vickers indentations made at the highest load
of the machine (1 kg). There are no observable cracks in these coatings,
even after significant substrate compliance (indentation marks are 9 Pm deep
into 1 Pm thick coatings). The coating hardness was quite high ranging from
18 to 30 GPa, and for most other typical hard coatings at these loads, cracks
in the corners of the indentations are expected.
Thus, novel nanocomposite designs of tough tribological coatings are
very promising. They explore fundamentally different concepts of the
toughness improvement, when compared to macrocomposite materials such
a) b) c)
Figure 3. Knoop and Vickers indenation marks into the surface of 1 Pm thick (a)
TiC/DLC, (b) WC/DLC, and (c) YSZ/Au tough nanocomposite coatings. Indents were
performed with the maximum available load of 1000 gm, providing about 9 Pm
indentation depth due to the steel substrate deformation. In all cases, there were no cracks
at the indention corners, which serve as stress risers.
5
as cermets. Furthermore, their design can be taken to the next level by
realizing that both matrix and nanograins can serve as reservoirs of solid
lubricants. These lubricants will be then released in a friction contact in the
course of sliding. The active tribological role of matrix and encapsulated
grains is a basis for the tough and low friction “chameleon” coatings, which
are discussed in the next section.
3. “CHAMELEON” TRIBOLOGICAL COATINGS
The objective of “chameleon” tribological coating designs is to provide a
reversible self-adjustment of surface chemistry, structure, and mechanical
behavior in the friction contact, depending on applied loads and operational
environment to maintain low friction and prevent wear.
In order to achieve reversible adaptation, two additional design concepts
were developed and combined with the tough nanocomposite coating
concepts (design concept numbering is continued here from Section 2):
(4) solid lubricant reservoirs are introduced as amorphous or poorly
crystalline inclusions to minimize reduction in composite hardness and
elastic modulus, since crystalline solid lubricants are typically very soft
[1,36];
(5) friction forces and surface reactions with the environment are used to
generate a lubricious transfer film or “skin” at the tribological contact, which
can self-adjust with each environmental change [1,36]; i.e,. coating
components serve as reservoirs to supply material for the “tribo-skin”, where
formation of a lubricating film with the required chemistry and structure
reduces friction.
Figure 4 presents a schematic of a nanocomposite coating design that
exhibits “chameleon”
behavior. This design was
implemented in the
fabrication of the
YSZ/Au/DLC/MoS2 and
WC/DLC/WS “chameleon”
2
coatings where an amorphous
matrix and a hard
nanocrystalline phase (e.g.,
YSZ or WC) were used to
produce optimum mechanical
performance and load support.
Nanocrystaline and
amorphous Au, MoS2, and Figure 4. Schematic of a conceptual design of the
YSZ/Au/MoS2/DLC tribological coating with
DLC were added to achieve
chameleon-like surface adaptive behavior.
chemical and structural
