Nanocrystalline carbide/amorphous carbon composites
A. A. Voevodin,
a)
S. V. Prasad, and J. S. Zabinski
Wright Laboratory, Materials Directorate, WL/MLBT, Wright-Patterson Air Force Base, Ohio 45433-7750
͑Received 24 January 1997; accepted for publication 11 April 1997͒
Nanocrystalline TiC/amorphous carbon (a-C͒ composite films were synthesized at near room
temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of
titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray
diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated
from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites
encapsulated in a sp
3
bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable
plasticity ͑40% in indentation deformation͒ at loads exceeding their elastic limit. They were also
found to have a high scratch toughness in addition to a low ͑about 0.2͒ friction coefficient. The
combination of hardness and ductility was correlated with film phase composition and structural
analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites
make them beneficial for surface wear and friction protection. © 1997 American Institute of
Physics. ͓S0021-8979͑97͒03114-9͔
I. INTRODUCTION
Nanocomposite films with microstructures comprising of
nanocrystalline grains in an amorphous matrix can produce
unique mechanical and tribological properties. For example,
Vepreck et al.
1,2
developed super-hard ͑50–55 GPa͒ nano-
composites, where plastic deformation was suppressed in or-
der to maximize hardness. Materials with suppressed plastic-
ity can be brittle. Therefore, they are not ideal for
applications where high contact stresses are experienced as,
for example, in bearings, where improved toughness and low
friction coefficient are desirable. Gleiter and co-workers
3,4
have reported an approach to impart room temperature duc-
tility in brittle ceramics by reducing their grain size to na-
nometer levels. More recently, Prasad and Zabinski
5
devel-
oped this concept further to improve tribological properties
of nanocrystalline oxides.
A further development in this direction is the incorpora-
tion of nanocrystalline ceramic grains into a lubricious ma-
trix such as amorphous carbon (a-C͒. Typically, the tech-
niques used for production of nanocrystalline/amorphous
composite films utilize substrate temperature as high as 500–
600 °C to promote crystalline phase formation.
1,2
Such tem-
peratures can considerably limit the choice of substrate ma-
terial and reduce benefits from film application. Moreover, at
these temperatures hard sp
3
bonded a-C phase transforms
into a graphitelike phase with sp
2
bonding. Thus, other
methods for the activation of crystal formation are needed to
produce composite films with a hard a-C matrix.
An alternate method was reported recently, where car-
bide and carbonitride crystallite formation was achieved by
intersection of low energy metal plasma produced by mag-
netron sputtering with high energy plumes produced by
pulsed laser ablation of graphite.
6
This technique was named
magnetron assisted pulsed laser deposition ͑MSPLD͒ and
was used to deposit films of crystalline TiC, TiCN, and
hydrogen-free a-C with diamondlike characteristics. Films of
TiC and a-C were found to be very hard with hardnesses of
27 and 60 GPa, respectively. Additionally, a-C had a low
coefficient of friction ͑typically around 0.1͒ in a variety of
sliding conditions.
7
Preliminary investigations in the Ti–C
system showed a gradual Ti→TiC→a-C transition with in-
creasing carbon content.
8
The studies indicated that even at
100 °C, a two phase region in the TiC→a-C transition was
formed.
8
This initial claim was based on x-ray photoelectron
spectroscopy ͑XPS͒ data of carbon. The formation of this
two phase film at near room temperature with the potential of
having enhanced mechanical properties was very interesting.
This was explored further with the goal of producing nano-
crystalline TiC/a-C composites and results are reported here.
II. EXPERIMENTAL DETAILS
Details of the MSPLD configuration, operation, and pro-
cess control are discussed in Ref. 6. Depositions were per-
formed on metallographically polished 440C steel substrates.
Prior to deposition, the substrates were cleaned in an 1 keV
Ar discharge for 15 min, which raised their temperature to
about 50–80 °C. There was no additional substrate heating
or biasing during deposition. Carbon plumes with energy of
several hundred electron volts were produced by pulsed ex-
cimer laser ͑KrF, 17 ns, 200 mJ͒ ablation of graphite at about
10
9
W/cm
2
and 7 Hz laser pulse frequency. A titanium
plasma flux with energy of several electron volts was pro-
duced with an unbalanced magnetron operating in 0.2 Pa Ar
atmosphere at 12 W/cm
2
. Both carbon and titanium fluxes
were intersected on the substrate surface, which was 60 mm
away from the graphite target and 150 mm away from the
titanium target. Films were grown to a thickness of 0.5
mat
a deposition rate of 10 nm/min.
Film chemistry was analyzed with XPS technique using
a Surface Science Instruments M-Probe. A monochromatic
source of Al K
␣
x rays was used to induce photoelectron
emission. Relative peak areas were compared to calculate
elemental composition, neglecting electron escape depth
variations. Sample surface was cleaned with 4 keV Ar ion
a͒
Electronic mail: voevodaa@ml.wpafb.af.mil
855J. Appl. Phys. 82 (2), 15 July 1997 0021-8979/97/82(2)/855/4/$10.00 © 1997 American Institute of Physics