Mechanical and tribological properties of tungsten carbide sputtered coatings
Release Date£º[2013-8-21 13:50:36]    Total access[643]Times
 

Tungsten Carbide (WC) hard coatings have been obtained on steel substrates by r.f. magnetron sputtering process. Two layercoatings have been deposited in order to improve adhesion on steel. The lower layer was tungsten metal and the upper WC layerwas obtained by reactive sputtering of the tungsten target in Ar and methane gas mixture. AES and SIMS confirmed that the WClayer composition depends on the reactive sputtering gas composition. Film microhardness were measured by microindentation and coating adhesion by microscratch. Measurement results showed that high hardness coatings can be prepared at a relative lowtemperature and that good adhesion on steel is achieved with the two layer coating. Nanowear measurements showed a noticeable dependence of this applied functional property on the coating compositions.

1. Introduction
Tungsten Carbide (WC) thin films have been classically used as protective hard coatings due to their
good mechanical properties [1]: high hardness and corrosion resistance and low wear properties, that are
sustained up to 400 oC [2,3]. Recently, this material hasincreased its technological interest because of its use in
composite coatings WC-C [4-7].Different processes, such as plasma spraying,physical vapor deposition (PVD) and mostly chemicalvapor deposition (CVD) have been used for depositing WC coatings. Typically, CVD processes are carried out at
temperatures above 500 oC, which do not allow the
coatings to be applied on hardened steels without significantly affecting the substrate mechanical properties.
Sputtering deposition is a PVD process that is increasingly used in industrial hard coating deposition
because it is a low temperature process and it uses unpolluting products. In the present work, r.f. magnetron sputtering process has been used to obtain WC/W bilayer coatings on steel substrates. The sputtering was carried out
from a pure W metal target and using pure Ar or Ar/CH4 mixture as sputtering gas. The W intermediate layer was deposited in order to improve the adhesion of the WC film to the steel substrates. The reactive sputtering process we
have used allows obtaining both WC and W layers in a single deposition process. The coatings have been analyzed in their
composition in order to determinethe gas mixture
proportion and the reactive sputtering parameters that allow
the obtention of the optimum WC film. The bilayer
coatings have been characterized in their hardness,
adhesion and wear behavior.
2. Experimental details
The deposition system used was a r.f. magnetron
sputtering working at a rather high sputtering gas pressure
and at a high current density (Table 1).
The sputtering cathode was a 35 mm diameter and 5
mm thick pure tungsten target and was continuously water
cooled. The substrates were located at distance of 5 cm.
The chamber was evacuated to 10-5 mbar before the
deposition by a turbomolecular pump. The sputtering gases
were Ar or Ar/CH4 mixture in the case of reactive
sputtering. The total chamber pressure and the relative gas
fluxes were controlled by separate mass flow controllers.
Coatings were deposited onto carbon steel
substrates, which were previously hardened to 9 GPa,
polished with 1 mm diamond paste, etched in dilute HNO3
and ultrasonically cleaned in alcohol and acetone. The
substrates were mounted on a resistively heated holder with
controlled constant temperature during the deposition
process. The target and the substrates were separately presputtered
in Ar plasma during 10 min. before deposition. In
all the runs, the substrate temperature was fixed at 350 oC.
This temperature was chosen for optimal adhesion and
deposition rates. The W intermediate layers were deposited
Table 1. Sputtering conditions for the deposition of WC/W
coatings
Target pure W metal
Substrates hardened carbon steel
Target-substrate distance 5 cm.
Sputtering gas Ar
Reactive sputtering gas Ar/CH4 mixtures
Substrate temperature 350 oC
Sputtering gas pressure 5x10-2 mbar
Sputtering parameters 500-600V, 150mA, ~80W
r.f. power density 12W/cm2with pure Ar gas sputtering. The WC layers were deposited
with Ar/CH4 gas mixtures. We essayed different increasing
methane concentrations: 7%, 17%, 24% and 31%.
The morphology and thickness of the bilayer
coatings were determined from cross-sectional SEM
micrographs. Samples were cut for observation using a
diamond saw. Compositional Auger analysis was done on
the coatings surface and also deep in the coating bulk by
means of Ar ion depth etching. The homogeneity of the
layers was analyzed by SIMS depth profiling with oxygen
ion etching.
The microhardness of the bilayer coatings was
measured by the dynamical microindentation method with
a Nanotest 550 instrument (MicroMaterials Ltd., UK). The
load-penetration curves were obtained using a Berkovich
diamond indenter and hardness values were deduced by the
Oliver&Pharr analysis method [8]. The adhesion of the
coatings to the steel substrate was evaluated by the
microscratch method using a spherical diamond indenter of
50 mm radius. Nanowear tests were carried out using a
scanning probe microscope (SPM) [9,10] and the AFM
image of the resulting wear marks were obtained for
increasing number of wear cycles.
3. Results and discussion
The W layer is deposited on the steel substrate by
sputtering the W target with pure Ar gas. The process
causes a considerable substrate temperature increase that is
automatically balanced by a reduction of the substrate
holder heating power. After the prefixed deposition time of
1 hour, the Ar gas is substituted by an Ar/CH4 mixture
without any intermediate step. The WC deposition
proceeds during one hour.
From the cross-section SEM micrographs (Fig. 1),
the bilayer coating structure can be observed with a gradual
interface transition. The W lower layer shows a definite
columnar dense structure and the upper WC layer appears
to be dense without any columnar feature. The coating
surface is flat, smooth and scattered with numerous low
height spherical bumps. The deposition rates were obtained
Fig. 1. Cross-section SEM micrograph of a WC/W bilayer coating
on steel.
0.0
0.5
1.0
1.5
2.0
2.5
0 10 20 30 40
Gas composition (%CH4)
Deposition rate (mm/h)
Fig. 2. Deposition rate as a function of the reactive sputtering gas
composition.
from the layer thickness measured on the SEM
micrographs. The pure W deposition process presents the
highest deposition rate (2.2 mm/h). The reactive sputtering
processes have lower deposition rates (Fig. 2) that decrease
to 0.4 mm/h for a 31% of CH4 in the gas mixture.
The composition of the WC layers in atomic percent
was deduced from the surface Auger analysis and also from
the bulk Auger analysis performed in craters etched to the
half of the layer thickness. The bulk compositions have
been represented in figure 3. In all the samples, surface
analysis show a composition richer in carbon than the
respective bulk composition. Samples obtained with
reactive sputtering gas concentrations of 17 and 24% CH4
show a W/C atomic composition approximately 40% in W,
that may correspond to the quasi-stochiometric b-WC1-X
cubic phase. However, the composition of the layer
obtained with the 7% CH4 gas mixture approaches the pure
tungsten and the very thin layer obtained with 31% CH4 is
a carbon rich layer.
SIMS depth profiles show that layer compositions
are homogeneous in depth. In the profile of figure 4, that
corresponds to the coating obtained with 17% CH4, it can
be seen a smooth transition between the WC and W layers.
The sample obtained with the 31% CH4 gas mixture shows
a pronounced increase of carbon content near the film
surface.
Hardness values were measured by
microindentation with a maximum load of 20 mN in order
to avoid interlayer and substrate effects. Hardness
0
20
40
60
80
100
0 10 20 30 40
Gas composition (%CH4)
W/C atomic composition (%)
Fig. 3. W/C atomic composition in the layer as a function of the
reactive sputtering gas composition.
Superficies y Vacío 9, 276-279, Diciembre 1999 ãSociedad Mexicana de Ciencia de Superficies y de Vacío
278
105
103
101
0 10 20 30 40 50 60 70 80
Sputter time (min)
Intensity (c/s)
C
Fe
W
Fig 4. SIMS depth profile of the WC layer for the 17% CH4
coating.
values obtained for the upper layer depend on the
layer composition. Films deposited from 17% and 24%
CH4 gas mixtures showed the highest hardness: 24 and 26
GPa, largely superseding the W layer hardness. The
hardened steel substrate did not change its hardness value
(9 GPa) before and after coating deposition.
The scratch tests were produced on the coatings
starting at 0 load and increasing it at a constant rate of 32.5
mN/s up to a maximum of 2000 mN. Friction forces were
measured during the scratch tests with a tangential force
sensor added to the diamond probe. Figure 5 shows the
friction coefficient vs normal load along the scratch for
both, the WC/W sample deposited with 24% CH4 and an
analogous WC sample deposited without the W interlayer.
In the lower load range, both samples showed a very low
friction coefficient. Beyond a threshold load, the friction
coefficient increases due to the onset of the cohesive failure
of the coatings. An abrupt change of friction coefficient
appears at the critical load, LC, due to the adhesive failure
of the coatings. The WC/W samples with the W interlayer
showed higher critical loads, LC, than the WC single
coatings.
The wear test was carried out by multiple dry sliding of a
200 nm radius diamond tip on the sample surface. The scan
area was 3 mm x 3 mm and the constant applied load was 10
mN. Figure 6a plots the depth of the wear marks as a
function of the number o4. Conclusions
Magnetron sputtering of a tungsten metal target in
an Ar/CH4 gas mixture proves to be a convenient method
for the deposition of wear-resistant hard coatings on
hardened steel. A WC/W bilayer coating can be obtained in
a single deposition process at a temperature lower than the
hardened steel annealing point. The W intermediate layer
provides a good adhesion of this coating onto the steel and
the WC overlayer provides a great reduction of the wear
rate under dry sliding. A range of coating compositions and
mechanical behaviors can be obtained by varying the
methane concentration in the sputtering gas.
Acknowledgements
J. Esteve and E. Martinez acknowledge the financial
support of the CICYT of Spain Government under contract
MAT96-0552 and the DGR of the Generalitat de
Catalunya. G. Zambrano and P. Prieto acknowledge the
financial support of the CYTED (project VIII.7A).

 

 


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