Transfer layer evolution during
friction in W-C:H coatings
The case study focuses on the development of nanocomposite W-C:Hcoatings with high hardness and simultaneously, with a lower coefficient offriction. We studied the W-C:H coating system with different hydrogenatedcarbon matrix content obtained during a hybrid PVD-PECVD process fromhydrocarbons (mostly acetylene) added into sputtering atmosphere. The
coatings were prepared using three different PVD techniques (DC magnetron
sputtering, HiPIMS, HiTUS) and the contents of hydrogenated carbon werecontrolled via different additions of hydrocarbon gas.
Since the friction behavior of the coatings studied herein is controlled by theformation of a transfer layer on the ball, while the focus of our earlier works wasto study the basic relationships between hardness and coefficients of friction,
the focus turned to the investigation of the evolution of the transfer layer itself.

The authors of this case study
are Frantisek Lofaj, Lenka
Kvetkova and Margita Kabatova.
They work at the IMR which ispart of the Slovak Academy ofSciences and it was founded in1955. It is focused on the studyof advanced metallic (mainlysteel) and ceramic materials,
powder metallurgy materials
and biomaterials with strongemphasis on mechanical andfunctional properties.

It is important to note that that the humidity is a very important factor in
friction in hydrogenated carbon (based) coatings. Several W-C:H coatings
with different amounts of carbon and hydrogen were tested in humid air andin flowing nitrogen with reduced humidity. Up to now, conventional optical
microscopy, SEM/EDS, SEM/FIB (Figure 1) and Raman spectroscopy wereused to evaluate different aspects of the transfer layer formation. However,
we have introduced the information obtained with the 3D optical profiler,
Plu neox, which provides further qualitative and quantitative information
about the transfer layer within the whole contact area.
CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGS

1

Figure 1. SEM (and light microscopy) micrographs of the transfer layers formed on
the worn ball cap during friction with various W-C:H coatings and under different
conditions.

This approach is not absolutely new, already in 2003 Scharf and Singer[Tribol. Letters, vol. 14, No1 and No2] applied Raman combined withoptical profilometry in 2D to evaluate thickness of the transfer layer in a-C:Hnanocomposite coatings. To the best of our knowledge, we are not aware ofany study where Confocal 3D optical profilometry is applied in nanocomposite
W-C:H coatings and in such an extensive range.

■ Measurements
On a steel bearing ball with a diameter of 6 mm, a worn ball cap with the transferlayer is formed during friction and strongly affects the resulting coefficient offriction. The diameter of the worn cap is around 200-400 μm and the transfer
layer covers only some parts of this contact area: a thick and dense but heavily
cracked layer develops along the leading edge while a much thinner transfer
layer occurs in the central part of the contact area.
The 3D optical profiler Plu neox in Confocal mode and using a 20X objective
magnification will be used to compute:
1.		 The amount of material of the ball lost due to wear.
2.		 The thickness of the transfer layer on the worn ball cap and its distribution
over the contact area. Based on this information, the correlations betweenthe deposition conditions, composition and coating structure related tofriction/wear resistance and transfer layer can be established.
2CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGSTo obtain this data, we took 3D topographies of the initial ball, the worn ball cap
after testing (some examples are shown in figures 2, 3 and 4) with and without
the transfer layer and subtracted them from each other. The 3D distributionof the thickness of transfer layer can be obtained as the difference between
the last two images whereas the worn volume of the ball as the difference
between the initial ball and of the ball without the transfer layer.

Figure 2. 3D topography acquired using Confocal technique(2D and 3D view). Transfer layers evolved on the worn ballcap of a steel ball during friction against W-C:H coating inhumid air and flowing nitrogen atmospheres.

The Plu neox is intended to measure the volumetric thickness of the transfer
layer on the worn ball cap and its distribution over the whole contact area.
This provides important information about the role of the transfer layer in theimprovement of the friction performance of W-C:H coatings and subsequent
optimization of the coatings structure and composition to obtain the best
tribological performance in different environments.

Figure 3. 3D topographies of the same tribological test applied in Humid air (a) andflowing nitrogen (b) maintaining the same W-C:H coating conditions.
CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGS

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Figure 4. Topography of the scratched area in the contact zone with the transfer layer

filling the grooves; the groove profiles were also revealed by a FIB cross section.

■ Results
It has been measured the evolution of the transfer layer and steel ball wear
in the studied W-C coating/steel ball system during friction test up to apredetermined length. Results are shown in the next table:

4Sliding distance/COF(m/-)

Transfer layer volume(µm3)

Avg. transfer layer
formation rate (µm3/N.m)

Worn volume of theball (µm3)

Average wear rate ofthe ball (µm3/N.m)

30 / 0.68

11646

1 553

24034

3205

150 / 0.79

10580

288

89281

2381

500 / 0.58

9238

74

52582

421

2000 / 0.75

15100

30283975

568

CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGS■ Conclusions
The main goal, the 3D visualization of the transfer layer formed during frictionof W-C:H coatings, was achieved. The treatment of the surface topographies of
the contact areas with and without the transfer layer allowed us to subtract thecontribution of the volume of the ball lost due to its wear and to quantify only
the volume of a transfer layer. Thus, the key data for the understanding of thecorrelations between the formation of transfer layer and friction performanceof W-C:H coatings can be obtained.
We used the Plu neox in Confocal mode using a 20X objective to investigate
the topography of the contact surfaces after friction tests between a steel balland W-C:H coatings. This combination provided sufficient resolution within
one shot. In limited cases, a 150X objective and stitching of the images applied
to obtain higher details may be required. However, for general purposes andconsistent conditions, a 20X objective was sufficient.
The contact surfaces were observed by conventional light microscopy inbright field and in DIC modes as well as in SEM, both in SE and BSE modes.
Both techniques provided valuable information, but the quantitative
information concerning volume of the studied layers was not available. The 3D
optical profiler was able to provide additional information about topography,
including not only 3D images with extensive possibilities to manipulate theirgraphics but also to obtain line profiles, volumetric and surface data (besides
data from linear measurements, available also from light and scanning electron
microscopy). Thus, confocal microscopy became a complimentary techniquewith the importance comparable to that of well-established ones such SEM
and light microscopy. Moreover, the additional quantitative information
obtained at the same time makes 3D optical profilometry more advantageousthan the conventional light microscopy, especially when uneven surfaces
must be imagined. Thus, confocal microscopy seems to be the best technique
for the purpose of our study.
With Sensofar Plu neox, the required information was obtained easily. We use it
often in our laboratory for the observation of contact surfaces after tribologicaltests because of its ease and speed, good graphic user interface and its
quantitative outputs. It also became the preferred choice over conventional
contact profilometers that were often applied for the same purpose in thepast.

CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGS

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■ References
There are no direct publications describing the above mentioned work ontransfer layers (the work is still ongoing) but it has been part of longer andmore comprehensive studies with various aspects of structure vs. mechanicalvs. tribological properties of W-C:H coatings which were already reported inthe following studies:1.		 F. Lofaj, P. Hviščová, P. Zubko, D. Németh, M. Kabátová, Mechanical andtribological properties of the High Target Utilization Sputtering W-C
coatings on different substrates, Int.J. Refractory Metals and Hard Materials, 80(2019) 305-314.
2.		 F. Lofaj, M.Kabátová, M. Klich, D. Vaňa, J. Dobrovodský, The comparison ofstructure and properties in DC magnetron sputtered and HiPIMS W-C:H
coatings with different hydrogen content, Ceram. Int., 45 (2019) 9502-9514
3.		 F. Lofaj, M. Kabátová, M. Klich, D. Medveď, V. Girman, Tribological behavior ofhydrogenated W-C/a-C:H coatings deposited by three different sputtering
techniques, Cerâmica, 65 (2019) 58-69.
4.		 F. Lofaj, M. Kabátová, L. Kvetková, J. Dobrovodský, V Girman, Hybrid PVDPECVD W-C:H coatings prepared by different sputtering techniques: The
comparison of deposition processes, composition and properties, Surf. Coat.
Technol. 375 (2019) 839-853.
5.		 F. Lofaj, M. Kabátová, L. Kvetková, J. Dobrovodský, The effects of deposition
conditions on hydrogenation, hardness and elastic modulus of W-C:Hcoatings, J. Eur. Ceram. Soc., 40 (2020) 2721-2730.
6.		 F. Lofaj, M. Kabátová, J. Dobrovodský, G. Cempura, Hydrogenation andhybridization in hard W-C:H coatings prepared by hybrid PVD-PECVD
method with methane and acetylene, Int. J. Refractory Met. Hard Mat., 88(2020) 105211.

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CASE STUDY TRANSFER LAYER EVOLUTION DURING FRICTION IN W-C:H COATINGSSENSOFAR is a leading-edge technology
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