Surface characterization of
tribological coatings: wear
properties, thickness, androughness
The Institute for Research in Advanced Materials and Mathematics (INAMAT2)
is a multidisciplinary research institute that has common objectives such as
the development, dissemination and transfer of knowledge from various
fields. The materials engineering department has a long experience in studies
related to coatings and the surface improvement of materials.
With the S mart 3D profilometer, it is possible to more precisely measure
the volume loss of the wear tracks after the friction and wear tests, since itmeasures the real shape of the wear track.
Our research focuses on improving tribomechanical properties of DiamondLike Carbon (DLC) coatings, deposited by the novel PVD technique HighPower impulse magnetron sputtering (HiPIMS) with positive pulses on toolsteels. These coatings are of great interest to the industry because of their
extraordinary properties: excellent wear resistance, very low friction coefficient,great hardness, or biocompatibility. The aim of these studies was to improve
the tribomechanical properties, such as, adhesive or wear resistance, of theDLC coatings on different steel substrates.

CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESSThe Institute for Researchin Advanced Materials andMathematics (INAMAT2) fromPublic University of Navarre (UPNA)
is a multidisciplinary research
institute that brings together
researchers from different scientificareas (Mathematics, Physics,
Chemistry and Materials Science).
The study was carried out byAdrián Claver (left) and José
Antonio García Lorente (right).

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With the Sensofar´s 3D optical profiler we expected to better characterize thesurfaces, with greater accuracy in the measurements and higher quality 2Dand 3D images.
Coated and uncoated round and plane samples have been measured. Different
steels and ceramics have been used as substrate and high-quality coatings,
such as DLC, are deposited on them. In these samples we carry out friction
and wear tests with a pin-on-disc tribometer and after that we measure thewear tracks.

Figure 1. (a) Friction and wear tests being carried out on one of the samples. (b) Imageof one of the samples after a friction and wear test.

Figure 2. Sample used to determine the thickness of the coating.

There are many key parameters in studying the tribomechanical propertiesof a surface or a coating: nano-hardness, adhesion between the coating andthe substrate, roughness, thickness of the layers or wear properties. With theS mart profilometer, we can measure the superficial roughness, the thicknessof the coatings and the volume loss after the friction and wear tests, which isnecessary to then calculate the wear coefficient.
In industrial applications wear resistance is a very important property since
it can determine the correct performance and provide a longer life of thematerials. Determining this value is not easy, since many factors intervene:
there are changes in the wear mechanisms and both the pin and the discsuffer wear.

2CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESSThe characterization of the wear is not so simple, and with the Sensofar´sprofilometer we are able to achieve the most realistic representation of thewear suffered during the friction in wear tests and to better measure volumeloss.

■ Measurements
VOLUME MEASUREMENT
For the friction and wear tests we used 6 mm diameter alumina balls, with asurface maximum roughness of Ramax 0.050 µm and a hardness of around
1650 HV, and different samples of coated and uncoated tool steels as the disks.
In the tests, the disks rotate against the pin under the following conditions: 40
N load, 200 rpm and 20,000 cycles, and the tests were repeated three times
at 8, 10 and 12 mm (track radii) for each sample. As a result of this test, a wear
track appears from which the volume loss is measured to determine the wear
coefficient.
The volume loss, measured to later calculate the wear resistance, was measured
using Confocal technology. The objectives were chosen depending on thewidth of the wear tracks, using the 5X and 10X ones for the uncoated samples,
where the widest wear tracks were observed, and the 20X and 50X ones forthe coated samples that presented the narrower tracks.

Figure 3. Measurements of the volume loss of the coating acquired by the S mart 3D

optical profilometer (10X EPI).

CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESS3Figure 4. Image of the wear track of a (a) WC-C and (b) ta-C coated sample acquired by

Confocal profilometer (10X brightfield objective).

The wear coefficient was determined in two ways to achieve the most accurate
value; following the standard ASTM G99 and straight from the Confocal
measures of volume loss. When Confocal profiler measurements are useddirectly, the measured volume loss value is extrapolated to the entire wear
track and with that volume loss value the wear coefficient is calculated.

Figure 5. Comparative graph of the wear coefficient values obtained for each sample

measured by standard ASTM G99 (orange plot) and Confocal (blue plot).

The main goal, to characterize the wear resistance of the coatings in the mostaccurate way, was achieved with the S mart 3D profilometer. Thanks to thefact that this profilometer measures the volume loss after the friction and wear
tests taking into account the real shape of the wear track, it can be said that4

CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESS

the wear coefficient values obtained by Confocal are lower and more realistic
than those calculated following the standard ASTM G99.
THICKNESS AND ROUGHNESS MEASUREMENT
Moreover, it was possible to easily measure the superficial and profile roughnessof the samples, then contrasting the values obtained by using two different
technologies for measuring it, Confocal and Interferometry. In this way, it
was possible to ensure the precision of the roughness values obtained. The
substrate’s roughness can be a determining factor in the adhesion between
the coating and the substrate.

Figure 6. Measurements of the superficial roughness (0.08 mm cut off) of a coated toolsteel sample by Confocal microscopy with the 50X objective.

Figure 7. Measurements of the profile roughness of a coated tools steel sample byinterferometry (10XDI).

CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESS

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Finally, the thickness of the coatings was measured in a fast and simple way. As well
as for roughness, the thickness was measured both with Confocal and Interferometry
in order to corroborate the results. The main advantage of this measurement is thatit was possible to measure the thickness without the need for destructive testing.

Figure 8. Measurements of the thickness of the coating acquired by the S mart 3D

optical profilometer.

For the roughness and thickness measurements both Confocal (50X) andInterferometry (10X) were used. This way we tried to achieve the highest
resolution in depth to obtain the most accurate values.

6CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESS■ Conclusions
There are many parameters that are important to study the tribomechanical propertiesof a surface or a coating such as nano-hardness, adhesion between the coating andthe substrate, roughness, thickness of the layers or wear properties. Sensofar’s opticalprofiler can assess all these properties, as well as being more accurate than othermethods of characterization. Finally, it should be noted that the versatility, speed
and ease of use of the S mart profilometer gives us the option of using different
techniques for corroborating the values obtained.

■ References
1.Claver A, Jiménez-Piqué E, Palacio JF, Almandoz E, Fernández de Ara J, Fernández I,Santiago JA, Barba E, García JA. Comparative Study of Tribomechanical Propertiesof HiPIMS with Positive Pulses DLC Coatings on Different Tools Steels. Coatings.
2021; 11(1):28.

CS23-05A-EN-UPNA

2.García JA, Rivero PJ, Barba E, Fernández I, Santiago JA, Palacio JF, Fuente GG,
Rodríguez RJ. A Comparative Study in the Tribological Behavior of DLC Coatings
Deposited by HiPIMS Technology with Positive Pulses. Metals. 2020; 10(2):174.

CASE STUDY SURFACE CHARACTERIZATION OF TRIBOLOGICAL COATINGS: WEAR PROPERTIES,THICKNESS AND ROUGHNESS7SENSOFAR is a leading-edge technology
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