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Ing. Tomáš Vítů, Ph.D.

2024, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 238 (1), p. 26-43), ISSN 1350-6501

Aerospace actuation gearboxes operate in low-temperature environments where increased lubricant viscosity leads to significant no-load power losses. Replacing fluid lubricants with coatings applied to the gear teeth is one potential approach to improving gearbox efficiency. Here we develop an approach to determining average wear rates of coated gears using a power-recirculating test stand, profile measurements and a model of the tooth contact. Worn gears are inspected using scanning electron imagery, and energy dispersive X-ray and Raman spectroscopy to understand the wear mechanisms and failure modes. Average coefficients of friction are determined at 20°C and −40°C using a power-absorbing test stand and isolation of tooth friction losses by calculation. These methods are then demonstrated on a promising C/Cr composite coating.

2023, Surface and Coatings Technology, 469, ISSN 0257-8972

Laser texturing is a well-established method used to customize the tribological performance of PVD coatings. To achieve this enhancement, two distinct strategies can be employed: laser texturing of the substrates prior to coating deposition, or laser texturing of the already deposited coating. Technological and economic problems of tuning laser parameters for different types of substrates can be significantly eliminated by laser texturing of already coated materials. Nerveless, the effect of structural changes caused by laser ablation on friction performance is not well studied. In this work, we investigated the differences in these two strategies on example of commercially available a-C:H/WC gradient coatings. The XPS, FIB, SEM and EDS results revealed that femtosecond laser texturing causes minor oxidation and graphitization of the a-C:H/WC coatings but only in a few top nanometers. Macroscale dry tribological tests reveal no statistically significant difference in tribological performance between samples manufactured using two different strategies irrespective of pattern type and tribotest parameters. This means femtosecond laser texturing of the coated substrates can be implemented in industry as a rapid, cost-, and environment-friendly strategy.

2022, Surface Engineering, 38 (10-12), p. 939-947), ISSN 0267-0844

For the first time, a dynamic beamshaping technology has been utilized for the efficient production of periodic nanostructures on top of AlTiN coating to enable dry machining without costly and environmentally hazardous cutting fluids. First, a variety of periodic nanostructures with periods in a range of 740–273 nm were produced utilizing different wavelengths. Additionally, beamshaping technology increased productivity by 4008% up to 105 cm2 min−1 by shaping the Gaussian beam into a rectangular beam of 500 × 30 μm. To simulate the application load and resulting heat production during manufacturing, friction analysis was performed at room and elevated temperature to 500°C. The analysis revealed a significant reduction in the friction coefficient – up to 27% and 19% at room temperature and 500°C, respectively. The combination of these results demonstrates that the proposed method can be scaled up for the mass production of functionalized machining tools for dry machining.

2022, Tribology International, 167, ISSN 0301-679X

Sliding of the WSC coated ceramics (ball-on-disc configuration) was investigated under vacuum condition at multiple loads (2–18 N), and each experiment was followed by detailed Raman and SEM analysis of both the wear track and the wear scar (on the ball). Main finding is that under low loads (up to 8 N), wear is polishing, carbon structure becomes more ordered and the number of WS2 monolayer increases. Above 8 N, wear transitions into an abrasive regime interrupting the aforementioned processes increasing both friction and wear. Furthermore, roles of the coating components were differentiated: WS component is responsible for the low friction, whereas the carbon part is responsible for excellent wear properties.

2021, Coatings, 11 (11), ISSN 2079-6412

The coating system MoN-Ag is an interesting candidate for industrial applications as a low friction coating at elevated temperatures, due to the formation of lubricous molybdenum oxides and silver molybdates. Film deposition was performed by high-power impulse magnetron sputtering and direct current magnetron sputtering. To facilitate a future transfer to industry Mo-Ag composite targets have been sputtered in Ar/N2 atmosphere. The chemical composition of the deposited MoN-Ag films has been investigated by wavelength dispersive X-ray spectroscopy. Morphology and crystallographic phases of the films were studied by scanning electron microscopy and X-ray diffraction. To obtain film hardness in relation to Ag content and bias voltage, the instrumented indentation test was applied. Pin-on-disc tribological tests have been performed at room temperature and at high temperature (HT, 450 °C). Samples from HT tests have been analyzed by Raman measurements to identify possible molybdenum oxide and/or silver molybdate phases. At low Ag contents (≤7 at.%), coatings with a hardness of 18–31 GPa could be deposited. Friction coefficients at HT decreased with increasing Ag content. After these tests, Raman measurements revealed the MoO3 phase on all samples and the Ag2Mo4O13 phase for the highest Ag contents (~23–26 at.%).

2021, Journal of Materials Engineering and Performance, 30 (6), p. 4117-4125), ISSN 1059-9495

The present work investigates chemical and structural modifications of molybdenum disulphide thin coatings induced by macroscale friction in both reactive (air) and inert conditions. Chemical and micro-structural modifications were analyzed in detail by Raman spectroscopy. We found no traces of oxide formation even when sliding was performed in the air. However, the formation of a well-adhered tribofilm, related to superior lubricating properties, was detected only in inert conditions. We can conclude that, in ambient condition, it is water physisorption and not oxidation, which impairs good lubrication. At the same time, Raman spectra indicated a re-crystallization effect induced by sliding.

2021, Wear, 480, p. 1-10), ISSN 0043-1648

Transition metal dichalcogenides such as MoS2 are widely used as solid lubricants for vacuum applications. On the other hand, diamond-like carbon coatings exhibit excellent sliding properties in the ambient environment. Our Mo-S-C coatings deposited by pulsed d.c. sputtering combine both structures to obtain stable properties regardless of the testing conditions. The coatings were studied using HR-TEM and Raman spectroscopy, revealing amorphous nature of the coating. The tribological properties were evaluated by pin-on-disc method. The results showed high lubrication ability in all the testing conditions. HR-TEM and Raman spectroscopy were employed to show the structural characteristics of the wear traces. Our results indicate that the low-friction effect should be attributed to carbon structure re-arrangement since expected wear-induced MoS2 formation was not observed.

2020, Journal of Cultural Heritage, 42, p. 1-7), ISSN 1296-2074

Salt degradation is a well-known but still poorly understood problem. Determination of crystallization pressure that growing crystals exert on the pore walls represents a challenge solved by authors from different points of view. Nevertheless, few papers are aimed at the experimental measurement of the crystallization pressure magnitude. A novel high precision device able to detect repulsive forces generated by a crystal at the crystal/glass interface has been designed. Although some problems with determining the correct contact area of the confined crystal surface, which is most probably not atomically smooth, still exist, the results are comparable to data from other experimental studies. Crystallization experiments were performed with sodium chloride under 30 + 2% and 60 + 2% relative humidity (RH) conditions and with sodium sulfate in 30 + 2% RH. The disjoining pressure values were variable but did not exceed 1 MPa. Special interest was aimed at determination of disjoining pressure of sodium sulfate during phase transition after wetting, since this phenomenon creates most damage during standard crystallization tests. The disjoining pressure values were between 0.957 and 3.159 MPa - sufficiently high to overcome the tensile strength of most of the porous building materials.