International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
Tribological behaviour of W-S-C coated ceramics in a vacuum environment
125
2022
K. Simonivic
T. Vitu
A. Cammarata
A. Cavaleiro
T. Polcar
ict2310289
23rd International Colloquium Tribology - January 2022 289 Tribological behaviour of W-S-C coated ceramics in a vacuum environment K. Simonovic Czech Technical University in Prague, Faculty of Electrical Engineering, Advanced Materials Group, Prague, Czech Republic Corresponding author: kosta.simonovic@fel.cvut.cz, simonovic.kosta@gmail.com T. Vitu Czech Technical University in Prague, Faculty of Electrical Engineering, Advanced Materials Group, Prague, Czech Republic A. Cammarata Czech Technical University in Prague, Faculty of Electrical Engineering, Advanced Materials Group, Prague, Czech Republic A. Cavaleiro SEG-CEMUC, Department of Mechanical Engineering, University of Coimbra, Portugal T. Polcar Czech Technical University in Prague, Faculty of Electrical Engineering, Advanced Materials Group, Prague, Czech Republic 1. Introduction In extreme environments such as high vacuum (pressure below ), extremely low or high temperatures (below 200 K and above 500 K), or those with high level of radiation exposure (dose equivalent above ), liquid lubricants and greases become ineffective and solid lubricants remain the only possible choice for reducing friction. From the application point of view, they are characterised by their low weight, making equipment lighter and cheaper; moreover, their long service life is advantageous for locations where service access is difficult or impossible [1]. Transition metal dichalcogenides (TMD) are a group of elements known for their excellent self-lubricating properties. The low friction related to TMD originates from the high anisotropy of the hexagonal crystal structure, which combines strong intra-planar bonds between chalcogenides and metal atoms, and weak bonding between the adjacent metal-chalcogenide layers. These easily breakable interlayer bonds facilitate sliding, and are responsible for the low coefficient of friction, while the strong intra-planar bonds give durability to the material [2]. In tribological applications, TMDs are used either as lubricant additives [3] or as solid coatings [4]. The unique frictional properties of TMD-based coatings are due to the formation of a thin tribolayer composed of pure crystalline TMD with basal planes oriented parallel to the sliding direction. The formation of such a tribolayer depends on contact conditions, where the contact pressure is the critical factor [5, 6]. However, tests under vacuum are often limited to single load investigations. Therefore, we investigated the tribological behaviour of magnetron sputtered W-S-C coatings over a range of applied loads, followed by detailed physicochemical surface characterisation. As the testing was done in a vacuum environment, we can exclude external contamination and better understand the factors underpinning the measured tribological properties (i.e., friction and wear). Moreover, considering TMD-based solid lubricant coatings are primarily used as lubricants for high-accuracy moving components and advanced controlling mechanisms in the aerospace industry, we decided to use lightweight ceramic substrates instead of steel or other high-density materials. 2. Experimental Tribological tests were performed using multiple loads (2 - 18 N), whereby the load was increased by 2 N in each subsequent experiment. Overall, nine individual loads were applied, and each test was repeated three times to ensure the repeatability of the results. Counter body for all the tests was 100Cr6 ball (∅ = 6 mm). Sliding speed and number of cycles were kept constant at 0.05 and 5000, respectively. The working pressure of the tribometer was . Each tribological test was followed by detailed Raman and SEM analysis. Moreover, obtained Raman spectra were complemented with Density Functional Theory calculations. 290 23rd International Colloquium Tribology - January 2022 Tribological behaviour of W-S-C coated ceramics in a vacuum environment 3. Results The coefficient of friction (COF), calculated from the last 30% of sliding cycles (1500 cycles), is shown in Figure 1. The coefficient of friction (Figure 3) does not follow a simple trend as a function of load. Specifically, there are two distinct regions: 1) up to 10 N of normal load, at which the coefficient of friction steadily decreases with the increase of load, with a sudden drop at 8 N, and 2) above 10 N of normal load at which the coefficient of friction is significantly higher. The wear volume and wear rate of the coated rings are presented in Figure 2. Note that the sliding distances varied over the loads as the number of cycles was kept constant. Figure 1: Average values of the coefficient of friction Two wear regimes are immediately distinguishable. Up to 10 N, the wear volume is almost constant, and, consequently, the wear rate steadily decreases. Once the load is higher than 10 N, we see an increase of 68% in the wear volume, indicating that wear behaviour enters a different regime. Again, the wear volume in this load region is almost independent of load. Moreover, SEM images (not shown) reveal that up to 10 N wear is polishing, while above 10 N it transitions to abrasive. Figure 2: Wear volume and wear rate of the disc for the investigated loads. Dashed lines are a guide for the eye. 3.1 Raman analysis summary Spectra taken from the wear scar of the ball are shown in figure 3. Following deconvolution of the spectra, and analysis (not shown), it was found that: 1) Compared to the as-deposited surface, the G peak position is significantly shifted upward (10 - 25 cm -1 ). However, until 8 N of normal load the upward shift of the G peak is increasing with the load. For loads above 8 N, the G peak position returns to lower values; 2) the D peak is shifted downwards by 10 - 30 cm -1 (compared to the as-deposited surface), with the highest shifts observed for loads of 8 N and 10 N; 3) the Full-Width-Half-Maximum (FWHM) of the G peak remained constant, and 4) the I D / I G intensity ratio was significantly lower compared to the as-deposited surface and reached a minimum value of 0.83 at 8 N. 23rd International Colloquium Tribology - January 2022 291 Tribological behaviour of W-S-C coated ceramics in a vacuum environment Figure 3: Raman spectra taken from the centre of the wear scar. 4. Conclusion Trough complex Raman analysis we have been able to understand mechanism that are behind measured friction and wear data. Moreover, we have supported our results with DFT calculation linking theory, and macroscopic tribology. Finally, we have differentiated individual roles of the coating components. References [1] Miyoshi K. Solid Lubricants. In: Wang QJ, Chung Y-W, editors. Encyclopedia of Tribology. Boston, MA: Springer US; 2013. p. 3159-65. [2] Pimentel JV, Danek M, Polcar T, Cavaleiro A. Effect of rough surface patterning on the tribology of W-S-C-Cr self-lubricant coatings. Tribology International. 2014; 69: 77-83. [3] Totolin V, Rodríguez Ripoll M, Jech M, Podgornik B. Enhanced tribological performance of tungsten carbide functionalized surfaces via in-situ formation of low-friction tribofilms. Tribology International. 2016; 94: 269-78. [4] Scharf TW, Prasad SV. Solid lubricants: a review. Journal of Materials Science. 2012; 48: 511-31. [5] Vitu T, Huminiuc T, Doll G, Bousser E, Matthews A, Polcar T. Tribological properties of Mo-S-C coating deposited by pulsed d.c. magnetron sputtering. Wear. 2021; 480-481: 203939. [6] Vuchkov T, Evaristo M, Yaqub TB, Polcar T, Cavaleiro A. Synthesis, microstructure and mechanical properties of W-S-C self-lubricant thin films deposited by magnetron sputtering. Tribology International. 2020; 150: 106363.
