## 4 Conclusions  
This article focuses on presenting the changes in temperature rise depending on different foil bearing designs and materials, the use of the test chamber cooling system and the different number of cycles. The experimental tests of the foil bearing were carried out in a special measuring chamber in which the conditions prevalent in a vapour microturbine fed with a low-boiling medium were simulated. Multi-cycle research (implementing run-up, constant speed running and run-down phases while maintaining the stability of the operating conditions) provides answers to many very important questions regarding the characteristics of top and bump foils under different load conditions or different states of aggregation of the low-boiling medium that has been used as a lubricant. Research shows that changing bearing clearance and assembly preload significantly affects heat generation and foil wear. A comparison of foil wear results obtained for the same material pair with diameters of 25.9 mm and 26 mm with different surface roughness shows that wear increases as the bearing clearance decreases. The maximum weight loss value of the top foil made of CuAl7 for a bearing housing diameter of 25.9 mm is 0.00226 g, while for a bearing housing diameter of 26 mm, it is 0.00088 g.  
A crucial finding of the conducted study was the demonstration of a very large effect of the sleeve's surface roughness on both the structural and operational characteristics of the entire foil bearing. The increase in roughness (Ra) from 0.532 \(\upmu\)m to 0.699 \(\upmu\)m could have resulted in a significant increase in structural stiffness, consequently resulting in an elevation of the bearing's operating temperature. The obtained results showed that this thread requires further analysis and regular experimental research to thoroughly investigate the effect of the surface roughness of the part of the foil bearing on its characteristics. The proposed cooling system fulfils its role when the temperature in the measuring chamber rises sharply due to the reduced bearing clearance. The temperature in the chamber stabilizes at the level at which the low-boiling medium maintains its state of aggregation. For the bearings with foils made from the material combination 2.0321/CuAl7, the maximum temperature at the end of the test with cooling applied was about 26.3 \({}^{\circ}\)C. Without external cooling, this temperature was 32.9 \({}^{\circ}\)C. In the case of the second material combination (CuAl7/Inconel), the temperature reached a value of 37.7 \({}^{\circ}\)C during the test without cooling. However, with the application of cooling, the maximum temperature dropped below 30 \({}^{\circ}\)C. Therefore, it was possible to carry out the test procedure in a start/stop cycle under quasi-steady conditions in the test chamber. The analysis of the actual friction torque curve of the foil bearings over 200 test cycles showed no significant changes or disturbances. On the one hand, the bearing parts wore out during the tests, which is also the effect of the running-in process. After conducting a series of experiments, it was also found that the 200-cycle tests only partially used the possibilities and research potential of the multi-repetition test. However, it is a good source of new knowledge about the actual properties of foil bearings. Therefore, the tests described in the article will be continued. Conceptualization, W.M. ; methodology, W.M. and B.M. ; software, B.M. and W.K. ; validation, B.M., G.Z. and W.K. ; investigation, B.M., P.B. and W.M. ; resources, W.M. and B.M. ; data curation, B.M., W.K., P.B. and G.Z. ; writing--original draft preparation, B.M., W.M. and G.Z. ; writing--review and editing, G.Z. and W.M. ; visualisation, B.M. and G.Z. ; supervision, W.M. ; project administration, W.M. ; funding acquisition, W.M. All authors have read and agreed to the published version of the manuscript.