## 1 Introduction
Liquid-lubricated foil bearings, like aerodynamic gas bearings, do not need an external supply because they use fluids from their surroundings. An example of such bearings is the liquid hydrogen-lubricated foil leaf bearings presented in the work [30]. They were developed for a cryogenic turbopump to increase its service life, where they replaced the rolling bearings previously used. The conducted research confirmed the high load capacity and low frictional losses, as well as the excellent dynamic properties of the rotor supported by such bearings. The tested foil bearings also showed no measurable wear after 130 start/stop cycles. Examples of leaf-type foil bearings lubricated with liquid hydrogen and oxygen can also be found in work [31]. The advantages of using bump-type foil bearings in cryogenic turbopumps are presented in the work [32]. The authors of this publication demonstrated that the use of a lubricating medium in liquid form in foil bearings allows a threefold increase in load capacity while maintaining very good dynamic properties even at high rotational speeds. On the other hand, the work [33] presents a method of modelling and analysing leaf-type  
Figure 1: The bump-type foil bearing design. foil bearings lubricated with liquid nitrogen, which uses the finite element method and the Navier-Stokes equations, considering the properties of the actual lubricating medium. The work [34] presents a numerical model of a foil bearing lubricated with a gaseous and liquid low-viscosity lubricant. As advantages of such a solution, the authors pointed out the low frictional losses (the possibility of improving the efficiency of the machine), the elimination of dynamic seals, the reduction of the size of the bearing and the simplification of the design and the possibility of encapsulating the entire machine. As an example of using foil bearings lubricated with a low-boiling liquid medium, they presented the rotor of an energy microturbine. Modelling of foil bearings lubricated with liquid and gaseous refrigerant is also discussed in the work [35]. The proposed model describes the vapour/liquid transition and local thermal approach and makes it possible to determine the three-dimensional temperature distribution within the lubrication gap. The results obtained confirm the broad applicability of foil bearings, also in turbomachines used in refrigeration. Foil bearings can also be lubricated with water. [PERSON] et al. published an experimental study on a prototype of this bearing type [36]. Utilising water as a lubricant for foil bearings enables the development of environmentally friendly machines such as hydroelectric turbines and water pumps. The initial findings from the research on water-lubricated foil bearings demonstrated an exceptionally low coefficient of friction (approximately 0.01) and a load capacity of about 300 kPa [36]. The examples of foil bearing design solutions presented above and the utilisation of diverse lubricants contribute to the multidirectional development of this bearing technology. A comprehensive wide state-of-the-art overview of the development of foil bearing technology can be found in several publications [7; 37]. Various modelling methods [18; 19; 23; 24] and experimental research [22; 23; 24; 26] of foil bearings are described in available publications. Although the dynamic properties of rotating systems can be predicted quite accurately using advanced foil bearing modelling techniques [19; 23; 38], numerical models cannot accurately assess tribological properties and bearing life. In this regard, experimental research is required. This article describes an experimental study aimed at determining the thermal properties and wear behaviour of foil bearings made of various materials, depending on bearing geometry, surface roughness, lubrication and cooling conditions. For this purpose, start/stop tests were performed according to a well-defined procedure.