# Gas Foil Bearing Technology Enhanced with Smart Materials
## 1 Introduction  
Gas Foil Bearings (GFBs) are one of the types of fluid film bearings (also known as slide bearings) which are specifically dedicated to support lightly-loaded shafts and rotors in small-size machines operating at rotational speeds up to several hundred thousand rpm [1, 2, 3]. In GFBs, a gaseous medium (preferably air) is employed as a lubricant keeping required clearance between the surfaces of moving parts. Figure 1 visualizes the cross-sectional view of a GFB. A shaft in a GFB is elevated over the inner surface of the bearing's bushing due to hydrodynamic effect. Specifically, while GFBs' run-up, a wedge-shaped air film is developed between the moving and stationary parts (at the so-called lift-off speed) due to their relative velocity (see Figure 1 for reference, where the hydrodynamic pressure profile is also marked). The resultant elasto-damping properties of a GFB, however, depend on both structural and fluidic parts of the shaft's supporting layer. In a bump type foil bearing its structural part is created with a set of two specialized thin foils made of superalloy, e.g., Inconel. They are known as top and bump foils. The former foil is made as a rolled sheet of metal to provide a proper mechanical interface between the rotating shaft and the remaining parts of the bearing. Moreover, its inner surface is covered by a special anti-friction material to reduce the undesired effects of a dry friction that is observed during run-up and spin-out. The bump foils, in turn, construct a three-dimensional structure to assure required elasticity that is complimentary to the air film. GFBs exhibit interesting characteristics [4]. First of all, they are technically maintenance-free devices. As previously mentioned, the air may be used to lubricate the bearing. Hence, the operating GFB may consume this medium without limits taking it from the surroundings. The used lubricant is also clean and does not degrade over time. Effectively, its replacement is not an issue as for the liquid based solutions. Moreover, a gaseous medium transported via compressor may also become a lubricant in the installed bearings. It is also worth noting that the energy losses, due to the friction forces, are lower in the case of GFBs compared to their oil and grease counterparts. GFBs also make use of gas for cooling down the bearing's installation. GFBs resist harsh operational conditions, especially in terms of thermal properties [5]. The bearings may work at high temperatures, i.e., at several hundred degrees Celsius. This advantageous capability results from the fact that all GFBs' components are made of metallic materials. Nevertheless, it should be emphasized that excessive temperature gradients may expose the bearing to the high risk of a damage. However, this issue is discussed in detail in Section 2. Moreover, keeping the operational conditions (primarily the load and rotational speed) within the allowed limits assures continuous generation of the hydrodynamic pressure profile required to maintain correct position of the shaft's journal. Finally, the construction of a GFB is quite straightforward. Its mechanical components can be easily manufactured using metallic solids and foils. The only inconvenience, which is of practical concern that has been also experienced by the authors of the present work, may be access to retail quantities of the semi-finished superalloy components. Besides, keeping desired material properties may be also a challenge. Hence, only trustworthy suppliers should be considered to assure the purchase of high quality products. Having made the use of various advantages of GFBs, the following applications of the mentioned bearings are found to be successful [4; 6; 7; 8; 9; 10]: compressors, turbocharges, blowers, microturbines, motors, air cycle machines, auxiliary power units, aeronautical and medical equipment, installations for food and water treatment.