# Gas Foil Bearing Technology Enhanced with Smart Materials
## 7 Discussion
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline
**Method Used to Enhance the GFBs’ Performance** & **Advantages / Potentials** & **Drawbacks / Inconveniences** \\ \hline
1. GFB equipped with SMAwires substitute pump foils [27] & Straightforward active method to control the bearing’s stiffness via thermal activation—managing heating (making use of electric current) and cooling (with dedicated pipes with fluids) processes; capability of the bearings’ damping control. & Relatively high inertia of the control process due to the nature of the thermally induced mechanical responses of SMAs, e.g., compared to the PZT-based solutions; Requirement of an effective cooling method \\
2. Concept of a GFB equipped with SMA wires installed on a top foil [20, 21] & Straightforward passive method dedicated to reduce mechanical vibrations via energy dissipation available for the phenomenon of superelasticity; experimentally validated SMA wire models for both isothermal and non-isothermal phase transformations dedicated for future use in GFB models \\
3. Application of PZT valves to control radial air injection (active lubrication) [30, 35] & Active method with experimentally proven efficiency; fast operation that allows for quick response of the GFB when subjected to the external radial excitation (sudden shocks); high potential of further investigation; reduced demands regarding manufacturing tolerances; advantageous growth of damping coefficients \\
4. Application of PZT composites—MFC to modify elasto-damping properties of the structural parts of the GFBs’ supporting layer [4, 31, 34] & Active method; capability of precise adjustment of the GFBs’ foils; fast operation; very high precision of positioning the GFBs’ structural parts \\
5. Concept of application of radially oriented PZT stacks to control the GFBs’ clearance/preload [32] & Active method; capability of precise adjustment of the GFBs’ foils; fast operation; very high precision of positioning the GFBs’ structural parts \\
6. Application of circumferentially oriented PZT and flexure-based mechanism amplifiers to control the GFBs’ clearance/preload [26, 28, 29] & Active method; capability of activation of large and fast deformations of the foils to adjust the clearance; experimentally tested; multiple-lobe clearance; controllable GFBs’ stiffness and the shaft’s journal eccentricity \\
7. Hybrid bearing (ball bearing and GFB) equipped with PZTs and electromagnetic coils [33] & Unique features not met in other applications—advantageously combines functionalities of both types of bearings; capability of effective change of elasto-damping properties of the bearing; interesting perspective of further research \\
8. Applications of TEMs and TCs to GFBs for thermal management [8, 37, 38] & Unique features not met in other applications; capability of effective control of the temperature filed in a top foil; interesting perspective of complex power system used to supply TEMs \\ \hline \end{tabular}
\end{table} 
Table 1: Properties of smart material-based GFB approaches. Finally, it should be noted that the presented solutions should be adapted to industrial conditions. Even though the desired capabilities are experimentally confirmed for most of the referenced applications and concepts, additional tests are still required to develop the presented installations to the more demanding operational cases and obtain robust technical solutions.