# Gas Foil Bearing Technology Enhanced with Smart Materials
## 7 Discussion  
Presented in Sections 4-6, a variety of the applications of smart materials to enhance the properties of GFBs is unquestionable. In reference, the known SMA, PZT, TEM and TC--based approaches are discussed below making use of their major characteristics collected in Table 1.  
All the above-referenced applications seem to exhibit complementary features important in view of the GFBs' performance. Specifically, SMA-based solutions are efficient in terms of energy dissipation to reduce the undesired mechanical vibrations of the shaft's journal. Moreover the construction of a GFB-SMA is relatively simple. Unfortunately, the high inertia of the involved physical phenomena (a low change rate of elasto-damping properties induced by thermal activation) considerably limits the scope of active approaches. However, SMA based installations offer demanded control functionality in cases when long-term and low-rate changes of the GFB's properties are required and satisfying. Adequate mechanical actuations may be generated via shape memory effects in response to the low frequency changes on the operational conditions (loads). Nevertheless, SMAs exhibit the desired characteristics to develop passive solutions. In these solutions the phenomenon of superelasticity is employed, which does not require any thermal activation. As already mentioned, advantageous mechanical energy dissipation is possible thanks to the mechanically initiated phase transformations. The wider hysteresis of the SMA component's stress-strain characteristics is the larger amount of energy that may be effectively collected from the bearing to improve the stability of its operation via a reduction in mechanical vibrations. Due to the specific properties of piezoelectric materials, PZT/MFC based installations provide very precise and fast control over the mechanical displacement. Hence, an active PZT/MFC system may react rapidly to precisely adjust the bearing's preload and clearance. As a result, the above-referenced functionality may provide the conditions required to maintain the air film, and, hence, assure GFBs' stability under harsh operational parameters. It is worth noting that circumferentially distributed PZTs allow one to utilize a multiple-lobe clearance. Hence, the bearing's air film may be formed relatively freely. Unfortunately, only a limited amount of energy may be dissipated via piezoelectric components. Moreover, the construction of a GFB-PZT is complicated, especially in cases when the PZT stacks are built-in into the bearings bushing. High voltage may be also required to supply the PZTs and MFCs and obtain desired strains. Extraordinary thermal properties are found for the TEM and TC--based approaches to GFBs. As proven during experimental investigations, effective thermal management in a GFB may be performed making use of both TC sensors and TEM-based heat energy sink/source control. The risky temperature gradients may by reduced to prevent from thermal stability loss. However, the referenced solutions exhibit a couple of major disadvantages, namely, design and structure complexity and nontrivial selection of control algorithms that would lead to the desired heat energy flows within the entire body of the bearing. In the authors' opinion, extraordinary properties of GFBs could be achieved for the combined solutions GFB-SMA-TEM and GFB-PZT/MFC-TEM. The capability of simultaneous control on the mechanical and thermal characteristics of the bearing seems desired due to potential further reduction in the risk of operational stability loss. In fact, the bearing may adequately and rapidly react to the changing environmental and loading conditions to prevent both reduction in the air film thickness and loss of thermal stability. 3. Application of PZT valves to control radial air injection (active lubrication) [30, 35]
4. Application of PZT composites--MFC to modify elasto-damping properties of the structural parts of the GFBs' supporting layer [4, 31, 34]
5.