In this paper, a hypothesis for the operating tribological mechanisms and phenomena occurring in compliant surface gas foil bearings subjected to low ambient pressure conditions, such as occur at high altitude or in soft vacuum, will be presented and discussed. Both theoretical and experimental evidence supporting the proposed hypothesis will be presented to show that, under low ambient pressure conditions (i.e., something akin to starved fluid film lubrication), the shaft is supported by a combination of hydrodynamic and morphological elements. The theoretical treatment of the compressible fluid film in a simple gas bearing is highly nonlinear in-and-of-itself, and especially more so when combined with a compliant surface supported on a frictional-elastic foil foundation. Adding a “molecularly starved gas film” to this highly nonlinear system, one encounters a very interesting and complex system that, heretofore, has not been considered. When operating compliant foil gas bearings in a near or soft vacuum, the term hydrodynamic may be considered oxymoronic in that there is little or no apparent fluid/gas to provide “a full hydrodynamic” action. However, theoretical and experimental evidence of compliant surface foil gas bearings operating at low ambient pressures show that they do continue to work and, in fact, can do so quite well given the appropriate compliancy and other factors, as yet to be discussed. In this paper, the situation will be addressed based upon the experimental evidence that resulted in the essential hypothesis that there are elements at work that go above and beyond purely hydrodynamic phenomenon or so-called solid lubrication. These elements include both tribological and morphological interactions, which are at work at all times and it is the respective ratios of hydrodynamic and morphological elements that characterize operation. Evidence is presented to the effect that, even when hydrodynamic effects dominate, morphological interactions contribute to bearing performance and load-carrying capacity and that, when morphological effects dominate, third body and surface elements impart to the interface many of the characteristics and effects of a hydrodynamic film. Thus, by combining classical Reynolds equation modified for compressible media with the quasi-hydrodynamic/continuum equations and the appropriate rheological and morphological parameters, meaningful solutions for foil bearing operating with extreme low-pressure boundary conditions are possible, and which result in increased load-carrying capacity contrary to classical hydrodynamic theory.
