| AVS 59th Annual International Symposium and Exhibition | |
| Tribology Focus Topic | Tuesday Sessions |
| Session TR-TuA |
| Session: | Molecular Origins of Friction and Wear |
| Presenter: | S. Misra, University of Maryland, College Park |
| Authors: | S. Misra, University of Maryland, College Park B. Hanrahan, University of Maryland, College Park R. Ghodssi, University of Maryland, College Park |
| Correspondent: | Click to Email |
Microball bearings have been successfully utilized in several micro-machinery applications, providing low friction, low wear contact, long lifetimes, and device robustness. On all size scales, rolling friction arises from a combination of volumetric mechanical properties, surface chemical properties, and bearing geometries. On the micro-scale, surface effects are enhanced relative to volume, and geometries are dictated by microfabrication techniques. Due to these unique factors, there is not a comprehensive understanding of the fundamental source of rolling friction in microscale systems, but adhesion is theorized to dominate. Vapor-phase lubrication has been implemented to change the chemistry of the surface, specifically addressing the adhesive compon ent of micro-scale rolling friction, leading to reduced friction and enhancing the overall understanding of the system. Future microball bearing supported microsystems will benefit from the work presented here due to a greater knowledge of the influence of adhesion on micro-rolling friction.
A custom silicon micro-turbine supported on ball bearings serves as the platform for the study of rolling friction. 440C stainless steel microballs (285 micrometers diameter) are housed in deep reactive ion-etched silicon raceways. The tribological properties of this device have been the focus of numerous previous studies. The micro-turbine is operated such that normal load and turbine speed can be independently controlled for normal load-resolved spin-down friction testing. The spin-down testing methodology reveals the relationship between friction torque and normal load, which is used to understand the fundamental sources of rolling friction. Vapor saturation techniques have been integrated within the turbine actuation scheme by bubbling nitrogen gas through heated liquid and then using it to actuate and provide normal load for the micro-turbine. A condenser is employed before the output to assure no liquid condenses within the raceway.
A water vapor-lubricated micro-turbine has demonstrated a 43% reduction of friction versus dry nitrogen at a normal load of 50mN in spin-down testing as well as a 37% increase in overall turbine performance. Additionally, with the introduction of vapor, the relationship between friction torque and normal load was fundamentally changed, revealing the significant influence of adhesion to the system. Vapor lubrication adsorbed on the surface of the ball and raceway lowers their surface energies, reducing the effect of adhesion. These results show the first conclusive demonstration of the adhesive component to micro-scale rolling friction by using vapor phase lubrication.