AVS2012 Session TR-TuA: Molecular Origins of Friction and Wear

Ionic solids have simple and well known crystalline structures while spanning several orders of magnitude in wear rates; this makes them excellent candidates for fundamental studies in wear. Wear experiments on the (001) surface of rock-salts, including NaCl and MgO, revealed that the material wear rates have significant dependency on crystallographic wear direction. The materials experienced maximum wear when sliding in the <100> family of directions and minimum wear when sliding in the <110>. For MgO the wear rate in the <100> direction was approximately three times that in the <110> direction. Wear experiments were performed at angles relative to the [100] direction in six degree increments revealing wear as a sinusoidal function of direction with 90 degree periodicity. These results offer a direct link between material structure and the wear properties of a material.

Polytetrafluoroethylene (PTFE) is an excellent candidate material for solid lubrication applications due to its low friction coefficient and chemical inertness; however, its use is limited due to its high wear rate. Unfilled PTFE is known to suffer from subsurface crack propagation and subsequent delamination during sliding. To combat this high wear mode, fillers of various sizes have been added to a PTFE matrix to increase the wear resistance by typically one or two orders of magnitude by fracture toughening and load support mechanisms. Nanocomposites are the state of the art in ultra-low wear performance in fluoropolymer systems; polymer blends have been shown to achieve nearly zero wear with wear rates below 1 x 10^-8 mm^3/Nm. These low wear rates are consistently accompanied by visually distinct tribofilms at the interface of the bulk material and counterface. Experiments suggest that a combination of mechanical and tribochemical mechanisms are responsible for the development of these tribofilms and consequent ultra-low wear behavior.

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.

We have performed a magnetic probe microscopy study of levitation and atomic-scale friction for Fe on YBCO (Tc = 92.5K) in the temperature range 65 - 293 K, to explore electronic contributions to friction at the atomic scale. The samples were prepared with oxygen-depleted surfaces, with thin semiconducting surface layers present atop the bulk. Below Tc, the friction coefficient was observed to be constant at 0.19 and exhibited no correlation with the strength of superconducting levitation forces observed below Tc. The friction coefficient exhibited a change in slope within experimental error of Tc that increased progressively above Tc and reached 0.33 by room temperature. The results were analyzed within the context of underlying atomic-scale electronic and phononic mechanisms that give rise to friction, and it is concluded that contact electrification and static electricity play a significant role above Tc. Quartz crystal microbalance studies of sliding friction studies of molecularly thin films in the presence and absence of magnetic fields, were also performed for both paramagnetic oxygen and diamagnetic nitrogen films on substrates in various magnetic states.

[1] I. Altfeder and J. Krim, J. Appl. Phys. (2012), in press

Supported by NSF and AFOSR