Macro- to Nanoscale Wear Prevention via Molecular Adsorption

Abstract

As the size of mechanical systems shrinks from macro- to nanoscales, surface phenomena such as adhesion, friction, and wear become increasingly significant. This paper demonstrates the use of alcohol adsorption as a means of continuously replenishing the lubricating layer on the working device surfaces and elucidates the tribochemical reaction products formed in the sliding contact region. Friction and wear of native silicon oxide were studied over a wide range of length scales from macro- to nanoscales using a ball-on-flat tribometer (millimeter scale), sidewall microelectromechanical system (MEMS) tribometer (micrometer scale), and atomic force microscopy (nanometer scale). In all cases, the alcohol vapor adsorption successfully lubricated and prevented wear. Imaging time-of-flight secondary ion mass spectrometry analysis of the sliding contact region revealed that high molecular weight oligomeric species were formed via tribochemical reactions of the adsorbed linear alcohol molecules. These tribochemical products seemed to enhance the lubrication and wear prevention. In the case of sidewall MEMS tests, the lifetime of the MEMS device was radically increased via vapor-phase lubrication with alcohol.

Vapors in the ambient—A complication in tribological studies or an engineering solution of tribological problems?

“…Vapor phase lubrication of silicon oxide surfaces has been shown to be effective and reliable on the macro to the nano-scales [214,215]. Pentanol vapor lubrication completely prevented failure of MEMS devices; as Fig.…”

“…14 shows, a MEMS device operated in dry conditions failed after less than 10,000 cycles (less than two min at a 100 Hz operation). In contrast, when the MEMS device was operated in the presence of pentanol vapor, the device ran for 11 days without failing or any sign of wear [215]. Under prolonged operation of the MEMS device at low partial pressure of pentanol vapor, a high molecular weight material was detected within the sliding area [215].…”

“…Organic layers are susceptible to degradation even under normal storage conditions. Hard coatings are prone to wear under prolonged sliding which limits the device lifetime [212,213]. Moreover, a conformal coating using organic and inorganic coatings is hard to achieve on surfaces buried in complex microstructures [212].…”

“…In contrast, when the MEMS device was operated in the presence of pentanol vapor, the device ran for 11 days without failing or any sign of wear [215]. Under prolonged operation of the MEMS device at low partial pressure of pentanol vapor, a high molecular weight material was detected within the sliding area [215]. This material is called tribo-polymer and it is an undesired product as it indicates materials wear during sliding [216].…”

Studies of the viscoelastic properties of water confined between surfaces of specified chemical nature.

“…Recent work, however, has demonstrated that gas-phase alcohols may provide a solution to the friction problem, in that when MEMS devices are exposed to low vapor pressures of short chain alcohols, devices with contacting, moving parts can run for billions of cycles without failure and with no visible wear [185]. While this appears to be a successful lubrication strategy, in order to move towards applications, a number of questions still need to be addressed.…”