Tribochemical Polymerization of Adsorbed n-Pentanol on SiO₂ during Rubbing: When Does It Occur and Is It Responsible for Effective Vapor Phase Lubrication?

Abstract

The origin and role of tribochemical reaction products formed while sliding silicon oxide surfaces in the presence of adsorbed alcohol molecules in equilibrium with the vapor phase were studied. Wear and friction coefficient studies with varying contact loads and n-pentanol vapor environments were used to determine under what operating conditions the tribochemical reaction species was produced. Imaging time-of-flight secondary ion mass spectrometry and microinfrared spectroscopy found that hydrocarbon species with a molecular weight higher than the starting vapor molecules are produced when there is wear of the SiO(2) surface. When the n-pentanol vapor lubrication is effective and the silicon oxide surface does not wear, then the tribochemical polymerization products are negligible. These results imply that the tribochemical polymerization is associated with the substrate wear process occurring due to insufficient adsorbate supply or high mechanical load. The tribochemical reactions do not seem to be the primary lubrication mechanism for vapor phase lubrication of SiO(2) surfaces with alcohol, although they may lubricate the substrate momentarily upon failure of the alcohol vapor delivery to the sliding contact.

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

“…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]. Wear of the silicon oxide can occur due to insufficient supply of the lubricating molecules from the surrounding vapor [216].…”

“…This material is called tribo-polymer and it is an undesired product as it indicates materials wear during sliding [216]. Wear of the silicon oxide can occur due to insufficient supply of the lubricating molecules from the surrounding vapor [216]. Section 4.2 offers a more detailed discussion about the mechanism and the causes for forming undesirable tribo-products.…”

“…Insufficient or intermittent delivery of vapors to the contact area can result in failure [216]. Pentanol vapor, for example, was found to be a good lubricant for self-mated silica contact, a system very important for electronics applications.…”

“…Pentanol vapor, for example, was found to be a good lubricant for self-mated silica contact, a system very important for electronics applications. However, when the amount of vapor supplied to the contact area was inadequate, friction became chaotic and silicon was severely worn [216]. The worn areas contained hydrocarbon deposit with molecular weight much higher than that of pentanol.…”

“…The worn areas contained hydrocarbon deposit with molecular weight much higher than that of pentanol. The deposit is called a tribo-polymer and is formed by a mechanochemical process catalyzed by the wear debris [216]. Mechanochemistry is a term that describes nonthermal chemical reactions occurring on solid surfaces solely due to mechanical processes like shear, repetitive impact, and tensile and compressive stresses.…”