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

Friction 3(2): 85–114 (2015) DOI 10.1007/s40544-015-0083-5 ISSN 2223-7690 CN 10-1237/TH REVIEW ARTICLE Vapors in the ambient—A complication in tribological studies or an engineering solution of tribological problems? Ala ALAZIZI, Anthony J. BARTHEL, Nicholas D. SURDYKA, Jiawei LUO, Seong H. KIM* Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA Received: 04 February 2015 / Revised: 14 April 2015 / Accepted: 15 May 2015 © The author(s) 2015. This article is published with open access at Springerlink.com Abstract: Tribology involves not only two-body contacts of two solid materials—a substrate and a counter-surface; it often involves three-body contacts whether the third body is intentionally introduced or inevitably added during the sliding or rubbing. The intentionally added third body could be lubricant oil or engineered nanomaterial used to mitigate the friction and wear of the sliding contact. The inevitably added third body could be wear debris created from the substrate or the counter surface during sliding. Even in the absence of any solid third-body between the sliding surfaces, molecular adsorption of water or organic vapors from the surrounding environment can dramatically alter the friction and wear behavior of solid surfaces tested in the absence of lubricant oils. This review article covers the last case: the effects of molecular adsorption on sliding solid surfaces both inevitably occurring due to the ambient test and intentionally introduced as a solution for engineering problems. We will review how adsorbed molecules can change the course of wear and friction, as well as the mechanical and chemical behavior, of a wide range of materials under sliding conditions. Keywords: vapor phase lubrication; environmental effect 1 Introduction The adsorption of molecules on solid surfaces from the gaseous environment is inevitable unless the solid surface is inert or has extremely low surface energy [1, 2]. The chemistry of the molecular impingement, adsorption, and reaction on solid surfaces is the core of gas separation and catalysis. Considerable research has been conducted focusing on improving adsorption or reaction selectivity and understanding adsorption behavior in heterogeneous catalysis [3]. This is especially true for nano-scale catalyst particles [4] and porous solids such as metal-organic frameworks (MOFs) and zeolites [5, 6]. Poisoning or deactivation of catalyst adsorption sites by an undesired reaction product or trace contaminant can quickly render the catalyst useless [7]. Gas phase chemical sensors and detectors also depend on the process of gas and vapor adsorp*Corresponding author: Seong H. KIM. E-mail: tion. This field has seen tremendous growth with research focusing on new detection methods, increased sensor selectivity and robustness, and expanded contaminant tolerance. Many good review articles cover both commercialized sensors as well as current areas of research [8−12]. Although invisible thus often ignored, the same processes of gas impingement, adsorption, and reaction at the solid surface play critical roles in tribology too. For example, the effects of oxygen or humidity on tribological measurements have long been recognized and documented in the literature. Adsorbed vapors can be essential for super-lubricious behavior [13] or they can deteriorate lubricity [14]. They can prevent wear [15] or result in catastrophic adhesive wear and tribochemical reactions [16]. Adsorbed vapors also play a crucial role in nano-scale contact by controlling adhesion and interfacial shear [17, 18]. This article will give an overview of the environmental effects on tribology focusing on the influence of adsorbed molecules on friction, wear, and surface Friction 3(2): 85–114 (2015) 86 structure of metals, ceramics, glasses, oxides, carbon materials, and polymers. The effect of surface roughness on vapor-phase lubrication will then be discussed. That will be followed by a section investigating the application of vapor phase lubrication in the lubrication of microelectromechanical system (MEMS) and in mechanochemical synthesis from adsorbed vapors under sliding contact. 2 Environmental effect on tribological and interfacial properties of materials Macro-scale tribological tests with controlled vapor environments primarily rely on two test methods to determine the behavior of friction and wear. These are fretting and ball-on-flat tests. Other important test methods, such as the four-ball test often used for liquid lubricants [19], are generally less common for vapor tests. In a fretting test, two bodies in contact undergo a periodic oscillatory displacement with high frequency to simulate intermittent or unintended contact between unlubricated surfaces [20]. Ball-on-flat tests, also called pin-on-disk or pin-on-flat, consist of a spherical ball in contact with a flat counter-surface. Ball-on-flat tests can be conducted in a bi-directional mode during which the ball reverses direction along the wear track, or in a continuous mode conducted on a revolving disk. 2.1 Environmental effect on friction and wear of metals Metals are perhaps the most extensively studied class of tribological materials because of their wide usages in diverse industries. Thus, a considerable portion of the literature is concerned with oil lubrication on industrially relevant alloys. The fundamental problem of the effect of environmental vapors on tribology of metals, which gets less attention, generally concentrates on pure metals. Therefore, the extent to which one or a few physisorbed molecular layers can change friction and wear across length scales has not been fully appreciated. This can be seen in tribology test reports that state the test conditions to be “humid” without measuring the humidity, or tests that simply state “air” without investigating the amount or type of vapors that may adsorb. Even tests that investigate friction and wear on metals in a vapor environment frequently do not give mechanistic explanations for observed results. The effect of water vapor on tribological performances of steels is a commonly studied subject. Initial fretting tests showed mixed results; some reported the largest wear in dry air and a monotonic decrease with increasing relative humidity (RH) [21], while others showed maximum wear at intermediate RH [22]. Unfortunately many investigations did not attempt to explain the trends that were found, but those that did often reported the formation of an oxide layer to be a prominent factor affecting friction and wear of steel. The majority of recent literature on steels generally agrees that increased RH causes a decrease in wear. The friction and wear of mild and carbon steels were shown to decrease with increasing RH in pin-on-disk experiments when tested over the range of 60%80% RH, although lower RH values were not investigated and mechanism wa (...truncated)