Surface chemistry of tribochemical reactions explored in ultrahigh vacuum conditions Javier Lara-Romero a,1 , Rafael Maya-Yescas a,1 , Jose ´ Luis Rico-Cerda a,1 , Jose ´ Luis Rivera-Rojas a,1 , Fernando Chin ˜as Castillo b, * , Matey Kaltchev c , Wilfred T. Tysoe d a Faculty of Chemical Engineering, Universidad Michoacana de San Nicola ´s de Hidalgo, Ciudad Universitaria, Edificio M, C.P. 58060, Morelia, Michoaca ´n, Me ´xico b Mecanical Engineering Department, Instituto Tecnolo ´gico de Oaxaca, C.P. 68030, Oaxaca, Oaxaca, Me ´xico c Department of Chemistry and Physics, Milwaukee School of Engineering, Milwaukee, WI, 53211, USA d Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA Received 18 January 2005; received in revised form 14 September 2005; accepted 15 September 2005 Available online 25 October 2005 Abstract The thermal decomposition of model extreme-pressure lubricant additives on clean iron was studied in ultrahigh vacuum conditions using molecular beam strategies. Methylene chloride and chloroform react to deposit a solid film consisting of FeCl 2 and carbon, and evolve only hydrogen into the gas phase. No gas-phase products and less carbon on the surface are detected in the case of carbon tetrachloride. Dimethyl and diethyl disulfide react on clean iron to deposit a saturated sulfur plus carbon layer at low temperatures (¨600 K) and an iron sulfide film onto a Fe + C underlayer at higher temperatures (¨950 K). Methane is the only gas-phase product when dimethyl disulfide reacts with iron. Ethylene and hydrogen are detected when diethyl disulfide is used. D 2005 Elsevier B.V. All rights reserved. Keywords: Tribological coatings; Molecular beam; Chlorinated hydrocarbons; Sulfurized hydrocarbons 1. Introduction Extreme-pressure (EP) lubricants are used in a wide range of metalworking applications such as severe stainless steel forming operations or fine blanking heavy parts for the automotive industry. Chlorinated and sulfurized hydrocar- bons are commonly added as additives to a base fluid to synthesize lubricants that can be used under these condi- tions, where interfacial temperatures in excess of 1000 K can be attained. The additives thermally decompose at the hot interface to reactively form a solid lubricating film [1– 5]. Extreme-pressure lubrication can be described as a dynamic process where the resulting film thickness at the lubricated interface arises from a balance between the film growth rate and the removal rate. The nature of the film and growth rate depends on the chemical structure of the additive used. Small chlorinated hydrocarbons (methylene chloride, chloroform and carbon tetrachloride) have been used as model additives [6–8]. It has been shown that methylene chloride and chloroform form films that consist of FeCl 2 and which incorporates carbonaceous particles [6,7]. When using carbon tetrachloride, iron carbide is formed leading to a much more effective lubrication [8]. Sulfur- containing EP additives generally comprise molecules con- taining sulfur linkages and are modeled using dialkyl disulfides (dimethyl disulfide and diethyl disulfide) [9–11]. They thermally decompose to form films that consist of FeS [12]. Molecular beam experiments carried out in ultrahigh vacuum conditions can mimic the tribological interface where a hot clean iron surface is exposed to the EP additive. The advantage of this strategy is that the gas-phase products can be monitored, the nature of the films determined and the film growth rate measured. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.108 * Corresponding author. E-mail addresses: lararom@zeus.umich.mx (J. Lara-Romero), fernando-chinas@mail.com (F.C. Castillo). 1 Tel.: +52 4433273584x116; fax: +52 4433273585x102. Thin Solid Films 496 (2006) 463 – 468 www.elsevier.com/locate/tsf