78 Tribology Online Japanese Society of Tribologists http://www.tribology.jp/trol/ Vol. 17, No. 2 (2022) 78-85. ISSN 1881-2198 DOI 10.2474/trol.17.78 Article Study on Reaction Mechanism of Sulfur and Phosphorus Type Additives Using an Acoustic Emission Technique Miho Morita 1) *, Shodai Tachiyama 1) , Ko Onodera 1) and Alan Hase 2) 1) EMG Lubricants Godo Kaisha, 6-1 Ukishima-cho, Kawasaki-ku, Kawasaki, Kanagawa 210-9526, Japan 2) Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan *Corresponding author: Miho Morita (morita.miho@eneos.com) Manuscript received 03 January 2022; accepted 24 March 2022; published 30 April 2022 Abstract Sulfur and phosphorus additives are used in lubricants as extreme-pressure and antiwear agents, which are typically used together to ensure reliability over a wide range of lubrication conditions. However, the working mechanism of the combined additive system has not been clearly defined due to difficulties obtaining information on the material surface where these additives work. This is because this surface is constantly being worn during testing. Therefore, in situ analysis applying an acoustic emission (AE) technique was proposed. AEs are elastic stress waves generated during the deformation and fracture of solids, which can be measured in real-time, providing information with respect to the magnitudes and types of damage. In this paper, an application of the AE helps to clarify how each additive acts on the surface in real-time. The working mechanism to understand improved reliability using both sulfur and phosphorous additives was investigated by the AE technique, along with conventional surface analysis methods. It is concluded that wear reducing properties were improved by the reaction of sulfur additives to remove the protruded parts, followed by the reaction of phosphorous additives to form a protective antiwear film. Keywords in situ measurement, acoustic emission (AE), lubricants, sulfur additives, phosphorus additives, extreme-pressure agents, anti-wear agents, low wear, smooth surface, frequency analysis Copyright © 2022 Japanese Society of Tribologists This article is distributed under the terms of the Creative Commons BY-NC-ND 4.0 License. 1 Introduction Improving efficiency with the use of lubricants contributes to better fuel economy in automobiles, and hence movement toward lowering viscosity has continously progressed [1]. Lubricants with reduced viscosity form thinner oil films and may undermine the reliabilities of the hardware in which they are used. Therefore, enhancing the reliablity performance of lubricants, such as the antiwear and antiseizure performance, will be crucial for low viscosity lubricants. To ensure the reliablity performance of lubricants, sulfur and phosphorous-containing additives (S and P additives, respectively) are used as extreme pressure (EP) and antiwear (AW) agents. S additives, such as sulfurized olefins and esters, adsorb on nascent surfaces under severe sliding conditions after their disulfide bonds are broken, and then a metal sulfide is formed, thereby preventing seizure by lowering the melting temperature [2]. P additives, such as alkyl phosphate and alkyl phosphite, adsorb onto metal oxides and produce an iron phosphate and polyphosphate on the surface that can prevent metal surfaces from being worn, via a hydrolysis reaction [3]. As described above, S additives operate under severe conditions, whereas P additives work under relatively mild conditions where they are not exposed to a nascent surface. Accordingly, a combination of S and P additives is often used to ensure reliability over a wide range of lubrication conditions [4]. Therefore, an understanding of the working mechanism of the combined additive system will be important to developing technology to enhance the reliability of low viscosity lubricants. Because the working conditions of S and P additives are different from one another, how the additives work on surfaces will change moment to moment. They work by adsorbing and chemically reacting on the sliding surface, which is changing the shape and properties from the early stages to steady state. Thus, it is important to understand the additive primarily working at that moment and the transition of the working additive. This cannot be understood by analyzing the test pieces after each test; in situ/operando analysis is needed to enable it. Various techniques in terms of in situ observation of the sliding surface have been reported, such as interferometry as well as Fourier Transform Infrared Spectroscopy (FT-IR) [5, 6] and Raman spectroscopy [6, 7]. These techniques provide important information to understand chemical behavior in the interfaces, contributing to clarification of mechanism, however, they are