Tribochemical investigation of ZDDP tribofilm
Creators: name: Ueda, Mao
Publisher: name: Imperial College London
Year: 2021

description: The current trend for using lower-viscosity lubricants with the aim of improving fuel efficiency of mechanical systems means that machine components are required to operate for longer periods in thin oil film, mixed and boundary lubrication conditions, where the risk of surface damage is increased. For this reason, the role of tribofilms generated from the antiwear additive zinc dialkyldithiophosphate (ZDDP) in providing surface protection has become increasingly important. However, the properties, performance and the mechanisms of tribofilm formation are not fully understood. Therefore, this thesis aims to further understand the tribochemical behaviour of ZDDPs. Several inter-connected areas of research are described in this thesis. These all investigate the formation of tribofilms by ZDDP and the impact of tribofilm formation on wear. Taken together they contribute to our understanding of the mechanisms of tribological behaviour of ZDDPs and should assist in the design of lubricants and rubbing components. Firstly, the evolution of ZDDP tribofilm properties, in particular, tribofilm durability and the origins of this durability, are examined. It is found that ZDDP tribofilms undergo a structural transformation during rubbing from a predominantly amorphous structure to one that is nanocrystalline, resulting in the tribofilm becoming much stronger and more durable. Secondly, the reaction mechanisms of tribofilm formation on various non-ferrous metal and non-metallic materials are studied, both by ion-implanting various alloying elements into steel surfaces, and by using non-metallic rubbing materials. It is found that a potentially important factor in the formation of ZDDP tribofilms is the presence and concentration of ferrous and/or non-ferrous metal atoms at the surface. Non-ferrous metals may act as adsorption sites for ZDDP in a similar manner to Fe in steel, enabling formation of ZDDP tribofilms. Thirdly, to further understand the antiwear performance of ZDDP, its impact on the wear of a-C:H DLC in the presence of molybdenum dialkyldithiocarbamate (MoDTC), which is known to be problematic, is investigated. It is well known that MoDTC can produce very high wear of DLC and that this can be mitigated by the presence of ZDDP in the lubricant. From this research it is shown that ZDDPs reduce DLC wear in the presence of MoDTC mainly by forming thick antiwear tribofilms and reducing the ratio of MoO3/MoS2 in the tribofilm. Fourthly, although ZDDP is very effective for reducing most types of wear, it is known that ZDDP can promote one particular form of wear, micropitting wear. In this research a new method of studying micropitting that enables both micropitting and tribofilm formation to be studied in parallel is developed which is key to assessing the impact of lubricant chemistry on micropitting. This is used to show that the influence of ZDDP on micropitting originates from its tendency to prevent running-in of the contacting surfaces. The mechanism by which a widely-used black oxide coating limits micropitting is explored and it is shown that this relatively soft coating provides adequate running-in even in the presence of ZDDP-containing oils. Finally, the impact of friction on micropitting is studied by isolating its effects from those of running-in, by controlling separately the formation of ZDDP and MoDTC tribofilms. Results show that friction has a very significant impact on micropitting.