Effect of Dispersant Concentration With Friction Modifiers and Anti-Wear Additives on the Tribofilm Composition and Boundary Friction

Abstract

To extend drain intervals and improve efficiency new engine oils with increased dispersant concentration and reduced viscosity are required. Low viscosity engine oils can increase the prevalence of boundary friction at low temperature and increase its severity at higher temperatures. As a result, combinations of organic and inorganic friction modifiers (FM) will be used to reduce boundary friction across a range of temperatures, also preventing damage to vehicle catalysts. This paper presents an experimental case study of such new generation of fully-formulated engine lubricants with varying concentrations of polyisobutylene succinimide dispersant, organic and inorganic FM. Representative conditions pertaining to those encountered at the top dead centre reversal of the piston compression ring-cylinder liner contact are created, and the generated friction is measured through use of a sliding-strip tribometer. Subsequently, X-Ray Photoelectron Spectroscopy (XPS) is used to determine the composition of the formed surface tribofilms in order to explain the observed frictional characteristics. The key interactions and frictional behaviour of the dispersant and friction modifiers are highlighted across a range of operating temperatures.

On the Role of Friction Modifier Additives in the Oil Control Ring and Piston Liner Contact

“…Further study of organic friction modifiers and ZDDP has shown that the interaction of the two additives is dependent on the chemistry and concentration and can result in a positive (lower friction) [20,21] or negative [22] effects (higher friction). Umer et al [8] conducted experiments on ferrous samples which showed that organic friction modifiers and dispersants interfered with the formation of ZDDP tribofilms which is consistent with other literature [22][23][24][25][26]. Umer et al also discuss that ZDDP and inorganic friction modifier molybdenum dithiocarbamates (MoDTC) can work together to form a low friction tribofilms [27][28][29][30].…”

“…The lubricants are described in in Table 2:. The remaining additive package remained consistent across the lubricants as shown in Table 3, the kinematic viscosity for all the candidate lubricants at 40°C is 38.3-40.3 cSt and at 100°C is 7.48-7.55 cSt [8]. Antioxidant Aminic octyl/butyl diphenylamine and phenolic ester (0.5wt%)…”

“…Umer et al [8] observed that the presence of an inorganic friction modifier successfully lowered the friction level with both low (3%) and high (9%) dispersant levels. They also similarly observed that the friction did not reduce when organic friction modifier was used with a low level of dispersant (3%) when tested on steel liners, suggesting this to be due to an antiwear film being formed when a low level of dispersant was present in the lubricant.…”

“…Friction modifiers form part of the additive package, and their purpose is to reduce friction when there is boundary or mixed regime lubrication in the contact. Friction modifiers present in a lubricant are classified as either organic or inorganic [8,9]. Inorganic friction modifiers are typically organomolybdenum-type compounds and they reduce friction in sliding contact by being broken down into low shear strength compounds and are activated by temperature [8,[10][11][12].…”

“…Friction modifiers present in a lubricant are classified as either organic or inorganic [8,9]. Inorganic friction modifiers are typically organomolybdenum-type compounds and they reduce friction in sliding contact by being broken down into low shear strength compounds and are activated by temperature [8,[10][11][12]. Organic friction modifiers however, are physiosorbed (no chemical change, bonding from intermolecular forces such as Van der Waals) to the surfaces during sliding and form monolayers of molecules which prevent direct contact of the surfaces [8,13,14].…”