Effect of engine oil formulation on soot wear

Abstract

High levels of soot in engine lubricants are frequently reported to induce high wear rates on engine components. Engine bearings, camshaft and crankshaft, piston rings, cylinder walls and timing chain are some of the engine parts that are most affected by wear induced by soot [1-3]. There has been extensive research to determine mechanisms by which dispersed soot particles in lubricants increase wear. In recent years attention has focused on the concept that soot particles abrade zinc dialkyldithiophosphate (ZDDP) films as rapidly as they form, leading to a high rate of corrosive-abrasive wear. The main evidence for this is that when both ZDDP and the soot surrogate carbon black (CB) are present in a lubricant, this can lead to much higher wear than if either ZDDP or CB is absent. This project aims to understand further the interaction of soot with lubricant additives and its impact on wear and to derive concrete conclusions and identify control measures.

To explore soot wear, wear tests and antiwear film formation and removal tests have been conducted in a High Frequency Reciprocating Rig (HFRR) and a Mini Traction Machine with Spacer Layer Imaging (MTM-SLIM). CB dispersions in model lubricants based on solutions of ZDDP and dispersant in GTL base oils have been studied using Dynamic Light Scattering (DLS) and rheology techniques. Wear volumes have been measured and wear scars and tribofilms analysed using Scanning White Light Interferometry (SWLI), Scanning Electron Microscopy with Electron Dispersive x-ray Spectroscopy (SEM-EDS), Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and Transmission Electron Microscopy (TEM) with EDS. In addition, HFRR holders have been modified to allow an in-situ observation of the contact with an optical fast speed camera and a sapphire window. Finally, different hardness of HFRR discs have been used to observe the effect of steel hardness on soot wear.

The HFRR wear tests confirmed the harmful effect of the ZDDP when present in solutions containing CB and dispersant. The results from MTM-SLIM and surface characterisation techniques suggest a corrosive abrasion mechanism. It has been also observed that the dispersant type and concentration have large influence on wear induced by CB in ZDDP containing oils. Most dispersants showed a maximum of wear at 0.02 % wt N which is close to the concentration used in fully formulated engine oils. A few dispersants do not show this effect however and maintain low wear over the whole dispersant concentration range.

The optical HFRR results showed how CB enters and aggregates at the contact while the different steel hardness results showed at least twenty times more wear when very hard surfaces are used in oils containing both ZDDP and CB compared to when either component is absent or when softer discs were used.

The key finding of this project is that when CB is present with ZDDP, an overlaying zinc phosphate film is not formed to stop iron sulphide formation and the latter reaction proceeds more readily than in the absence of CB. The high wear rate observed is then controlled by the abrasion of this iron sulphide layer by CB. Some dispersants that are highly functionalised are able to protect the iron sulphide film from abrasion.