Tribological and Tribochemical Behaviour of Machine Elements Having DLC/Cast Iron Interfaces and Lubricated with MoDTC-Type Friction Modifier

Due to the importance of reducing CO2 emissions and improving fuel economy, many researchers have focused on studying the cam/follower contact and the ways to reduce friction (improve engine efficiency) and avoid wear (enhance engine durability). This thesis addresses aspects of tribology and tribochemistry of a cam/follower interface with consideration of how the surface/lubricant interaction affect the system performance.
A Mini Traction Machine (MTM) was used to evaluate different lubricant formulations as well as to examine the tribofilm formation under pure sliding and sliding/rolling contacts. A Single Cam Rig (SCR), taken from 1.25L FORD Zetec (SE) engine, was also used in this work in order to investigate the friction, wear and Tribochemical performance of the camlobes/tappets as a function of tappet clearance and type of coating. In addition, a new technique of measuring tappet rotation using a giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet has also been developed in this study.
Raman Spectroscopy, Scanning Electron Microscope (SEM), Energy-Dispersive X-Ray (EDX), X-ray Photoelectron Spectroscopy (XPS), Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were performed on the surfaces to understand the tribochemical interactions between oil additives and the cam/follower interface.
Results obtained from MTM tribometer showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS2, FeMoO4 and not MoO3. In addition, it was observed that the MoS2/FeMoO4 ratio depends on test conditions and affects the friction performance.
The results obtained from SCR tribometer closely support those in the MTM tribometer and that link between tribometer and component testing is discussed. In addition, the chemistry of the tribofilm derived on camlobes and tappets varies as a function of tappet clearance and cam profile. In terms of tappet rotation, results showed that Molybdenum Dialkyl Dithiocarbamate (MoDTC) and Diamond-Like Carbon (DLC) coating promoted the rotation of the tappet. Furthermore, the tappet rotation is strongly dependent on oil formulation, clearance, speed/temperature, and surface roughness of the coating.