Tribochemical Analysis of Si-Doped and Non-Doped Diamond-Like Carbon for Application Within the Internal Combustion Engine

Due to the ever-increasing global drivers focused on increasing fuel economy in tandem with decreasing the environmental impact of automobile usage; the automotive sector is rapidly embracing widespread use of Diamond-Like Carbon (DLC) coatings. DLC coatings have the potential to reduce the required level of many traditional oil additives that can negatively impact on both the environment and certain parts of the car engine, specifically the catalytic converter. Furthermore DLC shows promise with regards to reducing friction and can be highly efficacious at reducing wear. The field of research into DLC is ever-developing and many examples of doped DLCs exist. Currently, there is no firm consensus on which dopants are best to include in the DLC matrix when it is being employed within the automotive field. Adding to this the lack of a sufficient understanding of how current engine oil additives interact with DLC; the motivation for undertaking an in-depth analysis of both a-C:H and Si-DLC with current engine oils is clear. This thesis addresses these issues and presents evidence on how both Si-DLCs and a-C:H DLCs interact with current engine oil additives to reduce wear in the engine. The fundamental tribochemistry governing DLC’s interactions at the interfaces are explored with specific reference to wearing of Si-DLCs. Tribological experiments are undertaken to emulate certain conditions within an engine using both reciprocating pin-on-plate tribometers and pin-on-disc tribometers. A novel Si-DLC is created and tested to explore the effect of tri-doping on the coating. Advanced surface analysis techniques are used to gain a full understanding of what processes have taken place at the interfaces. This includes use of X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry and scanning light interferometry. Key findings include the effect that Si doping has on the DLC coating with regards to structure, friction and wear. The fundamental observation that the Si-DLCs examined consistently exhibited wear at higher rates when compared to the a-C:H