Abstract
The reinforcement of coatings with diamond particles results in superior tribological performance for automotive applications. In addition to improving the coating’s bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribo-chemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C-O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication and ex situ chemical analysis by means of XPS, AES and EELS provide evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations.
Highlights
Lubricated diamond/steel tribocouples exhibit amazingly small friction coefficients and wear under boundary lubrication conditions (Mehan and Hayden, 1981)
We investigated the interfacial tribochemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts using reactive atomistic simulations and an on-line tribometer linked to chemical analysis
Reactive classical molecular dynamics simulations of tungsten (100) sliding against diamond (100) under high pressure revealed the radicalization of hydrocarbon molecules by proton transfer to the tungsten surface and their subsequent chemisorption onto dangling C bonds on the diamond surface
Summary
Lubricated diamond/steel tribocouples exhibit amazingly small friction coefficients and wear under boundary lubrication conditions (Mehan and Hayden, 1981). Since piston rings slide against metallic cylinder liners, the fundamental tribological mechanisms governing friction and wear in lubricated metal/diamond contacts are of academic (Mehan and Hayden, 1981) and of industrial interest (Esser et al, 2004). Already used in technical applications, the mechanisms leading to ultralow friction in lubricated metal/diamond contacts are not well-understood. It is unclear whether diamond undergoes a crystalline/amorphous phase transition (Pastewka et al, 2011) or whether chemical mixing leads to the formation of carbides. The role of the lubricant is still elusive It certainly plays an important role for friction reduction under boundary lubrication. Is it chemically inert and does it strongly physisorbs to the surface? Or is it susceptible to tribochemical reactions and chemisorbs on one of the tribopartners? These questions cannot be answered by macroscopic tribological experiments
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