| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuE |
| Session: | Next-generation Protective Coatings and Tribological Applications |
| Presenter: | Masahiro Saito, Tohoku University, Japan |
| Authors: | M. Saito, Tohoku University, Japan N. Miyazaki, Tohoku University, Japan Y. Ootani, Tohoku University, Japan N. Ozawa, Tohoku University, Japan M. Kubo, Tohoku University, Japan |
| Correspondent: | Click to Email |
MoDTC friction modifier for engine oil is well known to reduce friction under boundary lubrication where friction surfaces locally contact each other. MoDTC decomposes during friction and then, tribo-film of 2D-MoS2 layers oriented at sliding interface is formed from the decomposition products of MoDTC, leading to low friction [1] . It is well known that the effect of MoS2 largely depends on sliding condition. However, understanding of the dependency is not sufficient because of the difficulty of in situ observation at the sliding interface although sliding condition optimization is important to improve friction property. As one cause of this dependency, it is considered that the defect structure in MoS2 tribo-film has some influence on friction. Thus, in order to investigate influence of defect structures in MoS2 tribo-film generated from MoDTC at sliding interface on friction behavior, we performed molecular dynamics simulation by using reactive force field (ReaxFF), which takes into account the chemical reactions.
In the simulation, we prepared models in which some MoS2 layers are sandwiched between two diamond-like carbon (DLC) substrates because DLC is a coating material used in engine cylinders. Here, we investigated the influence of the defects by comparing the friction behavior of the MoS2 layers with and without defect structures. As the defect structures, we considered grain boundary structures in the MoS2 layer and a distorted layer structure in which layers unoriented at the interface. In order to simulate friction process, one DLC substrate was fixed and the other was slid at 100 m/s with 3 GPa of a contact pressure.
The friction simulation the model without defect showed that the contact surface of DLC and MoS2 was the sliding interface. The simulation the model with grain boundaries showed that the contact surface of MoS2 and MoS2 was the sliding interface since C-S bonds were formed between DLC and MoS2 around grain boundaries. In addition, the MoS2 layers with grain boundaries showed higher friction force because the grain boundaries in neighboring MoS2 layers interact each other. The simulation the model with distorted layer structure showed that the MoS2 edge adhered to DLC since C-S and Mo-C bonds were formed between DLC and MoS2. The shear stress acting on the unstable MoS2 edge accelerated the chemical bond formation. In summary, we found that interactions between MoS2 grain boundaries and formation of chemical bonds between MoS2 and DLC sliding surface around defect are significant frictional resistance.
[1] J. Graham et al., J. Tribol., 44 (2001) 4