FRICTION AND WEAR MICRO-MECHANICS OF EPOXY REINFORCED BY GRAPHENE BASED FILLERS

Open Access
- Author:
- Venkataraman, Karthik Srinivas
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 11, 2021
- Committee Members:
- Seong Kim, Outside Unit & Field Member
Andrea Arguelles, Major Field Member
Albert Segall, Chair & Dissertation Advisor
Jacques Riviere, Major Field Member
Albert Segall, Program Head/Chair - Keywords:
- Graphene
Epoxy
Friction
Wear
Ball on disk testing - Abstract:
- Graphene has been gaining much prominence owing to its excellent mechanical, thermal, and electrical properties. Indeed, polymer-graphene composites have demonstrated superior mechanical properties as compared to neat polymers and are well-suited as materials for seals and bearings applications. Given these advantages, the objective of this work is to understand the friction and wear mechanics of polymers reinforced by graphene-based fillers. Composites of several commonly used polymers - HDPE, Epoxy, POM, PPS, Novolac, and PI were prepared with a wide set of graphene/graphite fillers and tribologically evaluated in a ball-on-disk (BoD) configuration at different sliding velocities ranging from 0.047 to 1.61 m/s. It was found that only the epoxy + 5wt.% industrial graphite oxide (iGO) system demonstrated a lower wear rate. Fillers with no functional groups for interaction did not affect the friction and wear response of polymers. Thus, filler-matrix interaction was identified as key for preparing composites with enhanced lubricity. To further understand the factors necessary to improve the tribological response of polymers and to identify the filler composition that maximized the synergistic effect of adding graphene, epoxy composites with different weight loadings – 0, 0.5, 1, 2, and 5 wt.% of industrial graphite oxide (iGO) were prepared and tribologically evaluated at the same conditions. The addition of iGO improved the friction and wear resistance of epoxy against a stainless steel counterface; the presence of functional groups on the surface of iGO led to a stronger interaction with the matrix and enhanced adhesion between the dislodged polymer and the counterface. The wear mode for the epoxy-stainless steel system was identified as being predominantly adhesive and it was the nature of the transfer layer rather than the bulk polymer that dictated the tribological response. iGO also arrested the crack propagation in the matrix, enhancing the wear resistance of the composites. Epoxy with 1 wt. % iGO exhibited an order of magnitude improved wear resistance and the best combination of mechanical and wear properties. At loadings beyond 1 wt.%, the addition of iGO promoted filler-filler interaction leading to a weak interface with the matrix and hence the beneficial effect on the addition of iGO was not observed. Uniform dispersion, interaction with the matrix, and sufficient loading of the filler were identified as important factors that affected the mechanical and tribological response of polymer-graphene composites. Furthermore, epoxy-iGO composites prepared by two techniques – solvent dispersion and speed mixing were tribologically evaluated and the results compared. Solvent dispersion exfoliated the filler to a larger extent, thus creating a shorter interface with the matrix. Speed mixed samples demonstrated a longer interface owing to the reduced sonication times. The length of the interface had a direct impact on the level of mechanical reinforcement provided by the filler. In fact, longer interfaces led to the composites having a higher hardness and better tribological response. The role of post-curing on the tribological response of the epoxy composites was also investigated. Post-curing the epoxies beyond the glass transition temperature led to thermal degradation of the sample with a significant drop observed in the mechanical properties. However, this degradation from glassy to a rubbery state slightly improved the wear; The increased mobility of the system that facilitated easy shearing of the polymer and thus, enhanced lubricity was responsible for the improved wear resistance. Overall, this work identified the friction and wear mechanisms for epoxy-iGO systems. The knowledge gained from this work can be used to manufacture components that are environmentally more compatible, demonstrate a higher shelf life, and exhibit superior mechanical and tribological properties.