Tribological Performance of Polymer Based Self-lubricating Coatings

Abstract: The thesis comprises the two parts in each chapter: the first part focuses on the development and characterization of polyimide (PI) based composite coatings on a steel substrate. In order to improve the tribological performance of polyimide coatings, the fillers i.e. multi-walled carbon nanotubes (MWCNTs) and Graphene (GP) were added into PI and conducted friction test at elevated temperatures ranging from room temperature (RT) to 200°C. Also, the influence of fillers (MWCNTs and GP) materials into PI coatings surface, mechanical and tribological properties of polyimide composites coatings are measured. The addition of MWCNTs and GP reduces the friction coefficient as well as wear volume at elevated temperatures 50°C, 100°C and sometimes at 150°C. These temperatures play a vital role to form a lubrication layer in the contact interfaces at certain load and operating conditions. In these cases, three weight percentage (3wt%) of MWCNTs and GP into polyimide composites showed low friction and high wear-resistant as compared to other PI composites. Besides, by adding these two fillers into pure PI improved the mechanical properties such as micro-hardness and nanoindentation. The scanning electron microscope (SEM) was used to observe the wear mechanism of the composite coatings worn surfaces. The consequences expose that the fatigue wear mechanisms were predominant in the worn surfaces. Moreover, the thermal study of the polyimide composite coatings was conducted using thermal gravimetric (TG) to analyze the behavior of composite coatings at high temperatures. The results showed that the PI coatings with MWCNTs and GP have high thermal stability at 60% sample residue. In the second part-an epoxy coatings with filler materials e.g. hexagonal boron nitride (h-BN) and expanded graphite (EG) were made and conducted their tribological i.e. friction coefficient and wear performance. Also the perfect mixing ratio 4:1 (80 wt% base epoxy matrix and 20 wt% curing agent) was determined on the basis of stoichiometric ratio to cure the epoxy accurately. Therefore, seven samples with a various weight percentage (wt%) were prepared i.e. pure epoxy, epoxy with 5wt%, 10wt%, 15wt% of h-BN and EG. All the prepared samples ran at two different loading 2 N and 4 N conditions with 5 Hz frequency, 300 rpm and 30 minutes duration. The epoxy with h-BN showed low friction as compared to EG where EG has better wear-resistant behavior than h-BN.