Abstract
In the pursuit of environmentally friendly and sustainable transportation strategies, proton-exchange membrane fuel cells (PEM-FC) have emerged as a promising technology to decarbonise powertrains especially in heavy-duty vehicles. Oil-free bearings, the so-called Air Foil Bearings (AFBs), are crucial to the robust operation of the PEM-FC by preventing the contamination of the hydrogen fuel cell stack. However, during start-up and coast down of the PEM-FC, there is contact between the top foil of the AFB and the journal, which leads to high friction forces and durability concerns for the AFBs. The objective of this work is to identify the coating system that minimizes the friction forces for AFB applications. With this aim, tribological pin-on-disc tests were performed using coated Inconel discs and coated tool steel pins under room temperature conditions and at 150°C. Additionally, the development of a test rig to measure the tribological performance of coated AFBs under real life applications is discussed. In the pin-on-disc tests, the coated disc represented the top foil of the AFB while the coated pin represented the journal. Four solid lubricants were investigated for the discs, while two types of DLC coatings (monolithic and multilayer) were used for the pins. The worn surfaces have been investigated via microscopy and elemental analysis. The adhesion strength of the coatings was evaluated through indentation (VDI 3198) and scratch tests. The tribosystem of multilayer DLC against sulfur/molybdenum based solid lubricant coating afforded the lowest frictional response and friction stability compared to the other coating systems under evaluation.
