POSS-Grafted PI/MoS2 Gradient Composite Coatings with Enhanced Tribological Properties Under Atomic Oxygen Irradiation

The irradiation of atomic oxygen (AO) severely restricts the application of polymeric lubricating coatings in low Earth orbit (LEO). Herein, octa- and mono-amino polyhedral oligomeric silsesquioxanes (POSSs) were chemically bonded onto polyimide/molybdenum disulfide (PI/MoS2) composite coatings with a gradient structure based on Si density. The gradient coatings presented better wear resistance under different loads; notably, the wear rate decreased by 83.5%. Additionally, the effects of AO exposure on the surface morphologies, chemical structure, and tribological properties of the gradient coatings were investigated in detail. The results indicated that the mass loss and wear rates under AO irradiation decreased significantly, which can be attributed to the passivated network-like SiO2 layer that covered the coating surface after AO irradiation. As a result, the addition of POSS significantly improved the tribological properties and AO resistance.