Sliding wear of polyimide-C60 composite coatings

The sliding friction and wear characteristics of pyrimidine-containing polyimide (PI) coatings and PI-C60 composite coatings on steel substrates have been investigated using a flat-on-ring friction geometry. The wear behavior of the coatings was found to proceed through three stages: a rapid increase in wear at the beginning of the friction experiment, a steady state condition with a moderate increase in wear, and a catastrophic wear behavior in the latest stage of the friction process. The friction coefficient of PI and PI-C60 composite coatings was found to be only slightly determined by the conditions of material preparation and the presence of C60. The wear resistance of these materials is, however, strongly influenced by the latest stages of the imidization reaction. In the initial two stages of the friction process, a strong reduction in specific wear rate (from values of ws∼15×10−7 down to ∼2×10−7 mm3/N m at pressures of 1 MPa÷3 MPa) was observed for both neat PI- and PI-C60-coatings when they were treated in the region between 320 and 420 °C. The introduction of C60 was found to improve further the wear characteristics of the coatings down to specific wear rates below 2×10−7 mm3/N m when the coatings treatment took place at higher temperatures (420 °C). The structural analysis data of the polyamic acid-C60 solutions and PI-C60 coatings enabled us to suggest that the role of the C60 molecules in reducing the wear of composite coatings is due to the interactions between C60 and the PI.