Observation of anomalous low friction and wear behavior of polyimide in alkaline environment under high load

Polyimide (PI) was first discovered to exhibit excellent tribological properties in alkaline environments. PI is widely used because of its low thermal expansion coefficient, excellent mechanical properties, and resistance to high and low temperatures. The high manufacturing cost of PI makes its waste recycling a hot topic. The most common way of recycling is to hydrolyze PI in strong alkaline environments (C(OH−)>2 mol/L). Therefore, the intuition is that the friction performance of PI decline in highly corrosive environments. However, this paper found a counter-intuitive phenomenon. PI exhibits excellent tribological properties in alkaline environments. This paper studied the behavior of ball-on-disk contact between ceramic ball and PI disk tribopair in an alkaline environment. Results show a very low coefficient of friction (COF, 0.013) and wear rate (1.4 × 10−6 mm3/NM) in sodium hydroxide (NaOH) solution with pH = 13 under the average Hertzian contact pressure of 60 MPa. The COF and wear rate decreased by 80% and 63.4%, respectively, compared with those in solutions with pH = 7. The worn surface was characterized to explore the tribological mechanism. NaOH reacts with PI to form a polymer containing sodium ions during the friction process. The polymer adheres to the friction surface and improves the friction performance of PI. The chemical reaction breaks the C–N–C chemical bond of PI, thereby reducing the molecular rigidity and improving the adhesion of the PI surface. This paper provides a new modification method for improving the tribological properties of PI.