Robust low friction mechanism of HNT–PVPA composite coating based on microviscosity regulation

Reducing the coefficient of friction is a critical method for improving the service life and enhancing the efficiency of artificial implants. Maintaining a robust low-friction effect is essential for optimal artificial implant performance. This work utilizes the mechanism of the interaction between the interfacial charge and microviscosity to design a composite coating for titanium alloys modified with halloysite nanotubes/poly(vinylphosphonic acid) (HNT–PVPA). Compared with that of the pure PVPA coating, the coefficient of friction of the composite coating-polytetrafluoroethylene (PTFE) system stabilized at a low-friction state of approximately 0.008, with a 13.40% improvement in the load-bearing capacity. This low-friction state is maintained over a wide range of speeds and for extended periods. Furthermore, the study reveals that the electrical property differences between the inner and outer walls of halloysite nanotubes induce specific aggregation of anions and cations. These ions increase the microviscosity around the tube wall by forming hydrogen bonds with water molecules and attracting water molecules to form hydronium cations, contributing to the low-friction mechanism. The HNT–PVPA composite coatings also enhance the toughness of the coating in the body fluid environment by stabilizing the crosslinked core region against perturbations from multivalent cations. The results provide a new approach for achieving low-friction composite polymer coatings with improved frictional properties in biotribology.