The tribo-piezoelectric microscopic coupling mechanism of ferroelectric polymers and the synchronous online monitoring of load distribution/roller speed for intelligent bearings

The tribo-piezoelectric microscopic coupling mechanism of ferroelectric polymers remains elusive due to the complicated interfacial electron transfer behavior and dipole-dipole interactions, limiting the development of high-performance tribo-piezoelectric hybrid nanogenerators. Herein, we take PVDF-Cu as an example to investigate the tribo-piezoelectric coupling mechanism at the ferroelectric polymer/metal interface under compression via first-principles calculations and KPFM experiments. It is revealed that the local deformation of the ferroelectric β-phase PVDF molecular chain not only concentrates the LUMO (lowest unoccupied molecular orbital) in the deformed region and lowers the LUMO energy level, but also changes the orientation and polarity of the dipoles, thus enhancing the triboelectric and piezoelectric charges. Furthermore, this work identifies dipole-dipole interactions and intramolecular/intermolecular electron transfer as the key intrinsic factors that promote the synergistic enhancement of triboelectricity and piezoelectricity. Specifically, the intramolecular and intermolecular electron transfer within the β-phase PVDF induced by the interfacial electron transfer enhances the dipole polarization, thus enhancing the piezoelectric effect, which in turn generates stronger dipole-dipole interactions to provide additional electrostatic potential energy for electrons in the polymer, finally enhancing the interfacial triboelectric effect again. On this basis, we propose the microscopic coupled electrification model at the ferroelectric polymer/metal interface and clarify that the triboelectric and piezoelectric effects dominate at low and high pressures, respectively. Inspired by the above microscopic mechanism, we regulate the ferroelectric β phase of PVDF film to develop a tribo-piezoelectric coupled pressure sensor(T-PPS) with high sensitivity(0.95 V/kPa) and wide measurement range(>10 MPa). In addition, benefiting from the excellent sensing characteristics of T-PPS, we propose a novel strategy based on pressure fluctuation detection of the bearing outer ring, enabling the high-precision synchronous online testing of load distribution and roller speed for intelligent bearings. This method can address the problems of existing measurements such as the destruction of bearings structure, interference with cage motion, and low integration.