Integrated printing of high-strength, high-shape-retaining polyimide and its composite gradient structures for enhanced tribological properties

The incorporation of nanofillers into the entire matrix of polyimide (PI) to improve its tribological properties has been extensively investigated. However, the integrated gradient structure of PI is hardly mentioned due to the limitation of the molding method. Fortunately, the free manufacturing advantages of 3D printing present an opportunity for this. 3D printing PI that simultaneously meets high performance and high shape retention is critical for the tribological process, but it is also extremely challenging. In this paper, a highly soluble polyimide precursor, polyamic acid (PAA), was first obtained by molecular design and simulation calculation. Then 2-(dimethylamino)ethyl methacrylate (DMAEMA) was added to PAA as a rheology modifier and photocrosslinking site to obtain an ink suitable for direct ink writing (DIW) printers. By optimizing the ink formulation, the resulting 3D printed PI has highlighted tensile strength (~126 MPa), outstanding Young's modulus (~3.5 GPa), high hardness (~380 MPa), remarkable thermal stability (Td~415 °C, Tg~220 °C), low linear shrinkage (11%), and excellent light transmittance and transparency. Furthermore, molybdenum disulfide (MoS2) was selected as the solid lubricant to prepare a PI/MoS2 composite ink, which realized the integrated printing of PI with different structures. The results show that the 3D printed gradient PI/MoS2 exhibits a stable/low coefficient of friction (COF) and minimal wear with a 68% reduction in wear rate compared to the 3D printed pure PI.