Comparative Study on the Tribological Properties of Polyimides Fabricated by Vat Polymerization 3D Printing and Traditional Casting

Polyimide has been used in various fields where extreme environments are necessary because of its high thermal and chemical stability, outstanding mechanical properties, and excellent tribological properties. While in order to meet more and more requirements from different applications, increasing efforts have been made, including the development of monomers, new structures, composites, and processing techniques, etc. Very recently, the emerging 3D printing of polyimide has attracted extensive attention due to its fascinating merits by combining the flexible design and free-form manufacturing of 3D printing with the outstanding comprehensive performance of polyimide. To date, polyimides have been realized by the stereolithography appearance, the direct ink writing and the digital light processing. Despite of the various architectures including gear, retainer, and even bearing and the good properties such as high thermal stability and mechanical strength that have been successfully achieved with the 3D-printed polyimides, few studies on the tribological properties of 3D-printed polyimides. Therefore, it still remains questionable for the 3D printing of polyimides to be promising in moving parting manufacturing. Accordingly, in order to probe the effects of vat polymerization 3D printing on the tribological properties polyimides, the comparison investigation on the tribological properties, thermal stabilities, and mechanical properties of the polyimides with different material recipes fabricated by vat polymerization 3D printing and casting techniques was conducted in this work. The 3D-printed polyimde employed herein was realized in a previous literature, and therefore, for reasonable comparison, the polyimide with the same molecular structure was used for all the other samples, although the material recipes were different for meeting the requirements of different preparation techniques. The results showed that compared with the thermosetting polyimides obtained by casting, the 3D-printed samples exhibited the thermal decomposition temperature reduced by 80 ℃, the friction coefficient increased by 0.08, and the wear rate increased by 9.0 × 10⁻⁶ mm³/(N·m). It can be conclude that the 3D-printed polyimide exhibited decreased tribological properties and heat resistance. This was attributed to the addition of the reactive diluents and cross-linking agents in the 3D printing ink, which were necessary to satisfy the requirements of vat polymerization 3D printing. While it was also found that the incorporation of reactive diluents and cross-linking agents in the 3D-printed polyimides was positive for the hardness and modulus. Briefly, the comprehensive performances of the 3D-printed polyimides, including the tribological properties, were generally suppressed in order to meet the requirements of vat polymerization 3D printing by both the oligomers with low molecule weight and the incorporation of reactive diluents and cross-linking agents in ink. Nevertheless, because of the advantages of vat polymerization 3D printing technology at the integrally-formed, high precision and free-forming, the 3D printing of polyimide was still significant and promising to realize the integrated intelligent manufacturing of high-performance lubrication devices with high precision and structural complexity in the engineering field. Whereas, it has to admit that extensive further efforts on the improvement of 3D printing of polyimide must be conducted to realize its practical application.