抄録
100% cellulose nanofiber (CNF) moldings have attracted attention as next-generation biomass-derived sliding materials. However, the effects of fiber diameter and fiber orientation on their boundary lubrication properties remain poorly understood. In this study, three types of 100% CNF moldings with different fiber diameters (coarse/fine) and fiber orientations (in-plane/out-of-plane) were fabricated, and ball-on-disk friction tests were performed under poly-α-olefin (PAO4) lubrication. Friction surface analyses were conducted using a nanoindenter, polarized Raman spectroscopy, Raman spectroscopy (crystallinity index and hydrogen bond form), and atomic force microscopy (AFM) adhesion force mapping. Results showed that fiber diameter had little effect on boundary lubrication performance within the range examined for the in-plane moldings, whereas fiber orientation exerted a dominant influence. After running-in, in-plane moldings exhibited a marked increase in friction coefficient, while out-of-plane moldings maintained low friction. Surface analysis revealed that mechanical defibration—breakage of inter-fiber hydrogen bonds—occurred only in in-plane moldings during sliding, generating dangling bonds that increased surface adhesion and friction. In out-of-plane moldings, no mechanical defibration was observed, but surface-layer CNF fibers buckled in the sliding direction and transformed into an in-plane orientation, causing a reduction in elastic modulus. These findings demonstrate that the boundary lubrication properties of 100% CNF moldings are governed by the defibration behavior determined by fiber orientation.
