Tribological anisotropy of 3D-printed PEEK composites: Effects of fiber orientation and multi-scale particle reinforcement

Friction and wear anisotropy are commonly observed in additively manufactured fiber-reinforced polymer composites, however, the extent to which such behavior depends on material architecture rather than print-induced orientation remains insufficiently understood. In this study, fused filament fabrication (FFF) was employed as a controlled platform to investigate the combined effects of fiber orientation and multi-scale particulate reinforcement on the orientation-dependent tribological behavior of polyetheretherketone (PEEK) composites. Two composite systems were examined: PEEK-A was reinforced with short carbon fibers and graphite, exhibiting strong directional sensitivity, whereas PEEK-B additionally incorporated submicron TiO 2 and ZnS as well as nanoscale SiO 2 , to modify load transfer and interfacial behavior. Three testing orientations and a wide range of load-speed (F N ·v) conditions were used to analyze orientation-dependent tribological behavior. PEEK-A showed pronounced tribological anisotropy, with fiber-dominated sliding in the parallel directions and matrix-dominated damage when fibers were normal to the sliding direction. In contrast, PEEK-B exhibited a significantly reduced sensitivity to sliding orientation, driven by a stiffened matrix and the formation of particle-enriched transfer films that provide protection. An orientation-aware modeling approach was applied to both composite systems and was found to capture the experimentally observed trends in tribological behavior. Differences in fitted parameters reflect material-dependent responses arising from filler architecture and interfacial characteristics. Overall, this work provides experimental insights into how fiber alignment and multi-scale filler integration can be combined to mitigate tribological anisotropy in FFF-printed PEEK composites, offering practical guidance for the design of wear-resistant composite components.