Effect of Annealing on the Room-Temperature Dry Sliding Tribological Behavior of Additively Manufactured Neat and Graphene-Reinforced PLA

Abstract The influence of annealing on the dry reciprocating tribological behavior of fused filament fabrication (FFF)-printed neat polylactic acid (PLA) and graphene-reinforced PLA was investigated. Specimens were annealed at 90, 100, and 120 °C for 60, 120, and 240 min and tested under dry reciprocating sliding against a 6-mm Al 2 O 3 ball according to ASTM G133. Friction was obtained from the tangential-to-normal force ratio, while the specific wear coefficient was determined from gravimetric volume loss normalized by normal load and sliding distance. Worn surfaces were examined by stereomicroscopy and scanning electron microscopy. Annealing substantially improved the wear resistance of both materials: relative to the as-printed condition, the specific wear coefficient decreased by up to approximately 74% for neat PLA and 93% for graphene-reinforced PLA. Friction followed a different trend: among the annealed conditions, the mean coefficient ranged from approximately 0.51–0.73 for neat PLA and from 0.48 to 0.54 for graphene-reinforced PLA. The wear reduction was associated with concurrent annealing-induced changes in crystallinity and mechanical properties; these relationships are therefore interpreted as phenomenological correlations rather than independent causal relationships. Graphene-reinforced PLA exhibited lower wear than neat PLA throughout the investigated conditions; the separation between the two material datasets was not fully described by the bulk properties considered here. Microscopy showed surface morphologies consistent with extensive plastic deformation, tearing, and delamination-like damage in neat PLA and with milder grooving, localized cracking, and compacted debris in graphene-reinforced PLA. Under the investigated dry reciprocating sliding conditions, annealing and graphene reinforcement were associated with lower wear.