Load-adaptive tribology of in - situ compositionally graded Ti-6Al-4V/graphite composites produced by powder metallurgy

Abstract This study investigates the tribological behaviour of an in situ fabricated compositionally graded Ti-6Al-4V/graphite surface architecture produced by powder metallurgy, in which a graphite-containing surface layer (5 wt% graphite) is metallurgically bonded to a monolithic Ti-6Al-4V substrate to decouple lubrication and load-bearing functions through the depth. Reciprocating dry sliding tests were performed against a 10 mm Al 2 O 3 ball under 2.5 N and 5 N using a 5 mm stroke, 6.37 Hz frequency, and a fixed sliding distance of 250 m (n = 3 per condition). The frictional response was correlated with 3D surface topography, hardness gradients, worn-surface morphology, EDX spot analysis, and X-ray diffraction (XRD) phase analysis to evaluate graphite retention and potential TiC formation; XRD indicates retained graphite while TiC-related reflections are weak, suggesting only minor TiC (if any). Wear performance was quantified via wear-track volume loss (3D profilometry) and the specific wear rate. To assess the sensitivity of the graded surface to near-surface mechanical conditioning, a secondary surface severe plastic deformation step (shot peening) was applied. The graded architecture exhibits a clear hardness hierarchy, with the graphite-containing surface layer (∼400–430 HV) transitioning to the Ti-6Al-4V substrate (∼350–380 HV). Shot peening increases near-surface hardness to ∼470–510 HV (first ∼400–500 μm) and generates a peak–valley topography. At 2.5 N, all conditions operate in a moderate-to-high friction regime; the graphite-containing surface shows pronounced friction scatter consistent with a discontinuous third-body regime. At 5 N, graphite-containing graded surfaces transition to stable low-friction sliding ( μ ̅ ≈ 0.15–0.16), accompanied by more extensive residue coverage and a carbon-enriched third body confirmed by EDX. The secondary surface strengthening treatment does not change the load-triggered lubrication mechanism but improves friction regularity and worn-track uniformity by stabilizing third-body accommodation. Overall, the results highlight the in situ graded Ti-6Al-4V/graphite architecture as the primary driver of load-adaptive tribological performance.