Residual-Stress Engineering of Fullerene-Derived Tribofilms: A Sin²? Assessment of Tensile-Layer Suppression in Steel Contacts

Abstract

Residual tensile layers that form beneath sliding asperities play a decisive role in initiating delamination wear. In this work we use the sin²? X-ray diffraction method to quantify the stress state of steel surfaces protected by ? 0.5 wt % Renox modified-fullerene (C??-NP) lubricant additive. Diffraction from the {311} ferrite reflection at six ? tilts yields a linear lattice-strain gradient of 0.00105 ± 0.00003, which converts – through Prevey’s plane-stress formulation with E = 180 GPa and ? = 0.30 – to an in-plane tensile stress of 115 ± 4 MPa. This value lies at least a factor of three below the delamination threshold reported for the same alloy in the Renox project appendix, confirming that the self-assembled 1–3 nm fullerene film lowers the subsurface stress to a mechanically benign level. Sensitivity analysis shows that plausible ±5 % variations in E and ? shift ? by only ±6 MPa, underscoring the robustness of the result. Because a full sin²? scan requires fewer than ten minutes, the method offers a rapid, nondestructive metric for process control. The findings establish residual-stress relaxation – alongside friction reduction – as a critical performance attribute of fullerene-based tribofilms and provide a straightforward quality-assurance tool for their industrial deployment.