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<p>To enable in situ, machining-compatible surface modification, sub-nanosecond laser irradiation in polyalphaolefin (PAO) oil was used to directly form amorphous Fe–C layers on medium-carbon steel. In this study, the structural and tribological characteristics associated with the resulting low-friction behavior are investigated. Laser irradiation in oil enables simultaneous carbon incorporation and liquid-confined ultra-fast quenching, producing a fully amorphous Fe–C-rich layer (~ 150–200&#xa0;nm). Ball-on-disk tests show sustained low friction (<i>μ</i> ≈ 0.10–0.12) only under sufficiently developed irradiation within an intermediate pulse energy range around 500–650&#xa0;μJ; lower energies yield fragile/discontinuous layers, whereas excessive energy (1000&#xa0;μJ) promotes crystallization as the solidification path approaches the TTT nose. AFM lateral-force measurements reveal progressive microscale friction reduction upon repeated sliding, accompanied by indentation evidence of softening limited to the topmost ~ 5&#xa0;nm. Raman spectroscopy and cross-sectional transmission electron microscopy further demonstrate friction-induced partial structural ordering and carbon enrichment within a thin surface zone (~ 5–10&#xa0;nm), while the underlying layer remains fully amorphous. These results suggest that the observed low-friction behavior is closely associated with a hierarchical structure combining a load-bearing amorphous Fe–C base layer with a dynamically formed, shear-facilitating surface tribo-layer.</p>

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Elucidation of the Low-Friction Mechanism in Sub-nanosecond Laser-Induced Amorphous Fe–C Layers Formed in Lubricating Oil

  • 期刊论文

  • 发表日期：

    2026年3月31日

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  • Xiaoxu Liu,
  • Masato Yamanaka,
  • Haruka Sasai,
  • Satoru Maegawa,
  • Shingo Ono,
  • Fumihiro Itoigawa

摘要

To enable in situ, machining-compatible surface modification, sub-nanosecond laser irradiation in polyalphaolefin (PAO) oil was used to directly form amorphous Fe–C layers on medium-carbon steel. In this study, the structural and tribological characteristics associated with the resulting low-friction behavior are investigated. Laser irradiation in oil enables simultaneous carbon incorporation and liquid-confined ultra-fast quenching, producing a fully amorphous Fe–C-rich layer (~ 150–200 nm). Ball-on-disk tests show sustained low friction (μ ≈ 0.10–0.12) only under sufficiently developed irradiation within an intermediate pulse energy range around 500–650 μJ; lower energies yield fragile/discontinuous layers, whereas excessive energy (1000 μJ) promotes crystallization as the solidification path approaches the TTT nose. AFM lateral-force measurements reveal progressive microscale friction reduction upon repeated sliding, accompanied by indentation evidence of softening limited to the topmost ~ 5 nm. Raman spectroscopy and cross-sectional transmission electron microscopy further demonstrate friction-induced partial structural ordering and carbon enrichment within a thin surface zone (~ 5–10 nm), while the underlying layer remains fully amorphous. These results suggest that the observed low-friction behavior is closely associated with a hierarchical structure combining a load-bearing amorphous Fe–C base layer with a dynamically formed, shear-facilitating surface tribo-layer.