Traditional diamond-like carbon (DLC) films suffer from prolonged friction running-in stage, unstable atmospheric lubrication performance and accelerated interfacial oxidative wear, severely restricting their service application in precision bearing stainless steel parts. To break through the performance limitation of single-element doped DLC coatings, improve comprehensive tribological stability of composite modified DLC films, and clarify the undisclosed correlation between Mo/O atomic ratio, film microstructure evolution and interfacial friction wear mechanism, the work aims to explore the regulation law of the Mo/O atomic ratio on tribological properties and reveal an original interfacial cyclic deterioration mechanism. The 9Cr18 martensitic stainless steel substrate was sequentially treated with ethanol-acetone ultrasonic oil removal, ion bombardment cleaning and surface polishing to unify initial substrate roughness. Mo and O co-doped DLC films with six gradient Mo/O atomic ratios were fabricated by direct current pulse magnetron sputtering. Fixed sputtering power, pulse frequency, working air pressure and deposition duration were adopted, and Mo/O atomic ratio was accurately regulated only by adjusting the spatial distance between substrate samples, Mo target and oxygen auxiliary target without changing other process parameters. AFM and SEM-EDS were used to test surface roughness, cross-section morphology and elemental distribution. Raman spectroscopy and high-resolution TEM were applied to characterize carbon bond structure, graphitization degree and internal defect characteristics. Ball-on-disk friction-wear tests were implemented under room-temperature atmospheric environment, fixed normal load, rotating speed and friction stroke to obtain friction curve and wear rate data. Combined with first-principles density functional theory calculation, oxygen adsorption energy and molecular interfacial binding energy of molybdenum oxide phases were quantitatively simulated. Key comparative experimental data showed that with the rise of Mo/O atomic ratio, film island-like grain growth was optimized effectively, average surface roughness decreased from 4.53 nm to 1.32 nm, and Raman ID/IG ratio increased by 27.6%, which remarkably promoted amorphous carbon transformation and expanded sp2 carbon domain volume. Core novel findings were summarized as follows: high Mo/O ratio eliminated intra-film molybdenum oxide agglomeration defects and realized homogeneous element distribution. It also inhibited physical-chemical oxygen adsorption at friction contact interface in atmospheric environment. For tribological properties, the optimized sample shortened friction break-in period from 14 min to 2 min, reduced average friction coefficient from 0.19 to 0.05, and cut volumetric wear rate to the minimum value of 3.4×10-7 mm3/(N·m). Worn pair characterization proved that high Mo/O ratio accelerated uniform dispersion and interfacial transfer of sp2 carbon domains, forming dense, low-friction lubricating transfer film on GCr15 ball surface. First-principles simulation verified that MoOx compounds possessed high negative oxygen adsorption energy, which enabled spontaneous capture of atmospheric oxygen. This work proposes an original self-circulating degradation mechanism for low Mo/O ratio DLC films for the first time: low Mo/O ratio induces excessive Mo oxide phase precipitation, enhances oxygen adsorption capacity of film system, consumes metallic Mo active components and further reduces Mo/O atomic ratio, forming irreversible performance vicious cycle and triggering microstructure deterioration and tribological failure. By contrast, appropriately increased Mo/O ratio suppresses oxide nucleation, promotes in-situ carbon graphitization and high-quality transfer film formation, synergistically optimizing friction state and anti-wear performance. The work provides novel mechanism support and process optimization strategy for tribological doped DLC coating design.
Key words
magnetron sputtering /
DLC /
friction wear /
wear mechanism /
doping /
first-principles
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