Understanding the interfacial behaviors during superlubricity process of a-C:H film coated on the rough bearing steel surface

Hydrogenated amorphous carbon (a-C:H) film can achieve superlubricity in inert gas atmosphere in the previous reports, but the roughness (at atomic scale or several nanometers range) of most employed substrate surfaces for coating is much smaller than the grade of engineering application. Aiming at disclosing the influence mechanism of asperities of rough surface with a-C:H film on superlubriciy, in this work, the a-C:H film was deposited on the rough bearing steel surface (arithmetic mean height Sa ~ 128 nm) by ion beam deposition method. The tribological tests show that the friction coefficient can maintain a superlow level in the range of 0.004 ~ 0.008 for at least 80,000 sliding cycles. Multiwavelength Raman spectra indicate that more rubbing cycles lead to the consumption of the hydrogen atoms and the increasing of the sp2 clustering of a-C:H film coated on counterpart balls. The hydrogen content after the maximum cycles rubbing is reduced by 3.95–7.85% in contrast to the fewer friction cycles. The estimated contact pressure of the rough surface rises more than 4 times as compared with the apparent contact pressure of 1.17 GPa for the smooth surface. The results highlight that although the hydrogen atoms in summits of surface are consumed during friction, the materials keeping the original hydrogen content of a-C:H film in the valley gradually participate in rubbing and undertake the role of superlubricity. These findings can provide a guidance towards a possibility of the application of highly-hydrogenated a-C:H film coated on the moving parts of industry equipment.