Earlier we reported about superlubricity achieved with a mixture of water and 30 [wt%] glycerol by researchers from Tsinghua University, Beijing. The same investigators just published a paper using this as a base lubricant and improving its wear resistance by creating a colloidal solution with nano-diamonds.
The nano-diamonds were synthesized by detonating an oxygen deficient mixture which is known as detonation nano-diamond (DND). The size distribution was found to be in the range from 50 – 120 [nm] with an average of 90 [nm]. This rather large size is due to aggregation owing to the large surface area of a single grain which has a typical size of 5 [nm].
Experiments were carried out with a ball on disc tribometer. A 10 [mm] ball was pressed on the disc with 3 [N] normal force at 0.15 [m/s] sliding velocity. After a running in period the same low friction coefficient of 0.006 was found upon repeating the tests from the previous publication.
Compared to the base lubricant lower wear was observed in the experiments with the colloidal solution. It was found that only 0.01 [wt%] of DND is enough to achieve a reduction of 42 [%] in the ball wear scar diameter. More importantly the presence of DND did not affect the earlier found superlubricity of the base lubricant. Roughening of the surfaces, however, was found for the colloidal solution due to the presence of the DND.
Based on the ball wear scar size the superlubricity was determined to be in the mixed lubrication regime and close to hydrodynamic lubrication (EHL). Implying that most of the load is carried by the hydrodynamic effect. The addition of DND thus moved the system closer to EHL owing to the reduction in wear. This was confirmed by stepwise lowering the sliding velocity effectively terminating the superlubricity.
The mechanism behind the wear reduction is the earlier described hydrogen bond layer on, in this case, both the steel surface and the DND surface preventing direct contact. This allows the DND to roll between two contacting asperities as shown in the image below. However, EHL is needed to sustain this mechanism otherwise the pressure exceeds the bearing capacity of the hydrogen bond layer and superlubricity will disappear.
The authors conclude that the base lubricant combined with (any) hydrophilic or hydrogen bond rich nano-particle can achieve superlubricity and will be subject of future investigations.
The details can be found in the original article: Chen, Z., Liu, Y., and Luo, J., 2016, “Superlubricity of nanodiamonds glycerol colloidal solution between steel surfaces,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 489, pp. 400–406.
Image credit: Chen, Z., Liu, Y., and Luo, J., 2016, “Superlubricity of nanodiamonds glycerol colloidal solution between steel surfaces,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 489, pp. 400–406.