Tuning dry friction with micro-honeycomb patterns

honeycomb-pattern
Adapted from the original article

Controlling friction is one of the top priorities for many tribologists. The friction in bearings has to be reduced to increase the energy efficiency of numerous devices, while friction in transmission systems has to be increased for effective power transmission.

Recently, a joined group of researchers from various universities in China addressed the frictional properties of metallic glasses. These metallic alloys posses a number of interesting properties, such as an isotropy, high elasticity, high hardness, superior corrosion resistance, low internal friction. One of the promising applications of metallic glasses is the development of highly hydrophobic surfaces (using micro-patterning) of long lifetime.

The long term performance of the metallic glass based hydrophobic surface will be determined by the frictional and consequently wear behavior, which are in turn controlled by the surface pattern. The question is then, what type of pattern would be most suitable for the friction minimization and lifetime maximization?

In case of lubricated contacts, friction can be reduced by the surface patterning. Optimally designed patterns improve load carrying capacity of the lubricant film. It is also suggested that the textured areas may serve as oil pockets, but also help to release the wear particles from the contact by trapping in the patterns.

In case of dry friction, the surface patterning, for example honeycomb structures, can also be used to tune friction. When the surface pattern is imposed on the surface, the real contact area is altered and it is well known that in micro-scale, the friction force is proportional to the real contact area  S_a and shear strength \tau:

F^{'}  \propto \tau S_a

The real contact area is proportional to the apparent contact area, and hence the decrease of the pitch of the honeycomb reduces the real contact area. Therefore, the friction is reduced. However, as it was discussed by the researchers, the decrease of friction is only seen down to a ceratin value and increase in friction is seen aftewards. This is ascribed to the effect of ploughing which start to dominate when the real contact area is so small (local contact pressure is high) that the pattern start to penetrate to the surface. The friction force of a spherical asperity is proportional to the penetration depth, which in turn scales with the asperity load  F_s as  h \propto F_s^{2/3}. On the other hand, the  F_s \propto F_n/S_a, where  F_n is the normal load. Taking this into account, one can get:

 F_f^{''} \propto h \propto {(\frac{F_n}{S_a})}^{2/3}

Therefore, the total friction coefficient is composed of two components and can be written as follows:

 F_f = F_f^{'} + F_f^{''} = \propto \tau S_a  + {(\frac{F_n}{S_a})}^{2/3}

The corresponding friction coefficient  \mu = F_f/F_n is shown in Fig. 1. A clear optimum can be seen which is a result of a competition of two friction effects.

friction variation
Fig. 1, Re-used from the original article

Further details can be found in the original article: Ning Li, Erjiang Xu, Ze Liu, Xinyun Wang and Lin Liu, Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces, doi:10.1038/srep39388.

Aydar Akchurin
About Aydar Akchurin 35 Articles

Editor-in-Chief, PhD (Tribology), Researcher at ASML, Eindhoven, the Netherlands. Expertise in modeling of lubrication, friction and wear.

5 Comments

  1. Embrace ploughing!

    Ploughing plastic deformation = beneficial work-hardening of the metal surface (when using SGANs to nanopolish).

    Further, true laminar flow of the lubricant (over the surface) is impossible in the presence of surface texturing. Further still, reduction in the actual contact area (versus nominal area) by texturizing is the chemistry for disaster; as you then must distribute the load over less mass. With lubricant shear conditions, you cannot count on hydrodynamic lubrication principles to save the day.

    In short, stop coating/texturizing and start nanopolishing.

  2. This is mostly interesting for dry contact. As long as I understand, they create hydrophobic surfaces using texture and they want to optimize also the lifetime of the texture by minimizing friction.

    • Here is the portion of the discussion that prompted the comment above:

      “In case of lubricated contacts, friction can be reduced by the surface patterning. Optimally designed patterns improve load carrying capacity of the lubricant film. It is also suggested that the textured areas may serve as oil pockets, but also help to release the wear particles from the contact by trapping in the patterns.”

  3. Oh, yes, but further analysis was considering only dry contact situation. So this is more interesting from that standpoint.

    • Yes, it is agreed that these glassy coatings may well be only suitable for dry contacts, yet there is extensive discussion of lubricated contacts (and wet experimental data) in the original Paper. Consider the following excerpt from the original Paper’s discussion section…

      “The above experimental results clearly reveal reduced coefficients of friction for the patterned Zr-based BMG surfaces under dry and wet conditions, as compared with the smooth surface. In general, surface patterns or asperities were regarded as the oil pockets to reduce friction in two ways, one is the secondary lubrication effect, namely the surface textures act as a secondary oil source thus supplying lubricant continuously to the contact area to reduce friction and retard galling. The other is hydrodynamic effect wherein the surface asperity builds an additional hydrodynamic pressure which separates both rubbing surfaces, increases the oil film thickness and enhances load bearing capacity. This is the main reason for what we observed in Fig. 2c, wherein the friction coefficient under wet condition is lower than that under dry condition. However, the friction coefficient is sensitive to the variation of pitch in both conditions, demonstrates the possibility to tuning apparent tribological performance by surface patterning.”

      As you know, there was also discussion of adhesion theory and the evils of plastic deformation.

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