Self-healing tribocatalytic coatings

Friction is inevitable but wear is controllable! This is the mantra used to develop any mechanical system, especially when they involve rotating and sliding parts. Machine lubrication works wonders in such systems, they help in reducing the impact of shear stress on vulnerable surfaces. But we have a catch here! Though traditional liquid lubricants reduce friction and wear, its exposure to high contact load may lead to failure of the solid surfaces. Making its use both ironic and counterproductive.

This issue paved the way to the development of solutions (or additive packages) which are pre coated on surfaces which must endure a subtantial amount of loading and hence they are highly prone to wear. These coatings help improve the reliability of the sliding surfaces. Carbon based films such as graphene, nanodiamonds and DLC (diamond like carbon) films gained importance in the field of tribological research. Though this seems to solve our problems, unfortunately the good news ends here. If the coating cannot self-recover, these coatings will eventually fail, . In other words, if the coating has a self-healing capability, which will be possible in the presence of catalytically reactive environments, these contacts tend to perform the best.

In this experiment conducted by a group of researchers from the USA, the potential use of MeN-Cu based tribocatalytic coatings in alkane environment (decane, dodecane and hexadecane) are tested. The nano composites used in this experiment are MoN-Cu, VN-CU and MoVN-Cu. VN-Cu has the lowest hardness among its counterparts owing to its 5.9 wt.% of Cu presence while MoVN-Cu has the highest hardness of 30.3 GPa and an elastic modulus of 346 GPa. The grain sizes of the coatings are calculated to be 7-8 nm, 12-13 nm and 7-8 nm for MoN-Cu, VN-Cu and MoVN-Cu respectively.

A set of tribological tests were performed to determine the best coating. Pin on ball disk tribometer equipped with a 200 ml liquid cell and heating stage is used. Al2O3 balls with a roughness of 20 nm is used as counter bodies. The reciprocating mode is at 2 Hz with a 1.4 mm total stroke length at a maximum linear speed of 0.44 m/s. A 1N load at 50 °C is used to test decane, followed by wear track analysis. These results are compared with 55-60 HRC uncoated steel. MoN-Cu nanocomposite stood out with the lowest co-efficient of friction for this particular load case and also by creating protective film material buildup. When this is analyzed with Raman spectroscopy, high intensity formation of DLC (Diamond like carbon) is observed. With this observation an interesting interpretation is that, though it is considered good to have high surface hardness, it has to be low enough to release catalytic centers for material buildup. It is also important to note that all three coatings exhibited lower wear rates when compared to the reference.

Fig1. A graph representing wear track vs applied load correlation at different temperatures (Taken from [1])

Considering the exemplary performance of MoN-Cu when compared to its counterparts, this combination is taken for further testing. There loads vary from 0.25-1 N and temperatures of 25 °C- 30 °C immersing in decane, dodecane and hexadecane. During the application of constant pressure and lower temperature the COF (co-efficient of friction) is not steady with high values. With the increase of temperature, the COF tends to get steadier. In the case of dodecane, increasing contact loads higher wear track width formation. In Hexadecane the wear rate reduces. One common phenomenon that occurs in all the three cases is that the formation and replenishment of carbon-based tribo film occurs simultaneously. But the overall result varies because of the domination of one mechanism over the other. To attain better understanding on the effect of alkanes on the DLC film formation a series of Raman maps were generated for the wear tracks during sliding. Though the formation of DLC is significant for all the three environments but the rate of the film formation is high for hexadecane.

The three coatings have shown potential to become good lubricating surfaces. Material buildup was observed in the contact areas for all the considered cases. In conclusion, all the three nanocomposites showed near to zero wear volume. This encourages the selection of coatings layers that have the ability for material build whie, at the same time, reduce friction.

[1] Asghar Shirani, Yuzhe Li, Osman Levent Eryilmaz & Diana Berman, Tribocatalytically-activated formation of protective friction and wear reducing carbon coatings from alkane environment. https://doi.org/10.1038/s41598-021-00044-9