Continued from page 14

With graphene being a two-dimensional material, it offers unique friction and wear properties that are typically not seen conventionally. Besides its well-established thermal, electrical, optical and mechanical properties, graphene can serve as a liquid or colloidal lubricant for bearings.

Graphene is ultrathin even with multiple layers, so it can be applied to systems with oscillating, rotating and sliding contacts to reduce friction and wear, as well as protecting bearings from corrosion when exposed to water, a process commonly known as tribo-corrosion. This is due to graphene exhibiting a slippery texture, which could potentially make it an excellent lubricant.

For certain applications such as heavy loads, greases are the lubricant of choice. These are simply oils mixed with a thickener. Although greases have greater lubricant drag than oils or dry lubricants, a reduced fill can provide a low torque bearing while also being appropriate for high speed applications.

Certain disadvantages of grease lubricants must be overcome to meet the demands of modern equipment operating conditions. Now, grease with a graphene additive is being explored to further reduce frictional torque, potentially making this type of lubricant suitable for applications where it wouldn’t have previously been considered.

A 2019 study conducted by the Institute of Machine Design and Tribology3

at Leibniz University, looked to

reach a conclusion regarding the frictional properties of graphene as a grease additive.

It investigated the frictional torque for sliding contacts and rolling bearing application. For the latter, two bearing test rigs were used to monitor the pivoting movement for more than one million test cycles. The results showed the frictional torque levels for the grease lubricant without a graphene additive compared to graphene grease in a series of thicknesses.

The frictional torque for the pure grease lubricated bearing averaged values of 150 Newton Millimetres

(Nmm), while the graphene platelets with a thickness of 6-8nm reduced frictional torque by approximately 80 percent and proved to be the most successful lubricant.

The study concluded that the thickness of the graphene layers affected the coefficient of friction (COF), but all thicknesses tested reduced friction significantly.

Surface treatments The process of adding a graphene coating to bearings is relatively simple compared to adding a traditional lubricant — graphene does not require any additional processing steps, other than just sprinkling a small amount of solution or spraying the solution on the surface, making this process simple, environmentally friendly and cost effective.

While grease with a graphene additive, reduces friction, a dry lubricant will reduce frictional torque even further. Research shows4

that the small number

of layers in graphene not only reduces friction in steel seven times more, but also wear-and-tear 10,000 times more, reducing tribo-corrosion.

Despite this, a one size fits all approach never works for bearing lubrication, so although adding a graphene coating may be more effective at reducing friction than graphene grease, this is not the only design consideration.

It is obvious that innovations in graphene and newly discovered materials have real potential as both dry lubricants and as a grease additive. Once fully developed, graphene could have positive impacts on many mechanical applications that could lead to tremendous energy savings.


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