Microscopic and spectroscopic investigation of the carbon spheres after the tribological experiments illustrated their excellent mechanical and chemical stability.

Coefficient of friction and sliding speed versus time for neat oil and 3.0 CS-Oil using POD apparatus under (a) 22.2 and (b) 93.7 N applied normal load (error ±5%). Credit: ACS, Alazemi et al. Click to enlarge.

The team attributed the significantly better tribological performance of the hybrid lubricant to the perfectly spherical shape and ultra-smooth surface of carbon sphere additive filling the gap between surfaces and acting as a nanoscale ball bearing.

Reducing friction by 10 to 25 percent would be a significant improvement. Many industries are trying to reduce friction through modification of lubricants. The primary benefit to reducing friction is improved fuel economy.

—Farshid Sadeghi, Cummins Distinguished Professor of Mechanical Engineering and corresponding author

The spheres are 100-500 nanometers in diameter, a range that generally matches the surface roughness of moving components in engines. The spheres are able to help fill in these areas and reduce friction, said mechanical engineering doctoral student Abdullah A. Alazemi, lead author of the paper.

Friction and wear are primary factors in energy loss and failure in engines, with the majority of friction and wear losses occurring in the boundary and mixed lubrication regimes during engine startup/shutdown and low-speed operation.

In the boundary lubrication regime, lubricating surfaces generally are in contact with each other despite the fact that a fluid is present. While in the mixed lubrication regime, a thin lubricant film with an average thickness between 0.01 and 1 μm separates the lubricating surfaces. The tribological performances of traditional fluid lubricants do not meet the demands of new generation mechanical devices. As a result, there is continuous research for improving the tribological performance of lubricants.

Previous reports suggested that performance of oil lubricants can be improved by adding solid particles. These additives are beneficial in the boundary lubrication regime, where surface contact occurs even in the presence of fluid lubricant. … In spite of the good tribological behavior of current solid oil additives, there are several concerns about the complex synthetic methods used to create them and their toxicity and high cost. Another serious issue is performance degradation on prolonged use due to the poor mechanical and chemical stability of the solid additives.

… In order to mitigate these drawbacks, we have developed an ultrasonic assisted method for the rapid synthesis of ultra-smooth carbon submicrosphere oil additives possessing excellent mechanical and chemical stability.

—Alazemi

The spheres are created using ultrasound to produce bubbles in a fluid containing a chemical compound called resorcinol and formaldehyde. The bubbles expand and collapse, generating heat that drives chemical reactions to produce polymer particles. These polymeric particles are then heated in a furnace to about 900 ˚C, yielding the perfectly smooth spheres.

People have been making these spheres for about the last 10 years, but what we discovered was that instead of taking the 24 hours of synthesis normally needed, we can make them in 5 minutes.

—Vilas Pol, an associate professor of chemical engineering at Purdue University

Funding was provided by Purdue’s School of Chemical Engineering. Electron microscopy studies were performed at the Birck Nanotechnology Center in Purdue’s Discovery Park.

Future research will include work to determine whether the spheres are rolling like tiny ball bearings or merely sliding. A rolling mechanism best reduces friction and would portend well for potential applications. Future research also will determine whether the resorcinol-formaldehyde particles might themselves be used as a lubricant additive without heating them to produce pure carbon spheres.

Resources

• Abdullah A. Alazemi, Vinodkumar Etacheri, Arthur D. Dysart, Lars-Erik Stacke, Vilas G. Pol, and Farshid Sadeghi (2015) “Ultrasmooth Submicrometer Carbon Spheres as Lubricant Additives for Friction and Wear Reduction” ACS Applied Materials & Interfaces doi: 10.1021/acsami.5b00099