UK researchers say this opens the door for more environmentally-friendly lubricants

Tribology (the study of friction and wear) might not be the first thing that comes to mind when considering the energy crisis but close to a quarter (23%, or 119 EJ) of the world’s total energy consumption originates from tribological contacts. 20% (103 EJ) goes towards overcoming friction, and the remaining 3% (16 EJ) accounts for the replacement of components that have failed due to wear. The main route to reducing this energy loss is through the design of effective, efficient lubricants that can form a protective barrier between rubbing surfaces.

Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118742] researchers from the University of Leeds report on a novel lubricant formulation that incorporates nanodiamonds which, they say, significantly reduces the coefficient of friction and improves the wear performance of steel plates at industrially-relevant temperatures.

They started with fairly typical constituents – a polyalphaolefin (PAO) base oil, which is the most common synthetic base oil employed in industrial and automotive lubricants, and glycerol monooleate (GMO); an extensively-used organic friction modifier. The primary additive was zinc dialkyl dithiophosphate (ZDDP). While ZDDP has long been utilised as a lubricant additive in car engines due to its impressive anti-friction and anti-wear performance, in high concentrations it can have a negative impact on the fuel combustion catalysts that reduce emissions. The researchers chose to incorporate a maximum of 0.2 wt% ZDDP in their test formulations. As a secondary additive, they used nanodiamonds (NDs). These ultrafine carbon-based particles are known to have outstanding thermal and chemical stability, and are extremely hard, imparting excellent wear resistance to surfaces. NDs were added at 0.05 wt% to four of the nine test formulations.

The tem used a range of analytical techniques, including high-resolution TEM, Fourier transform infrared spectroscopy, dynamic light scattering, and rheometry to characterise each of the lubricants. This analysis showed that in the formulations that included NDs, the NDs were carboxylated, which helped them disperse well in the oil medium. Evidence of particle agglomeration was found in some samples after 30 days. As a result, all tribological tests were carried out immediately following preparation of the nanolubricants.

A reciprocating (sliding) tribometer with steel surfaces was used to measure the tribological performance of each lubricant. In order to simulate real engine conditions in internal combustion vehicles and hybrid vehicles, two elevated test temperatures were chosen: 50 °C and 80 °C. PAO oil alone exhibited the highest overall coefficient of friction at both temperatures; close to 0.11. In contrast, nanolubricant PGZN (98.75 wt% PAO, 1 % GMO, 0.2% ZDDP, 0.05% ND) had a measured friction coefficient of 0.06. This was the lowest value measured at 50 °C. The nanolubricant’s performance was even more impressive at 80 °C, where an average friction coefficient of 0.04 was measured. In all cases, the formulations that contained nanodiamonds outperformed those without NDs.

Following the sliding tests, the resulting wear surfaces were characterised. Here, ZDDP was the star. The additive was shown to help form thick, solid, pad-like tribofilm on the rubbing surfaces, preventing metal-to-metal contact and minimising the volume of material lost through wear. The inclusion of NDs had a polishing effect on the contacting

surfaces, further protecting them from wear. Structural analysis of the tribofilm formed between rubbing surfaces at 80 °C showed that the inclusion of NDs resulted in a significantly thicker film – on average, PGZ’s tribofilm was 3-5 nm thick after two hours of sliding, while PGZN’s was 20-22 nm thick. The authors attribute the nanolubricant’s low friction coefficient to the formation of this thick layer.

Their proposed mechanism for tribofilm formation involves adsorption of GMO onto the steel surface, followed by sliding-induced decomposition of ZDDP to form a film. This film then acted as a matrix for the NDs to embed and mechanically interlock themselves in place, increasing the film hardness and robustness, and reducing the measured friction coefficient. They write, “The experimental findings presented demonstrate the potential to develop environmentally friendly low sulphated ash, phosphorus, sulphur (SAPS) lubricants by utilising ND/GMO/low concentration ZDDP synergy.”

Future work will include developing a molecular dynamic simulation of the mechanism in order to verify their experimental results.


A.K. Piya, L. Yang, A. Al Sheikh Omar, N. Emami, A. Morina. “Synergistic lubrication mechanism of nanodiamonds with organic friction modifier,” Carbon 218 (2024) 118742. DOI: 10.1016/j.carbon.2023.118742