Investigation of the chemical structure and antiwear performance of biochar used as a grease additive

The aim of the work was to investigate the influence of the microstructure of biochar additives on the antiwear performance of lithium greases. The novelty of this work is the investigation of the possibilities of effectively replacing the critical raw material (graphite), used as an anti-wear additive to greases, with biochar derived from agricultural waste and to study the impact of biochar on the friction surface. The initial grease based on paraffin oil and lithium stearate and four greases additionally enriched with 3 wt% or 5 wt% of biochar produced during the pyrolysis of flax or wheat biomass were tested. All the greases had an NLGI grade of 2. The biochar additives were examined using FTIR, and the I D /I G ratio was determined based on Raman spectra. Friction tests were carried out in a roller-block system made of 100Cr6 bearing steel. After the tests, the volumetric wear of the blocks was determined using a profilometer and the chemical structure of the friction surfaces was examined using SEM/EDS and XPS. The research has confirmed that a higher content of biochar in the grease reduces the volumetric wear of the blocks more effectively. The best tribological properties were demonstrated by the grease containing 5 wt% of flax biochar, which resulted in a 24% reduction of wear compared to the wear of the block lubricated with the model grease. It was found that additives with more highly ordered graphite structures (I D /I G =0.33) and without oxygen functional groups showed better antiwear performance than biochar additives containing such groups but with a more disordered structures (I D /I G =0.44). The wear tracks included iron oxides and thin layers of greases that were not removed when the blocks were ultrasonic cleaned after the tests. Further analysis will allow to determine the mechanism of boundary layer formation. Then researchers will be able to present the results in a subsequent paper. • Wheat- and flax-based biochar can be an effective AW additive to plastic greases • AW properties of the grease depend on the content of ordered graphite structures • During friction, a boundary layer of the grease is formed