The application of biochar as an alternative to graphite in mineral greases – Tribological and rheological property studies

This study aims to investigate the possibility of replacing a critical raw material - graphite - with an ecological biochar material used as a functional additive to mineral greases thickened with lithium stearate. Biochar is obtained by pyrolysing wheat (Triticum aestivum) straw at 500°C in an inert nitrogen atmosphere. To compare the properties of mineral greases including biochar (BW) and graphite (G) additives, six grease compositions were prepared, taking into account the analysis of the effect of carbon additive concentrations (1 %, 3 %, and 5 % BW, and 1 %, 3 %, and 5 % G). Graphite (f < 35 µm) used in commercially available mineral greases serves the purposes of comparative studies. A grease made from a mixture of mineral oils and thickened with lithium stearate is the base grease. No significant changes resulting from replacing graphite with ecological biochar were observed in the rheological tests. The values of the parameters responsible for anti-scuffing properties also suggest that completely replacing graphite with eco-friendly biochar material has no negative effect on these properties of the grease (3 % G - 2617 ± 111 N/mm², 3 % BW - 2595 ± 220 N/mm²). Anti-wear tests reveal the micrometer graphite used has a detrimental effect on surface wear. However, the BW addition raises the wear limit by up to 43 % compared to the base grease. The analysis of the wear traces, along with the determination of surface roughness, demonstrated that the protective mechanism of the biochar-added grease is a consequence of several overlapping processes resulting from the specific properties of biochar.