Soot Nanoparticles Generated from Tribofilm Decomposition under Real Engine Conditions for Identifying Lubricant Hazards

Lubrication of an internal combustion engine is critical for energy and material losses. Engine lubricants contain a number of functional additives including zinc dialkyldithiophosphate, which is a commonly used antiwear additive that forms by in situ decomposition a protective interface at the metal surface. Here, we present a detailed nanoscale investigation of carbonaceous soot nanoparticles generated from real engine conditions. By combining macroscale XPS with high-resolution STEM-EELS-EDX, we reveal that such a soot nanoparticle matrix contains also 3–5 nm ZnO-based nanoparticles with additions of phosphorus and sulfur, originating from the organometallic antiwear additive. Under the consideration of the obtained chemical information on the carbonaceous matrix and (ZnO:P,S) nanoparticles and the generally known suggestion of potential toxicity for soot nanoparticles, our method allows us to predict nanoparticle-based hazards from mechanochemical applications and also their formation mechanism. These are critical information and also the basis of toxicity assessment, both for theoretical predictions and experimental testing for the estimation of overall life-cycle analysis, including the environmental impact. Our results unravel the tribofilm decomposition under real field conditions and hint toward potentially unidentified toxicological nanoparticle hazards with respect to organophosphate-containing lubricants.