Excerpt from the actual article:
...
To correlate XANES results with HR-TEM and gain detailed insight of embedded compounds, interplanar spacing of nanocrystalline region is measured and detailed in Table 3. Calculated values were compared with similar d-spacing of compounds possible from XANES results. Based on the XANES findings, it was speculated to detect various compounds such as ZnS, ZnSO4, Zn3(PO4)2, CaO, CaSO4, Ca3(PO4)2, FeS, Ca(OH)2, but no apparent match was found. Matching of interplanar spacing indicates the presence of only Ca-based compounds, hydroxyapatite (Ca5(PO4)3OH) and carbonate hydroxyl apatite (Ca5(PO4,CO3)3OH) in the soot structure. It appears that Zn phosphates, sulfates or sulfides are present in amorphous form, whereas crystalline particles are dominated by Ca-based compounds. The origin of Ca compounds is from the detergents such as alkyl benzene sulfonates wherein Ca occurs as CaCO3 used in engine oil formulations. These detergents are used to suspend insoluble polar debris and neutralize acidic byproducts of combustion. Also, they contribute to protective tribofilms formed on engine parts. Many researchers have studied the influence of Ca detergent chemistry on the tribofilm formation and have reported the formation of small chain calcium phosphate films on the rubbing surfaces.(53,59−61) The detection of chemically bonded or mechanically incorporated calcium phosphates in the turbostratic soot structure indicates either possibility of cross-interaction of soot particles with detergent decomposition products in engine oil or removal of Ca-based tribofilms by trapped soot during engine operation. One of the mechanism responsible for embedment of these nanocrystalline particles can be three-body wear which is a proposed wear model in several studies.(36,37,62,63,93) It can be postulated that soot particles/agglomerates in crankcase film can get trapped between two surfaces in motion and can experience three-body wear condition at extreme pressure and temperature, leading to the incorporation of nanocrystalline particles originating from calcium detergent chemistry in soot structure. These nanocrystalline particles had been reported to have higher hardness.(13,51,58) Thus, the presence of such mechanically embedded hard nanocrystalline particles in soot structure will make it more abrasive in nature and might promote soot-induced abrasive engine wear.
...
4. Conclusions
The variations in the carbonaceous structure and chemical composition of crankcase soot were mapped using XANES and HR-TEM. XANES analysis demonstrated the presence of zinc polyphosphate, calcium sulfate, calcium phosphate, and iron sulfate, and phosphate. The identification of short chain zinc polyphosphate suggested the interaction of soot with the antiwear tribofilm. HR-TEM results showed the nanocrystalline particles of hydroxyapatite and carbonate hydroxyl apatite embedded in the periphery of the turbostratic carbon structure of the soot. It was concluded that the mechanical embedment of the hard nanocrystalline particles during the three-body contact of soot particles in the crankcase oil altered its structure and could make it more abrasive in nature.
In this study, a different approach using high-temperature XRD was used to depict and correlate the oxidation stability with interplanar lattice spacing changes of soot. Results from XANES, HR-TEM, HT-XRD, BET techniques, and EDS analysis of the residue (left behind after oxidation) indicated that the interaction of lubricant additives and crankcase soot extracted from Mack-T-12 dynamometer engine test influences its structure and chemistry, which in turn affects its ease of oxidation.
This research work also focused on understanding the detrimental effect of increased levels of soot in lubricating oil on antiwear properties of additives and elucidating the mechanism by which abrasive soot promotes high wear in boundary lubrication conditions. The proposed dominant wear mechanism is the abrasion wear due to the rapid removal of the protective antiwear tribofilms by the abrasive soot particles/agglomerates. ZDDP exhibited the worst performance at a higher level of soot, whereas ionic liquid, DEDTP exhibited better antiwear performance. This indicates that ionic liquids may serve as a secondary antiwear protection for fully formulated diesel engine lubricants.