Realizing the effective conversion and utilization of CO₂ is of great significance for alleviating the greenhouse effect and protecting the ecological environment. A strategy of in situ conversion of CO₂ to carbon-based friction film without additional energy and catalyst is put forward to enhance the tribological properties of the lubrication system. The tribological properties of three kinds of olamine solutions after CO₂ absorption were examined by a four-ball friction testing machine. The experimental results indicated that the friction coefficient of olamine solutions decreased significantly after CO₂ absorption. The mechanism study revealed that the alkamine solution can react with CO₂ to form carbamate, and the negatively charged carboxylic acid group in carbamate can be adsorbed on the positively charged metal surface and form a dense molecular brush structure, thereby prevented the direct contact of friction and reduced friction. At the same time, the carboxylic acid base group in the CO₂-absorbing alkylamine solution was more strongly bound to the metal substrate than that in the unabsorbed CO₂ alkylamine solution, which made it easier for alkylamine to undergo biochemical reactions under friction conditions and generate carbon-based friction film, further reducing friction. Three alcohol amines were selected as absorption media, namely ethanolamine (MEA), 2-methyl-2-amino-1-propanol (AMP), and 3-aminopropanol (3-AP). All were diluted to a weight concentration of 25% (water 60g, alcohol amine 20g), and CO₂ was bubbled into them for 20, 40, and 60 min, respectively. Separately, the ethanolamine solutions with absorption time of 0 min, 10 min, 20 min, 30 min, 40 min, 50 min and 60 min were investigated by a four-ball friction tester (30 minutes, 98N, 300r/min) The friction coefficients of MEA, AMP, and 3-AP before absorbing CO₂ were 0.19, 0.20, and 0.29, respectively. After absorbing CO₂ for 60 minutes, they were reduced to 0.13, 0.13, and 0.15, respectively. The friction coefficients of MEA and AMP decreased by about 33% after absorbing CO₂, while the friction coefficient of 3-AP decreased by about 50%. But it was found that the diameter of the wear scars increased for all three alcohol amine solutions after CO₂ absorption Because AMP exhibited the best anti-wear and friction reduction performance before and after CO₂ absorption, AMP was selected as object for further study. Initially, at a constant load of 98N, the AMP solution with CO₂ absorption for 40 minutes was compared with that without CO₂ absorption under rotational speed of 300, 600, 900 and 1 200 r/min. Subsequently, load conditions were varied to 98, 147, 196 and 245 N while maintaining a constant speed of 300 r/min. Finally, two different AMP solutions underwent a one-hour friction experiment under a load of 98 N and speed of 300 r/min. After the experiment, the friction coefficient decreased and the wear increased after absorbing CO₂. The worn surface was analyzed using non-contact three-dimensional surface profilers, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) to examine element distribution, elemental content and chemical states. It was observed that the wear scars generated from AMP solutions without CO₂ absorption appeared relatively clean, whereas the wear scar from AMP solutions with CO₂ absorption for 40 min was surrounded by a significant number of black substances. In other words, the AMP solution that had absorbed CO₂ for 40 min formed a carbon-based tribofilm after undergoing four-ball friction test. The EDS scanning was utilized to analyze the elemental composition of the two wear scars. It was observed that, compared with AMP solutions without CO₂ absorption, the carbon content of wear scar from AMP solutions with CO₂ absorption had increased from 3.19% to 6.49%, nitrogen content had increased from 0.13% to 0.46%, and iron content had decreased from 94.48% to 90.76%.

The oxygen content remained relatively unchanged. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence of C=O, C-C and C-O bonds in the C1s spectrum, with binding energy peaks at 288.9, 288.4, 285.3, 285.1 and 284.8 eV. The N1s spectrum had shown a binding energy peak at 399.7eV, indicated the existence of a C-N bond. Additionally, the O1s spectrum had exhibited peaks at 532.7, 532.0, 531.6, 530.6, 530.3 and 529.9 eV corresponding to C-O, C=O and Fe-O bonds; while the Fe2p spectrum had displayed peaks at 724.4, 724.4, 711.0 and 712.0 eV indicated the presence of Fe - O and Fe₃O₄. During the process of friction behavior, the metal substrate had acquired a positive charge, led to the absorption of carboxylate anions in the solution and their accumulation on the metal substrate surface, formed a molecular brush structure. Meanwhile, the hydroxyl group at the other end of the anion had entered into the water phase. Under external loads, shearing force was more likely to occur, resulted in a reduction of friction coefficient. In cases where alcohol amine solution had not absorbed CO₂, it became difficult for anions in the solution to ionized and enriched on the metal substrate to form a molecular brush structure, thus hindered effective reduction of friction coefficient. Additionally, carboxylate ions ware prone to chemical reaction with metal substrates under high shear stress conditions, led to formation of carbon-based tribofilms and further reduction in coefficient of friction. However, the tribofilm had poor bearing capacity and was easy to be damaged during the friction process. When the tribofilm was damaged, the metal substrate strongly bound by anions was taken away, resulting in wear. With the progress of the friction test, new carboxylate anions ware adsorbed on the metal substrate again, and the above process was repeated constantly, resulting in increased wear of the metal substrate.