Tribological Behaviors of Tetrazole Based Protic Ionic Liquids as Additives in Water

Six examples of protic tetrazole based ionic liquids were synthesized by a simple neutralization reaction from substituted tetrazoles and ethanolamine compounds. The tribological performances and mechanism were further investigated when these tetrazole based ionic liquids were employed as additives in water. The structures of the synthesized ionic liquids were confirmed by FTIR. Furthermore, it was found that the tetrazole based ionic liquids had good solubility in water and were noncorrosive. The tribological behaviors of the tetrazole based ionic liquid additives were tested by amicro-vibration friction and wear testing machine (SRV-V) and three-dimensional profilometer. The results demonstrated that the average friction coefficient was about 0.15 and the wear volumes lied below 6×10⁻⁴ mm³, indicating the tetrazole based ionic liquid additives can effectively improve the tribological performances of pure water. In particular, the friction coefficient was reduced by about 66% and the wear volume decreased by about 85% when 2.0% of 5-benzyltetrazole-N-butyldiethanolamineionic liquid (BTBE₂) was subjected in water. Additionally, these tetrazole based ionic liquids also greatly increased the extreme pressure property of water. Due to the presence of rigid phenyl ring and heteroatomic elements in the structure, especially the sulfur element in PTTBE₂, the seizure load was up to 750 N. The formation of physical adsorption film on the friction interface was further investigated by means of contact angle (CA) and quartz crystal microbalance (QCM). The CAs produced by the lubricant system containing tetrazole based ionic liquids were about 60^o in comparison to 80^o of water, indicating that a thicker physical absorbed film was generated. The ECR values of lubricant system adding BTBE₂ and PTBE₂ were relatively high, demonstrating that a strong covalent-binding effect existed between the surfaces of steel friction pairs. As a result, the tribochemical reactions occurred and a useful tribochemical film formed on the contact surface. Moreover, SEM micrographs showed that water containing tetrazole ionic liquid additives showed regular and small plow grooves, and much smaller wear. The XPS displayed that Fe₂O₃ and FeO were produced on the surface of friction pairs, indicating that a tribochemical protective film composed by the versatile organic and inorganic compounds formed during the friction procedure. Based on the above results, a possible lubricant mechanism was proposed based on the unique of structure these tetrazole based ionic liquids including the portic typemolecules, the rich electron heteroatoms and the rigid phenyl rings, which led to the formation of the powerful physical absorbed films and tribochemical films. As consequences, the combination of the above physical absorbed films and tribochemical films effectively enhanced the order and structural strength of the friction film on the contact surfaces and reduced the transverse shear force in the sliding process, which straightly led to excellent tribological performances of this tetrazole ionic liquid-water lubricant system.