263780 An Approach to Low Friction by Combination of Mesoporous Titanium Dioxide Film and Ionic Liquid
Titanium (Ti) coated with native dense titanium dioxide (TiO2) layers, as a robust substrate, provides advantages relative to silicon and the other metals in microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) due to its superior fracture toughness, better electrical conductivity, and excellent corrosion resistance. However, poor friction on TiO2 surface affects MEMS/NEMS performance and in some cases, can even prevent devices from working, limits the durability and reliability of Ti on MEMS/NEMS.
Ionic liquid (IL), is called green solvents for their non-volatile, non-flammable and high thermal and chemical stabilities, having attracted increasing attentions on MEMS/NEMS. Its characteristics are also just what high-performance lubricants demand in MEMS/NEMS. However, the IL films form on solid surfaces by the way of weak physisorption rather than strong chemical bonding. Some approaches have been tried with chemical functional groups to fix IL film as a fluidic nature for reducing friction. However, the functional groups are susceptible to oxidation, hydrolysis or thermal degradation. Consequently, there exists an ongoing need for geometrical roughness-induced nano-patterned structures to hold IL films as lubricants to reduce friction. Atomic force microscopy (AFM) with a nanoscale sensitivity, is utilized to explore the molecular-level factors that govern in nanofrictional fields.
In this work, mesoporous TiO2 film was fabricated on Ti based on titanate to provide nano-patterned pores due to the existence of small amount of TiO2(B). 1,2-propanediol/choline chloride deep eutectic solvent, as an IL, is held in the nano-patterned pores to from stable IL films. The nano-patterned pores induced by TiO2(B) could provide a fishing net-like support to contain IL films and ensure the maintenance of the IL within the pores by capillary force. In comparison, dense TiO2 film was also prepared to support IL film, but it is not IL-philic enough, and IL can not from a stable film on the TiO2 surface. Obviously, only IL weak physisorption on support could not form a stable lubricating film. By AFM, the friction coefficients of mesoporous TiO2 film and IL filled in mesoporous TiO2 film are measured respectively as 0.0026 and 0.0011. IL addition is in a large part responsible for the friction coefficient of mesoporous TiO2 film decreasing to more than 1/2. With the support of nano-patterned mesoporous TiO2 film, IL could form a stable film to act as a molecular-level lubricant for reducing the friction. It will have a strong impact on a broad range of applications of Ti on including but not limited to MEMS/NEMS. It is also valuable in fundamental understanding of interfacial phenomena in nanostructures to provide a bridge between science and engineering.