Defect induced luminescence from surface modified calcium fluoride nanoparticles for in-situ temperature monitoring of lubricating oil

The utilization of functional nano-additives provides us a novel research idea to deal with the in situ temperature sensing/monitoring of lubricating oil. In this work, monodispersed and luminescent calcium fluoride (CaF2) nanoparticles with surface modification were synthesized by a hydrothermal approach, which were employed as nano-additives for polyalphaolefin (PAO6) lubricating oil. Because of the surface modification of hydrophobic oleic acid molecule, the as-prepared CaF2 nanoparticles exhibited excellent oil solubility. The tribological results further suggested that after dispersing in PAO6 lubricating oil, the CaF2 nanoparticles could maintain its original friction and wear properties. In addition to the above basic characteristics for lubricating nano-additives, it was attractive to find that the as-synthesized CaF2 nanoparticles exhibited a doublet luminescence characteristic. The intensity ratio of the two emission peaks presented a nearly linear relationship to the surrounding temperature, which was also effective for CaF2 nanoparticles dispersed in PAO6 base oil. By comprehensive consideration of the oil dispersibility, tribological properties and temperature responsiveness, the as-synthesized CaF2 nanoparticles should be promising nano-additives for temperature monitoring of lubricating oil.