SWCNH as additives to improve thermal and tribological properties of conventional lubricating oils for refrigerant and air conditioning applications

At present, the refrigeration industry is working towards an equilibrium between environmental safeguard and energy saving, so the conventional working fluids in refrigerator compressors are being replaced by more ecological solutions. Among the natural refrigerants, CO2 seems to be the most promising and it is being studied for possessing many useful features. A critical component in all refrigeration and air-conditioning systems is the compressor, which contains both the lubricant and the coolant and operates at high contact pressure and temperature. In the last years, the tendency for high speeds, loads and efficiency is continuously raising, and the sliding conditions are becoming increasingly severe on moving components. The introduction of alternative refrigerants and lubricants changes the strictness of the tribological contacts, increasing operational failure probability for traditional designs. In this context, the tribology of critical couplings and the studies on new materials and surface solutions are under intense investigation. Recent research has focused on developing wear resistant surface modifications and new more efficient lubrication systems. Variations of the applied refrigerants usually require the design of refrigeration apparatus to be adapted and lubricant to be re-selected, since the ability of the oil to provide adequate lubrication and its compatibility with the refrigerant affects the energy efficiency, reliability, and durability. Recently nanolubricants have been widely studied as alternative solutions to conventional lubricant oils, because of interesting improvements of friction decrease performances and load-carrying capability. Adding nano-objects to lubricating liquids can reduce friction and wear in surfaces that operate in sliding contact. Of particular interest are nanomaterials such as Single Walled Carbon Nanohorns (SWCNH). In this work, a study on the tribological and thermal properties of nanolubricants for vane-on-roller systems is carried out, by testing suspensions containing different weight concentrations of SWCNH in oil. Poly-Alkylene Glycol (PAG) has been selected as the base fluid to prepare nanolubricants, and suspensions have been characterized both as regards the viscosity and the stability over time. Tribological and thermal behavior of produced nanolubricants is evaluated through wear tests and thermal diffusivity photo-acoustic measurements at different temperatures. Nanolubricants containing up to 1%wt in PAG oil were prepared by a two-step method and tribologically characterized through rotational ball-on-disk wear tests. Experiments were carried out at 25° and 70°C and wear tracks were analyzed by optical and electron microscopy and stylus profilometry. PAG based nanolubricants achieved improvements in coefficient of friction. In fact, the coefficient of friction was significantly lower for nanofluids if compared with the values recorded for the base oil. The friction force reduction could be attributed to the mobility of the nano-objects in addition to the reduced contact area obtained from the presence of nanoparticles sited between sliding surfaces. Analyses carried out on wear tracks revealed that nanohorns played an important role in protecting surface against wear phenomena. SWCNH in lubricant oils lower the wear amount compared to raw oil. Nanohorns could prevent the counter body surface to enter in direct contact with the sample surface and carry part of the applied load, thus reducing wear of the substrate through rolling and sliding activity. No exfoliation nor protective film formation was supposed, since no residuals of broken nanohorns nor carbon film were detected on wear tracks. Finally, Stribeck tests were performed on the most promising system, to evaluate benefits brought by SWCNH in all lubrication regimes. Thus, this study demonstrates the ability of SWCNH to reduce friction and wear phenomena under boundary lubrication conditions, which are the most severe for tribological couplings.