Nowadays, refrigeration and air conditioning industries are working towards an equilibrium between environmental safeguard, energy saving and improvement of efficiency in devices. Thus, the conventional working fluids in refrigerator compressors are being replaced by more ecological solutions, like CO2. A critical component in all these 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 sliding conditions are becoming increasingly severe on moving components because of the continuous raise of applied speeds and loads. The introduction of substitute refrigerants and lubricants changes the strictness of the tribological contacts, increasing operational failure probability for traditional designs. Thus, recent research has focused on developing wear resistant surface treatments and innovative more efficient lubrication systems. The variations of the applied refrigerants usually require the re-design of refrigeration apparatus and re-selection of the lubricant, 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 new solutions, thanks to interesting improvements of friction decrease performances and load-carrying capability. Adding nano-objects to lubricating fluids can reduce friction and wear on sliding surfaces. Of particular interest are nanoparticles such as Single Walled Carbon Nanohorns (SWCNHs). In this work, a study on the tribological properties of nanolubricants for vane-on-roller systems is carried out, by testing suspensions containing different weight concentrations of SWCNH in oil. Suspensions have been characterized both as regards the viscosity and the stability over time prior to carry out tribological tests. Nanolubricants containing different concentrations of nano-additives in oil were prepared by a two-step method and tribologically characterized through rotational ball-on-disk wear tests. Wear tracks were analyzed by optical and electron microscopy and stylus profilometry. SWCNH 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 raw oil. The friction force reduction could be attributed to the mobility of the nano-objects in addition to the reduced contact area obtained by the presence of nanoparticles sited between sliding surfaces. SWCNHs in lubricant oils lowered 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.
Improved tribological properties of Single-Walled Carbon Nanohorns based nanolubricants: refrigeration applications
V Zin;F Agresti;S Barison;L Colla;M Fabrizio;C Pagura
2015
Abstract
Nowadays, refrigeration and air conditioning industries are working towards an equilibrium between environmental safeguard, energy saving and improvement of efficiency in devices. Thus, the conventional working fluids in refrigerator compressors are being replaced by more ecological solutions, like CO2. A critical component in all these 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 sliding conditions are becoming increasingly severe on moving components because of the continuous raise of applied speeds and loads. The introduction of substitute refrigerants and lubricants changes the strictness of the tribological contacts, increasing operational failure probability for traditional designs. Thus, recent research has focused on developing wear resistant surface treatments and innovative more efficient lubrication systems. The variations of the applied refrigerants usually require the re-design of refrigeration apparatus and re-selection of the lubricant, 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 new solutions, thanks to interesting improvements of friction decrease performances and load-carrying capability. Adding nano-objects to lubricating fluids can reduce friction and wear on sliding surfaces. Of particular interest are nanoparticles such as Single Walled Carbon Nanohorns (SWCNHs). In this work, a study on the tribological properties of nanolubricants for vane-on-roller systems is carried out, by testing suspensions containing different weight concentrations of SWCNH in oil. Suspensions have been characterized both as regards the viscosity and the stability over time prior to carry out tribological tests. Nanolubricants containing different concentrations of nano-additives in oil were prepared by a two-step method and tribologically characterized through rotational ball-on-disk wear tests. Wear tracks were analyzed by optical and electron microscopy and stylus profilometry. SWCNH 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 raw oil. The friction force reduction could be attributed to the mobility of the nano-objects in addition to the reduced contact area obtained by the presence of nanoparticles sited between sliding surfaces. SWCNHs in lubricant oils lowered 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
