Study On The Tribological Properties And Lubricant Mechanisms Of Nanolubricants For Magnesium Alloys Rolling

Magnesium and its alloys are promising materials in transportation,electronics industries or aerospace for their excellent properties,such as low density,high specific strength and electromagnetic compatibility.Rolling is an important plastic processing for magnesium alloys.So far,there are no suitable lubricants for the rolling process of Mg alloy,even at some conditions,the lubricants used for Al alloy rolling is casually used and the result is not satisfied.The application of conventional oil-based lubricant in Al alloy rolling processes relies heavily on sulphur,chlorine and phosphorous-containing additives.These additives cause negative effects on the environment even at low concentrations during the disposal of waste fluids.In this work,the effects of the nanolubricants on the tribological behavior for magnesium alloy/steel contacts were studied using a tribometer.The morphology and composition of worn surfaces were characterized by means of OLS4000 profilometer,field emission scanning electron microscopy?FESEM?and X-ray photo-electron spectroscopy?XPS?.Correlations between the tribological properties and the morphology of the worn surfaces were found,thereby improving our understanding of the behavior of nanoparticle addition to lubricating system.AZ31 Magnesium alloy sheets were processed by rolling without lubrication and with nanolubricant so as to investigate rolling performance by measuring rolling force as well as the surface roughness of the sheets.Main results were obtained as follows:?1?The tribological properties of MoS₂ nanolubricants with 5# machine oil?without other additives?,30# machine oil?with special additives?and rapeseed oil as base lubricants were investigated by tribological tests.The results showed that the tribological properties of the 5# machine oil and rapeseed oil have been improved by adding the MoS₂ nanoparticles,and the lubricant properties of 5# machine oil with nano-MoS₂ was more pronounce than that of rapeseed oil with nano-MoS₂.However,the lubricant properties of 30# machine oil with nano-MoS₂ were comparative with pure 30# machine oil.?2?The tribological properties of nano-SiO₂ and nano-MoS₂ as oil additives for magnesium alloy/steel contacts were investigated by controlling the concentration of the nanoparticles in the base lubricant and test conditions.Three sets of contact conditions were used to evaluate the effect of concentration,the capacity of carrying load and the stability of the lubrication film,respectively.It was found that nano-MoS₂ has better anti-wear property than nano-SiO₂,while nano-SiO₂ has better friction reduction property than nano-MoS₂.In addition,the load carrying capacity and lubrication film stability of the nano-MoS₂ were more pronounced as compared with nano-SiO₂.In view of the irrespective special tribological characteristics,it was worth to study how the two nanoparticles behave together as lubricant additive and try to further optimize the comprehensive tribological performances.Further,the lubrication mechanism was discussed in detail.?3?The water containing graphene or graphene oixde exhibit excellent lubrication properties as compared with the pure water for AZ31 magnesium alloy/steel contacts.The SiO₂/graphene hybrid nanolubricants exhibited superior lubrication performances than individual nano-SiO₂ nanolubricants or graphene nanolubricants,whereas the lubricant properties of the SiO₂/graphene oxide hybrid nanolubricants and pure graphene oxide nanolubricants were similar.?4?AZ31 magnesium alloy sheets were processed by rolling without lubrication and with nanolubricant so as to investigate rolling performance.The SiO₂/MoS₂ hybrid oil-based nanolubricant showed reduction of rolling force by 14.5% and reduction of surface rougness by 55.9 % in contrast with dry rolling for nominal reduction of 10%.The SiO₂/graphene hybrid water-based nanolubricant showed reduction of rolling force by 12% and reduction of surface rougness by 42.4 % in contrast with dry rolling for nominal reduction of 10%.The SiO₂/graphene oxide hybrid water-based nanolubricant showed reduction of rolling force by 13.1% and reduction of surface rougness by 28.8 % in contrast with dry rolling for nominal reduction of 10%.?5?In conclusion,the SiO₂/MoS₂ hybrid oil-based nanolubricant and the SiO₂/graphene hybrid water-based nanolubricant were more prounced in terms of lubricant properties in comparison with single nanolubricants.The excellent lubrication properties of the SiO₂/MoS₂ hybrid oil-based nanolubricant and the SiO₂/graphene hybrid water-based nanolubricant were attributed to the physical synergistic lubricating actions of nano-SiO₂ and nano-MoS₂ or graphene during the rubbing process.While the lubricant properties of the SiO₂/graphene oxide hybrid water-based nanolubricant and pure graphene oxide water-based nanolubricants were similar.Even so,production costs for the SiO₂/graphene oxide may be decreased by substituting SiO₂ for partial graphene oxide.