Tribological Mechanism of Graphene and Ionic Liquid Mixed Fluid on Grinding Interface under Nanofluid Minimum Quantity Lubrication

Graphene has superhigh thermal conductivity up to 5000 W/(m·K),extremely thin thickness,superhigh mechanical strength and nano-lamellar structure with low interlayer shear strength,making it possess great potential in mini-mum quantity lubrication(MQL)grinding.Meanwhile,ionic liquids(ILs)have higher thermal conductivity and better thermal stability than vegetable oils,which are frequently used as MQL grinding fluids.And ILs have extremely low vapor pressure,thereby avoiding film boiling in grinding.These excellent properties make ILs also have immense potential in MQL grinding.However,the grinding performance of graphene and ionic liquid mixed fluid under nano-fluid minimum quantity lubrication(NMQL),and its tribological mechanism on abrasive grain/workpiece grinding interface,are still unclear.This research firstly evaluates the grinding performance of graphene and ionic liquid mixed nanofluids(graphene/IL nanofluids)under NMQL experimentally.The evaluation shows that graphene/IL nanofluids can further strengthen both the cooling and lubricating performances compared with MQL grinding using ILs only.The specific grinding energy and grinding force ratio can be reduced by over 40%at grinding depth of 10μm.Work-piece machined surface roughness can be decreased by over 10%,and grinding temperature can be lowered over 50℃at grinding depth of 30μm.Aiming at the unclear tribological mechanism of graphene/IL nanofluids,molecular dynamics simulations for abrasive grain/workpiece grinding interface are performed to explore the formation mechanism of physical adsorption film.The simulations show that the grinding interface is in a boundary lubrication state.IL molecules absorb in groove-like fractures on grain wear flat face to form boundary lubrication film,and graphene nanosheets can enter into the grinding interface to further decrease the contact area between abrasive grain and workpiece.Compared with MQL grinding,the average tangential grinding force of graphene/IL nanofluids can decrease up to 10.8%.The interlayer shear effect and low interlayer shear strength of graphene nanosheets are the principal causes of enhanced lubricating performance on the grinding interface.EDS and XPS analyses are further carried out to explore the formation mechanism of chemical reaction film.The analyses show that IL base fluid happens chemical reactions with workpiece material,producing FeF_(2),CrF_(3),and BN.The fresh machined surface of workpiece is oxidized by air,producing NiO,Cr_(2)O_(3) and Fe_(2)O_(3).The chemical reaction film is constituted by fluorides,nitrides and oxides together.The combined action of physical adsorption film and chemical reaction film make graphene/IL nano-fluids obtain excellent grinding performance.

Research on digital twin technology and its application in intelligent operation and maintenance of highspeed railway infrastructure

Purpose–This paper analyzes the application of digital twin technology in the field of intelligent operation and maintenance of high-speed railway infrastructure from the perspective of top-level design.Design/methodology/approach–This paper provides a comprehensive overview of the definition,connotations,characteristics and key technologies of digital twin technology.It also conducts a thorough analysis of the current state of digital twin applications,with a particular focus on the overall requirements for intelligent operation and maintenance of high-speed railway infrastructure.Using the Jinan Yellow River Bridge on the Beijing–Shanghai high-speed railway as a case study,the paper details the construction process of the twin system from the perspectives of system architecture,theoretical definition,model construction and platform design.Findings–Digital twin technology can play an important role in the whole life cycle management,fault prediction and condition monitoring in the field of high-speed rail operation and maintenance.Digital twin technology is of great significance to improve the intelligent level of high-speed railway operation and management.Originality/value–This paper systematically summarizes the main components of digital twin railway.The general framework of the digital twin bridge is given,and its application in the field of intelligent operation and maintenance is prospected.

Tribological mechanism of carbon group nanofluids on grinding interface under minimum quantity lubrication based on molecular dynamic simulation

Carbon group nanofluids can further improve the friction-reducing and anti-wear properties of minimum quantity lubrication(MQL).However,the formation mechanism of lubrication films generated by carbon group nanofluids on MQL grinding interfaces is not fully revealed due to lack of sufficient evidence.Here,molecular dynamic simulations for the abrasive grain/workpiece interface were conducted under nanofluid MQL,MQL,and dry grinding conditions.Three kinds of carbon group nanoparticles,i.e.,nanodiamond(ND),carbon nanotube(CNT),and graphene nanosheet(GN),were taken as representative specimens.The[BMIM]BF4 ionic liquid was used as base fluid.The materials used as workpiece and abrasive grain were the single-crystal Ni–Fe–Cr series of Ni-based alloy and single-crystal cubic boron nitride(CBN),respectively.Tangential grinding force was used to evaluate the lubrication performance under the grinding conditions.The abrasive grain/workpiece contact states under the different grinding conditions were compared to reveal the formation mechanism of the lubrication film.Investigations showed the formation of a boundary lubrication film on the abrasive grain/workpiece interface under the MQL condition,with the ionic liquid molecules absorbing in the groove-like fractures on the grain wear’s flat face.The boundary lubrication film underwent a friction-reducing effect by reducing the abrasive grain/workpiece contact area.Under the nanofluid MQL condition,the carbon group nanoparticles further enhanced the tribological performance of the MQL technique that had benefited from their corresponding tribological behaviors on the abrasive grain/workpiece interface.The behaviors involved the rolling effect of ND,the rolling and sliding effects of CNT,and the interlayer shear effect of GN.Compared with the findings under the MQL condition,the tangential grinding forces could be further reduced by 8.5%,12.0%,and 14.1%under the diamond,CNT,and graphene nanofluid MQL conditions,respectively.