| As the internal combustion engine is strengthened,the frictional interface of piston ring-cylinder liner（PRCL）is subjected to higher mechanical and thermal loads.This requires the lubricating oil with both low fluid friction resistance and excellent boundary lubrication at high loads and low speeds.It can inhibit the mutual contact between the micro-asperities on the PRCL interface to improve the friction and wear performance.Graphene,which has unique laminar structure and slip properties,can be added to lubricating oil as a solid lubricant additive to meet the demand of the great-performance lubricants for the strengthening internal combustion engine.Graphene was prepared by liquid plasma method and three graphene-based lubricant additives were prepared.The friction and wear properties of different graphene-based additives were studied on a piston ring reciprocating liner test rig simulating the conditions of peak cylinder pressure.The effects of temperature and loading conditions on friction,wear and anti-scuffing performance were obtained.Under the synergistic action of ZDDP in lubricating oil,the friction reduction and wear resistance mechanism of graphene-based additives was discussed.The main conclusions are as follows:（1）Graphene（G）flakes were prepared by liquid plasma method,then high cathodic potential induced reduction of[Mo S4]2-precursor was used to generate graphene-Mo S2（G+Mo S2）material.Trihexy（tetradecy）phosphine bis（2,4,4-trimethylpentyl）hypophosphite ionic liquid（L）was introduced and mixed with the prepared grapheme.These additives were combined with RP-4652D lubricating oil with ZDDP respectively to prepare three lubricating oil containing graphene-based additives（4652D+G,4652D+G+Mo S2,4652D+G+L）.（2）The friction and wear properties of graphene-based additives were investigated under different temperature conditions（150°C,180°C,210°C,240°C）.The results showed that temperature had a great influence on the friction and wear properties of graphene-based additives.At 150°C and 180°C,4652D+G+Mo S2 additive had betterfriction reduction performance.At 210°C and 240°C,4652D+G+L had better friction reduction performance.In the range of 150～240°C,4652D+G+Mo S2 showed better wear resistance.For 4652D+G,the soft laminated graphene was brought to the honing grooves to fill in the honing grooves and reduced the contact of the micro-asperites on the PRCL interface.At the same time,graphene with slip characteristics formed a boundary physical adsorption film on the frictional surface and participated in the wear process to reduce friction.For 4652D+G+Mo S2,the friction reduction and wear resistance performance were affected by unoxidized Mo S2 and tribochemical products such as phosphate decomposable by ZDDP.For 4652D+G+L,the degree of tribochemical reaction had a great influence on the friction reduction and wear resistance of ionic liquid mixed graphene additive.The variation of the type and relative content of phosphate in the tribochemical reaction film reflects the degree of tribochemical reaction.（3）In the starvation experiment,the order of the anti-scuffing time was 4652D+G+L>4652D+G+Mo S2>4652D+G>4652D.Compared with 4652D,the anti-scuffing time of4652D+G,4652D+G+Mo S2 and 4652D+G+L lubricants extended by77%,256%and 344%,respectively.The results showed that the graphene-based lubricant additive exhibited excellent anti-scuffing performance.With its unique lamellar structure and self-lubricating properties,graphene and graphene-Mo S2 additives can act as lubricating medium on the PRCL interface to avoid the mutual contacts of micro-asperities.The ionic liquid mixed with graphene additive could synergistically react with ZDDP to generate large amounts of phosphate in the boundary lubricating film.This may be an important reason for maintaining its anti-scuffing performance. |
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