Research On Graphene-related Lubricant Additives For Marine Engine Base On Low-Sulfur Fuel Condition

With the implementation of International Maritime Organization’s"2020 Sulphur Restriction Order",low-sulfur fuel has become the major choice for many shipping companies.However,the use of low-sulfur fuel in marine engine is likely to cause abnormal wear of cylinder liner and piston rings.This thesis addressed the issue of abnormal wear of cylinder liner and piston rings in marine engine when burning low-sulfur fuel（hereinafter referred to as low-sulfur conditions）,and the wear mechanisms and inhibition methods using lubricant additives has been studied.Firstly,cylinder liner-piston ring materials,cylinder oil,and cylinder oil residue obtained from the ship were used as test material and test oil sample,the corrosion and wear mechanism of cylinder liner was studied by simulating different working conditions of marine engine burning low-sulfur fuel.On this basis,potassium borate/graphene nanocomposite lubrication additives were designed and prepared by plasma-assisted ball milling,and the structure of the additives were characterized and analyzed.Molecular dynamics methods were then employed to theoretically analyze the micro-frictional behavior of potassium borate/graphene lubricant additives.Finally,friction and wear test and engine bench test were conducted to test and analyze the wear inhibition behavior and mechanism of potassium borate/graphene lubricant additives.The main contributions of this thesis are shown as follows:（1）Cylinder liner-piston ring materials,cylinder oil,and cylinder oil residue obtained from a marine engine burning low-sulfur fuel were used as test material and test oil sample,the marine engine operating conditions were simulated,and the static immersion test,electrochemical test,and corrosion-wear test were carried out.The corrosion loss,corrosion rate,corrosion morphology,corrosion products,and the interaction between corrosion-wear of cylinder liner materials were studied.The results indicated that the cylinder liner specimens produced chemical corrosion and electrochemical corrosion in normal working conditions and acidic working conditions（where cylinder oil alkalinity was less than the alkalinity required to neutralize the sulfur content of low-sulfur fuel）,and chemical corrosion was inhibited under alkaline conditions（where cylinder oil alkalinity was greater than the alkalinity required to neutralize the sulfur content of low-sulfur fuel）.The products of chemical corrosion and electrochemical corrosion of the cylinder liner samples were Fe SO₄,Fe₃O₄,Fe₂O₃,Fe O,and Fe S.Wear was the main cause of liner material loss,while corrosion was a small proportion of liner material loss.（2）Potassium borate/graphene nanocomposite lubricant additives were prepared by plasma-assisted ball milling and its microstructure were characterized and analyzed,and the dispersion stability and clean dispersion performance were tested.The results showed that the potassium borate/graphene composite lubricant additives were prepared by plasma-assisted ball milling for 20 h,the potassium borate/graphene composite lubricant additives had approximately8 layers of graphene,the size of potassium borate particles was about 10-50 nm,and the nano-particles of potassium borate were uniformly distributed on graphene.The surface of the potassium borate/graphene composite lubricant additives was modified by oleic acid.The composite lubricating additives had excellent dispersion stability in cylinder oil.Potassium borate has cleaning,dispersing and solubilizing effects on scavenge box residue of marine engine burning low-sulfur fuel.（3）A friction pair model of iron-oil-iron was established by molecular dynamics,simulated the reciprocating motion of piston rings on the cylinder liner surface,and calculated and simulated the micro-frictional behavior of potassium borate/graphene lubricant additives.The analysis included the radial distribution function,load carrying capacity,oil film thickness,and thermal conductivity’s effects on lubricant performance.The microstructural study helped elucidate the micro-reduction and anti-wear mechanisms of potassium borate/graphene nanocomposite lubricant additives.The results of computational simulation showed that potassium borate/graphene nano-composite lubrication additives had excellent dispersion performance in the cylinder oil,potassium borate/graphene nano-composite lubrication additives can improve the thermal conductivity of the cylinder oil.They could enhance the thermal conductivity of cylinder oil,reduce oil film temperature,increase oil film thickness,effectively improve the lubrication conditions of marine engine cylinder liner under low-sulfur conditions,and minimize friction and wear.（4）The cylinder liner and piston rings,cylinder oil,and cylinder oil residue obtained from the ship were used as test material and test oil sample,the pin-disc friction pair was used to simulate the reciprocating motion of marine engine cylinder liner and piston rings under different conditions.The anti-friction,anti-wear,and anti-corrosion properties of potassium borate/graphene in cylinder oil and cylinder oil residue were investigated under different working conditions.The test results indicated that potassium borate/graphene with mass fraction of 0.75 wt.%significantly improved the anti-friction,anti-wear and anti-corrosion properties of cylinder oil and cylinder oil residue under various working conditions.During the friction process,potassium borate/graphene adsorbed and deposited on the wear surface and undergo tribochemical reaction with the wear surface to form a friction film composed of Fe O,Fe₂O₃,Fe₃O₄,B₂O₃and Fe₂B.B₂O₃had a smooth surface,high-temperature resistance,and Fe₂B had high hardness,good thermal stability,and strong wear and corrosion resistance.B₂O₃and Fe₂B improved the high temperature resistance,corrosion resistance and anti-friction and anti-wear properties of cylinder oil.（5）Cylinder oil residue was used as lubricant,bench test was conducted with a ZS1100engine to investigate the anti-friction,anti-wear,and anti-corrosion effects of potassium borate/graphene lubricant additives during actual engine operation.The test results showed that adding 0.75 wt.%potassium borate/graphene to cylinder oil residue reduced the contents of metal elements in the engine oil sump,such as Fe,Cr,Mo,and Ni of lubricating oil.The size of metal particles reduced signficantly,and the wear on the cylinder liner surface was reduced.Potassium borate/graphene produced a friction chemical reaction on the surface of the cylinder liner,and generated a layer of friction film composed of Fe O,Fe₂O₃,Fe₃O₄,B₂O₃,and Fe₂B,which had anti-friction and anti-wear effects and corrosion inhibition properties.Potassium borate/graphene lubricant additives significantly reduced the friction and wear of cylinder liner in engine burning low-sulfur fuel.