Corrosion And Wear Properties Of 404C Stainless Steel And Its Surface Carbon Based Films In Methanol Solution

The development of methanol-fuelled vehicles will not only help accelerate the implementation of the national"dual carbon"target in the transport sector,but also help to ensure national energy security and promote energy diversification in the automotive industry.However,metal parts of methanol vehicle engines are subject to the interaction of wear and corrosion during service,which reduces the performance and service life of the parts and severely limits their application in the methanol environment.Diamond-like carbon（DLC）films have excellent properties such as high hardness,low friction,wear resistance,corrosion resistance and chemical inertness,and are a promising new metal surface protection material.Therefore,in this paper,404C stainless steel was selected as the base material and H-DLC films and Si:H-DLC films were prepared on the surface of 404C stainless steel by non-equilibrium magnetron sputtering.The frictional wear properties,corrosion resistance and corrosion wear properties of 404C stainless steel,H-DLC films and Si:H-DLC films were investigated.The specific contents and results of this thesis are as follows:（1）The effects of applied load and formic acid concentration on the frictional wear properties and corrosive wear properties of 404C stainless steel were investigated by frictional wear tests and electrochemical tests.Electrochemistry shows that:with the increase of load,the corrosion current density of 404C stainless steel decreases.When the load is 10N,the corrosion current density is minimum 2.17×10^-10 A/cm².When the concentration of formic acid increases,formic acid as a weak electrolyte,the smaller the degree of ionisation,the smaller the corrosion tendency of the metal in the medium.Friction results show that:404C stainless steel friction coefficient and wear rate are reduced with the increase of load,when the load is 10N,404C stainless steel friction coefficient minimum 0.75,wear rate of 0.93×10^-5 mm³·N^-1·m^-1.In the methanol environment,with the increase of load,404C stainless steel surface Cr-rich passivation film generation faster than the rate of removal,resulting in a lower coefficient of friction.When the concentration of formic acid increases,the strong corrosiveness of formic acid leads to an increase in surface roughness,which leads to an increase in the coefficient of friction.（2）H-DLC films were prepared on the surface of 404C stainless steel by non-equilibrium magnetron sputtering,and the effects of applied load and formic acid concentration on the frictional wear properties and corrosive wear properties of H-DLC films were investigated.The electrochemistry shows that the corrosion potential of H-DLC thin films corrects and the corrosion current density decreases as the load increases.When the formic acid concentration increases,the corrosion current density of the H-DLC film increases and the corrosion resistance becomes poor.the corrosion current density of the H-DLC film(2.87×10^-11 A/cm²)is reduced by 5 orders of magnitude compared to the substrate(4.85×10^-6 A/cm²)and has excellent corrosion resistance.Friction tests show that the coefficient of friction of the H-DLC film in air（0.048）is reduced by approximately 94%under high loads compared to the substrate（0.78）and the wear rate(0.97×10^-5 mm³·N^-1·m^-1)is reduced by an order of magnitude compared to the substrate(1.6×10^-6 mm³·N^-1·m^-1).This is attributed to the formation of a carbon transfer film on the counterbalance ball during the friction process,reducing the coefficient of friction and greatly improving the friction and wear reduction performance of 404C stainless steel.（3）The Si:H-DLC films were prepared on SUS404C surface by non-equilibrium magnetron sputtering method,and the effects of different loads and formic acid concentrations on the friction wear and corrosion wear properties of Si:H-DLC films were investigated.The electrochemistry showed that the corrosion potential of Si:H-DLC films tended to be positive and the corrosion current density decreased as the load increased.the corrosion current density of Si:H-DLC films（2.67×10-10 A/cm²）decreased by 4 orders of magnitude compared with that of the substrate(4.85×10^-6A/cm²),and when the formic acid concentration was 0.5%,the minimum corrosion current density was 1.74×10^-11 A/cm²,which has excellent corrosion resistance.This is attributed to the fact that the higher the concentration of formic acid after corrosion wear,the smaller the ratio of sp²/sp³ and therefore the better the corrosion resistance.The friction results showed that the friction coefficient of Si:H-DLC film（0.08）was reduced by 53%compared to SUS404C（0.17）in methanol solution due to the generation of silicon rich transfer film on the counterbalance sphere during the friction process thus reducing the friction coefficient.