Study On The Plasma Electrolytic Nitriding On The Surface Of Piston Ring In Solution And Perfromances Of The Modified Layer

As one of the most important components of internal combustion engines,the piston ring is not only subjected to confront the erosion of gas with high temperature and high pressure,but also have to withstand the effects of severe wear and large impact load.Therefore,it is essential to conduct proper treatment on the surface of piston ring,which helps to increase the surface hardness,improve the surface lubrication state,reinforce the resistance to wear,and thus to extend its service life.Nitriding has been a common method to dispose the piston ring surface.After nitriding,the piston ring surface has higher hardness,stronger wear resistance,and the core part of piston ring has stronger toughness.Plasma electrolytic nitriding is an efficient technology for iron surface treatment with less energy composition.This technology can not only improve the performance of piston rings,but also help solve some problems of traditional surface nitriding technology such as long processing cycle,high energy consumption and tedious operation of piston rings.In this paper,an electrolyte was prepared by using formamide as penetrant and NH₄Cl as conductive salt,in which the volume ratio of formamide to 10%NH₄Cl solution is 4:1.Then,piston rings were treated with plasma electrolytic nitriding technology in the electrolyte through working as cathode while a stainless steel rod serving as anode.The structure,element distribution and phase composition of the PEN modified layer on the surface of the treated piston rings were analyzed,and the effects of operating parameters such as working voltage,processing time,duty cycle and frequency on the hardness,surface contour,surface roughness,and tribological properties of the PEN modified layer on the surface of the treated piston rings were also investigated.The main research content are as follows:?1?The effects of operating parameters on the structure of the PEN modified layer on the treated piston ring surface and the distribution of infiltrating elements were studied.A scanning electron microscope was used to observe the structure of the PEN modified layer on the surface of the treated piston rings.It was found that the PEN modified layer was composed of compound layer,diffusion layer and transition layer in order from the outside to the inside.The thickness of the PEN modified layer increased with rising the operating voltage,extending the processing time,increasing the duty cycle,and reducing the current frequency.The vitality of operating parameters influencing the thickness of the PEN modified layer is ranked as working voltage,processing time,frequency and duty cycle.Besides,an energy distribution analyzer was used to analyze the distribution of elements in the PEN modified layer.The results show that the infiltrated nitride was mainly distributed in the compound layer and the diffusion layer,and its content shows a decreasing trend from the surface to the inside of the piston ring.?2?The effects of operating parameters on the phase composition of the PEN modified layer on the treated piston ring surface were also studied.An X-ray diffractometer was used to analyze the phase composition of the PEN modified layer on the treated piston ring surface.It was found that the infiltrated nitride existed as solid solutions in the modified layer and the main phase composition of the modified layer were Fe_2-3N and Fe₄N.Moreover,increasing the working voltage and extending the processing time appropriately can help N atoms to diffuse effectively and then form iron-nitrogen compounds in the treated piston ring surface.However,changing the duty cycle and frequency had little effect on the phase composition in the modified layer.?3?The influencing factors and distribution laws of the cross-section hardness of the PEN modified layer were analyzed.A microhardness tester was used to test the hardness of the PEN modified layer.It was found that the hardness distribution of the modified layer increased first and then decreased from outside to inside.In general,the diffusion layer presented the maximum hardness,and it increased gradually with rising the operating voltage,extending the processing time,increasing the duty cycle and reducing the current frequency.Besides,the maximum hardness of the PEN modified layer can reach higher than twice of the substrate.?4?The influencing factors of roughness and surface contour of the PEN piston ring surface and their changing rules with operating parameters were analyzed.A surface profilometer was used to measure the surface contour and surface roughness of the PEN piston ring.The results show that the surface contour of the piston ring was rougher because of the pits and bumps on the surface of the PEN piston rings which produced by the impact of the plasma explosion.Furthermore,operations such as increasing the operating voltage,extending the processing time,increasing the duty cycle,and reducing the frequency can cause the increase of the surface roughness.?5?The influencing factors of friction coefficient and wear rate of the PEN piston ring surface and their changing rules with operating parameters were analyzed.A tribological testing machine was used to test the tribological properties of untreated piston rings and PEN piston rings.The results show that the friction coefficient of PEN piston rings increased due to the higher surface roughness.However,the improvement in surface hardness and oil storage capacity may effectively suppress the wear.Moreover,the wear rate of PEN piston ring decreased with increasing the processing time and reducing the current frequency.For example,under the processing time of 15min,the wear rate of the PEN piston ring was 46.8%lower than that of the untreated piston ring;while under the processing frequency of 500Hz,the wear rate of the PEN piston ring was 40%lower than that of the untreated piston ring.On account of the analysis of the wear rate curve,the working voltage of 260V and the duty cycle of 40%were preferred for the plasma electrolytic nitriding process on the surface of the piston ring.