Tribological Properties Of The Surface Modified Piston Rings Under High Strengthening Conditions

The cylinder liner and piston ring are one of the most crucial friction pairs in the internal combustion engine,the fuel efficiency,system durability and even the life of internal combustion engine are affected directly by their lubrication and friction properties.With the internal combustion engine moves toward high-power density and low fuel consumption,the engine pressure and temperature increase significantly,all of these lead to a worse condition for cylinder liner and piston ring.Thus,improving fuel efficiency of cylinder liner and piston ring and prolonging the service life of internal combustion engine are of vital significance in the development of modern internal combustion engine.There are many technologies that can improve the tribological properties of piston rings,and surface treatment is a common and effective method.To improve the performance of piston ring coating through different surface treatment processes,which can enhance the friction and wear capability of piston ring under severe working conditions and improve its scuffing resistance.Nitriding(Nitriding)coating can effectively reduce its wear loss and specific wear rate;chromium nitride(Cr N)coating has good toughness,wear resistance and tensile properties.Goetze diamond coating(GDC),as an enhanced version of the chromium-based ceramic composite coating,composed of a Cr matrix and reinforced microdiamond particles embedded in an ultra-fine cracked mesh,it has excellent wear resistance;diamond-like carbon coating(DLC)has extremely high hardness(up to 90 GPa),elasticity and low coefficient of friction.In this study,four piston rings with different surface treatment processes and two cylinder liners were orthogonal matched to research the tribological properties of each pair.Piston rings include Nitriding coating,Cr N coating,GDC,and DLC.In addition,chrome-plated cylinder liner and boron-phosphorus alloy cast iron cylinder liner were used.When the different piston rings are matched with the cylinder liner,their tribological properties are different.Therefore,exploring a suitable pair for optimizing the engine piston group is necessary.Specific research contents and conclusions are as follows:(1)In order to simulate the boundary lubrication state of the cylinder liner-piston ring system at the top dead center of stroke in the actual internal combustion engine,the reciprocating friction and wear test developed and designed by the research group is used to simulate the reciprocating motion form of the friction pair,the contact state and the boundary lubrication state between the friction pair near the top dead center of the stroke in the actual internal combustion engine.The cylinder liner and piston ring used in the actual diesel engine are selected as the experimental objects to evaluate the extreme pressure,anti-friction and antiwear performances of different cylinder liner-piston ring under various complicated running conditions.After test,the worn morphology of the cylinder liner and piston ring surfaces were observed and analyzed by scanning electron microscope to research the wear mechanism.The results show that when piston rings paired with chrome-plated cylinder liners,the friction performance of DLC rings and different cylinder liner pairs is optimal at low temperatures.At 240°C,the DLC ring also showed the best frictional performance at the beginning of the high load wear phase,with the cylinder pulling phenomenon occurring after two hours.In contrast,when paired with cast iron cylinder liners,no significant sudden change in friction,i.e.severe adhesive wear,occurred at either temperature.In terms of wear,the overall wear of the friction pair increases with temperature.When paired with a chrome-plated cylinder liner,DLC paired cylinder liner wear is the most serious at high temperatures.This is because at high temperatures and high load DLC coating,internal stress is more likely to occur flaking phenomenon,and then produce serious adhesion phenomenon.The DLC paired cylinder liner,honing pattern is not filled,still has the role of collecting grinding chips,showing a better wear performance.(2)In addition to the extreme pressure,anti-friction and anti-wear performances,in order to more systematically evaluate the comprehensive tribological characteristics of different cylinder liner-piston ring,the scuffing resistance can also not be ignored.Using oil break test and interrupted scuffing test methods evaluate the scuffing resistance of the cylinder linerpiston ring,and finally determine the optimal matching.Meanwhile,observing the crosssectional morphology of the cylinder liner at different time to analysis the evolution process of scuffing and provide theoretical supports for further reducing the risk of scuffing and improving its scuffing resistance.The results show that of the four types of piston ring,DLC has the most vigorous resistance to scuffing due to the formation of a mixed friction chemical layer of carbides and oxides during the lean oil process.Under oil broken conditions,the oil film between the friction pairs is continuously depleted,allowing direct metal contact and eventually leading to scuffing.The frictional force of the frictional pair tends to drop suddenly before the scuffing occurs,probably due to the cylinder liner’s reduced roughness.Still,the DLC frictional pair’s frictional force drops suddenly and rises later without serious adhesive wear.Scuffing did not occur due to the presence of a complex of carbon deposits and oxides on the cylinder liner surface,thus containing the scuffing.