Research On Ultra-Smooth And Low-Damage Chemical Mechanical Polishing Of 9Cr18Mo Space Bearing Rings

The machining accuracy of transmission parts such as high-speed bearings is the key factor that limits the performance,service life and reliability of space moving parts and mechanisms.Improving the machining accuracy of transmission parts such as bearings can greatly improve the performance,service life and reliability of spacecraft.At present,the traditional grinding technology is generally used to process the inner and outer rings of space high-speed bearings.Due to the limitation of the pure machining principle,in the machining process,the contact load must be greater than the plastic yield limit of the material,so it is difficult to improve the surface quality on the original basis,and damage will occur.When the space high-speed bearing works under some special working conditions,the thickness of the lubricating film may decrease sharply,resulting in various poor lubricating conditions.If the surface quality of the core components of the bearing is not high at this time,and the height of rough peaks on the surface of the bearing friction pair is larger than the thickness of the lubricating film,the rough peaks are in direct contact with each other,which will easily lead to serious wear of the bearing.On this basis,by means of ultra-precision polishing,the surface quality of the core components of space high-speed bearings can be further improved,the roughness of the workpiece surface can be reduced,the film thickness ratio can be increased,and the rough peaks between friction pairs can be prevented from directly contacting,a better lubrication state can be achieved,and the service performance of the bearings can be improved,and then the spacecraft performance,life and reliability can be improved.In this thesis,the chemical mechanical polishing technology is applied to the ultra-precision machining of bearings,the type of space bearing is B7004C,and the material is9Cr18Mo.Among them,chromium is one of the important components of 9Cr18Mo bearing steel.It is of great significance for ultra-precision polishing of bearing steel to study the effect of chromium content on chemical mechanical polishing of chromium-containing steel.Therefore,firstly,the influence of chromium content on chemical mechanical polishing is explored,then a polishing fluid for ultra-smooth polishing of 9Cr18Mo bearing steel is developed,and the related polishing mechanism is analyzed.Finally,a chemical mechanical polishing device for the bearing ring is built to realize ultra-smooth and low damage polishing of the bearing inner and outer ring surface.The main contents of this thesis are as follows:(1)The effect of chromium content on chemical mechanical polishing of chromium steel was investigatedFour typical samples of pure iron,GCr15 bearing steel(chromium content is about 1.5wt%),9Cr18Mo bearing steel(chromium content is about 16.3 wt%)and pure chromium are selected to study the effect of chromium content on chemical mechanical polishing of chromium steel under the action of oxidation and complexation.With the increase of chromium content,the material removal rate of steel decreases.Among them,the corrosion of low chromium steel such as GCr15 is basically the same as that of pure iron,and the chemical reaction of iron on the surface is dominant.When analyzing the material removal mechanism,we mainly pay attention to iron.When the chromium content is high,such as 9Cr18Mo bearing steel,the chromium oxide in the surface film increases,the corrosion rate decreases sharply,and the mechanical strength of the film is high,so it is difficult to remove.When analyzing the material removal mechanism,we mainly pay attention to chromium and iron elements.In addition,in order to achieve ultra-smooth polishing of high chromium steel,the removal of iron and chromium in the steel should be synchronized.(2)The mechanism of ultra-smooth and low damage chemical mechanical polishing of 9Cr18Mo bearing steel is revealedThe effect of hydrogen peroxide concentration on the polishing properties of 9Cr18Mo bearing steel without a complexing agent was studied.The results show that the material removal rate and surface roughness of 9Cr18Mo bearing steel decrease with the increase of hydrogen peroxide concentration.Electrochemical experiments show that the addition of hydrogen peroxide enhances the surface corrosion resistance of 9Cr18Mo bearing steel.TEM and XPS experiments show that under the action of hydrogen peroxide,a surface film with uniform distribution of iron oxide and chromium oxide is formed on the surface of 9Cr18Mo bearing steel,and the surface film is uniformly distributed on the surface of 9Cr18Mo bearing steel.With the increase of the concentration of hydrogen peroxide,the surface film becomes denser,and the ratio of iron to chromium in the surface film is closer to that of the substrate.Under the action of wear particles,the iron oxide and chromium oxide in the surface film can be removed uniformly and synchronously.Hence,the surface roughness is low,and the SEM results also show that the polished surface is smooth.When the concentration of hydrogen peroxide is 1 wt%,the surface roughness S_a of 9Cr18Mo bearing steel reaches the lowest value of 0.6 nm,there is no machining damage on the subsurface.(3)Build a contact chemical mechanical polishing device for bearing rings and realize ultra-smooth and low damage polishing of bearing ringsIn order to apply the plane chemical mechanical polishing process of 9Cr18Mo bearing steel to the surface polishing of 9Cr18Mo space bearing and outer ring,a contact chemical mechanical polishing device for the bearing ring was designed and built.The polishing head and fixture of the inner and outer ring are designed pertinently,and the polishing head,bearing ring and sample rotation module are highly coaxial through the first self-aligning module and the second self-aligning module.On this basis,the polishing module is rotated at high speed for polishing,the rotational speed is 280 r/min,the sample rotation module is rotating at low speed,and the rotational speed is 10 r/min.Combined with the previous results,the chemical mechanical polishing fluid formula is optimized to achieve uniform polishing of the bearing inner and outer ring surface and make the bearing ring surface roughness S_a close to 1 nm,and the roundness is significantly improved.To sum up,this thesis explored the chemical mechanical polishing process of 9Cr18Mo bearing steel,and realized the ultra-smooth and low damage polishing of the ring of 9Cr18Mo space bearing,which is helpful to expand the ultra-precision machining method of bearing and lay the foundation for improving the running stability of space high-speed bearing.