Construction Of Chemical Mechanical Polishing Oxidation Removal Model For Bearing Steel And Its Application

As important basic mechanical components,bearings have been used in aerospace,rail transit,wind power and other fields.The performance of bearings determines the service life of the equipment.The research shows that reducing the surface roughness of the bearing contact area can significantly improve the service life of the bearings.The surface roughness S_a of bearing inner race and rolling element is from 0.2μm down to 0.04μm,and the less damage caused by surface processing,the longer the service life of the bearing,and can significantly reduce the noise and vibration during operation.As an ultra-smooth and low-damage surface processing technology,chemical mechanical polishing（CMP）has been gradually applied to the processing of bearing steel.For metal CMP,the material removal mechanism is generally considered as alternating processes of oxidation and removal.Based on this mechanism,metal CMP slurries usually contain oxidizers for oxidation,complexing agents for complexation and dissolution,and abrasive particles for mechanical removal.Generally,complexing agents,such as glycine,can only react with metallic oxides instead of metal.In order to achieve efficient removal,a metal must first be oxidized to its corresponding oxidation states.It can be seen that oxidation is vital to metal CMP.Therefore,the material removal mechanism of bearing steel can be understood through the study of oxidation,so as to provide a theoretical basis for improving the surface quality of bearing.Based on the polishing results and the characterization results of electrochemical,static corrosion,XPS and SEM technology,this paper constructs the oxidation removal model of bearing steel polishing.By applying oxidizers to the model,this paper expounds on the influence mechanism of oxidation and further selects the oxidant with application prospect to the ultra-precision machining of the outer ring of wind power bearing.The main contents of the paper are summarized as follows:（1）The oxidizers for GCr15 steel CMP are divided into three categoriesIn the GCr15 steel CMP,the effects of eight representative oxidizers on the polishing properties were studied,including hydrogen peroxide,potassium iodate,potassium periodate,sodium hypochlorite,potassium chlorate,potassium persulfate,iron nitrate and potassium perchlorate.The results show that potassium perchlorate does not react with iron and has no effect on the material removal rate.For the other seven oxidizers,the material removal rate showed three trends with the increase of oxidizer concentration.The first trend is that it rises rapidly first,then decreases,and then levels off;The second trend is that it increases rapidly and then levels off;The third trend is that it almost linear increases.Electrochemical and static corrosion results show that,for the first trend,when the oxidizer concentration is low,the reaction layer on the surface is easy to remove,and when the concentration increases,the reaction layer on the surface becomes difficult to remove.For the second trend,due to the pitting effect of chloride ions,the reaction layer will become easier and easier to remove with the increase of oxidizer concentration.For the third trend,loose and easily removed black substances will be formed on the surface.For the forth trend,potassium perchlorate does not react with iron at room temperature and pressure.It shows that the three polishing trends are due to the difference in oxidation mechanism.Therefore,oxidizers can be divided into three categories:hydrogen peroxide,potassium iodate and potassium periodate as the first category,potassium chlorate and sodium hypochlorite as the second category,potassium persulfate and ferric nitrate as the third category.（2）The oxidation removal model of bearing steel for chemical mechanical polishing was establishedThe effects of three categories of oxidizers on the surface composition of GCr15 steel were investigated.For the first two categories,the material removal rate is positively correlated with the Fe²⁺content,and negatively related to the Fe³⁺content.For the third category,the increase of surface Fe₃O₄ content can lead to an almost linear increase in material removal rate.It shows that oxidizers affect the surface composition through different oxidation paths,resulting in different material removal rate trends.By considering the chemical action of oxidizer and complexing agent and the mechanical action of abrasive particles,a material removal model of GCr15 steel is established based on chemical kinetics theory and steady-state theory.The model can analyze the oxidation mechanism quantitatively by establishing the relationship between material removal rate and iron and its oxide content.According to the different oxidation paths,the model is applied to three categories of oxidizers,and the CMP data are fitted with the model.It is found that the fitting results agree with the CMP data.By analyzing the fitting results,it is found that for the first two categories of oxidizers,it is required to form more Fe²⁺to achieve a high MRR of GCr15 steel,while more Fe₃O₄ for the third category.That is,the oxidation path can be changed by changing the category of oxidizer,and the oxidation strength can be changed by changing the concentration of oxidizer,so as to achieve material removal at different rates.（3）The action mechanism of potassium iodate in the slurries is clarified and applied to the ultra-precision machining of wind power bearingIn the GCr15 steel CMP,the synergistic effect of three complexing agents and potassium iodate on the polishing properties of GCr15 steel was investigated,including glycine,formic acid and oxalic acid.With the increase of complexing agent concentration,the material removal rate first increases rapidly and then tends to be stable.Oxalic acid has the strongest complexation ability.Accordingly,oxalic acid was the appropriate complexing agent for potassium iodate in the slurries.The electrochemical results proved the complexation of oxalic acid in the slurries.When oxalic acid is added into the slurries,oxalic acid reacts with divalent iron on the surface to form soluble complexes,which inhibits the oxidation of divalent iron to trivalent iron.At this time,the surface divalent iron content increases,and the material removal rate increases rapidly.With the continuous increase of oxalic acid concentration,limited by the concentration of potassium iodate,the amount of iron oxidized to divalent iron is limited,and the material removal rate tends to be stable.Static corrosion experiments further optimized the slurry.The optimized slurry was applied to polishing the inner surface of the outer ring of wind power bearing through two processing methods of sub-aperture and full-aperture.The sub-aperture method can obtain the surface roughness of 42.7 nm,and 17.7 nm can be obtained by the full-aperture method.The feasibility of potassium iodate in the slurries for curved surface polishing is preliminarily verified.This paper provides a useful reference for the oxidizer selection of GCr15 steel CMP.It provides a new method for applying CMP technology to curved surface machining,which is of far-reaching significance for developing ultra-precision machining of basic mechanical parts such as bearings in the future.