Chemical Mechanical Polishing Methord Of Single Crystal Diamond At Room Temperature And Its Material Removal Mechanism

Diamond is an outstanding industrial material,which has excellent chemical and physical properties such as the high hardness,great thermal conductivity,ultra-high chemical stability,high electron mobility and extremely wide band gap.It has been applied in ultra-precision machining,electronic information,aerospace,national defense and other fields.In order to ensure the service performance and service life,all kinds of diamond devices put forward high requirements for diamond surface quality.Diamond is a typical difficult-processsing material.The current ultra-precision processing technology cannot meet the requirements of diamond in the above-mentioned application fields,which limits the practical applications of diamond.Chemical Mechanical Polishing(CMP)is a manufacture method combining mechanical force and chemical effect,which causes extremely low damage to the material during polishing.At the same time,the cost of CMP is lower when compared with some non-contact or vacuum polishing methods,thus it has great potential in diamond polishing.However,the ultra-high chemical stability of diamond makes that common oxidants have great difficulty to react with it at room temperature,leading to extremely low material removal rate(MRR).Therefore,many CMP methods for polishing diamond under heating condition have been proposed.Although those processes have improved MRR,problems such as the volatilization of polishing slurry,the thermal deformation of polishing disc are arisen,which will lead to unpredictable machining precision and surface quality.In addition,the mechanism of microscopic material removal of diamond in CMP process is rarely studied,which provide limited theoretical guidance for the diamond CMP process.Therefore,it is necessary to further study the diamond CMP technology at room temperature and its material removal mechanism.This paper investigates the CMP process of diamond at room temperature and reveal the atomic scale material removal mechanism through the method of combining simulation calculation and experimental research.The main research contents are as follows:(1)The most widely used(100)crystallographic plane diamond was used as workpiece.Molecular Dynamics(MD)simulation method based on ReaxFF force field was used to investigate the feasibility of the CMP process of diamond at room temperature under the environment of strong oxidized component·OH.The simulation result shows that:After interact with·OH,a soft layer didn't form on the diamond surface while it is often formed during conventional CMP process.There is only some C=O,C-H and C-OH structures are formed on the diamond surface and some of the carbon atoms are oxidized.With the abrasive particle sliding,carbon atoms are removed from the substrate in the form of CO,CO2 or adhesion to the abrasive particle,and the removed carbon atoms are mainly from the first layer of matrix.Thus theoretically this method can realize the atomic level machining.The higher the concentration of ·OH,the stronger its ability to oxidize diamond,which means the higher MRR.Therefore,Fenton agent,which can produce a large amount of ·OH has great potential to be the oxidant suitable for the CMP process of diamond at room temperature.(2)Conducting experiment by using Fenton agent and hydrogen peroxide which can produce ·OH as oxidant,to polishing(100)crystallographic plane diamond at room temperature.Then the effect with potassium ferrate and potassium permanganate are compared,which are two commonly used oxidants in diamond polishing.The result shows that the best surface quality after polishing was obtained when using Fenton agent as oxidant.In addition,the polishing effects of four different abrasive particles,diamond,cubic boron nitride,boron carbide and alumina were compared,and diamond powder was selected as optimized abrasive particle.Finally,a new Fenton polishing slurry suitable for CMP process of diamond at room temperature was developed.(3)Through experiments,the effect of polishing load,rotating speed on the CMP process of(100)crystallographic plane diamond at room temperature has been investigated,and the process parameters was optimized.Under the optimized process parameters and using the developed Fenton polishing slurry,the surface roughness of diamond was decreased greatly from Ra 3.5 nm to Ra 0.7 nm in 2 h after CMP,proving that this is a high efficiency polishing method.In order to furtherly improve the surface quality,silica sol polishing slurry was used to finish polish,from this a combined diamond CMP method at room temperature which had a better balance between efficiency and surface quality were developed.Through this method,an ultra-smooth surface with a local roughness of Ra 0.166 nm was obtained in 4 h.Compared with the existing local heating CMP process using potassium ferrate polishing slurry,the polishing time of this method can be reduced by about 50%in the case of obtaining similar surface roughness.(4)Through polishing experiment and Molecular Dynamics simulation based on ReaxFF force field,the influence of mechanical sliding and Fenton agent on the CMP process of(100)crystallographic plane diamond at room temperature was investigated.The result shows that:Mechanical sliding plays two roles in diamond CMP at room temperature,on the one hand,mechanical sliding can break the weakened C-C bonds to remove the material from substrate through shearing;on the other hand,mechanical sliding can promote the formation of C-O-C on diamond surface,which provides conditions for the generation of CO2;the improvement of mechanical force can promote the removal of carbon atoms;carbon atoms on the diamond surface are oxidized in Fenton agent,meanwhile the bond order of C-C bonds become smaller which connected to these carbon atoms,leading to the carbon atoms easier to be removed.After the introduce of Fenton agent,the MRR of diamond CMP at room temperature was risen from 3.34 nm/min to 7.12 nm/min?(5)Contrast experiments among diamonds of different crystallographic plane of(100),(110),(111)has been conducted to investigate the feasibility of Fenton agent on the CMP of(110)and(111)diamond at room temperature.The result shows that:Fenton polishing slurry has effect on all of the three diamonds during CMP,among which the best effect is to(100)crystallographic plane diamond.The MRR is anisotropy among the three diamonds during CMP,and the order of MRR is(100)>(110)>(111).Molecular Dynamics(MD)simulation method based on ReaxFF force field was used to reveal the mechanism of anisotropy MRR:The chemical adsorption anisotropy on diamond surface results in different number of weakened CC bonds,which then lead to the different MRR of the diamonds with different crystallographic planes.