6061铝合金近原子尺度化学机械抛光工艺优化

Optimization of Chemical Mechanical Solution for Close-to-Atomic Scale Polishing of 6061 Aluminum Alloy

针对6061铝合金难以在化学机械抛光中获得高表面质量的问题,研究了抛光液中氧化剂(H2O2)、缓蚀剂(BTA)的质量分数和pH值对表面粗糙度的影响。基于响应曲面法设计实验,采用方差分析(ANOVA)检验各参数对表面粗糙度影响的显著性,分析参数的交互影响,得出抛光液的最佳组分配比。结果表明,6061铝合金的表面粗糙度随着pH值的增大先增大后减小再增大,随着H2O2和BTA质量分数的增加先减小后增大。在优选的抛光液参数组合(pH值为9.7,H2O2的质量分数为0.57%,BTA的质量分数为1.16%)下,实现最低表面粗糙度Sa为0.31 nm,获得了近原子尺度的超光滑表面。

Objective 6061 aluminum alloy is widely employed in aerospace,optical manufacturing,and other fields in the production of mirrors due to its high reflectivity,easy machining,and sound stability after forming.However,at present,the surface roughness of aluminum mirrors using grinding and diamond turning machining methods is large,and the diffraction and scattering effects are too serious,which reduces the performance of aluminum mirrors.As an ultra-precision machining technology to obtain high quality surface,chemical mechanical polishing technology has been widely studied in the polishing of aluminum and its alloys.However,due to the presence of Mg_(2)Si hard particles and other elements in 6061aluminum alloy,it is difficult to obtain ultra-smooth surfaces at present,and the influence of various parameters on the polishing process is still unclear.To this end,we investigate the influence of the parameters in the polishing solution on the surface quality of 6061 aluminum alloy and optimize a chemical mechanical polishing solution for 6061 aluminum alloy.The polishing solution can remove surface defects and obtain a good surface with a surface roughness of less than 0.4 nm.Therefore,our research can fill the current gap in the ultra-precision machining of 6061 aluminum alloy and contribute to the performance improvement of aluminum alloy mirrors.Methods Firstly,the chemical mechanical polishing experiments of 6061 aluminum alloy are carried out by single factor control of different oxidant H_(2)O_(2),corrosion inhibitor BTA concentration,and pH from acidity to alkaline in the polishing solution.After polishing,the Zygo interferometer is adopted to characterize the surface morphology and roughness to explore the effect of each parameter acting alone.Then,according to the influence law of each single factor,based on the response surface method(RSM),we set up a three-factor,three-level experiment,and utilize ANOVA to test the effect of degree of each parameter on the surface quality.The surface composition of the aluminum part is measured by an X-ray photoelectron spectrometer(XPS)and the interaction of parameters is analyzed to predict the composition ratio of the polishing solution which can obtain the best surface quality.Finally,atomic force microscopy(AFM)is leveraged to further measure the surface roughness after polishing and evaluate the accuracy of the predicted results.Results and Discussions The surface quality of 6061 aluminum alloy is poor when only abrasive grains and pH alone act(Fig.4),which helps further reduce the surface roughness after adding a certain concentration of H_(2)O_(2).However,the surface inevitably produces deeper craters(Fig.6),the surface quality is significantly improved with the addition of BTA,and the surface roughness is reduced to a sub-nanometer scale(Fig.8).This is because aluminum as an amphoteric metal corrodes severely with both acids with too small a pH and bases with a large pH,and neutral aluminum is difficult to remove as it is too stable.After adding H_(2)O_(2) and BTA,the aluminum first acts with H_(2)O_(2) in a weak alkaline environment to promote the oxidation reaction of aluminum,thus generating Al_(2)O_(3),AlOOH,and Al(OH)_(3) oxide layers(Fig.10).Then BTA forms a passivation film on the oxide layer surface.The raised area on the surface of the aluminum parts makes it easy to contact with the abrasive particles and polishing pad to achieve material removal.However,the pits are protected from excessive corrosion by the passivation film to achieve local ultra-smooth polishing(Fig.11).Finally,RSM is adopted to optimize the composition ratio of the polishing solution to achieve a balance between oxidation reaction,passivation reaction,and mechanical action during the polishing.The ultra-smooth surface of 6061 aluminum alloy at a close-to-atomic scale is achieved(Fig.12).Conclusions The effects of various parameters on the chemical mechanical polishing of 6061 aluminum alloy are studied and a polishing solution for close-to-atomic scale polishing of 6061 aluminum alloy surface is optimized.The results of single-factor experiments show that 6061 aluminum alloy can obtain the best surface quality when pH is in weak alkalinity,and the surface roughness decreases first and then increases with the rising H_(2)O_(2) and BTA concentration.XPS energy spectra show that H_(2)O_(2) in a certain concentration in the polishing solution can promote aluminum oxidation under the alkaline environment,and BTA can form a passivation film based on the oxide layer to inhibit the oxidation.The optimized polishing solution parameters include a pH value of 9.7,a H_(2)O_(2) mass fraction of 0.57%,and a BTA mass fraction of 1.16%,and the surface roughness of the aluminum mirror decreases from 140 nm to 0.31 nm.The error with the predicted value of RSM is less than 10%,and the ultra-smooth surface close-to-atomic scale is obtained.