Investigation Of Chemical Mechanical Polishing For Titanium Alloy And Nickel Based Alloy

Titanium alloy and nickel-based alloy are versatile engineering materials and have been exploited in the fields of aviation and aerospace,petrochemical industry,military equipment and microelectronics,owing to their distinguished mechanical properties and anti-oxidation at high temperature,as well as the excellent corrosion resistance properties.High precision instruments such as MEMS devices call for high-quality surfaces,and surface planarizations are used to provide smooth surfaces.At present,the planarization of metal alloy relies on mechanical polishing,chemical polishing and electrochemical polishing,while these methods are inefficient,poor processing quality and often involve the use of harmful chemicals.In this paper,a novel environment-friendly slurry for alloy was developed,and chemical mechanical polishing（CMP）experiments were conducted on Ti-6Al-4V titanium alloy and Hastelloy C 2000 nickel-based alloy.Nano-scale smooth surfaces were obtained by CMP,and the polishing mechanism of alloy was studied using electrochemical and X-ray photoelectron spectroscopy（XPS）technologies.The novel CMP slurry for alloy mainly consists of colloidal silica,hydrogen peroxide,organic acid additives and deionized water,eliminating the use of HNO₃,HF and other hazardous substances.The optimized slurry for Ti-6Al-4V contains the food additives malic acid and citric acid as pH regulators and the pH is adjusted about 3.5 to 4.5.The surface is very smooth without any defects,and surface roughness Ra and rms are 0.684 nm and 0.856 nm,respectively,after CMP for 20 min.Results of dynamic potential polarization curves show that the organic acids promote the corrosion reaction of Ti-6Al-4V,and the corrosion rate increases with H2O2 concentration,which is in agreement with the CMP experiments.An oxide layer mainly consisted of TiO2,Al2O₃ and a small amount of TiO,Ti2O₃ and VO2 is detected on the surface of Ti-6Al-4V after CMP and soaked in slurry,while there were only metallic Ti,Al and V in 10 nm depth under the surface of CMP samples.The XPS experiments reveal the slurry reacts with the metal and generates an oxide layer on the top surface of Ti-6Al-4V,during the CMP process.A two step polishing process of mechanical polishing and CMP is used in the planarization of C 2000 nickel based alloy,the final polished surface is mirror-like,and the surface roughness Ra and rms are 0.705 nm and 0.958 nm,respectively.To analyze the effects of each component of the slurry and mechanical action on C 2000 surface,XPS energy spectrum of 5 C 2000 samples with different processing methods were conducted.The results declare that the oxidation reaction between H2O2 and C 2000 improves the content rates of CrO₃,Ni（OH）2 and MoO₃.And in the malic acid solution,NiO and Ni（OH）2 are partly dissolved.Moreover,high valence oxides of Cr2O₃,MoO₃,NiO and Ni（OH）2 are prior to being removed by the mechanical action,during the CMP process.Therefore,the alloy firstly reacts with the polishing solution and produces an oxide film layer on the surface,which is easy to remove,and then the oxide film layer is removed by the mechanical action.In short,the surface quality is constantly improved in the alternating process of chemical reaction and mechanical reaction,and finally a nano-scale smooth surface is achieved.