| Recent years, a great attention has been attracted by titanium and its alloy's microarc oxidation (MAO) coating because of their series of excellent properties. MAO is a complex process controlled by multiple factors, and different process parameters can bring tremendous impactions on the characteristics of the coating. And its theoretical and experimental research is far from comprehensive and sufficient. Thus, there is still much work to do on titanium and its alloy's MAO coating's formation mechanism and their physical and chemical characteristics. In this paper, MAO technology has been used to producing MAO coating on commercially pure titanium TA2 using a constant working voltage. In addition, a comprehensive, systematic study has been proceed with the changing rules of the coating growth characteristics and physical and chemical properties under different process parameters'conditions.The study of this paper is divided into the three following parts:In the first part of the paper, in the Na3PO4 treating solution , the impacts of the MAO coating thickness, surface morphology and phase composition caused by the amplitude, frequency and duty cycle of voltage pulse as well as the treating time have been detailed studied, and the growth process of the coating has been analyzed. The results showed that: there are micro-pores everywhere on the surface of the coating, and the coating is mainly made up of anatase and rutile TiO2. The thickness of the coating changed between 6.3~27.0μm. As the amplitude of voltage pulse increased, the thickness of oxide coating, the micro-pores'size and surface roughness, and the relative content of rutile TiO2 increased gradually; while the micro-pores'density and the relative content of anatase TiO2 gradually decreased. As the frequency of voltage pulse increased, the thickness of MAO coating, the micro-pores'size and surface roughness and the relative content of rutile TiO2 reduced gradually; while the micro-pores'density and the relative content of anatase TiO2 gradually increased. As the duty cycle of voltage pulse increased, the thickness of the coating, the micro-pores'size first increased and then trended to decrease slightly; the density of micro-pores and the relative content of anatase TiO2 decreased gradually; while, the relative content of rutile TiO2 increased gradually. As the treating time increased, the thickness of the coating, the roughness of coating's surface and the relative content of rutile TiO2 increased gradually, while the relative content of the anatase TiO2 decreased gradually. MAO coating growth process is made up of three stages, including the formation stage of the initial passive coating, the rapid growth stage of MAO coating and the partial growth stage of MAO coating.In the second part of the paper, the changing rules of the MAO coating's thickness, microstructure and phase structure under various concentrations of Na3PO4, Na2SiO3 and NaAlO2 treating solution have been studied separately, and the effect of solution conductivity to the process of MAO has been analyzed. The results showed that: As the concentration of Na3PO4 treating solution increased, MAO coating thickness, the average micro-pores'size, surface roughness and the relative content of rutile TiO2 increased gradually, while the density of micro-pores and the relative content of anatase TiO2 gradually decreased. The coating prepared in Na2SiO3 treating solution was made up of the outside loose granular layer and the interior dense layer. The loose layer was made up of amorphous SiO2, and the dense layer was made up of a large number of anatase TiO2 and rutile TiO2 and a small amount of amorphous SiO2. As the concentration increased, the thickness of the dense layer and loose granular layer and the relative content of the rutile TiO2 gradually increased, while the relative content of anatase TiO2 decreased. The MAO coating prepared in NaAlO2 treating solution was mainly made up of Al2TiO5, but a small amount of Al2O3 was found in the coating prepared in high concentrations of NaAlO2 treating solution. As the concentration increased, coating thickness and surface roughness gradually increased, and the average size of micro-pores first increased and then decreased, while the micro-pores density kept decreasing. In Na3PO4, Na2SiO3 and NaAlO2 treating solution, the conductivity increased separately with the increase of their concentration. And the critical voltage of the specimens gradually decreased with the increase of conductivity, while the MAO coating thickness gradually increased accordingly.In the third part of this paper, the physical and chemical properties including MAO coating's electrochemical impedance spectroscopy, corrosion resistance, micro-hardness and coating-binding strength, etc. have been studied systematically. The results showed that: The corrosion resistance of the specimens processed with MAO technique significantly increased; As the amplitude, frequency and duty cycle of voltage pulse as well as peak value of voltage pulse as well as the processing time increased, the corrosion potential of the specimen showed the trend to first increase and then decrease; As the concentration of Na3PO4, Na2SiO3 and NaAlO2 increased, the specimens'corrosion potential increased gradually, and the corrosion potential of specimens prepared in Na3PO4 and NaAlO2 was higher than the specimen prepared in Na2SiO3. In H2SO4 solution, the corrosion rate of specimens processed with MAO technique significantly decreased; In H2SO4 and HNO3 solution, the corrosion rate of specimens prepared in same treating solution had the same changing rules, and the corrosion rate in H2SO4 slightly higher than that in HNO3; The corrosion rate of specimens prepared in different treating solution was different, The corrosion rate of specimen prepared in Na3PO4 is largest, followed by the specimen prepared in NaAlO2, and the specimen prepared in Na2SiO3 is smallest; As the corrosion time increased, the corrosion rate of specimens prepared in Na3PO4 and NaAlO2 decreased gradually, while the corrosion rate of specimen prepared in Na2SiO3 showed the trend to first increase and then decrease. The reasonable use of equivalent circuit model was useful to analyze EIS of specimens and commercially pure titanium. The EIS of coating specimen reflected the electrochemical behavior between coating and treating solution, which is corresponding to the surface state and structure of coating, while the EIS of commercially pure titanium reflected the electrochemical behavior between metal and treating solution, which is corresponding to the double-layer structure on the surface of commercially pure titanium. The corrosion resistance of Titanium could be characterized by Rp, and the electric charge transfer resistance (Rt) in the equivalent circuit can be used to assess the corrosion resistance of the coating. The microhardness of the specimens processed with MAO technique significantly increased; As amplitude, frequency and duty cycle of voltage pulse and processing time increased, the microhardness on the surface of specimens showed the trend to first increase and then decrease; At different location away from coating and substrate interface, the microhardness of the specimen prepared in the same solution was different, The hardness of the coating near the substrate was the highest, and the hardness of the coating at the outer surface was the lowest, the hardness of the coating from the substrate to the outer surface decreased gradually; The microhardness of the coating prepared in different treating solution was different, The microhardness of coating prepared in NaAlO2 was biggest, followed by the coating prepared in Na2SiO3, and the coating prepared in Na3PO4 is smallest. MAO coating and substrate had a high bonding strength; As amplitude, frequency and duty cycle of voltage pulse increased, the bonding strength between coating and substrate showed the trend to first decrease and then increase; The specimens prepared in different treating solution had different bonding strength between coating and substrate, the specimen prepared in NaAlO2 had highest bonding strength between coating and substrate, followed by the specimen prepared in Na3PO4, and the specimen prepared in Na2SiO3 is smallest.
|
PDF Full Text Request