Effect Of Post-treatment And Nano Si₃N₄ On The Phase Structure And Performance Of Phosphate Conversion Coating On Titanium

Zinc phosphate conversion coating has been widely used in metal surface corrosion,in order to improve the coating layer adhesion and surface lubrication and so on.On the other hand,because of its high specific strength,non-magnetic,good stability and many other excellent performance,titanium has played an important role in the native economy.However,its application has been limited due to easy oxidation,sensitivity to adhesive wear,poor adhesion to coating and other defects.In this paper,zinc phosphate conversion coating has been prepared on titanium surface by a facile chemical conversion method under room temperature.Firstly,the conversion formula of zinc phosphate conversion coating on the surface of titanium under normal temperature was studied by discussing factors such as parameters of pretreatment process,basic intermediate transformation process and post-treatment process.The phase,microstructure,mechanical properties and corrosion resistance of the conversion coatings were analyzed systematically by X-ray diffraction?XRD?,field emission scanning electron microscopy?FE-SEM?,acoustic emission scratches and three-electrode electrochemical workstations.The results showed that the conversion coating mainly consists of Zn3?PO4?2·4H2O,and the technological conditions have little effect on the phase of the conversion coating.The effect of alkali washing,pickling and activation was irreplaceable each other for the preparation of the high quality conversion coating.The conversion time and temperature had little effect on phase of the conversion coating.The thickness of conversion coating prepared under 25? and 75? had significant difference,rather than binding and corrosion resistance was not much difference.As the conversion time was prolonged between 0.5 and 10 minutes,the grain size of the conversion coating increased and crystal the number increased,which grain shape varied from a single sheet to a rough strip,forming a more compact conversion coating.Different orders of drying and rinsing also had an impact on microstructure and corrosion resistance of the conversion film.The film which prepared by drying before rinsing had higher density because precipitated phase of residual conversion solution have filled the void of grains.Precipitated phase has covered grain surface,which makes grains appear smaller.This phenomenon has improved corrosion resistance of the film.Meanwhile formed porous structure help to improve the painting properties of titanium and its alloys.Adding nano-hard particles in the conversion coating did not only change the phase of the conversion coating,but also themselves did not have phase changes.When the doping amount of Si₃N₄ is between 0.5 and 2.0 g/L,most of particles were agglomerated in grooves or voids between the zinc phosphate grains,and some were embedded on the sides of the grains and others were attached to the bare substrate.It is speculated that the nano Si₃N₄ particles which embedded on the side of the grain and attached to the bare substrate may act as nucleation centers to promote nucleation and growth of the transformed film.In this paper,with the increase of nano Si₃N₄ doping,the amount of zinc phosphate increased.The grain size firstly increased and then decreased,and the thickness of the conversion film increased.The grain size was the smallest when the doping amount of Si₃N₄ is 0.5 g/L.It was found that adding nano-Si₃N₄ particles in the conversion coating had improved corrosion resistance of the conversion coating.With the increase of the doping amount,the corrosion voltage increased first and then decreased,but the corrosion current had opposite variation.Additionally,Si₃N₄ can be detected in the residual conversion coating after the electrochemical workstation detects,which indicating that the added Si₃N₄ particles not only lie in surface but also the internal grains and even the grains,which is the reason for improving corrosion performance.It can be found that Addition of Si₃N₄ particles in the conversion coating improves the binding force between the conversion coating and the matrix.