Study On Improving The Coating Hardness Of Micro Arc Oxidation On TC4 Alloy

Micro-arc oxidation technology is a way to grow a layer of ceramic coating on the surface of titanium alloy without changing the internal structure and mechanical properties of the alloy.But,recent research of coating hardness is low.In some special environment such as intense friction,it needs a higher coating hardness.So studying a micro-arc oxidation process which can improve the hardness of the coating has become an urgent need.In this paper,the micro-hardness of the coatings was studied by the method of different main salt solution,different electrical parameters and different organic additives.The micro structure,element distribution and phase composition of the coatings were analyzed by SEM,EDS and XRD.It was found that the coating phase composition was one of the main factors affecting the coating micro-hardness.The micro-hardness of the coating in the aluminate solution is the highest,but the coating base is the worst,the micro-hardness in the zirconium salt solution is inferior,and the hardness of the coating in the phosphate and silicate solution is no significant difference.In the silicate,phosphate,aluminate and zirconium salt solution,the arc voltage of TC4 alloy is relatively low,and the surface roughness is the smallest.The arc voltage decreases with the increase of the main salt concentration.In the zirconium salt solution,the thickness of the micro arc oxidation coating increases gradually with the increase of oxidation time.The coating surface porosity and roughness increases,the ZrO2 content in the coating decreases,the surface hardness increases first and then decreases.With the increase of the voltage,the number of surface micro-pores decreases and the pore diameter increases,the surface density decreases.The micro-hardness increases first and then decreases with the increase of the voltage.The optimum technological parameters for preparing high hardness micro arc oxidation coating in the zirconium salt solution are as follows: the oxidation time is 15 min,the voltage is 420 V,the duty cycle is 15%,and the pulse frequency is 500 Hz.Alcohol additives and amine additives can make the coating surface structure dense and micro-porous distribution.The solution with the addition of glycerol,N,N-dimethyl formamide,urea can prepare a micro-arc oxidation coating generating new phase TiC to improve the coating micro-hardness.In alcohol additives,glycerol can improve the hardness of the coating.And in amine additives,N,N-dimethyl formamide,triethanolamine,urea can increase the hardness of the coating.While urea improves micro-hardness of the coating is the most significant.On the whole,the amine additive is more effective than the alcoholic additive to improve the micro-hardness of the coating.