Study On Micro Arc Oxidation Coatings With High Emissivity On The Surface Of Ti-6Al-4V

Aircrafts play an important strategic and transport role in the fields of military transport.China also has invested a lot of energy in the development of aircrafts in recent years.Titanium and its alloys are widely used as structural components of aircrafts due to their better specific strength,corrosion resistance and heat resistance.However,the friction between shell and atmosphere as well as the flame radiation of the nozzle will lead to excessive temperature of the fuselage during flight.A long period of oxidation process at high temperature will change the structure of metals,thus causing damage to their mechanical properties.The high infrared emissivity coatings can release the energy through heat conduction,which can improve the flight life of aircrafts by reducing the rise of temperature.In this study,the coatings with high infrared emissivity were prepared on the surface of Ti-6Al-4V alloy by micro arc oxidation.Alkali metal salts KMnO4,FeSO4 and K2ZrF6 were added into the three film forming agent systems of Na2SiO3-(NaPO3)6,(NaPO3)6 and Na2SiO3-NaPO3)6 to form electrolytes separately.In order to optimize the best doping concentration and oxidation voltage of each system,the additive concentration and oxidation voltage were changed,and the effect of vacuum heat treatment on the performance of coatings was studied.The influence of the amorphous content of MnO2,FeSO4 and ZrO2,thickness and roughness of coatings,surface porosity on the infrared emissivity was also studied.The changes in phase composition,microstructure and chemical state of the coatings were studied by XRD,SEM,EDS and XPS.The tensile test machine and Fourier infrared spectrometer were used to obtain the bonding strength and infrared emissivity of the coatings.In addition,the influence factors on infrared emissivity under different conditions were discussed.The results showed that the main phases existed in the coatings were rutile-TiO2,anatase-TiO2 and Ti.The elements in electrolyte all participated in the film forming reaction and the basic film forming agent existed as the amorphous form of SiO2,NaH2PO4 and Na4P2O7,the alkali metal salts existed as the amorphous form of MnO2,FeSO4,ZrO2 and ZrSiO4 respectively.The existence of amorphous oxides could promote the formation of local energy levels and lattice defects,which was beneficial to the increase of infrared emissivity.The additive concentration and oxidation voltage of different electrolytes all had influence on the bonding property of the coatings.The bonding strength of the coating prepared in Na2SiO3-(NaPO3)6-KMnO4 electrolyte reached the maximum of 40.2 MPa when the concentration of KMnO4 was 0.8 g/L and the oxidation voltage was 540 V;the bonding strength of the coatings prepared in(NaPO3)6-FeSO4 electrolyte decreased with the addition of FeSO4,and increased first and then declined as the voltage rose,but all the values were greater than 27 MPa;most of the bonding strength of the coatings prepared in Na2SiO3-(NaPO3)6-K2ZrF6 electrolyte exceeded 18 MPa;the results indicated the good bonding property between the coatings and substrate.Alkali metal salts were added to two basic electrolytes of Na2SiO3-(NaPO3)6 and(NaPO3)6,then the change of infrared emissivity of the coatings was studied.The infrared emissivity of the coating prepared in the Na2SiO3-(NaPO3)6 electrolyte reached 0.88 when the KMnO4 concentration was 0.8 g/L and the oxidation voltage reached 540 V,which had a 10%improvement compared with the coating without KMnO4 addition.The infrared emissivity of the coating prepared in the(NaPO3)6-FeSO4 electrolyte was up to 0.91,the corresponding FeSO4 concentration and voltage were 9 g/L and 450 V respectively.The value was basically close to the standard blackbody radiation value(0.93),and it was 31.9%higher than the coating without FeSO4 addition.By adding K2ZrF6 to the Na2SiO3-(NaPO3)6 electrolyte,the infrared emissivity of the coating could reach 0.87 when the concentration of K2ZrF6 was 6 g/L and the oxidation voltage was 480 V,which was 7.4%higher than the coating without K2ZrF6 addition.The best composition ratio of the electrolyte was:(NaPO3)6-9 g/L FeSO4;The optimum process condition was:the oxidation voltage:450 V,the frequency:600 Hz,the duty cycle:15%and 20 min for the oxidation time.In order to simulate the application environment of coatings in space,the samples with higher infrared emissivity were treated by vacuum heat treatment.It was found that there was no change in the existing state of elements,but the density,bonding strength and infrared emissivity of the coatings were all improved.After the vacuum heat treatment,the infrared emissivity of the coating prepared in the Na2SiO3-(NaPO3)6-KMnO4 system was increased from 0.88 to 0.91.Similarly,the infrared emissivity of the coating prepared in the Na2SiO3-(NaPO3)6-K2ZrF6 system was increased from 0.87 to 0.91,it can be seen that the two values were almost the same as that of the standard blackbody.