In-situ Preparation And Properties Of High Emissivity Coating On Titanium Alloy

Titanium alloy couldn’t radiate the heat accumulated on the surface effectivelydue to its high solar absorbance and low emissivity rate, which has greatly limited itsapplications in the aviation and aerospace fields. Thermal control coating is one of themost important technologies to solve thermal control problems currently. In this work,thermal control coating was in-situ prepared by plasma electrolytic oxidation ontitanium alloy. A systematic study was conducted on the effects of concentration,reaction time, electrical parameter for phosphate, aluminate and silicate system,respectively. Thickness, roughness, SEM, XRD was characterized. Solar absorbance,emissivity before and after annealing, and the thermal shock property was alsoperformed. Low solar absorbance and high emissivity coatings were prepared byoptimizing the condition.The coating prepared in phosphate system is dark, and its thickness androughness are small and the coating is consisted of amorphous oxides from varioussalts according to EDS and XRD characterization. Both solar absorbance andemissivity are high, with the values of about0.9and0.85, respectively. Furthermore,the solar absorbance decreased a lot while the emissivity is stable after annealing.Besides, the coating’s color turns shallow on a macro level and the micro-sized cracksemerge and extend after thermal shock for40times.The coating prepared in aluminate system is gray or gray white, its thickness androughness are a little bigger than that in phosphate system and the coating is consistedof Al₂TiO₅, Al₂O₃and TiO₂. Both solar absorbance and emissivity are around0.6and0.8, respectively. Furthermore, the solar absorbance increased and the emissivitydecrease after annealing. Besides, the micro-sized cracks emerge and the spall occursa bit after thermal shock for40times.The coating prepared in silicate system is white, its thickness and roughness aremuch bigger than those in the above two systems. The coating is consisted ofamorphous SiO2from EDS and XRD characterization. The optimized reactioncondition is as follows: the concentration of sodium silicate and sodiumhypophosphite is10g/L and1g/L, respectively; the frequency was500Hz; thecurrent density was10A/dm²; the reaction time was45min. The correspondingcoating’s absorbance and emissivity reach0.37and0.93, respectively. Furthermore,both the solar absorbance increases and the emissivity decreases a little afterannealing. Besides, there is no crack on the surface of the coating, but slightly spalls occur after thermal shock for40times, presenting the good thermal shock resistance.