Preparation Of Colored Coating On Titanium Alloy By Micro-arc Oxidation And Research On Thermal Stability

The color coated treatment of titanium alloy can meet the requirements of theoptical properties in aerospace field, the identification function in medical field and theaesthetics in daily use. As a result, the preparation of the colored film on titanium alloyhas received much attention. In this paper, yellow, blue, green and black ceramiccoating was preparted respectively by plasma electrolytic oxidation on titanium alloyTC4. The influence of process paramenters have been studied in this paper. Thecomposition and structure of the coatings were characterized by SEM, XRD, XPS,Layer Thickness Meter and Roughness Measuring. The thermal stability of the coatingshave been studied by baking experiment. In the meantime, the corrosion resistance andthe adhesive strength of the coatings were researched by electrochemical process andtensile experiment, respectively.In NaOH-phosphate electrolyte, the coating which thichkness was about10μm wasmade. The coating is made up of rutile and anatase TiO2with porous structure. Theelectrolyte composition influenced the color of coatings: the coatings turn yellow andbrown from gray with phosphate concentration increasing; the color turn yellow frombrown with NaOH concentration increasing. In electorlyte with2.64g/L phosphate,4g/LNaOH and1g/L NaAlO2, yellow coating were made under electrical parameters whichwere1000HZ,45%duty cycle and constant voltage380V for10min. The coatingchromatic value is L*=53.13, a*=4.66, b*=29.02. The coating was yellow because of Velement in substrate deposited on coating as vanadate which chemical valence were+4and+5. The compose of electorlyte influenced the deposited of V element on coating, asa result the colore of coating changed. After baking in different high temperature, thecolor turn darke from yellow.Black ceramic coating was prepared in phosphate-silicate electrolyte with NH4VO3.The coating consisted of amorphous phase and thickness was about9μm. When theconcentration of vanadate was increasing, the color turn black. The electricalparamenters influenced the adhesive strength of coating instead of the color. With thefrequency and voltage increasing or duty cycle decreasing, the adhesive strengthbetween coating and substrate. The black coating chromatic value which made byoptimized measurement is L*=8.81, a*=-0.37, b*=1.33. After baking in different hightemperature, the coating color did not change under200oC, but the color turn brownwith the tempreture increasing. The coating crystallized when baking in300oC.Black ceramic coating was prepared in phosphate electrolyte with Fe, Co, Ni andtungstate, which coating was amorphous, and the thickness was increasing with reactiontime getting longer. The electrical paramenters influenced the adhesive strength heavily.With the frequency increasing or duty cycle decreasing, the adhesive strength between coating and substrate. The black coating chromatic value which made by optimizedmeasurement is L*=14.88, a*=-1.72, b*=1.79. After baking in different hightemperature, the coating color did not change under200oC, but the color turn dusty bluewith the tempreture increasing. The coating crystallized when baking in300oC.