Preparation Of MAO Coatings On Ti Alloys In Zirconate System And Research On Thermal Controlproperty

Alloy TC4has low emissivity and high absorption, which seriously affect itsapplication used to be structure material in aerospace industry. To improve its thermalcontrol properties, in this article, high emissivity and low absorption thermal controlceramic coating was prepared by plasma electrolyte oxidation on the surface of TC4inthe Zirconium potassium fluoride and phosphate mixed solution. The coating wascharacterized by SEM, XRD, EDS, thickness and roughness; its thermal controlproperties were studied by infrared emitting ability and radiometer; and spark dischargebreakdown phenomenon during the plasma electrolyte oxidation was also analyzedpreliminary by optical emission spectroscopy.In zirconate solution with high content acid, white ceramic films was prepared onthe surface of alloy TC4. It included two layers: the inner layer is compact, while theexternal layer is porous in white color. When the reaction time is short, the film isthinner and it is consist of TiO2and (TiZr)O2; as the time goes on, the film becamethicker and crystal composition changed to be KZr2(PO4)3、K5Zr(PO4)3, which werezirconium phosphate. Based on its thermal control properties, the better treatmentcondition we achieved like the following: the concentration of zirconium potassiumfluoride, sodium phosphite and ortho-phosphoric acid are6g/L,0.5g/L and2mL/L,respectively, current density is more than12A/dm2, the Frequency is50Hz, duty ratio is25~45%, reaction time is more than20min. Under this condition, the film we preparedhas emissivity more than0.960and absorption less than0.320. After40heat shocks, theemissivity decreased a little while the absorption increased slightly.In zirconate solution both with weak content acid and weak base have similarcomposition and crystal structure with high content acid solution. In the weak contentacid solution, the more the current density was, the more emissivity and less absorptionbecame, vice versa. With duty ratio increasing, the emissivity decrease firstly and thenincrease, and the absorption has a similar change; as the time was longed, the absorptiondecreased while emissivity fluctuated a little. The emissivity ranged from0.88to0.91and the absorption ranged from0.335to0.400. In weak base solution, as the currentdensity increased, the emissivity kept stable while the absorption decreased; as the timeextended, the absorption decreased and emissivity increased. The emissivity rate was around0.96and the absorption ranged from0.285to0.400under this electrolytesolution. The emissivity and absorption both have little change in these two systemsafter40thermal shocks.By collecting and analyzing the spark during the reaction, metallic element andpositive ion were found to be the main sparking elements such as sodium ion, potassiumion, titanium ion, hydrion and so on, but these elements only have small amount contentin the films, these sparks provide energy for the MAO process. Anion and Anionicgroup were sintered on the surface to be part of the films while sparking.