Preparation And Radiation Ability Of Aluminum Contained Composite Oxides Coatings On Titanium Alloys

Titanium alloys are widely used in hyper-sonic vehicle industries due to their high strength-to weight ratios.However,with the pursuit of higher speeds,the aerodynamic heating rate has to be drastically increased,which would lead to the machenical failure,oxidation and oxygen embrittlement of the titanium structures and the damage of electronic devices inside the hyper-sonic vehicles.To solve this problem,it is important to minimize the damage caused by aerodynamic heating via preparation of heat-dissipating coatings with high emissivity.In this paper,TiO₂based,Al₂O₃ based and aluminum-chromium phosphate based high emissivity coatings with high oxidation resistance were prepared on TA15 substrates by plasma electrolytic oxidation,cathodic plasma electrolytic oxidation and slurry method,respectively.The phase composition,chemical composition,microstructures,valence states of the elements and functional groups of the binders were measured by XRD,EDS,SEM,XPS and FT-IR.And the shear strength,spectral emissivity and oxidation resistance at 700oC of the coatings were evaluated.We prepared TiO₂ ceramic coatings containing rutile and anatase phases in electrolyte containing Na₂CO₃.The coating showed good shearing strength with substrate,but the emissivity value in the 3-8?m waveband was not satisfied.To further enhance the infrared radiation performance of the coatings,ceramic coatings containing Al₂TiO₅ phase were prepared in the electrolyte containing NaAlO₂,with improved emissivity value in the 3-8?m waveband.And,we have prepared ceramic coatings containing?-Al₂O₃ and?-Al₂O₃ on TA15 substrates in acidic electrolyte,and doped Fe and Mn elements into the Al₂O₃ coatings.The XRD results showed that the doping of Fe and Mn do not induce new phases in the coatings.The SEM results indicated that the morphology of spherical stacks disappeared with inceasing doping Fe and Mn.The shearing strength between Al₂O₃ coatings and TA15substrate were satisfied.The higher spectral emissivity indicated that the radiation ability of the coatings could be improved with the introduction of Fe and Mn elements.The emissivity value of the coating prepared in the electrolyte with Mn to Al mole ratio 8%were improved by 26.7%in the 3-8?m wave band,and 46.4%in the waveband 8-20?m.Meanwhile the introduction of Fe and Mn element has little effect on the oxidation resistance of the Al₂O₃ coatings.Aluminum-chromium phosphate based heat-dissipating coatings are prepared by the slurry method.Aluminum-chromium phosphate was selected as binder,and the mixture of Al₂O₃,SiC,AlN was employed as curing agent.Mixed oxides,mixed oxides with SiC and mixed oxides with BN were selected as fillers.The emissivity of coating with mixed oxides as filler was 0.74 in the 3-8?m waveband and 0.87 in the 8-20?m waveband.The mixed oxides were sintered at various temperatures.And the composites were obtained by sintering at 1050oC and 1250oC.The results show that the aluminum-chromium phosphate coating prepared with the filler sintered at 1050oC display the maximum emissivity which is much higher than the coatings prepared by the plasma electrolytic oxidation and cathodic plasma electrolyte deposition methods because of the Mn_1.5Cr_1.5O₄ phase.Further sintering the filler at 1250oC decreased the emissivity of the coatings.However,the shearing strength of the aluminum-chromium phosphate based coating is much lower.The aluminum-chromium phosphate coating and plasma electrolytic oxidation coating were combined to easily fabricate heat-dissipating coating with high shear strength and oxidation resistance.The composite coating has similar radiation performance with the aluminum-chromium phosphate coating.Furthermore,much higher shearing strength was achieved because of strong mechanical and chemical bonding between aluminum-chromium phosphate coating and plasma electrolytic oxidation coating.In the mean time the introduction of plasma electrolytic oxidation coating can further improve the oxidation resistance of aluminum-chromium phosphate coating.