Preparation And Sheering Properties Of High Thermal Emission Coatings On Ti Alloy

When a flight flying at high speed, its surface fricts with atmosphere, and produces lots of heat. For this reason, the temperature of flight vehicle's surface goes up obviously. And the high temperature will bring disadvantageous influence to the properties of the flight's material. The high temperature oxidation has became an important question, which the flight vehicle must face on. The thermal protection coatings with high emissivity can resolve the question of thermal oxidization effectively. Thus, the stability of the flight's material is guaranteed.In this paper, the ceramic coatings were prepared in situ on Ti-6Al-4V alloy by mirco-arc oxidation (MAO) in trisodium phosphate electrolyte system which contains transition metal salts or silicon carbide nano-particles. And the effects of different additives on the emissivity of the film is discussed. Ferrous sulfate, cobalt acetate, nickel acetate and SiC nanoparticles are selected for further research. Effects of additives concentration on the film's structure, composition, shear strength and emissivity are investigated. The shear strength of the film and the key factors of the emissivity are analysized.The phase composition and morphology of the coatings are studied by X-ray diffraction (XRD) and the Scanning electron microscopy (SEM). The distribution of elements is examined by energy dispersive spectrum (EDS). The bonding strength between the film and the substrate are studied by shear strength test, and the emissivity of film is measured using the spectral emissivity measurement system.The results shows that the phase composition and coating structure are affected by the electrolyte concentration. The ceramic films are composed mainly of rutile and anatase TiO₂. The content of rutile TiO₂ increase with the increase of electrolyte concentration. Moreover, the thickness of ceramic films increase with the increase of reaction time. And the shear strength of the coating decrease with increase of electrolyte concentration. The ceramic coating and substrate engaged each other on the interface, and integregated closely. The coatings have the characteristics of a metallurgical bond.The content of Si, Fe, Ni and Co in the film increase with the electrolyte concentration increase.When electrolyte contains cobalt acetate, the ceramic coatings prepared in situ on Ti-6Al-4V alloy by mirco-arc oxidation have the highest emissivity. However, with the increase of the concentration of cobalt acetate, the emissivity of the coating increased first and then decreased. When the concentration of cobalt acetate is up to 4.0g/L, the coating has the highest emissivity, and have a higher shear strength. Effect of time on shear strength and frequency on emissivity are investigated. In conclusion, when cobalt acetate concentration is 4.0g/L, the voltage is 350V, the frequency is 100Hz and the reaction time is 10min, the emissivity of the coating is 0.76-1.0 when the wavelength from 3μm to 8μm and the shear strength is 7.20MPa.