High emissivity coatings on titanium alloy prepared by micro-arc oxidation for high temperature application

Micro-arc oxidation coatings were prepared on Ti6Al4V alloy in Na3PO4-based electrolyte with different additives such as FeSO4, Co(CH3COO)(2), Ni(CH3COO)(2), and K2ZrF6. The composition, structure, surface morphology, and chemical state of the coatings were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis system, and X-ray photoelectron spectroscopy, respectively. The spectral emissivity of the coating was measured by Fourier transform spectrometer apparatus. The bonding strength between the ceramic coating and substrate was studied by tensile strength test and shear strength test. The thermal shock resistance of the coatings was evaluated too. The results showed that the thermal emission of the coatings increased significantly with adding additives. The average spectral emissivity value of the coating with adding Co(CH3COO)(2) is about 0.91 at wavelength of 3-20 mu m. All the coatings showed a tensile strength higher than 30 MPa, and a shear strength higher than 10 MPa. In addition, after subjected to severe thermal shocking for 40 cycles, there was no peeling off of the coating occurred, the coatings possessed good thermal shock resistance.