Black PEO coating with enhanced thermal stability on titanium alloy and its thermal control properties

Abstract A black thermal control ceramic coating with high absorption (200–2500 nm, >0.95) and emission (2.5–16 μm, >0.92) was successfully prepared on the surface of Ti6Al4V alloy by plasma electrolytic oxidation with the coloring additives of NH4VO3, FeSO4 and Co(CH3COO)2. The coating composition, surface morphology and thermal control performance were studied in detail. Results showed that the transition metal elements in the coating had various valence states, which presented Ti4+, Fe3+, Co2+, Co3+, V3+, V4+ and V5+, and existed in the form of oxides. Among them, the total content of V3+ and V4+ was increased with the increase of PEO time. Besides, there were some micro-pores and nano-pores on the coating surface. With the increase of PEO reaction time, the number of micro-pores got reduced while their size was increased. Before 20 min of reaction, the improvement of the absorption and emission was mainly due to the semiconductor properties, d-d transition and infrared stretching vibration of transition metal oxides. As the reaction proceeds, the complex porous structure on the surface also played a major role in improving the absorption and emission. Moreover, the coating showed excellent thermal stability in air up to 300 °C or oxygen deficient environment up to 500 °C, which provided a possibility for practical application.