Microstructure and high temperature properties of two-step voltage-controlled MAO ceramic coatings formed on Ti2AlNb alloy

A two-step voltage-controlled microarc oxidation (MAO) method has been used to produce ceramic coatings (NA-2st) on Ti2AlNb alloy. For a comparative study, one-step voltage-controlled MAO ceramic coatings (NA-1st) were also formed on Ti2AlNb alloy. The NA-2st coating with a relatively compact microstructure is composed of Al2TiO5, R-TiO2, α-Al2O3 and γ-Al2O3 phases. The adhesive strength of MAO coatings was tested by a direct pull-off method. Isothermal oxidation tests were carried out at 800 °C in a muffle furnace in air. Normal spectral emissivity of MAO coatings was measured at 600 °C in the infrared wavelength range of 3-20 μm. High temperature tribological properties of MAO coatings were evaluated by using a ball-on-disc friction and wear tester at 600 °C. Due to its compact microstructure and high content of Al2TiO5 phase, the NA-2st coating exhibits better high temperature properties, such as good oxidation resistance, high infrared emissivity, low friction coefficient and small wear rate.