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

Abstract

• MAO coatings on Ti 2 AlNb alloy consist of Al 2 TiO 5 , R-TiO 2 , Al 2 O 3 and Nb 2 O 5 . • Two-step voltage technique enhances the densification of MAO coatings. • MAO coating improves the thermal radiation property of Ti 2 AlNb alloy. • MAO coatings protect Ti 2 AlNb substrate from the high temperature oxidation. • Two-step MAO coatings exhibit better high temperature properties. A two-step voltage-controlled microarc oxidation (MAO) method has been used to produce ceramic coatings (NA-2st) on Ti 2 AlNb alloy. For a comparative study, one-step voltage-controlled MAO ceramic coatings (NA-1st) were also formed on Ti 2 AlNb alloy. The NA-2st coating with a relatively compact microstructure is composed of Al 2 TiO 5 , R-TiO 2 , α-Al 2 O 3 and γ-Al 2 O 3 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 Al 2 TiO 5 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.