Gradient Cr–AlN composite coating prepared on alumina-forming austenitic steel

A gradient composite coating of Cr–AlN with an outer Cr-rich layer and an inner AlN-rich layer was deposited on the alumina-forming austenitic (AFA) steel surface by pack-cementation chromising. The phase composition, element distribution, and microstructure of the coating were characterised and analysed. The microhardness and the friction and wear performance at 600 °C were tested. The results show that the outer Cr-rich layer mainly consists of a polycrystalline Cr–Fe solid solution and a small amount of Cr₂C and (Cr, Fe)₂N_1−x particles. The inner layer contains plenty of AlN particles with different sizes dispersed on the Cr–Fe solid solution. The formation of AlN particles is closely related to the spontaneous chlorination and nitridation reaction. The addition of Y₂O₃ particles in the encapsulated powder promotes the uniformity of Cr plating and the growth of AlN particles. The gradient composite coating remarkably improves the microhardness and high-temperature wear resistance of AFA steel.