Microstructure and properties of Cr-AlN composite coating prepared by pack-cementation on the surface of Al-containing ODS steel

An Al-containing ODS steel was coated with a composite Cr-AlN layer by pack-cementation treatment. Constituent phases and microstructures of the coating layer were carefully characterized. The results show that the Cr-layer is a Cr-Fe solid solution with {Cr,Fe} 2 N 1-x particles and Cr 2 C particles, while the AlN-layer is composed of AlN particles and α-Fe matrix. The formation of Cr 2 C and {Cr,Fe} 2 N 1-x particles in the Cr-layer is due to the drag effect of the strong carbide-forming element Cr on the carbon in ODS steel and nitridation reaction of NH 4 Cl and Cr, Fe, respectively. During high-temperature pack-cementation, N atoms in the air diffuse into the ODS steel substrate and react with Al in the steel under the catalytic action of yttria , resulting in the formation of the AlN particles. Due to the limited diffusion depth of N and the high nitrogen concentration in the surface layer, the density and size of AlN particles are distributed in a gradient. The particle size is small and high on the side of the Cr-layer, while the particles on the side near the substrate are coarse and low in density. The composite coating greatly improves the surface hardness and friction resistance of the ODS steel.