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. Costituent 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)N1-x particles and Cr2C particles, while the AlN-layer is composed of AlN particles and alpha-Fe matrix. The formation of Cr2C and {Cr,Fe}(2)N1-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 NH4Cl 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.