Microstructure and mechanical properties of 17-4PH steel plasma nitrocarburized with and without rare earths addition

17-4PH stainless steel was plasma nitrocarburized at 500 °C with and without rare earths (RE) addition. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope equipped with an energy dispersive X-ray analyzer, X-ray diffraction, hardness tests and pin-on-disc tribometer. The results show that rare earths atoms can diffuse into 17-4PH steel surface and change the microstructure of the nitrocarburized layer. The incorporation of RE elements increases the layer thickness and the hardness of the nitrocarburized layer up to 29% and 70–120 HV, respectively. The friction coefficients and wear rates of the nitrocarburized specimens are apparently lower than that of un-nitrocarburized one. The wear mechanisms of steel specimen plasma nitrocarburized with and without RE addition are different mainly due to the differences in the microstructure, the phase proportion and the hardness of the modified layer.