Enhanced microhardness of four modern steels following nitrogen ion implantation

Samples of two kinds of hot-work steels (Orvar Supreme, QRO 90 Supreme), one cold-work steel (Sverker 21) and one ultra-high-strength aircraft steel (AerMet 100) were implanted with 120 keV N2+ ions (60 keV/N+) at doses ranging from 5×1015 to 1.2×1017 ions/cm2 at room temperature. Also implanted into the four steels were 60 keV C+ ions using the same doses. The Knoop microhardnesses on the sample surfaces increased by 31–72% at load of 25 gf after nitrogen ion implantation. There was no obvious effect on microhardness after carbon ion implantation. Tribological properties of the nitrogen ion implanted samples were evaluated. A heat-treatment study of nitrogen ion implanted (8×1016 ions/cm2) AerMet 100 and Sverker 21 showed that the hardness reduction arising from heat treatment was clearly lower for the implanted samples heat-treated at 400°C. The microhardness of the implanted layers alone was calculated by the Jönsson–Hogmark model. The layer thickness used for the calculation of implanted-layer hardness was obtained from a molecular dynamic simulation code (TRIM). The residual stress of the surface after nitrogen ion implantation was measured by X-ray diffraction. Friction and wear properties of the implanted samples were also measured. Nanohardness and Youngʹs modulus were measured for the nitrogen ion implanted layers. Grazing angle X-ray diffraction (GXRD) analysis shows the possible formation of nitride phases after nitrogen ion implantation. After nitrogen implantation, samples were depth profiled by AES (Auger Electron Spectrometry) to confirm the depth distribution of the implanted nitrogen ions. Implanted samples showed increased hardness, wear resistance and surface compressive stress. By using X-ray photoelectron spectroscopy (XPS), Cr2N phases were found in Orvar Supreme and Sverker 21 samples implanted with nitrogen ions. These two steels have higher content of Cr (5.2% and 12%), and thus their hardness increase is greater than in the steels with lower Cr content.