Studies of N-ion-implanted stainless steels oriented for industrial applications

We have built an industrial 100-keV ion implanter for treating metal parts in order to increase wear resistance. 70-keV N ions of >5×1016/cm2 were implanted into the surface polished stainless steel 420 (SS420) with average surface roughness (Ra) of 0.04 μm, and wear resistance of N-ion-implanted specimen at the mild abrasive condition was investigated. When the beam incidence was 45° with respect to the specimen surfaces, nitrogen was detected up to at least 300 nm from the surface, as measured with Auger electron spectroscopy (AES). X-ray photoelectron spectroscopy (XPS) analysis showed that the implanted N formed mostly Cr2N without postirradiation annealing. Hardness profiles of the specimens were obtained with nanoindentation technique as a function of distance from the surface before and after ion implantations. The peak hardness of 14 Gpa formed at approximately 50-nm depth from the N-ion-implanted surface was about at least two times higher than nonirradiated specimen. Along with the hardness measurement, ball-on-disc wear resistance test was conducted with 500-gf alumina ball. The wear track to the onset point of abrupt increase in the frictional coefficient was about 5 m for the N-implanted specimen, while wear took place for the pristine sample as soon as the test started. On the other hand, when 1000-gf ball was used for the wear test, the difference in the wear track between the pristine and N implanted specimens was smaller than 500-gf ball. After the ion-beam irradiation, the surface roughness was reduced from Ra=0.04 μm to Ra=0.02, μm as measured with a high-resolution surface roughness tester. We found that the ion implantation prolonged the lifetime of the metal parts subjected to mild abrasive environment, like hair clipper blades.