Carbon, Nitrogen, and Oxygen Ion Implantation of Stainless Steel
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Mat. Res. Soc. Symp. Proc. Vol. 396 0 1996 Materials Research Society
due to diffusion of gas from the ion source. High temperature implants were achieved by employing the ion beam itself to heat the workpiece. The LANL device was operated at 27-50 kV, j = 1-4 mA/cm2 at the workpiece over a 20-pts pulsewidth and 500-2000 Hz pulse repetition rate. Workpiece temperatures were controlled by adjusting the repetition rate. Gas sources included N2 , NH 3 , or CH 4 introduced at 3 -5xlO4 torr. Plasmas were created with a 500-900 W, 13.56 MHz, capacitively-coupled rf source. Workpieces consisted of cold-rolled stainless steel coupons, type 12Khl8N10T (comparable to AIS1321), composed of 0.12% (wt) C, 18% Cr, 10% Ni, 0.7% Ti, 1-2% Mn. They were polished with diamond paste to a surface roughness of 150 nm or less. X-ray diffraction (XRD) of unimplanted samples revealed primarily a FCC y phase with lattice parameter of 0.359 nm, with relatively small concentrations (< 3%) of (x phase. Implanted doses ranged between 0.5 to 3 X 1018cm-2. Workpiece temperatures were maintained between 25 and 800°C. The composition and microstructure of the implanted materials were examined with XRD, Rutherford backscattering (RBS), and nuclear reaction analyses (NRA) of the 12C(d,p)13 C, 14N(d,c) 12C, and 1O(d,p)17 0 reactions with a 0.9-MeV deuteron beam, and with a 8.9 MeV alpha particle beam. Depth profiles of the implanted nitrogen were calculated after comparing the measured NRA spectrum with that produced by a TiN08. 2 reference specimen. X-ray photoelectron spectroscopy (XPS) was also used to infer depth profiles. Carbon and oxygen profiles were similarly determined with U8 steel and Ta 20 5 reference materials, respectively. Vickers microhardness was measured at both 50 and 200 g loads, with indent depths ranging between a few tenths to several microns, depending on microhardness. More sensitive surface nanohardness and modulus measurements at depths between 20 to 800 nm from the surface were made with a Nano-Indenter®II [8]. Wear properties were investigated with pin-on-disk test with a 6-mm diameter ruby pin, a sliding speed of 0.94 m/min, a wear track diameter of 3 mm, and in a 10% relative humidity atmosphere. Samples were tested with a load of 27 g and contact stress of 533 MPa. The yield strength of the untreated stainless steel is 200 MPa, so the conditions chosen resulted in highly accelerated wear. RESULTS Implant distribution profiles as determined from RBS and NRA are plotted in Figs. 1 and 2. At elevated temperatures, deeper penetration (when compared to the < 100 nm ballistic ion range) of C and N are observed. Dramatic changes in C profile are observed between 390°C and 480'C. Examination of N profiles obtained with the same implanted dose (lxlO 1cm-2) but with pulse ion current densities of 3 and 10 mA/cm2 reveal different spectra (Fig. 1). A lower N concentration is observed at 10 mA/cm2 and 765°C, when compared with the 3 mA/cm 2 implant that was performed at 613'C. These results suggest an increased diffusion coeffici
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