Tribological and Mossbauer Studies of Ion-Implanted Iron
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TRIBOLOGICAL AND MOSSBAUER STUDIES OF ION-IMPLANTED IRON D.L. WILLIAMSON*, YI QU*, RONGHUA WEI**, W.S. SAMPATH**, AND P.J. WILBUR** *Department of Physics, Colorado School of Mines, Golden, CO 80401 "**Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523
ABSTRACT A modified pin-on-disc wear test technique and conversion electron M6ssbauer spectroscopy (CEMS) have been used to characterize the tribological and microstructural properties, respectively, of pure Fe implanted with N and Ar ions at high current densities. CEMS measurements were made before and after wear testing. For the lubricated, mild adhesive wear conditions used here, no evidence was found for iron-nitride dissolution or N atom migration. Disordering of -y'-Fe4 N did occur as a result of the wear process. All Nimplanted surfaces were tribologically superior to the Ar-implanted surface and an extremely wear-resistant surface layer about 30-50 nm thick was produced with a dose of 8x1016 N atoms/cm2 , at a dose rate of 100 UA/cm 2 . However, once this layer was worn away the wear rate returned sharply to that of unimplanted pure Fe. A high retained N dose has been observed even for a dose rate of 750 AA/cm2 during which the sample reached 280"C. INTRODUCTION Tribological improvements induced by ion implantation of engineering materials are well documented [1-3]. However, the mechanisms by which such improvements are realized remain poorly understood, in part due to the complexity of the surface modification produced by this highly non-equilibrium process and in part due to the inherent complexity of the substrate materials typically selected for study. In addition, the relatively shallow depth of the modified layer (-100 nm) leads to special problems in both microstructural analysis and tribological testing. The lack of durability of such a shallow layer and the long processing time are often-cited disadvantages of ion implantation. Based on the above considerations we have implemented a systematic program of research with the following major aspects: (1) use of metallurgically simpler substrates (pure Fe here); (2) use of the nondestructive, near-surface-sensitive, microstructural probe known as conversion electron M6ssbauer spectroscopy (CEMS) applied to both the as-implanted and worn surface; (3) development of a modified pin-on-disc wear test that generates a large worn area for the M6ssbauer studies; (4) and use of a unique, high current density, broad beam ion implantation system, based on ionrocket technology. This paper describes some effects of high current density N and Ar implantation into pure Fe on the near surface microstructure as determined by CEMS. Results of tribological testing of some of these surfaces are also presented together with CEMS results after various stages of wear. Mat. Res. Soc. Symp. Proc. Vol. 128. t1989 Materials Research Society
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EXPERIMENTAL METHODS Ion Implantation The ion implanter used in this work has been designed specifically for the high dose level that is required f
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