Enhancement of Ferrous Alloy Surface Mechanical Properties by Nitrogen Implantation
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ENHANCEMENT OF FERROUS ALLOY SURFACE MECHANICAL PROPERTIES BY NITROGEN IMPLANTATION
JOHN T.A. POLLOCK, MICHAEL J. KENNY*, PETER J.K. PATERSON,** *CSIRO Division of Chemical Physics, Lucas Heights Research Laboratories, NSW, 2232, Australia. ** Applied Physics, R.M.I.T., Melbourne, VIC. 3000, Australia.
ABSTRACT Clarification of the relationship between nitrogen profiles and wear behaviour has been sought by studying nitrogen implanted mild steel. Implant energy and dose wer_ in the ranges 25 - 65 keV and 0.9 - 3 x 10 1ions cm , respectively. Wear characteristics were measured with a ball-on-disc system followed by interferometric analysis. Nitrogen distributions before and after wear were determined by Auger electron spectroscopy and Rutherford backscattering methods, and compared with wear track profiles. On balance, the data offers qualified support for nitrogen migration at low loads, although nitrogen was not detected for wear depths >2 times the implant depth. Observations of wear on the abrader ball-bearing surface and the role of oxygen in the wear process are reported.
INTRODUCTION The effectiveness of nitrogen implantation as a wear-reducing agent is well established for many steels, particularly those which are not highly hardened by prior heat treatment (1-4). Improved properties are usually attributed to nitride formation and/or Cottrell atmosphere hardening. Evidence for these structural changes has been reasonably well established in recent years (5,6,7). However, the mechanism by which wear enhancement persists beyond the original implantation depth has yet to be clearly resolved. By inference, nitrogen retention by microstructural modification must occur during the wear of the implanted layer. Dissolution of nitrides due to localised heating at shearing asperities, together with thermal diffusion down the accompanying steep temperature gradient is the most accepted proposition (8). In contrast with the former assertion of deep diffusion (8,9), recent experiments have shown that migration occurs at best over short distances (4,10,11). The present work compares Auger electron spectroscopy (AES) nitrogen profiles to interferometric profiles of the same tracks and show in some detail, the relationship between wear condition and residual nitrogen.
EXPERIMENTAL PROCEDURE Disc samples (12 mm dia.) of bright mild steel were polished metallographically to 1 Pm diamond before implanting with unanalysed nitrog n at energies of 25 - 65 keV to nominal tluxes ?f 0.9 - 6 x 10 ions cm- , using beam current of 20 - 25 x 10 A cm- . Implanted and unimplanted discs were subsequently annealed at 400 K for 6 h. Mat. Res. Soc.
Symp. Proc. Vol.
27 (1984)
DElsevier Science Publishinq Co.,
Inc.
692
Wear measurements were made with a machine based on the pin-on-disc Sample speed and contact time principle, but using a ball-bearing abrader. between abrader (a 1.0 cm dia. carbide ball-bearing) and disc were normalised for various wear conditions so that tracks could be worn at Contact velocity was 0.8 m s- with vario
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