The Relationship Between Crystal Structure and Hardness of Nitrogen Implanted Iron Surface Layers
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THE RELATIONSHIP BETWEEN CRYSTAL STRUCTURE AND HARDNESS OF NITROGEN IMPLANTED IRON SURFACE LAYERS Takanobu FUJIHANA
Advanced Technology Inc.,
Nase 84, Totsuka, Yokohama, Kanagawa, 245 JAPAN
Yoshio OKABE Saitama Institute of Technology, Okabe-machi,
Ohsato, Saitama, 362-02 JAPAN
Masaya IWAKI RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama, 351-01 JAPAN
A study has been made of the relationship between crystal structure and hardness of N-implanted Fe-surface layers. The substrates used were polycrystalline Fe-sheets 1 mm thick with a purity of 99.9 %. The ion implantation of "N* was performed with doses ranging from 7.5x10' 6 to 1x10 10 ions/cm2 at an energy of 100 keV. The substrate temperatures during implantation were kept at -40, 20, 100 and 200 'C. The crystal structure of N-implanted layers was identified by X-ray diffraction method (XRD). The near surface hardness was measured by a Knoop hardness tester. The XRD patterns revealed that the iron-nitride peculiar to each implantation condition was formed, and the iron-oxide was embedded in the nitrides in case of higher temperature implantation. N-implantation led to hardness increase, whose degree also depended on implantation conditions. The comparison between the structure and hardness of implanted layers indicated that, among the phases of iron-nitrides produced by 20 'C implantation, s-phase is the hardest and the y-Fe 20 and y '-Fe 4 N formed by 200 'C implantation are harder than the &-phase nitrides. These results are discussed with respect to the surface composition estimated by Rutherford backscattering spectrometry (RBS) using 1.5 MeV 4 He*-ions. 1. INTRODUCTION Over the last decade, N-ion implantation into iron and steels has been employed for fundamental studies on these surface layers and its applications. In particular, a great improvement has been made of tribological properties such as wear and hardness [1-31. In order to research the improvement mechanisms, the composition and structure in N-implanted layers have been investigated by means of Auger electron spectroscopy (AES) [4], conversion electron M'dssbauer spectroscopy (CEMS) [51, high voltage electron microscopy (HVEW) [61and so on, but these mechanisms have not sufficiently been made clear. We previously reported that the surface hardness of the quenched stainless steel (AISI 420) is at its maximum with a dose of 5x10 1 7 ions/cm2 in case of 75 keV N-implantation at near room temperature [3]. The phenomenon can also be seen for N-implanted pure-iron [7]. Even as regards these results, the relationship between crystal structure and hardness of N-implanted surface layers are not clear yet. In this report, N-ion implantation into Fe-sheets has been carried out at various substrate temperatures to form iron-nitrides. The composition and crystal structure of implanted layers are evaluated by Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD), respectively. The near surface hardness measured by a Knoop hardness tester is discussed
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