Takanobu Fujihana

Mechanical and electrochemical properties of various tool steels implanted with nitrogen or NO ions

Abstract A study has been made of the tribological properties of various tool steels implanted with nitrogen or NO ions in order to investigate practical applications of nitrogen implantation for the improvement in durability of injection moulds for plastics. The substrates used were prehardened steels, maraging steels, alloy tool steels and stainless steels. The nitrogen or NO ion implantation was performed with doses of (1–5) × 10 17 ions cm −2 at 150 ke V. Surface analyses were carried out by means of secondary ion mass spectrometry, electron spectroscopy for chemical analysis, Auger electron spectroscopy and scanning electron microscopy. The near-surface hardness of implanted steels was measured with a Knoop hardness tester. Wear tests were carried out under unlubricated conditions in a pin-on-disc machine. Aqueous corrosion was estimated by means of cyclic voltammetry in aqueous solutions. The results are as follows. 1. (1) Implanted nitrogen atoms form a gaussian-like distribution and various nitrides. 2. (2) Nitrogen implantation causes the surface hardness to increase and the wear process to change from adhesive wear to oxidative wear. The change in wear process improves the wear resistance. 3. (3) NO implantation causes the corrosion to improve more than nitrogen implantation does. 4. (4) The surface hardness and wear resistance are correlated. This relationship may be useful in selecting material for practical applications.

Effects of implantation temperature on the hardness of iron nitrides formed with high nitrogen dose

Abstract A study has been made of the effect of substrate temperature during implantation on the change of hardness and crystal structure of surface layers induced by high dose nitrogen implantation into iron. The substrates used were polycrystalline iron sheets 1 mm thick with a purity of 99.9%. The ion implantation of N+ was performed with a dose of 5 × 1017 ions/cm2 at an energy of 100 keV. The substrate temperatures during N+ implantation were kept at −40, 20, 100 and 200° C. The composition of implanted layers was estimated by Rutherford backscattering spectrometry (RBS) using 1.5 MeV 4He+ ions. The identification of crystal structure produced by implantation was investigated by X-ray diffraction method (XRD). The near surface hardness was measured by a Knoop hardness tester. The results of RBS measurements revealed that the nitrogen depth profiles were nearly Gaussian-like shapes for implantation below 100° C and became rectangular-like for the 200° C implantation. The XRD patterns revealed that ζ-Fe2N was formed in all implanted layers and γ′-Fe4N and γ-Fe2O3 appeared with ζ-Fe2N matrix for the high temperature implantation. The increase in hardness corresponded to the increase in the temperature during ion implantation. It is concluded that the nitrogen implantation at a high temperature into iron is effective in enhancing the increase in hardness, which is attributed to the formation of hard nitrides.

X-ray photoelectron spectroscopy characterization of high dose carbon-implanted refractory metals

Abstract A study has been made of the depth dependence of atomic fraction and chemical bonding states of carbon-implanted group IVa to VIa transition metals, by means of X-ray photoelectron spectroscopy combined with an Ar + sputtering. Implatation of 12 C + was performed with 10 18 ions cm -2 at 100 keV, and at room temperature. A gaussian-like carbon distribution, predicted by the range theory, was observed even following such a very high dose. The implanted carbon combined with both matrix metal and carbon itself to form metallic carbides and graphitic solid state carbon. The carbon-enriched carbides were also found by the deconvolution of C 1s spectra. The depth profile of C-metal bonded carbon was trapezoidal, whereas C-C bonded carbon exhibited a gaussian profile corresponding to the net carbon distribution. The binding energy of core electrons of metals shifted nearly linearly with carbon content at the region deeper than the average projected range and at the shallower region it was large and approximately constant, which indicate the formation of carbides with carbon vacancies and stoichiometric carbides respectively. These results will be discussed with respect to the energy deposition effects accompanied by ion implantation.

CRYSTAL STRUCTURE OF CARBON-IMPLANTED TITANIUM, VANADIUM AND CHROMIUM

Abstract The dose dependence on crystal structures and lattice parameters of carbon-implanted layers of polycrystalline titanium, vanadium and chromium plates has been investigated by X-ray diffraction (XRD) measurements. Implantations of 12 C + -ions were performed at doses between 1 × 10 17 and 2 × 10 18 ions/cm 2 at 100 keV, and at room temperature. The formation of titanium carbide is characterized only in the B1(NaCl)-type structure in the entire implant dose range, and its lattice parameter mostly agrees with that of monocarbide TiC at higher doses. For vanadium, the B1-type carbide grows preferentially with the dose increment. Its lattice parameter suggests the formation of nearly monocarbide V 4 C 3 at the highest dose. For chromium, only a cubic carbide peculiar to carbon implantation, of which the lattice of chromium atoms is fcc having lattice parameter equal to the diagonal line segment size of the bcc lattice, is found to crystallize at doses over 5 × 10 17 ions/cm 2 . Carbides of titanium and vanadium formed by carbon implantation correspond to those expected by a standard equilibrium with the system of carbon and the respective metal. The formation process of a particular phase will be discussed for chromium carbide.

Characterization of the surface layer of various metals implanted with nitrogen

Abstract The surface layers of various metals (Al, Ti, V, Fe, Ni, Co, Cu, Zr, Nb, Mo, Sn, Ta and W) which were implanted with nitrogen at doses of 3 × 10 17 and 1 × 10 18 N cm -2 were analysed by means of Rutherford backscattering spectrometry and X-ray diffraction. For the lower dose, metal in the implanted layer was partially nitrided. In some targets (V, Ti, Zr and Ta) a solid solution was created, accompanying an isotropic enlargement of the lattice for the cubic metal (V, Ta) and an anisotropic c -direction enlargement for the hexagonal metal (Ti, Zr). When an amount of nitrogen large enough to saturate the implanted region of the targets was implanted, the most stable nitride was created in this region, and the amount of retained nitrogen increases with a decrease of the heat of formation of the nitride. This result shows that retention of nitrogen in the target is affected strongly by the reactivity of the metal to nitrogen.

Ion bombardment into inner wall surfaces of tubes and their biomedical applications

Abstract A study has been made of ion bombardment into the inner wall surfaces of tubes to develop hybrid type, small diameter artificial vascular grafts. Substrates used were polystyrene with an inside diameter of 2 mm and segmented polyurethane (SPU) coated glass tube with an inside diameter of 1.5 mm. Ne-ion bombardment into inner wall surfaces of tubes was performed at an energy of 150 keV with an average fluence of 4 × 10 14 ions/cm 2 at an incident angle of about 88.3°. The surface modification of inner wall surfaces was examined by X-ray photoelectron spectroscopy, which showed the presence of amorphous carbon structures in the inner surfaces of Ne bombarded tubes. Endothelialization was performed on the Ne-bombarded SPU coated inner wall of a glass tube, although it could not be done without ion bombardment. A femoral artery has been replaced by the new artificial graft, and exposed to blood for 24 hours. The new graft demonstrated 100% patency. The development of artificial vascular grafts will be feasible by ion beam modification of inner wall surfaces of tubes.

Characterization of oxide layers induced by oxygen ion implantation into Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W

Abstract The mechanism of oxide formation induced by ion implantation with and without mass-analyzed oxygen ions into nine refractory metals of groups IVa, Va, and VIa in the periodic table was investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). O2+ ions were implanted at an energy of 75 keV per atom with dose ranging from 1 × 1017 to 2 × 1018 ions cm-2 at room temperature using the RIKEN low current ion implanter. The SIT-100 direct ion implanter was used for oxygen ion implantation without mass separation at an acceleration voltage of 80 kV at room temperature. Profiles of implanted oxygen were measured by XPS and RBS and the oxides induced by implantation were identified by XRD. XRD patterns showed almost the same process of oxide formation for both mass-analyzed and non-mass-analyzed implantation. In the case of titanium, however, an effect of energy deposition during the formation of oxides was found by comparing oxides induced by ion implantation with and without mass separation. The formation process was approximately classified into three groups in relation to the groups and periods of the periodic table of elements. The stoichiometric ratio of oxides induced by implantation with high doses is discussed based on data obtained by XRD, XPS and RBS. Finally, the aging behavior of oxides kept in an evacuated desiccator for about 1 year is discussed.

Microstructure and mechanical properties of high dose nitrogen-implanted iron, chromium and titanium sheets☆

A study has been made of the mechanical properties of high dose nitrogen-implanted iron, chromium and titanium sheets. The ion implantation of N+ ions was performed with doses ranging from 7.5 × 1016 to 1 × 1018 ions cm−2 at an energy of 100 kev. The substrate temperature during implantation was kept at about 20°C using a low beam current density and water cooling. The depth profile of implanted nitrogen was estimated by Auger electron spectroscopy (AES) combined with Ar+ sputtering. The surface structure produced by implantation was identified by X-ray diffraction (XRD). The near-surface hardness and friction coefficient were measured by a Knoop hardness tester and a Bowden-Leben-type friction-testing machine, respectively. AES measurements revealed that nitrogen formed a Gaussian distribution for a low dose and error functional or trapezoid-like distributions for the highest dose, and the nitrogen concentration did not exceed a certain stoichiometric ratio. The XRD patterns showed that nitrides peculiar to substrates such as Fe2N, CrN and TiN were formed at room-temperature implantation. The hardness of iron had a maximum at a dose of 1 × 1017 ions cm−2, and the increase in hardness of chromium and titanium corresponded to the increase in nitrogen dose. The friction coefficients of iron and chromium decreased stably at the highest dose, but that of titanium scattered widely as the dose increased. These mechanical properties are discussed with respect to the surface composition and structure.

Crystal structure of carbon-implanted Group 4 transition metals

Abstract The dose dependence of the crystal structures and lattice parameters of carbon-implanted titanium, zirconium and hafnium has been investigated by X-ray diffraction (XRD) measurements. Implantations of 12 C + -ions were performed at doses between 1 × 10 17 and 1.5 × 10 18 ions cm −2 at 100 keV. The beam current density was limited to around 10 μA cm −2 and the specimen was mounted on a water-cooled holder. XRD patterns showed that only one carbide phase with the B1 (NaCl) type structure was characterized for all metals over the whole implant dose range, and its lattice parameter was mostly in agreement with that of monocarbide at doses over 1 × 10 18 ions cm −2 . A solid solution was observed at doses less than 5 × 10 17 ions cm −2 . For zirconium and hafnium, the crystallized C1 (CaF 2 ) type oxide other than the B1-type carbide and solid solution was found. These results will be discussed from the viewpoint of composition and crystal structure.

Surface Composition and Electrochemical Properties of High-Dose Carbon-Implanted Iron

The effect of high-dose C+-implantation on the anodic dissolution properties of iron was studied by multisweep cyclic voltammetry in an acetate buffer solution of pH 5.0. Implantation of 12C+ was performed with a dose of 1?1018 ions/cm2 at an energy of 100 keV. The XPS was used to analyze the depth profile and chemical bonding state of carbon in the surface layers of C+-implanted iron. High-dose C+-implantation was extremely effective in suppression the anodic dissolution of iron in the solution. The dissolution process is discussed from the depth profiles measured before and after electrochemical treatments.