Kingo Azuma

Effect of nitrogen plasma-based ion implantation on joint prosthetic material

Abstract In total hip arthroplasty (THA), Ultra High Molecular Weight Polyethylene (UHMWPE) wear debris generated at the articular surface has been recognized as a long-term cause of loosening and failure of artificial hip joints due to osteolysis. The technique of ion implantation has been used to improve wear resistance of the metal femoral head. The Plasma-Based Ion Implantation (PBII) technique is more suitable for complex shaped implants such as femoral head and femoral prosthesis due to three-dimensional ion implantation effects. The effect of pulse voltage and cooling of the substrate of the PBII process for Co–Cr-based materials were examined in terms of wear resistance by the pin-on-disc wear test. The wear resistance of Co–Cr alloy with high nitrogen-ion implantation dose was superior to the untreated Co–Cr alloy. Their corrosion resistance has also been examined with an anodic polarization measurement. High dose and cooling of PBII process proved to be effective in enhancing corrosion resistance of Co–Cr alloy. In this study, it was confirmed that the improvement of wear and corrosion resistance of Co–Cr alloy modified by PBII, as well as high pulse voltage and cooling of the substrate for PBII were the most effective treatments.

Corrosion resistance of TiN coatings produced by various dry processes

The corrosion resistance of a TiN surface prepared by plasma-based ion implantation (PBII) was compared with that of TiN coating films prepared by sputtering deposition, plasma spraying, and shielded vacuum arc deposition. The corrosion test with the potentiodynamic polarization curve shows that the PBII sample had the best corrosion resistance. The SEM observation indicates that there was no pinhole on the TiN surface prepared by PBII. However, a lot of pinholes were observed in the TiN coating films prepared by the other dry coating processes.

Surface treatment of steel by short pulsed injection of high-power ion beam

Abstract A hydrogen-high power and short pulsed ion beam (PPIB) with ion energy of 160 keV, ion beam current density of 500 A/cm 2 and pulse duration (FWHM) of 65 ns was injected into a high-speed tool steel (SKH51), producing an austenitic phase in nm-sized polycrystals on the surface layer of target. The average crystal size in the austenitic phase was estimated to be 40 nm, which is 1/170 of crystal size before the PPIB injection. The heat affected depth is estimated to be approximately 1.0 μm from the surface that is in good agreement with the ion range calculated by TRIM code.

Uniform coating of thick DLC film on three-dimensional substrates

Abstract Uniform coating of the thick diamond-like carbon (DLC) on the cylindrical pipe, triangular prism and trench of aluminum alloy was studied with the hybrid processing system of plasma-based ion implantation and deposition. In this system, the RF pulse for plasma generation was applied to a substrate with the high-voltage pulse for ion implantation through a joint matching network and a single electrical feed-through. The DLC film with the thickness of a few μm was prepared using the toluene gas (C6H5CH3) and the negative high-voltage pulse (−10 to −20 kV, 2–4 μs and 1–4 kHz). The DLC film thickness profile estimated from the cross-sectional SEM observation was extremely uniform on the surfaces of cylindrical pipe and triangular pole. For the trench with the aspect ratio of 1, the film thickness of top surface was about 1.3 times larger than that of bottom and 1.9 times than the sidewall.

Properties of thick DLC films prepared by plasma-based ion implantation and deposition using combined RF and H.V. pulses

Abstract A diamond-like carbon (DLC) film with a thickness of a few μm was prepared on an aluminum alloy by the hybrid process of plasma-based ion implantation and deposition using acetylene gas. The residual compressive stress of the DLC film determined from the substrate curvature decreased with increasing the negative high voltage for ion implantation and was about 0.2 GPa at the high voltage of −20 kV. Ion implantation also served to produce a graded interface between the DLC film and the substrate material. The relaxation of residual stress in the film allowed us to produce thick DLC films of high adhesion. A wear resistance test by the ball-on-disc method showed that the DLC film on an aluminum alloy substrate had the low friction coefficient of about 0.1.

Surface Modification of High-Speed Tool Steel by Repeated Irradiations of Intense Pulsed Ion Beam

Surface modification of a high-speed tool steel (SKH51) has been performed by irradiation of the proton intense pulsed ion beam (IPIB) with the ion energy of 180 keV, the ion current density of 460 A/cm2, and pulse duration of 65 ns. By 1-pulse irradiation of the IPIB, the second phase carbides such as VC, Fe6C, and Cr6C are dispersed from the irradiated surface, and the average grain size decreases to 240 nm corresponding to approximately 1/30 of the untreated one. Repeated irradiations of the IPIB result in the reduction in grain size and conversion of an α-Fe structure to a γ-Fe one. The SKH51 treated by 10-pulses irradiation has a single phase of γ-Fe, and the grain size is reduced to approximately 1/170 of that of the untreated one. In addition, the 10-pulses-irradiated surface has an improved wear resistance.

Electrical and Optical Characteristics of High-Power Pulsed Sputtering Glow Discharge

Droplet-free metal-plasma sources are promising for enhanced adhesion of deposited films with a smooth surface. High-power pulsed sputtering (HPPS) plasma is an arc-free discharge and a glow-discharge plasma with instantaneous power consumption of several tens of kilowatts, although the average power is the same as conventional sputtering discharge systems. Sputtered metallic species are significantly ionized. In this paper, two metal-plate cathode targets are positioned in parallel to form a gap. A magnetic field with a strength of 0.3 T is oriented parallel to the electric field, and a Penning discharge is generated at the gap. When a negative pulse voltage is applied to the two targets with the same polarity, a glow plasma is generated at the gap. Gas ions are first produced and accelerated toward the target, where metals are sputtered and simultaneously ionized. The plasma source is compact in size (60 times 60 times 60 mm3), with a gap length of 10 mm. The pulsed voltage is rectangular in shape, with amplitude ranging from -600 to -1500 V and pulse-widths of 30 and 100 mus. The repetition rate of the applied pulse is 625 Hz. Electrical and optical characteristics are investigated to determine fundamental characteristics of the HPPS glow metal plasma, and the plasma density are estimated using an electrode immersed in the HPPS glow plasma. The electrode is set nearby the plasma source. With a current-limiting resistor of 10 Omega and an applied voltage of -1200 V, the peak discharge current is approximately 76 A, which results in a peak instantaneous power consumption of approximately 30 kW at an argon gas pressure of 2 Pa. In this case, the consumed energy per pulse is 0.8 J, which corresponds to an average power of 500 W at a repetition rate of 625 Hz. The peak power density at the target surface is approximately 1.3 kW/cm2. The ion density at the holder-electrode set, positioned near the HPPS plasma source, is estimated to be on the order of 1016 m-3, and it is time dependent. Optical emissions from titanium ions with a charge of +1 and excited titanium atoms are observed. Emissions are also observed from argon atoms and ions. Ions are extracted at the holder electrode. It is found that the titanium-ion spectrum intensity is proportional to the argon-ion spectrum intensity. The waveform of the ion current has a sharp peak at the initial stage followed by a stationary state. Thus, the holder electrode is immersed in the plasma, and a transient ion sheath is formed around the holder electrode. It can be shown that the stationary ion current is proportional to the ion density at the surface of the holder electrode.

Profile of implanted nitrogen ions in Al alloy for mold materials

Abstract Nitrogen ions were implanted into an aluminum alloy (Al-7Si) for casting mold materials by plasma-based ion implantation to enhance wear resistance and hardness of the target. A negative high-voltage pulse of 10 kV with a pulse width of 10 μs and repetition rate of 100 Hz was applied to the target immersed in the nitrogen plasma produced by filament discharge. Auger electron spectroscopy (AES) analysis indicated that the depth profile of implanted nitrogen had two peaks, corresponding to N + and N 2 + ions, for short implantation times less than approximately 30 min. For implantation times longer than 30 min, however, the two peaks merged with each other. The AES analysis also indicated the formation of AlN by implantation of nitrogen ions. The AlN was formed in the surface region within the nitrogen ion range. A wear-resistance test by the ball-on-disc method showed that nitrogen ion implantation resulted in a reduction of the friction coefficient from 0.6 to 0.1 and the wear resistance was increased by one order of magnitude. It was also found that nitrogen ion implantation enhances the hydrophilic properties.

Structural analysis of a high-speed tool steel irradiated by an intense pulsed-ion beam

The surface of a high-speed tool steel has been modified by an intense pulsed-ion beam (IPIB) that has an ion energy of 180 keV, an ion current density of 460 A/cm/sup 2/, and a pulse duration of 65 ns. Transformation of the crystalline structure in the sample surface layer by IPIB-irradiation was observed by a glancing angle X-ray diffraction and a transmission electron microscopy. The second-phase carbides, V/sub 4/C/sub 3/, Cr/sub 6/C, and Fe/sub 6/C, were dispersed from the irradiated surface layer. A martensitic matrix in the surface layer was converted into an austenitic matrix by ten-pulses-irradiation of the IPIB. After repeated IPIB-irradiation of 100 pulses, a mixed phase of the austenite containing 2.0 wt.% carbon and a cementite was formed in the surface layer. The crystalline size of the sample was reduced from 7 /spl mu/m for the untreated to 41 nm after 100-pulses-irradiation. The Vickers hardness of the treated surface was Hv=1030 corresponding to 1.2 times of the untreated one. In addition, the friction coefficient of the treated surface was reduced from 1.2 to 0.9, and the wear resistance of the surface was significantly improved.

Microstructure of Al-alloy surface implanted with high-dose nitrogen

Abstract Nitrogen ion implantation by plasma-based ion implantation with a negative bias voltage of 10 kV led to the formation of AlN on the surface layer of a casting aluminum alloy (Al–7Si) sample. The AES analysis of the ion-implanted sample indicated that the depth profile of nitrogen ions implanted at room temperature was approximated by a Gaussian profile with a center of ion range (27 nm). The AFM observation showed the changes in surface morphology with nitrogen ion implantation. The surface roughness of the unimplanted sample, which was mechanically polished before ion implantation, had a mean-average roughness of R a =13.9 nm. After implantation, on the other hand, the surface roughness decreased with increasing the ion dose, and was reduced to R a =8.2 nm at a dose of 2.2×10 18 cm −2 . Plasma ion implantation led to both implantation and smoothing simultaneously.