Yasuaki Sugizaki

Improvement of corrosion resistance of titanium by co-implantation

Abstract A number of investigations have successfully represented the beneficial effects of ion implantation in improving the corrosion resistance of metals. In this paper, we review studies on improving the corrosion resistance of titanium by co-implantation with metal ions. These studies have demonstrated that co-implantation is a possible process for attaining excellent corrosion protection of titanium against aggressive environments. The electrochemical behavior of titanium implanted with tantalum, molybdenum, nickel ions, or combinations of two of these ions, have been investigated in boiling sulfuric acid solution. The polarization curves of tantalum-implanted titanium are similar to that of unimplanted titanium, showing the active region followed by passivation. However, anodic polarization current densities are considerably decreased in the active and passive regions, and tantalum implantation suppresses the dissolution of titanium. The corrosion potentials of titanium implanted with nickel or molybdenum are shifted in the noble direction with an increase in fluence, and reside in the passive region of titanium at higher fluences. No significant change is observed in the passive current densities. Nickel and molybdenum implantation preferentially promotes the passivation of titanium in such an aggressive environment. These effects are diminished in a relatively short-term immersion. Titanium implanted with a combination of molybdenum and tantalum ions, or a combination of tantalum and nickel ions, exhibits stable passivation behavior with low passive current densities. The corrosion rate estimated from the polarization curves is approximately 0.1 mm year−1 for co-implantation with tantalum and nickel ions. Co-implantation provides long-term protection for immersion. The outstanding corrosion resistance of titanium is achieved by the complementary effect of co-implantation with these metal ions.

Influence of nitrogen implantation on the hydrogen absorption by titanium

The influence of nitrogen implantation on the hydrogen absorption by titanium has been investigated as a function of fluence. Titanium was implanted with nitrogen ions at an energy of 35 keV in the fluence range of 1 × 1015 to 8 × 1017ions/cm2. The implanted specimens were cathodically charged with hydrogen and the absorbed hydrogen was analyzed quantitatively. Hydrogen depth profiles were also determined by using an elastic recoil detection technique. The structural changes of the implanted layer accompanying the range of fluences were examined by a glancing angle X-ray diffraction method. It was found that nitrogen implantation significantly affected the hydrogen absorption by titanium at fluences above 6 × 1016ions/cm2. At fluences between 6 × 1016and 2 × 1017ions/cm2, the hydrogen contents were markedly increased and the excess hydrogen was preferentially distributed in the implanted layer where the implanted nitrogen distorted the titanium lattice. However, the hydrogen content steeply decreased at fluences above 2 × 1017ions/cm2. The suppression of the hydrogen absorption was caused by the retarding effect of titanium nitride precipitation on the hydrogen migration. It was concluded that the hydrogen absorption of the nitrogen implanted titanium depended on the behavior of the implanted nitrogen in the titanium lattice.

Formation of TiB2 films and BFe interdiffused layers by dynamic mixing

Steel specimens were treated by B ion implantation combined with simultaneous Ti evaporation (dynamic mixing), and the structure of the synthesized films and the formation of BFe interdiffused layer were investigated. B ion beam current density and Ti deposition rate were varied, 15–130 μA/cm2, and 0.03–0.27 nm/s, respectively. The ion energy was maintained at 40 keV. The phase of the synthesized film was found to change from αTi to αTi + TiB2 and then to TiB2 by decreasing the Ti deposition rate or increasing the B ion current density. The TiB phase was not observed in any condition. The formation of a BFe interdiffused layer beneath the synthesized film was found to be enhanced by decreasing the Ti deposition rate. The thickness of the BFe interdiffused layer reached up to 3 μm, which was about 30 times the thickness of the 40 keV B ion implanted layer in steel as predicted by the TRIM simulation program. These thin films synthesized by the dynamic mixing showed higher adhesion compared with those deposited by the conventional rf magnetron sputtering.

Effect of ion beam modification on hydrogen absorption by metals

Abstract Excessive hydrogen absorption by metals, generally causing precipitation of hydrides, leads to degradation of the mechanical properties. Recently, some studies have successfully demonstrated that ion implantation is effective in suppressing hydrogen absorption by the metals. In the present paper, we have reviewed the published studies concerned with modifying the interactions between metals and hydrogen. The results of these studies have provided different ways in preventing the hydrogen penetration into the bulk. These are based on either modifying the hydrogen evolution reaction or retarding the hydrogen diffusion into the bulk. Noble metals such as platinum and palladium are catalytically active, and ion implantation with such ions can prevent the hydrogen absorption through stimulating the recombination and desorption reactions of hydrogen in its evolution reaction. The implanted foreign elements such as titanium, which has a strong affinity for hydrogen, are responsible for hydrogen gettering. Although the gettering enhances hydrogenation in the implanted region, the distribution of hydrogen over the bulk can be inhibited. This results in suppressing the hydrogen embrittlement of the metals. Trapping behaviors are also observed in titanium implanted with nitrogen ions at relatively low fluences. However, nitrogen implantation at higher fluences causes the formation of titanium nitride in the implanted layer, which acts as a barrier to inward migration of hydrogen. As a consequence, a barrier layer such as titanium nitride can completely inhibit hydrogen absorption. These results have demonstrated that ion implantation offers an effective way to prevent hydrogen embrittlement.

Electrochemical behavior of titanium implanted with nickel and tantalum ions

Electrochemical behavior of titanium implanted with nickel and tantalum ions and a combination of the two has been investigated as a function of fluences. The polarization curves of the implanted titanium were potentiodynamically measured in the active and passive regions in a boiling 10 wt.% sulfuric acid solution. Electrochemical measurements revealed that nickel ion implantation significantly promoted the passivation of titanium with an increase of fluence and the corrosion potentials resided in the passive region at fluences above 1 × 1016 ions/cm2. Tantalum ion implantation was effective in reducing the anodic current densities in the active and passive regions as fluences increased. This effect was reduced at fluences above 7 × 1016 ions/cm2. At such fluences, the concentration of the implanted tantalum species decreased due to the etching effect of sputtering. On the other hand, the polarization curves of the co-implanted titanium exhibited more stable passive behavior with considerably low current densities. It was concluded that an excellent corrosion resistance of titanium was achieved by complementary effects of nickel and tantalum ion implantations.

Outward transport of substrate atoms during dynamic ion mixing

Abstract The transport of deposited atoms and substrate atoms during dynamic ion mixing was investigated by Auger electron spectroscopy (AES). Mild steel substrates were treated by dynamic ion mixing, in which Ne, Ar or Kr ion implantation and Ti deposition were carried out simultaneously. Before the treatment, Ta markers were embedded in the surface region of the steel by Ta ion implantation in order to confirm the location of the original substrate surface. The distance of outward transport of Fe atoms (ΔFe) gradually exceeds that of the inward transport of Ti atoms (ΔTi) as the ion/atom arrival ratio (i.e. the ratio of impacting ions to condensing atoms) increases. The same Δ Fe Δ Ti ratio is attained with a lower ion/atom arrival ratio when the ion mass is larger and ion energy is lower, i.e. when the sputter yield of the ion implantation is larger. The outward transport of substrate atoms is thought to be caused by the reaction of the sputtered Fe atoms the depositing Ti atoms during the process.

The effect of ion-mixed Ti on the BFe interdiffusion during dynamic mixing

Abstract The effects of ion-mixed Ti on the B Fe interdiffusion during a dynamic mixing treatment on mild steel were investigated. The experiment and the resulting improvements in the mechanical surface properties are described in this paper. For the experiment, mild steel specimens were implanted with 40 keV B ions to fluences of 1 × 1018−5 × 1018 ions/cm2. Separate steel specimens were treated by dynamic mixing processes, in which B-ion implantation to fluences of 1 × 1018−5 × 1018 ions/cm2 and Ti evaporation at a fixed deposition rate of 0.05 nm/s were carried out simultaneously. For the B-ion implanted steel, the thickness of the B Fe mixed layer in the substrate was limited to less than 0.6 μm, and an accumulation of implanted B species in the surface region took place when the fluence exceeded 3.5 × 1018 ions/cm2. The B-ion implantation had little effect in improving the wear property of the steel. For the dynamic mixing-treated steel, TiB2 film was synthesized on the surface and B Fe interdiffusion was found to have taken place beneath the TiB2 film during the treatment. The thickness of the B Fe interdiffused layer reached up to 3 μm, about five times the thickness of the B Fe mixed layer resulting from only the B-ion implantation. This dynamic mixing treatment was far more effective in improving the wear property of the steel than the B-ion implantation.

Boronizing of steel by outward transport of Fe atoms during dynamic ion mixing

Abstract Mild steel substrates were treated by B-ion implantation or dynamic ion mixing in which B-ion implantation and Ti deposition were carried out simultaneously. Before the treatments, some substrates were implanted with C ions to confirm the location of the original substrate surface. The compositional depth profiles of the surface regions were obtained by using Auger electron spectroscopy (AES). For the B-ion implantation, an accumulation of the implanted B species takes place followed by a growth of the B layer as the B fluence is increased. The Fe matrix is separated during the process with outward transport of Fe atoms caused by the growth of the B layer. For the dynamic ion mixing, the outward transport of Fe atoms is enhanced during the process which is thought to be caused by not only the growth of the B layer but also the reaction between the sputtered Fe atoms and the depositing Ti atoms at steel surface. The TiFe reaction causes an incorporation of Fe atoms into the growing B layer resulting in the formation of a several μm thick boronized layer.

Effect of structural change of α-SiC film synthesized by dynamic ion mixing on surface hardness and friction coefficient

Abstract Mild steel specimens were treated by dynamic ion mixing in which Si-deposition and 40 keV C-ion implantation were carried out simultaneously. A mixed layer of Si phase and α-SiC phase is synthesized when the Si-deposition rate is 0.10 nm/s and the C-ion current density is 20 μA/cm 2 . The Si phase transforms to α-SiC phase, which grows to form a monolithic α-SiC layer as the C-ion current density increases up to 30 μA/cm 2 . With a further increase in the C-ion current density or a decrease in the Si-deposition rate, the implanted C species accumulate to form a graphite phase, resulting in the growth of a graphite layer. The friction coefficient during a reciprocal motion wear test decreases and the surface hardness increases as the C-ion current density increases from 20 μA/cm 2 to 30 μA/cm 2 , i.e. as the α-SiC phase grows to form the monolithic α-SiC layer. The friction coefficient increases and the surface hardness decreases with a further increase in the C-ion current density or a decrease in the Si-deposition rate, i.e. as the graphite layer grows.

MODIFICATION OF METAL SURFACES BY ION IMPLANTATION

This study is specifically directed toward ion beam surface modification of metals. Our current results of the improvement in the corrosion resistance of titanium and the wear resistance of steel are reviewed in this paper. Ta/Ni co-implantation to titanium is effective in preventing the general corrosion in a sulfuric acid solution. B-Fe interdiffused layer formed on steel during the dynamic mixing with Ti-deposition and B-ion implantation significantly increases the duration of the low friction coefficient during the reciprocal motion wear test.