Published papers

In this study, a rapid oxynitriding technique based on induction-heating was developed for the titanium alloy Ti6Al4V, and its effects on surface characteristics of the alloy were investigated. The surface microstructures of the alloy were characterized by employing a scanning electron microscope, electron back-scatter diffraction, X-ray diffraction and nano-indentation tests. Induction-heating in air for 60 s caused the formation of a hardened layer on the Ti6Al4V alloy surface, which can be attributed to the diffusion of oxygen and nitrogen atoms. When water quenching was performed after heating, transformation to an acicular martensitic ¡A phase occurred, resulting in increased in substrate hardness. However, cracks were initiated at the surface of the oxynitrided alloy. The cracks were eliminated through fine particle bombarding treatment, whereas the hardened layer formed by the oxynitriding treatment remained. The obtained results thus indicate that the oxynitriding technique developed in this study can enhance the wear resistance and tensile strength of the Ti6Al4V alloy within a short period of time.

Surgery, radiation therapy, and chemical therapy have been reported as the main treatments for cancer, which is one of the deadliest reported diseases. However, because of the high invasiveness of patients, cancer hyperthermia has been studied as a non-invasive treatment. Hyperthermia uses a difference in thermal tolerance between normal and cancer cells and provides an affected part thermal stimulation to kill cancer cells selectively. To develop effective conditions for hyperthermia, an in vitro study to evaluate the thermal tolerance is required. However, because the existing cell culture vessels cannot control the culture temperature, genuine thermal tolerance of cells cannot be investigated appropriately. To reveal the critical temperature proper for hyperthermia, we developed a culture device controlling culture temperature. With the developed device, culture temperature was regulated considerably more quickly than the conventional method with the existing vessels, which enabled us to study the thermal tolerance of cells appropriately. For the control of culture temperature, the device has a titanium culture substrate, where a Peltier element is adhered. Because the biocompatibility of the device was confirmed, the difference in thermal tolerance between normal and cancer cells was investigated using the developed device as well as normal human dermal fibroblasts and Michigan Cancer Foundation-7 as model cell species. Therefore, it was confirmed that cancer cells were more sensitive to thermal stimulation than normal cells, which was qualitatively consistent with previous studies. Thus, our developed device can be used to investigate the thermal tolerance of each cell species, which will contribute to the development of cancer hyperthermia.

Fatigue tests under axial loading were conducted on steel with a shallow hardened layer induced by induction hardening, and in situ X-ray stress measurements were performed to investigate the relaxation of residual stresses during fatigue. The residual stresses were relaxed owing to tensile loading and not compressive loading. The two conditions that bring about this phenomenon are (i) a high peak of tensile residual stress just below the hardened layer, and (ii) the hardened layer coinciding with the compressive residual stress field that prevents the yielding of the compressive residual stress field under compressive loading. In this case, tensile yielding occurred just below the hardened layer under tensile loading, the residual stresses are redistributed, and the compressive residual stress on the material surface is relaxed. The experimental results also showed that the fatigue fracture morphology changed depending on the residual stress relaxation behavior.

Fatigue tests under axial loading were conducted on steel with a shallow hardened layer induced by induction hardening, and in situ X-ray stress measurements were performed to investigate the relaxation of residual stresses during fatigue. The residual stresses were relaxed owing to tensile loading and not compressive loading. The two conditions that bring about this phenomenon are (i) a high peak of tensile residual stress just below the hardened layer, and (ii) the hardened layer coinciding with the compressive residual stress field that prevents the yielding of the compressive residual stress field under compressive loading. In this case, tensile yielding occurred just below the hardened layer under tensile loading, the residual stresses are redistributed, and the compressive residual stress on the material surface is relaxed. The experimental results also showed that the fatigue fracture morphology changed depending on the residual stress relaxation behavior.

Controlling cell orientation is of paramount importance in bioengineering processes. While several surface modification techniques have emerged to guide cell orientation, they often involve complex, repetitive procedures for each experiment. Thus, a streamlined approach for cell orientation is necessary. In this study, we present a potentially reusable metallic culture surface that induces an anisotropic cell orientation due to its unique geometric morphology. Using a femtosecond laser, periodic nanostructures were produced on the metallic culture surface, resulting in a distinctive macro stripe design composed of both laser-treated and mirrored areas. Myoblast cells cultured on this surface showed a pronounced orientation. The macro stripe design provided stronger control of cell orientation compared to the simple laser-treated surface with periodic nanostructures. Initial random cell adherence was followed by migration toward the mirror surface, culminating in the desired orientation. This shift can be attributed to the mirrored areas providing superior cell adhesion and reduced wettability compared to the laser-treated areas. This innovative culture surface has significant potential for advancing bioengineering endeavors, especially in the field of tissue engineering.

Atmospheric-controlled induction-heating fine-particle peening (AIH-FPP) was applied to low-alloy, low-carbon steel AISI 4120 to achieve rapid carburizing within minutes and enhance fatigue strength. The shot particles used during AIH-FPP were specifically created by mechanically milling a mixture of steel particles and carbon powders. The specimens were analyzed using laser microscopy, optical microscopy, X-ray diffraction for residual stress measurement, a micro-Vickers hardness tester, and an electron probe micro-analyzer. Axial fatigue tests and fracture surface analyses via scanning electron microscopy and energy-dispersive X-ray spectroscopy were also conducted. AIH-FPP effectively transferred carbon to the treated surface and facilitated its diffusion in a short period, creating a martensite layer with high hardness and compressive residual stress, which resulted in increased fatigue strength. The fatigue strength of the AIH-FPP-treated specimens was influenced by the conditions of reheating and quenching, which altered the prior austenitic grain size and stress concentration at the specimen surface. Notably, specimens treated with AIH-FPP followed by reheating and water-injection quenching exhibited fatigue strength comparable to that of conventional gas-carburized specimens, owing to the formation of a carburized layer with a small prior austenite grain size.

The effects of surface characteristics on the axial fatigue properties and wear resistance of the GBIH-nitrided titanium alloy were investigated to improve the fatigue strength and wear resistance of Ti-6Al-4V alloy rapidly nitrided by induction-heating in a nitrogen atmosphere with blowing nitrogen gas (GBIH-nitriding). The fatigue strength of the GBIH-nitrided Ti-6Al-4V alloy was lower than that of the mirror-polished specimen owing to the formation of a brittle nitrogen compound layer and grain coarsening. Fatigue tests using specimens prepared by combining GBIH-nitriding and particle collision treatments revealed that the formation of nitrogen diffusion layers at the outermost surface and the generation of compressive residual stress without forming surface asperities increased the fatigue strength of the GBIH-nitrided Ti-6Al-4V alloy. Moreover, the formation of a nitrided layer and suppression of surface asperities effectively improved the wear resistance of the Ti-6Al-4V alloy. Therefore, formation of a nitrogen diffusion layer and the introduction of compressive residual stress without grain coarsening, a compound layer, and surface asperities increased the fatigue strength and wear resistance of the GBIH-nitrided alloy.

In situ X-ray stress measurements were conducted to investigate the relaxation of compressive residual stress induced by fine particle peening during fatigue. To quantitatively assess the stability of the compressive residual stress, a concept of relaxation threshold stress was proposed. The relaxation threshold stress, which indicates the stability of the compressive residual stress, showed a direct correlation with specimen hardness. If the combined value of the compressive residual stress and compressive applied stress surpass the relaxation threshold stress, then the compressive residual stress relaxes. The relaxation threshold stress can be used to estimate the relaxation that occurs during fatigue.