Keijiro Nakasa

Bio: Keijiro Nakasa is an academic researcher from Hiroshima Kokusai Gakuin University. The author has contributed to research in topics: Delamination & Coating. The author has an hindex of 12, co-authored 115 publications receiving 596 citations. Previous affiliations of Keijiro Nakasa include Hiroshima University.

Relation between delamination of thin flims and backward deviation of load–displacement curves under repeating nanoindentation

Abstract: The delamination behavior of sputter coated TiN and AlN films have been examined from the load–displacement curves under repeated nanoindentation loading. When the penetration depth is either much larger or much smaller than the film thickness, the curve progresses in forward direction that represents the deformation of the substrate or film. On the other hand, a backward deviation is observed when the penetration depth is comparable with the film thickness. Observation of the indents by a scanning electron microscope (SEM) and an atomic force microscope (AFM) shows that cracking and extrusion of the film occur when the backward deviation appears. It suggests that the partially delaminated film releases the compressive residual stress and causes an upward bending due to the elongation of the delamintated film, which resists the penetration of the indenter in the consecutive cycle.

Observation by atomic force microscope of corrosion product during pitting corrosion on SUS304 stainless steel

Abstract: Using in situ observation by atomic force microscope, it was found that the corrosion product plays an important role on the growth of corrosion pits on SUS304 steel in NaCl solution and the current density for pit growth was much higher than that applied on the entire surface.

Influence of ultraviolet light irradiation on corrosion behavior of weathering steel with and without TiO2-coating in 3 mass% NaCl solution

Abstract: In this study the influence of ultraviolet (UV) light on the corrosion behavior of weathering steel with and without TiO 2 coating has been investigated in 3% NaCl solution, using anodic polarization, dipping and dropping tests. In polarization tests a large negative shift of corrosion potential appeared for the TiO 2 -coated specimen in UV light. The weight loss of the steels with and without TiO 2 film increased under UV light in the dipping test, while they decreased in the dropping test. The rust formed on the weathering steel showed a semiconductor like behavior, similar to that shown by TiO 2 film.

Backward deviation and depth recovery of load–displacement curves of amorphous SiC film under repeating nanoindentation

Abstract: The deformation and delamination behaviors of sputtered amorphous SiC film have been studied from the load–displacement curves under repeated nanoindentation loading When the penetration depth is either much larger or much smaller than the film thickness, the curve progresses in forward direction that represents the deformation of the substrate or film On the other hand, when the penetration depth is comparable with the film thickness, large backward deviation and depth recovery around the minimum load are observed Observation of the indents by an atomic force microscope proved that delamination starts from the portions beneath the indenter and progresses towards the shoulder of the indent This behavior is strongly connected with the existing large residual compressive stress in the film The difference in the delamination behaviors between SiC and TiN, AlN films is also discussed

Superhydrophobicity produced by vapor deposition of a hydrophobic layer onto fine protrusions formed by sputter-etching of steels

Abstract: Argon ion sputter-etching was applied to W–Mo–Cr–V high speed steel, Cr–Mo–V die steel, and type 304 and 316 austenitic stainless steels. Vapor-deposition of the polyethylene (PE) or 1H, 1H, 2H, 2H, -Perfluorodecyltriethoxysilane (PFDS) was carried out on the sputter-etched surface. The hydrophobicity was evaluated by measuring the contact angle (CA) of a water droplet on the surface. The sputter-etching formed fine and dense pillar- or cone-shaped protrusions with a diameter of 100 nm to 1 μm on the tool steels and the type 316 steel but cone-shaped protrusions with a diameter of more than 1 μm on the type 304 steel. The surfaces of all the steels just after the sputter-etching revealed CAs smaller than 4.5°, or superhydrophilicity. On the other hand, all the steels that were sputter-etched and vapor-deposited with PE or PFDS produced CAs of more than 150°, or superhydrophobicity, except that the CA of the type 304 steel with micron-sized protrusions was 146°. The adhesion of the PFDS layer to the protrusions is much stronger than the PE layer, i.e. it is difficult to remove the PFDS layer by heating at 473 K and ultrasonic cleaning in acetone.

Corrosion resistance of TiO2 films grown on stainless steel by atomic layer deposition

Abstract: Titanium dioxide (TiO2) films have been deposited onto stainless steel substrates using atomic layer deposition (ALD) technique. Composition analysis shows that the films shield the substrates entirely. The TiO2 films are amorphous in structure as characterized by X-ray diffraction. The electrochemical measurements show that the equilibrium corrosion potential positively shifts from − 0.96 eV for bare stainless steel to − 0.63 eV for TiO2 coated stainless steel, and the corrosion current density decreases from 7.0 × 10− 7 A/cm2 to 6.3 × 10− 8 A/cm2. The corrosion resistance obtained by fitting the impedance spectra also reveals that the TiO2 films provide good protection for stainless steel against corrosion in sodium chloride solution. The above results indicate that TiO2 films deposited by ALD are effective in protecting stainless steel from corrosion.

Mechanics of crack curving and branching — a dynamic fracture analysis

Abstract: A comparative study on crack curving and branching criteria in dynamic fracture mechanics shows that the criteria based on “advanced cracking” concept correlated best with available experimental data. The crack branching criterion requires as a necessary condition, a critical dynamic stress intensity factor, K Ib, and a sufficient condition involving the crack curving criterion. The criteria are used to predict crack curving and crack branching in dynamic photoelastic experiments involving Homalite-100 and polycarbonate fracture specimens, as well as bursting steel and aluminum pipes.

The size effect in KICresults

Abstract: Fracture toughness results are often connected with varying thickness effects. Many results are contradictory, with some results indicating no size effect, some indicating increasing toughness with decreasing thickness and others indicating decreasing toughness with decreasing thickness. In this paper the causes for different thickness effects are discussed. It is shown that most of the observed size effects are due to invalid tests. The only test parameters to be regarded as valid are those that correspond to the initiation of crack extension. The theoretical thickness effects for both ductile and brittle fracture initiation are evaluated. It is shown that ductile fracture initiation is thickness independent if B ≥ α ( J / σ y ). The thickness effect in brittle fracture is explained by applying a mechanism-based statistical cleavage fracture model. A theoretical thickness correction for cleavage fracture is derived, and its validity is confirmed for a variety of materials.

A study on the N-, S- and Cl-modified nano-TiO2 coatings for corrosion protection of stainless steel

Abstract: Nano-titania coatings doped with anions of nitrogen, sulfur and chlorine have been supplied on the surface of 316L stainless steel by a sol–gel process and dip-coating technique. The measurements of XRD, SEM, ATR-IR, Raman and XPS were carried out to characterize the chemical composition and structure for the prepared samples. The corrosion performances of the coating in 0.5 M NaCl were evaluated by electrochemical impedance spectroscopy (EIS) and polarization measurements. According to the measurements of EIS and electrochemical polarization, the N-modified TiO 2 nano-coatings show a highest corrosion resistance among the prepared coatings. It is revealed, from the SEM, XRD and Raman characterizations, that the surface of N-modified TiO 2 nano-coatings are more compact and uniform, relatively well-crystallized and able to act as an optimal barrier layer to metallic substrates. The XPS analysis confirms the presence of low concentration of N element in two forms, atomic β-N (interstitial state) and chemisorbed γ-N 2 on the surface of TiO 2 nano-coatings. It is suggested that the addition of nitrogen is beneficial to improve the compact structure and enhance the hydrophobic property.

Effect of oxygen and hydrogen on mechanical properties of commercial purity titanium

Abstract: Commercial purity (CP) titanium is used primarily in applications requiring exceptional corrosion resistance. The CP titanium alloys contain small amounts of oxygen. The mechanical properties of titanium, such as tensile and fatigue strengths, can be enhanced with oxygen additions, but care must be taken not to compromise toughness and ductility. Hydrogen, at concentrations below the commercially accepted limit, can also be detrimental to some of the mechanical properties of CP titanium, particularly under multiaxial stress and at both high and low strain rates. Hydrogen decreases ductility and creep resistance without, however, lowering the tensile strength. Although some of these effects result from the formation of a hydride phase, transmission electron microscope evidence indicates that residual interstitial hydrogen lowers the energy for dislocation generation and propagation. These hydrogen effects are more pronounced in oxygen alloyed titanium than in high purity titanium.