Yasuo Kogo

Measurement of adhesive force between mold and photo-curable resin in imprint technology

Imprint lithography using photocurable resin is the most promising technique compared with other imprint lithography techniques, because it can complete a pattern transfer at room temperature. Thus, it would be able to implement practical mass-production lithography. In a previous report, however, a part of the solidified polymer was ripped away, because of strong adhesive force between the mold and solidified polymer. In order to improve this phenomenon, release coating of quartz mold and development of a photocurable resin are necessary. In this paper, we describe a measurement method of adhesive force between mold and resin using a tensile tester and, furthermore, durability of release coating material.

High damping capacity due to two-step phase transformation in Ni-Ti, Ni-Ti-Cu, and Fe-Cr-Mn alloys

Abstract In high damping materials, phase transformation is one of the most effective mechanisms. In this study, both Ni–Ti or Ni–Ti–Cu alloy and Fe–Cr–Mn alloy were investigated. Ni–Ti and Ni–Ti–Cu alloy were made by lamination and diffusion process and the phase transformation were B2→B19′ in the former and B2→B19→B19′ in the latter. The damping capacity was higher in B19 than in B19′, which were observed in two-step transformation. In Fe–Cr–Mn martensitic transformation was obtained through γ→α′ or γ→e→α′ depending on composition and heat treatment. The maximum damping capacity was obtained in the e phase, which was observed in a two-step transformation. For those cases, the d-electron concept was applied. Parameters B 0 and Md were used. B 0 represents the bond order which is related with bonding strength. Md represents the energy level of d-electron, which is related with electronegativity or atomic size of the elements. In the B 0 vs. Md map for Ni–Ti and Ni–Ti–Cu alloy the region for B19 formation was shown. In Fe–Cr–Mn the region for e formation by cold work was also shown. Those areas corresponded to high damping regions. Therefore those maps would be effective for finding high damping region in each alloy.

Effect of Stress Concentration on Tensile Fracture Behavior of Carbon-Carbon Composites:

The effect of stress concentrations on tensile fracture behavior of carbon-carbon (C/C) composites was investigated using circularly holed specimens and double-edge-notched (DEN) specimens. As for the circularly holed specimens, the tensile fracture stress was much higher than that estimated from the maximum stress criterion, which suggest that major stress relaxation mechanisms should exist. On the other hand, the linear elastic fracture mechanics can be applied to the DEN specimen, which means the damaged zone should be small enough compared with the notch length. In order to discuss the magnitude of the stress relaxation, damaged regions of the two kinds of testing geometry were estimated using the point stress criterion. The estimation led to remarkable difference in the size of the damaged regions, which will explain the difference in the magnitude of the stress relaxation. Through the observations of fractured specimen, it was deduced that not only the shear deformation but delamination along fiber bundles and opening of transverse crack would relax the stress concentrations. The other mechanism was also proposed based on the testing results, that is strength increase in the damaged region.

Interfacial shear strength of C/C composites

Abstract Fiber-bundle push-out, single-fiber push-in, and single-fiber push-out tests were conducted in order to examine the applicability of these methods for determining the interfacial shear strength of carbon–carbon composites. The fiber-bundle push-out test resulted mostly in fractures along the fiber/matrix interface but created a small amount of fractures in the matrix. Hence, the evaluated strength was regarded as an approximate value. In order to precisely evaluate the interfacial strength, push-in and push-out tests for a single fiber were performed using a micro-Vickers indentation tester. In these tests, the load has to be placed within a target fiber, and the indentation should not extend to the matrix. This condition restricted the load that could be applied to a carbon fiber. Within this limit, a single carbon fiber could not be pushed-in. For the sake of load reduction, single-fiber push-out tests were conducted using thin specimens. The thickness appropriate for a single-fiber push-out specimen was estimated based on the interfacial shear strength obtained by the bundle push-out tests. Below this thickness, single-fiber push-out tests could be successfully performed.

Tensile Strength of Carbon-Carbon Composites: I - Effect of C-C Density

The tensile fracture stress and strain of carbon fiber-reinforced carbon matrix composites (C–Cs) were examined as functions of the bulk density. When the density increased, the interfacial strength of the C–Cs monotonically increased, and the tensile fracture strain decreased. In contrast, the tensile fracture stress was improved and degraded in the regions of density lower and higher than 1.6 g/cm3, respectively. Two tensile fracture mechanisms of the examined C–Cs were identified with the transition at the density of 1.6 g/cm3. In the low-density region, load transfer capability across fiber–matrix interfaces was shown to have an important role, and in the high-density region, stress concentrations at matrix-crack tips were presumed to be a major factor for the tensile fracture of C–Cs. This suggests that the most important interfacial property for tensile fracture is not interfacial sliding but debonding stress.

Internal friction of TiNi alloys produced by a lamination process

Abstract Recently, the demand for higher damping materials with higher strength has been requested from the precision machine industries. Of these materials, the TiNi alloy has excellent characteristics with high damping capacity and high strength. However, with respect to practical use, its cold rolling is difficult. In order to solve this problem, we examined the production of the TiNi alloy from the Ti–Ni laminated material by the solid-phase diffusion method. In this study, to investigate the best processing conditions to make a high damping TiNi alloy with high strength, the effects of the material composition, annealing time and cooling rate of the water quenching on the internal friction and tensile strength were examined. As a result, a material with an internal friction of δ=0.14 at 250 K and ultimate tensile strength about 800 MPa was obtained.

Internal friction of carbon–carbon composites at elevated temperatures

The internal friction of unidirectionally and cross-ply laminated C/C composites was investigated at elevated temperatures up to 2500 K in transverse and torsional vibration modes. The internal friction of the C/C composites was found to decrease monotonically with increasing temperature up to 1200 K in transverse measurements. In a higher temperature range, it was almost constant up to 2000 K in torsional vibration mode, and significant increase was observed at higher temperatures. For any type of specimen and measurements, no peak was observed in this temperature range. It was also found that the internal friction of the C/C composites could be predicted by the simple rule of mixture without consideration of the contribution by the interface.

Application of three-dimensionally reinforced carbon-carbon composites to dovetail joint structures

Abstract In development of an air-turbo-ramjet engine with an expander cycle (ATREX) for a space plane, application of carbon–carbon (C/C) composites plays an important role to achieve high performance. Above all, dovetail joints are one of the key structures to realize the turbine system made of C/C composites. In this study, the feasibility of dovetail joints made of three-dimensionally reinforced C/C composites was investigated. Tensile tests were carried out on simplified dovetail joint models. The shoulder angles of the dovetail joint and fiber volume fractions of the C/C composites were taken as parameters to optimize the shape of the dovetail joint. Finite element analyses were also carried out for various cases. Comparison between the experimental and the calculated results suggests that fracturing of the dovetail joint was controlled by the average shear stress, which implies that the shear stress concentration on the shoulder was relaxed during the fracture process. It was also shown that the dovetail joint made of C/C composites is feasible for use in the ATREX engine.

Multiwalled Carbon Nanotube Monoliths prepared by Spark Plasma Sintering (SPS) and their Mechanical Properties

Three types of multiwalled carbon nanotube (MWCNT) monoliths without any binders were obtained by spark plasma sintering (SPS) treatment at 2000 degrees C under 80 MPa sintering pressure. Three MWCNTs with different diameters: thin (slashed circle20-30 nm, CNT Co., Ltd., Korea), thick (slashed circle100 nm, Nano Carbon Technologies Co., Ltd., Japan) and spherical thin (slashed circle20-30 nm, granulated diameter = 1-3 microm, Shimizu Corporation, Japan) were employed for SPS. SEM observation confirmed that these materials maintained the nanosized tube microstructure of raw CNT powder after SPS treatment. The densest monolith was prepared with the spherical MWCNTs. The mechanical properties of this material were estimated by the dynamic hardness test. The elastic modulus of the monolith did not depend on the difference of MWCNTs, but the hardness of spherical MWCNTs was higher than that of thick MWCNTs. The high density and hardness of the spherical MWCNTs were caused by the high packing density during the SPS process because of its spherical granulation. Thus, the spherical MWCNTs were most useful for the MWCNT monolith preparation with the SPS process and its application as a bone substitute material and a bone tissue engineering scaffold material was suggested.

Examination of strength-controlling factors in C/C composites using bundle composites

C/C composites unidirectionally reinforced by one fiber bundle (the bundle C/C composites) were fabricated and tested in tension. Through the comparison between the tensile strength of the bundle C/C composite and that of the laminated one, effects of lamination on the tensile strength of the C/C composites were examined. Results suggested that defects in the laminae, such as transverse cracks, have no effects on the tensile strength of the C/C composites. This means the strengthcontrolling factors are included in the bundle. As one of the factors in the bundle, the effect of bonding strength of fiber/matrix interface was examined by using the fiber bundles with different surface treatments. In addition to surface oxidized and non-oxidized fiber bundles, an attempt was made to form porous carbon coating layers around the fibers to reduce the bonding strength intentionally. Results suggested that the bonding strength of the fiber/matrix interface, which was evaluated by bundle pushout tests, have significant effect on the tensile strength of C/C composites. It was also shown that the porous coating layers were effective in improving the tensile strength of C/C composites.