Papers by Author: Takayuki Kitamura

Abstract: Chiral sculptured thin films (STFs) Glancing-angle deposition (GLAD) thin films are nanoengineered to meet the requirements of a variety of applications such as micro filters, sensors, and waveguides due to their unique frequency characteristics which cannot be achieved by conventional solid materials. For the design, it is necessary to understand the elastic properties of STFs. To facilitate this, we report on our newly developed advanced micro-scale vibration testing process. In the testing, specially designed micro-specimens with surface areas of tens by tens of microns are excited using a piezoelectric (PZT) actuator and the resonance frequencies are detected by a laser device in the vertical or lateral directions successfully. The anisotropy elastic modulus of STFs composed of helical nanosprings are identified on the basis of vibration testing. The thin film shows strong characteristic anisotropy that the solid one hardly can attain. The micro-scale testing technique can be extended to other materials and microstructures.

Abstract: In order to investigate the effect of microscopic structure on fatigue behavior of nanoscale components, a resonant fatigue experiment is conducted using a nanocomponents specimen where the test section is composed of a single crystalline Si substrate, a 200 nm thickness Cu polycrystalline film and a SiN amorphous layer. In the specimen, only the Cu portion plastically deforms because the yield stress is lower than those of other materials. The shape and the crystalline orientation of each grain on the surface of Cu portion are specified by means of EBSD. Although crystallographic slip bands with a width of a few tens of nanometers appear only in a grain of Cu portion, the grain is different from that expected by the Schmid factor. A FEM analysis, which takes into account the deformation anisotropy of grains, reveals that shear stress to generate slip bands is concentrated on the grain owing to the deformation constraint by neighboring crystals and components.

Abstract: Directionally solidified (DS) superalloys, which have elongated large grains, are used for gas-turbine blades. Since the grain size is not small enough in comparison with the crack size observed often in a real component, the inhomogeneous microstructure due to the aligned grains may strongly affect the crack propagation property. Center-cracked-plate specimens with three different orientations, TP0 with the parallel DS axis to the load, TP90T with the perpendicular DS axis to the load and the crack propagation direction, and TP90L with the perpendicular DS axis to the load and parallel to the crack propagation direction, are subjected to high-temperature (1143K) fatigue. The specimens after the tests show the transgranular cracking perpendicular to the load axis in TP0, intergranular one parallel to the DS axis in TP90L, and intergranular / transgranular one parallel to the DS axis in TP90T. The crack propagation rate da/dN shows a good correlation with effective stress intensity factor range
Keff in each cases. However, the magnitude of da/dN at a same
Keff in TP90L is relatively higher than that in TP0, and that in TP90T remarkably fluctuates. A microscopic observation of TP90T reveals that the high da/dN is caused by the intergranular cracking, while the low da/dN is observed on the transgranular cracking. A procedure for the prediction of crack propagation is proposed on the basis of the two types of cracking; transgranular and intergranular cracks perpendicular to the load axis. The validity of the proposed procedure is discussed by the comparison of tested and predicted results of crack propagation in a fatigue condition.

Abstract: The plasticity of a copper (Cu) nano-component is experimentally evaluated by a cantilever specimen with multi-layered structure. The cantilever is monotonically loaded by a diamond tip and the deflection at the free-end is precisely measured by a transmission electron microscope (TEM). The plastic deformation of the Cu nano-component is successfully monitored through the non-linear behavior of applied load, P, and cantilever deflection, δ. The plastic constitutive quation of the Cu component is inversely analyzed by finite element method (FEM) assuming that the component obeys the Ramberg-Osgood law. The parameters in the R-O law (σ0, n and α) are optimally fitted to reproduce the experimentally evaluated P-δ relation. The resultant parameter set is derived as (σ0, n, α) = (345 MPa, 3.2, 1.25). The Cu nano-component has a much higher yield stress and a hardening rate compared with the ones in a bulk Cu.

Abstract: Atomistic simulations using molecular dynamics (MD) method are conducted to check the conditions of the onset of fracture at the interface edges with a variety of angles. The simulations are facilitated with model bi-material systems interacting with Morse pair potentials. Three simulation models are considered, i.e. the interface edges with angles 45°, 90° and 135°, respectively. The simulation results show that, at the instant of crack initiation, the maximum stresses along the interfaces reach the ideal strength of the interface; also, the interface energies just decrease to below the value of the intrinsic cohesive energy of the interface. And the onset of fracture at the interface edges with different geometries is controlled by the maximum stresses or the cohesive interfacial energy.

Abstract: The interface strength of low-dimensional nano-components such as films and islands formed on substrates has been investigated in this project, and the focus is put on the mechanics of crack initiation from the free interface edge and propagation along the interface. The series of experiments elucidates the applicability of fracture mechanics concept on the structures. We proposed experimental methods for evaluating the initiation strength of an interface crack in submicron films and islands deposited on substrates. The initiation is governed by the singular stress field, and the criterion is prescribed by the stress intensity parameter. Using special loading apparatus built in a TEM, we developed a crack initiation method for nano-components and the role of plasticity on the delamination is clarified. Subcritical crack growth along an interface between submicron films under fatigue was also investigated by modified four-point bend method.

Abstract: The diamond structure of single crystal silicon transforms to other structures under mechanical stress. We investigate the structural transformation of diamond cubic structure to betatin structure in silicon
under uniaxial stress using atomistic simulation on the basis of the Tersoff potential. As a result, under extensive compressive strain, the structural transformation from Si-I to Si-II is found.

Abstract: Using the first-principles calculation, the elastic constant C44 of Ag/Al multilayers with different modulation periods from 0.43 nm to 2.27 nm has been evaluated in order to examine the effect of atomic and electronic structures on it. With increasing modulation period, C44 decreases and becomes close to that obtained by the conventional mixing rule, however, the difference of 8 % still remains at the modulation period of 2.27 nm. As C44 correlates with the average interplanar spacing, the decrease of C44 can be explained by the decrease of the charge density in the stacking direction due to the increase of the average interplanar spacing. The difference in the electronic structure is included in the effect of atomic structure.

Abstract: The ideal strength of a nano-component, which is the maximum stress of the structure, provides an insight into the mechanical behavior of minute material. We conducted tensile simulations for cylindrical-shaped Cu nano-wires composed of an atomic chain as a core wrapped around by shell(s) with the structure of (111) layers in an fcc crystal. The results are compared with Cu atomic chain and sheet which are components of the nanowire. Young’s moduli and the ideal strengths of the wires are less than a single atomic chain and a sheet. The mechanical strength of the wire is weakened by the following three factors: (A) Change in electron arrangement caused by combining core and shell; (B) Larger interatomic distance (inherent tensile strain) of the outer shell introduced by the mismatch of atomic layers due to the curvature difference; (C) Mismatch between shells due to curvature difference. Factor (A) reduces the bonding strength in the shell(s) that occupy a greater part of the wire. 5-1 wire, which consists of a core and a shell, is weaker than the single atomic chain and the single sheet due to (A) and (B). 10-5-1 wire, consisting of a core and two shells, has less strength than 5-1 wire due to (C) in addition to (A) and (B).

Abstract: Molecular dynamics (MD) simulations are performed to study the onset of fracture at the free edges of bi-material interfaces. The objective is to see whether a unified criterion could be formulated for crack initiation at interface edges with different angles or not. The simulations are facilitated with model bi-material systems interacting with Morse pair potentials. Three simulation models are considered, i.e. the interface edges with angles 45°, 90° and 135°, respectively. The simulation results show that, at the instant of crack initiation, the maximum stresses along the interfaces reach the ideal strength of the interface; also, the interface energies just decrease to below the value of the intrinsic cohesive energy of the interface. These findings revealed that the onset of fracture at the interface edges with different geometries could be controlled by the maximum stresses or the cohesive interfacial energy.