A few studies on the synthesis of titanium aluminum nitride (Ti,Al)N films have attracted interests, in particular as an alternative to the widely used TiN films, to obtain wear-protective hard coating with higher oxidation resistance at elevated temperature. Here, the (Ti,Al)N films were prepared by depositing Ti and Al metal vapor under simultaneous irradiation of N ions, that is ion mixing and vapor deposition (IVD) technique. With an increase of evaporation Al/Ti and/or transport ratio (Ti + Al)/N, a single-phase of NaCl structure in the films transformed into that of wurtzite structure through a two-phase mixture consisting of NaCl and wurtzite structure. Based on the revelation of two-phase structure, therefore, the optimum preparation conditions for wear- and corrosion-resistive hard coating were identified. The (Ti,Al)N films were also highly resistant against oxidation. Consequently, the results suggest that the Al oxide layers formed on the top of (Ti,Al)N films during elevated temperature oxidation tests protect the films from further oxidation.
Diamond-like carbon (DLC) is of great interest due to superior properties as a coating material, e.g., high hardness, high wear resistance and chemical stability. Recently, a focused ion-beam assisted chemical vapor deposition (FIB-CVD) method is proposed as a new technique for fabricating three dimensional micro- and/or nano- structures of DLC. Processing conditions, microstructures and mechanical properties of this material are, however, not clear sufficiently so far. In this study, therefore, processing conditions of DLC using the FIB-CVD method were examined. Microstructures of the DLC were also analyzed. Mechanical properties were investigated in consideration of applying for structural material of MEMS and/or NEMS. DLC samples were deposited by the FIB system with Ga+ ion-beam. Source gas was supplied by heating of phenanthrene powder. The deposition condition was varied by controlling ion-beam irradiation. The surface topology of deposited DLC was measured by atomic force microscopy in order to obtain the deposited volume and the deposition rate of the samples. Microstructure of the DLC was characterized by using a high-resolution transmission electron microscopy (HRTEM). Mechanical properties of the DLC were examined using a nano-hardness tester. Various deposition experiments indicated that the fabricating of DLC structure required suitable conditions of ion-beam irradiation. HRTEM images and diffraction images, which showed halo patterns, showed that the DLC had amorphous structures. Energy dipersive spectroscopy analysis indicated that Ga was included 3∼5 at.% and distributed homogeneously in all over the samples. Vickers hardness and Young's modulus were in the range of 1100-1300 HV and 130-140 GPa, respectively. These properties were obtaind similarly even though the deposition conditions were changed in our study.
The WC-Co cemented carbides with the WC grain sizes of 0.4-4.5 μm and the Co contents of 10-15% were machined by focused 30 keV Ga+ ion beam (FIB) at doses of 5.0×1016∼2.5×1018 ions/cm2 and their surface roughness was investigated by atomic force microscope (AFM), where FIB machining was carried out by scanning the nominal spot sizes of 100 and 300 nm, the beam overlap of 37.5, 50 and 67% and the pixel dwell time of 100 and 227 μs. Experimental results on effects of microstructure on the surface roughness showed that the decrease in WC grain size and averaged phase width of Co is required to reduce the surface irregularities for the FIB micro-machining of WC-Co cemented carbides. Moreover, it was revealed from the viewpoint of effects of machining conditions on the surface morphology that small surface irregularities is obtained by FIB machining with larger beam diameter, larger overlap and shorter dwell time.
We have proposed a novel bonding process using Ag metallo-organic nanoparticles with average particle size of around 11 nm, which can be alternative to lead-rich high melting point solders. The influences of bonding parameters on the bondability of Cu-to-Cu joints were examined based on the measurement of the shear strength of the joints and the observation of fracture surfaces and cross-sectional microstructures. The joints using Ag metallo-organic nanoparticles had strength ranging from 10 MPa to 50 MPa depending on bonding conditions. From the results, the bonding conditions providing the joint strength equivalent to those using the lead-rich high melting point solders were proposed.
In this paper, the contribution of magnetoelastic-anisotropy effect on perpendicular-magnetic anisotropy of Co/Pt superlattice thin films is studied considering the strain-dependent elastic constants. We measured the effective-magnetic-anisotropy energy changing the Pt-layer thickness (2 to 20 Å) keeping the Co-layer thickness unchanged (4 Å). The effective-magnetic-anisotropy energy changed depending on the Pt-layer thickness and showed a maximum near dPt=12 Å. This result is explained by calculating the magnetoelastic-anisotropy energy considering the change of the elastic constants caused by the lattice misfit. We measured anisotropic elastic constants of Co/Pt superlattice thin films by resonance-ultrasound-spectroscopy method coupled with laser-Doppler interferometry and the picosecond laser-ultrasound method. The measured elastic constants showed elastic anisotropy between the in-plane and out-of-plane directions. They agreed with those predicted from the strain-dependent elastic-constant model. The elastic constants show correlations with the perpendicular-magnetic anisotropy.
We have measured the complete set of elastic moduli Cij of an Fe-Si-B amorphous-alloy thin film using thickness resonance with electromagnetic acoustic resonance (EMAR) and newly developed resonance ultrasound spectroscopy (RUS)/laser methods. From the thickness-resonance measurements, out-of-plane shear moduli (C44 and C55) were found to be 73.2 and 71.0 (GPa). This indicates that the amorphous-alloy thin film shows transverse-isotropy elastic symmetry. RUS/laser measurements gave the remaining moduli : C11=193.6, C33=234.3, C23=72.1, C12=79.8 and C44=72.1 (GPa). From the ratios of the elastic modulus, (C33/C11=1.21 and C44/C66=1.26), the amorphous Fe-Si-B thin film shows elastic anisotropy between in-plane and the out-of-plane directions in the range of 20-30%. These results also indicate that this ultrasonic technique is also applicable for measuring all Cij of other thin films.
The Young's modulus of a polysilicon thin microelement was evaluated using the previously developed mechanical testing system for thin tensile microelements. The tested part of the specimen is 250 μm in length, 10 μm in width, 3.8 μm in thickness. The Young's modulus of the tested polysilicon thin film was 133±10 GPa. To analyze the effect of the crystal orientations and the grain size on the Young's modulus, two-dimensional finite element models in plain strain condition were developed using a Voronoi structure. The number of grains in a model of a 10 μm square area was changed from 23 to 1200. The crystal orientation of the film was analyzed by means of an electron back-scattering diffraction pattern (EBSP) method. The front surface of the film did not have the oriented structure. Therefore, random crystal orientation was given to each grain of the FEM models. When the number of grains increased, the Young's modulus converged on about 170 GPa and its scatter caused by the different sets of the random orientation was reduced. However, the Young's modulus obtained by the FEM analysis was larger than the value obtained by the tensile tests.
SiC and SiCN films were deposited on titanium and tool steel substrates in an RF magnetron sputtering apparatus with a SiC target using a mixture of argon and nitrogen gas. A micro edge-indent method was newly proposed to evaluate delamination strength of the thin films, and the relation between the delamination energy and wear strength of the films was examined. The results show that the micro edge-indent test is effective to evaluate the delamination energy of thin films, and the delamination energy of SiC film is larger for Ti substrate than tool steel substrate. The maximum delamination energy of the SiC and SiCN films are obtained when the substrate is not heated and the nitrogen gas flow rate is about 1.0 cm3/min. The wear delamination cycles of SiC film decreases with increasing substrate heating temperature, and that of SiCN film decreases with increasing nitrogen gas flow rate. The delamination cycles of the films almost correspond with the delamination energy before wear tests, but they are also affected by friction coefficient and hardness of films.
Micro-Vickers indentation tests in conjunction with acoustic emission (AE) monitoring were performed for TiN films deposited on three kinds of substrates, soft and hard steel by physical vapor deposition (PVD) method. We aimed to evaluate the intrinsic strength of thin films. In this study, three types of substrate with different hardness were employed. One was a WC-Co cermet, denoted as hard substrate, while the others were mild steel (SS400) and metastable austenitic stainless steel (SUS304), denoted as soft substrate. Equidistance step-wise terrace, or rippled cracks were observed and detected AEs only for soft substrates materials. Detected AE waveforms were analyzed using the polarity pattern in order to classify the cracking behavior. Most AEs detected were produced by Mode-I cracking with opening vector in the direction parallel to the film surface. Internal stress in the films was analyzed by the finite element method (FEM) to study the influence of substrates on the fracture mechanism. It was revealed that bending stress was developed by indenter penetration and it induced rippled crack initiation for the soft substrate materials. It was also found that the rippled cracks initiated at a certain distance of spacing. Using the FEM and the value of cracking space (Lc), we proposed the method to determine the bending strength of the thin films.
A micro-sized fracture toughness testing technique has been applied to investigate the fracture properties of colonies in a fully lamellar Ti-46Al-5Nb-1W alloy. Micro-sized cantilever specimens with a size≈10×20×50 μm3 were prepared by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into these specimens by focused ion beam machining. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness values obtained were in the range 1.4-6.9 MPam1/2. Fracture surface observations indicate that these variations are due to differences in local lamellar orientations ahead of the notch. These fracture toughness values are also lower than those having been previously reported in conventional samples. This may be due to the absence of significant extrinsic toughening mechanisms in these micro-sized specimens. Fracture mechanisms of these alloys are also considered on the micrometer scale. The results obtained in this investigation give important and fundamental information on the development of TiAl based alloys with high fracture toughness.
The fatigue crack initiation and propagation behaviors were examined for commercially-pure iron films of 100 μm thickness. The cold rolled films were annealed and adhered to cover a circular through hole in a base plate. The base plate was subjected to constant cyclic stress with a stress ratio of R = 0. In order to discuss correlation between fatigue crack propagation and change of crystal orientation, crystal orientation on the surface of the film materials was measured before and after fatigue testing. The crystallographic information of these films was analyzed using the Electron Back-scatter Diffraction (EBSD) system and the quantitative evaluation method for the crystal rotation angle and the rotation axis, the rotation direction with fatigue was developed. As a result, the crystal rotation angle near the fatigue crack is larger than that apart from the crack and the crystal rotation was quantitatively larger around the transgranular crack than the intergranular crack.
Fatigue tests of thin wires made of commercially pure iron or aluminum were conducted in air. The fatigue damage of the Materials was successfully measured with either an A. C. potential method or a digital microscope. For the A. C. potential method, constant current of 600 mA with 888 Hz was applied to the specimen. In pure iron, the value of electric potential was increased for a specimen whose diameter was 0.20 mm, however it was almost constant for a specimen of 0.60 mm in diameter. In pure aluminum, the value of potential was increased for every specimen diameter. In this Material also the change of A. C. potential was larger for smaller specimens. The change of A. C. potential value was detected before the specimen deformation observed with the digital microscope. Two types of fracture morphology were observed with digital microscope. Those were with or without fatigue cracks. In the latter case, the specimen was necked at the final stage of fatigue process.
In order to clarify the deformation behavior of bulk metallic glasses from atomistic viewpoint, an amorphous Ni-Al binary alloy is made and subjected to tension by means of molecular dynamics simulation. The initial equilibrium is made by a melt-quench simulation of a single crystal in which Ni or Al atoms are randomly arranged on the fcc lattice point. Both the radial distribution and Voronoi analysis attests that the initial equilibrium is in amorphous state. The amorphous metal is then subjected to tension with the strain rate of 1.0×109/s under periodic boundary condition, showing nonlinear stress-strain curve from the initial stage of loading. The curve shows the peak stress of about 3.0 GPa at the strain of 0.07. After the peak the stress slightly decreases and the metal deforms with constant stress of about 2.4 GPa. There is no remarkable change in the radial distribution function and Voronoi polyhedron during the deformation so that the metal maintains the short-range order of amorphous structure. Then we have calculated the elastic stiffness matrix, which is evaluated by the stress and elastic coefficients, at each atom point during tension to evaluate the local stability. Contrary to the crystal, many atoms show negative value even in the initial equilibrium of amorphous metal. These “unstable” atoms turn out to be in the outer-shell of the local cluster such as (0, 0, 12, 0) icosahedron or in the boundary of the local clusters. On the other hand, the center atoms of the local clusters show high stability, resulting in the system stability of about 2×1012 GPa6. It is also demonstrated that the change in the local stability reveals the collapse or reconfiguration of local clusters.
In this paper a rectangular crack vertical to a bimaterial interface under mixed model loading is considered. The solution utilizes the body force method and requires Green's functions for perfectly bonded semi-infinite bodies. The formulation leads to a system of hypersingular integral equations in which the unknowns are three types of crack opening displacements. In the numerical calculation, the unknown body force densities are approximated by using fundamental density functions and polynomials. The results show that the present method yields rapidly converging numerical results for various aspect ratios of a rectangular crack near a interface. Then, the effect of a free boundary on stress intensity factors of a rectangular crack subjected to tension is discussed by comparing the results of an elliptical crack with a same area. Moreover, distributions of stress intensity factors are indicated in tables and figures with varying the shape of crack, distance from the interface, and elastic constants. Meanwhile, the effect of crack shape on the stress intensity factors in term of root area is investigated. It is found that the stress intensity factors are mainly controlled by the root area parameter almost independent of the crack shape. At last, the stress intensity factors of a rectangular crack in a infinite body are calculated under tension and shear stress at infinity.
The influence of various basic factors of combustion gas flow conditions on the recession rate of silicon nitride has been experimentally clarified, and the recession rate equation is deduced using the dependence of influential factors on the recession rate and the mass transfer theory. The exposure tests are performed under various gas flow conditions (T = 1200∼1500°C, P = 0.2∼0.7 MPa, V = 40∼200 m/s, PH2O = 30∼90 kPa, PO2 = 20∼45 kPa, t = 10 h). Recession rates mainly depend on water vapor partial pressure, pressure, gas temperature, and Reynolds number represented the gas flow conditions inside the specimen holder. The dependent on oxygen partial pressure is extremely low for silicon nitride. The recession rates of silicon nitride in combustion gas flow are expressed in the form exp (-E/RT) · (PH2O)n · Re0.8/P, and the predicted recession rates of silicon nitride shows good agreement with reported exposure test results under a gas turbine condition.
In order to understand recession behavior and the amount of recession of Lu2Si2O7 in the combustion gas flow, sintered Lu2Si2O7 specimens were manufactured by hot pressing and exposed under various combustion gas flow conditions (T = 1300∼1500°C, P = 0.3 MPa, V = 150 m/s, PH2O = 27∼69 kPa, t = 10 h). After the exposure tests, etch pits, which are assumed to form due to volatilization of SiO2 in the grain boundary phase, were observed at the surface of specimen. The amount of Lu2SiO5 phase at the surface of specimen increased with the increase of gas temperature or water vapor partial pressure. A corresponding decrease in the amount of Lu2Si2O7 phase was observed. Furthermore, by using the average weight loss rate for exposure times of ten hours, the influence of gas temperature and water vapor partial pressure on weight loss rate was examined, and the amount of recession under gas turbine conditions was calculated. Superior environmental resistance was shown relative to Si3N4, SiC and Al2O3.
The mechanical and X-ray elastic constants of ceramics used for solid oxide fuel cells (SOFCs) were measured and compared with the predictions based on the models of elastic deformation of polycrystals and multiphase materials. The elastic compliances of single crystal of scandia stabilized zirconia (10ScSZ) for electrolyte were determined from the experimental values of X-ray elastic constants as follows : s11 = 2.21 TPa-1, s12 = -0.10 TPa-1, s44 = 24.5 TPa-1. The X-ray elastic compliances increase with the orientation function. The X-ray elastic compliances for doublet peaks can be obtained from those of each peak and the intensity ratio. The experimental values of the mechanical elastic constants of anode composed of zirconia and nickel (Ni-3YSZ) or nickel oxide (NiO-3YSZ) agreed fairly well with the prediction based on the self consistent model. The experimental values of the X-ray elastic constants of anode showed about 10 to 15% deviation from the prediction based on the self consistent model, because of the inhomogeneous mixture of zirconia and nickel (or nickel oxide). The experimental determination of the X-ray elastic constants was recommended for X-ray stress analysis of anode.