Tatsuhiko Aizawa

Friction and wear properties of CrAlN and CrVN films deposited by cathodic arc ion plating method

Abstract Chromium nitride (CrN) films are superior to the titanium nitride film in corrosion and wear resistances, and friction behavior. CrN has been widely applied to the molding dies, machine parts and sliding part. In the present paper, additions of aluminum and vanadium into CrN films were performed with an expectation of improvement in tribological properties. CrN, CrAlN and CrVN were deposited by a cathodic arc ion plating. Deposited films were characterized by X-ray diffraction for crystal structure identification and energy-dispersive X-ray spectroscopy for chemical composition analysis. Diffraction peaks that appeared were similar in position and orientation in all films because the crystal structure and the lattice constant for CrN, VN and AlN are close to each other. The composition of the film deposited with Al and V was estimated to be Cr70Al30N and Cr50V50N, respectively. Knoop hardness test showed that CrAlN was harder than CrN and CrVN. Friction and wear tests were carried out by a ball-on-disk tribometer with stainless steel and cemented carbide balls as a counter material, with and without lubricant. Flaking occurred on CrN with stainless steel ball in wear tests without lubrication by ball-on-disk tribometer, but that did not occur on CrAlN and CrVN films. In the case of the wear test with cemented carbide ball, depth of wear track on CrN film reached to the substrate. The friction coefficient was almost the same for CrN and CrAlN films; however, that for CrVN film was lower than other films, in motor oil. V addition into CrN film successfully improved its tribological properties.

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Abstract Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10−3 s−1. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.

Thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1-x prepared via bulk mechanical alloying and hot pressing

Abstract In the present paper, starting from elemental Bi, Sb and Te granules with 5 N purity and 1–5 mm diameter, p-type single phase (Bi 2 Te 3 ) x (Sb 2 Te 3 ) 1− x ( x =0.20, 0.225, 0.25, 0.275 and 0.30) thermoelectric materials with high densification (>99% theoretical density) were prepared via bulk mechanical alloying (BMA) and hot pressing (HP). Their thermoelectric properties, are less composition sensitive in the composition range of x ≤0.25 and are comparable to that of those materials prepared by travelling heater method (THM); the figures of merit of (Bi 2 Te 3 ) x (Sb 2 Te 3 ) 1− x are about 3×10 −3 /K. When x >0.25, the figures of merit of (Bi 2 Te 3 ) x (Sb 2 Te 3 ) 1− x decreased rapidly with increasing x .

Hydrogen absorption of nanocrystalline palladium

Abstract Thermodynamic properties of hydrogen in nanocrystalline Pd (Pd-n) were determined by pressure–composition ( P–C ) isotherms. Pd-n was prepared by repeated compression–extrusion cycles in the die cavity of a high speed forging apparatus. It was found that Pd obtained was composed of nano-sized Pd grains, i.e. ∼10 nm, which were strained in 〈111〉 and 〈100〉 by 1.3 and 2.4%, respectively. From P–C isotherm measurements over the temperatures from 298 to 373 K, it was found that the hydrogen solubility in the α phase region for Pd-n was significantly larger than that for coarser grained Pd. On the other hand, Pd-n showed lower plateau pressures than coarser grained Pd. In addition to the above, the maximum hydrogen solubility of Pd-n was almost 50% lower than that for coarser grained Pd. The differences in the hydrogen solubility and plateau pressure between Pd-n and coarser grained Pd will be discussed under the consideration of the structure of nano-sized Pd grains. The drastic reduction in the maximum hydrogen solubility for Pd-n could be related to the viewpoint of strain of the octahedron in f.c.c. Pd.

Synthesis and formation mechanisms of molybdenum silicides by mechanical alloying

Abstract The synthesis and formation of MoSi 2 , Mo 5 Si 3 , and Mo 3 Si compounds by the mechanical alloying of Mo Si powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10–80 at.% Si. The formation of molybdenum silicides, especially MoSi 2 , during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of α MoSi 2 during mechanical alloying at 67 at.% Si (MoSi 2 stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80 at.% Si, however, the formation of MoSi 2 proceeded by the gradual formation of both the α and /gb phases instead of the MSR mode. The formation of Mo 5 Si 3 during mechanical alloying was characterized by a slow reaction rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to the direct formation of Mo 3 Si. The Mo 3 Si phase appeared only after brief annealing at temperatures of 800°C and above.

Effect of aluminum concentration on friction and wear properties of titanium aluminum nitride films

Abstract It is well known that the hard thin ceramic films are utilized to industrial applications such as cutting tools, molding dies and sliding parts, because of its effect of reducing the wear. Recently, the demand of TiAlN film is expanding, because it has a high oxidation resistance at elevated temperature as well as good wear resistance. In this paper, friction and wear properties of TiAlN films having various concentration ratios of Ti/Al have been studied. The films were deposited by a cathodic arc ion plating method on cemented carbide substrate with a thickness of approximately 3 μm. Composition of the deposited film was evaluated by energy dispersive X-ray spectroscopy. Crystal structure of the deposited film was characterized by X-ray diffraction. Hardness of the films was measured by a nano-indentation tester. Friction and wear tests were carried out by a pin-on-disk tribometer at 300, 473, 673 and 873 K. Compositions of the films deposited from cathode materials that have atomic ratios (Ti:Al) of 50:50, 34:66 and 25:75 in TiAl alloys were Ti 0.6 Al 0.4 N, Ti 0.42 Al 0.58 N and Ti 0.3 Al 0.7 N, respectively. The crystal structure of Ti 0.6 Al 0.4 N Ti 0.42 Al 0.58 N and Ti 0.3 Al 0.7 N were found to be a B1, B4+B1 and B4, respectively. Nano-hardness decreased with increasing Al content in the films. There is no correlation between friction coefficient of each film and test temperature. The width of wear track for each film decreased above the temperature of 673 K. Ti 0.6 Al 0.4 N film was worn less than the other films at 873 K.

Tribology of dry deep-drawing of various metal sheets with use of ceramics tools

Abstract Lubricant is essential in the conventional metal forming to reduce the friction between tool and workpiece and forming energy, to increase the forming limit, to shorten the forming process, and to prolong the tool life by prevention of galling and seizure. Huge amount of wasted lubricants has become a great nuisance of environmental issues. Dry deep-drawing is attractive toward zero emission of lubricants but it has great difficulty in application to metal sheets in general. As one of the most promising methods, use of ceramic die, are highlighted because of their high tribological properties. In the present paper, practical possibility of dry deep-drawing is studied using ceramic dies. Various ceramic die materials are prepared and actually applied to deep-drawing for various sheet materials without lubrication. The limiting draw ratio (LDR) as well as the drawing load, were used as a parameter to evaluate the friction between tool and blank. Each ceramic material has its own compatibility to sheet materials. Ceramic dies can be applied to deep-drawing of mild steel and pure copper sheets with success, while they failed to be utilized for titanium sheet. Deep-drawability of ceramic dies might be dependent on the covalency of ceramics: i.e. LDR(SiC)>LDR(Si3N4)>LDR(Al2O3)>LDR(ZrO2). In case of deep-drawing for metal-alloy sheets, pretreatment to form the adhesive tribo-coating is effective to improve the workability when using alumina and zirconia dies. The adaptive design of ceramic die to each metallic sheet material is indispensable to realize the dry forming in practice.

Synthesis of Mg2Ni alloy by bulk mechanical alloying

Abstract Mg 2 Ni was synthesized by a solid-state reaction from the constituent elemental powder mixtures via bulk mechanical alloying. Since homogeneous refining and alloying takes place efficiently by repeated forging, the process time can be reduced to one fiftieth of time duration necessary for conventional mechanical milling and attrition. This MA Mg 2 Ni alloy can be obtained as a high dense powder compact with a relative mass density in the order of 85%; its average powder particle size ranges from 1 to 3 μm. The prepared Mg 2 Ni alloy has a sufficiently high reaction rate to hydrogen even at relatively lower temperatures and a potential to form a ternary hydride on the pressure-composition isotherms from 523 to 623 K. The enthalpy and entropy for ternary hydride formation were estimated to be −28. 7 KJ/mole-H and −46. 8 J/K/mole-H, respectively. The present bulk mechanical alloying was found to be adaptive to fabrication of single phase Mg 2 Ni in high productivity.

Experimental study for the improvement of crashworthiness in AZ91 magnesium foam controlling its microstructure

Abstract Metallic foams are expected to be used as the impact energy absorber material because of their unique deformation characteristics, which almost constant compressive stress appears in a wide range of strain. This phenomenon is well known as the regime of collapse plateau. It is very important to know strain rate dependence of the plateau stress, and the impact energy for suitable design of automotive components. Only limited amount of mechanical response data of metallic foams under dynamic loading are, however, available comparing with those of polymeric foams. In this study, the absorbed energy of an open-celled magnesium foams with a relative density of 0.03–0.06 is evaluated at a dynamic strain rate of ∼103 s−1 in compression by using the split Hopkinson pressure bar apparatus. In order to investigate the effect of microstructure in the solid material, solution treatment and aging are performed to all the specimens and then examined for the same strain rates. Peak stress and plateau stress per (relative density)3/2 for as-received and heat treated AZ91 foams showed the strain rate dependence, which decreased by the heat treatment. Therefore, it is possible to control the absorption energy of the AZ91 metallic foam by means of microstructural improvement, which controls the ductility in the solid material.

Hydrogen absorption and electrochemical properties of Mg2-xNi (x = 0 - 0.5) alloys prepared by bulk mechanical alloying..

Abstract Thermodynamic properties of hydrides of Mg 2− x Ni alloys produced by bulk mechanical alloying were determined from the pressure–composition isotherms for absorption over the temperatures from 623 to 423 K. The van’t Hoff plots for the plateau pressures of the isotherms clearly indicated the existence of high- and low-temperature hydrides with different entropy and enthalpy for hydride formation. The phase transition temperature was 525 K for Mg 2.0 Ni and decreased with increasing value of x . Chemical and electrochemical behaviors of Mg 2− x Ni alloys in an alkaline solution were precisely determined. It was found that hydrogen absorption in Mg 2− x Ni alloys takes place by only immersing the alloys in 6 M KOH solution. On the other hand, during electrical charging hydrogen content increased with the quantity of charged electricity and reached the maximum value. However, the hydrogen content decreased afterwards because the corrosion of the alloys in the alkaline solution is significant.