Ichiro Shiota

SiC/C multi-layered coating contributing to the antioxidation of C/C composites and the suppression of through-thickness cracks in the layer

Abstract Silicon carbide/carbon multi-layered coatings have been formed on the surfaces of C/C composites. The multi-layered coatings were deposited by the CVD method at 1200°C using SiCl 4 , C 3 H 8 and H 2 gases. The principal idea of the present multi-layered coatings is to suppress the through-the-thickness coating cracks by making the thickness of each SiC and carbon layer lower than the corresponding critical thicknesses, h c s. The values of h c s for the SiC and carbon coatings were 0.2 μm and above 15 μm, respectively. The role of the SiC layers is oxidation protection and that of the carbon layers is to mechanically isolate each SiC layer. Based on theoretical and experimental discussions, the SiC/C multi-layered coating was shown to successfully suppress the through-thickness coating cracks.

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.

In situ synthesis of titanium-aluminides in coating with supersonic free-jet PVD using Ti and Al nanoparticles

This paper presents supersonic free-jet PVD (SFJ-PVD) as a new coating technology for structural materials. In SFJ-PVD, coating film is formed by high velocity impact of solid nanoparticles to base materials. This method is composed of evaporation and deposition processes. In the evaporation process, the source material evaporates to form nanoparticles in an inert gas atmosphere. In the deposition process, nanoparticles are deposited on base materials to form coating film with supersonic gas flow. The gas flow is generated by pressure difference between the evaporation chamber and the deposition chamber. The flow of the gas is accelerated through a specially designed supersonic nozzle at the supersonic flow of 3.6 Mach number. With SFJ-PVD, mixed Ti and Al nanoparticles produce high-density coatings of titanium-aluminides without voids and cracks. They synthesize titanium-aluminides on the substrate at 800 K.

The strength of carbon fibre-reinforced nickel composites produced by electro-deposition and hot-pressing

Carbon fibre-nickel composites were made by electro-deposition and hot-pressing. The composites with low volume fraction of carbon fibres had a higher strength than the value obtained by the law of mixtures using the strength of annealed nickel. The current density became lower when the deposited nickel became thicker. The effects of annealing on deposited nickel were also observed and were related to the current density. Nickel deposited at low current density was hard to anneal.

Fracture Behavior of Carbon–Carbon Composites with Cross-Ply Lamination

Tensile fracture behavior of carbon–carbon composites with 0 /90 laminations were examined using double end-notched specimens. In the composites, the volume fractions of the 0 - and 90 -layers were systematically varied to observe the variation of fracture patterns as a function of the shear strength, which increased with the 0 -ply fraction, r. The fracture mode of the notched C–Cs was shown to transfer from Mode I to Mode II when r increased, with the transition occurring at r slightly higher than 0.5. When Mode II fracture occurred, complete notch insensitivity resulted. In contrast, when Mode I fracture was observed, slight notch sensitivity appeared. To evaluate fracture behavior more deeply, compact tension tests were performed. Resultant crack-resistance (R-) curves clearly explain the transition behavior from Mode I to Mode II. The R-curves also suggested that the slight notch sensitivity in the Mode I fracture was due to small scale splitting that appeared near the notch tips.

Development of FGM thermoelectric materials in Japan-the state of the art

Two times higher performance than a traditional thermoelectric material can be expected if the proper carrier concentration gradient is tailored to fit the temperature gradient. Performing a stepwise change of carrier concentration is also a method for practical application. That is a fundamental concept of energy converting functionally graded materials (FGM). It is essential to choose a proper material for each part to fit the temperature gradient. The proper material is a material with the proper carrier concentration and a proper compound to match the temperature of each part along the temperature gradient. Joining of these FGM materials and fitting electrodes with FGM interfaces are also core techniques, because thermal stress relaxation caused by the difference of thermal expansion coefficients is important at a high temperature. Joining two Bi/sub 2/Te/sub 3/ samples with carrier concentrations n/sub c/ of 1.0 and 4.5/spl times/10/sup 25/ was done by the ordinal soldering technique or diffusion bonding. The specific temperature range of the Seebeck coefficient /spl alpha/ for the joined Bi/sub 2/Te/sub 3/ is extended from 50 to 100 K, and the value of /spl alpha/ at the valley between the two materials with different n/sub c/ was higher than both materials. The sintered n-type PbTe FGM with 3 layers of n/sub c/=3.51, 2.60 and 2.26/spl times/10/sup 25/ was prepared by hot pressing. The effective maximum power P/sub max/ of the FGM at the temperature difference of /spl Delta/T=310 K is 150 Wm/m/sup 2/ and is about 7% larger than that of the layer with n/sub c/=3.51/spl times/10/sup 25/ whose P/sub max/ is the greatest in all layers.

Crystal grain size dependence of thermoelectric properties for sintered PbTe by spark plasma sintering technique

Thermal and electrical conduction parameters of the sintered PbTe with different crystal grain sizes Ds were measured over the temperature range from 77 to 350 K, to examine the possibility of improvement in thermoelectric performance for sintered PbTe by reducing D. The starting powders with particle sizes of 6, 39, 180 and 380 /spl mu/m were obtained by pulverizing an undoped PbTe boule prepared by the Bridgman method. Sintering was carried out by the spark plasma sintering technique (SPS) under the conditions of sintering temperature of 732-803 K, sintering duration of 60-90 min and sintering pressure of 45 MPa. All the sintered compacts, whose conduction type was p-type, had an apparent relative density of more than 99%. Thermal conductivity /spl kappa/ decreased with reducing D below 200 K, but showed no dependence on D above 200 K. Long-wave phonons are scattered efficiently at grain boundaries only below 200 K. Hall mobility /spl mu//sub H/ of the sintered compacts with different Ds were almost same above 200 K. It is concluded that thermoelectric performance for sintered PbTe is not directly affected by controlling D.

Fabrication of nanostructure composites in functionally graded coatings with supersonic free-jet PVD

Graded Al/AlTi and Al/Al-Si coatings are prepared by depositing nanoparticles with supersonic free-jet PVD (SFJ-PVD). The SFJ-PVD has been developed as a new coating method in which a coating film is formed by depositing nanoparticles with very high velocity onto a substrate. The high velocity of nanoparticles is produced by the supersonic gas flow of inert gas. A smooth, compact and defect-free microstructure is formed both at the interface between substrates and coating films and inside the coating films. The microstructures of Al/AlTi and Al/Al-Si coating films have very fine grain size. Mixing Ti and Al nanoparticles by depositing them onto a substrate produces in-situ syntheses of g-TiAl and a2-Ti3Al intermetallic compounds on the substrate. It is confirmed with nano-indentation hardness tester that graded coatings have graded hardness corresponding to the gradation of composition.

Measurement of fiber/matrix interface properties of C/C composites by single fiber and fiber bundle push-out methods

Interfacial fracture stresses of carbon/carbon composites were measured by indentation methods. Two types of test methods, namely, single fiber push-out, and bundle fiber push-out tests were conducted. Both methods successfully gave fiber/matrix interface mechanical properties, especially debonding behavior. However, when the interface was strong, the single fiber push-out test encountered technical difficulty in processing the extremely thin specimen required to realize the fiber push out. On the other hand, the bundle fiber push-out test gave a good estimation of interfacial fracture stresses.

Suppression of through-the-thickness cracks in SiC coating on C/C composites

Two concepts are proposed to minimize coating cracks on the surface of C/C composites. The first idea of a multi-layered coating is based on the phenomenon that there exists a critical coating thickness below which no coating cracks appear. This phenomenon is experimentally and theoretically confirmed for a SiC coating. Thus, by alternately laminating thin SiC and carbon layers, coating-cracking is expected to be suppressed for a thick and dense coating. The second concept is the use of a fiber-reinforced coating. By the fiber reinforcement, the toughness of a ceramic coating should be improved. In this paper, both concepts are shown to be promising to reduce through-the-thickness coating cracks. In addition, problems especially related to oxidation behavior are discussed in relation with the two-coating systems.