Tsuneaki Matsudaira

Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W2B, WB, W2B5 and WB4 were formed, although the boride phase having the composition of the highest boride, WB12, did not appear. The formation of W2B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W2B5 and WB4 was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W2B5 was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB4 was prepared with a large excess boron content. However, the formation behaviour of WB4 showed that WB4 is metastable above 1400° C. The stability of WB4 phase could be increased by the presence of excess boron.

Synthesis of niobium boride powder by solid state reaction between niobium and amorphous boron

Abstract Niobium boride powders were synthesized by solid state reaction between niobium metal powder and amorphous boron powder. The formation of niobium borides was found to be dependent on temperature. Single phases of the stable borides, NbB and NbB2, were formed by heating mixed powders corresponding to the stoichiometric compositions at 1000 °C for 60 min. A single phase of Nb3B4 was obtained at a higher temperature of 1800 °C as a result of the promoted diffusion of boron atoms in niobium metal. All synthesized powders were well dispersed and had particle sizes of 5–10 μm. The sinterability of the synthesized NbB2 powder was evaluated at high pressure (4 GPa) and temperature (1600 °C) for 15 min; a single-phase sintered compact of NbB2 was formed with a relative density of 98% and a Vickers microhardness of 2600 kg mm−2.

Effects of added c-BN seed crystals on the reaction sintering of c-BN accompanied by a conversion from h-BN to c-BN

Reaction sintering behaviour of c-BN which is accompanied by a conversion from h-BN to c-BN was investigated under high pressure (7 GPa) and temperature (1700°C) conditions for 30 to 60min. A high conversion yield of c-BN in the sintered compact was attained by adding fine-grained c-BN seed crystals (particle size 0.5 to 8μm) to h-BN powder in the presence of 1 wt% NH4NO3 as a catalyst. An induced transformation from h-BN to c-BN occurs over a large surface area of c-BN seed crystals, which results in the formation of direct interparticle bonding between c-BN grains in the sintered compact. A fully dense sintered compact of c-BN (bulk density 98% theoretical) was obtained from the specimen of 70wt% h-BN with 30wt% added c-BN crystals having a particle size of 0.5μm. This c-BN compact had an average microhardness of 5100 kg mm−2 and a specific dielectric constant of 10.0 at a frequency of 1 MHz.

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB2 powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB2 formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB2 formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB2 powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB2 powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB2 formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB2 powder with an average grain size of 15μm was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB2 powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB2 powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.

Effect of Oxygen Potential Gradient on Mass Transfer in Polycrystalline α-Alumina at High Temperature

The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina scales formed on heat-resistant alloys, was evaluated at a temperature of 1873 K. Mass transfer along grain boundaries (GBs) in an alumina wafer exposed to a large oxygen potential gradient (dμO), where both oxygen and aluminum mutually diffuse along GBs, was analyzed using 18O2 and SIMS. 18O was concentrated at GB ridges on the high oxygen partial pressure (PO2(hi)) surface and along the GBs near the PO2(hi) surface. 18O adsorbed on the surface diffused almost immediately to surface GBs, resulting in the formation of new alumina by reaction with aluminum diffusing outward along the GBs. Oxygen GB diffusion coefficients in the vicinity of the PO2(hi) surface were determined from the 18O depth profile along each GB for the 18O map of the cross section of the exposed alumina wafer. The oxygen GB diffusion coefficients were comparable to the values calculated from the oxygen permeability constants assuming an electronic conductivity and were obviously lower than those of oxygen GB self-diffusion without an oxygen potential gradient.

Control of Polymorphism and Mass-Transfer in Al2O3 Scale Formed by Oxidation of Alumina-Forming Alloys

Thermal barrier coatings (TBCs) are widely used for hot section components of gas turbine engines to protect the underlying metals from the high operating temperatures; they serve to both increase the engine efficiency and improve the durability of components. TBC systems typically consist of a Ni-based superalloy substrate, an alumina forming-alloy bond coat and an yttria-stabilized zirconia topcoat. When TBCs are exposed to high temperatures in oxidizing environments, a thermally grown oxide (TGO) develops on the bond coat surface underneath the top coat. Fracture of TBCs progresses in the vicinity of the TGO, which attains a critical thickness during thermal cycling operations (Evans et al., 2001). Thus, suppressing oxidation of the bond coat is anticipated to enhance the durability of TBCs. Alumina-forming alloys tend to form metastable Al2O3 polymorphs such as gammaand theta-phases in oxidizing environments at 1100-1450 K (Brumm et al., 1992, Tolpygo et al., 2000). The thermodynamically stable alpha-Al2O3 typically forms at longer oxidation times and/or higher temperatures. Metastable oxide scales consisting of the gammaand thetaphases contain lattice defects so that they hardly act as a protective layer to further oxidation of the alloys compared with a alpha-Al2O3 scale (Brumm et al., 1992). It is also well known that the transformation of the alpha-phase from metastable polymorphs involves large changes in volume (~13%) and morphology. Metastable oxides tend to form on the bond coat during coating with the topcoat in an oxygen-containing atmosphere at high temperatures, so that they are present at the interface between the bond coat and topcoat. The transformation of these oxides to the stable alphaphase at higher operating temperatures promotes topcoat spalling. Thus, in order to enhance the durability of TBCs, the as-processed TBC microstructures should contain a thin layer of alpha-Al2O3 at the interface with no the metastable oxides. The standard gritblasting procedure that is used in practical applications to prepare the bond coat for topcoat deposition promotes the formation of alpha-Al2O3. However, it also results in severe contamination of the coating surface and accelerates oxidation of the bond coat (Tolpygo et al., 2001). Some studies have revealed that a pre-oxidation step that forms a alpha-Al2O3 TGO on the bond coat without grit blasting reduces further oxidation of the bond coat and improves the durability of the TBC (Tolpygo et al., 2005, Nijdam et al., 2006, Matsumoto et al., 2006, 2008). Although it is generally preferable to perform pre-oxidation above 1450 K to produce a thin alpha-Al2O3 oxide and avoid producing metastable oxides, high temperature

Grain Boundary Engineering of Alumina Ceramics

Oxygen permeability through alumina wafers was evaluated at high temperatures up to 1923 K to elucidate the mass-transfer mechanisms of polycrystalline alumina and serve as a model for protective alumina film formed on heat-resistant alloys. Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher oxygen partial pressure (PO2) surface side to the lower PO2 surface side, along with the simultaneous GB diffusion of aluminum in the opposite direction to maintain the Gibbs–Duhem relationship. Oxygen GB diffusion coefficients in the vicinity of the PO2(hi) surface were lower than those of oxygen GB self-diffusion without an oxygen potential gradient (dµO). When dµO was applied to the wafer, the oxygen and aluminum fluxes at the outflow side of the wafer were significantly larger than those at the inflow side. Ln (Y and Lu) and Hf segregation at the GBs selectively reduced the diffusivity of oxygen and aluminum, respectively. Thus, the mesoscopic arrangements of segregating dopants, which were selected by taking into consideration the behavior of the diffusion species and the role of dopants, enabled the alumina film to have enhanced oxygen shielding capability and structural stability at high temperatures. Furthermore, the GB diffusion data derived from the oxygen permeation experiments were compared to those for alumina scale formed by the so-called two-stage oxidation of alumina-forming alloys.

Effect of Dopant Configuration on Oxygen Shielding Properties of Polycrystalline Alumina

The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina layers, under an oxygen potential gradient ΔPO2 was evaluated at a temperature of 1873 K. When mutual grain boundary (GB) diffusion of oxygen and aluminum occurred in wafers subjected to a steep ΔPO2, the oxygen and aluminum fluxes at the inflow side of the wafer were significantly smaller than those at the outflow side. It was noteworthy that Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. It was found that a wafer with a bilayer structure, in which a Lu-doped layer was exposed to a low partial oxygen pressure (PO2) and a Hf-doped layer was exposed to a high PO2, exhibited excellent oxygen shielding properties at high temperatures.

Control of Polymorphism and Mass-transfer in Al2O3 Scale on Alumina Forming Alloys

The transformation from metastable polymorphs to stable alpha-Al2O3 in the scale formed on a CoNiCrAlY alloy is accelerated under lower oxygen partial pressure (PO2), where both Al and Cr in the alloy are simultaneously oxidized, resulting in the formation of a dense and monolithic alpha-Al2O3 scale. Under higher PO2, where all components of the alloy are oxidized, the transformation is retarded and (Co,Ni)(Al,Cr)2O4 is also produced. The oxygen permeability in polycrystalline alpha-Al2O3 wafers exposed to steep oxygen potential gradients is evaluated at high temperatures to investigate the complicated mass-transfer phenomena through the scale formed on the alloy. The diffusion of Al and O species, which are responsible for the oxygen permeation along the grain boundaries of Al2O3, is dependent on the formation of an oxygen potential gradients. For Lu-doped Al2O3 polycrystals, it was found that Lu depressed the mobility of oxygen, but did not directly influence the migration of Al.