Papers by Author: Tatsuo Tabaru

Abstract: Aluminum nitride (AlN) thin films formed on the heat-resistant alloy substrates were heated to 1100K. Cracking was observed in the AlN film formed on the stainless steel substrate (SUS430), while no crack was seen in that on the nickel-base superalloy substrate (IN750X). The electrical impedance measurements, X-ray diffraction analysis and finite element method calculation have been conducted to discuss the relationship between the cracking and the stress introduced into the AlN films. The AlN film cracking would be significantly affected by grain refinement of AlN.

Abstract: Nb-base in-situ composites with the base composition of Nb-18Si-2HfC were prepared by conventional arc-melting. Their microstructures and mechanical properties, such as high-temperature strength and room temperature fracture toughness, were investigated to elucidate the effects of Re alloying. The in-situ composites predominantly have eutectic microstructures consisting of an Nb solid solution (NbSS) and Nb5Si3. The compressive strength increased with the increasing Re contents at 1470K and not at 1670 K. The strengthening effect observed at 1470 K is higher than that by W and Mo. Re alloying of about 2 % is valuable for improving both the high temperature strength and room temperature fracture toughness of Nb-18Si-2HfC base materials.

Abstract: Aluminum nitride (AlN) is a promising Acoustic Emission (AE) sensor element for high-temperature environments such as gas turbines and other plants because AlN maintains its piezoelectricity up to 1200°C. Highly c-axis-oriented AlN thin-film sensor elements were prepared on silicon single crystals by rf magnetron sputtering. Both ordinary-temperature AE sensors and high-temperature AE sensors have been developed using these elements. In this paper, to study effects of d33 and thickness of AlN elements on sensor sensitivity, AlN elements with d33 from 2 to 7 pm/V and thickness from 3 to 9 /m were prepared. It is confirmed that the AE sensor sensitivity increased with d33 and thickness of AlN elements. The sensitivity of the high-temperature AE sensor was also improved by a design of the sensor structure. The sensor characteristics were evaluated at elevated temperatures from 200 to 600°C. It was confirmed that the AE sensor works well at 600°C and does not deteriorate.

Abstract: Nb-base in-situ composites, which have the base composition of Nb-18Si-5Mo-5Hf, have been investigated in microstructure, hardness (Hv*), Young’s modulus (E), tensile properties and fracture behavior. The microstructures of all composites examined consist of NbSS matrix and Nb5Si3 secondary phases. No secondary phase such as Nb2C appeared. The crystal structure of Nb5Si3 is Mn5Si3-type when C replaces 2mol%-Nb, though typical structures of a (Cr5B3-type) and b (W5Si3-type) as in the base composition when W replaces. W addition is effective in increasing Hv* and E of both phases as expected. However, C alloying is somewhat beneficial only in Nb5Si3 with a noticeable negative effect in NbSS. Furthermore, the composite exhibits the highest strength at 1473 K, while the base composite exhibits the highest at room temperature. The fracture behavior is independent of the compositions and it is controlled by cleavage fractures of Nb5Si3, decohesion of NbSS/Nb5Si3 interface and ductile rupture of NbSS depending on the testing temperatures.

Abstract: Nb base in-situ composites with the base composition of Nb-5Mo-2W-18Si were prepared by conventional arc-melting and induction heating floating zone melting followed by directional solidification. To investigate the effect of HfC addition, Nb was replaced with 0, 1 and 2 mol% HfC. The in-situ composites predominantly have an eutectic microstructure consisting of Nb solid solution (NbSS) and (Nb,Mo,W))5Si3 (5-3 silicide). The strength at 1470 K and 1670 K increases without fracture toughness decreasing, with increasing the HfC content. Directional solidification also improves the strength at the high temperature. The slip band under the shearing stress occurs in the NbSS during plastic deformation, which contributes to suppress microcrack propagation. It seems that HfC addition reinforces the bonding strength at grain boundary or NbSS/5-3 silicide interface.