減圧プラズマ溶射を用いた耐酸化セラミック皮膜の作製および評価

抄録

In order to improve the oxidation resistance of Nb base alloys as a promising high temperature structural material in the next generation, plasma sprayed ceramic coatings were prepared on Nb and their oxidation behavior and thermal shock property were investigated. Al₂O₃, MoSi₂ and Al₂O₃-MoSi₂ were chosen as the coatings by considering the oxidation resistance and the difference in thermal expansion. The plasma sprayed Al₂O₃ was laser-treated for densification. Oxidation test was carried out in air at 1473 K for 100 h and mass change during exposure was measured. The thermal shock test was performed by water quenching after holding for 50 h at 1473 K using Nb coated with Al₂O₃-30 vol%MoSi₂. To investigate the effect of reacted phases between a coating and Nb on thermal shock properties, Al₂O₃ under-coating was employed.
Laser-treatment on a porous Al₂O₃ coating leads to densification due to elimination of most pores but yields many macrocracks. The oxidation test of Nb plasma sprayed with Al₂O₃ or MoSi₂ indicates that the coatings are spalled at an early stage. In contrast, Al₂O₃-MoSi₂ coated Nb exhibits excellent oxidation resistance and mass change after 100 h exposure is less than 5 mg/cm². It is suggested that SiO₂ formed on MoSi₂ acts as a protective layer against oxygen penetration, and porous Al₂O₃ in the layered Al₂O₃/MoSi₂ structure relaxes the thermal stress generated in the coating. In the thermal shock test of Al₂O₃-MoSi₂ coated Nb, spalling occurs at a reaction layer consisting of Mo₅Si₃ and Nb₅Si₃ which form between Nb and the coating. A lot of Kirkendall voids are produced in the Mo₅Si₃ reaction layer probably because of higher diffusivity of Si than Nb. The spalling is caused by propagation of cracks due to thermal stress throughout the voids. In the Al₂O₃ under-coated sample, there exists no reaction layer after the thermal shock test, thereby resulting in no spalling.