Manabu Matsubara and Shuji Hanada
Institute for Materials Research, Tohoku University, Sendai 980-8577
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.
(Received July 16, 1998; In Final Form September 9, 1998)
plasma spray, niobium base alloy, ceramic coating, oxidation resistance, oxidation test, thermal shock test
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