On the high-temperature reactions between advanced ceramics and a cobalt-base alloy

Abstract

Abstract The incorporation of advanced ceramics or ceramic composites into land-based turbines may potentially increase their efficiency due to increased gas inlet temperatures and/or reduced cooling requirements. In some proposed designs, incorporation of ceramics would require intimate contact of the ceramic and a metallic structure at elevated temperatures. Since many advanced ceramics contain Si, and since turbines are usually constructed out of Ni or Co-base alloys, the formation of Co or Ni silicides is expected at the ceramic/metallic interfaces. The goals of the present work are (a) an assessment of the microstructures and reaction products formed at ceramic/metallic interfaces during annealing at elevated temperatures and (b) a description of the kinetics of the reaction zone formation. Reaction couples consisting of the Co-base alloy ECY 768 and various advanced ceramics were prepared by hot-pressing in vacuum and subsequent annealing in vacuum or air at temperatures ranging from 1173 to 1323 K and for times ranging between 10 and 1000 h. Two types of silicon nitride (NT154 and AS800) and four whisker-reinforced alumina ceramics containing different weight fractions of SiC whiskers (ARtuff CC5500, S7, SX and CC7000) were investigated. The reaction zones consisted primarily of (Cr, Co)carbosilicides. The maximum reaction zone thicknesses after 1000 h at 1323 K were in most cases below 50 μm. Whereas the reaction zones were well defined in the case of the silicon nitrides, the Al 2 O 3 /SiC w materials developed multiple silicide layers within the reaction zone. With increasing SiC whisker content, the reaction zones increased in thickness for a given heat treatment. A non-linear fitting procedure was employed to describe the reaction zone thicknesses in terms of apparent activation energies and pre-exponential factors. Whereas this procedure worked well for the silicon nitrides annealed in a vacuum, substantial scatter occurred for air anneals. The ARtuff materials always showed substantial scatter which is attributed to their layered reaction microstructure.