A chromia forming thermal barrier coating system

Abstract

Conventional thermal barrier coating (TBC) systems consist of an insulating ceramic topcoat, a bond coat for oxidation protection and the underlying superalloy designed to combat the oxidising conditions in aero-and land-based gas turbines. Under high-temperature oxidation, the use of an alumina forming bond coat is warranted, thus all current TBC systems are optimised for the early formation of a dense, protective thermally grown oxide (TGO) of alumina. This also offers protection against Type I hot corrosion but a chromia layer gives better protection against Type II corrosion and intermediate temperatures, the conditions found in land-based gas turbines. In this paper the authors present the first known results for a chromia forming TBC system. Tests have been performed under oxidising conditions, up to 1000 h, at temperatures between 750 8C and 900 8C, and under Type I (900 8C) and Type II (700 8C) hot corrosion conditions up to 500 h. Under all these conditions no cracking, spallation or degradation was observed. Examination showed the formation of an adherent, dense chromia TGO at the bond coat / topcoat interface. These initial results are very encouraging and the TGO thicknesses agree well with comparable results reported in the literature.

Steam oxidation of Super 304H and shot-peened Super 304H

“…The values for the shot-peened Super 304H oxidised in air at 700 C correlates well with the K p values of chromia growth from the literature[23]. The oxides formed on the…”

“…and Cr -Mn-rich (j) layers grown on pickled Super 304H oxidised in steam. The K p values are compared to known K p values of chromia growth on different metals after oxidation in dry atmospheres ()[23]. The shot-peened super 304H appears to form chromia at 700 C in air.…”

High-Temperature Oxidation of NiAlCr–Ca and NiAlCr–Sr Alloys in Air

“…High-temperature oxidation resistance of metallic materials can be improved by a few methods such as alloying with Al [3] and Cr [4], applying a thermal barrier coating (TBC) [5,6], and oxidationresistant metallic coatings [7,8]. TBC works as a barrier layer that reduces the heat flow rate, while metallic coatings are designed to work as a bond coat for TBC's [9] that forms protective thermally grown oxides (TGO) as in the case of alloying, such as alumina (α-Al 2 O 3 ) [10] or chromia (Cr 2 O 3 ) [11]. TGO scales are desired to be dense, slow-growing, compatible with the alloy, well-adhered, free of stress, free of defects such as pores and cracks, composed of a compound with low ionic conductivity, and stable.…”

Modelling of the interdiffusion and oxidation of a multilayered chromia forming thermal barrier coating

“…The structure of the bond coat is based on a SmartCoat system containing alternate layers rich in aluminium and chromium. Previous studies on this TBC have been published presenting the concept and preliminary oxidation data . In this paper, further development of the bond coat is presented with the inclusion of an aluminium‐rich layer within the structure.…”