Characterization of Chemical Vapor‐Deposited (CVD) Mullite+CVD Alumina+Plasma‐Sprayed Tantalum Oxide Coatings on Silicon Nitride Vanes After an Industrial Gas Turbine Engine Field Test

Abstract

Silicon nitride ceramic vanes coated with chemical vapor‐deposited (CVD) mullite, CVD alumina, and plasma‐sprayed tantalum oxide were exposed to field tests in an industrial gas turbine engine. Results varied due to expected non‐uniformities in the CVD coating microstructures, but dense CVD mullite/alumina showed excellent stability and protective capacity after 1148 h of engine testing. Surfaces without CVD coatings experienced massive intragranular subsurface oxidation and/or rapid recession of the ceramic substrate due to volatilization of silica species formed by oxidation. These results suggest that thin (<5 μm), dense, high‐purity CVD mullite and CVD alumina are viable components for an environmental barrier coating system to protect structural ceramics in combustion environments.

Silicon Nitride for High‐Temperature Applications

“…Several oxide ceramic material systems have been evaluated as candidate EBC material for Si 3 N 4 182–190 . A summary of the corrosion behavior of the possible candidates to be used as corrosion‐resistant EBC material has been provided in Sidebar 4 183…”

“…EBC systems containing CVD Al 2 O 3 , mullite, and plasma‐sprayed Ta 2 O 5 have been investigated as protective layer on AS 800 188,189 . This layer system was found to be effective in engine tests at about 1100°C for >1000 h. At higher temperatures, however, reactions between the Al 2 O 3 and the SiO 2 formed during oxidation would weaken the stability and functionality of the EBC layer.…”

“…Several oxide ceramic material systems have been evaluated as candidate EBC material for Si 3 N 4 . [182][183][184][185][186][187][188][189][190] A summary of the corrosion behavior of the possible candidates to be used as corrosion-resistant EBC material has been provided in Sidebar 4. 183 A three-layer EBC system has been developed in a joint research of NASA, General Electric and Pratt and Whitney for SiC/SiC combustor liners.…”

“…187 EBC systems containing CVD Al 2 O 3 , mullite, and plasmasprayed Ta 2 O 5 have been investigated as protective layer on AS 800. 188,189 This layer system was found to be effective in engine tests at about 11001C for 41000 h. At higher temperatures, however, reactions between the Al 2 O 3 and the SiO 2 formed during oxidation would weaken the stability and functionality of the EBC layer. Furthermore, the hot gas stability of these materials was found to be insufficient, especially Al 2 O 3 and mullite formed volatile hydroxides (Al(OH) x ) in hot gas atmospheres.…”

Stability of Mullite Protective Coatings for Silicon‐Based Ceramics

“…Other failure mechanisms are clearly possible (e.g., failure could be initiated at one of the internal interfaces associated with the buried silica layer). It is also worth noting that recent data show that improved coating life in a combustion environment can be obtained by depositing a thin, coherent layer of alumina on top of a mullite coating 29 . This configuration is significantly different from a system where mullite transforms to alumina; thus, these results do not contradict the proposed failure mechanism associated with Fig.…”