Hybrid phlogopite mica seals with glass interlayers were evaluated in terms of materials degradation in a combined ageing and thermal cycling test. Three glass interlayers were investigated: a standard Ba-Ca-Al silicate glass (G18), a modified Ba-ca-Al silicate glass with a nucleation agent added (G18m), and a borosilicate glass (G6). The hybrid phlogopite mica seals were aged at 800 degrees C for similar to 500 to similar to 1000 h in moist, dilute hydrogen fuel, and then subjected to short-term thermal cycling between similar to 100 and 800 degrees C. Seals with G 18 and G18m glass interlayers showed very poor thermal cycle stability after isothermal ageing. Seals with borosilicate glass interlayers showed very good thermal cycle stability with constant, low leakage (< 0.01 sccrn cm(-1) at 0.2 psi) after ageing for 508 h followed by 56 thermal cycles. Post-mortem analyses showed degradation of the mica due to interaction with the interlayer glasses. Leak paths were identified and correlated to the measured leak rates. (c) 2005 Elsevier B.V. All rights reserved.