Substrate effect on the high temperature oxidation behavior of a pt-modified aluminide coating.: Part II:: Long-term cyclic-oxidation tests at 1,050°C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 degrees C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclic-oxidation tests at 1,050 degrees C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 degrees C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 degrees C for 100 h and cyclic-oxidation tests at 900 degrees C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the beta to gamma' transformation leads to the formation of a two-phase beta/gamma' layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this beta to gamma' transformation occurs gradually by an inward transformation of beta leading to the formation of a continuous layer of gamma' phase, parallel to the metal/scale interface.