SEM micrographs of thermal cycling tested samples in air are displayed in Fig.1 with their references for compari-son. No macroscopic spallation has been noticed on any coatings after thermal cycling tests. Indeed, lutetium and yttrium silicate materials have been chosen for their coefficient of thermal expansion in agreement with the substrate(see Table 1) as well as for their good thermal cycling resistance. Therefore, the thermal cycling tests validate the yttrium and lutetium silicates choice as envi-ronmental barrier coating material regarding their thermal cycling resistance properties. Moreover, the porous structure and vertical cracks promote the coating resistance to thermal cycling tests.
Fig.1 Representative BSE-SEM micrographs of L(1), LZ(2),Y(3) and YZ(4) heat-treated coatings before(a) and after(b) thermal cycling test
The major phase of L coatings is the lutetium disilicateLu2Si2O7, with additional traces of lutetium mono-silicate Lu2SiO5, due to a lack of silica during the deposition process. The mono-silicate phase is reduced to traces so that L coatings composition can be considered as homo-geneous and as a result no thermal stress can be generated from composition variation.
Table 4 Coefficient of thermal expansion(CTE)of bulk materials
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