Crack healing phenomena were observed in mullite and mullite + Yb 2 SiO 5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiC f /SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles; however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 • C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO 2 phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al 2 O 3 in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y 2 SiO 5 . The indentation load-displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb 2 SiO 5 EBC demonstrated the best durability during repetitive thermal shocks.