Revealing the oxidation growth mechanism and crack evolution law of Si-HfO2/Yb2Si2O7/Yb2SiO5/high-entropy hafnate thermal/environmental barrier coatings during thermal cycling

The Si-HfO 2 /Yb 2 Si 2 O 7 /Yb 2 SiO 5 /(Dy 0.2 Ho 0.2 Er 0.2 Tm 0.2 Lu 0.2 ) 2 Hf 2 O 7 thermal/environmental barrier coating (T/EBC) protects ceramic matrix composites (CMCs) from turbine multi-corrosive media erosion. Challenges persist in the thermal cycling performance of multi-layered T/EBC, notably in understanding the oxidation of the Si-HfO 2 bond coat, compatibility of its mixed thermally grown oxide (m-TGO) with adjacent layers, and the evolution of cracks caused by thermal cycling. Utilizing plasma spraying physical vapor deposition (PS-PVD), this T/EBC on CMC substrates withstands up to 200 hours at 1450 ℃ to 1550 ℃. The m-TGO oxidation follows a parabolic growth curve, with oxygen diffusion activation energies of 160.31 kJ/mol from 1450 ℃ to 1500 ℃, and 125.16 kJ/mol from 1500 ℃ to 1550 ℃. Thermo-mechanical calculations indicate that elastic strain energy accumulation causes interlaminar cracks between m-TGO and adjacent layers. Controlling mud crack density is key to preventing the stress attraction at the tips of bifurcated cracks, thereby avoiding the formation of interlaminar cracks. • The oxidation growth of m-TGO follows a parabolic model, and oxygen is transported in molecular form within the T/EBC. • The residual stress at the CMC/m-TGO interface is the highest in the multilayer T/EBC, promoting interlaminar cracks. • The energy release rate at the CMC/m-TGO and m-TGO/Yb 2 Si 2 O 7 interfaces significantly increases near the cristobalite phase transition temperature. • The stress field at the tips of adjacent vertical mud-cracks may promote the lateral propagation of bifurcated cracks.