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Abstract: With the aid of three-point bending test the mechanical behavior of the NiCrAlY coating systems with or without barrier layer was analyzed. Together with the results from tensile adhesion test the damage mechanism of the investigated systems was discussed. It is found that the damage behavior was mainly dependent on the interfacial adhesion and the overlayer property. With a strong or weak interfacial adhesion and the brittle or ductile overlayer different damage mechanism was developed during test. The presence of residual stress and brittle precipitates or flaws at the coating/substrate interface influenced the interfacial adhesion property. The thermal exposure treatment improved the interfacial strength and the overlayer ductility. [Copyright &y& Elsevier]

Abstract: The microstructural evolution of the NiCrAlY/CrON system during vacuum heat treatment and thermal exposure was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The elastic modulus and residual stress of the coating system were measured by nanoindentation and photo-stimulated luminescence spectrum (PSLS) methods, respectively. It was shown that the series reaction of β→γ′→γ took place in the NiCrAlY coating, which contributed to a decrease in the Young’s modulus. The replacement of Cr2O3 by Al2O3 during thermal exposure resulted in an increase of the Young’s modulus of the barrier layer. A change in the residual stress in the barrier layer was due to a combination of thermal and growth stresses. [Copyright &y& Elsevier]

Abstract: Thermal shock cycling behaviors of D-gun sprayed TBCs with a hollow spherical ZrO2–8Y2O3 (HSP-YSZ) top coat and NiCrAlY bond coat on directionally solidified Ni-base superalloys DZ125 were investigated at high temperature (1100°C)↔room temperature (RT) repeatedly by water quenching. Scanning electron microscopy (SEM) was used to characterize the coating microstructure and failure morphology. The results showed that failure of the D-gun sprayed TBC starts with crack initiation along the splats boundary in the ceramic top coat and the non-alumina oxides. The cracks propagate and coalesce with the increasing thermal cycling. The extensive cracking of the rapidly formed non-alumina oxides, resulting from the depletion of aluminum in the bond coat, aids to delamination of the outer ceramic layer. The stress distributions in TGO layer at different thermal shock cycles was measured by luminescence spectroscopy to investigate the failure mechanism of TBC system. [Copyright &y& Elsevier]

Abstract: NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated specimens were investigated by cycling between high temperature (900, 1000 and 1050°C) and room temperature (forced water quenching). The experienced attack during thermal shock cycling was studied by gravimetry, XRD, SEM and EDS. The results showed that bare alloys and coated specimens experienced aggravating oxidation under thermal shock cycling conditions. The coated specimens exhibited excellent thermal shock resistance and provided enough protective effect on the superalloys under thermal shock cycling conditions. The accelerated oxidation mechanism and crack formation and propagation in the coatings were also investigated. The accelerated oxidation can be attributed to the accelerating oxide formation and breakdown due to compressive stresses accumulation in the oxides due to oxide growth at high temperature and thermal mismatch during cooling. The formation and propagation of cracks formed in coatings can be attributed to tensile stresses in oxides generated by irreversible cyclic creep accumulation in the metallic coatings and the “wedging” effect of oxide protruded into the coatings. [Copyright &y& Elsevier]

Abstract: NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated superalloys were investigated by cycling between high temperature (900, 1000 and 1050°C) and room temperature (forced water quenching). The microstructure evolution in coatings and the underneath substrates was studied. The results showed that phase transformation from γ′ to γ occurred at the surface of the coated and bare specimens due to the consumption of Al and Ti by oxidation. A directional coarsening of the γ′ precipitates (γ′ rafting) just beneath the γ′-impoverishing zone (or interdiffusion zone) is observed. This kind of rafting is believed to come into being from the compressive stresses by temperature gradients due to high cooling rate during cooldown. The relationship between microstructure evolution and cracks formation and propagation is also discussed. [Copyright &y& Elsevier]