MCrAlY coatings, with good balance of oxidation, hot corrosion, and mechanical properties, are widely used on turbine engine blades/vanes both as standalone overlays and as bond coats for thermal barrier coatings (TBCs). However, in long-term service at high temperature, element interdiffusion between coating and underlying alloy greatly impairs its stability, shortens its service life. The introduction of diffusion barrier is an effective way to minimize the effect of element interdiffusion. By arc ion plating (AIP) technique, MCrAlY coating systems with or without diffusion barrier were deposited. With the aid of XRD, AFM, PSLS, SEM (equipped with EDS) and TEM, the morphologies, phase structures and chemical compositions of the coating systems were analyzed; the diffusion barrier ability and oxidation performance were evaluated. Using nanoindentation method, tensile adhesion test, three-point bending test and photo-stimulated luminescence spectroscopy technique, the mechanical properties of the coating systems were investigated; the influence of microstructural evolution on the mechanical properties were discussed; the relationship of the barrier ability of diffusion barrier and interfacial adhesive strength of the coating systems was clarified.
By controlling the O2 and N2 gas flow rates, the diffusion barriers with the different microstructure were deposited using Cr target. It was CrN phase in the CrN diffusion barrier and CrN and Cr2O3 phases in the CrON layer; during thermal treatment the reaction of CrN→ Cr2N→ TiN and Cr2O3→ Al2O3 took place. The diffusion barrier deposited with NiCrAlYSi alloy target was governed by Al2O3 phase after vacuum heat treatment.
With the addition of diffusion barrier the oxidation performance of the coating systems was improved due to the reduced interdiffusion. The formation of TiN in CrN diffusion barrier consumed the Ti element from DSM11 substrate and blocked the interdiffusion. But the coatings were attacked by oxidation eventually with the destruction of continuous diffusion barrier layer. The coating systems with CrON interlayer or diffusion barrier deposited with NiCrAlYSi target exhibited much effective protection for DSM11, which attributed to the excellent diffusion barrier ability of Al2O3 layer formed in the barrier layer. In NiCrAlY coating, γ'-Ni3Al and α-Cr particles were precipitated during vacuum heat treatment; besides the transformation of β-NiAl→ γ'–Ni3Al→ γ-Ni, the coarsening of grains, twins, interlaced dislocation and slip bands were detected after thermal exposure.
The microstructural evolution of the coating systems contributed to the change of mechanical properties. The transformation of β→ γ'→ γ resulted in the decrease of Young`s modulus in NiCrAlY coating. The replacement of Cr2O3 by Al2O3 brought on the increase of Young`s modulus of the barrier layer. The change of residual stress in the barrier layer was due to the combined result of thermal and growth stresses and was influenced by the formation of defects, which also had an effect on the measurement of Young`s modulus .
The introduction of diffusion barrier weakened the interfacial adhesive strength of the systems. The tensile adhesion test results showed that the strength was 44.1 MPa and 70.3 MPa for annealed NiCrAlY/CrON/DSM11 and NiCrAlY/CrN/DSM11 specimens respectively; 60.1 MPa and 63.6 MPa (with a bond coat) for annealed systems deposited with NiCrAlYSi alloy target by continuous method. After thermal exposure, the interfacial adhesive strength of the three systems was beyond the glue adhesive strength. During three-point bending test, the system with CrON interlayer failed in the interface; the system with CrN diffusion barrier ruptured in the overlayer after vacuum heat treatment and was damaged from the precipitates in the barrier layer after thermal exposure; the damage of single NiCrAlY coating originated from the defects formed on the coating surface or in the NiCrAlY/DSM11 interface. The interdiffusion helped to the improvement of the interfacial bonding and the overlayer ductility.
Here diffusion barrier layer with the conflicts between interdiffusion barrier and interfacial adhesion is appropriate with good balance of barrier ability and interfacial strength.
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