Volume 8, Issue 10 (November 2011)
Influence of Protective Coatings on Damage and Lifetime of Alloy 247 DS in Thermomechanical Fatigue and Bending Tests
The present work was focused on the damage mechanisms and lifetime of a directionally solidified, oxidation protection coated Ni-base superalloy under out-of-phase thermomechanical fatigue (OP TMF) loading. The main scope of the work was the analysis of crack formation and propagation in the coating depending on its fracture strain. Two coating systems applied by low pressure plasma spraying were investigated: (i) MCrAlY-coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl top coat. The fracture strains of the coatings were determined at T = 20°C up to 600°C by four-point bending with acoustic emission analysis. The MCrAlY-coating did not influence significantly the TMF lifetime of the superalloy, whereas duplex-coatings reduced it drastically. This correlates with the fracture strains of the coatings that are for the MCrAlY-coating distinctly higher and for the duplex coating lower than the mechanical strain range of the TMF tests. Thus, the damage process was controlled by propagation of cracks formed after few TMF cycles in the duplex coated material and by slower initiation and propagation of fatigue cracks in the MCrAlY coated alloy. Moreover, crack branching at the interface between coating and substrate alloy affects crack penetration into the substrate. In addition to measurement of TMF life, the early stages of crack initiation and propagation in the duplex coating were studied in more detail after TMF tests that were interrupted after defined cycle numbers as well as during in situ bending tests in the scanning electron microscope. Furthermore, the influence of pre-oxidation on crack propagation is discussed.