Abstract
In this study, a peg-nail structured laser modification was implemented on thermal barrier coatings (TBCs) by a pulsed Nd:YAG laser. The effects of the peg-nail structured laser modified units on microstructure and thermal shock resistance of the TBCs were investigated. The results indicated that the surface roughness of the peg-nail structured laser modified units was less than that of the as-sprayed coating. The laser modified region, which penetrated the whole YSZ top coating and the partial NiCoCrAlYTa bond coating, consisted of a network of segmented cracks perpendicular to surface and a dense dendritic columnar structure associating with a thin discontinuous equiaxed grains on its surface. The interface between the top coating and the bond coating exhibited a metallurgical bond to some degree after the peg-nail structured laser modification. XRD analysis revealed that the as-sprayed coating was composed of nonequilibrium tetragonal (T′) phase, cubic (C) phase and a small amount of monoclinic (M) phase, however, the last phase disappeared after laser modification. The thermal shock tests indicated that the cyclical lifetime of the as-sprayed TBCs were enhanced around 3–5 times by the peg-nail structured laser modification after 130 thermal shock cycles. The TBCs’ failure was due to the oxide scale growth of bond coating and the thermal expansion mismatch stress during thermal shock cycles. Furthermore, both improvement on strain accommodation and delay of cracks propagation resulting from network segmented cracks perpendicular to the surface and dendritic columnar structure in the peg-nail structured units, formation of metallurgical bonding which enhances bonding strength, creating the lamellar and columnar alternate microstructure with toughness and rigidity hybrid characteristics for providing more compliance to the coating, were all considered as the main reasons of improving thermal shock lifetime of peg-nail structured laser modified TBCs.
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