Thermal cyclic response of laser-glazed plasma-sprayed ZrO 2-19.5wt.%Y 2O 3/Ni-22Cr-10Al-1Y thermal barrier coatings

Abstract

A pulsed CO 2 laser with a cyclindrical focal lens has been employed to glaze the surface layer of plasma-sprayed ZrO 2-19.5wt.%Y 2O 3/MCrAlY thermal barrier coatings (TBCs). The effects of laser glazing on the thermal cyclic response of these coatings have been evaluated. Results of this research showed that laser glazing enhanced the thermal cyclic life of plasma-sprayed TBCs by a factor of about 2–6 depending on the heating cycle length and the definition of cyclic life. The effect on the life improvement was more pronounced for specimens with a short heating cycle length. The thermal cyclic life of laser-glazed TBCs increased with increasing input energy, i.e. increased with increasing melting depth, while the overlap had no significant influence on the thermal cyclic life. The laser-glazing process did not have an evident effect on reducing the oxidation rate of the bond coat. Segmented cracks produced by laser glazing were beneficial for accommodating the strain which resulted either from the thermal cycle or from oxidation of the bond coat. Improving the strain tolerance was the major enhancement mechanism for TBC life extension. The failure of laser-glazed TBCs was similar to that of plasma-sprayed TBCs, which was initiated and propagated mainly within the top coat near the top coat-bond coat interface just above the higher asperities. The laser parameters and heating cycle length had no influence on the fracture morphologies. After cyclic tests no phase transformation was found in either the plasma-sprayed or laser-glazed top coats.