Mechanisms of spallation of solution precursor plasma spray thermal barrier coatings

The durability and failure mechanisms of solution precursor plasma spray (SPPS) thermal barrier coatings (TBCs) are investigated. SPPS TBCs exhibit an average life of 1018 cycles in a 1-h 1121 °C cycle furnace test, that is more than 2.5 times of that of a commercial APS TBC in the same test. Failure of the SPPS TBC starts with crack initiation along the unmelted particles in the ceramic top coat and the non-alumina oxides. The cracks propagate and coalesce with thermal cycling. The extensive cracking of the rapidly formed non-alumina oxides, resulting from the depletion of aluminum in the bond coat, leads to the development of a large separation between the TBC and substrate. When a crack of sufficient size emerges, the TBC separates from the metal substrate by large scale buckling. The greatly improved durability of the SPPS TBCs compared to APS TBCs on the same substrate and bond coat is attributed to the reduced stress near the bond coat-SPPS ceramic interface as a result of the vertical microcracks in the SPPS microstructure.