On the effect of ageing on the erosion of EB-PVD TBCs

Thermal barrier coatings (TBCs) have been used in gas turbine engines for some four decades, and the erosion resistance of these coatings has been well documented over the years by various different groups. However, as far as can be ascertained most of the laboratory research on erosion resistance of the coatings has been conducted on the coatings in the as-received condition. The effect of service conditions on the erosion rate of the coatings has not been addressed. This paper looks at the effects of thermal ageing on the erosion rate of electron beam physical vapour-deposited TBCs by testing the erosion resistance of coatings that have received various different heat treatments and comparing them to those in the as-received condition. Initially two different ageing heat treatments were used 1500 °C for 24 h, aimed at sintering the ceramic columns, and a less severe ageing of 1100 °C for 100 h. It was found that both of the heat treatments resulted in a significant increase in the erosion rates when compared to the as-received samples. Subsequent to the initial tests, it was found that heat treating the TBC for 30 h at 1100 °C resulted in an insignificant increase in the erosion rate. For the samples given the higher temperature heat treatment, the increase in the erosion rate was attributed to the fact that the columns were partially sintering together which enabled cracks to propagate into neighbouring columns, as opposed to stopping at column boundaries as occurs in samples that have not been heat treated. This results in an increase in erosion rate due to the fact that more material is removed per impact event.