Nano and microhardness testing of aged EB PVD TBCs

Abstract

Previous studies on the erosion of electron beam physical vapour deposited thermal barrier coatings (EB PVD TBCs) has shown that aging the coatings at between 1100 and 1500 à °C before erosion increases the erosion rate. These changes in the erosion rate were attributed to a number of factors including changes to the nanoporosity within the coatings as well as phase changes within the coatings. Such changes in the morphology of the coatings should be measurable as changes in their hardness. Thus, it was decided to ascertain the effect that the aging had on the hardness of the coatings. Since, during erosion, the size of the interaction zone between the impacting particle and the coating is in the same range as the size of the individual columns of the coating, it was decided to measure the change in the hardness of the columns as well as the coating as a whole. It was found that the aging increased the hardness of both the coating as a whole and the individual columns of the coatings. The microhardness of the coating was found to increase from 2.5â  3.5 GPa in the as-received condition to 4.5â  6 GPa after 100 h at 1100 à °C and to 7.5â  8 GPa after 24 h at 1500 à °C. The nanohardness of the individual columns on the other hand was found to increase from 18 GPa in the as-received condition to 35 GPa after aging. This paper discusses the increases in hardness due to aging in terms of the sintering and morphological changes that occur in the coating. The difference in the nano- and microhardness results are discussed in terms of the relative size of the indents and column size and the associated interactions that occur under the different indenter heads a