Effect of heat treatment on elastic properties of separated thermal barrier coatings

Elastic response behavior of four different plasma-sprayed deposits has been investigated using depth-sensing micro-indentation technique. Due to the high degree of porosity and inhomogeneity of the coatings, the characteristic elastic moduli were found to be in the range of 20-75% of that of the dense bulk material (200 GPa). Considering the wide variation of properties, 150 data points were generated with five different indentation loads for each coating, and statistical tools were employed to represent the scatter of the data. The characteristic elastic moduli of all the coatings were observed to be almost doubled when the magnitude of indentation load was reduced from the highest (1000 mN) to the lowest (30 mN). The coatings were subsequently heat treated at 1100 degrees C, the operational temperature of a gas turbine, for 2, 25, and 100 h, and in all the coating grades the corresponding elastic moduli increased significantly. However, the stiffening effect was not uniform in two grades and was more pronounced for the smaller indentation loads. The increase in elastic modulus is attributed to elimination of fine porosity and sintering neck formation, an assumption also supported by the results of mercury porosimetry.