A thermal barrier coating system consisting of a NiCoCrA/Y bond coat and cerium-stabilized zirconia ceramic coating was sprayed on to a metallic substrate. Ageing at 400 and 800 degrees C for 100, 500, and 1000 h was performed. Microhardness measurements of as-sprayed and heat-treated samples were used to evaluate microstructural variations throughout the thermally sprayed coating after different ageing conditions. Forty readings were taken at both the bond coat and ceramic coating positions within the thermal barrier coating (TBC) system and adjusted by subtracting the two largest and two smallest readings. Both data sets were statistically analysed to assess whether they belonged to Weibull or Gaussian (or "normal") distributions. This study has established that the homogeneity of coatings, at least as measured by a microhardness test, varies during service and, thus, may influence the lifetime.