Changes in porosity and Young's Modulus due to sintering of plasma sprayedthermal barrier coatings

Increased efficiencies in energy conversion systems are the driving forces for the development of new and improved materials as part of operating comp onent such as gas turbines. Due to the large amounts of energy produced, ev en the smallest changes of the inlet temperatures result in a considerably smaller demand of fuel to produce the same amount of energy. In order to pr otect the material of the turbines against heat and oxidation, yttria stabi lized zirconia is used. Ni-base super alloy samples (IN 617) were coated with a two-layer thermal-b arrier coating system. The first layer was a vacuum plasma sprayed MCrAlY c oating, which was corrosion resistant and acted as a bond coat also. The se cond layer was atmospheric plasma sprayed and consisted of a 7 wt.% Y2O3-st abilized ZrO2 ceramic which acted as the thermal barrier. As other examinat ions have shown, two very important properties to characterize a thermal ba rrier coating are the porosity level of the ceramic coating and its Young's Modulus, as these have a significant effect on the strain tolerance and st ress level and hence on the durability of the coating. By varying the plasm a spraying parameters, two different microstructures and therefore porosity levels were produced. To simulate high-temperature exposure inside a turbi ne, the samples were annealed at 1100 degrees C for 2, 50 and 100 h. The di fferent ceramic coatings were characterized in terms of porosity and mean Y oung's Modulus before and after annealing. Due to its significant effect on the value of Young's Modulus, the porosity distribution was investigated a lso. The changing of the porosity level was measured with a mercury porosim etry. The effect of sintering on the mean Young's Modulus and hardness was examined with an instrumented depth-sensing micro-indentation technique. On e coating showed significant sintering detected through optical microscopy, increasing Young's Modulus and decreasing the porosity level, whilst the o ther coating showed no sintering at all, most probably due to having a diff erent powder composition. (C) 1999 Elsevier Science S.A. All rights reserve d.