Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

Thermal barrier coatings have mainly been used in land based gas turbines and aircraft engines. Their main purpose is to ensure protection against wear, oxidation, hot corrosion, fly ash damage and also provide thermal insulation to the metallic components of gas turbines. Normally, the commercial thermal barrier coating systems are comprised of yttria-stabilized zirconia based ceramic top layer, a metallic bond layer and a thermally grown oxide layer. However, such systems have been seen to suffer from critical issues that lead to destructive effects such as the undesirable phase transformation and sintering that occurs at temperatures above 1170^°C.

Increasing demand for more efficient next generation gas turbines has championed for the need to have a higher temperature limit for the yttria-stabilized zirconia based thermal barrier coatings. Gadolinium zirconate, an alternative ceramic top coat material having better thermal properties than yttria-stabilized zirconia, has been seen as a potential replacement. It has already been proved to possess superior Ca–Mg–Al–Silicate and hot corrosion attack resistance characteristics than yttria-stabilized zirconia.

Mustafa Guven Gok and Gultekin Goller at Istanbul Technical University and Hakkari University in Turkey, investigated the thermal shock behavior and the resistance against hot corrosion and Ca–Mg–Al–Silicate attack of gadolinium zirconate/ceria-yttria stabilized zirconia based thermal barrier coating having multilayered and functionally graded designs. They hoped to achieve better thermal cycling and thermal shock resistance using this design as opposed to the single layered design. Their research work is now published in the peer-reviewed journal, Journal of the European Ceramic Society.

The researchers begun conducting their empirical test by selecting gadolinium zirconate/ceria-yttria stabilized zirconia based multilayered and functionally graded thermal barrier coating systems having 2, 4, 8 and 12 layers as coating samples. The coatings were subjected to thermal shock tests in a tube furnace and latter thrown directly into cold water. Hot corrosion attack and Ca–Mg–Al–Silicate resistance test was then performed simultaneously and observations made. A CO₂ laser beam was used to heat specimens. During the test, simultaneously with heating, a water cooling was performed from the back surface of the specimens to generate a thermal gradient on the specimens.

The authors observed that the gadolinium zirconate/ceria-yttria stabilized zirconia functionally graded coatings displayed better thermal shock resistance than multilayered and single layered gadolinium zirconate coatings. Secondly, they noted that the microstructural characterizations showed that the reaction products were penetrating locally inside of the yttria-stabilized zirconia. On the other hand, a reaction layer having ∼6 µm thickness between Ca–Mg–Al–Silicate and gadolinium zirconate was seen. This reaction layer was observed to inhibit further penetration of the reaction products inside of the functionally graded eight layered gadolinium zirconate/ceria-yttria stabilized zirconia coating.

The results obtained from the Ca–Mg–Al–Silicate and hot corrosion tests show that surfaces of yttria-stabilized zirconia and functionally graded eight layered gadolinium zirconate/ceria-yttria stabilized zirconia coating react differently with the Ca–Mg–Al–Silicate and hot corrosion products. It has been seen that Ca–Mg–Al–Silicate and hot corrosion products penetrated inside of the yttria-stabilized zirconia coating through micro cracks and pores. The coming into existence of the reaction layer having ∼6 µm thickness on the surface of gadolinium zirconate/ceria-yttria stabilized zirconia coating inhibits further penetration of Ca–Mg–Al–Silicate reaction products into the coating. Moreover, the thermal shock life time of the gadolinium zirconate/ceria-yttria stabilized zirconia coatings having multilayered and functionally graded designs is longer than single layered gadolinium zirconate thermal barrier coating hence it can be evaluated as an alternative thermal barrier coating material for yttria-stabilized zirconia coatings.