Influence of the Thermal Barrier Coatings Design on the Oxidation Behavior

The properties of two different types of thermal barrier coatings (TBCs) were compared to improve the surface characteristics on high temperature components. These TBCs consisted of a duplex TBC and a five-layered functionally graded TBC. NiCrAlY bond coats were deposited on a number of Inconel-738LC specimens using high velocity oxy-fuel spraying (HVOF) technique. For duplex coating, a group of these specimens were coated with yttria stabilized zirconia (YSZ) using plasma spray technique. Functionally graded NiCrAlY/YSZ coatings were fabricated by plasma spray using co-injection of the two different powders in a single plasma torch. The amount of zirconia in functionally graded coatings were gradually increased from 30 to 100 vol. pct. Microstructural changes, thermally grown oxide (TGO) layer growth and damage initiation of the coatings were investigated as a function of isothermal oxidation test at 970℃. As a complementary test, the performance of the fabricated coatings by the optimum processing conditions was evaluated as a function of intense thermal cycling test at 1100℃. Also the strength of the adhesive coatings of the substrate was also measured. Microstructural characterization was analyzed by scanning electron microscopy (SEM) and optical microscopy whereas phase analysis and chemical composition changes of the coatings and oxides formed during the tests were studied by XRD (X-ray diffraction) and EDS (energy dispersive spectrometer). The results showed that microstructure and compositions gradually varied in the functionally graded coatings. By comparison of duplex and functionally graded TBCs oxidation behavior (duplex failure after 1700 h and funcitionally graded TECs failure after 2000 h), thermal shock test and adhesion strength of the coatings, the functionally graded TBC had better performance and more durability.