CMAS behavior of yttrium aluminum garnet (YAG) and yttria-stabilized zirconia (YSZ) thermal barrier coatings

Calcium magnesium aluminosilicate (CMAS) that is formed from the ingested deposits in gas turbines degrades thermal barrier coatings (TBCs), especially for the most widely used material; yttria-stabilized zirconia (YSZ). In the present work, we examine the behavior of yttrium aluminum garnet (YAG) as an alternative material for TBCs. CMAS interaction studies were conducted by making composite pellets of YAG-CMAS and YSZ-CMAS powders. These pellets, after being subjected to heat treatment between 1100 °C and 1500 °C were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS), which showed YAG to be almost inert to CMAS whereas YSZ exhibited significant phase changes. To test the behavior of TBCs with YAG and 8YSZ as the topcoat material in a CMAS environment, cyclic furnace tests were conducted in which a controlled amount of CMAS was applied and then the samples were cycled to failure. In addition, to simulate the continuous accumulation of CMAS expected in service, a cyclic furnace test was devised in which a small dose of aqueous solution of CMAS was applied on TBC specimens at the start of every cycle until the samples were cycled to failure. In all these tests YAG TBCs outperformed YSZ in terms of durability. The mechanisms of CMAS attack are described and the relative resistance of YAG and YSZ is shown to be consistent with the Optical Basicity (OB) theory.