The effect of CMAS penetration on the microstructure and failure of the TBCs applied by APS/APS method

This study investigates the effect of CMAS (CaO-MgO-Al2O3-SiO2) penetration with different dosages on TBCs applied by the APS/APS method. For this purpose, CMAS powder at dosages of 5 and 15mg/cm2 was applied on the surface of laboratory samples. The samples, which were heated for 4 h, were made by Inconel 738 with BC and TC layers. CMAS sediments were melted at 1200 °C and penetrated the pores and cracks in the microstructure of TBCs. It was observed that at a low dosage of CMAS elements, horizontal cracks appear in the middle parts of the TC layer. However, crack formation was close to the TC and BC interface at a high CMAS dosage. The reaction of CMAS with the Y in the ceramic layer leads to its discharge. XRD analysis has shown a phase change in the ceramic layer from YSZ-t to YSZ-m caused by the CMAS attack. This phase change is accompanied by an increase in volume, causing buckling of the TC. Further, the presence of CMAS in the ceramic layer made a CTE gradient in this layer, which also intensifies the buckling. In this study, OOF2 software was used to determine the ceramic layer's thermal conductivity changes using SEM images. To validate the numerical results calculated by OOF2 software, the LFA method has been used to determine the thermal conductivity changes in the ceramic layer. It was observed that the presence of CMAS elements increased the thermal conductivity of the TC layer by 49.5 %, leading to an increase in the growth rate of the TGO layer by 1.5 times. This phenomenon caused the degradation of the TC layer to occur more severely.