**4.3. Top coat degradation**

Top coat degradation is another parameter that governs TBC failure. The ceramic top coat has a tendency to crack due to stress generated from thermal expansion mismatch between the three layers of the TBC system. When the top coat cracks, oxygen easily diffuses to the bond coat leading to the catastrophic failure of the TBC system. Significant research is being carried out to improve the microstructure, mechanical properties, and stability of the ceramic top coat [12]. TBCs are subject to many kinds of degradation, e.g., erosion, foreign object damage (FOD), oxidation, etc., which deteriorate the integrity and mechanical properties of the whole system. Moreover, a new type of damage has been highlighted, i.e., corrosion by molten Calcium-Magnesium-Alumino Silicates, known as CMAS with the aim to increase the turbine inlet temperature. Basu et al. studied interactions between YSZ materials synthesized via the solgel process and synthetic CMAS powder via a step-by-step methodology. However, CMAS can cause faster sintering of the ceramic and thereby, leading to loss of strain tolerance in the protective coating. Further, a dissolution/re-precipitation mechanism between YSZ and CMAS resulted in the transformation of the initial tetragonal YSZ into globular particles of monoclinic zirconia. In addition, CMAS infiltrated both EB-PVD and sol-gel YSZ coatings at 1,250°C for 1 h [12]. Thompson and Clyne [13] deposited a vacuum plasma spray (VPS) MCrAlY bond coat and atmospheric plasma spray (APS) zirconia top coat onto a nickel superalloy substrate. They measured the stiffness of detached top coats by cantilever bending and also by nanoin‐ dentation technique. Measurements were made on as-sprayed specimens and after various heat treatments. Significant changes were detected in the Young's modulus of the heat treated top coat. The rate of sintering was found to be a function of temperature and weather. The coating was detached with the substrate during heat treatment. During high temperature exposure the effects of stiffening of the top coat on the stress development within the TBC system was included by using a well-known, modified numerical model. Sintering of the top coat enhanced debonding at the top coat-bond coat interface resulting in top coat spallation under service conditions [13]. It has been found by Abubakar et al. [14] that the use of low grade fuels in land-based turbines in Saudi Arabia results in hot corrosion due to the diffusion of a molten salt (V2O5) into the top coat of the TBCs. Consequently, volumetric expansion of the coating occurs due to the tetragonal-to-monoclinic transformation of zirconia in the planar reaction zone near the surface of the coating. They used a phase field model for estimating the kinetics of microstructure evolution during the corrosion process at 900 °C and close agreement between numerical and experimental results was achieved. The transformation-induced stresses were predicted by coupling the phase transformation with elasticity. The result showed that the coating spallation occurred due to very high compressive stress development within the coating cross section [14].
