Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.     

Thick thermal barrier coatings with different segmentation crack densities: Microstructure analysis and thermal oxidation behavior

Thick thermal barrier coatings (TTBCs) have been developed to increase the lifetime of hot section parts in gas turbines by increasing the thermal insulating function. The premeditated forming of segmentation cracks was found to be a valuable way for such an aim without adding a new layer. The TTBC introduced in the current study are coatings with nominal thickness ranging from 1 to 1.1 consisting of MCrAlY bond coat and 8YSZ top coat deposited by air plasma spray technique (APS). TTBCs with segmented crack densities of 0.65 mm^?1 (type-A) and 1 mm^?1 (type-B) were deposited on a superalloy substrate by adjusting the coating conditions. It was found that the substrate temperature has an influential role in creating the segmentation crack density. The crack density was found to increase with substrate temperature and liquid splat temperature. The two types of coatings (type-A and B) with different densities of segmentation crack were heat-treated at 1000 °C (up to 100 h) and 1100 °C (up to 500 h). The variation of hardness measured by indentation testing indicates a similar trend in both types of coatings after heat treatments at 1000 °C and 1100 °C. Weibull analysis of results demonstrates that higher preheating coating during the deposition results in a denser YSZ coating. The growth rate of TGO for TTBCs was evaluated for cyclic and isothermal oxidation routes at 1000 °C and 1100 °C. The TGO shows the parabolic trend for both two types of coatings. The Kps value for two oxidation types is between 5.84 × 10^?17 m²/s and 6.81 × 10^?17 m²/s. Besides, the type B coating endures a lifetime of more than 40 cycles at thermal cycling at 1000 °C.     

Thermal shock resistance of thermal barrier coating with different bondcoat types and diffusion pre-coating

This paper investigates the resistance of two types of thermal barrier coatings and compares their behavior with common coatings. Coatings’ layers in the first and second target sample were fabricated as HVOF/APS/APS (two bondcoats and one topcoat) and APS/APS (one bondcoat and topcoat) with diffusion pre-coating, respectively. Also, to accurately compare the behavior of these two types of coatings with conventional coatings used in gas turbines, this paper explored the resistance of three types of coatings applied as APS/APS, HVOF/APS, and HVOF coatings against thermal shock. In order to create shock loading, five types of laboratory samples were heated under regular cycles and cooled down with water. During the experiment, the sample changes caused by thermal shock loading were investigated through visual inspections. Then, after the experiment, the SEM images were leveraged to inspect the changes. In addition, changes in the structure of coating layers and their degradation process were studied. The results show that using two bond layers increases the resistance and life of the coating against heat shock by up to 1.40 times. Among the samples with one band coat, the sample with a diffusion coating applied under the BC showed the best performance. The sample life increased by 1.25 times compared to the common APS/PAS coating.