Tailoring approach to predict the microstructure of thermal barrier coatings by adjusting the distribution of splat morphologies
Final Report Abstract
The project combines the investigations of the influence of indirect splat-affecting parameters (gun current, standoff distance, and flow of plasma gases; Argon and H2) and direct splat-affecting parameters (spray angle, gun traverse speed, and substrate temperature) on the splat type distribution to improve the thermal properties of DVC TBCs. The goal is to design the top coating of the TBC system that is dense vertically cracked with a porosity level higher than 10 %. The effects of indirect splataffecting parameters were analyzed indirectly, hence the name, in two stages. Firstly, the influences of indirect splat-affecting parameters on the melting state and inflight particle temperature and velocity at the time of impact on the substrate’s surface were investigated. Secondly, the influence of impact temperature and velocity on the splat formation, relative fractions of splat types, and coating porosity and microstructure were analyzed. Unexpectedly, the plasma jet of both spray guns showed voltage fluctuations that have almost no effect on the average temperature and velocity of the inflight particles, and consequently, on the distribution of the splat type. Since the SinplexPro 90 plasma gun provides a more stable plasma arc across a wide range of settings compared to the MB-F4 gun, it was utilized throughout the entire project to conduct the experiments. Based on the performed analyses, one parameter set, among the other sets, was selected to be used in the next work package because it generated a higher fraction of disk-like splat type and produced a consistent layer structure that is eligible to form dense vertical cracks. Concerning the effect of direct splat-affecting parameters, it was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats up to 60 %, and coating porosity while shifting the spray angle distorts the disk-like splats and decreases porosity. The effect of gun traverse speed is found to have almost no effect on the splat formation but has a significant effect on the generation of segmentation cracks and porosity. This result draws attention to the effect of the powder feed rate parameter, which is not included in the current work. It was also found that distributions with dominant fully molten splats are eligible to generate dense segmentation cracks as long as they are sprayed at low gun traverse speeds. Surprisingly, it was found that depositing on a hot steel substrate plays only a secondary role in the formation of the segmentation cracks. The effect of a hot substrate on the density of segmentation cracks is only observable when deposition takes place at low gun traverse speeds. The parameter set that is selected after investigating both indirect and direct splataffecting parameters was further investigated by an optimization process. Gun traverse speed and flow of the plasma gas (argon) were varied with the focus on examining the interaction between them to increase the level of porosity and segmentation cracks at the same time. Although depositing with gun traverse speeds lower than 1.5 m/s generates dense segmentation cracks, the produced coatings showed poor lamellar interconnection. Therefore, it is recommended to increase the feed rate of the feedstock to compensate for the effect of slow gun traverse speed. The regression model is a very useful tool in predicting the optimal setting for process parameters based on the results of conducted experiments. Since there was no parameter indicating the coating’s lamellar interconnection (coating quality), the model further proposed deposition with very low gun traverse speeds in order to increase the density of segmentation cracks. Coatings that are produced using the optimal set parameters showed a density of segmentation cracks of about 4.5 crack/mm, which is very close to the work’s target, and a porosity of about 5.5 area%, which is too low. Splat analysis during the parameter optimization revealed that splat type distributions of dominant fully molten and disk-like splats are essential to generate segmentation cracks. Many organic materials can act as PFAs effectively, however, suitable bulk density and morphology should be taken into account to avoid problems of powder flowability. Additionally, the weight fraction of the PFA will depend on the difference in the bulk density of the feedstock and PFA powders. Spherical morphology and a high ratio of width to length (for rod-like particles) of the organic PFAs are necessary for good flowability in the powder feeding process. More investigations are required to reveal the effect of PFA parameters on the added porosity. To evaluate the design procedure of the desired TBC coating, three coatings that represent the design stages were examined by performing four tests. The coating of the final stage (YSZ-20%PFA) demonstrated lower thermal conductivity, higher thermal shock resistance, and moderate adhesion strength and wear resistance compared to the coatings of the earlier design stages. At the earlier stages, the design procedure focuses on producing a high-quality layer structure by selecting the parameters and conditions that lead to forming higher fractions of disk-like splats. Expectedly, the obtained coatings were dense vertically cracked and possessed dense structures with very low levels of porosities. However, the parameter set and conditions led to the favorable porosity distribution (larger numbers of fine-sized pores) were included in an optimization process. In the last stage of the procedure, the design aimed to increase the coating porosity utilizing PFAs without affecting the density of segmentation cracks or the layer interconnection. The detriments caused by the added porosity (the reduction in wear resistance and adhesion strength) could be tolerated for the sake of improving the thermal properties of the designed TBC.
Publications
- Influence of Introducing an Organic Pore- Forming Agent on the Porosity and Microstructure of Alumina Coatings Produced by the Atmospheric Plasma Spray Process. In: Journal of Thermal Spray Technology, 28(8) (2019), p 1919-1932
Tillmann, W.; Khalil, O.; Abdulgader, M.
(See online at https://doi.org/10.1007/s11666-019-00934-5) - Porosity Characterization and Its Effect on Thermal Properties of APS-Sprayed Alumina Coatings. In: Coatings, 9(10), (2019), p 601
Tillmann, W.; Khalil, O.; Abdulgader, M.
(See online at https://doi.org/10.3390/coatings9100601) - Influence of direct splat-affecting parameters on the splat-type distribution, porosity, and density of segmentation cracks in plasmasprayed YSZ-coatings. In: Journal of Thermal Spray Technology, 30(4), (2021), p 1015- 1027
Tillmann, W.; Khalil, O.; Baumann, I.
(See online at https://doi.org/10.1007/s11666-021-01180-4) - Influence of spray gun parameters on inflight particle’s characteristics, the splat-type distribution, and microstructure of plasmasprayed YSZ coatings. In: Surface and Coating Technology, 406 (2021) 126705
Tillmann, W .; Khalil, O.; Baumann, I.
(See online at https://doi.org/10.1016/j.surfcoat.2020.126705)
