| 1. |
- Gupta, Mohit Kumar, 1986-
(författare)
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Design of Microstructures in Thermal Barrier Coatings : A Modelling Approach
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plasma sprayed Thermal Barrier Coating systems (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. The material that is most commonly used in these applications is Yttria Stabilized Zirconia (YSZ) because of this ceramic’s favourable properties, such as low thermal conductivity, phase stability to high temperature, and good erosion resistance. The coating microstructures in YSZ coatings are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating’s final thermal and mechanical properties, and the service lives of the coatings. Determination of quantitative microstructure–property correlations is of great interest as experimental procedures are time consuming and expensive.This objective of this thesis work was to investigate the relationships between coating microstructure and thermal-mechanical properties of TBCs, and to utilise these relationships to design an optimised microstructure to be used for next generation TBCs. Simulation technique was used to achieve this goal. Important microstructural parameters influencing the performance of TBCs were identified and coatings with the identified microstructural parameters were designed, modelled and experimentally verified. TBCs comprising of large globular pores with connected cracks inherited within the coating microstructure were shown to have significantly enhanced performance. Low thermal conductivity, low Young‘s modulus and high lifetime were exhibited by these coatings. The modelling approach described in this work can be used as a powerful tool to design new coatings as well as to achieve optimised microstructures.
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| 2. |
- Torkashvand, Kaveh, 1990-
(författare)
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Tribological behaviour of HVAF-sprayed WC-based coatings: : Role of process variables and binder chemistry
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tungsten carbide (WC) based metallic matrix coatings sprayed using high velocity air fuel (HVAF) technique have attracted increasing attention as they show excellent tribological performance in various wear conditions. In the HVAF method, particles’ in-flight temperature and velocity can be influenced by the process variables including nozzle configuration as well as feedstock particle size range. On the other hand, the chemistry of metallic binder can also play a key role in determining both properties and performance of these coatings. In this thesis, characteristics and tribological behaviour of HVAF-sprayed WC-based coatings were investigated employing four different nozzle configurations (4L2, 4L4, 5L2and 5L4), and three different feedstock particle sizes of WC-CoCr feedstock powder (5/20, 5/30 and 15/45 μm). Also, characteristics and performance of coatings processed with four different WC-based feedstocks comprising alternative binders to traditionally used CoCr (namely CoCr, NiMoCrFeCo,FeNiCrMoCu and FeCrAl) were investigated. Characteristics of the coatings were explored by conducting scanning electron microscopy observations, microindentation testing and X-ray diffraction analysis. Performance of the coatings was evaluated by conducting sliding wear, dry jet erosion wear and sand rubber wheel abrasion wear testing. Moreover, material removal mechanisms in the coatings subjected to the above tests were investigated through post wear analysis.X-ray diffraction analysis showed that no considerable phase change compared to the starting feedstock was observed in any of the coatings sprayed by HVAF technique. It was revealed that decrease in powder particle size range can result in an improvement in microstructural characteristics, such as homogeneity and density, as well as hardness of the coatings. Besides, it can lead to a substantial improvement in wear performance of the coatings. It was shown that using various nozzle configurations does not result in any considerable change in characteristics or performance of the HVAF-sprayed WC-CoCr coatings. It was further shown that, by decreasing particle sizes from coarse to medium or fine, a significant difference can be observed. While wear mechanisms for medium and fine feedstock coatings were dominated by ploughing and fracture of individual carbide grains, for the coarse feedstock coatings ploughing and grooving along with the local removal of coating material were observed.Hardness values for all the four coatings with different binder chemistries were in a narrow range of 1100 – 1300 HV0.3. WC-NiMoCrFeCo and WC-FeNiCrMoCu coatings showed better or comparable sliding wear performance compared to WC-CoCr coating (as reference). WC-FeNiCrMoCu and WC-FeCrAl coatings showed comparable performance under erosion conditions while all the three binder alternatives yielded slightly inferior coating performance under abrasion wear, compared to the reference coating. While ploughing was the common wear mechanism in all the four coatings, pitting was noted in coatings with CoCr and FeNiCrMoCu binders in case of sliding wear.
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| 3. |
- Uczak de Goes, Wellington, 1990-
(författare)
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Suspension plasma sprayed thermal barrier coatings for internal combustion engines
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The upward trend in internal combustion engine efficiency is likely driven by the depletion of fossil fuels. Since no replacement in sight can deliver energy comparable to the conventional oil, there is a need to use it more rationally and effectively. Thermal barrier coatings have been seen for a long time as a solutionto increase the thermal efficiency of gas turbine engines but suffer from the lackof strong applicability in internal combustion engines. This is due to the different restrictions when comparing the environment on the gas turbines and in internal combustion engines. To overcome this problem and, at the same time, expand the application field of thermal barrier coatings, more efforts need to be devoted.In this work, different top coat materials using various deposition techniques were evaluated and categorized in three different thermal barrier coating (TBC) architectures. The first was the lamellar yttria-stabilized zirconia (YSZ) top coat deposited by atmospheric plasma spray (APS), used as a reference sample. The second architecture was a columnar suspension plasma spray (SPS) TBC with YSZ and gadolinium zirconate (GZO) top coat. The SPS process can produce avariety of microstructures, and they were, for the first time, tested in an internal combustion engine. The third architecture was an SPS top coat, with an additional layer on the top, called a sealing layer of either metallic or ceramic material, both never investigated in a diesel engine application earlier.For the thermophysical properties investigation, a combination of laser flashanalysis (LFA) and modeling with object-oriented finite element (OOF) was employed to understand the properties in all the applications. The performance of the coatings was evaluated in two different ways, by thermal cyclic tests, basedon the TBCs behavior under cyclic thermal loads and by single-cylinder engine experiment. The characterization of the coatings was done by scanning electron microscope (SEM) before and after the thermal cyclic tests.The performance properties were correlated with coatings microstructure and thermophysical properties. It was shown that a columnar TBC produced by SPS had a superior engine efficiency in the single cylinder engine experiment.
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