|Tipo di tesi||Tesi di dottorato di ricerca|
|Titolo||Protezione di componenti per la produzione di energia attraverso tecniche di ingegneria delle superfici: sviluppo di bondcoat rinforzati tramite dispersione di ossidi e coating diffusivi a base cromo.|
|Titolo in inglese||Protection of components for energy production by surface engineering techniques: development of oxide-dispersion-strengthened bondcoats and Cr-rich diffusion coatings.|
|Settore scientifico disciplinare||ING-IND/22 - SCIENZA E TECNOLOGIA DEI MATERIALI|
|Corso di studi||Scuola di D.R. in MODELLISTICA, SIMULAZIONE COMPUTAZIONALE E CARATTERIZZAZIONE MULTISCALA PER LE SCIENZE DEI MATERIALI E DELLA VITA|
|Data inizio appello||2014-02-27|
|Disponibilità||Accessibile via web (tutti i file della tesi sono accessibili)|
Per proteggere i componenti dei moderni sistemi Turbogas per la produzione di energia,si utilizza negli stadi più caldi una barriera termica multistrato (Thermal Barrier Coating, TBC), formata da un layer ceramico esterno e da un rivestimento metallico più interno. Il coating ceramico garantisce l’isolamento termico, quello metallico (bondcoat) protegge il materiale dai fenomeni degradativi. Negli stadi più freddi, come alternativa più economica alla termospruzzatura del bond coating metallico, si impiegano rivestimenti diffusivi ottenuti tramite pack-cementation.
Thermal barrier coatings (TBC) are applied on gas-turbine and aero-engine components to extend their durability, by protecting the base material from thermal degradation. TBCs are usually based on a metallic bondcoat and an external ceramic layer: the former protects the substrate from oxidation and hot corrosion, the latter provides the thermal insulation. To protect the colder stages of gas-turbines, pack-cementation coatings can be employed as a cost-effective alternative to thermally sprayed bondcoats. The continuous rise of the turbine inlet temperature, necessary for achieving higher thermal efficiency, requires the use of more durable TBCs. The aim of the present work was the development of protective coatings, applied using different surface engineering techniques, in order to extend the working life of gas-turbine components. The study was focused on the systematic investigation of the process parameters as well as on the evaluation of the high temperature behavior of thermally sprayed bondcoats and diffusion coatings obtained by pack-cementation. The first part concerned the deposition of Oxide-Dispersion-Strengthened (ODS) CoNiCrAlY bondcoats. The addition of oxide particles reportedly improves the creep resistance and reduces the mismatch between the coefficient of thermal expansion (CTE) of the different layers, enhancing the high-temperature behavior of the TBC. ODS powders were obtained by milling CoNiCrAlY and Al2O3 particles. The milling process was carefully investigated, by varying the milling speed, the milling time, the ball-to-powder mass ratio and the Al2O3 amount. Powders were then sprayed by High Velocity Oxygen Fuel (HVOF) and Air Plasma Spray (APS) onto Inconel 738 substrates. Two different systems were deposited: a full ODS bondcoat and a hybrid TBC based on a thin ODS layer applied above a standard bondcoat. ODS coatings deposited by APS exhibit high porosity and oxidized zones, whereas HVOF coatings are denser and more homogenous, even if some porosities are still present. The high-temperature behavior was evaluated by performing thermal cycling oxidation and isothermal oxidation tests. The microstructure of the coatings strongly influenced the thermal cycling resistance. ODS coatings deposited by APS failed after a limited number of cycles, whereas coatings deposited by HVOF showed a better resistance. The amount of hard phase embedded in the metal matrix affects the CTE and therefore the thermal shock resistance. The thickness of the thermally grown oxide (TGO) layer depends on the duration of the isothermal oxidation test. During the early stage the TGO is based on Al2O3. Increasing the duration of the test brought to the formation of a mixed-oxide layer. The analysis revealed that despite the addition of oxide particles, the metal matrix of the thin ODS layers is well connected, allowing the outward diffusion of a proper amount of Al from the standard bondcoat, necessary to form the oxide scale. The second part of the work was focused on the deposition of Cr-rich diffusion coatings. The addition of Cr is found to be a good improvement against hot-corrosion. Inconel 738 substrates were coated using a pack-chromizing process, varying the pack-mix composition and the duration of the thermal treatment. The characterization tests demonstrated that the diffusion mechanisms started before reaching the process temperature, leading to the formation of a thin Cr-rich layer on the external surface of the substrates. After a complete treatment, the coating consists of a three-layer structure: a Cr-rich outer layer, an intermediate layer based on Al-rich oxide particles and an inner layer consisting of Ti- and Cr-carbides and nitrides.