Related resources
Search for item elsewhere
University researcher(s)
Academic department(s)
FABRICATION AND CHARACTERIZATION OF THERMAL BARRIER COATINGS
[Thesis]. Manchester, UK: The University of Manchester; 2015.
Access to files
-
FULL-TEXT.PDF (pdf)
Abstract
New methods in the fabrication of top coat and bond coat have been introduced to improve the efficiency and performance of advanced thermal barrier coatings (TBCs).i. Top coat. Thick yttria-stabilized-zirconia (YSZ) coatings (300-400 μm) have been fabricated by using electrophoretic deposition (EPD) method. The EPD coatings have more favorable microstructures with uniformly distributed porosity and stronger bonding, in comparison with conventional air-plasma spray (APS) coatings. ii. Bond coat. Pt-diffused single γ’-phase bond coat has been fabricated by applying selective etching prior to the electroplating of Pt on CMSX-4 single crystal superalloys. The concern on the compromised scale adhesion caused by the depletion of Pt is effectively avoided, as Pt remains stable in a coherent γ’-phase layer after long-term diffusion and oxidation. Considerable cost of Pt could also be reduced. Commercial TBCs, comprising an electron beam physical vapour deposition (EBPVD) top coat, a Pt-enriched intermetallic bond coat and a CMSX-4 single crystal superalloy, have also been investigated focusing on the failures that typically occurred at the scale/alloy interface. Advanced characterization techniques have been used to study the chemical factors (Al, Pt, S, Hf, etc.) that determine the durability of TBCs. Mechanisms have been discussed that control the TBCs behaviours of diffusion, oxidation, and adhesion. i. Diffusion. A depletion of Pt near the scale/alloy interface inevitably occurs at high temperatures, which significantly weakens the scale adhesion. Mechanisms controlling the diffusion of Pt in Ni-based single crystal superalloys at high temperatures have been investigated focusing on the evolution of phase, microstructure, and composition. It was found that Pt has negative chemical interactions with Al, Ti and Ta, all of which could stabilize Pt in β- and γ’-phases, and therefore avoid the depletion of Pt. ii. Oxidation. Selective oxidation behaviour of Ni-based superalloys has been studied by using thermodynamic calculations, which is mainly affected by alloy compositions, oxygen partial pressures and temperatures. It was found that the formation of a protective α-Al2O3 scale is more favoured under lower oxygen partial pressures and higher temperatures. The additions of Al and Pt in Ni-based superalloys could also promote the formation of Al2O3 and the exclusion of NiO and spinel. The additions of reactive elements (RE), however, are less effective and may even cause severe internal oxidations due to a competitive oxidation between Al and RE.iii. Adhesion. Sulphur effect in TBCs mainly refers to a segregation of sulphur at the scale/alloy interface, which significantly deteriorates the scale adhesion to alloys. High resolution secondary ion mass spectrometry (Nano-SIMS) was employed to trace sulphur in commercial TBCs. The undesired “sulphur effect” on scale adhesion was suggested to be caused by the formation of residual sulphides beneath the scale with weaker ionic bonding to alloy cations, rather than a segregation of sulphur atoms. Possible solutions have been suggested to alleviate the sulphur effect in TBCs.